Transitory versus Persistent Effects of Connectivity in Environmentally Homogeneous Metacommunities

Abstract

While the effect of habitat connectivity on local and regional diversity has been analysed in a number of studies, time-dependent dynamics in metacommunities have received comparatively little consideration. When local patches of a metacommunity are identical in environmental conditions but differ in initial community composition, dispersal among patches may result in homogenization of local communities. In a microcosm experiment with benthic ciliates, we tested the hypothesis that the effect of connectivity on diversity is time-dependent and only transitory, with the degree of connectivity affecting the time to homogenization but not the final outcome. Six microcosms were connected to a metacommunity with one of three levels of connectivity. The six patches differed in initial community composition but were identical in environmental conditions. We found a time-dependent and transitory effect of connectivity on local and regional richness and on local Shannon diversity, while Bray-Curtis dissimilarity and regional Shannon diversity were persistently affected by connectivity. Local richness increased and regional richness decreased with connectivity during the initial phase of the experiment but soon converged to similar values in all three connectivity treatments. Local Shannon diversity was unimodally related to time, with maximum diversity reached sooner with high than with medium or low connectivity. Eventually, however, local diversity converged to similar values irrespective of connectivity. At the regional scale, Shannon diversity was persistently lower with high than with low connectivity. While initial differences in community composition vanished with medium and high connectivity, they were maintained with low connectivity resulting in persistently high beta and regional diversity. The effect of connectivity on ciliate community composition translated down to the algal resource, as stronger dominance of the superior competitor with high and medium connectivity resulted in stronger depletion of the resource.

Cross‐scale drivers of plant trait distributions in a fragmented forest landscape

“…Community assembly involves multiple ecological processes operating at different spatial scales (Ricklefs , Brown 2014). At local scales, physical environmental factors, such as soil properties and topography, play critical roles in the assembly of ecological communities (Weiher and Keddy , De Cáceres et al ). For example, Cornwell and Ackerly () found that plant traits showed a close relationship with topography and soil moisture in a coastal landscape in California.…”

“…Habitat fragmentation transforms large expanses of habitat into numerous smaller patches of lower total area, isolated from each other by a matrix of habitats unlike the original (Wilcove et al 1986). Habitat fragmentation thus alters both the total amount and the spatial configuration of remaining habitat (Didham et al ). Widespread fragmentation means that the majority of local community dynamics now takes place within small habitat fragments in close proximity to anthropogenic edges (Broadbent et al 2008).…”

“…Widespread fragmentation means that the majority of local community dynamics now takes place within small habitat fragments in close proximity to anthropogenic edges (Broadbent et al 2008). Moreover, fragmentation can reshape the hierarchical effects of ecological processes operating at multiple spatial scales, from landscape‐ to patch‐ down to local‐scale (Didham et al ), yet there has been little emphasis placed on how cross‐scale interactions might influence community assembly when habitats are fragmented (Shackelford et al 2017, Bosco et al 2019).…”

“…Spatial configuration of habitat in the landscape, and patch attributes such as size and shape, can also critically influence the abiotic environment (Ewers and Didham ) through increases in perimeter‐to‐area ratio that cause greater exposure to external factors (Ries et al ). Such local‐scale edge effects in turn influence ecological processes, such as environmental filtering (HilleRisLambers et al ); the abiotic conditions that select for the establishment of a group of species in a given habitat (Aguilar‐Trigueros et al 2017).…”

“…Here, we test whether the interacting effects of habitat fragmentation operating across multiple spatial scales drive predictable shifts in plant traits during community re‐assembly of a previously deforested and fragmented area. An ideal system for testing these effects is the Thousand Island Lake (TIL) fragmentation project in eastern China, in which more than 1000 islands were created during inundation of a water reservoir 60 yr ago (Yu et al ). Islands of different sizes and degrees of isolation were formed within archipelagos of varying landscape context, with the surrounding water matrix being inhospitable to terrestrial plant survival.…”