American Society of Naturalists

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“Ecological limits as the driver of bird species richness patterns along the east Himalayan elevational gradient”

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Matthew Schumm, Alex E. White, K. Supriya, and Trevor D. Price (May 2020)

Himalayan bird species pack into niche space where resources are more abundant, at middle elevations

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For decades, ecologists have been trying to understand why some places have many more species of plants and animals than others. In the east Himalaya, about 100 more bird species breed in the middle elevations (1000-2000 meters above sea level) than at the bottom of the mountain or in high-elevation forests. Many of the species that live together are closely related to each other, look similar, and eat similar insects and spiders. These birds’ similarity despite their coexistence has been taken to imply that competition for food is a relatively unimportant factor in determining which bird species are found where. However, Matthew Schumm and colleagues challenge this view. They show that the pattern of bird species numbers and species traits in the east Himalaya matches the pattern of prey insect numbers and sizes available at different elevations. This implies that amount and type of available food is likely a major determinant of where bird populations can survive and breed.

The authors compare bird body and beak shapes and sizes and feeding habits with insect numbers and sizes at various elevations. They show that larger numbers of small insects are present at the middle elevations in the summer. The greater number of small insects appears to support the coexistence of closely related small-bodied and small-beaked insect-eating bird species. The authors then look at patterns of bird traits and insect numbers along other mountain ranges in tropical Ecuador, Tanzania, and New Guinea and find that patterns of bird beak size match patterns of insect size. The authors argue that resource availability likely plays an important role in shaping bird diversity patterns globally.

Common, small, and insectivorous. The white-browed fulvetta <i>Alcippe castaneceps</i> and yellow-throated fulvetta <i>Alcippe cinerea</i>.<br/>(Photos by Tom Stephenson).
Common, small, and insectivorous. The white-browed fulvetta Alcippe castaneceps and yellow-throated fulvetta Alcippe cinerea.
(Photos by Tom Stephenson).

Abstract

Variation in species richness across environmental gradients results from a combination of historical non-equilibrium processes (time, speciation, extinction) and present-day differences in environmental carrying capacities (i.e., ecological limits, affected by species interactions and the abundance and diversity of resources). In a study of bird richness along the sub-tropical east Himalayan elevational gradient, we test the prediction that species richness patterns are consistent with ecological limits using data on morphology, phylogeny, elevational distribution, and arthropod resources. Species richness peaks at mid-elevations. Occupied morphological volume is roughly constant from low to mid-elevations, implying more species are packed into the same space at mid-elevations compared with low elevations. However, variance in beak length, and differences in beak length between close relatives decline with elevation, a consequence of the addition of many small insectivores at mid-elevations. These patterns are predicted from resource distributions: arthropod size diversity declines from low to mid elevations, largely because many more small insects are present at mid-elevations. Weak correlations of species mean morphological traits with elevation also match predictions based on resources and habitats. Elevational transects in the tropical Andes, New Guinea, and Tanzania similarly show declines in mean arthropod size and mean beak length, and in these cases likely contribute to declining numbers of insectivorous bird species richness along these gradients. The results imply conditions for ecological limits are met, although historical non-equilibrium processes are likely to also contribute to the pattern of species richness.