Authors: Eric R. Pianka, Laurie J. Vitt, Nicolás Pelegrin, Daniel B. Fitzgerald, and Kirk O. Winemiller
Ecologists begin to construct a Periodic Table of Niches
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| Redsands study area in the Great Victoria Desert of Western Australia – 55 species of lizards, including the thorny devil Moloch horridus, occur in sympatry here. (Credit: Eric R. Pianka) |
Five prominent ecologists present new insights into a concept first suggested by the late Robert H. MacArthur over half a century ago. MacArthur suggested a Periodic Table of ecological niches could be constructed similar to the chemist’s Periodic Table of Elements, which are ordered by a combination of their atomic number (protons), configuration of electrons, and certain chemical properties.
Throughout, the paper features a back-and-forth interplay among outright natural history, niche theory, state-of-the-art multivariate statistical analyses, and downloadable rotatable 3D plots.
Ecological niches are so complex that the notion they could somehow be ordered in a predictable way has eluded ecologists. The niche defines how species live, reproduce, and interact with other organisms and their environments. Consequently, niches are multidimensional. Visualizing and understanding such complex systems is difficult because humans perceive only three dimensions.
In a major synthesis appearing in The American Naturalist, a niche ordination and classification framework is presented based on extensive ecological data gathered by the first two authors during the course of two lifetimes of fieldwork conducted in diverse habitats on four continents over the last half-century. A key prediction of this theory is that distantly related species on different continents should evolve to fill identifiably similar niches, and should exhibit similar sets of ecological traits, referred to as evolutionarily convergence by ecologists. To test this prediction, data were assembled on five major niche dimensions (habitat, diet, life history, metabolism, and defense, each with 7-15 variables) for 134 species representing 24 of the 38 extant lizard families.
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| Three-dimensional depiction of Periodic Table of Niches. Each box represents a lizard niche (n1−n12). Species pairs from different geographic regions or different evolutionary lineages (clades) independently evolve traits allowing them to effectively use similar niches in their respective habitats (ecological convergence). Two such convergent pairs are indicated: 1) blue lines indicate the convergent pair Phrynosoma from North America and Moloch from Australia and 2) green lines indicate the convergent pair Polychrusfrom Brazil and Chamaeleo from Africa. In reality, ecological niches are multidimensional. (Credit: Laurie J. Vitt) |
Lizard niche dimensions have evolved in concert; transitions in life history and trophic traits occur in synchrony. Natural dichotomies include activity time, forging mode, parity mode (egg laying/live bearing), and habitat. Patterns are repeated: Australian desert lizards solve problems in the same ways that African and American lizards do, even when they are not evolutionary related.
Convergent evolution among lizards around the world is one of the pillars for the idea of a Periodic Table. Based on multivariate analyses of 50+ lizard niche dimensions, just three
capture 61.7 % of the total variation indicating that lizard niches are tightly constrained. To more easily visualize their results, the authors produced rotating 3-dimensional graphics (link to 3-dimensional graphics) that allow exploration of constraints and tradeoffs in the evolution of lizard niches. These reveal the manner in which species overlap or separate out based on habitat, body size, foraging mode, diet, life history, metabolism, defensive tactics, and/or time and place of activity. One hundred distantly-related convergent species pairs from different geographic regions stay close to each other as niche space is rotated (link to 3D plot showing some convergent pairs), thus confirming periodicity in lizard niche dimensions, and strongly supporting the utility of a Periodic Table. Extending this approach to other taxa should lead to a wider understanding of niche evolution.
Lead author Eric Pianka points out, “Summarizing major ecological traits in such simple schemes will allow ecologists to predict how species might react to new environmental conditions, the invasive potential of species, and even inform us about how niches have evolved in the past and will evolve in the future, all of which have direct bearing on impacts of climate change.”
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