Ontogenetic shifts within the selfish herd: predation risk and foraging trade-offs change with age in colonial web-building spiders

We tested for the existence of density dependence in annual adult rangeland grasshopper (Orthoptera: Acrididae) data from Montana, USA (1951–1991). Statistical density dependence was, in the sense of a stochastic equilibrium or return tendency, detected in all of the grasshopper mean density time-series from the three major physiographic regions of the state, Northern Glaciated Plains, Southern Unglaciated Plains, and Western Mountains. Parameters were estimated for a model that described the stochastic equilibrium characteristics of regional mean densities. The analyses showed that rangeland grasshopper regional densities fluctuate according to gamma distribution with a mean of 6.1–6.3 grasshoppers per m². Further, when regions exhibit outbreaks, the resulting infestation period (duration of outbreak) is short, spanning only a few generations.