Sensitivity of tropical montane birds to anthropogenic disturbance and management strategies for their conservation in agricultural landscapes.

Tropical montane bird communities are hypothesized to be highly sensitive to anthropogenic disturbance because species are adapted to a narrow range of environmental conditions and display high rates of endemism. We assessed avian sensitivity at regional and continental scales for a global epicenter of montane bird biodiversity, the tropical Andes. Using data from an intensive field study of cloud forest bird communities across 7 landscapes undergoing agricultural conversion in northern Peru (1800-3100 m, 2016-2017) and a pan-Andean synthesis of forest bird sensitivity, we developed management strategies for maintaining avian biodiversity in tropical countrysides and examined how environmental specialization predicts species-specific sensitivity to disturbance. In Peru, bird communities occupying countryside habitats contained 29-93% fewer species compared with those in forests and were compositionally distinct due to high levels of species turnover. Fragments of mature forest acted as reservoirs for forest bird diversity, especially when large or surrounded by mixed successional vegetation. In high-intensity agricultural plots, an addition of 10 silvopasture trees or 10% more fencerows per hectare increased species richness by 18-20%. Insectivores and frugivores were most sensitive to disturbance: abundance of 40-70% of species declined in early successional vegetation and silvopasture. These results were supported by our synthesis of 816 montane bird species studied across the Andes. At least 25% of the species declined due to all forms of disturbance, and the percentage rose to 60% in agricultural landscapes. The most sensitive species were those with narrow elevational ranges and small global range sizes, insectivores and carnivores, and species with specialized trophic niches. We recommend protecting forest fragments, especially large ones, and increasing connectivity through the maintenance of early successional vegetation and silvopastoral trees that increase avian diversity in pastures. We provide lists of species-specific sensitivities to anthropogenic disturbance to inform conservation status assessments of Andean birds.

Sensibilidad de aves montanas a perturbaciones antropogénicas y estrategias de manejo para su conservación en paisajes agrícolas Resumen Se ha hipotetizado que las comunidades de aves tropicales montanas son sumamente sensibles a la perturbación antropogénica porque las especies están adaptadas a una reducida gama de condiciones ambientales y tienen altas tasas de endemismo. Evaluamos la sensibilidad aviar a escalas regional y continental para un epicentro global de biodiversidad de aves montanas, los Andes tropicales. Utilizando datos de un estudio intensivo de campo de comunidades de aves de bosques nublados en 7 paisajes bajo conversión agrícola en el norte de Perú (1800 - 3100 m, 2016-2017) y una síntesis pan-Andina de sensibilidad de aves de bosque, desarrollamos estrategias de manejo para el mantenimiento de la biodiversidad de aves en campiñas tropicales y examinamos cómo la especialización ambiental predice la sensibilidad de cada especie a la perturbación. Las comunidades de aves ocupando hábitats campestres tropicales en Perú contenían 29 - 93% menos especies en comparación con las de bosques y tuvieron una composición distinta debido a los altos niveles de recambio de especies. Los fragmentos de bosque maduro fungieron como reservorios para la diversidad de aves de bosque, especialmente cuando eran extensos y estaban rodeados por vegetación secundaria mixta. En las parcelas con actividad agrícola intensiva, la adición de 10 árboles silvopastoriles o 10% más de cercos por hectárea incrementó la riqueza de especies en 18 - 20%. Las insectívoras y frugívoras fueron más sensibles a la perturbación: la abundancia de 40 - 70% de especies declinó en la vegetación secundaria temprana y en la silvopastura. Estos resultados fueron sustentados por nuestra síntesis de 816 especies de aves montanas estudiadas en los Andes. Por lo menos 25% de estas especies declinaron debido a todas las formas de perturbación, y el porcentaje incrementó a 60% en paisajes agrícolas. Las especies más sensibles fueron aquellas con rangos altitudinales estrechos y extensiones de distribución pequeñas, las insectívoras y carnívoras y las especies con nichos tróficos especializados. Recomendamos la protección de fragmentos de bosque, especialmente los extensos, y el incremento de la conectividad mediante el mantenimiento de vegetación secundaria temprana y árboles silvopastoriles que incrementan la diversidad de aves en los pastizales. Proporcionamos listas de la sensibilidad de cada especie a la perturbación antropogénica para contribuir a las evaluaciones del estatus de conservación de aves Andinas.

Network structure of avian mixed-species flocks decays with elevation and latitude across the Andes.

Birds in mixed-species flocks benefit from greater foraging efficiency and reduced predation, but also face costs related to competition and activity matching. Because this cost-benefit trade-off is context-dependent (e.g. abiotic conditions and habitat quality), the structure of flocks is expected to vary along elevational, latitudinal and disturbance gradients. Specifically, we predicted that the connectivity and cohesion of flocking networks would (i) decline towards tropical latitudes and lower elevations, where competition and activity matching costs are higher, and (ii) increase with lower forest cover and greater human disturbance. We analysed the structure of 84 flock networks across the Andes and assessed the effect of elevation, latitude, forest cover and human disturbance on network characteristics. We found that Andean flocks are overall open-membership systems (unstructured), though the extent of network structure varied across gradients. Elevation was the main predictor of structure, with more connected and less modular flocks upslope. As expected, flocks in areas with higher forest cover were less cohesive, with better defined flock subtypes. Flocks also varied across latitude and disturbance gradients as predicted, but effect sizes were small. Our findings indicate that the unstructured nature of Andean flocks might arise as a strategy to cope with harsh environmental conditions. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Wet and dry extremes reduce arthropod biomass independently of leaf phenology in the wet tropics.

Warming temperatures are increasing rainfall extremes, yet arthropod responses to climatic fluctuations remain poorly understood. Here, we used spatiotemporal variation in tropical montane climate as a natural experiment to compare the importance of biotic versus abiotic drivers in regulating arthropod biomass. We combined intensive field data on arthropods, leaf phenology and in situ weather across a 1700-3100 m elevation and rainfall gradient, along with desiccation-resistance experiments and multi-decadal modelling. We found limited support for biotic drivers with weak increases in some herbivorous taxa on shrubs with new leaves, but no landscape-scale effects of leaf phenology, which tracked light and cloud cover. Instead, rainfall explained extensive interannual variability with maximum biomass at intermediate rainfall (130 mm month-1 ) as both 3 months of high and low rainfall reduced arthropods by half. Based on 50 years of regional rainfall, our dynamic arthropod model predicted shifts in the timing of biomass maxima within cloud forests before plant communities transition to seasonally deciduous dry forests (mean annual rainfall 1000-2500 mm vs. <800 mm). Rainfall magnitude was the primary driver, but during high solar insolation, the 'drying power of air' (VPDmax ) reduced biomass within days contributing to drought related to the El Niño-Southern Oscillation (ENSO). Highlighting risks from drought, experiments demonstrated community-wide susceptibility to desiccation except for some caterpillars in which melanin-based coloration appeared to reduce the effects of evaporative drying. Overall, we provide multiple lines of evidence that several months of heavy rain or drought reduce arthropod biomass independently of deep-rooted plants with the potential to destabilize insectivore food webs.

El aumento de las temperaturas está incrementando los extremos de precipitación, pero las respuestas de los artrópodos a las fluctuaciones climáticas siguen siendo poco conocidas. Aquí, utilizamos la variación espaciotemporal en el clima montano tropical como un experimento natural para comparar la importancia de los factores bióticos versus abióticos en la regulación de la biomasa de artrópodos. Combinamos datos de campo intensivos de artrópodos, fenología de las hojas y clima in situ a lo largo de un gradiente altitudinal de 1700 a 3100 m y un gradiente de precipitación, junto con experimentos de resistencia a la desecación y modelos multi-decenales. Encontramos evidencia limitada para los factores bióticos con aumentos débiles en algunos taxones de herbívoros en arbustos con hojas nuevas, pero no hubo efectos a escala de paisaje en la fenología de la hoja, que rastreaba la luz y la cubierta de nubes. En cambio, las precipitaciones explicaron la amplia variabilidad interanual con una biomasa máxima en precipitaciones intermedias (130 mm mes−1 ), ya que los tres meses de precipitaciones altas y bajas redujeron los artrópodos a la mitad. Basándose en 50 años de precipitación regional, nuestro modelo dinámico de artrópodos predijo cambios en el momento de los máximos de biomasa dentro del bosque nuboso antes de que las comunidades de plantas hicieran la transición al bosque seco estacional caducifolio (precipitación media anual 1000-2500 mm vs. <800 mm). La magnitud de las lluvias fue el principal factor, pero durante la alta insolación solar, el "poder de secado del aire" (VPDmax ) redujo la biomasa en cuestión de días, lo que contribuyó a la sequía relacionada con El Niño-Southern Oscillation (ENSO). Destacando los riesgos de la sequía, los experimentos demostraron la susceptibilidad de toda la comunidad a la desecación, excepto en el caso de algunas orugas en las que la coloración a base de melanina parece reducir los efectos de la desecación por evaporación. En resumen, proporcionamos múltiples líneas de evidencia de que varios meses de fuertes lluvias o sequías reducen la biomasa de artrópodos independientemente de las plantas de raíces profundas con el potencial de desestabilizar las redes alimentarias de los insectívoros.

Functional response traits and altered ecological niches drive the disassembly of cloud forest bird communities in tropical montane countrysides.

Anthropogenic disturbance contributes to global change by reshaping the ecological niche space available to biological communities. Quantifying the range of functional response traits required for species persistence is central towards understanding the mechanisms underlying community disassembly in disturbed landscapes. We used intensive field surveys of cloud forest bird communities across seven replicate landscapes undergoing agricultural conversion in the Peruvian Andes to examine how a suite of 16 functional response traits related to morphology, diet, foraging behaviour and environmental niche breadth predict (1) species-specific abundance changes in countryside habitats compared to forest and (2) differential changes to the ecological niche space occupied by communities. Our analyses relied on (1) hierarchical distance sampling models to examine the functional predictors of abundance change across the agricultural land use gradient while accounting for imperfect detection and (2) n-dimensional hypervolumes to quantify the expansion and contraction of ecological niche space in countryside habitats. Key traits related to increased abundance in early successional and mixed-intensity agricultural areas included (1) morphological adaptations to dense understorey habitats, (2) plant-based diets (flowers, fruit and seeds) and (3) broad elevational range limits and habitat breadth. Species occupying mixed and high-intensity agricultural land use regimes had mean elevational range limits 20%-60% wider than species found within forests. Collectively, ecological niche space expanded within agricultural habitats for traits related to diet and environmental niche breadth, while contracting for foraging and dispersal traits. Such changes were driven by species with unique functional trait combinations. Our results reveal the dynamic changes to ecological niche space that underly community structure in disturbed landscapes and highlight how increased niche breadth can ameliorate disturbance sensitivity for generalist species. We emphasize that functional traits can be used to predict changes in community structure across disturbance gradients, allowing insights into specific mechanisms underlying community disassembly beyond emergent patterns of functional diversity. By identifying key functional trait groups that align with different countryside habitats, we demonstrate how conservation practitioners can contribute to the retention of avian functional diversity in agricultural landscapes throughout the world.

La perturbación antropogénica contribuye al cambio global al remodelar el espacio de nicho ecológico disponible para las comunidades biológicas. Cuantificar la gama de rasgos de respuesta funcional requeridos para la persistencia de las especies es fundamental para comprender los mecanismos que subyacen al desensamble de la comunidad en los paisajes perturbados. Utilizamos muestreos de campo intensivos de las comunidades de aves del bosque nublado en siete paisajes replicados convertidos a uso agrícola en los Andes peruanos para examinar cómo un conjunto de 16 rasgos de respuesta funcional relacionados con la morfología, dieta, comportamiento de forrajeo, y la amplitud del nicho ambiental predicen (1) cambios en las abundancias de especies específicas en paisajes agrícolas ("countrysides") en comparación con el bosque y (2) cambios diferenciales en el espacio del nicho ecológico ocupado por las comunidades. Nuestros análisis se basaron en (1) modelos jerárquicos de muestreo por la distancia para examinar los predictores funcionales del cambio de abundancia a través del gradiente de uso de suelo para agricultura teniendo en cuenta la detección imperfecta, y (2) "n-dimensional hypervolumes" para cuantificar la expansión y contracción del espacio de nicho ecológico en los hábitats agrícolas. Los rasgos clave relacionados con el aumento de la abundancia en áreas agrícolas de sucesión temprana y de intensidad mixta incluyeron (1) adaptaciones morfológicas a hábitats de sotobosque denso, (2) dietas basadas en plantas (flores, frutas y semillas), y (3) amplios límites de rango de elevación y amplitud de hábitat. Las especies que ocupan regímenes de suelo agrícola mixto y de alta intensidad tenían límites de rango de elevación promedio 20%-60% más amplios que las especies que se encuentran en los bosques. En conjunto, el espacio del nicho ecológico se expandió dentro de los hábitats agrícolas para los rasgos relacionadas con la dieta y la amplitud del nicho ambiental, mientras que se contrajo para los rasgos de forrajeo y dispersión. Dichos cambios fueron impulsados por especies con combinaciones de rasgos funcionales únicos. Nuestros resultados revelan los cambios dinámicos en el espacio del nicho ecológico que subyacen a la estructura de la comunidad en los paisajes perturbados y destacan cómo una mayor amplitud del nicho puede mejorar la sensibilidad a las perturbaciones para las especies generalistas. Enfatizamos que los rasgos funcionales pueden utilizarse para predecir los cambios en la estructura de la comunidad a través de gradientes de perturbación, lo que permite comprender los mecanismos específicos que subyacen al desensamble de la comunidad más allá de los patrones emergentes de diversidad funcional. Al identificar grupos de rasgos funcionales claves que se alinean con diferentes hábitats agrícolas como en "countrysides" demostramos cómo los profesionales de la conservación pueden contribuir a la retención de la diversidad funcional de las aves en los paisajes agrícolas en todo el mundo.

Adaptations to light predict the foraging niche and disassembly of avian communities in tropical countrysides.

The role of light in partitioning ecological niche space remains a frontier in understanding the assembly of terrestrial vertebrate communities and their response to global change. Leveraging recent advances in biologging technology and intensive field surveys of cloud forest bird communities across an agricultural land use gradient in the Peruvian Andes, we demonstrate that eye size predicts (1) the ambient light microenvironment used by free-ranging birds, (2) their foraging niche, and (3) species-specific sensitivity to agricultural land use change. For 15 species carrying light sensors (N = 71 individuals), light intensity levels were best explained by eye size and foraging behavior, with larger-eyed species using darker microenvironments. Across the cloud forest bird community (N = 240 species), hyperopic ("far-sighted") foragers, (e.g., flycatchers), had larger eyes compared to myopic ("near-sighted") species (e.g., gleaners and frugivores); eye size was also larger for myopic insectivores that foraged in the forest understory. Eye size strongly predicted sensitivity to brightly lit habitats across an agricultural land use gradient. Species that increased in abundance in mixed intensity agriculture, including fencerows, silvopasture, and pasture, had smaller eyes, suggesting that light acts as an environmental filter when communities disassemble in a human-disturbed landscape. We suggest that eye size represents a novel functional trait contributing to terrestrial vertebrate community assembly and sensitivity to habitat disturbance.

Emulating natural disturbances for declining late-successional species: a case study of the consequences for cerulean warblers (Setophaga cerulea).

Forest cover in the eastern United States has increased over the past century and while some late-successional species have benefited from this process as expected, others have experienced population declines. These declines may be in part related to contemporary reductions in small-scale forest interior disturbances such as fire, windthrow, and treefalls. To mitigate the negative impacts of disturbance alteration and suppression on some late-successional species, strategies that emulate natural disturbance regimes are often advocated, but large-scale evaluations of these practices are rare. Here, we assessed the consequences of experimental disturbance (using partial timber harvest) on a severely declining late-successional species, the cerulean warbler (Setophaga cerulea), across the core of its breeding range in the Appalachian Mountains. We measured numerical (density), physiological (body condition), and demographic (age structure and reproduction) responses to three levels of disturbance and explored the potential impacts of disturbance on source-sink dynamics. Breeding densities of warblers increased one to four years after all canopy disturbances (vs. controls) and males occupying territories on treatment plots were in better condition than those on control plots. However, these beneficial effects of disturbance did not correspond to improvements in reproduction; nest success was lower on all treatment plots than on control plots in the southern region and marginally lower on light disturbance plots in the northern region. Our data suggest that only habitats in the southern region acted as sources, and interior disturbances in this region have the potential to create ecological traps at a local scale, but sources when viewed at broader scales. Thus, cerulean warblers would likely benefit from management that strikes a landscape-level balance between emulating natural disturbances in order to attract individuals into areas where current structure is inappropriate, and limiting anthropogenic disturbance in forests that already possess appropriate structural attributes in order to maintain maximum productivity.