Sensitivity of Tropical Insectivorous Birds to the Anthropocene: A Review of Multiple Mechanisms and Conservation Implications

Abstract

Epigraph: “The house is burning. We do not need a thermometer. We need a fire hose.” (P. 102, Janzen and Hallwachs, 2019). Insectivorous birds are declining widely, and for diverse reasons. Tropical insectivorous birds, more than 60% of all tropical birds, are particularly sensitive to human disturbances including habitat loss and fragmentation, intensive agriculture and pesticide use, and climate change; and the mechanisms are incompletely understood. This review addresses multiple, complementary and sometimes synergistic explanations for tropical insectivore declines, by categorizing explanations into ultimate vs. proximate, and direct versus indirect. Ultimate explanations are diverse human Anthropocene activities and the evolutionary history of these birds. This evolutionary history, synthesized by the Biotic Challenge Hypothesis (BCH), explains tropical insectivorous birds' vulnerabilities to many proximate threats as a function of both these birds' evolutionary feeding specialization and poor dispersal capacity. These traits were favored evolutionarily by both the diversity of insectivorous clades competing intensely for prey and co-evolution with arthropods over long evolutionary time periods. More proximate, ecological threats include bottom-up forces like declining insect populations, top-down forces like meso-predator increases, plus the Anthropocene activities underlying these factors, especially habitat loss and fragmentation, agricultural intensification, and climate change. All these conditions peak in the lowland, mainland Neotropics, where insectivorous bird declines have been repeatedly documented, but also occur in other tropical locales and continents. This multiplicity of interacting evolutionary and ecological factors informs conservation implications and recommendations for tropical insectivorous birds: (1) Why they are so sensitive to global change phenomena is no longer enigmatic, (2) distinguishing ultimate versus proximate stressors matters, (3) evolutionary life-histories predispose these birds to be particularly sensitive to the Anthropocene, (4) tropical regions and continents vary with respect to these birds' ecological sensitivity, (5) biodiversity concepts need stronger incorporation of species' evolutionary histories, (6) protecting these birds will require more, larger reserves for multiple reasons, and (7) these birds have greater value than generally recognized.

Light and thermal niches of ground‐foraging Amazonian insectivorous birds

“…The Neotropics are a hotspot of avian diversity, with Amazonia holding the world's highest richness of suboscine passerines (Harvey et al, 2020). These birds largely comprise understory insectivores, a group highly sensitive to disturbance and therefore useful as indicators of change in rainforest ecosystems (Bregman et al, 2014; Powell et al, 2015a; Sherry, 2021). Approximately 20% of the Amazon rainforest had been removed by 2018 (Artaxo, 2019; da Cruz et al, 2021), but the footprint of disturbance is even greater as clearing degrades habitat beyond deforested areas (Bregman et al, 2014).…”

Guild‐level response to Hurricane Carlotta among birds in a Mexican wetland

“…Nonetheless, we anticipate that an intensifying disturbance regime due to climate change will strongly influence the composition of avian terrestrial insectivores in mature mangroves from hurricane‐prone regions. Tropical insectivores are ecologically sensitive to anthropogenic disturbance (Sherry 2021), making sustainable disturbance dynamics in their remaining natural habitats vital. Understorey species may be negatively affected by shifts in forest structure (Krauss & Osland 2019) and mangrove management should aim to buffer disturbance‐sensitive species by optimizing forest resilience, while harnessing carbon sequestration capacity.…”

Grand challenges at the frontiers of bird science

“…Yet, sample sizes of trait measurements are often small for many taxa, particularly for species endemic to the most biodiverse places on earth, so there remains much to learn in terms of geographic variation in natural history traits. Furthermore, some natural history information is nearly or entirely absent for most species, such as details of dietary items and how diets vary across time and space (Sherry et al, 2020;Sherry, 2021). The paucity of such information hamstrings our efforts to understand population declines in the thousands of species that consume insects.…”