How ecosystems change Conservation planning must accommodate changes in ecosystem composition to protect biodiversity By Anne E. Magurran


uman impacts on the planet, including anthropogenic climate change, are reshaping ecosystems in unprecedented ways. To meet the challenge of conserving biodiversity in this rapidly changing world, we must understand how ecological assemblages respond to novel conditions (1). However, species in ecosystems are not fixed entities, even without human-induced change. All ecosystems experience natural turnover in species presence and abundance. Taking account of this baseline turnover in conservation planning could play an important role in protecting biodiversity. More than 150 years ago, Darwin observed that taxa “favoured by any slight change of climate” will increase in numbers, whereas other, less-favored species “must decrease” (2). In these few words, Darwin touches on two ideas key to understanding the fate of ecosystems. On the one hand, he suggests that the number of individuals and species in an ecosystem will tend toward some equilibrium level. On the other, there is constant turnover in the identities and abundances of species in any locality. Darwin’s views were probably shaped by per448

sonal experience: In Western Europe, the 1850s were unusually cold. But they provide a context for understanding the fate of ecological communities at a time when climate change is remodeling the natural world at a pace that exceeds historical levels. Darwin’s proposition that ecological communities track fluctuations in climate is supported by time series collected over decades to reveal how rapidly ecosystems change. For instance, Barceló et al. recently reported that the fish species now present in the near-shore assemblage of the Skagerrak to the south of Norway differ from those caught during a colder period in the 1960s and 1970s (3). The current community composition is more similar to that seen during a warmer phase in the 1930s and 1940s. These community shifts arise because species adjust their ranges in response to temperature (4), with coldfavoring fish moving south in colder years and warm-favoring ones extending north in warmer years. Total species richness increases in the Skagerrak during warmer phases and declines during cooler ones. Barceló et al. saw no systematic trend over the duration of the eight-decade time series (2). However, the recent appearance of previously unrecorded species such as the Eu-

ropean anchovy (Engraulis encrasicolus), European pilchard (Sardina pilchardus), and tub gurnard (Trigla lucerna) reinforces the idea that even if the species richness of an assemblage does not change, its composition can shift markedly (see the figure). Great strides have been made in modeling vulnerability to climate change on a species-by-species basis (5), but it is also important to understand how entire communities respond to novel conditions (6). Ecological communities reorganize over time as a result of changes in both α diversity (such as a change in species richness) and β diversity (change, or turnover, in species composition). Human actions in the form of land use change and pollution can drive down the number of species in local assemblages (7). However, in many communities, such as the Skagerrak, average species richness has remained constant over recent decades (3, 8–10). Indeed, as Darwin suggested (2), ecological processes may help to regulate local α diversity (11), and seemingly unvarying richness can mask substantial change in community structure (12). Temporal α diversity is thus not always an informative gauge of the severity of human impacts on the natural world (see the figure). In contrast, investigations of temporal β diversity suggest that community reorganization is occurring at rates that exceed historical baseline turnover (1, 8). For instance, the 66 freshwater fish found in the rivers of Centre for Biological Diversity, School of Biology, University of St. Andrews, St. Andrews, Scotland, UK. E-mail: [email protected] SCIENCE

29 JANUARY 2016 • VOL 351 ISSUE 6272

Published by AAAS




Beyond baseline turnover. The Caroni River system in Trinidad supports a rich fish fauna, including the introduced tetra, Copella arnoldi (13). Today’s species turnover rate in the rivers of Trinidad and Tobago is roughly double that seen historically.


Time 1


Time 2

local and regional scales. Communities are increasingly dominated by the same cosmopolitan species, rather as shopping districts around the world have become populated by familiar brands. For example, the common carp (Cyprinus carpio) and goldfish (Carassius auratus), species that originated in Asia, now inhabit freshwaters in 48 and 42 U.S. states, respectively (14). Further, groundfish assemblages of α Diversity (species richness)

the islands of Trinidad and Tobago (see the photo) include six introduced species (13). Local extinctions offset these gains, but the presence of exotic species alongside natural colonists means that the contemporary turnover rate has approximately doubled relative to the historical level. Invasive species are a prominent feature of many assemblages and have contributed to greater biotic homogenization at both




β Diversity (turnover)

Time 1

Species lost from the assemblage Species lost from the region Introduced species Species shifting range due to climate change

the Northeast Atlantic have become progressively more similar in species composition over the past three decades (12). Thus, biotic homogenization may also be linked to climate change. The homogenization of Northeast Atlantic fish communities parallels an evening-out of water temperature (a consequence of ocean warming) over the same period. Conservation is sometimes regarded as synonymous with preservation. Yet paradoxically, to protect biodiversity, conservation managers will increasingly need to accommodate species turnover in their planning. A first step is to report changes both in abundance of individual species and in species composition. Identifying which taxa are native to a locality or biome and which are new to it will be crucial information, both to support native taxa and to ameliorate the impact of invasive ones. More data on contemporary rates of turnover, relative to baseline rates, would be invaluable. Scale is also important. Day-to-day conservation management often occurs at a local scale, but conservation strategy needs to be enacted at an appropriate scale to accommodate changes in species distributions. Further, managed change can support conservation. In Europe, rewilding takes advantage of farmland abandonment and is contributing to the recovery of megafauna (15). Finally, we have little understanding of how changes in composition will affect ecosystem function (1). However, new insights into how different species respond to warming will help to provide early warning of accelerated turnover in ecosystems (6). It will also be important to investigate evolutionary change alongside ecological change (2). ■


Time 2

Additional turnover due to species invasions and climate change

Observed turnover

Baseline turnover Time 1

Time 2

Toward managing biodiversity change. (A and B) A regional assemblage at two time points. The taxa observed in the sampled assemblage (white rectangle) change over time as a result of local and regional species loss. These losses may be offset by gains due to range shifts and introductions of exotic species. In this example, there is no change in α diversity (such as species richness) (C), even though there are substantial changes in assemblage composition (D). A major challenge in conservation planning is managing turnover that exceeds baseline turnover, arising from human impacts such as invasive species and climate change. SCIENCE

1. J. M. Pandolfi, C. E. Lovelock, Science 344, 266 (2014). 2. C. Darwin, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (John Murray, 1859). 3. C. Barceló, L. Ciannelli, E. M. Olsen, T. Johannessen, H. Knutsen, Global Change Biol. 10.1111/gcb.13047 (2015). 4. I.-C. Chen, J. K. Hill, R. Ohlemüller, D. B. Roy, C. D. Thomas, Science 333, 1024 (2011). 5. E. S. Poloczanska et al., Nat. Clim. Change 3, 919 (2013). 6. R. D. Stuart-Smith, G. J. Edgar, N. S. Barrett, S. J. Kininmonth, A. E. Bates, Nature 10.1038/nature16144 (2015). 7. T. Newbold et al., Nature 520, 45 (2015). 8. M. Dornelas et al., Science 344, 296 (2014). 9. M. Vellend et al., Proc. Natl. Acad. Sci. U.S.A. 110, 19456 (2013). 10. S. R. Supp, S. K. M. Ernest, Ecology 95, 1717 (2014). 11. S. K. M. Ernest, J. H. Brown, K. M. Thibault, E. P. White, J. R. Goheen, Am. Nat. 172, E257 (2008). 12. A. E. Magurran, M. Dornelas, F. Moyes, N. J. Gotelli, B. McGill, Nat. Commun. 6, 10.1038/ncomms9405 (2015). 13. D. A. T. Phillip et al., Zootaxa 3711, 1 (2013). 14. F. J. Rahel, Science 288, 854 (2000). 15. S. Ceaușu et al., Conserv. Biol. 29, 1017 (2015).

10.1126/science.aad6758 29 JANUARY 2016 • VOL 351 ISSUE 6272

Published by AAAS


ECOLOGY. How ecosystems change.

ECOLOGY. How ecosystems change. - PDF Download Free
2MB Sizes 6 Downloads 11 Views