Diversity is maintained by seasonal variation in species abundance

Abstract

Background: Some of the most marked temporal fluctuations in species abundances are linked to seasons. In theory, multispecies assemblages can persist if species use shared resources at different times, thereby minimizing interspecific competition. However, there is scant empirical evidence supporting these predictions and, to the best of our knowledge, seasonal variation has never been explored in the context of fluctuation-mediated coexistence.

Results: Using an exceptionally well-documented estuarine fish assemblage, sampled monthly for over 30 years, we show that temporal shifts in species abundances underpin species coexistence. Species fall into distinct seasonal groups, within which spatial resource use is more heterogeneous than would be expected by chance at those times when competition for food is most intense. We also detect seasonal variation in the richness and evenness of the community, again linked to shifts in resource availability.

Conclusions: These results reveal that spatiotemporal shifts in community composition minimize competitive interactions and help stabilize total abundance.

Figure 1

Seasonal groupings in the fish assemblage at Hinkley Point. Dendrogram: four seasonal groups of species identified by cluster analysis based on the seasonal fluctuation term in the model. This fluctuation term is driven by the water temperature and month effects, as identified in the fitted generalized additive models (GAMs). Box plots: the pattern of the ln-scaled relative abundances for each seasonal cluster: winter (group 1), spring (group 2), autumn (group 3) and summer (group 4).

Figure 2

Abundance of the community and the seasonal groupings through time. Top: numerical abundance (ln) of the community through time. Bottom: the modeled seasonal component of the total relative abundance (ln) of the seasonal groupings shown in Figure 1 (winter (blue), spring (green), autumn (orange) and summer (red)) over the 31-year study.

Figure 3

Representation of spatial guilds in each of the seasonal groups. The expected (black circles with 95% confidence intervals) and observed number (red squares) of different types of spatial guild in each seasonal group. The expected number of spatial guilds occupied in a given season was derived using a randomization test (see methods for details).

Figure 4

Monthly variation in biomass. Box plots for fish (top) and crustaceans (bottom) showing the monthly variation in total biomass (wet weight in g.).

Figure 5

Temporal trends in the diversity of the assemblage. (a) The seasonal pattern of species richness (with 95% confidence limits) in the community. The plot shows the mean monthly values for both the core species (blue) and the entire community (green). (b) The seasonal pattern of diversity as measured by the exponential form of the Shannon index [34,35], again with 95% confidence limits, and shown for both the core (blue) and entire (green) community. As the Shannon index takes both evenness and richness into account the seasonal differences in the trends can be attributed to differences in evenness.