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Understanding the Ecology of Extinction: Are We Close to the Critical Threshold?

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Understanding the Ecology of Extinction: Are We Close to the Critical Threshold? Ann. Zool. Fennici 40: 71–80 ISSN 0003-455X Helsinki 30 April 2003 © Finnish Zoological and Botanical Publishing Board 2003 Understanding the ecology of extinction: are we close to the critical threshold? Tim G. Benton Institute of Biological Sciences, University of Stirling, FK9 4LA, UK (e-mail: [email protected]) Received 25 Nov. 2002, revised version received 20 Jan. 2003, accepted 21 Jan. 2003 Benton, T. G. 2003: Understanding the ecology of extinction: are we close to the critical threshold? — Ann. Zool. Fennici 40: 71–80. How much do we understand about the ecology of extinction? A review of recent lit- erature, and a recent conference in Helsinki gives a snapshot of the “state of the art”*. This “snapshot” is important as it highlights what we currently know, the tools avail- able for studying the process of extinction, its ecological correlates, and the theory concerning extinction thresholds. It also highlights that insight into the ecology of extinction can come from areas as diverse as the study of culture, the fossil record and epidemiology. Furthermore, it indicates where the gaps in knowledge and understand- ing exist. Of particular note is the need either to generate experimental data, or to make use of existing empirical data — perhaps through meta-analyses, to test general theory and guide its future development. Introduction IUCNʼs 2002 Red List, a total of 11 167 species are currently known to be threatened (http:// Few would argue that managing natural popu- www.redlist.org/info/tables/table1.html), though lations and their habitats is one of the greatest this is a gross underestimate of the true value, as, challenges for humans in the 21st century. Some for example, the status of most terrestrial inver- species require that management be targeted at tebrates is poorly known. Considering birds and them to manipulate their persistence, typically mammals, where the status information is more because they are economically important (e.g. readily available, some 16% of all species are harvested, or pest species) or politically impor- threatened (2329 out of 14 709). tant (e.g. “headline conservation” species). More The scientifi c study of extinction, and the generally, it is important for a number of reasons processes that drive it, is one that has been a focus to manage biodiversity. For example, it can be of scientifi c study over recent decades. However, argued that conserving biodiversity may ensure in comparison with other areas of ecology it has ecosystem function, is ethically important, is not been as intensely studied (for example, a aesthetically pleasing, and safeguards the pos- Web of Science keyword search for the period sibility of future exploitation. Understanding 1981–2002 indicates that there are about 4 the processes that lead to population persistence, times more papers with keywords “life history or its fl ipside, extinction, is therefore an enor- and population” than “extinction and population mous challenge for ecologists. According to the and conservation”: 4079 to 992). One catalyst * Extinction Thresholds, organised by the Spatial Ecology Programme at the University of Helsinki, held in Helsinki 2–5 September 2002 72 Benton • ANN. ZOOL. FENNICI Vol. 40 for studing extinction has been the realisation The second technique uses “pair approximation” that populations exist in a spatial context, and methods that allow the effects of spatial structure global persistence is the result of processes hap- in habitat loss on equilibrium metapopulation pening at a smaller scale. The study of models of size and extinction threshold (Ovaskainen et such metapopulations shows that there are often al. 2002). In contrast to deterministic theory, thresholds and non-linearities in their extinction stochastic metapopulation theory is now being behaviour. In deterministic models, an extinc- developed (Frank & Wissel 2002, Ovaskainen tion threshold occurs when a marginal change in & Hanski 2003). The major conceptual change some parameter (such as habitat fragmentation) this has on the extinction threshold is that, in causes a change in the probability of extinction the stochastic case, extinction is not only always from close to zero to close to one. As such, the possible, but in the long-run, almost sure; so concept bears much in relation to the proverbial instead of a threshold below which the popula- camel, whose back was broken when loaded tion becomes deterministically extinct, one can with an extra wisp of straw. The term “extinction consider time to extinction and fi nd the thresh- threshold” was coined by Lande (1987), though olds below which extinction by the specifi ed the existence of threshold behaviour in spatially- time becomes highly probable. These different implicit Levins-type population models had been approaches make quite different assumptions, evident from Levinʼs (1969) initial work and had but they draw similar broad conclusions, which been previously discussed by, amongst others, emphasise the generality of the results. Hanski (1985). Extinction threshold behaviour Alongside theoretical advances in the study was later examined in the spatially explicit meta- of extinction thresholds, go conceptual ones. An population model of Bascompte and Sole (1996), important conceptual advance is the concept of and has since been found to be a general property the “extinction debt”. Extinction of a popula- in a range of different model formalisations. The tion (whether a single population in a patch, or extinction threshold is determined by a number of a metapopulation) is a stochastic process, so factors, notably the demography of the organism it is possible for habitat fragmentation to have (especially its reproductive rate), the rate at which occurred on a faster timescale than the extinc- it disperses across the habitat, the pattern of the tion processes. Thus, populations may be suf- habitat in space (especially the fragmentation), fi ciently fragmented to prevent re-colonisation and the survival of organisms in the non-habitat when they do go extinct — there is therefore a environment (the “matrix”) (Fahrig 2001). “debt” of extinctions to be paid as a consequence The study of extinction thresholds has largely of past habitat loss (Tillman et al. 1994, Hanski been driven by theory. Many approaches have & Ovaskainen 2002). The concept of debt can be involved numerical investigation, because, until extended beyond that due to habitat loss, per se. recently, analytical results required an assump- For example, if there are extinctions then mutu- tion of uniform distributions of habitat over ally dependent species may be driven extinct, space. Recently, however, analytical approaches even if the habitat remains hospitable (as may have been developed which greatly allow the occur to a plant when a seed disperser goes generality of the extinction threshold concept to extinct: Pacheco & Simonetti 2000). be elucidated. One approach is spatially realistic The existence of extinction thresholds in metapopulation theory (Hanski & Ovaskainen metapopulation models, and the concept of the 2000, Hanski 2001, Ovaskainen & Hanski 2003). lagged effect of previous losses on persistence, This body of theory includes two main tech- has helped to stimulate interest and focus the niques. One is based on a spatial matrix model, minds of ecologists and conservationists. Two with the matrix describing the effect of the land- important practical messages stand out clearly scape on population colonisations and extinc- from the recent literature. Firstly, the non-linear tions. As with standard matrix models (Caswell behaviour of the metapopulation dynamics 2001), the dominant eigenvalue describes the strongly suggests that even if populations have population growth rate, and, if it goes below 1.0 persisted with historical patterns of habitat loss, the population goes deterministically extinct. survival of even trivial amounts of extra frag- ANN. ZOOL. FENNICI Vol. 40 • Understanding the ecology of extinction 73 mentation is not certain. Secondly, the existence This is necessarily a somewhat personal account. of extinction debts suggests that even if popula- To add some perspective to this analysis, I also tions persist currently, they may still have little undertook a quantitative review of the literature future even if further habitat fragmentation does on extinction. I scanned editions of Conserva- not occur. Both messages indicate that under- tion Biology, Animal Conservation, Biological standing extinction is urgent, as small amounts Conservation, Journal of Animal Ecology and of extra habitat loss though fragmentation could Journal of Applied Ecology from 2000 to present precipitate an anthropologically driven “mass (November 2002), and found 261 papers that extinction”, if it is not already in progress. included the term “extinction” in the abstract or The study of extinction thresholds and keywords. For each paper, I scored it for: associated concepts has been driven by theory. A search of Web of Science using the phrase 1. generality of the “question” (“general” or “extinction threshold” yielded 24 papers over “specifi c”), the period 1996–2002, of these the majority 2. generality of the results (“general or spe- are purely theoretical (63%, n = 15) in that they cifi c), investigate extinction in general models, 8 (33%) 3. whether it was empirically based or theoreti- are more empirically motivated in that they cal (or a combination), parameterise models with data from single spe- 4. how closely it was related to
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