Effects of experimental small-scale grassland fragmentation on the extent of grazing damage in Trifolium repens seedlings

摘要

We examined the extent of grazing damage to seedlings of white clover (Trifolium repens) in experimentally fragmented calcareous grasslands. The experimental set-up consisted of 12 blocks distributed over three sites. Each block contained one large (4.5 X 4.5 m), one medium (1.5 x 1.5 m) and two small (0.5 x 0.5 m) fragments and corresponding control plots. Fragmentation was maintained by frequently mowing the area between the fragments. A fence around the study sites excluded large herbivores. Seedlings of T repens planted in petri dishes were exposed to invertebrate herbivores in fragments and control plots for 2-3 days both in summer and autumn. In summer-unlike autumn-grazing intensity expressed as leaf biomass loss per petri dish and day was 43% lower in fragments than in control plots. Grazing intensity in summer was positively correlated with relative gastropod density (number of individuals from 12 species), but not with relative grasshopper density (number of individuals from 15 species). In autumn, grazing intensity was correlated neither with gastropod nor with grasshopper density. It is suggested that a combination of factors can explain the seasonal difference in fragmentation effect on grazing intensity. Different herbivore species are active at different seasons and/or under different environmental conditions. Furthermore, different herbivore species react differently to the type of grassland fragmentation chosen in the present study. We conclude that fragmentation effects on grazing depend on which of the potential herbivore species react to fragmentation and which of them are active during the investigation period. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved. [References: 60]