In a 2017 publication in Global Ecology and Biogeography, we collated a novel quantitative volumetric dietary dataset for 308 lizard species worldwide from the field and literature. This novel dataset enabled us to test seven competing hypotheses posited to explain dietary niche breadth, focusing on those that are thought to either cause, or be influenced by, the latitudinal diversity gradient.
 Notomabuya frenata (photo Alison Gainsbury) | A species’ niche breadth is defined as the suite of environments or resources that the species can inhabit or use. Niche breadth is often invoked to explain the latitudinal diversity gradient. The latitudinal diversity gradient is the increase in species richness or biodiversity that occurs from the poles to the tropics. Despite this pattern having been recognized for over 200 years, the processes that drive and maintain the latitudinal diversity gradient remain unclear. We investigated which processes are important drivers of global lizard dietary niche breadth patterns, focusing on the relationship between niche breadth and species richness. |
A major tenant explaining greater species richness in the tropics is interspecific competition. Dietary niche breadth has long been hypothesized to decrease from the poles toward the tropics, as the numbers of competitors increase. Geographical variation in niche breadth is also hypothesized to be linked to high ambient energy levels, water availability, productivity and climate stability – reflecting an increased number of available prey taxa. Range size and body size are also hypothesized to be strongly and positively associated with niche breadth. We sought to determine which of these factors is associated with geographical variation in niche breadth across broad spatial scales and thus potentially drive the latitudinal diversity gradient.
| Overall, our findings are consistent with the notion that climate is an important predictor of dietary specialization, with both less rainfall and more stable temperatures associated with narrower dietary niches. Trophic interactions between lizard species and their arthropod prey are sensitive to climate. It is likely that climatic conditions not only affect these interactions but also alter the functional role of other vertebrate predators in terrestrial ecosystems. The sensitivity of dietary niche breadth to climate has important implications for essential ecosystem functions that maintain increased species richness in the tropics, such as food web stability and energy flow. |  Norops meridionalis (photo: Alison Gainsbury) |
The synergistic effects of a narrow dietary niche and small range size augments the vulnerability of species to habitat loss and climate change. Based on our findings, the ‘competitionist’s paradigm’ seems to be the exception rather than the rule in explaining the latitudinal diversity gradient.
Author: Alison Gainsbury
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