Abstract
Habitat fragmentation may affect the dispersal behaviour of individuals across the landscape. If there is a high boundary contrast between the remaining fragments and the matrix, individuals should tend to stay inside the fragments, but the configuration of the landscape, i.e. the level of fragmentation and isolation distance between fragments, may modulate this. To test these ideas, we made several mark-recapture experiments with the ladybird Eriopis connexa (Germ.) in manipulated alfalfa model landscapes (30 × 30 m). Specifically we evaluated (i) ladybird movement and permanence in alfalfa and bare ground areas, (ii) how they move across fragment/matrix boundaries, (iii) how their movement between fragments within a landscape is affected by fragmentation level and isolation distance, and (iv) how their permanence in the landscape is affected by fragmentation level and isolation distance. The frag- mented alfalfa landscapes had a constant level of habitat loss (84%), but varied in the level of habitat fragmentation (4 or 16 frag- ments), and isolation distance between fragments (2 or 6 m). We also sampled aphid abundance in the different landscapes through time. We found that ladybirds stay longer and displace less in alfalfa than on bare ground, and that they move less from the alfalfa fragments to the neighbouring bare ground matrix than vice versa. At the landscape level, ladybirds had a higher inter-fragment movement when fragments were closer, they tended to remain in less fragmented landscapes, regardless of their isolation distance. Aphid abundance increased in time, but it was similar in all landscapes. Ladybird movement within fragments, in the matrix and in the boundary between both habitats explains why ladybirds concentrate within alfalfa fragments. However, their dispersal move- ments at the landscape level do not reflect the higher abundance sometimes found for several species of ladybirds in more frag- mented landscapes. At this larger scale, other mechanisms may have a stronger influence in determining the abundance of ladybirds.
Highlights
A landscape is a heterogeneous area of land, containing habitat patches of different kinds, arranged in a particular configuration
In experimentally fragmented alfalfa landscapes, we found that E. connexa, and other ladybird species, concentrates inside alfalfa fragments surrounded by a bare ground matrix, and were positively affected by the level of habitat fragmentation
We created an experimental model system composed of 12 landscapes 30 × 30 m (3 replicates of 4 types) of alfalfa varying in levels of fragmentation and isolation distance between fragments: (a) 4–2 m, (b) 4–6 m, (c) 16–2 m (16 9 m2 fragments separated by 2 m), and (d) 16–6 m (16 9 m2 fragments separated by 6 m) (Fig. 1)
Summary
A landscape is a heterogeneous area of land, containing habitat patches of different kinds, arranged in a particular configuration. A landscape should not be a priori restricted to certain spatial scale, rather it should be defined by the scale relevant to organisms and the phenomenon under consideration (Wiens, 1992; Ims, 1995). Movement can be very complex and can occur at various hierarchical spatial scales for different activities. Foraging and mate-finding will usually occur at small spatial scales, probably within the same habitat, while searching for new habitats, and migration between regions or towards hibernation sites, occur at larger scales (Ims, 1995). The movement decisions at one level may be affected by spatial structures at levels below and above in the hierarchy (Kotliar & Wiens, 1990)
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