Streams are experiencing a dramatic loss of species diversity, largely as a consequence of anthropogenic impacts such as climate change and habitat loss. However, limited knowledge about the mechanisms maintaining diversity limits our ability to conserve stream communities. Moreover, diversity is a multiscale phenomenon; persistence at larger scales is necessary for presence locally, while local interactions scale up to affect regional persistence. These scaling relationships complicate conservation interventions that are often local but seek to achieve larger-scale outcomes.
Empirical research and conceptual models place strong emphasis on the roles of environmental variation, dispersal, and connectivity for structuring stream communities. We present a multiscale theory that addresses these factors and discuss its implications for the conservation of ecological diversity in streams. We develop simulation models that account for natural history and analyse their outcomes using coexistence theory. This approach quantifies the mechanisms maintaining diversity and investigates how natural history contributes to those mechanisms. We emphasize issues of scale that link local communities to the maintenance of diversity at the watershed scale.
We show that environmental variation is key to diversity maintenance in river systems, in keeping with many conceptual ideas and empirical understanding. Specifically, interactions between environmental variation, dispersal, and stream structure are critical for both local composition and the maintenance of diversity regionally, because these interactions control the extent to which organisms are able to concentrate in favourable locations. This concentration is often the dominant driver of diversity maintenance and typically overwhelms other processes such as extinction-colonization dynamics. One outcome is that the locations with the most favourable environmental conditions have outsize importance, and their conservation is likely to be disproportionately beneficial. Similarly, conserving sites with high local diversity may be counterproductive, as high local diversity tends to be an outcome of regional diversity and may weaken watershed-scale diversity maintenance.