Transitory versus Persistent Effects of Connectivity in Environmentally Homogeneous Metacommunities

<div><p>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.</p> </div

Local, regional, and beta richness of the prey community over the course of the experiment.

<p>The experiment compared an unconnected control (UN) and metacommunities with low prey dispersal (LP), high prey dispersal (HP), low predator and prey dispersal (LPP) and high predator and prey dispersal (HPP). Low connectivity is shown with open symbols, high connectivity with closed symboles. Circles mark the treatments with prey dispersal only, and triangles are the treatments with predator and prey dispersal. The unconnected control is shown with dotted line and filled squares. Values are means ± SE (n = 3).</p

Ciliate biovolume (10³ µm³; n = 15), mortality rate (day⁻¹) in the single prey species experiment, and mortality rate and selectivity in the mixed prey species experiment.

<p>Ciliate biovolume with standard error in brackets. Mortality rates and selectivity were averaged over 10,000 random draws from 5 replicates, 2.5 and 97.5 percentiles in brackets.</p

Resource abundances in the three connectivity treatments.

<p>(A) algae, (B) bacteria, and (C) flagellates, means ± SE, n = 3.</p

Algal biovolume (A) and total bacterial biovolume (B) in the metacommunities.

<p>Treatment abbreviations and symbols as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0029071#pone-0029071-g002" target="_blank">Fig. 2</a>. Additionally, resources were measured in isolated basins without animals (WA), shown with dashed line and open squares. Values are means ± SE (n = 2 for WA, n = 3 for all other treatments).</p

Beta diversity in the three connectivity treatments.

<p>Dissimilarity of local communities in a metacommunity was calculated as Bray-Curtis distance. Values are means ± SE, n = 3.</p

Local and regional diversity with low, medium, and high connectivity.

<p>The measures of diversity used were species richness (A, C) and the Shannon-Wiener index (B, D), computed at the local (A, B) and regional scale (C, D), respectively. Values are means ± SE, n = 3.</p

P-values and results of Tukey's post-hoc tests of one-way rm-ANOVAs testing for treatment effects on local, regional, and beta richness and on the abundances of the five ciliate species.

<p>UN = unconnected, LP = low prey dispersal, HP = high prey dispersal, LPP = low predator and prey dispersal, HPP = high predator and prey dispersal; P-values<0.05 are bold; n = 3.</p

Experimental design of the metacommunity experiment and initial distribution of the predator.

<p>Four microcosms were connected according to one of three connectivity levels. Microcosms either remained unconnected (UN), were connected to an open square by tubing of 15 cm length (LP, LPP), or were connected to a closed square by tubing of 5 cm length (HP, HPP). Predatory copepods were introduced into one of the four microcosms on day 7 of the experiment. Copepods were either free to disperse to neighbouring basins (LPP, HPP) or prevented from dispersing by a 100 µm mesh in the middle of the connections (LP, HP).</p

Final copepod abundance (A) and dry weight (B).

<p>Only those basins to which the predator had access were considered for calculations. Values are means ± SE (n = 3).</p