Landscape structure and locust swarming: a satellite's eye view

Abstract

Desert locust Schistocerca gregaria outbreaks consistently start in the same places, suggesting that certain landscapes are particularly favourable for outbreaking. Outbreaks are generated by multiplication, concentration and gregarisation of locust populations. Previous research has shown how small‐scale vegetation patterns in desert ecosystems influence locust gregarisation; the present study examines the effects of large‐scale landscape structure on locust multiplication and concentration. NOAA/AVHRR satellite imagery was used to relate abundance and spatial distribution of resources at the landscape scale to the historical record of locust outbreaks. Threshold NDVI values were investigated to define what constitutes ‘resource’ for locusts. The first part of the study showed that abundance and spatial distribution of resource were not sufficient to distinguish between outbreak and non‐outbreak areas in the western part of the locust distribution area. Thus, outbreak danger zones cannot be identified by landscape structure at this spatial resolution. The second analysis investigated spatio‐temporal patterns of vegetation growth in two locust breeding areas with very different landscape structure; in both cases, the patterns differed significantly between outbreaking and non‐outbreaking years. In Mauritania, a flat homogeneous desert landscape, both resource abundance and fragmentation were higher in outbreaking years. On the Red Sea coast, a fragmented landscape, resource spatial distribution was consistent between years, and abundance alone was a significant predictor of outbreaking. High resource abundance promotes locust multiplication, and contraction of resource into small patches increases locust concentration; these two mechanisms explain how landscape structure influences locust outbreaking.