Abstract
Species distributions are limited by a complex array of abiotic and biotic factors. In general, abiotic (climatic) factors are thought to explain species’ broad geographic distributions, while biotic factors regulate species’ abundance patterns at local scales. We used species distribution models to test the hypothesis that a biotic interaction with a tree, the Colombian oak (Quercus humboldtii), limits the broad-scale distribution of the Acorn Woodpecker (Melanerpes formicivorus) in the Northern Andes of South America. North American populations of Acorn Woodpeckers consume acorns from Quercus oaks and are limited by the presence of Quercus oaks. However, Acorn Woodpeckers in the Northern Andes seldom consume Colombian oak acorns (though may regularly drink sap from oak trees) and have been observed at sites without Colombian oaks, the sole species of Quercus found in South America. We found that climate-only models overpredicted Acorn Woodpecker distribution, suggesting that suitable abiotic conditions (e.g. in northern Ecuador) exist beyond the woodpecker’s southern range margin. In contrast, models that incorporate Colombian oak presence outperformed climate-only models and more accurately predicted the location of the Acorn Woodpecker’s southern range margin in southern Colombia. These findings support the hypothesis that a biotic interaction with Colombian oaks sets Acorn Woodpecker’s broad-scale geographic limit in South America, probably because Acorn Woodpeckers rely on Colombian oaks as a food resource (possibly for the oak’s sap rather than for acorns). Although empirical examples of particular plants limiting tropical birds’ distributions are scarce, we predict that similar biotic interactions may play an important role in structuring the geographic distributions of many species of tropical montane birds with specialized foraging behavior.
Highlights
Understanding the factors that explain species’ distributional limits is a fundamental goal of ecology and biogeography [1]
Because our study focuses on Acorn Woodpeckers in the Northern Andes, we limited our species distribution modeling to an extent between 69°W and 82°W longitude and 7°S and 11°N latitude, which does not include the nearest neighboring population of Acorn Woodpeckers present in the highlands of Costa Rica and Panama
Few studies have marshaled quantitative evidence that biotic interactions limit distributions of tropical species [58,59,60], and the influence of biotic interactions with important food resources on the distributional limits of tropical birds has seldom been previously considered in a species distribution modeling framework
Summary
Understanding the factors that explain species’ distributional limits is a fundamental goal of ecology and biogeography [1]. Many examples demonstrate that species interactions can influence distributional limits at fine spatial scales (e.g., for competition) [11,12,13,14,15], and the influence of biotic factors such as interspecific competition can explain non-random abundance patterns at regional scales [16,17]. The relative paucity of examples where biotic factors explain species’ geographic limits supports the Eltonian noise hypothesis, which posits that biotic interactions seldom influence species’ geographic extents [18]. Investigating the influence of biotic interactions on species’ distributions is an active arena of research [10,19,20,21], and an increasing number of case studies demonstrate that biotic interactions can be important factors influencing species’ distributions [22,23,24,25]
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