The relative importance of abiotic, biotic, and spatial factors in structuring the stream macroinvertebrate metacommunity in a temperate rainforest

Elsa Katrina Toskey

doi:10.7273/000005304

We investigated how a stream macroinvertebrate metacommunity (i.e., multiple assemblages in a region linked by dispersal) was structured by local abiotic and biotic variables versus regional spatial and dispersal variables and whether this varied by network position (i.e., upstream versus downstream) or dispersal mode (i.e., aquatic versus aerial dispersers). We collected 143 benthic macroinvertebrate samples and associated abiotic and biotic data related to local habitat and species interactions across three river basins on the Olympic Peninsula (Washington, USA). We assessed macroinvertebrate assemblage composition in relation to environmental and spatial variables using mixed effects linear regression and partial redundancy analysis. We found that water temperature, bankfull width, canopy cover, sestonic algal biomass, and benthic algal biomass were the main factors associated with macroinvertebrate diversity and abundance. Water temperature, substrate grain size, bankfull width, and salmonid density were the main local factors associated with macroinvertebrate assemblage composition. Overall, 19.7% of the variation in taxonomic assemblage composition was explained by local variables and 7.4% by overland spatial distance (p = 0.014). The relative importance of local variables was higher for aerially dispersing taxa than for aquatically dispersing taxa. The relative importance of local (i.e., abiotic and biotic) versus regional (i.e., overland spatial) variables did not vary by network position. Our results indicated that local abiotic and biotic variables acted together with regional (i.e., dispersal and spatial variables) to shape the macroinvertebrate metacommunity composition across river basins. However, the relative importance of local habitat variables and spatial processes depended on the dispersal mode, with evidence of dispersal limitation for aquatic dispersers. The local (i.e., abiotic and biotic) and regional (i.e., spatial) portions of explained variance in macroinvertebrate metacommunity composition did not vary by network position, indicating that either we did not test this across a sufficiently large longitudinal spatial scale or that the network position hypothesis may not be valid for the insect-dominated macroinvertebrate metacommunity in this stream system. Our study provides insight into the mechanisms and processes underlying the coexistence of stream macroinvertebrates and underscores the importance of dispersal and isolation in modulating the strength of the effect of the local habitat on metacommunity organization. Additionally, because our findings yielded evidence of dispersal limitation in upstream reaches for aquatically dispersing taxa, restoration and conservation work should also focus on maintaining or improving stream connectivity, particularly for poorly dispersing taxa in headwater reaches.

The relative importance of abiotic, biotic, and spatial factors in structuring the stream macroinvertebrate metacommunity in a temperate rainforest

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