Evidence for biotic resistance in highly diverse tropical rivers
The theory of biotic resistance suggests that higher diversity should confer stronger resistance to invasion. The current reorganization of the earth’s freshwater fish fauna presents a critical test of this theory; however, the evidence for biotic resistance is mixed. Small-scale, experimental studies of aquatic systems have found some support for biotic resistance. In contrast, analyses of regional patterns across river basins have generally found positive correlations between the number of natives and non-natives. One possible explanation for this discrepancy is that these studies are testing different phases of the invasion process. Large-scale studies often use river basin checklists, which are more relevant to the initial invasion step (i.e., introduction). Biotic interactions occur on a local scale and may be more appropriately tested by reach-scale data reflecting the establishment and spread phases of the invasion process. Using a dataset of 226 reach-scale surveys assembled from the literature, we use a generalized linear model to test whether number of native fish species is related to the number of non-native species. In addition, we use a log-linear model comparing native diversity in species’ native and introduced ranges to test whether invader origin influences patterns of non-native species.
There was large variation in the number of non-native species found in low diversity sites, whereas high diversity sites (richness > 100) contained few non-native species. A significant negative effect of native species richness was found, along with a positive effect of degree of human impact. Highly impacted sights contained a higher number of non-native species on average, but that number quickly decreased with increasing native diversity. A total of 145 introduced species were recorded in 473 unique species by basin introductions. Significantly more cases (n = 436) were recorded in which the non-native species originate from a more diverse fish fauna. There were significant interactions between species origin and degree of impact as well as species origin and habitat type. These results are consistent with previous studies finding that human activities are the primary driver of global patterns of non-native species; however, they also suggest a role for biotic resistance. While numerous cases of tropical fishes being introduced into temperate waters were recorded, few successful introductions moved in the opposite direction. Issues of scale and relative differences in species richness between donor and receiving fauna may explain the discrepancies often noted between small-scale and large-scale tests of biotic resistance.