Tropical frugivorous birds are both more specialized and generalized than temperate frugivorous birdsSubmitted by editor on 21 February 2017.
Frugivorous bird assemblages at high latitudes consist mainly of bird species with an omnivorous diet containing both fruits and a wide range of other food types. This is here exemplified with the North American Turdus migratorius eating both earthworms and fruits of Cornus florida. Painting by Jon Fjeldså.
By Bo Dalsgaard, Matthias Schleuning, Pietro K. Maruyama, D. Matthias Dehling, Jesper Sonne, Jeferson Vizentin-Bugoni, Thais B. Zanata, Jon Fjeldså, Katrin Böhning-Gaese, Carsten Rahbek
The tropics with its unique diversity of species are in many ways special, but how specialized are tropical species? A long-held tenet in ecology is that tropical species are more specialized and have a finer partitioning of niche space than those in temperate systems (e.g. Dobzhansky 1950, MacArthur 1969, Schemske 2009). A priori, one might indeed expect that species with broad niches inhabit temperate environments, as lower productivity and higher seasonal and inter-annual fluctuations in climate and resources should select for species traits that allow foraging on a broad range of food types. However, it remains debated whether biotic specialization in fact decreases toward temperate regions, or if this is just a “zombie idea” (Dynamic Ecology Blog).
Recently, there has been a renewed interest in evaluating the latitudinal gradient in biotic specialization using species interaction networks, i.e. studies tediously recording interactions between co-occuring species assemblages. Most effort has been put on mutualistic interactions between plant assemblages and their animal pollinators and seed dispersers. These studies have found idiosyncratic results, e.g. plant-hummingbird networks were more specialized toward the tropics (Dalsgaard et al. 2011), whereas plant-frugivore networks were most specialized in temperate regions (Schleuning et al. 2012, 2014).
In this study, we revisited latitudinal patterns of specialization for avian frugivore assemblages by using two complementary ways to measure specialization. We first scored whether frugivorous birds were obligate or opportunistic frugivores, i.e. whether birds almost exclusively relied on fruit diet or used a wide array of food types. Tropical assemblages largely consisted of obligate frugivores, up to about 50-60% of the species within a given assemblage, whereas very few species relied solely on fruit diet at high latitudes and, instead, ate many other food types, such as insects and other invertebrates. Second, we mapped all pairwise interactions between co-occurring frugivorous birds and their food plants and used network tools to estimate the niche overlap in food plants among the bird species. Here we identified an increasing level of specialization with increasing latitude, i.e. temperate networks were the most specialised networks. Thus, depending on the scale at which we measured the biotic specialization of frugivorous birds, we found contrasting latitudinal specialization patterns. This shows that avian frugivore assemblages are more specialized on fruit diet in the tropics, but tropical birds have a generalized fruit diet and share many of their fruit recourses. The evolution of obligate frugivory in the tropics may thus have resulted in a high overlap of fruiting plants among avian frugivores.
Our results highlight that a better understanding of geographical patterns of specialization in resource-consumer interactions requires comparing different approaches to measure biotic specialization in ecological communities.
Tropical frugivore bird communities mainly consist of obligate frugivourous species, which form fairly generalized interaction networks with their fruit plants, i.e. the birds often share fruit recourses. This is here exemplified with the South American Tangara chilensis and Chiroxiphia boliviana both eating berries from a Miconia sp., as illustrated in the painting to the left and highlighted in red in the network to the right. In the network, bird species are shown to the left and plant species to the right; link thickness reflects the pairwise frequency of interaction, and bar size illustrates the total number of interactions for each species. Painting by Jon Fjeldså.
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