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The question of why some groups of animals and plants flourish while others do not has puzzled biologists for centuries. One way to address the question is to look for special features or abilities shared by the successful groups. We know that smaller organisms like insects or bacteria are more diverse than larger organisms (birds, mammals), and that the warmer habitats of the tropics generate more animal and plant diversity than temperate areas.
The ability to eat new foods also helps explain the incredible number of species among the herbivorous insects. Just like insects on land, many crustaceans – the 70,000 species of crabs, lobsters and their relatives – eat plants and seaweeds in the kelp forests and coral reefs in the sea, and in streams and lakes around the world. Some crabs even climb mangrove trees to feed on leaves, and others eat seedlings from the rainforest floor.
Poore et al. (2017) showed that the ability to eat seaweeds and plants promotes diversity among crustaceans, just as it does among herbivorous insects. To do this, they examined the evolutionary tree of crustaceans and found animals eating plants in at least 31 different lineages. Then, to test whether plant-feeding promotes diversity, they compared the number of species in each plant-feeding group with their nearest relatives. These sister comparisons showed that the herbivores had, on average, 21 times more species than their nearest relatives - crustaceans eating live animals, microbes or decaying organic material. The geographic distributions of plant-feeding and sister taxa were analysed to examine whether shifts to plant feeding have facilitated increases in range size and to test the likelihood of contrasts in richness being confounded by possible regional differences in richness (latitude, biogeographic regions). The records from OBIS for each clade were analysed to estimate range size, latitudinal range and the occurrence in the biogeographic realms of Spalding et al. (2007). These analyses detected that plant-feeding clades did, on average, have larger range sizes, and that the increases in their richness could not be explained by disproportionate sampling in the tropics or in certain biogeographic regions.
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