Tests for Similarity and Convergence of Finch Communities

Convergence at the community level occurs when whole groups of organisms become more similar than their ancestors in a similar environment. I suggest a method for indirectly assessing community convergence, and apply it to several characteristics of finch communities in different habitats worldwide. The method is based on the analysis of variance, and has several advantages over other indirect tests for convergence. Advantages include: (a) convergence is tested by comparing the variance between replicate communities within habitats to the variance between habitats, using a large and representative sample of habitat types; (b) the method estimates degree of convergence, a useful index for contrasting convergence of different community traits; (c) historical constraint on community convergence may be viewed as an additional component of total variation in the community character, and its importance can be estimated and compared with the effect of habitat type. The method revealed significant convergence of finch species number, mean finch size, mean finch shape (body size relative to beak size), and standard deviation in shape. Standard deviation in finch size may also have been convergent, but the pattern was not significant. From nearly 30 to >60% of the variability in these community traits was attributable to an effect of similar habitat type. Significant variability in one trait (mean shape) was also attributable to geographic region, apparently an effect of differences between regions in the phylogenetic origin of finches present. Hence, historical factors appear to have constrained convergence, but only in one of the five finch community traits. I also suggest some methods to evaluate community similarity, and apply these to four of the five finch community traits (species number excepted). Similarity of communities within habitat types was unrelated to the degree of convergence. For example, finch communities within habitat were significantly similar in only one convergent trait, mean size. Mean shape, also convergent, was significantly different between communities within habitats. Similarity, as defined here, indicates that sizes of individual species within communities are nonrandomly spaced. Interspecific interactions may thus be involved in the convergence of at least one trait, mean finch size.