The extinction vortex

25 08 2008

One for the Potential list:

vortexFirst coined by Gilpin & Soulé in 1986, the extinction vortex is the term used to describe the process that declining populations undergo when”a mutual reinforcement occurs among biotic and abiotic processes that drives population size downward to extinction” (Brook, Sodhi & Bradshaw 2008).

Although several types of ‘vortices’ were labelled by Gilpin & Soulé, the concept was subsequently simplified by Caughley (1994) in his famous paper on the declining and small population paradigms, but only truly quantified for the first time by Fagan & Holmes (2006) in their Ecology Letters paper entitled Quantifying the extinction vortex.

Fagan and Holmes compiled a small time-series database of ten vertebrate species (two mammals, five birds, two reptiles and a fish) whose final extinction was witnessed via monitoring. They confirmed that the time to extinction scales to the logarithm of population size. In other words, as populations decline, the time elapsing before extinction occurs becomes rapidly (exponentially) smaller and smaller. They also found greater rates of population decline nearer to the time of extinction than earlier in the population’s history, confirming the expectation that genetic deterioration contributes to a general corrosion of individual performance (fitness). Finally, they found that the variability in abundance was also highest as populations approached extinction, irrespective of population size, thus demonstrating indirectly that random environmental fluctuations take over to cause the final extinction regardless of what caused the population to decline in the first place.

What does this mean for conservation efforts? It was fundamentally the first empirical demonstration that the theory of accelerating extinction proneness occurs as populations decline, meaning that all attempts must be made to ensure large population sizes if there is any chance of maintaining long-term persistence. This relates to the minimum viable population size concept that should underscore each and every recovery and target set or desired for any population in trouble or under conservation scrutiny.

CJA Bradshaw

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Synergies among extinction drivers

24 08 2008

Hopefully one for the Potential list:

© J. Hance

Brook, BW, NS Sodhi, CJA Bradshaw. (2008) Synergies among extinction drivers under global change. Trends in Ecology and Evolution 23, 453-460

A review my colleagues, Barry Brook and Navjot Sodhi, and I have just published in Trends in Ecology and Evolution demonstrates how separate drivers of extinction (e.g., habitat loss, over-exploitation [hunting, fishing, etc.], climate change, invasive species, etc.) tend to work together to heighten the extinction probability of the species they affect more than the simple sum of the individual effects alone.

In what we termed ‘synergies’, the review compiles evidence from observational, experimental and meta-analytic research demonstrating the positive and self-reinforcing actions of multiple drivers of population decline and eventual extinction. Examples include experimental evidence that wild radishes experiencing inbreeding depression have lower fitness than expected from simple population reduction (Elam et al. 2007), inter-tidal polychaetes succumb to pollution effects much more so at low densities than when populations are abundant (Hollows et al. 2007), and habitat fragmentation, harvest and simulated climate warming increase rotifer extinction risk up to 50 times more than expected from the additive effects of the threatening processes (Mora et al. 2007).

We argued that conservation actions only targeting single drivers will more than likely be inadequate because of the cascading effects caused by unmanaged synergies. Climate change will also interact with and accelerate ongoing threats to biodiversity, so the importance of accounting for these interactions cannot be understated.

CJA Bradshaw

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Classics: Minimum Viable Population size

21 08 2008

‘Classics’ is a category of posts highlighting research that has made a real difference to biodiversity conservation. All posts in this category will be permanently displayed on the Classics page of

Too-Few-CaloriesShaffer, M.L. (1981). Minimum population sizes for species conservation. BioScience 31, 131–134

Small and isolated populations are particularly vulnerable to extinction through random variation in birth and death rates, variation in resource or habitat availability, predation, competitive interactions and single-event catastrophes, and inbreeding. Enter the concept of the Minimum Viable Population (MVP) size, which was originally defined as the smallest number of individuals required for an isolated population to persist (at some predefined ‘high’ probability) for some ‘long’ time into the future. In other words, the MVP size is the number of individuals in the population that is needed to withstand normal (expected) variation in all the things that affect individual persistence through time. Drop below your MVP size, and suddenly your population’s risk of extinction sky-rockets. In some ways, MVP size can be considered the threshold dividing the ‘small’ and ‘declining’ population paradigms (see Caughley 1994), so that different management strategies can be applied to populations depending on their relative distance to (population-specific) MVP size.

This wonderfully simply, yet fundamental concept of extinction dynamics provides the target for species recovery, minimum reserve size and sustainable harvest if calculated correctly. Indeed, it is a concept underlying threatened species lists worldwide, including the most well-known (IUCN Red List of Threatened Species). While there are a host of methods issues, genetic considerations and policy implementation problems, Shaffer’s original paper spawned an entire generation of research and mathematical techniques in conservation biology, and set the stage for tangible, mathematically based conservation targets.

Want more information? We have published some papers and articles on the subject that elaborate more on the methods, expected ranges, subtleties and implications of the MVP concept that you can access below.

CJA Bradshaw

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Global warming and biodiversity extinction

14 08 2008

My colleague Barry Brook recently posted a discussion on the impacts of climate change on biodiversity extinction rates and patterns. A very good introduction to the subject.

CJA Bradshaw

Captive breeding for conservation

7 08 2008

My first attempt at this potentially rather controversial section of Inspired by my latest post (30/07/2008), I must comment on what I believe is one of the biggest wasters of finite conservation (financial) resources – captive breeding for population recovery. The first laureate of the Toothless category goes to 7 authors (Snyder et al.) who I believe deserve at least a round of beers for their bold paper published way back in 1996 in Conservation BiologyLimitations of captive breeding in endangered species recovery.

The paper describes basically that in most situations, captive breeding for population recovery is ill-conceived, badly planned, overly expensive and done without any notion of the particular species’ minimum viable population size (the population size required to provide a high probability of persistence over a long period). Examples of ridiculous cloning experiments done in the name of ‘conservation’ (one example with which I am familiar is the case of the SE Asian banteng cloning experiment – these conservation-challenged scientists actually claimed “We hope that the birth of these animals will open the way for a new strategy to help maintain valuable biodiversity and to respond to the challenge of large-scale extinctions ahead.” after spending amounts that would make Bill Gates blush). Come on! Minimum viable population sizes number in the thousands to tens of thousands (e.g., Brook et al. 2006; Traill et al. 2007), not to mention the genetic diversity necessary for persistence captive populations generally lack (see Frankham et al. 2004).

In the spirit of ecological triage, we must focus on conservation efforts that have a high probability of changing the extinction risk of species. Wasting millions of dollars to save a handful of inbred individuals (insert your favourite example here) WILL NOT, in most cases, make any difference to population viability (with only a few exceptions). Good on Snyder et al. (1996) for their analysis and conclusions, but zoos, laboratories and other captive-rearing organisations around the world continue to throw away millions using the ‘conservation’ rationale to justify their actions. Rubbish. I’m afraid there is little evidence that the Snyder et al. paper changed anything. (post original published in Toothless 31/07/2008).

CJA Bradshaw

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