Stage-structured harvesting and its effects: an empirical investigation using soil mites

1. Population dynamics results from an interplay between the environmental state and population density. With many organisms there is structure to the life history, and this structure has important consequences for the population's density dependence and its interaction with environmental noise, and therefore its population dynamics. Perturbing population structure, such as through harvesting, may therefore affect the way that the populations respond to stochastic environmental variation. 2. We conducted three experiments on populations of soil mites kept under controlled conditions and harvested a constant proportion of eggs, juveniles or adults. The experiments (a) minimized environmental variability, (b) created environmental variability by randomizing food supplies, and (c) provided excess food and repeatedly subdivided the populations to maintain them below carrying capacity. 3. We find that harvesting different stages has marked effects on stage structure, which differ between constant and variable environments. For example, harvesting adults decreases the number of adults, harvesting eggs increases the number of adults and harvesting juveniles has no effect under near constant conditions, whereas in a variable environment harvesting adults and juveniles reduces adult numbers, but harvesting eggs has no effect. 4. As well as changing the mean age structure, harvesting can change the variance of the different stages. For example, in a constant environment harvesting juveniles does not change the variability in juvenile numbers or population size, but in a variable environment harvesting juveniles increases the variability in the size of the juvenile class and hence the total population variability. 5. For populations that are kept at one-third to one-half carrying capacity (approximately where the maximum sustainable yield should result) harvesting of the different age classes has marked positive effects on the population growth rates. Harvesting different age classes causes changes in the density dependence, which explains the way in which the population parameters respond. 6. In conclusion, the population response to harvesting depends on the stage/age structure and the way it changes with harvesting and environmental conditions. Managing economically important populations, subject to harvesting, should consider structured life histories.