Abstract
It is well known that forest fragmentation reduces fecundity in several avian species, including wood thrush, Hylocichla mustelina, a migratory songbird that has been declining for several decades. However, I found that landscape-scale density in wood thrush was lower and population declines steeper in higher quality, less-fragmented landscapes (an inverse buffer effect) than in poor quality landscapes. These patterns suggest that wood thrush was not limited by availability of breeding habitat but that declines were primarily driven by non-breeding season events. A two-season model predicts that if this hypothesis is correct, breeding population trends will be negatively related to the strength of density dependence (b’) in the breeding season. To test this, a site-dependence model was used to construct fecundity-density curves and showed that landscape fragmentation affected the shape of density dependence. In good quality landscapes, the onset of strong density dependence was much more abrupt than in poorer quality landscapes and the realized strength of density dependence, b’, was lower in good quality landscapes. Population trends were negatively associated with b’, providing support for the non-breeding limitation hypothesis. The combination of the negative associations of trends with b’ and b’ with landscape quality explain the inverse buffer effect.
Highlights
Forest fragmentation has been shown to be a major component of breeding habitat quality for several forest-dwelling bird species that experience lower fecundity in smaller forest patches or more fragmented landscapes[1]
The strong sensitivity of fecundity in wood thrush to forest fragmentation as well as correlations between local trends and breeding habitat loss as well as the strong contribution of fecundity to spatial or temporal variation in growth rates suggest that declines are driven in large part by breeding habitat loss or fragmentation and that wood thrush population growth is limited by availability of breeding habitat or productivity[9,10,11]
A full annual cycle, spatial network population that allows for density dependence in both non-breeding survival and fecundity indicates that, while breeding habitat loss and fragmentation have affected regional population changes, the primary driver of the species-wide population declines in wood thrush is non-breeding habitat loss and/or fragmentation[12]
Summary
Forest fragmentation has been shown to be a major component of breeding habitat quality for several forest-dwelling bird species that experience lower fecundity in smaller forest patches or more fragmented landscapes[1]. No relationship between |∆n| and b′ would be expected in a population that is limited by habitat availability in both seasons To test these predictions for wood thrush, I estimated trends and density from the North American Breeding Bird Survey (BBS)[8,15] in breeding landscapes with varying landscape quality (i.e., levels of fragmentation). An approach to estimate the shape of density dependence for a particular component of pgr, such as fecundity or survival, has been to conduct or review field studies that measure the fecundity or survival in populations of different densities These studies often show that such vital rates decline with increasing density but small sample sizes and the degree of stochasticity often preclude an estimation of the exact shape of the relationship[22,23]. Application of this model to a real landscape and species requires knowledge of the distribution of forest patch sizes within the landscape, an estimation of fecundity as a function of patch size, and an estimation of the saturation density, potentially a function of patch size
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