ECOMORPHOLOGICAL DIVERSIFICATION IN LOWLAND FRESH-WATER FISH ASSEMBLAGES FROM 5 BIOTIC REGIONS
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This study investigates the relationships among species diversity, community structure, and convergent evolution among divergent fish faunas. Morphological traits can be used as surrogates for ecological data in the comparative study of community niche relationships. In the present study I examined 30 morphological features related to the ecology of the dominant fish species from lowland stream and backwater habitats in five widely separated geographic regions: nearctic Alaska, temperate North America, Central America, South America, and tropical Africa. The study regions exhibited a general gradient of species richness from a minimum of 6 dominant species at one of the high-latitude sites (65 N) to a maximum of 43 numerically dominant species at a neotropical site (8 N). Fishes from Alaskan sites near the edge of the polar circle tended to be larger than fishes at other sites. Mean values for most morphological characters varied little between regions, indicating similar faunal centroids in morphological space. Morphological diversification within fish assemblages was estimated from species similarities based on Euclidean distances plus species projections on the principal axes from multivariate analysis. The total morphological space encompassed by ichthyofaunas in both stream and backwater sites was generally concordant with the latitudinal and species-richness gradient, with low-diversity nearctic assemblages exhibiting little morphological diversification relative to high-diversity tropical faunas. The Central American assemblages showed a greater range of ecomorphological diversification than African assemblages that contained a few more species, and this pattern may be related to greater seasonal stability at the Central American site. Phenetic patterns of dispersion reflect ecological relationships in which greater numbers of coexisting species are associated with higher levels of niche diversification and ecological specialization, leading to enhanced resource partitioning. Without additional ecological information, a community morphological analysis cannot directly determine whether or not increased ecological specialization is associated with the addition of new resources on the fringes of resource space or with increased subdivision of previously utilized core resources. Based on ecological information gathered concurrently with the fishes used in this analysis, I conclude that close species packing in morphological space is associated with niche generalists rather than with niche compression. With the possible exception of the two high-latitude sites, assemblages that contained more fish species generally did not exhibit tighter packing in niche space than species-poor assem-blages, and this result was observed for comparisons both within and between regions. In contrast with several earlier studies, I interpret the lack of correlation between species richness and the average minimum distance between species in assemblage morphospace as being entirely consistent with the observed expansion of morphospace with species richness and latitude. Morphological patterns show that average spacing is held more or less constant as the variety of ecomorphological configurations increases in more-speciose fish assemblages. Even though morphological characters reflect community relationships, tight packing near the center of faunal morphospace actually indicates the opposite of species packing (niche compression) in the traditional sense of resource-utilization curves. The analysis supports the thesis that species interactions are important determinants of community morphological features, particularly in species-rich tropical regions. Relative to assemblages in the other four regions, the two Alaskan fish assemblages were hyperdispersed within a comparatively small morphological space. The Alaskan fish assemblages probably formed via geologically recent, and perhaps repeated, colonizations of polar freshwaters by species with evolutionary histories in more diverse southern and coastal fish faunas. I interpret evidence of greater niche diversification at lower latitudes within a habitat type as being derived primarily from the influence of competition and predation, whereas differences between habitats within regions seems to be associated with the combined effects of biotic interactions and differences in habitat volume and heterogeneity. Even though different regions within ecomorphological space were dominated by different fish orders, numerous ecomorphological convergences and several one-for-one ecological equivalents were identified within different biotic regions.