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 (Δλ) scaled to the rate of habitat loss (Δh) falls below −1% (Δλ/Δh = −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.

Habitat restoration is often recommended in conservation without first evaluating whether populations are in fact habitat limited and thus whether declining populations can be stabilized or recovered through habitat restoration. We used a spatially structured demographic model coupled with a dynamic neutral landscape model to evaluate whether habitat restoration could rescue populations of several generic migratory songbirds that differed in their sensitivity to habitat fragmentation (i.e., severity of edge effects on nesting success). Simulating a best-case scenario, landscapes were instantly restored to 100% habitat before, at, or after habitat loss exceeded the species' vulnerability threshold. The vulnerability threshold is a measure of extinction risk, in which the change in population growth rate (∆λ) scaled to the rate of habitat loss (∆h) falls below −1% (∆λ/∆h ≤ −0.01). Habitat restoration was most effective for species with low-to-moderate edge sensitivities and in landscapes that had not previously experienced extensive fragmentation. To stabilize populations of species that were highly edge sensitive or any species in heavily fragmented landscapes, restoration needed to be initiated long before the vulnerability threshold was reached. In practice, habitat restoration is generally not initiated until a population is at risk of extinction, but our model results demonstrate that some populations cannot be recovered at this point through habitat restoration alone. At this stage, habitat loss and fragmentation have seriously eroded the species' demographic potential such that halting population declines is limited more by demographic factors than the amount of available habitat. Evidence that populations decline in response to habitat loss is thus not sufficient to conclude that habitat restoration will be sufficient to rescue declining populations. Limitaciones Demográficas de la Capacidad de la Restauración de Hábitat para Rescatar Poblaciones en DeclinaciónResumen: A menudo, la restauración del hábitat es recomendada en conservación sin evaluar previamente si las poblaciones están limitadas por el hábitat y por lo tanto si las poblaciones en declinación pueden ser estabilizadas o recuperadas mediante la restauración del hábitat. Utilizamos un modelo demográfico estructurado espacialmente acoplado con un modelo de paisaje neutral dinámico para evaluar si la restauración del hábitat podría rescatar poblaciones de varias aves migratorias genéricas con distinta sensibilidad a la fragmentación del hábitat (i.e., severidad de los efectos de borde sobre eléxito de anidación). Al simular el mejor escenario, los paisajes fueron restaurados instantáneamente a 100% hábitat ya sea antes, durante o después de que la pérdida de hábitat excedió el umbral de vulnerabilidad de las especies. El umbral de vulnerabilidad es una medida del riesgo de extinción, en la el cambio en la tasa de crecimiento poblacional (∆λ) en relación con la tasa de pérdida de hábitat (∆h) es menor a 1% (∆λ/∆h ≤ −0.01). La restauración del hábit...

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Grasslands are often grazed by cattle and many grassland birds nest on the ground, potentially exposing nests to trampling. We tested for trampling risk introduced by cattle to nests of endangered Florida Grasshopper Sparrows (Ammodramus savannarum floridanus) using experimentally paired grids of artificial nests (i.e., clay targets) similar in size to nests of Florida Grasshopper Sparrows and counted the number of clay targets that were broken in paired grazed and ungrazed enclosures. Clay targets in grazed grids were trampled 3.9% more often than their respective ungrazed grids, and measurements of cattle presence or density were correlated with the number of broken clay targets, suggesting that excluding cattle during breeding is an important management recommendation for the Florida Grasshopper Sparrow. Trampling rates within grazed enclosures were spatially homogeneous with respect to cattle infrastructure such as supplemental feeding troughs and fences, and forests and stocking density were poor predictors of trampling rates when excluding ungrazed grids. We used population viability analysis to compare quasi-extinction rates, intrinsic growth rates, and median abundance in grazed and ungrazed Florida Grasshopper Sparrow aggregations to further understand the biological significance of management aimed at reducing trampling rates during the breeding season. Simulations indicated that trampling from grazing increased quasi-extinction rates by 41% while reducing intrinsic growth rates by 0.048, and reducing median abundance by an average of 214 singing males after 50 years. Management should avoid grazing enclosures occupied by Florida Grasshopper Sparrows during the nesting season to minimize trampling rates. Our methods that combine trampling experiments with population viability analysis provide a framework for testing effects from trampling on other grassland ground-nesting birds, and can directly inform conservation and management of the Florida Grasshopper Sparrow. Évaluation du risque de piétinement par le bétail de nids de passereaux menacés à partir d'expériences avec des nids artificiels et de simulations RÉSUMÉ. Les prairies sont souvent broutées par le bétail et de nombreux oiseaux de prairie nichent au sol, exposant ainsi potentiellement leur nid au piétinement. Nous avons testé le risque de piétinement causé par le bétail des nids du Bruant sauterelle de Floride (Ammodramus savannarum floridanus), une espèce menacée, au moyen de grilles expérimentales appariées de nids artificiels (c.à-d. disques d'argile), semblables en taille aux nids des Bruants sauterelles de Floride. Nous avons compté le nombre de disques d'argile qui ont été détruits dans les enclos appariés broutés ou non broutés. Les disques d'argile situés dans les grilles broutées ont été piétinés 3,9 % plus souvent que ceux qui sont sis dans les grilles non broutées appariées, et les mesures de la présence du bétail ou de la densité étaient corrélées avec le nombre de disques détruits; ces résultats nous mènent à recommander fortement...