David Johannes Kotze

Bio: David Johannes Kotze is an academic researcher from University of Natal. The author has contributed to research in topics: Biodiversity & Ecotone. The author has an hindex of 3, co-authored 3 publications receiving 262 citations.

No general edge effects for invertebrates at Afromontane forest/grassland ecotones

Abstract: The Afromontane region of South Africa is characterised by numerous small, remnant forests in a grassland matrix. The edges, or ecotones between forests and grasslands are usually sharp (typically just over a few metres) and are mainly maintained by both natural and, more recently, anthropogenic fires. We investigated epigaeic amphipod, carabid and ant distribution patterns across Afromontane forest/grassland ecotones and found little evidence to support the biological edge effect. Five of the fifty-two sampled species however, did increase significantly in abundance at the ecotone. Among these was a very distinct edge species, the amphipod Talistroides africana. Overall, carabids were more abundant and species rich in forests while for ants it was in the grasslands. Ants and carabids were both more abundant and species rich in spring and summer than in autumn and winter. More interestingly, the abundance and species richness patterns across the ecotone did not change with the passing of the seasons. We argue that a conservation strategy for the Afromontane forest patches must also incorporate the surrounding grassland. The grassland habitat is often perceived as less valuable than forest and, as a consequence, is subject to many anthropogenic disturbances such as fragmentation, cattle grazing and afforestation. Protecting grasslands around forest patches not only conserves the rich ant diversity, but also conserves the biota in the forests and at the edges, and would therefore be more meaningful in terms of the overall conservation of Afromontane biodiversity.

Invertebrate conservation at the interface between the grassland matrix and natural Afromontane forest fragments

Abstract: Diversity patterns of amphipods, carabid beetles and ants were investigated in five naturally-fragmented Afromontane forest remnants, and in the surrounding grassland matrix. Forests were architecturally similar. In contrast, grasslands surrounding these forests are subject to great differences in anthropogenic impacts. Consequently, transition from forest to grassland ranged from being abrupt (heavy disturbance) to gradual (little disturbance). Significantly different mean numbers of carabid individuals and species were captured between sites and multivariate analyses showed clear separation in carabid assemblage-structure with level of disturbance. Carabids were, furthermore, significantly more diverse in forests, compared to grasslands. Ants, however, were equally species rich between sites but were significantly more abundant and species rich in grasslands than forests. Amphipods, represented here by a single species, Talistroides africana, was significantly less abundant at highly disturbed sites and significantly more abundant in forests than grasslands. Results support the hypothesis that the dynamics of remnants are influenced by their surrounding landscape. Here, the dynamics of amphipods and carabids (predominantly forest taxa) were influenced by different disturbance regimes in grasslands surrounding these forests. Epigaeic ants, a predominantly grassland taxon here, also showed significant differences in assemblage-composition between sites with varying disturbance. Conserving Afromontane grasslands should be of prime concern because this will include the protection of forest/grassland ecotones and forest remnants.

Support for the multi-taxa approach in biodiversity assessment, as shown by epigaeic invertebrates in an Afromontane forest archipelago

Abstract: Often a single indicator invertebrate taxon is used for assessing changing landscape patterns. However, we argue here against the exclusive selection of a single group. Covariation in diversity patterns of spiders, carabids, staphylinids and ants were compared in and between five highly naturally-fragmented Afromontane forest patches (size range: 3.5–25.2 ha) in South Africa. Significantly fewer individuals and species were captured in smaller forest patches (<6 ha) for most of the taxa, except Formicidae, where a higher number of species were captured in medium-sized patches (7–9 ha). When sampling effort was standardized, a higher diversity (rarefaction and Simpson's diversity index) was obtained in the smaller patches for Carabidae, Staphylinidae and Formicidae. The only significant positive correlation between taxa, in terms of numbers of species, was between Carabidae and Staphylinidae. The other taxa showed only weak positive correlations between species richness, or negative correlations. Multivariate techniques showed significant species turnover between patch assemblages for each taxon, and also showed that some taxa are more similar in assemblage-structure than others. An index of complementarity showed that species compositions of the selected taxa varied greatly between forest patches of different sizes. Our results support the multi-taxa approach, in conservation studies, even at the level of taxonomically-related groups sharing a common habitat stratum.

The Theory of Island Biogeography

Abstract: Preface to the Princeton Landmarks in Biology Edition vii Preface xi Symbols Used xiii 1. The Importance of Islands 3 2. Area and Number of Speicies 8 3. Further Explanations of the Area-Diversity Pattern 19 4. The Strategy of Colonization 68 5. Invasibility and the Variable Niche 94 6. Stepping Stones and Biotic Exchange 123 7. Evolutionary Changes Following Colonization 145 8. Prospect 181 Glossary 185 References 193 Index 201

Confounding factors in the detection of species responses to habitat fragmentation

Abstract: Habitat loss has pervasive and disruptive impacts on biodiversity in habitat remnants. The magnitude of the ecological impacts of habitat loss can be exacerbated by the spatial arrangement -- or fragmentation -- of remaining habitat. Fragmentation per se is a landscape-level phenomenon in which species that survive in habitat remnants are confronted with a modified environment of reduced area, increased isolation and novel ecological boundaries. The implications of this for individual organisms are many and varied, because species with differing life history strategies are differentially affected by habitat fragmentation. Here, we review the extensive literature on species responses to habitat fragmentation, and detail the numerous ways in which confounding factors have either masked the detection, or prevented the manifestation, of predicted fragmentation effects. Large numbers of empirical studies continue to document changes in species richness with decreasing habitat area, with positive, negative and no relationships regularly reported. The debate surrounding such widely contrasting results is beginning to be resolved by findings that the expected positive species-area relationship can be masked by matrix-derived spatial subsidies of resources to fragment-dwelling species and by the invasion of matrix-dwelling species into habitat edges. Significant advances have been made recently in our understanding of how species interactions are altered at habitat edges as a result of these changes. Interestingly, changes in biotic and abiotic parameters at edges also make ecological processes more variable than in habitat interiors. Individuals are more likely to encounter habitat edges in fragments with convoluted shapes, leading to increased turnover and variability in population size than in fragments that are compact in shape. Habitat isolation in both space and time disrupts species distribution patterns, with consequent effects on metapopulation dynamics and the genetic structure of fragment-dwelling populations. Again, the matrix habitat is a strong determinant of fragmentation effects within remnants because of its role in regulating dispersal and dispersal-related mortality, the provision of spatial subsidies and the potential mediation of edge-related microclimatic gradients. We show that confounding factors can mask many fragmentation effects. For instance, there are multiple ways in which species traits like trophic level, dispersal ability and degree of habitat specialisation influence species-level responses. The temporal scale of investigation may have a strong influence on the results of a study, with short-term crowding effects eventually giving way to long-term extinction debts. Moreover, many fragmentation effects like changes in genetic, morphological or behavioural traits of species require time to appear. By contrast, synergistic interactions of fragmentation with climate change, human-altered disturbance regimes, species interactions and other drivers of population decline may magnify the impacts of fragmentation. To conclude, we emphasise that anthropogenic fragmentation is a recent phenomenon in evolutionary time and suggest that the final, long-term impacts of habitat fragmentation may not yet have shown themselves.

Ground beetles (Coleoptera: Carabidae) as bioindicators

Abstract: One of the primary goals of research on bioindicators is to identifyspecies or other taxonomic units that would reliably indicate disturbances inthe environment, and reflect the responses of other species or the overallbiodiversity. However, there is no perfect bioindicator and selecting the mostsuitable one depends to a great extent on the goal of the survey. In this paperwe examine the suitability of carabids as bioindicators. Carabids are frequentlyused to indicate habitat alteration. They have been used in grasslands andboreal forests where species number and/or abundances have been noted to changealong a habitat disturbance gradient. A common trend is that large, poorlydispersing specialist species decrease with increased disturbance while smallgeneralist species with good dispersal ability increase. Some species are notaffected by moderate disturbance. There is, however, not enough research todetermine how suitable carabids are for biodiversity studies, or how well theyrepresent the response of other species. We conclude that carabids are usefulbioindicators, but as crucial understanding of their relationship with otherspecies is incomplete, they should be used with caution.

Terrestrial invertebrates as bioindicators: an overview of available taxonomic groups

Abstract: Bioindicators, as taxa or functional groups, are widely used as indicators of environmental change, specific ecological factors or taxonomic diversity. The use of ecological, environmental and biodiversity indicators, is reviewed here. Although indicator taxa are considered to be generally unreliable as broad indicators of biodiversity, they may serve a useful function in identifying ecological characteristics or monitoring the effects of habitat management. Use of only a narrow range of taxa may be unreliable, and is particularly vulnerable to distortion by a small number of invasive species. Taxa also need to be selected to reflect the specific ecosystem being studied. It is recommended that isopods be used for soil systems (if there is sufficient local diversity), in some areas earthworms or mites may be useable but are generally too difficult to identify to be practically useful. In the ground layer indicator sets could include ants, millipedes, molluscs (snails in particular), ground beetles, harvestmen and gnaphosid spiders. Foliage-inhabiting indicators could comprise ants, chrysomelid leaf beetles, theridiid spiders and arctiid moths. Ants, orthopterans and butterflies may be appropriate for use in open habitats. These basic sets should be supplemented by other taxa where appropriate resources and taxonomic expertise are available.