American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Adaptation and latitudinal gradients in species interactions: nest predation in birds”

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Benjamin G. Freeman, Micah N. Scholer, Mannfred M. A. Boehm, Julian Heavyside, and Dolph Schluter (Dec 2020)

Nest predation in birds is thought to be highest in the tropics—but new research shows this is not true, likely because of adaptation

Read the Article (Just Accepted)

Evolution can alter geographic gradients in ecological interactions

Two Black-throated Mango (<i>Anthracothorax nigricollis</i>) nestlings barely fit in their nest. This nest was located on a dead branch one meter above a fast-flowing river, permitting close approach by canoe but presumably making it more difficult for non-avian nest predators to reach. Tropical birds exhibit many such behaviors that appear to be adaptations against nest predation. <br />(Credit: Benjamin Freeman)
Two Black-throated Mango (Anthracothorax nigricollis) nestlings barely fit in their nest. This nest was located on a dead branch one meter above a fast-flowing river, permitting close approach by canoe but presumably making it more difficult for non-avian nest predators to reach. Tropical birds exhibit many such behaviors that appear to be adaptations against nest predation.
(Credit: Benjamin Freeman)

The first bird nest Ben Freeman found in the tropics was a tidy cup tucked near the base of a grass clump, two Bluish Flowerpiercer eggs nestled inside. But the next day, the eggs were gone, consumed by a predator. Ben’s introduction to tropical breeding bird biology mirrored the consensus in the scientific literature that tropical birds face high levels of nest predation, illustrating the more general idea that interactions between species are fast and furious in the tropics.

A decade later, Ben, now a postdoc at the University of British Columbia, teamed up with graduate students Micah Scholer, Mannfred Boehm, Julian Heavyside, and supervisor Dolph Schluter to revisit this dogma with data. Surprisingly, they found that daily rates of nest predation for landbirds are essentially the same across latitudes from Canada to Chile. What accounts for the discrepancy between previous studies and their new analysis?

A Thick-billed Euphonia female (<i>Euphonia laniirostris</i>) pauses outside its nest, a cavity excavated inside a wasp nest. Tropical birds exhibit many behaviors that appear to be adaptations against nest predation. Some are about where the nest is located. For example, nesting inside an active wasp nest confers protection against predators. Others are behaviors the birds themselves exhibit. For example, euphonias do a “magic trick” that disguises the location of their nest: male and female euphonias fly to their nest together as a pair, then the male sharply veers to the side exactly as the female enters the nest and “disappears”. This makes it much more difficult to locate the nest, at least for in this human’s experience. This “magic trick” behavior has independently evolved at least 10 times in Neotropical birds (Gulson-Castillo et al. 2018, <i>Wilson Journal of Ornithology</i>).<br />(Credit: Benjamin Freeman)
A Thick-billed Euphonia female (Euphonia laniirostris) pauses outside its nest, a cavity excavated inside a wasp nest. Tropical birds exhibit many behaviors that appear to be adaptations against nest predation. Some are about where the nest is located. For example, nesting inside an active wasp nest confers protection against predators. Others are behaviors the birds themselves exhibit. For example, euphonias do a “magic trick” that disguises the location of their nest: male and female euphonias fly to their nest together as a pair, then the male sharply veers to the side exactly as the female enters the nest and “disappears”. This makes it much more difficult to locate the nest, at least for in this human’s experience. This “magic trick” behavior has independently evolved at least 10 times in Neotropical birds (Gulson-Castillo et al. 2018, Wilson Journal of Ornithology).
(Credit: Benjamin Freeman)

The authors propose that differential adaptation by tropical and temperate birds explains their unexpected result. When they control for a key life history trait-nesting period length, which is longer in the tropics- they find that tropical birds suffer higher rates of daily nest predation than temperate birds. This implies that if all birds had similar life history traits, then nest predation rates would indeed by greatest in the tropics. But tropical and temperate birds have diverged in their life history traits, in part driven by adaptation to differences in predation regimes.

The overall conclusion? If birds across all latitudes were the same, then nest predation rates would be higher in the tropics. But species can adapt to the negative interactions they experience, and these adaptations in turn influence interaction rates. The authors argue that evolution plays a key role in geographic gradients in interactions.


Abstract

Are biotic interactions stronger in the tropics? Here we investigate nest predation in birds, a canonical example of a strong tropical biotic interaction. Counter to expectations, daily rates of nest predation vary minimally with latitude. However, life history traits that influence nest predation have diverged between latitudes. For example, tropical species have evolved a longer average nesting period, which is associated with reduced rates of nest attendance by parents. Daily nest mortality declines with nesting period duration within regions, but tropical species have a higher intercept. Consequently, for the same nesting period length, tropical species experience higher daily nest predation rates than temperate species. The implication of this analysis is that the evolved difference between latitudes in nesting period length produces a flatter latitudinal gradient in daily nest predation than would otherwise be predicted. We propose that adaptation may frequently dampen geographic patterns in interaction rates.