Mating patterns and rates of biological invasion

Invasive nonnative plants are in the news because of their dramatic effects on our national agriculture, aquaculture, rangelands, water supplies, and other important resources (1, 2). Invasive plants also cause the decline of local biodiversity worldwide, with clear effects on native species and ecosystems (3, 4). For economic and ecological reasons, these invaders have been a primary focus of conservation and restoration efforts, although control programs seldom have the resources to keep up with the spread of new invaders (5). One of the conundrums of controlling invasive plants is that eradication is feasible only very early on, before a species has become widespread. However, a new nonnative plant is unlikely to attract attention or funding when it is only found here and there, especially when its populations are growing slowly and therefore seem to pose little immediate threat. Initially slow “lag phases” in population growth are suspected to be common in invasions (e.g., ref. 6), yet evidence that they occur has been mostly anecdotal. Demonstrating the occurrence of lag phases has been impeded by the nature of exponential growth. When numbers are small, population growth can be imperceptible, even if the growth rate is quite large. A lag phase implies that the rate itself increases over time. We need hard evidence that rates of population growth change over time, along with experiments to reveal the mechanism behind that change. In this issue of PNAS, Davis …