Habitat partitioning in riverine macrophyte communities

1. Habitat partitioning has been extensively studied in terrestrial plant communities. By comparison, few studies have examined habitat partitioning in riverine macrophyte communities. Riverine environments are often highly heterogeneous with respect to current speed, depth, bottom sediment fertility and other biologically important properties. Thus, the potential for habitat partitioning in riverine macrophyte communities is great. 2. The objective of this study was to investigate how, or if, macrophyte species are differentially distributed with respect to abiotic gradients in a large river in central British Columbia, Canada. 3. A survey of macrophyte biomass and associated measures of current speed, depth and sediment texture showed that coexistence occurred both with and without habitat partitioning. 4. Coexistence in the absence of habitat partitioning was depicted by Potamogeton gramineus, Potamogeton berchtoldii, Myriophyllum exalbescens, Callitriche hermaphroditica and Potamogeton pectinatus such that they had a minimum and average niche overlap (NO) of 65% and 84%, respectively, for all measured abiotic variables. Similarly, the spatial niche of E. canadensis was close to that of Ceratophyllum demersum and M. exalbescens. 5. While several taxa seemed to occupy the same spatial niche, some taxa were distinguished from one another in terms of the types of microhabitats they utilized. Mosses, for example, were often the only occupants of sites having current speeds greater than 0.6 m s^–1 and substrates of bare rock. Ranunculus aquatilis was most abundant at sites having current speeds between 0.4 and 0.6 m s^–1 whereas most other taxa were restricted to slower waters. Habitat partitioning across depth contours, which occurred between several taxa, was most evident between P. pectinatus and C. demersum (NO = 17%), C. hermaphroditica and C. demersum (NO = 3%), C. demersum and P. richardsonii (NO = 19%) and C. hermaphroditica and E. canadensis (NO = 29%).     

Population and species boundaries in the South American subterranean rodent Ctenomys in a dynamic environment

Subterranean rodents of the genus Ctenomys are an interesting system to assess the effects of habitat instability on the genetic structure of populations. The perrensi group is a complex of three species (C. roigi, C. perrensi and C. dorbignyi) and several forms of uncertain taxonomic status, distributed in the vicinity of the Iberá wetland in Argentina. Because of limited availability of suitable dry habitat, Ctenomys populations are distributed patchily around a vast mosaic of marshes, swamps and lagoons and become connected or isolated over time, depending particularly on the precipitation regime. Genetic variation at 16 microsatellite loci in 169 individuals collected in the area revealed eight clusters of populations which are thought to be evolutionary units, but which do not fit previous species limits. We interpret this lack of congruence between taxonomy and genetic structure as the result of a dynamic population structure. Where populations become connected, hybridization is possible. Where populations become isolated, rapid genetic divergence may occur. In the perrensi group, it appears that both of these factors disrupt the association between different genetic and morphological characters. The study of multiple characters is crucial to the understanding of the recent evolutionary history for dynamic systems such as this. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 368–383.