Knowing the factors that control plant population size is important for our basic understanding of nature and for developing methods for controlling weedy and invasive plants. Ecologists agree that plants are often limited by competition with other plant species for resources. However there is debate about how important insects that eat plants (herbivores) are in controlling plant populations. Previous tests of the influence of herbivores have considered only effects on plant reproduction under low-density conditions and have not accounted for how the influence of herbivores changes with plant density. This study will use field experiments coupled with mathematical models to determine the influence of insect herbivores on the population growth of a weedy plant over a range of plant densities. Invasive and weedy plants impose enormous economic and environmental costs in the USA. Much attention has focused on using bio-control agents, particularly insect herbivores, to control these plants. This approach assumes that herbivores strongly affect plant populations. This project will provide the most complete test of this assumption to date. It will also help train new scientists, involving students as assistants (learning research techniques, data analysis and computer programming), and as independent researchers (learning to design studies, and present results).
Knowing the factors that control plant population size is important for our basic understanding of nature and for developing methods for controlling weedy and invasive plants. Ecologists agree that plants are often limited by competition with other plant species for resources. However there is debate about how important insects that eat plants (herbivores) are in controlling plant populations. Previous tests of the influence of herbivores have considered only effects on plant reproduction under low-density conditions and have not accounted for how the influence of herbivores changes with plant density over the entire life of the plant. This study used field experiments over four years coupled with mathematical models to determine the influence of insect herbivores on the population growth of the weedy plant Carolina horsenettle (Solanum carolinense; Figure 1). For herbivores to control plant populations, they must alter the effects of plant density on individual plant success. Our studies show that this occurs in Carolina horsenettle. For example, plants receive more damage at intermediate densities than they do at high or low densities. Damage from herbivores also reduces plant reproduction, but these effects differ when plants are growing at higher or lower densities. Similarly, the plant’s ability to tolerate damage increases when plants are growing at higher densities. To determine whether these herbivore and density effects influence the plant population, we employed demographic models. These models use information about individual plants to predict what will happen to the population over time. For example, estimates of demographic transitions (e.g., the proportion of plants that increase in size or die during a given time period) can be used to calculate how fast the population is growing or shrinking. (Population growth is often measured with the variable lambda (λ); when λ>1, the population is growing.) If demographic transitions or population growth changes with density and herbivores (e.g., Figure 2), we can conclude that herbivores contribute to controlling the plant’s population. Our modeling results show that herbivores change the effects of plant density on both demographic transitions and population growth. Herbivore effects vary from year to year; sometimes they magnify the reduction in growth rate caused by higher plant density, and sometimes they counteract it. Herbivore damage also changes the way density affects the probability that an individual plant will grow, stay the same size, or shrink; for example, there are several cases where density affects these demographic transitions when herbivores are absent but not when they are present. Together, these results suggest that herbivores contribute to determining the population size of Carolina horsenettle. Invasive and weedy plants impose enormous economic and environmental costs in the USA. Our research suggests that the focus on using bio-control agents, particularly insect herbivores, to control these plants may not always be successful. Although our results allow for strong herbivore effects on plant populations, the effects may not always be in the desired direction. For example, plant populations sometimes grew faster without herbivores than with them, especially at high densities. Thus, bio-control is unlikely to be a panacea. This research project has also contributed to training new scientists in the USA. Over the five years of funding, we employed five full-time technicians (all of whom have since started or completed science graduate programs) and two post-docs (one of whom has started a tenure-track faculty position at a liberal arts college). We also employed many part-time undergraduate research assistants and nine full-time summer research undergraduates (six of whom have started graduate or professional school). For the summer research students, we created and ran a professional development seminar that addressed issues such as research ethics, applying to grad school, careers in ecology and evolution, how to give scientific presentations, and how to write a cover letter and CV. Over the life of this grant, more than half of the people employed by this project (including students) were women, and several have been students of color or from under-represented backgrounds (including African American, Asian, Dominican, Pacific Islander, Native Hawaiian, first-generation US Americans, or first-generation college students).
