Dispersal decreases diversity in heterogeneous metacommunities by enhancing regional competition

Abstract

Abstract. Experiments and models reveal that moderate dispersal rates between local communities can increase diversity by alleviating local competitive exclusion; in contrast, high dispersal rates can decrease diversity by amplifying regional competition. However, hitherto experimental tests on how dispersal affects diversity in the presence and absence of environmental heterogeneity are largely missing, although it is known that environmental heterogeneity influences diversity. For the first time we experimentally show that the interaction between dispersal rate and the presence of an environmental gradient with onaverage lower resource availability than the homogeneous control treatment affects diversity. In metacommunities of nine co-occurring species of marine benthic microalgae we factorially manipulated dispersal rate and the presence and absence of a light intensity gradient across local patches to test effects on local, regional, and beta diversity and to compare results to predictions from monoculture experiments. Although species in this experiment did not show resource partitioning along the light gradient as assumed by source-sink models, dispersal limitation maintained diversity in metacommunities with light gradients but not without. Local diversity and evenness were high under low light intensities when dispersal was limited and decreased with both increasing light intensities and dispersal rates. These diversity changes can be explained by the reduction of growth of the regional superior competitor at low light intensities alleviating its competitive strength. Increasing dispersal rate in turn compensated for the superior competitor's slow growth in those local patches with rather unfavorable light conditions and thus led to decreasing diversity and evenness. In contrast, diversity in the metacommunities without a light gradient was constantly low. Here, the superior competitor contributed 90% to total community biomass in all patches. High dominance, however, likely resulted from on-average higher resource availability (i.e., higher light intensities) compared to metacommunities with light gradient and not from patch homogeneity in itself.

Hydrologic connection between ponds positively affects macrophyte α and γ diversity but negatively affects β diversity

“…Our findings of a high contribution of b diversity and low contribution of a diversity to regional c diversity are consistent with previous studies of freshwater fishes and invertebrates (Stendera andJohnson 2005, Eros 2007) and terrestrial plants (Clough et al 2007). High dissimilarity between habitat patches has been explained by a strong dispersal limitation between the patches as well as a strong species-sorting process by environmental filters or competition (Capers et al 2010, Clarke et al 2010, Matthiessen et al 2010. For pond macrophytes in particular, differences in environmental conditions (Ray et al 2001, Capers et al 2010 as well as restrictions on propagule dispersal (Ray et al 2001, Akasaka andTakamura 2011) can strongly restrict species composition.…”

“…Thus, for an area with multiple habitat patches, the probability of losing a species through stochastic events is lower in areas with inter-patch dispersal compared to those without interconnections between patches, when propagules are moderately dispersed Loreau 2003, Leibold et al 2004). The total species number in habitat patches connected by moderate dispersal, therefore, is higher than in patches without connections when the overall surface areas of the patches are equal (Matthiessen et al 2010). When the overall surface areas are equal, the total number of species in multiple habitat patches without interconnections (i.e., several small patches) is generally higher than in a single large patch (Williams et al 2004, Wiersma andUrban 2005).…”

Interactions between pH and nutrients on benthic algae in streams and consequences for ecological status assessment and species richness patterns

“…A number of different factors interact in affecting benthic primary producer taxon richness, the most commonly investigated being nutrient supply and disturbance (Biggs & Smith, 2002;Cardinale et al, 2006), as well as dispersal limitations (Matthiessen et al, 2010). We show here that pH is an important additional factor influencing primary producer richness, and that underlying gradients in pH, when not accounted for, interfere with patterns in benthic primary producer richness to nutrient supply.…”

Effects of dispersal on total biomass in a patchy, heterogeneous system: Analysis and experiment

“…An interesting recent result from mathematics is that a popu- 4 lation in an environment in which resources vary spatially will reach 5 a higher total equilibrium biomass than the same population in an 6 environment with the same total resources but where resources are 7 distributed homogeneously [1][2][3], which they referred to as 'a curi- 8 ous fact indeed'. The mathematical result depends on the population 9 being able to diffuse in space.…”