Ladybird (Coleoptera: Coccinellidae) dispersal in experimental fragmented alfalfa landscapes

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 fragmented alfalfa landscapes had a constant level of habitat loss (84%), but varied in the level of habitat fragmentation (4 or 16 fragments), 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 movements at the landscape level do not reflect the higher abundance sometimes found for several species of ladybirds in more fragmented landscapes. At this larger scale, other mechanisms may have a stronger influence in determining the abundance of ladybirds.