The effects of arable field margin management on the abundance and species richness of Araneae (spiders)

The effects of ten different management regimes on the abundance and species richness of Araneae on pre-existing and newly expanded zones of field margins surrounding arable fields are compared in a large-scale experiment conducted between 1987 and 1991. Most of the management regimes involved varying the Liming and frequency with which the field margins were cut. There was no evidence of consistent temporal trends in the abundance and species richness of Araneae on either the existing or expanded zones of the field margins during the first four years of the experiment. However, lowest numbers were recorded before expansion of the margins in 1987, and highest numbers in the final sample, in 1991. Araneae remained less abundant, and less species rich, on the newly expanded than on the existing zones of the field margins four years after they were set-aside'. Culling reduced both abundance and species richness. Regimes which included culling in mid-summer had a greater impact which persisted for longer than did those involving cutting in spring and autumn. Removal of cut material was associated with lower species richness than leaving it in situ. Sowing the margins with a wild flower seed mixture was associated with increased abundance and species richness of Araneae. Annual spraying with a broad-spectrum, non-persistent herbicide reduced the abundance of Araneae but the effect was not detectable until several months after the application of the spray. Both the species richness and abundance of Araneae were positively correlated with a measure of vegetation height made on the field margins in September, on both the pre-existing and newly expanded zones of the margins. The extent to which management practices which benefit Araneae are compatible with both the management requirements for various other taxa. and the overriding constraint of effective pernicious weed control on field margins, is discussed.     

Edge effects and tropical forest canopy invertebrates

The term edge effect can be used to encompass a wide range of both biotic and abiotic trends associated with boundaries between adjacent habitat types, whether these be natural or anthropogenic. Edge effects have been shown to represent significant forces affecting both faunal and floral assemblages in fragmented ecosystems. Specific studies of faunal assemblages associated with habitat edges have revealed trends at all levels of biological organisation from individuals to communities.Studies of edge effects on invertebrates in tropical forests have been relatively scarce. In this paper we review the nature and organisation of edge effects, focusing upon the processes which may lead to detrimental consequences for both forest canopy invertebrates and the forests themselves. We present as a case study data illustrating the very large amount of variance (over 50%) in community structure that is predicted simply by abiotic (microclimatic) variables in both a tropical and a temperate forest edge. We summarise major features of edge effects amongst forest invertebrates, stress the inter-relatedness of edge and canopy biology, and present an agenda for study of the canopy as an edge.     

Transition matrix models and succession after release from grazing on Aldabra atoll

The process of succession after release from grazing by giant tortoises was investigated in three different vegetation types on Aldabra atoll. After four years there was very little change in two vegetation types: a tall mixed scrubland and an open single-species grassland. Probably in the former case grazing had no effect and in the latter case soil nutrients were limiting sward growth and establishment. In the third vegetation type, a patchwork of scrub and low vegetation, analysis of transition matrices showed that the succession was non-independent, non-stationary and depended on more than one past state (high order). This succession was extrapolated by simulation. The results suggested that other vegetation types studied were discrete associations, not merely different stages in the same grazing-mediated sere. The transition matrix approach proved useful in helping to generate initial hypotheses, but became too cumbersome for a realistic model of this succession. An experimental approach will be better for dealing with problems identified from the transition models. We are extremely grateful to the staff of the Seychelles Islands Foundation and of their Aldabra Research Station for permission to work on the island and for their assistance in the field.