The Population Dynamics of Grasshoppers and Locusts

Summary 1. The eggs of Acrididae are well adapted to survive adverse weather and mortalities in field populations from this cause are low in most years. The eggs of tropical species are generally only laid where conditions are suitable for growth, while an egg diapause is present in many temperate species, during which time eggs are resistant to extremes of both temperature and moisture. Only prolonged flooding or drought can cause high mortalities and then only at certain stages in the egg's development. All eggs must absorb water to complete their development. 2. Nymphal and adult populations are far more dependent on the right weather conditions. Prolonged wet, cloudy weather greatly increases the mortality of both. Newly hatched nymphs are particularly susceptible. The way in which wet weather affects survival is little understood, but since the rate of feeding of Acrididae is so dependent on warm sunny conditions, it is possible that inclement weather may lead to starvation. 3. Fecundity is also greatly reduced by cool wet weather and the level indicated by laboratory studies is rarely reached in the field. 4. While both survival and fecundity are enhanced by hot, dry conditions, both are dependent on the presence of green food, which can be destroyed by excessive drought. 5. These opposing requirements of Acrididae make their numbers extremely sensitive to variations in weather and cause large fluctuations in their populations. 6. The eggs of most acridids are laid in bare ground, while vegetation is required by nymphs and adults for food and shelter. These requirements are again opposing. High‐density populations are often found in natural and man‐made ecotones, in which vegetation and bare ground occur in a mosaic pattern, thus providing both in their maximum availability. Ecotones are often unstable and the extent of the two vegetation components (bare ground and plant cover) is greatly influenced by weather. Variations in weather may then further affect acridid populations through effects on the carrying capacity of the habitat. 7. In locusts, gregarious behaviour and swarming are brought about by increases in population density, and variations in the relative extent of the components of the vegetation mosaic may cause crowding of one or other of the stages in the life cycle. 8. The influence of weather on acridid populations is so marked that correlations have been found for a large number of species between population size and particular weather conditions. The most important factor determining numbers varies between different species occupying different habitats and geographical regions. 9. Acridid populations are attacked by a large array of natural enemies–diseases, parasites and predators. Little quantitative work has been done on their effects, but biological considerations show that it is extremely unlikely that any are sufficiently density‐dependent to act in the way suggested by Nicholson (1933, 1954, 1958). It is likely that they do little more than damp the peaks in population fluctuations. 10. Many locusts and swarming grasshoppers emigrate from their habitats when their populations are large. This density‐dependent emigration possibly occurs in all Acrididae, though it is less well documented for non‐swarming species. 11. Locusts change both biologically and morphologically when crowded. These changes appear to be an adaptation to life away from their permanent habitat. Basically they reflect a change in the individua's metabolism to favour greater mobility. These ‘phase’ changes also affect fecundity, and crowding results in a marked reduction in the number of eggs laid. 12. Population size may be controlled by emigration and phase changes. When numbers become high, emigration of part of the population brings them down again. In locusts, phase changes could theoretically reverse the trend at both the peaks and troughs in population size. 13. The dynamics of acridid populations are discussed in the light of the main theories in population control, and it is concluded that Milne's theory (1957a) is closest to the known facts.