Abstract - Conservation Ecology

ABSTRACT

We explore the thesis that stochasticity in successional-disturbance systems can be an agent of species extinction. The analysis uses a simple model of patch dynamics for seral stages in an idealized landscape; each seral stage is assumed to support a specialist biota. The landscape as a whole is characterized by a mean patch birth rate, mean patch size, and mean lifetime for each patch type. Stochasticity takes three forms: (1) patch stochasticity is randomness in the birth times and sizes of individual patches, (2) landscape stochasticity is variation in the annual means of birth rate and size, and (3) turnover mode is whether a patch is eliminated by disturbance or by successional change. Analytical and numerical analyses of the model suggest that landscape stochasticity is the most important agent. Landscape stochasticity increases the extinction risk to species by increasing the risk that the habitat will fluctuate to zero, by reducing the mean abundance of species, and by increasing the variance in species abundance. The highest risk was found to occur in species that inhabit patches with short lifetimes. The results of this general model suggest an important mechanism by which climate change threatens biodiversity: an increase in the frequency of extreme climate events will probably cause pulses of disturbance during some time periods; these in turn would cause wider fluctuations in annual disturbance rates and thus increase the overall level of landscape stochasticity. However, the model also suggests that humans can manipulate landscape stochasticity to reduce risk. In particular, if managed disturbances were more evenly distributed in time, attrition of the regional biota might be prevented. Other work on the connection between patch dynamics and extinction risk assumes the absence of landscape stochasticity and thus overlooks an important component of risk to biodiversity.

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INTRODUCTION

Birth-death processes can be considered to operate at a variety of levels of organization in biological systems and are tightly bound up with ideas about the persistence of species. The most familiar level of organization at which births and deaths occur is, of course, that of the individual organism, and the persistence of entire species has traditionally been expressed in terms of the birth rates and death rates at this level. Over the past few decades, it has become useful to broaden the focus and consider "births" and "deaths" at other levels, such as entire populations and even whole habitat patches (Levins 1969, Levin and Paine 1974, Hanski and Gilpin 1997). These birth-death processes are commonly referred to as metapopulation dynamics and patch dynamics, respectively.

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As we see it, there are two important limitations in the work to date. First, patch births in a given landscape are always assumed to be independent of one another, in clear contrast to the empirical situation in which climate fluctuations and contagious processes such as fires or floods cause patch births (and deaths) to be correlated in time (O'Neill et al. 1986, Busing and White 1993, Turner et al. 1993, Clark 1993, 1996, Malamud et al. 1998). Are conclusions about species extinction sensitive to this assumption?

Second, the body of work contains heterogenous assumptions about the statistical distribution of death times for patches. Sometimes the assumptions resemble situations in which patch deaths are caused by succession (Fahrig 1992); other times they resemble disturbance (Keymer et al. 2000, Amarasekare and Possingham 2001) or a mixture of the two (Brachet et al. 1999). Are conclusions about extinction sensitive to these assumptions (see also Johnson 2000)?

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KEY WORDS: biodiversity, catastrophe, dispersal, disturbance, extinction, landscape, metapopulation, patch dynamics, patchy population, succession.

Read the full manuscript at Conservation Ecology Online: http://www.consecol.org/vol6/iss2/art2/index.html

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