Inferring the effects of potential dispersal routes on the metacommunity structure of stream insects: as the crow flies, as the fish swims or as the fox runs?

Abstract

Summary1. Metacommunity research relies largely on proxies for inferring the effect of dispersal on local community structure. Overland and watercourse distances have been typically used as such proxies. A good proxy for dispersal should, however, take into account more complex landscape features that can affect an organism's movement and dispersal. The cost distance approach does just that, allowing determining the path of least resistance across a landscape. 2. Here, we examined the distance decay of assemblage similarity within a subarctic stream insect metacommunity. We tested whether overland, watercourse and cumulative cost distances performed differently as correlates of dissimilarity in assemblage composition between sites. We also investigated the effect of body size and dispersal mode on metacommunity organization. 3. We found that dissimilarities in assemblage composition correlated more strongly with environmental than physical distances between sites. Overland and watercourse distances showed similar correlations to assemblage dissimilarity between sites, being sometimes significantly correlated with biological variation of entire insect communities. In metacommunities deconstructed by body size or dispersal mode, contrary to our expectation, passive dispersers showed a slightly stronger correlation than active dispersers to environmental differences between sites, although passive dispersers also showed a stronger correlation than active dispersers to physical distances between sites. The strength of correlation between environmental distance and biological dissimilarity also varied slightly among the body size classes. 4. After controlling for environmental differences between sites, cumulative cost distances were slightly better correlates of biological dissimilarities than overland or watercourse distances between sites. However, quantitative differences in correlation coefficients were small between different physical distances. 5. Although environmental differences typically override physical distances as determinants of the composition of stream insect assemblages, correlations between environmental distances and biological dissimilarities are typically rather weak. This undetermined variation may be attributable to dispersal processes, which may be captured using better proxies for the process. We suggest that further modifying the measurement of cost distances may be a fruitful avenue, especially if complemented by more direct natural history information on insect dispersal behaviour and distances travelled by them.

Metacommunity structuring in Himalayan streams over large elevational gradients: the role of dispersal routes and niche characteristics

“…Glacial runoff can alter the water chemistry, creating unfavourable conditions for certain taxa, thereby acting as a dispersal barrier for instream dispersers (Cauvy‐Fraunié et al ., ). Results on the most important dispersal routes for stream metacommunities inhabiting dendritic networks have been equivocal (Landeiro et al ., ; Grönroos et al ., ; Cañedo‐Arguelles et al ., ; Kärnä et al ., ), but this, of course, depends on the environmental context. Cañedo‐Arguelles et al .…”

“…This is not surprising given sites in this catchment were less spatially isolated compared with the other two catchments, particularly for overland dispersers. Local environmental conditions tend to predominate over spatial structuring in streams (Siqueira et al ., ; Grönroos et al ., ; Kärnä et al ., ), but this depends on the level of connectivity and spatial extent of sites (Brown & Swan, ; Soininen, ; Tonkin et al ., 2016a). The putatively limited connectivity observed in our study among high elevation and headwater streams also seems valid at lower levels of organization and may generally drive elevated beta diversity among headwater regions at the scale of species and intraspecific genetic variation (Finn et al ., ; Geismar et al ., ).…”

“…Mass effects and dispersal limitation (high and low dispersal, respectively) provide opposite ends of the dispersal spectrum in metacommunities (Heino et al ., 2015a). Consequently, the structure of dispersal routes is central to controlling access to local communities, with stream networks providing a unique example (Cañedo‐Arguelles et al ., ; Kärnä et al ., ). While environmental conditions are often more important than spatial structuring in stream invertebrate communities (Siqueira et al ., ; Kärnä et al ., ), the dendritic arrangement of river networks (Altermatt, ) plays a key role in organizing metacommunities (Brown & Swan, ).…”

“…Consequently, the structure of dispersal routes is central to controlling access to local communities, with stream networks providing a unique example (Cañedo‐Arguelles et al ., ; Kärnä et al ., ). While environmental conditions are often more important than spatial structuring in stream invertebrate communities (Siqueira et al ., ; Kärnä et al ., ), the dendritic arrangement of river networks (Altermatt, ) plays a key role in organizing metacommunities (Brown & Swan, ). This structuring can elevate beta and decrease alpha diversity in headwaters compared with downstream sections (Finn et al ., ).…”

Riverine Ecosystem Management

“…This energetic state depends also on the position in the river network as conceptualized by the river continuum concept (Vannote et al 1980). At the same time, the local populations and communities are linked to meta-population and meta-community dynamics, which also depend on river network connectivity and overland distances to other network branches as well as in-stream distances to recolonization pools (Kärnä et al 2015). These aspects influence the resilience of river ecosystems to chemical effects, which is further discussed below in the context of mitigation.…”