Ayres Abstracts
| Ylioja, T., H. Roininen, M.P. Ayres, M. Rousi, and P.W. Price. 1999. Host-driven population dynamics in an herbivorous insect. Proceedings of the National Academy of Sciences 96(19):10735-10740. Full text in Acrobat PDF format. |
| Understanding the nature and relative importance of endogenous(density-dependent) and exogenous (density-independent) effects on population dynamics remains a central problem in ecology. Evaluation of these forces has been constrained by the lack of long time series of population densities and largely limited to populations chosen for their unique dynamics (e.g., outbreak insects). Especially in herbivore populations, the relative contributions of bottom-up and top-down effects (resources and natural enemies, respectively) have been difficult to compare because population data have rarely been combined with resource measurements. The feeding scars of a wood-mining insect (Phytobia betulae Kangas; Diptera: Agromyzidae) of birch trees (Betula pendula and B. pubescens) provided the most accurate time series of substantive length (47 and 65 years) ever analyzed for a biological population. Abundances were measured on individual plants with known growth histories. Analyses showed a powerful exogenous effect of stand age on temporal variation in insect abundance (58% and 32% of the variance in two populations). With the additional effects of variation among trees, 77 and 64% of the total variance in abundance was attributable to exogenous bottom-up effects of host plants. Endogenous effects were evident as immediate linear density-dependence, but only accounted for 10% of the total variance. Abundance of Phytobia is primarily a function of disturbance history, which produces a mosaic of different aged birch stands that harbor Phytobia populations of different sizes. Density-dependence tends to regulate local populations around levels determined by host suitability. |
| Betula; Phytobia; Population regulation; Population dynamics; Density-dependence; exogenous vs. endogenous forces; bottom-up vs. top-down effects; time series; host quality |