Assemblage-level diversity of life-history strategies along longitudinal gradients of river systems

Background/Question/Methods

Population responses to environmental conditions are mediated by life-history strategies, which represent an adaptive suite of intercorrelated reproductive and demographic traits.  Trade-offs in adaptive responses to environmental variation are tied to aspects of predictability and scale relative to generation time.  In fishes, for example, species exhibit combinations of traits that yield life-history strategies along a continuum between three endpoints characterizing adaptation to environments with stochasticity over short temporal and/or spatial scales (opportunistic strategists), relatively stable environments with strong effects of biotic interactions (equilibrium strategists), and environments with large-scale patchiness and cyclic or seasonal variation (periodic strategists).  However, for many, if not most, communities, a diversity of life-history strategies along that continuum may be simultaneously present.  Using 645 standardized surveys of stream fish assemblages across multiple river basins in the Great Plains of North America and a suite of 14 quantitative life-history traits for 93 species, we examined assemblage-level patterns of life-history diversity.  PCA was used to reduce the dimensionality of life-history traits, and three metrics of functional diversity (richness, evenness, and divergence) were calculated using species PC axis scores and relative abundances in each assemblage. Stream size, network structure and watershed characteristics were summarized using GIS.

Results/Conclusions

Functional richness was related to species richness by an asymptotic relationship, whereas functional evenness stabilized around intermediate levels as species richness increased and functional divergence appears to have a threshold where the maximum observed value declines as species richness increases past 15 species present (i.e. increasing redundancy).  Species richness and the three functional diversity metrics exhibited different (yet complementary) relationships with hydrology and stream characteristics. Stream size/stability was strongly related to species and functional diversity in unique ways. For example, functional richness was highest in intermediate reaches, and declined in both the largest and smallest streams due to reduced abundance of more specialized strategies, whereas functional divergence increased linearly with stream size.  In addition to the assemblage-level summary metrics of functional richness, evenness and divergence, the specific composition of life-history strategies along river gradients represented a transition to more periodic and equilibrium species in larger streams. Although assemblages may be comprised of species representing a diversity of life-history strategies, strong gradients in the relative composition and abundance of life-histories were tied to primary attributes of river systems representing a gradient of stochasticity to predictability/stability.  Such relationships may allow modeling of community attributes and ecosystem responses to environmental change.