A landscape‐scale field experiment reveals the importance of dispersal in a resource‐limited metacommunity

Abstract

Dispersal may play a strong role in driving species diversity across landscapes. Theoretically, dispersal permits species to remain extant within a metacommunity, even if they are periodically excluded from some local communities. Field tests of dispersal effects are difficult, and most non-experimental data suggest that environmental conditions play the predominant role in setting species diversity. However, most such studies cannot differentiate between patterns caused primarily by dispersal constraints vs. abiotic factors vs. biotic constraints (e.g., priority effects). In 22 km of a sand-bed stream in southeastern Australia, strong longitudinal gradients in the abiotic environment and detritus densities (resources) mean that downstream locations have abiotic conditions that may be physiologically stressful, low resource densities and low species diversities. We experimentally increased the retention of detritus over 40 m stream lengths along the gradient, with other sites acting as controls. If dispersal is constrained, or abiotic or biotic factors primarily control community structure, then we predicted that increasing resources would result in no change in species composition. Alternatively, if dispersal is common, we predicted that species diversity would increase at treatment sites through colonization by species able to tolerate abiotic conditions downstream and able to invade established communities. Invertebrates were sampled prior to manipulation and then four times (1, 4, 9 and 12 months) following manipulation. Detrital standing stocks increased by an order of magnitude at treatment sites. Over 1 yr, invertebrate densities and species richness also increased in treatment sites. Effect sizes were strong in middle and downstream areas, which were colonized by upstream species. Thus, faunal composition of the downstream treatment sites became more similar to upstream locales, and β-diversity across treatment sites declined as α-diversity increased. Out of 54 common taxa, roughly half responded to the experiment; responders and non-responders had similar proportions of upstream specialists and of different functional feeding groups. Contrary to many non-experimental studies, our results demonstrate that extensive dispersal can be very important and, for many species, potentially more important than the abiotic environment or biotic constraints in affecting community structure when adequate resources are available.

Combined effects of local habitat, anthropogenic stress, and dispersal on stream ecosystems: a mesocosm experiment

“…As a result, species may occur at environmentally non‐optimal sites. Thus, high connectivity and extensive dispersal can homogenize local assemblages (Chase and Ryberg , Lancaster and Downes ) and reduce the environmental control of community structure (“mass effects”; Mouquet and Loreau , Cadotte and Fukami ). There is, however, also contrasting evidence showing that dispersal can promote the divergence of local assemblages by means of priority effects, thereby increasing β‐diversity rather than decreasing it (Vannette and Fukami ).…”

Itinerant, nomad or invader? A field experiment sheds light on the characteristics of successful dispersers and colonists

“…When drift is the main source of colonists, manipulation sites could theoretically build up densities so high that these sites then contribute to the drift, which could therefore gradually shift the distribution of species in a downstream direction. We did observe such zonal changes during the experiment (Lancaster & Downes, ). For example, some upstream specialists (species that are restricted to or with highest densities in upstream locations) gradually colonised locations further and further downstream.…”

“…Downstream, sand‐affected locations have about half the species richness of upstream locations, which are less affected by sand and have higher densities of in‐stream detritus (Downes, Lancaster, Glaister, & Bovill, ). Our experiment boosted the densities of detritus, resulting in increases in species diversities at manipulation sites that were unequivocally caused by dispersal (Lancaster & Downes, ), with some taxa appearing to be invaders, as defined above. Over 12 months, the species composition of manipulation sites gradually converged on those of the more species‐diverse, upstream locations.…”

“…This was matched by 1.3×, 1.9× and 1.6× increases in species richness at manipulation sites compared with controls in upstream, middle and downstream zones, respectively. The species composition of communities in middle and downstream manipulation sites gradually became more similar to upstream sites, in strong contrast to controls, which showed only interannual variation and changes reflecting seasonality (Lancaster & Downes, ).…”

When to cut your losses: Dispersal allocation in an asexual filamentous fungus in response to competition

“…For most organisms, the ability to disperse is integral to persist in a fundamentally dynamic and often competitive environment (Edman, Gustafsson, Stenlid, Jonsson, & Ericson, ). However, the drivers that shape patterns in dispersal remain unclear for many groups of organisms, particularly for microorganisms such as fungi (Cadotte, Fortner, & Fukami, ; Fuhrman, ; Lancaster & Downes, ; Nemergut et al, ).…”