Patterns and mechanisms underlying ecoregion delineation in North American freshwater plants

Aim: Biogeographical regionalisations are actively studied in different ecosystems, because they increase our understanding on fundamental broad-scale patterns and can help us in the establishment of conservation areas. Thus, we studied how well existing freshwater ecoregions describe geographical delineation for inland water plants and which ecogeographical gradients explain them. Location: North America, excluding Mexico and remote islands. Taxon: Freshwater vascular plants of all taxa and different functional groups. Methods: Using newly available fine-grained data on freshwater plant distributions, we calculated internal homogeneity and cross-boundary heterogeneity among neighbouring ecoregions. We further integrated measures of community dissimilarity to assess whether the degree of within-ecoregion homogeneity and distinctness are driven by their relationships to species replacements and richness differences, and explored how a complex suite of ecogeographical mechanisms and plant life forms affect ecoregion delineation using spatially explicit regression routines. Results: We found a clear geographical patterning of ecoregion robustness for North American freshwater plants, with their communities being more internally homogeneous and more similar to one another in polar and subtropical inland waters. Surprisingly, the degree of internal homogeneity and ecoregion distinctness were almost equally driven by species replacements and richness differences. Considering different life forms, ecoregion delineation performed best for emergent and floating-leaved plants. Finally, within-ecoregion homogeneity and distinctness were best explained by annual mean temperature and terrain ruggedness, respectively, with mean water alkalinity, ecoregion area and Late Quaternary Ice Age legacies having supplementary effects. Main conclusions: Our findings emphasise that geographical regionalisations founded on a particular organismal group are not applicable for all taxa. Our study is a promising starting point for further investigations of geographical delineations for different freshwater taxa. These updated regionalisations can then be used for conserving different biotas in freshwaters, which are currently among the most threatened ecosystems in the world.

Mapping variations in the strength and breadth of biogeographic transition zones using species turnover

Biogeographic regions are widely regarded as real entities, or at least as useful summaries of the complex patterns of spatial concordance among species. The problem is that, whereas some parts of the transition zones between regions may be strong and abrupt, other parts of the same zones may be weak or broad, so that the corresponding parts of border lines drawn on maps, although convenient, are arbitrary constructs. One approach to investigating transition zones ascribes values to the area units themselves, by quantifying the spatial turnover among species within the surrounding neighbourhoods of areas on maps. Using data for bumble bee distributions world-wide, I show that quantitative measures of neighbourhood turnover can discover many of the transition zones that are found by classification techniques when applied to the same data. But unlike classification techniques, turnover measures, particularly when used in combination, can show how a transition zone varies along its length, not only in its strength (the proportion of species contributing to the zone) but also in its breadth (the degree of spatial overlap or the degree of coincidence among species replacements across it). For bumble bees at least, these transition zones are also negatively associated with areas that have a combination of both high species richness and high species nestedness.

Species Replacements among Orconectes Crayfishes in Wisconsin Lakes: The Role of Predation by Fish

To test the role of size- and species-selective predation by fishes in species replacements among congeneric crayfishes in northern Wisconsin lakes, we tethered the same three sizes of each of three species of Orconectes at six sites in the littoral zone of Trout Lake, Vilas County, Wise. Small crayfish (15–18 mm carapace length) were removed by fish at significantly higher rates than medium (23–25 mm) and large (33–35 mm) crayfish. Orconectes virilis was removed at significantly higher rates than O. propinquus and O. rusticus. These species-selection results suggest that predation by native fishes contributes to the previously documented replacement of the native crayfish O. virilis by the exotic crayfishes O. propinquus and O. rusticus. In combination with the observations of others that adult size of O. rusticus is typically larger than that of O. propinquus in northern Wisconsin lakes, our size-selection results suggest that fish predation may also hasten the ongoing replacement of O. propinquus by O. rusticus. Thus, fish predation is probably an important mechanism driving the replacement of O. virilis by O. propinquus and the replacement of both congeners by O. rusticus.