Predicting the impacts of harvesting using structured population models: the importance of density-dependence and timing of harvest for a tropical palm tree.
Structured population models are used in a range of forms to predict the long-term behaviour of populations of economic or conservation interest. Such models rarely include density-dependence and do not account explicitly for the ordering of events within a generation. We analysed a model for the harvesting of adults of the edible palm Euterpe edulis in which the role of density-dependence had been clearly defined. We modified the timing of harvesting in relation to the point in the life cycle at which populations were censused. It is shown that the timing, form and intensity of harvesting are all important in determining asymptotic population behaviour. If harvesting affects only those individuals that were recorded as being adults at the start of a year, then the model predicts that all adults may be harvested without population eradication. In contrast, if harvesting also affects individuals moving from the next smaller size class during the course of a year then populations can, under some forms of harvesting, tolerate much lower levels of harvesting. If density-dependence is not taken into consideration, predictions of population responses to harvesting may be erroneous. A review of transition matrices for woody plants indicates that many of these may have been derived from populations subject to strong population regulation. Synthesis and applications. In the specific case of E. edulis our model shows that, although populations appear to be robust to very high levels of harvesting, when modelled as affecting only reproductive adults, this conclusion may be sensitive to varying the timing and form of harvest, and to the assumption that only reproductive individuals are removed. Structure population models used to determine levels of harvesting should account for the existence of density-dependence as well as its timing.