What Causes a Locust Swarm: A Hierarchical Patch Dynamics Approach

Ecological phenomena act on various spatial and temporal scales. To understand what causes animal populations to build and decline depends heavily on abiotic and biotic conditions which vary spatiotemporally throughout the biosphere. One excel- lent example of animal populations dynamics is with locusts. Locusts are a subset of grasshoppers that undergo periodical upsurges called swarms. Locust swarms have plagued human history by posing significant threats to global food security. For example, the 2003-2005 desert locust (Schistocerca gregaria) swarm destroyed 80%-100% of crops in the impacted areas and cost over US $500 million in mitigation as estimated by the Food and Agriculture Organization of the United Nations. An integrative multi-scale approach must be taken to effectively predict and manage locust swarms. For my dissertation, I looked at the ecological causes of locust swarms on multiple scales using both the Australian plague locust (Chortoicetes terminifera) and desert locust as focal species. At the microhabitat scale, I demonstrated how shifts in the nutritional landscape can influence locust gregarization. At the field level, I show that locust populations avoid woody vegetation likely due to the interactive effect of plant nutrients, temperature, and predators. At the landscape level, I show that adaptations to available nutrient variation depends on life history strategies, such as migratory capabilities. A strong metapopulation structure may aid in the persistence of locust species at larger spatial scales. Lastly, at the continental scale I show the relationship between preceding vegetation and locust outbreaks vary considerably between regions and seasons. However, regardless of this variation, the spatiotemporal structure of geographic zone > bioregion > season holds constant in two locust species. Understanding the biologically relevant spatial and temporal scales from individual gregarization (e.g. micro-habitat) to massive swarms (e.g. landscape to continental) is important to accurately predicting where and when outbreaks will happen. Overall, my research highlights that understanding animal population dynamics requires a multi-scale and trans-disciplinary approach. Into the future, integrating locust re- search from organismal to landscape levels can aid in forecasting where and when locust outbreaks occur.