Using classification tree analysis to reveal causes of mortality in an insect population

摘要

1 Invasive species pose significant threats to native and managed ecosystems. However, it may not always be possible to perform rigorous, long-term studies on invaders to determine the factors that influence their population dynamics, particularly when time and resources are limited. We applied a novel approach to determine factors associated with mortality in larvae of the sawfly Profenusa thomsoni Konow, a leafminer of birch, and a relatively recent invader of urban and rural birch forests in Alaska. Classification tree analysis was applied to reveal relationships between qualitative and quantitative predictor variables and categorical response variables in a large data set of larval mortality observations. 2 We determined the state (living or dead) of sawfly larvae in samples of individual leaves. Each leaf was scored for variables reflecting the intensity of intra-specific competition and leaf quality for leafminers, year of collection and degree-days accumulated were recorded for each sample. We explored the association of these variables with larval state using classification tree analysis. 3 Leafminer mortality was best explained by a combination of competition and resource exhaustion and our analysis revealed a possible advantage to group feeding in young larvae that may explain previously observed patterns of resource overexploitation in this species. Dead larvae were disproportionately found in smaller leaves, which highlights the potential effect of competition on mortality and suggests that smaller-leaved species of birch will better able to resist leafminer damage. 4 We show that classification tree analysis may be useful in situations where urgency and/or limited resources prohibit traditional life-table studies.