Species abundance may be affected negatively by interspecific competition or positively by ecosystem productivity. If competition influences the abundance of the inferior competitor, then in the absence or low richness of superior competitors, the inferior competitor can exhibit higher abundance, a phenomenon known as density compensation. In turn, productivity is generally positively related to species richness. One explanation for the productivity-richness relationship lies in the More Individuals Hypothesis (MIH), which posits that increasing energy leads to increased abundance of all species. Therefore, when contrasting the density compensation hypothesis (DCH) and the MIH a paradox arises. On one hand, according to MIH, productivity can affect positively both competitor richness and abundance. On the other hand, according to the DCH, productivity increasing competitor richness can lead to a decrease in abundance by increasing competition. We tested if abundance of 79 phyllostomid bat species is related to potential competitor richness (PCR) or to productivity (actual evapotranspiration, AET), lending support to the DCH or MIH, respectively. Based on relative abundance data gathered from literature, we interpolated abundance within each species’ range. Then we correlated species abundances with PCR and AET and analyzed all correlation coefficients in a meta-analytical framework to draw a general conclusion.
Results/Conclusions
Abundances of frugivore species were not related to PCR or AET whereas abundances of nectarivore species were negatively related to AET and not to PCR. In contrast, abundances of animalivore species were negatively related to both AET and PCR, with effect sizes higher than those found for frugivores and nectarivores. High abundance of nectarivores in low productivity areas may be due to high availability of chiropterophilic flowers. The absence of a relationship between abundance and PCR in nectarivores may also indicate ecological flexibility, such as diet shifting. Competition appears to be stronger within the animalivore guild than in other guilds probably because resource availability may be more limited for prey- than for plant-eating bats, with most preys being inconspicuous and others presenting defense strategies. Conversely, many plants depend on animals for fruit dispersal and flower pollination, maximizing attractiveness of mutualists. When confronting mutually exclusive hypotheses on spatial variation in abundance, we did not find support for the MIH and we lend strong support for the DCH. Furthermore, based on feeding guilds analyzed here, we conclude that importance of competition is associated with the species trophic level.