Summary
Habitat connectivity and regional heterogeneity represent two factors likely to affect biodiversity across different spatial scales. We performed a 3 × 2 factorial design experiment to investigate the effects of connectivity, heterogeneity, and their interaction on artificial pond communities of freshwater invertebrates at the local (α), among-community (β), and regional (γ) scales. Despite expectations that the effects of connectivity would depend on levels of regional heterogeneity, no significant interactions were found for any diversity index investigated at any spatial scale. While observed responses of biodiversity to connectivity and heterogeneity depended to some extent on the diversity index and spatial partitioning formula used, the general pattern shows that these factors largely act at the β scale, as opposed to the α or γ scales. We conclude that the major role of connectivity in aquatic invertebrate communities is to act as a homogenizing force with relatively little effect on diversity at the α or γ levels. Conversely, heterogeneity acts as a force maintaining differences between communities.
Methodology
To test whether the effects of connectivity on biodiversity depended on regional heterogeneity we conducted an artificial pond mesocosm experiment at the Queen’s University Biological Station in the summer of 2007. In total, seventy-two 350-L ponds were set up, grouped into 18 regions of four ponds each. The experiment had a 3 × 2 factorial design with three levels of habitat connectivity (none, low, and high), and two levels of regional heterogeneity (homogeneous regions, heterogeneous regions) (Online Resource A). Each cell of the design contained three replicate regions.
Connectivity levels were defined by transferring set volumes of water (and any included organisms) between ponds within a region on a weekly basis. In regions with no connectivity, mock transfer events were performed by transferring samples of water back to the pond from which they originated; in regions with low connectivity, 3 L of water was transferred from each pond to every other pond within the region (total turnover per pond per week 9 L); in regions with high connectivity, 12 L of water was transferred (total turnover per pond per week 36 L). While our high dispersal treatment is relatively strong, our goal was to elicit strong responses to connectivity to ensure that the response of an extremely well mixed region would be observed. Prior to all dispersal events the ponds were gently homogenized with a paddle, and the necessary volume of water was transferred by tube sampler.
Heterogeneity was modified using pond substrate. Homogeneous metacommunities were composed of four ponds with identical substrate (bare tank with no added substrate), and heterogeneous metacommunities were composed of four ponds, each with a different substrate (bare tank, rock substrate, short artificial-macrophyte substrate, long artificial-macrophyte substrate). The same substrate combinations were present in each replicate. We chose substrate as the basis for our heterogeneity treatment as substrate has previously been shown to have effects on zooplankton community composition (Declerck et al. 2007), and we felt it would support clear niche differentiation among littoral communities.