PS 37-29 - Evidence towards a varying relationship between macroinvertebrate community density and habitat size in freshwater stream ecosystems

Abstract: The factors that influence community size are well studied throughout ecological literature. Physical factors such as habitat size are linked to abundance of species and in turn determine community size. In an era rampant with habitat fragmentation, understanding how habitat metrics relate to community size is vital to conservation. Stream macroinvertebrate community composition is known to be incredibly sensitive to environmental variables, in particular, stream size. Starting from small, first order headwater streams and moving down a river gradient to larger order streams, vegetation differences and organic input influence macroinvertebrate functional groups. The implications of this continuum on riverine food webs is wide ranging, shaping water chemistry, trophic structure, and community composition in a framework known as the River Continuum Concept (RCC). Since the introduction of the RCC, other research has focused on stream substrate or specific taxa in relation to variables described in the RCC model. However, the relationship between macroinvertebrate abundance and stream size is not well understood. To investigate this often disregarded assumption of no relation between these two variables, we analyzed 5 existing datasets of stream macroinvertebrate community surveys coupled with physical habitat measurements. We also collected one additional dataset using the same methodology as the other datasets. Macroinvertebrate community samples were collected using standardized kick net approaches. We used both wetted width (distance across the wetted stream channel) and drainage area as metrics for stream size depending on the dataset used.

As previously stated, it is often assumed that there is a null relationship between stream size and macroinvertebrate density, or in other words, that the same density of macroinvertebrates should be expected in suitable habitat regardless of stream size. Here, we hypothesized that there is a significant relationship between stream size and macroinvertebrate abundance, and that the direction of this relationship will vary depending on the community dynamics within the system. We found significant relationships between macroinvertebrate density and stream size across datasets. However, the strength and direction of this relationship varied across systems. Some datasets exhibited positive relationships, whereas some exhibited negative relationships, and all relationships had varying strengths. These results have important implications for water quality monitoring and habitat management. Current monitoring practices do not assume a link between these variables and investigating this could inform more informed data collection. Further contextualizing these results, metacommunity theory may provide a unique lens to better elucidate the mechanisms affecting this relationship.