Bush-crickets in small experimental grassland patches: Space use within patches and movement between them

Summary

In Norway, the practically flightless wart-biter (Decticus verrucivorus, Tettigoniidae) survives in small grassland patches and corridors. In this experiment, I created eight grassland patches of three different sizes to study how patch size affects movement within and between patches. The patches initially contained no wart-biters, and were populated with wart-biters at equal density in the start of the experiment. Half of the experiment insects were reared in the laboratory, while the other half were caught as adults; the latter were in better condition than the former (both sexes moved more, and the males sang more frequently and loudly). Emigration rate increased when patch area decreased, from 23% in the largest patches to 71% in the smallest. This is expected from theoretical studies. However, these studies assume that an animal's probability of emigrating is related to how often it encounters the patch edge. In contrast, we found that within patches, the probability of emigration was not influenced by how frequently an individual was in the edge zone. Thus, these results indicate that a different mechanism underlies the correlation between patch size and emigration rate. As an alternative mechanism, supported by the results, I suggest that the tendency to emigrate is counteracted by social cohesion created by male singing. I also found that males emigrated more than females, and wild-caught ("high-quality") animals emigrated more than animals reared in the laboratory ("low-quality"). Wart-biter males often congregate in clusters of equally-spaced males, and provide no parental care. In these two respects, their mating system resembles that of lekking species (e.g., black grouse). As in lekking species, we found that "high-quality" males, which sang frequently and loudly, tended to occupy the center of the medium-sized patches, while "low-quality" males stayed closer to the edges. The same division of space use was found for females, but low-quality females tended to move towards the center when male singing was very intensive. Similar patterns of space use was also found the only small patch that retained a stable population of both high- and low-quality animals of both sexes (the other small patches lost the high-quality animals due to emigration). The results indicate that in small and medium-sized patches, "high-quality" males defend mobile territories by singing louder and more frequently. Such mobile territories also appear in lekking birds when landmarks are lacking (e.g., on ice-covered lakes). In the large habitat patch, there was no spatial segregation between "high-" and "low-quality" animals, possibly of because higher costs of defending a centrally located territory.