Ecological equivalency among pure, hybrid, and superinvasive tiger salamander genotypes
Use of taxon substitutes for extinct/endangered species is a controversial conservation measure. We use the example of the endangered California tiger salamander (Ambystoma californiense; CTS), which is being replaced by hybrids with the invasive barred tiger salamander (Ambystoma mavoritum), to illustrate a strategy for evaluating taxon substitutes based on their position in a multivariate community space. Approximately ¼ of CTS’s range is currently occupied by “full hybrids” with 70% non-native genes, while another ¼ is occupied by “superinvasives” with the 4% of non-native genes that are invading most rapidly. Based on surveys of natural CTS breeding ponds, we stocked experimental mesocosms with realistic densities of the average breeding pond community. We used these mesocosms to evaluate ecological equivalency between pure CTS, full hybrids, and superinvasives by rearing all three and measuring the resulting pond communities. We also included a fourth treatment with no salamanders present to evaluate the community effects of eliminating Ambystoma larvae altogether.
We found that pure CTS and superinvasive larvae were ecologically equivalent, since their positions in multivariate community space were statistically indistinguishable and they did not differ significantly along any univariate community axes. Full hybrids were ecologically similar, but not equivalent, to the other two genotypes, since their position in community space was significantly different, but closer than the no-Ambystoma treatment. We conclude that, at least for the larval stage, superinvasives are adequate taxon substitutes for pure CTS and should be protected under the Endangered Species Act. The proper conservation status for full hybrids remains debatable.