Computer-generated null models as an approach to detect perceptual range in mark–re-sight studies – an example with grasshoppers

Abstract.  1. Dispersal and habitat detection are key factors for the colonisation of habitat fragments in heterogeneous landscapes. The ability to recognise a habitat from a certain distance should increase the survival chances of a dispersing individual; however, due to methodological problems there is little information on the perceptual range of most species.
2. In a field experiment, 44 individually marked grasshoppers of the species Oedipoda caerulescens (Orthoptera: Acrididae: Locustinae) were released into an unfamiliar, hostile environment at various distances from a patch of preferred habitat.
3. Whether individuals reached the habitat or not was measured, as well as the daily movement distances. The number of individuals that reached the habitat was tested against computer-generated predictions based on different underlying rules for the movement behaviour of individuals but not accounting for the ability to detect habitat from distance.
4. On the first day a significantly higher proportion of grasshoppers arrived in the habitat than predicted by any of the null models.
5. It was concluded that individuals of O. caerulescens are able to detect their preferred habitat from a distance.
6. Edge permeability was very low as none of the individuals left the habitat once they had reached it.
7. Additional analyses showed that individuals changed movement behaviour from a directed walk with great daily distances in unsuitable habitat to a walk with significantly shorter daily distances within the preferred habitat.
8. The problems that arose in the field experiment are discussed and recommendations are given for further studies.