Does isolation influence the relative role of environmental and dispersal‐related processes in stream networks? An empirical test of the network position hypothesis using multiple taxa

Abstract

Quantifying the relative importance of how local (environmental or niche‐based) and regional (dispersal‐related or spatial) processes regulate the assembly of communities has become one of the main research avenues of community ecology. It has been shown that the degree of isolation of local habitats in the landscape may substantially influence the relative role of environmental filtering and dispersal‐related processes in metacommunities. Dendritic stream networks are unique habitats in the landscape, where more isolated upstream sites have been predicted to be primarily structured by environmental variables, while more central mainstem rivers by both environmental and spatial variables (hereafter the network position hypothesis, NPH). However, the NPH has almost exclusively been tested for stream macroinvertebrates, and therefore its predictions warrant confirmation from multiple taxa. We examined the validity of the NPH for benthic diatoms, macrophytes, macroinvertebrates and fish in the Pannon Ecoregion, Hungary. Following the NPH we predicted a clear dominance of environmental over spatial variables in headwaters, and a larger effect of spatial variables in rivers compared to headwaters. We tested these predictions using variance partitioning analyses separately for the different taxa in headwater and in riverine habitats. We found large differences in the explained community variance when the impact of environmental (physical and chemical) and spatial (overland and watercourse distance) variables for various taxa was studied. In general, total explained variance was lower for the more passively dispersing plant taxa than for animal taxa with more active dispersal in both streams and rivers. However, similar to other studies, the total explained variance was low for both headwater streams and rivers. Community structure of diatoms could be best explained by both environmental and spatial variables in streams, whereas their community structure could not be explained by either variable group in rivers. The significance of environmental and spatial variables depended on the distance measure (overland versus watercourse) in the case of macrophytes. Community structure of macroinvertebrates could be explained by environmental variables in streams and by both environmental and spatial variables in rivers. Moreover, variation was explained by different predictors when macroinvertebrate taxa were divided into flying and non‐flying groups, suggesting the importance of dispersal mode in explaining community variation. Finally, community structure of fishes could be explained by both environmental and spatial variables in streams and only by environmental variables in rivers. In conclusion, we found no clear evidence of the NPH in our multi‐taxa comparison. For example, while patterns in macroinvertebrate communities seem to support the NPH, those in fish communities run counter with the predictions of the NPH. This study thus shows that different taxa may behave differently to isolation effects in stream ne...

A comprehensive examination of the network position hypothesis across multiple river metacommunities

“…More recently, ecologists have focused on determining how ecological factors such as ecosystem type, taxonomic group, natural disturbance and landscape connectivity influence the relative importance of environmental and dispersal factors on metacommunities (Cottenie 2005, Henriques-Silva et al 2013, Fernandes et al 2014. Particular attention has been given to landscape connectivity in river networks because of their dendritic nature that creates preferential (or obligatory, considering strictly aquatic taxa) connections between habitat patches (Fagan 2002, Brown and Swan 2010, Altermatt et al 2013, Zhao et al 2017, Schmera et al 2018). Hence, the spatial positioning of habitats within a stream network can play a crucial role in determining how communities are structured by dispersal and environmental filtering processes (Altermatt et al 2013).…”

“…Hence, the spatial positioning of habitats within a stream network can play a crucial role in determining how communities are structured by dispersal and environmental filtering processes (Altermatt et al 2013). Based on previous studies, Brown and Swan (2010) developed two predictions about community assemblages in river networks that were recently formalized into the network position hypothesis (NPH) by Schmera et al (2018). The NPH first predicts that 1) communities in headwater reaches are exclusively regulated by environmental processes because they are relatively more isolated and environmentally heterogeneous than downstream reaches (Brown and Swan 2010).…”

“…Hence, both environmental conditions and dispersal should account for compositional variation in downstream communities (Brown and Swan 2010). While the NPH has been highly influential in river ecology, few studies have tested its predictions explicitly and those that have were rather inconclusive (Tonkin et al 2016, Göthe et al 2017, Schmera et al 2018). Examples of inconsistencies include a lack of support for the NPH predictions across taxonomic groups (Schmera et al 2018), and the relative importance of environmental and dispersal processes being dependent on the systems being studied rather than network position (Datry et al 2016a, b, Tonkin et al 2016.…”

“…With previous findings in mind, an opportunity exists to determine whether the NPH predictions are general (i.e. applicable across all river networks), or context-dependent (Schmera et al 2018).…”

Ecological drivers of fish metacommunities: Environmental and spatial factors surpass predation in structuring metacommunities of intermittent rivers

“…On the other hand, DDR analysis is scale-dependent and one cannot truly separate the role of the pure and joint effects of environmental and spatial factors (Schmera et al, 2017); but our choice to use it helped us see the effects of the explanatory components in each site. In our study, the pRDA results indicated that pure environmental differences explained more variation than pure spatial differences, but it also reaffirmed the need for caution in community studies.…”