Ecology matters: Diet predicts genetic divergence in Neotropical birds

Exploring the consequences of dispersal capacity and demographic stability.

Almost one in four bird species breeds in the Neotropical lowlands. But where did all this diversity come from? Most research on avian speciation in this region focused on geographical barriers, such as the formation of rivers, the rise of mountains, or the fragmentation of forest habitats during Pleistocene. Apart from these extrinsic factors, however, the ecology of birds can also play a role. In a previous blog post, for example, I described how the habitat preference of certain species determined the likelihood of crossing land bridges and colonizing islands in Southeast Asia. Forest specialists needed forested areas to disperse and could not transverse the land bridges which consisted mostly of open vegetation. Generalist species, on the other hand, did use the land bridges and travelled freely between islands.

A similar reasoning can be applied to diet. Several studies on Neotropical birds found that species that eat the reproductive parts of a plant (e.g., fruits, seeds, flowers) have greater dispersal abilities than species that feed on arthropods. This relationship between diet and dispersal can probably be explained by the seasonal availability of the food sources. In the tropics, arthropods are present year-round, so insectivores do not have to move far to forage. Fruits and flowers, however, show a seasonal pattern, forcing frugivores and nectivores to disperse more in search of food. Moreover, the dynamics of food resources will have an influence on the demographic changes of the species: stable food availability will lead to stable populations, whereas fluctuating food sources will result in fluctuating populations. This all sounds very logical, but can we detect the consequences of these processes in the genetic make-up of a bird species?

Genetic Divergence

A recent study in the journal Ecology Letters used this information to make some predictions about the level of genetic divergence in Neotropical bird species. The year-round availability of arthropods and the limited dispersal capacity of insectivores should lead to clear genetic differences between populations with this diet. The situation for frugivores and nectivores – with unstable food sources and higher dispersal rates – will occasionally result in population expansions and consequent gene flow might reduce levels of genetic differentiation. The researchers tested these predictions by estimating genetic divergence in 56 Neotropical bird species (using the mitochondrial gene ND2) and correlating this estimate with several ecological traits. Based on literature, they determined the diet, forest use (interior vs. forest edge) and vertical stratum (canopy or understory) of the species. In addition, stable isotope analyses of nitrogen were used to get an independent measure of diet for a subset of species.

Statistical analyses revealed that diet was the main determining factor in predicting genetic divergene. The researchers write that “Birds species consuming plant products such as fruit, seeds and nectar have significantly less mitochondrial divergence between Belize and Panama than species consuming solely arthropods and species with mixed arthropod- and plant-based diets.” Exactly as expected based on the reasoning I explained above. Isn’t it wonderful when a hypothesis is supported by the data?

Statistical analyses indicated that frugivores have significantly lower genetic divergence than insectivores and mixed diet species. From: Miller et al. (2021) Ecology Letters.

Demography

But what about the stability of the populations: are insectivorous populations more stable than frugivores and nectivores? To test this hypothesis, the researchers calculated population growth rates using the software LAMARC. One of the outputs from this analysis – the statistic R2 – indicates population expansion. This statistic was significant for 12 of the 20 (60%) frugivore and nectivore populations, while only 6 of the 25 (23%) insectivore populations showed a significant population expansion. Similarly, the statistic g (a measure of population dynamics based on coalescent theory) pointed to more expanding populations in frugivores and nectivores compared to insectivores. These patterns are in line with the population dynamics we can expect based on the availability of the food sources.

Putting it all together, this study nicely shows how demographic fluctuations and differences in dispersal capacity associated with a particular foraging ecology have predictable effects on levels of genetic divergence. In other words, ecology matters.

References

Miller, M. J., Bermingham, E., Turner, B. L., Touchon, J. C., Johnson, A. B., & Winker, K. (2021). Demographic consequences of foraging ecology explain genetic diversification in Neotropical bird species. Ecology Letters24(3), 563-571.

Featured image: Blue-gray Tanager (Thraupis episcopus) © Mike’s Birds | Wikimedia Commons

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