Neutral theory as a predictor of avifaunal extinctions after habitat loss

Abstract

The worldwide loss of natural habitats leads not only to the loss of habitat-endemic species but also to further and protracted extinctions in the reduced areas that remain. How rapid is this process? We use the neutral theory of biodiversity to answer this question, and we compare the results taken with observed rates of avifaunal extinctions. In the neutral model, we derive an exact solution for the rate of species loss in a closed community. The simple, closedform solution exhibits hyperbolic decay of species richness with time, which implies a potentially rapid initial decline followed by much slower rates long term. Our empirical estimates of extinction times are based on published studies for avifaunal extinctions either on oceanic islands or in forest fragments, which span a total of six orders of magnitude in area. These estimates show that the time to extinction strongly depends on the area. The neutral-theory predictions agree well with observed rates over three orders of magnitude of area (between 100 and 100,000 ha) both for islands and forest fragments. Regarding the species abundance distribution, extinction times based on a broken-stick model led to better agreement with observation than if a log-series model was used. The predictions break down for very small or very large areas. Thus, neutrality may be an affordable assumption for some applications in ecology and conservation, particularly for areas of intermediate size.extinction debt | faunal relaxation | biodiversity loss | extinction rates T he primary driver of extinction in terrestrial environments today is habitat loss (1, 2). In particular, the loss of large portions of tropical rainforest through land conversion was estimated to lead to rates of species extinction hundreds of times higher than the background rate (3). However, the ensuing extinctions will not be realized immediately, because most of the species found in the destroyed habitat are also found in the remaining area. Instead, extinctions occur in the course of the following decades, centuries, or even millennia after the initial habitat contraction. This process has been called faunal relaxation (4). Another term used is extinction debt, which refers to the degree to which the species richness exceeds the speciescarrying capacity of the diminished area (5, 6). A good understanding of this phenomenon is required to explain why the extinctions forecasts are not immediately observed.Empirically, extinction is difficult to observe, because we must establish the absence of an organism simultaneously everywhere before we can be sure that it is extinct. As a result, organisms thought extinct frequently pop up again in subsequent surveys (7). Despite this, a large body of empirical work related to extinction debt has been assembled (review by Kuussaari et al.) (6), including various workarounds using natural experiments (4, 8, 9), microcosms (10), and historical data (11,12). An important and still unresolved part of the picture is to find the dynamic model of extinction in the comm...

The Unified Neutral Theory of Biodiversity and Biogeography at Age Ten

“…Neutral models provide a baseline expectation for important questions, such as the effect on biodiversity of habitat fragmentation [70,79,81]. Halley and Iwasa [80] recently tested neutral expectations of avifaunal extinctions following habitat loss and found that the neutral model performs extremely well. Approximate predictions of species loss under different scenarios of habitat destruction can also be derived from the theory [82], although we note that very different results might have been obtained had protracted speciation instead of point mutation speciation been used.…”

“…We suggest that the publication trend instead indicates that the value of neutral theory in conservation has gone unrecognised. Indeed, two papers appeared very recently each showing different uses of neutral theory in conservation modelling [79,80]. Neutral models provide a baseline expectation for important questions, such as the effect on biodiversity of habitat fragmentation [70,79,81].…”

Biogeographic patterns in ocean microbes emerge in a neutral agent-based model

“…After about 100 years, the population consists of about 10 OTUs, resident in relatively distinct spatial regions and the rate of OTU loss becomes limited by dispersal between these provinces (28). At that time, the model starts to predict higher OTU richness than a neutral theory model that does not consider dispersal limitation (31). The rate of OTU loss becomes low, but the populations continue to mix (28) and the probability of extinction remains greater than zero.…”

Why abundant tropical tree species are phylogenetically old

“…The taxon cycle that has occurred over intervals on the order of millions of years in several independent lineages of Lesser Antillean birds (20) provides some indirect empirical evidence for slow population dynamics over geologic time scales. However, recent studies also suggested that taxonomic turnover in very abundant clades, like birds (21,22) and planktonic foraminifera (23), is sometimes much faster than that predicted by purely ecological drift.…”