Community dynamics and demography of small mammal Populations in natural grasslands and rehabilitated ash dams

Abstract

While small mammal communities in various rehabilitated ecosystems have been well studied, no data are available for rehabilitated sites in the heavily transformed grasslands of South Africa. Small mammal community characteristics and successional trends on fly-ash disposal sites (ash dams), and in adjacent natural grasslands (control grids),at Hendrina Power Station (Mpumalanga) were therefore examined, and the success of current rehabilitation strategies at restoring sustainable small mammal populations was evaluated. Population sizes and biomass of small mammals on the ash dams and controls were similar and varied significantly on a seasonal basis, being highest in autumn and winter and lowest in summer. Such seasonal variation in population sizes is atypical of small rodents in the summer rainfall zone of southern Africa and apparently reflected localized immigration and emigration, both on a seasonal basis and also following a veld fire during 1998, as well as the effects of weather and resource availability on trapability in the wet summer months. Small mammal communities on all grids were dominated by Rhabdomys pumilio, numbers of which increased progressively with increasing rehabilitation age to peak at nine years. Mastomys coucha occurred in significantly higher numbers on the slopes of the ash dams, particularly those adjacent to agricultural fields and disturbed or undisturbed grasslands. Population sizes of this species, and thus community composition on the ash dam slopes, were apparently influenced by the ecological status of adjoining areas, perhaps reflecting recolonization opportunities and rates. Conversely, Tatera brantsii occurred in highest numbers on the plateaux grids surrounded by rehabilitated areas, and was absent from the controls, suggesting that numbers of this species, and the composition of the plateaux communities, was not markedly influenced by recolonization from adjacent unrehabiliated areas. Successional tendencies on the ash dam grids were characterized by changes in equitability, local extinctions of less common (more stenoeious) species and species replacements. Diversity and equitability increased with rehabilitation age between four to seven years post-rehabilitation (when these parameters were significantly higher than on the control grids) ,and then declined. dramatically to levels observed on the control grids. This decline reflected increased dominance of the communities by R. pumilio (especially on the slopes) and T. brantsii (on the plateaux). Small mammal species diversity and equitability generally increased with an increase in the diversity, equitability and richness of plant communities on the ash dam grids. Stepwise regression and ordination analyses, however, suggested that certain edaphic factors were the best predictors of small mammal community structure and dispersion. Given the general resemblance in small mammal community attributes and demography on the ash dams and controls, and similar successional tendencies evident in these areas, I conclude that current rehabilitation actions have been successful at restoring reasonably diverse small mammal communities over the short term. These communities do not, however, appear to be sustainable over the medium-long term, necessitating further management actions and changes to current rehabilitation strategies.