Branched networks, diversity patterns, and regional coexistence

Background/Question/Methods

Stream systems are characterized by a branched network structure. The branching itself is widely thought to be a significant driver of distinctive patterns of abundance and diversity. These patterns are likely to be modified by complex environmental variation and dispersal characteristics. The resulting mosaic of varying environmental and competitive conditions experienced by the organisms potentially alters the strength of coexistence and community composition at the scale of the whole stream. However, no rigorous theoretical link exists between branching structure, diversity patterns, and regional coexistence.

We use a simulation model to ask how branched networks interact with environmental variation and dispersal to determine patterns of abundance and diversity along streams. We then quantify the effect of these interactions on the strength of two coexistence mechanisms that arise from environmental variation: the storage effect and fitness-density covariance. The specific functional forms of these mechanisms identify the conditions under which stream characteristics influence the composition of the community at the scale of the whole stream. This quantification allows us to move beyond characterizing the patterns of local diversity to explore how these patterns and their underlying drivers relate to the strength of coexistence regionally.

Results/Conclusions

Branching patterns can generate distinct patterns of diversity along streams, but this depends on the relationship between dispersal and the length of the reaches. However, branching topology itself has little effect on the strength of coexistence at the scale of the whole stream, even in the conditions that lead to the most striking patterns of local diversity. Instead, the relationship between the scales of dispersal and environmental variation is of primary importance. However, stream branching does not exist in isolation in nature. Instead, branching is frequently correlated with patterns of environmental conditions or dispersal. We show that when branching is associated with factors that allow species to partition space, such as variation in environmental conditions between tributaries, it can strengthen coexistence. In contrast, associations that do not contribute to spatial partitioning, such as downstream increases in carrying capacity, do not affect the strength of coexistence.