On the importance of landscape history for assessing extinction risk

Assessment of extinction risk may depend not only upon the current state of the landscape and its projected trajectory of change, but also on its past disturbance history. We employed a spatially structured demographic model to evaluate extinction risk for several generic migratory songbirds within landscapes subjected to ongoing habitat loss and fragmentation. We generated different scenarios of dynamic landscape change using neutral landscape models, in which breeding habitat was systematically destroyed at various rates (0.5%, 1%, or 5% per year) and degrees of fragmentation, thus enabling us to determine the relative contribution of these factors to population declines. Extinction risk was assessed relative to the vulnerability threshold, the point where the change in population growth rate (Delta lambda) scaled to the rate of habitat loss (Ali) falls below -1% (Delta lambda/Delta h = -0.01). Our model predicts that songbirds are likely to exhibit lagged responses to habitat loss in landscapes undergoing rapid change (5% per year). In such scenarios, the landscape changed more rapidly than the demographic response time of the population, such that population growth rates never exceeded the vulnerability threshold, even though these species inevitably went extinct. Thus, songbirds in landscapes undergoing rapid change might not be assessed as at risk until the population's demographic potential has been seriously eroded, which would obviously compromise the success of management actions aimed at recovering the population. Furthermore, our model illustrates how assessment of a species' extinction risk may vary widely among landscapes of similar structure, depending upon how quickly the landscape achieved its current state. Thus, information on the current landscape state (e.g., amount of habitat or degree of fragmentation) may not be sufficient for assessing long-term population viability and extinction risk in the absence of information on the history of landscape disturbance.