Environmental Management (2014) 54:1131–1138 DOI 10.1007/s00267-014-0344-5
Effects of Livestock Grazing and Well Construction on Prairie Vegetation Structure Surrounding Shallow Natural Gas Wells N. Koper • K. Molloy • L. Leston • J. Yoo
Received: 9 January 2014 / Accepted: 12 July 2014 / Published online: 31 July 2014 Ó Springer Science+Business Media New York 2014
Abstract Short and sparse vegetation near shallow gas wells has generally been attributed to residual effects from well construction, but other mechanisms might also explain these trends. We evaluated effects of distance to shallow gas wells on vegetation and bare ground in mixed-grass prairies in southern Alberta, Canada, from 2010 to 2011. We then tested three hypotheses to explain why we found shorter vegetation and more bare ground near wells, using cattle fecal pat transects from 2012, and our vegetation quadrats. We evaluated whether empirical evidence suggested that observed patterns were driven by (1) higher abundance of crested wheatgrass (Agropyron cristatum) near wells, (2) residual effects of well construction, or (3) attraction of livestock to wells. Crested wheatgrass occurrence was higher near wells, but this did not explain effects of wells on vegetation structure. Correlations between distance to wells and litter depth were the highest near newer wells, providing support for the construction hypothesis. However, effects of distance to wells on other vegetation metrics did not decline as time since well construction increased, suggesting that other mechanisms explained observed edge effects. Cattle abundance was substantially higher near wells, and this effect corresponded with changes in habitat structure. Our results suggest that both residual effects of well construction and cattle behavior may explain effects of shallow gas wells on habitat structure in mixed-grass prairies, and thus, to be effective, mitigation strategies must address both mechanisms.
N. Koper (&) K. Molloy L. Leston J. Yoo Natural Resources Institute, University of Manitoba, 70 Dysart Rd., Winnipeg, MB R3T 2M6, Canada e-mail: [email protected]
Keywords Edge effects Grasslands Habitat structure Livestock grazing Natural gas
Introduction As energy development continues across the world, its impacts on the environment are also increasing. This may have impacts on many components of prairie ecosystems, including ungulates (Sawyer et al. 2006; Beckmann et al. 2012), birds (Ingelfinger and Anderson 2004; Aldridge and Boyce 2007; Gilbert and Chalfoun 2011; Hamilton et al. 2011), and soil and vegetation structure (Rowell and Florence 1993; Nasen et al. 2011). Above-ground infrastructure associated with shallow natural gas wells is relatively small (23.1 m2/well, 1 m high in our study) and affects a relatively small area compared with infrastructure required for oil, solar and wind energy, biodiesel and ethanol (McDonald et al. 2009), and thus its impact on the landscape might also be predicted to be relatively small. Nonetheless, habitat near shallow gas wells has been associated with increased bare ground, increased abundance of non-native plant species, decreased abundance and diversity of native plant species, and shallow litter (Berquist et al. 2007; Nasen et al. 2011), and thus, their footprint extends beyond the perimeter of the infrastructure itself. Edge effects associated with shallow gas wells might explain why pronghorn (Antilocapra americana; Beckmann et al. 2012), songbirds (Hamilton et al. 2011), mule deer (Odocoileus hemionus; Sawyer et al. 2006; 2009), and other species have abundances that vary with shallow gas well density. Mechanisms that explain the effects of shallow gas wells on wildlife and their habitats are not well known (Riley et al. 2012). However, identifying the reasons for effects of wells on vegetation structure (e.g., vegetation height,
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density, and litter depth) and amount of bare ground (hereafter, vegetation structure and percent bare ground are referred to as habitat structure) is critical for determining effective mitigation strategies (Walker et al. 2007). Some characteristics of shallow gas wells suggest that these wells should have few ecological effects. For example, shallow gas wells in southern Alberta are accessed using small, low-impact trails, which have substantially less ecological impact than roads (e.g., Sutter et al. 2000; but see Carpenter et al. 2010). In addition, the above-ground infrastructure of shallow gas wells is silent, and visits are required only once annually (CSA Group 2011), virtually eliminating negative effects of anthropogenic noise (Barber et al. 2009). Nonetheless, effects of well construction might last for many years (e.g., Smith et al. 1988; Nasen et al. 2011), and well drilling and maintenance could introduce non-native plants such as crested wheatgrass (CWG, Agropyron cristatum; Larson et al. 2001; Riley et al. 2012), which has a strong impact on vegetation communities and structure (Vaness and Wilson 2007). While CWG is no longer allowed for restoration of shallow gas wells in Alberta (Alberta Environment 2003a), introduction of nonnative species may have long-lasting ecological impacts (Vaness and Wilson 2007; Simmers and Galatowitsch 2010; Nasen et al. 2011). Another mechanism that might explain effects of shallow gas wells on habitat structure is that above-ground infrastructure, including exclusion fencing around wells, may attract livestock to the wells, and perhaps it is livestock that directly impact habitat. Rangelands cover more than 300 million ha in the US alone (Reeves and Mitchell 2011), and a majority of North American grassland ecosystems are used for livestock grazing (Knick et al. 2003; Fleischner 1994). Consequently, most native and tame grasslands that include shallow gas well sites are grazed by livestock (e.g., see also Hamilton et al. 2011; Nasen et al. 2011). Cattle grazing can strongly influence habitat structure, which impacts habitat suitability for wildlife (Chapman et al. 2004). However, the interaction between livestock and gas wells has not been examined. Cattle are known to spend increased time around water sources (Fontaine et al. 2004; Tate et al. 2003; White et al. 2001), supplemental feed (Sanderson et al. 2010; Tate et al. 2003), and paddock gates and shade (Sanderson et al. 2010). Increased livestock activity and grazing pressure around these sites (Putfarken et al. 2008; Owens et al. 1991) can lead to increased bare ground (Sanderson et al. 2010), nutrient load (Sanderson et al. 2010), and decreased litter depth (Fontaine et al. 2004), so if livestock are also attracted to shallow gas wells, this might explain why vegetation tends to be short and sparse near well sites. In our study, we first assessed whether habitat structure varied with distance to shallow gas wells in southern
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Alberta, Canada (Analysis 1). We then developed three hypotheses and associated predictions to compare the empirical support for proposed reasons for observed patterns (we were unable to evaluate effects of trails because maps of trail locations were unavailable). These hypotheses were as follows: (Analysis 2) if effects of distance to wells on habitat structure are observed because CWG is close to wells, and CWG alters habitat structure, then occurrence of CWG should be higher near wells, and occurrence of CWG should be correlated with structural variables that are similarly correlated with distance to wells; (Analysis 3) if effects of distance to wells on habitat structure are driven by cattle grazing, abundance of cattle should be higher near wells, and vegetation should be shorter and more sparse near wells; and (Analysis 4) if effects of distance to wells on habitat structure are driven by cattle, effects of distance should be either independent of well age or increase with well age (depending on cattle breeds and ages; e.g., Ksiksi and Laca 2000); conversely, if effects of distance to wells on habitat structure are driven by residual effects of drilling, effects of distance should be greater near younger wells.
Methods Study Area Our study took place within 100 km of Brooks, Alberta, Canada (50˚ 350 N, 111˚ 530 W). This was part of a larger program to evaluate effects of shallow gas extraction on grassland songbirds. For this study, we surveyed vegetation in 36, 258-hectare (1 9 1 mile section) flat or gently rolling sites. All sites consisted of native mixed-grass prairie, predominantly species such as needle-and-thread (Hesperostipa comata), blue grama (Bouteloua gracilis), junegrass (Koeleria macrantha), western wheatgrass (Pascopyrum smithii), prairie sage (Artemisia ludoviciana), and pasture sage (Artemisia frigida). While there were some occurrences of non-native species such as dandelion (Taraxacum officinale) and goatsbeard (Tragopogon dubius), only crested wheatgrass occurred more than occasionally (occurrencecwg = 2 % of quadrats). All sites were also surrounded by native prairie, to minimize edge effects, but contained a few roads or trails within or at the perimeter of sites. Roads and trails ranged in permanence and impact from grass-dominated two-track trails with some bare ground in the tire tracks, to two-track trails with mostly bare ground in the tire tracks but with grass between the trails, to a few gravel and dirt roads. Almost all shallow gas wells were accessed by trails, not roads. Both control and treatment sites were selected using the same methods. Potential sites in the study region were
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identified using a GIS land-use map overlaid with well locations. Grassland-dominated sites were ground-truthed to ensure that they were predominantly native and surrounded by grassland on all sides. All suitable sites that were either controls, or had leases held by Cenovus Energy, were used for our study, to maximize power. Nine sites were controls that did not contain shallow gas wells. The remaining 27 sites contained shallow gas wells at a range of densities, from 1 to 20 well pads/section (1–29 wells), from low–high for this region. Due to commingling, some well pads contain more than one well head. On average, the area directly impacted by one shallow gas well pad (the area within the exclosure fence) was 23.1 m2; the largest measured exclosure was 42.3 m2. Wells were constructed 1–44 years prior to our surveys. Well construction methods changed somewhat during this period to reduce environmental impacts (e.g., Alberta Environment 2003a; Desserud 2011; Nasen et al. 2011). The most ecologically important change to well construction was a gradual switch from ‘‘full-build’’ methods (requiring removal of all topsoil and vegetation at the drilling site, and, since the 1980s, subsequent revegetation) to allow almost all vegetation and topsoil to remain in place while drilling directly through the topsoil, greatly reducing the disturbance footprint (Alberta Energy and Utilities Board 2002a; Alberta Environment 2003b). This practice became gradually more common starting in the mid-1990s, but even today, full-builds may be used for new well construction where topography or soil conditions make it necessary (Alberta Environment 2003a, b). Restoration methods following construction also changed over time. In the 1980s, it became common to reseed well construction sites using CWG or Russian wildrye (Psathyrostachys junceus); however, it quickly became clear that non-native grasses created a habitat structure that permanently and greatly differed from surrounding native mixed-grass prairies (Alberta Energy and Utilities Board 2002a; Alberta Environment 2003a), so the practice was short-lived. Restoration with native rather than non-native species was recommended by the early 1990s (Alberta Environment 2003a). Although CWG is no longer used for site restoration, seed may be accidentally transferred among lease sites via vehicles. The need for reseeding may be avoided entirely by using no-strip or reduced disturbance construction methods, which have become increasingly common (Alberta Environment 2003b). In general, over the last several decades, disturbance during well construction has declined, while restoration methods have become more effective (Alberta Energy and Utilities Board 2002a; Nasen et al. 2011). Sites were grazed by the Eastern Irrigation District (EID) with cow–calf pairs at stocking rates ranging from 0.11 AUM/ha to 1.29 AUM/ha. Cattle were managed
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variably among our sites, representing the range of management strategies used in native pastures in our region, including rotational grazing among large fields (86 ha or larger), season-long grazing (late May to September or October), and fall grazing (after mid-August). While this broad range of cattle management regimes probably decreased our analytical power by increasing variance, it ensured that our zone of inference included those management conditions typical of the area (e.g., Quinn and Keough 2002). Grazing intensity was independent of well density (r = -0.19, P = 0.28). Habitat Structure (Analyses 1, 2, 4) In 2010 and 2011, we measured four habitat structure variables (vegetation height and density, litter depth, and percent bare ground), and one measure of non-native plant abundance (occurrence of CWG), within 1-m2 quadrats in each site. We selected these variables as they have a strong influence on vegetation structure and prairie songbirds at a local scale (Fisher and Davis 2009). To ensure that habitat quadrats were not only located randomly but also spatially covered each site, quadrats were located using a stratified random method. We placed a virtual 4x4 grid (16 squares) over each site, then randomly selected the center point of 10 of the 16 squares. One habitat quadrat was then placed at a random distance up to 100-m north of each center point, and another quadrat was placed at a random distance up to 100-m south of each center point. Habitat structure was measured in each site in both years, but quadrats were located in different locations in each year (n = 40 quadrats/site). For the current study, we used only those quadrats that were 1–400 m from well exclosures (n = 435). Habitat quadrats were formed by placing crossed meter sticks on the ground; thus, each quadrat was divided into four. Each quadrat was visually assessed by one researcher. Each researcher measured vegetation structure in all treatments, to minimize trends in observer bias among treatments. Vegetation height and litter depth were measured using a meter stick at the center and at each end of the crossed meter stick (five points per quadrat), and values from each of these points within each quadrat were averaged prior to analyses, so that we had one value per variable per quadrat. Percent bare ground and percent cover of CWG were estimated within each quadrat but were converted to the presence/absence (occurrence) values prior to analyses (see ‘‘Statistical Analysis’’, below). Vegetation density was estimated by placing a Wiens pole at the center and each end of the crossed meter stick and counting the number of blades or leaves of native vegetation touching the pole (Wiens 1969). These counts were summed within quadrats prior to analyses, so that we had one density value per quadrat.
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Cattle Abundance (Analysis 3) We assessed cattle abundance and activity (hereafter, cattle abundance) relative to distance to wells using counts of cattle fecal pats. While fecal pats may not be good indices of local grazing intensity over short periods (Kohler et al. 2006), they are good indices of cattle abundance, activity, and grazing intensity over time (Milchunas et al. 1989; Tastad 2013). Each individual cattle defecation event was counted as one fecal pat, as identified using size, shape, pattern, and direction of the droppings (Henderson 2009, unpublished data). Because we wanted to evaluate cumulative impacts of grazing intensity, we counted pats regardless of pat age (Milchunas et al. 1989). Data were collected in 2012 from 14 sites (10 with shallow gas wells, 4 controls). Of the gas well sites, 6 had low gas well pad density (1–8 well pads/section), while four had high gas well densities (9–20 well pads/section). We had insufficient resources to sample all sites for cattle responses to distance to wells, and our subsequent analyses suggest that we had sufficient power from those transects we surveyed to draw robust conclusions. At each gas well site, data were collected along three distance transects, each starting at a randomly selected well, road/trail (hereafter, trail), or control point with no infrastructure present. Each transect ran 5–200 m (5, 10, 25, 50, 100, and 200 m) from the starting point. All transects were a minimum of 300 m away from other transects. In control sites, data were collected from a control point and along a trail. Along each distance transect, and perpendicular to it, we placed a 50-m long, 2-m wide fecal pat transect, within which all cattle fecal pats were counted; for example, distance transects ran perpendicular to trails, and thus fecal pat transects were parallel to trails. This approach was necessary to obtain a sufficiently high sample size of fecal pats to estimate cattle relative abundance (Tastad 2012, pers. comm.; n = 0–28 pats/fecal pat transect). Statistical Analysis We first used generalized linear models or linear regression to determine whether Gaussian, Poisson, or negative binomial distributions best fit the residuals, using diagnostic plots and deviance/df ratios. Negative binomial distributions fit vegetation height, density, and litter depth residuals. Distributions of percent bare ground and abundance of CWG fit none of these distributions, even after transformations. In those cases, we converted data to occurrence and used a binomial distribution. Generalized estimating equations (GEEs) were used for subsequent analyses (PROC GENMOD, SAS 9.3). GEEs are semiparametric models that allow for hierarchical clustering of
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Fig. 1 Predicted effects of distance to shallow gas wells showing an increase of vegetation height and litter depth as distance increases, and a decrease in occurrence of bare ground, occurrence of crested wheatgrass, and abundance of cattle fecal pats as distance increases, in southern Alberta, Canada, 2010–2012
data (Hardin and Hilbe 2003), in our case years within plots and/or plots within sites. We used QIC to determine whether distance to infrastructure and well age should be logged to linearize them, and used the logged variable if that resulted in the better model fit (smaller QIC; Pan 2001). For all other analyses, we used a frequentist approach (Mundry 2011). We used an alpha value of 0.1 to decrease the likelihood of making Type II errors, which is a significant problem in conservation biology (Taylor and Gerrodette 1993). We modeled effects of log(distance) on vegetation height and density, litter depth, occurrence of bare ground, and occurrence of CWG. To evaluate whether the presence of CWG (whether seeded or introduced accidentally) could explain effects of log(distance) on habitat structure, we evaluated effects of the presence of CWG on the four habitat structure variables. To evaluate whether cattle abundance near wells might explain effects of distance to wells on habitat structure, we used GEEs to compare effects of distance to wells, distance to trails, and distance to an arbitrary point, on cattle abundance, using the independent variables log(distance), treatment (well, trail and control), and log(distance) * treatment. To evaluate whether effects of distance to well varied with well age, we used log of well age, because there was a marginal (e.g., occurrence of bare ground: DQIC = 0.0813) to large (vegetation height: DQIC = 2113.569) improvement in model fit with log(age) compared with the models assuming a linear effect of log age. We then used GEEs to evaluate effects of log(distance), log(well age), and their interaction on habitat structure.
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Table 1 Effects of distance to trails, shallow gas wells, and control points without infrastructure on cattle abundance in southern Alberta, 2012 b
SE
P
Slope control
-0.0055
0.0564
0.9228
Slope gas well
-0.4435
0.1317
0.0008
Slope trail Difference slope control: gas well
-0.1568 0.4381
0.0574 0.1392
0.0063 0.0017
Difference slope trail: gas well
0.2868
0.1286
0.0258
Difference slope control: trail
0.1513
0.0874
0.0833
Fig. 3 Distance to newer shallow gas wells had a greater effect on litter depth in southern Alberta, Canada, 2010–2011
(b = -0.0034, SE = 0.2652, P = 0.9899), or litter depth (b = 0.1995, SE = 0.3750, P = 0.5947). Analysis 3: Effects of Distance to Wells on Cattle Abundance Abundance of cattle declined as distance to wells increased (Table 1; Figs. 1 and 2). Cattle were also more abundant closer to trails, but the effect of distance was substantially less strong for distance to trails compared with distance to wells (Table 1; Fig. 2). There was no trend in abundance of cattle along control transects (Table 1). Fig. 2 Predicted effects of distance to shallow gas wells, trails, and controls (random points) on cattle fecal pat abundance in southern Alberta, Canada, 2012, showing negative effects of increasing distance to wells and trails on cattle fecal pat abundance
Results Analysis 1: Effects of Distance to Wells on Habitat Structure Vegetation height (b = 0.0947, SE = 0.0501, P = 0.0586) and litter depth (b = 0.2600, SE = 0.1485, P = 0.0800) were significantly shorter closer to wells, while occurrence of bare ground was higher closer to wells (b = -1.4995 SE = 0.2629, P \ .0001; Fig. 1). There was no effect of distance to wells on vegetation density (b = 0.0101, SE = 0.0567, P = 0.8588).
Analysis 4: Interactions Between Distance to Well and Well Age There was no significant interaction between distance to well and well age on vegetation density (b = -0.0002, SE = 0.1399, P = 0.9989), height, (b = -0.1771, SE = 0.1889, P = 0.3483), or occurrence of bare ground (b = 0.4862, SE = 0.8740, P = 0.5780). However, correlations between distance to wells and litter depth declined as well age increased, such that there was a strong increase in litter depth as distance to younger wells increased, but little correlation between distance to 40-year-old wells and litter depth (Interaction b = -0.4909, SE = 0.3013, P = 0.1033; Fig. 3).
Analysis 2: Effects of Distance to Wells on CWG
Discussion
Occurrence of CWG was higher closer to wells (b = -0.6717, SE = 0.3819, P = 0.0786; Fig. 1). Occurrence of CWG was positively correlated with amount of bare ground but was not significantly correlated with vegetation height (b = 0.2440, SE = 0.1542, P = 0.1136), density
Cumulatively, our results suggest that both residual effects of well construction, and cattle activity, are likely to contribute to the trend for vegetation near wells to be short and sparse. These changes may have important ecological consequences to habitat suitability for many prairie species
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sensitive to vegetation structure, including birds such as horned larks (Eremophila alpestris; Ingelfinger and Anderson 2004; Gilbert and Chalfoun 2011), greater sagegrouse (Centrocercus urophasianus; Aldridge and Brigham 2002), and chestnut-collared longspurs (Calcarius ornatus; Koper and Schmiegelow 2006); snakes (Klug et al. 2010); butterflies (Swengel and Swengel 2001; Collinge et al. 2003); grasshoppers and crickets (Onsager 2000); and many other species. Occurrence of crested wheatgrass was also higher near wells (see also Berquist et al. 2007; Nasen et al. 2011), perhaps because crested wheatgrass was introduced intentionally or accidentally during well construction. Nonetheless, our results do not support the hypothesis that effects of distance to wells on the measures of habitat structure that we evaluated here were driven by occurrence of crested wheatgrass. This is for two reasons: (1) because litter depth and vegetation height were sensitive to distance to wells but not occurrence of crested wheatgrass, and (2) because vegetation was taller farther from wells, while crested wheatgrass occurred more frequently closer to wells (whereas the non-significant trend was for vegetation to be taller where crested wheatgrass occurred more). Residual effects of well construction may explain some effects of distance to wells on litter depth. Construction activities disturb and admix the surface and subsurface of the prairie (Smith et al. 1988; Berquist et al. 2007), such that amount of bare ground increases, litter depth decreases (Nasen et al. 2011), and soil pH, nutrient cycling, and water holding capacity may be altered (Smith et al. 1988; Rowell and Florence 1993). Our results suggest that effects of well construction on litter depth decline over time, although they are still noticeable more than 10 years following well construction. One problem with interpreting these correlations is that drilling and restoration methods changed during the 44-year period since our first wells were constructed; thus, well age, and construction and restoration methods, are confounded. However, our results suggest it is more likely residual effects of drilling explain temporal trends. Effects of distance to wells on litter depth decreased with well age, suggesting recovery over time; if effects of construction and restoration method explained these trends, we would predict effects of distance to wells would increase with well age, as more recent practices are generally less invasive (e.g., Alberta Environment 2003a, b; Desserud 2011; Nasen et al. 2011). Nonetheless, the roles of well construction and restoration methods in explaining these patterns cannot be ruled out. Our results also suggest that gas wells may alter cattle distribution within grazed pastures, in the same way that water sources and supplemental feed locations can cause localized increases in cattle presence (Sanderson et al. 2010). Trails had a significant but much more subtle effect,
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perhaps because their length distributed cattle more widely than did the point-source wells. While the local effect of roads and trails was small, there might be significant cumulative effects. Our results suggest there is an important interaction between cattle foraging behavior and anthropogenic features. Livestock close to attractants can have significant effects on habitat structure (Sanderson et al. 2010; White et al. 2001, Fontaine et al. 2004). Our results suggest that, in contrast to a previous study, effects of drilling and cattle foraging extend beyond 40 m (Nasen et al. 2011), thus potentially greatly increasing their cumulative ecological impacts. Because cattle are likely to be present continually in these pastures, as cattle help to ensure both economic and ecological viability of native prairies (Fritcher et al. 2004; Derner et al. 2009), their cumulative effects over time may be greater than those of well construction, which decline as wells age.
Conclusions Incorrect assumptions about the mechanisms that explain effects of shallow gas wells on vegetation structure would divert management and conservation resources away from more effective alternatives. Previous studies have often attributed effects of wells on vegetation structure to soil and vegetation disturbances that occurred during construction (e.g., Smith et al. 1988; Hamilton et al. 2011; Nasen et al. 2011), and our results provide some support for this. Given the abundance of wells that fragment the prairie landscape, residual effects of well construction may have significant ecological impacts. Nonetheless, the additional role of livestock in impacting vegetation structure has, to date, been ignored. This is problematic because our results suggest that cattle behavior increases edge effects associated with shallow gas wells, but management strategies for reducing effects of well construction cannot mitigate effects of livestock. Attraction of livestock to wells could be mitigated or minimized by providing alternate rubbing posts; moving water, mineral, and food sources away from wells (Sanderson et al. 2010; Tate et al. 2003); or decreasing visibility of above-ground infrastructure. It is also important to mitigate effects of well construction by avoiding seeding of non-native species, cleaning drilling equipment and vehicles prior to entering lease sites to minimize accidental introduction of non-native species, and minimizing the area disturbed by drilling (Simmers and Galatowitsch 2010; Riley et al. 2012). However, these mitigation measures complement, rather than replace, efforts to mitigate effects of livestock. Our results suggest that both livestock activity and residual effects of well construction contribute to edge effects associated with shallow gas wells, so to be effective, mitigation strategies must address both mechanisms.
Environmental Management (2014) 54:1131–1138 Acknowledgments We thank three anonymous reviewers and Drs. Efroymson and Collins for their helpful reviews of the manuscript. J. Heese, of Cenovus Energy, provided helpful information on changes in well construction and restoration methods over time. We thank the Eastern Irrigation District and Cenovus Energy for allowing us access to their rangelands and lease sites for our research. Numerous research assistants and graduate students contributed to data collection. This research was supported by Cenovus Energy, Natural Sciences and Engineering Research Council, Canadian Foundation for Innovation, the Manitoba Research and Innovations Fund, and the Clayton H. Riddell Faculty of Environment, Earth and Resources Endowment Fund.
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Effects of Livestock Grazing and Well Construction on Prairie Vegetation Structure Surrounding Shallow Natural Gas Wells N. Koper • K. Molloy • L. Leston • J. Yoo
Received: 9 January 2014 / Accepted: 12 July 2014 / Published online: 31 July 2014 Ó Springer Science+Business Media New York 2014
Abstract Short and sparse vegetation near shallow gas wells has generally been attributed to residual effects from well construction, but other mechanisms might also explain these trends. We evaluated effects of distance to shallow gas wells on vegetation and bare ground in mixed-grass prairies in southern Alberta, Canada, from 2010 to 2011. We then tested three hypotheses to explain why we found shorter vegetation and more bare ground near wells, using cattle fecal pat transects from 2012, and our vegetation quadrats. We evaluated whether empirical evidence suggested that observed patterns were driven by (1) higher abundance of crested wheatgrass (Agropyron cristatum) near wells, (2) residual effects of well construction, or (3) attraction of livestock to wells. Crested wheatgrass occurrence was higher near wells, but this did not explain effects of wells on vegetation structure. Correlations between distance to wells and litter depth were the highest near newer wells, providing support for the construction hypothesis. However, effects of distance to wells on other vegetation metrics did not decline as time since well construction increased, suggesting that other mechanisms explained observed edge effects. Cattle abundance was substantially higher near wells, and this effect corresponded with changes in habitat structure. Our results suggest that both residual effects of well construction and cattle behavior may explain effects of shallow gas wells on habitat structure in mixed-grass prairies, and thus, to be effective, mitigation strategies must address both mechanisms.
N. Koper (&) K. Molloy L. Leston J. Yoo Natural Resources Institute, University of Manitoba, 70 Dysart Rd., Winnipeg, MB R3T 2M6, Canada e-mail: [email protected]
Keywords Edge effects Grasslands Habitat structure Livestock grazing Natural gas
Introduction As energy development continues across the world, its impacts on the environment are also increasing. This may have impacts on many components of prairie ecosystems, including ungulates (Sawyer et al. 2006; Beckmann et al. 2012), birds (Ingelfinger and Anderson 2004; Aldridge and Boyce 2007; Gilbert and Chalfoun 2011; Hamilton et al. 2011), and soil and vegetation structure (Rowell and Florence 1993; Nasen et al. 2011). Above-ground infrastructure associated with shallow natural gas wells is relatively small (23.1 m2/well, 1 m high in our study) and affects a relatively small area compared with infrastructure required for oil, solar and wind energy, biodiesel and ethanol (McDonald et al. 2009), and thus its impact on the landscape might also be predicted to be relatively small. Nonetheless, habitat near shallow gas wells has been associated with increased bare ground, increased abundance of non-native plant species, decreased abundance and diversity of native plant species, and shallow litter (Berquist et al. 2007; Nasen et al. 2011), and thus, their footprint extends beyond the perimeter of the infrastructure itself. Edge effects associated with shallow gas wells might explain why pronghorn (Antilocapra americana; Beckmann et al. 2012), songbirds (Hamilton et al. 2011), mule deer (Odocoileus hemionus; Sawyer et al. 2006; 2009), and other species have abundances that vary with shallow gas well density. Mechanisms that explain the effects of shallow gas wells on wildlife and their habitats are not well known (Riley et al. 2012). However, identifying the reasons for effects of wells on vegetation structure (e.g., vegetation height,
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density, and litter depth) and amount of bare ground (hereafter, vegetation structure and percent bare ground are referred to as habitat structure) is critical for determining effective mitigation strategies (Walker et al. 2007). Some characteristics of shallow gas wells suggest that these wells should have few ecological effects. For example, shallow gas wells in southern Alberta are accessed using small, low-impact trails, which have substantially less ecological impact than roads (e.g., Sutter et al. 2000; but see Carpenter et al. 2010). In addition, the above-ground infrastructure of shallow gas wells is silent, and visits are required only once annually (CSA Group 2011), virtually eliminating negative effects of anthropogenic noise (Barber et al. 2009). Nonetheless, effects of well construction might last for many years (e.g., Smith et al. 1988; Nasen et al. 2011), and well drilling and maintenance could introduce non-native plants such as crested wheatgrass (CWG, Agropyron cristatum; Larson et al. 2001; Riley et al. 2012), which has a strong impact on vegetation communities and structure (Vaness and Wilson 2007). While CWG is no longer allowed for restoration of shallow gas wells in Alberta (Alberta Environment 2003a), introduction of nonnative species may have long-lasting ecological impacts (Vaness and Wilson 2007; Simmers and Galatowitsch 2010; Nasen et al. 2011). Another mechanism that might explain effects of shallow gas wells on habitat structure is that above-ground infrastructure, including exclusion fencing around wells, may attract livestock to the wells, and perhaps it is livestock that directly impact habitat. Rangelands cover more than 300 million ha in the US alone (Reeves and Mitchell 2011), and a majority of North American grassland ecosystems are used for livestock grazing (Knick et al. 2003; Fleischner 1994). Consequently, most native and tame grasslands that include shallow gas well sites are grazed by livestock (e.g., see also Hamilton et al. 2011; Nasen et al. 2011). Cattle grazing can strongly influence habitat structure, which impacts habitat suitability for wildlife (Chapman et al. 2004). However, the interaction between livestock and gas wells has not been examined. Cattle are known to spend increased time around water sources (Fontaine et al. 2004; Tate et al. 2003; White et al. 2001), supplemental feed (Sanderson et al. 2010; Tate et al. 2003), and paddock gates and shade (Sanderson et al. 2010). Increased livestock activity and grazing pressure around these sites (Putfarken et al. 2008; Owens et al. 1991) can lead to increased bare ground (Sanderson et al. 2010), nutrient load (Sanderson et al. 2010), and decreased litter depth (Fontaine et al. 2004), so if livestock are also attracted to shallow gas wells, this might explain why vegetation tends to be short and sparse near well sites. In our study, we first assessed whether habitat structure varied with distance to shallow gas wells in southern
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Alberta, Canada (Analysis 1). We then developed three hypotheses and associated predictions to compare the empirical support for proposed reasons for observed patterns (we were unable to evaluate effects of trails because maps of trail locations were unavailable). These hypotheses were as follows: (Analysis 2) if effects of distance to wells on habitat structure are observed because CWG is close to wells, and CWG alters habitat structure, then occurrence of CWG should be higher near wells, and occurrence of CWG should be correlated with structural variables that are similarly correlated with distance to wells; (Analysis 3) if effects of distance to wells on habitat structure are driven by cattle grazing, abundance of cattle should be higher near wells, and vegetation should be shorter and more sparse near wells; and (Analysis 4) if effects of distance to wells on habitat structure are driven by cattle, effects of distance should be either independent of well age or increase with well age (depending on cattle breeds and ages; e.g., Ksiksi and Laca 2000); conversely, if effects of distance to wells on habitat structure are driven by residual effects of drilling, effects of distance should be greater near younger wells.
Methods Study Area Our study took place within 100 km of Brooks, Alberta, Canada (50˚ 350 N, 111˚ 530 W). This was part of a larger program to evaluate effects of shallow gas extraction on grassland songbirds. For this study, we surveyed vegetation in 36, 258-hectare (1 9 1 mile section) flat or gently rolling sites. All sites consisted of native mixed-grass prairie, predominantly species such as needle-and-thread (Hesperostipa comata), blue grama (Bouteloua gracilis), junegrass (Koeleria macrantha), western wheatgrass (Pascopyrum smithii), prairie sage (Artemisia ludoviciana), and pasture sage (Artemisia frigida). While there were some occurrences of non-native species such as dandelion (Taraxacum officinale) and goatsbeard (Tragopogon dubius), only crested wheatgrass occurred more than occasionally (occurrencecwg = 2 % of quadrats). All sites were also surrounded by native prairie, to minimize edge effects, but contained a few roads or trails within or at the perimeter of sites. Roads and trails ranged in permanence and impact from grass-dominated two-track trails with some bare ground in the tire tracks, to two-track trails with mostly bare ground in the tire tracks but with grass between the trails, to a few gravel and dirt roads. Almost all shallow gas wells were accessed by trails, not roads. Both control and treatment sites were selected using the same methods. Potential sites in the study region were
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identified using a GIS land-use map overlaid with well locations. Grassland-dominated sites were ground-truthed to ensure that they were predominantly native and surrounded by grassland on all sides. All suitable sites that were either controls, or had leases held by Cenovus Energy, were used for our study, to maximize power. Nine sites were controls that did not contain shallow gas wells. The remaining 27 sites contained shallow gas wells at a range of densities, from 1 to 20 well pads/section (1–29 wells), from low–high for this region. Due to commingling, some well pads contain more than one well head. On average, the area directly impacted by one shallow gas well pad (the area within the exclosure fence) was 23.1 m2; the largest measured exclosure was 42.3 m2. Wells were constructed 1–44 years prior to our surveys. Well construction methods changed somewhat during this period to reduce environmental impacts (e.g., Alberta Environment 2003a; Desserud 2011; Nasen et al. 2011). The most ecologically important change to well construction was a gradual switch from ‘‘full-build’’ methods (requiring removal of all topsoil and vegetation at the drilling site, and, since the 1980s, subsequent revegetation) to allow almost all vegetation and topsoil to remain in place while drilling directly through the topsoil, greatly reducing the disturbance footprint (Alberta Energy and Utilities Board 2002a; Alberta Environment 2003b). This practice became gradually more common starting in the mid-1990s, but even today, full-builds may be used for new well construction where topography or soil conditions make it necessary (Alberta Environment 2003a, b). Restoration methods following construction also changed over time. In the 1980s, it became common to reseed well construction sites using CWG or Russian wildrye (Psathyrostachys junceus); however, it quickly became clear that non-native grasses created a habitat structure that permanently and greatly differed from surrounding native mixed-grass prairies (Alberta Energy and Utilities Board 2002a; Alberta Environment 2003a), so the practice was short-lived. Restoration with native rather than non-native species was recommended by the early 1990s (Alberta Environment 2003a). Although CWG is no longer used for site restoration, seed may be accidentally transferred among lease sites via vehicles. The need for reseeding may be avoided entirely by using no-strip or reduced disturbance construction methods, which have become increasingly common (Alberta Environment 2003b). In general, over the last several decades, disturbance during well construction has declined, while restoration methods have become more effective (Alberta Energy and Utilities Board 2002a; Nasen et al. 2011). Sites were grazed by the Eastern Irrigation District (EID) with cow–calf pairs at stocking rates ranging from 0.11 AUM/ha to 1.29 AUM/ha. Cattle were managed
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variably among our sites, representing the range of management strategies used in native pastures in our region, including rotational grazing among large fields (86 ha or larger), season-long grazing (late May to September or October), and fall grazing (after mid-August). While this broad range of cattle management regimes probably decreased our analytical power by increasing variance, it ensured that our zone of inference included those management conditions typical of the area (e.g., Quinn and Keough 2002). Grazing intensity was independent of well density (r = -0.19, P = 0.28). Habitat Structure (Analyses 1, 2, 4) In 2010 and 2011, we measured four habitat structure variables (vegetation height and density, litter depth, and percent bare ground), and one measure of non-native plant abundance (occurrence of CWG), within 1-m2 quadrats in each site. We selected these variables as they have a strong influence on vegetation structure and prairie songbirds at a local scale (Fisher and Davis 2009). To ensure that habitat quadrats were not only located randomly but also spatially covered each site, quadrats were located using a stratified random method. We placed a virtual 4x4 grid (16 squares) over each site, then randomly selected the center point of 10 of the 16 squares. One habitat quadrat was then placed at a random distance up to 100-m north of each center point, and another quadrat was placed at a random distance up to 100-m south of each center point. Habitat structure was measured in each site in both years, but quadrats were located in different locations in each year (n = 40 quadrats/site). For the current study, we used only those quadrats that were 1–400 m from well exclosures (n = 435). Habitat quadrats were formed by placing crossed meter sticks on the ground; thus, each quadrat was divided into four. Each quadrat was visually assessed by one researcher. Each researcher measured vegetation structure in all treatments, to minimize trends in observer bias among treatments. Vegetation height and litter depth were measured using a meter stick at the center and at each end of the crossed meter stick (five points per quadrat), and values from each of these points within each quadrat were averaged prior to analyses, so that we had one value per variable per quadrat. Percent bare ground and percent cover of CWG were estimated within each quadrat but were converted to the presence/absence (occurrence) values prior to analyses (see ‘‘Statistical Analysis’’, below). Vegetation density was estimated by placing a Wiens pole at the center and each end of the crossed meter stick and counting the number of blades or leaves of native vegetation touching the pole (Wiens 1969). These counts were summed within quadrats prior to analyses, so that we had one density value per quadrat.
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Cattle Abundance (Analysis 3) We assessed cattle abundance and activity (hereafter, cattle abundance) relative to distance to wells using counts of cattle fecal pats. While fecal pats may not be good indices of local grazing intensity over short periods (Kohler et al. 2006), they are good indices of cattle abundance, activity, and grazing intensity over time (Milchunas et al. 1989; Tastad 2013). Each individual cattle defecation event was counted as one fecal pat, as identified using size, shape, pattern, and direction of the droppings (Henderson 2009, unpublished data). Because we wanted to evaluate cumulative impacts of grazing intensity, we counted pats regardless of pat age (Milchunas et al. 1989). Data were collected in 2012 from 14 sites (10 with shallow gas wells, 4 controls). Of the gas well sites, 6 had low gas well pad density (1–8 well pads/section), while four had high gas well densities (9–20 well pads/section). We had insufficient resources to sample all sites for cattle responses to distance to wells, and our subsequent analyses suggest that we had sufficient power from those transects we surveyed to draw robust conclusions. At each gas well site, data were collected along three distance transects, each starting at a randomly selected well, road/trail (hereafter, trail), or control point with no infrastructure present. Each transect ran 5–200 m (5, 10, 25, 50, 100, and 200 m) from the starting point. All transects were a minimum of 300 m away from other transects. In control sites, data were collected from a control point and along a trail. Along each distance transect, and perpendicular to it, we placed a 50-m long, 2-m wide fecal pat transect, within which all cattle fecal pats were counted; for example, distance transects ran perpendicular to trails, and thus fecal pat transects were parallel to trails. This approach was necessary to obtain a sufficiently high sample size of fecal pats to estimate cattle relative abundance (Tastad 2012, pers. comm.; n = 0–28 pats/fecal pat transect). Statistical Analysis We first used generalized linear models or linear regression to determine whether Gaussian, Poisson, or negative binomial distributions best fit the residuals, using diagnostic plots and deviance/df ratios. Negative binomial distributions fit vegetation height, density, and litter depth residuals. Distributions of percent bare ground and abundance of CWG fit none of these distributions, even after transformations. In those cases, we converted data to occurrence and used a binomial distribution. Generalized estimating equations (GEEs) were used for subsequent analyses (PROC GENMOD, SAS 9.3). GEEs are semiparametric models that allow for hierarchical clustering of
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Fig. 1 Predicted effects of distance to shallow gas wells showing an increase of vegetation height and litter depth as distance increases, and a decrease in occurrence of bare ground, occurrence of crested wheatgrass, and abundance of cattle fecal pats as distance increases, in southern Alberta, Canada, 2010–2012
data (Hardin and Hilbe 2003), in our case years within plots and/or plots within sites. We used QIC to determine whether distance to infrastructure and well age should be logged to linearize them, and used the logged variable if that resulted in the better model fit (smaller QIC; Pan 2001). For all other analyses, we used a frequentist approach (Mundry 2011). We used an alpha value of 0.1 to decrease the likelihood of making Type II errors, which is a significant problem in conservation biology (Taylor and Gerrodette 1993). We modeled effects of log(distance) on vegetation height and density, litter depth, occurrence of bare ground, and occurrence of CWG. To evaluate whether the presence of CWG (whether seeded or introduced accidentally) could explain effects of log(distance) on habitat structure, we evaluated effects of the presence of CWG on the four habitat structure variables. To evaluate whether cattle abundance near wells might explain effects of distance to wells on habitat structure, we used GEEs to compare effects of distance to wells, distance to trails, and distance to an arbitrary point, on cattle abundance, using the independent variables log(distance), treatment (well, trail and control), and log(distance) * treatment. To evaluate whether effects of distance to well varied with well age, we used log of well age, because there was a marginal (e.g., occurrence of bare ground: DQIC = 0.0813) to large (vegetation height: DQIC = 2113.569) improvement in model fit with log(age) compared with the models assuming a linear effect of log age. We then used GEEs to evaluate effects of log(distance), log(well age), and their interaction on habitat structure.
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Table 1 Effects of distance to trails, shallow gas wells, and control points without infrastructure on cattle abundance in southern Alberta, 2012 b
SE
P
Slope control
-0.0055
0.0564
0.9228
Slope gas well
-0.4435
0.1317
0.0008
Slope trail Difference slope control: gas well
-0.1568 0.4381
0.0574 0.1392
0.0063 0.0017
Difference slope trail: gas well
0.2868
0.1286
0.0258
Difference slope control: trail
0.1513
0.0874
0.0833
Fig. 3 Distance to newer shallow gas wells had a greater effect on litter depth in southern Alberta, Canada, 2010–2011
(b = -0.0034, SE = 0.2652, P = 0.9899), or litter depth (b = 0.1995, SE = 0.3750, P = 0.5947). Analysis 3: Effects of Distance to Wells on Cattle Abundance Abundance of cattle declined as distance to wells increased (Table 1; Figs. 1 and 2). Cattle were also more abundant closer to trails, but the effect of distance was substantially less strong for distance to trails compared with distance to wells (Table 1; Fig. 2). There was no trend in abundance of cattle along control transects (Table 1). Fig. 2 Predicted effects of distance to shallow gas wells, trails, and controls (random points) on cattle fecal pat abundance in southern Alberta, Canada, 2012, showing negative effects of increasing distance to wells and trails on cattle fecal pat abundance
Results Analysis 1: Effects of Distance to Wells on Habitat Structure Vegetation height (b = 0.0947, SE = 0.0501, P = 0.0586) and litter depth (b = 0.2600, SE = 0.1485, P = 0.0800) were significantly shorter closer to wells, while occurrence of bare ground was higher closer to wells (b = -1.4995 SE = 0.2629, P \ .0001; Fig. 1). There was no effect of distance to wells on vegetation density (b = 0.0101, SE = 0.0567, P = 0.8588).
Analysis 4: Interactions Between Distance to Well and Well Age There was no significant interaction between distance to well and well age on vegetation density (b = -0.0002, SE = 0.1399, P = 0.9989), height, (b = -0.1771, SE = 0.1889, P = 0.3483), or occurrence of bare ground (b = 0.4862, SE = 0.8740, P = 0.5780). However, correlations between distance to wells and litter depth declined as well age increased, such that there was a strong increase in litter depth as distance to younger wells increased, but little correlation between distance to 40-year-old wells and litter depth (Interaction b = -0.4909, SE = 0.3013, P = 0.1033; Fig. 3).
Analysis 2: Effects of Distance to Wells on CWG
Discussion
Occurrence of CWG was higher closer to wells (b = -0.6717, SE = 0.3819, P = 0.0786; Fig. 1). Occurrence of CWG was positively correlated with amount of bare ground but was not significantly correlated with vegetation height (b = 0.2440, SE = 0.1542, P = 0.1136), density
Cumulatively, our results suggest that both residual effects of well construction, and cattle activity, are likely to contribute to the trend for vegetation near wells to be short and sparse. These changes may have important ecological consequences to habitat suitability for many prairie species
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sensitive to vegetation structure, including birds such as horned larks (Eremophila alpestris; Ingelfinger and Anderson 2004; Gilbert and Chalfoun 2011), greater sagegrouse (Centrocercus urophasianus; Aldridge and Brigham 2002), and chestnut-collared longspurs (Calcarius ornatus; Koper and Schmiegelow 2006); snakes (Klug et al. 2010); butterflies (Swengel and Swengel 2001; Collinge et al. 2003); grasshoppers and crickets (Onsager 2000); and many other species. Occurrence of crested wheatgrass was also higher near wells (see also Berquist et al. 2007; Nasen et al. 2011), perhaps because crested wheatgrass was introduced intentionally or accidentally during well construction. Nonetheless, our results do not support the hypothesis that effects of distance to wells on the measures of habitat structure that we evaluated here were driven by occurrence of crested wheatgrass. This is for two reasons: (1) because litter depth and vegetation height were sensitive to distance to wells but not occurrence of crested wheatgrass, and (2) because vegetation was taller farther from wells, while crested wheatgrass occurred more frequently closer to wells (whereas the non-significant trend was for vegetation to be taller where crested wheatgrass occurred more). Residual effects of well construction may explain some effects of distance to wells on litter depth. Construction activities disturb and admix the surface and subsurface of the prairie (Smith et al. 1988; Berquist et al. 2007), such that amount of bare ground increases, litter depth decreases (Nasen et al. 2011), and soil pH, nutrient cycling, and water holding capacity may be altered (Smith et al. 1988; Rowell and Florence 1993). Our results suggest that effects of well construction on litter depth decline over time, although they are still noticeable more than 10 years following well construction. One problem with interpreting these correlations is that drilling and restoration methods changed during the 44-year period since our first wells were constructed; thus, well age, and construction and restoration methods, are confounded. However, our results suggest it is more likely residual effects of drilling explain temporal trends. Effects of distance to wells on litter depth decreased with well age, suggesting recovery over time; if effects of construction and restoration method explained these trends, we would predict effects of distance to wells would increase with well age, as more recent practices are generally less invasive (e.g., Alberta Environment 2003a, b; Desserud 2011; Nasen et al. 2011). Nonetheless, the roles of well construction and restoration methods in explaining these patterns cannot be ruled out. Our results also suggest that gas wells may alter cattle distribution within grazed pastures, in the same way that water sources and supplemental feed locations can cause localized increases in cattle presence (Sanderson et al. 2010). Trails had a significant but much more subtle effect,
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perhaps because their length distributed cattle more widely than did the point-source wells. While the local effect of roads and trails was small, there might be significant cumulative effects. Our results suggest there is an important interaction between cattle foraging behavior and anthropogenic features. Livestock close to attractants can have significant effects on habitat structure (Sanderson et al. 2010; White et al. 2001, Fontaine et al. 2004). Our results suggest that, in contrast to a previous study, effects of drilling and cattle foraging extend beyond 40 m (Nasen et al. 2011), thus potentially greatly increasing their cumulative ecological impacts. Because cattle are likely to be present continually in these pastures, as cattle help to ensure both economic and ecological viability of native prairies (Fritcher et al. 2004; Derner et al. 2009), their cumulative effects over time may be greater than those of well construction, which decline as wells age.
Conclusions Incorrect assumptions about the mechanisms that explain effects of shallow gas wells on vegetation structure would divert management and conservation resources away from more effective alternatives. Previous studies have often attributed effects of wells on vegetation structure to soil and vegetation disturbances that occurred during construction (e.g., Smith et al. 1988; Hamilton et al. 2011; Nasen et al. 2011), and our results provide some support for this. Given the abundance of wells that fragment the prairie landscape, residual effects of well construction may have significant ecological impacts. Nonetheless, the additional role of livestock in impacting vegetation structure has, to date, been ignored. This is problematic because our results suggest that cattle behavior increases edge effects associated with shallow gas wells, but management strategies for reducing effects of well construction cannot mitigate effects of livestock. Attraction of livestock to wells could be mitigated or minimized by providing alternate rubbing posts; moving water, mineral, and food sources away from wells (Sanderson et al. 2010; Tate et al. 2003); or decreasing visibility of above-ground infrastructure. It is also important to mitigate effects of well construction by avoiding seeding of non-native species, cleaning drilling equipment and vehicles prior to entering lease sites to minimize accidental introduction of non-native species, and minimizing the area disturbed by drilling (Simmers and Galatowitsch 2010; Riley et al. 2012). However, these mitigation measures complement, rather than replace, efforts to mitigate effects of livestock. Our results suggest that both livestock activity and residual effects of well construction contribute to edge effects associated with shallow gas wells, so to be effective, mitigation strategies must address both mechanisms.
Environmental Management (2014) 54:1131–1138 Acknowledgments We thank three anonymous reviewers and Drs. Efroymson and Collins for their helpful reviews of the manuscript. J. Heese, of Cenovus Energy, provided helpful information on changes in well construction and restoration methods over time. We thank the Eastern Irrigation District and Cenovus Energy for allowing us access to their rangelands and lease sites for our research. Numerous research assistants and graduate students contributed to data collection. This research was supported by Cenovus Energy, Natural Sciences and Engineering Research Council, Canadian Foundation for Innovation, the Manitoba Research and Innovations Fund, and the Clayton H. Riddell Faculty of Environment, Earth and Resources Endowment Fund.
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