Response of an algal model community to gradual and abrupt environmental change in isolated versus connected habitats

Background/Question/Methods

Climate change and habitat fragmentation are human-induced environmental changes with severe impacts on biodiversity and community composition. Many species have been found to shift their distributions in response to climate change. Fragmentation of habitats, however, may constrain species’ dispersal and prevent them from tracking environmental change, thus enhancing the negative effect of climate change on biodiversity. In a microcosm experiment, we analysed the effect of environmental change and habitat connectivity on biodiversity and community composition. We hypothesized that rising temperature would have a negative effect on biodiversity, particularly when the increase in temperature is abrupt rather than gradual and when habitats are isolated rather than connected. We assembled communities of 10 algal species from four different taxonomic groups in cool (20 °C) and warm (25 °C) habitats. In a factorial design, we tested for effects of environmental change (constant temperature, gradually increasing temperature, abruptly increasing temperature), habitat connectivity (isolated versus connected metacommunities), and landscape heterogeneity (homogeneously cool, homogeneously warm, and heterogeneous metacommunities).

Results/Conclusions

Environmental change affected both composition and diversity of our algal model communities. Warming resulted in a faster decline in species richness with time. When taking the dominance structure of the communities into account, gradual warming of initially cool habitats decreased local diversity, while rising temperatures in warm habitats increased diversity. These patterns were driven by high dominance of a small coccal cyanobacterium at an intermediate temperature and co-dominance of a filamentous cyanobacterium at high temperature. A number of species differed in their response to warming depending on whether change was gradual or abrupt. Connectivity affected diversity only in metacommunities that were spatially heterogeneous with respect to temperature. Habitat connectivity resulted in increased abundance of cyanobacteria in cool habitats through immigration from warm habitats. Our data suggest that environmental change can have complex effects on diversity and species’ abundances, depending on the rate of change, the initial environmental conditions, and the degree of habitat connectivity.