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We used House Sparrows (Passer domesticus) as a model species to assess responses to different levels of human visitation in a fragmented urban landscape. Regionally, we analyzed linear and nonlinear variation in breeding densities in relation to observed pedestrian rates in forest fragments. Locally, we tested experimentally the resource-use–disturbance trade-off hypothesis, which suggests that an increase in the frequency of human visitation decreases the frequency of resource use by an animal, assuming that individuals react to humans as if they were potential predators, and that responses depend upon the probability of visually detecting humans. Breeding densities peaked at intermediate pedestrian rates in two consecutive years. Consumption rates of artificial food within fragments were higher at intermediate pedestrian rates, which may indicate that this species uses people as cues to find food. Consumption rates decreased at high pedestrian rates as a result of low tolerance to people and a reduction in the time devoted to foraging. Our results imply a balance between attraction to and avoidance of humans to account for these nonlinear responses, and that the resource-use–disturbance trade-off hypothesis can be a useful mechanistic explanation for understanding the responses to humans of species that may receive a direct or indirect benefit from human presence. Probando la Hipótesis del Riesgo-Perturbación en un Paisaje Fragmentado: Respuestas No Lineales de Passer domesticus hacia Humanos Resumen. Se escogió a Passer domesticus como especie modelo para analizar su respuesta a distintos niveles de perturbación humana en un ambiente urbano fragmentado. Regionalmente, se analizaron las variaciones lineales y no-lineales en las densidades reproductivas con relación a la tasa observada de visitantes a fragmentos forestales. Localmente, se comprobó experimentalmente la hipótesis del balance entre el uso del recurso y perturbación, la cual analiza la relación entre la frecuencia de visita y la frecuencia de uso del recurso, suponiendo que los animales reaccionan hacia los humanos como si fuesen depredadores, y que las respuestas dependen de las probabilidades de detección de humanos. Las densidades reproductivas en dos años consecutivos fueron más elevadas con tasas de visita intermedias. Las tasas de consumo dentro de fragmentos mostraron un pico cuando la frecuencia de visita fue intermedia, lo que sugiere que esta especie utiliza a los humanos como indicadores de alimento; pero la tasa de consumo disminuyó con frecuencias de visita mas elevadas como resultado de la baja tolerancia hacia humanos y la reducción del tiempo dedicado a la alimentación. Nuestros resultados indican que un balance entre atracción y repulsión hacia humanos está involucrado en las respuestas no lineales y que la hipótesis del balance entre el uso del recurso y el disturbio puede ser una explicación apropiada para comprender las respuestas de especies que reciben un beneficio directo o indirecto de la presencia humana.

Loss, fragmentation and decreasing quality of habitats have been proposed as major threats to biodiversity world-wide, but relatively little is known about biodiversity responses to multiple pressures, particularly at very large spatial scales. We evaluated the relative contributions of four landscape variables (habitat cover, diversity, fragmentation and productivity) in determining different components of avian diversity across Europe. We sampled breeding birds in multiple 1-km 2 landscapes, from high forest cover to intensive agricultural land, in eight countries during 2001−2002. We predicted that the total diversity would peak at intermediate levels of forest cover and fragmentation, and respond positively to increasing habitat diversity and productivity; forest and open-habitat specialists would show threshold conditions along gradients of forest cover and fragmentation, and respond positively to increasing habitat diversity and productivity; resident species would be more strongly impacted by forest cover and fragmentation than migratory species; and generalists and urban species would show weak responses. Measures of total diversity did not peak at intermediate levels of forest cover or fragmentation. Rarefaction-standardized species richness decreased marginally and linearly with increasing forest cover and increased non-linearly with productivity, whereas all measures increased linearly with increasing fragmentation and landscape diversity. Forest and open-habitat specialists responded approximately linearly to forest cover and also weakly to habitat diversity, fragmentation and productivity. Generalists and urban species responded weakly to the landscape variables, but some groups responded non-linearly to productivity and marginally to habitat diversity. Resident species were not consistently more sensitive than migratory species to any of the landscape variables. These findings are relevant to landscapes with relatively long histories of human land-use, and they highlight that habitat loss, fragmentation and habitat-type diversity must all be considered in land-use planning and landscape modeling of avian communities.

Structural variations between edge and interior areas within forest fragments may bring about differences in food availability that may influence the selective behaviour of predators and prey population dynamics. The purpose of this paper was to assess patterns of artificial prey selection by wild birds (House Sparrow Passer domesticus and Rock Dove Columba liuia) between edge and interior areas of woodland, taking into account differences in prey frequency (10% of one food type and 90% of the other) and density (30 baitdm-' and 50 baitdm-'). Experiments were conducted at 24 plots in 3 forest fragments in the city of Madrid, Spain. Selectivity did not vary among parks or between densities. However, selectivity did vary with the frequency and location of baits, showing an antiapostatic trend (baits were preferred at low rather than at high frequencies) that was more pronounced at interiors than at edges. Two possible factors that may account for stronger anti-apostatic selection at edges are the higher densities of predators and pedestrians found there. However, there are many other possible explanations, and no specific conclusion can be supported with the current data. The results of this study also point out that site heterogeneity should be taken into account in the experimental design of future studies on frequency-dependent food selection by wild birds, particularly in fragmented landscapes.

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