II.10 Interactions between Plants and Herbivores
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II.10 Interactions between Plants and Herbivores Rebecca J. Morris OUTLINE 1. The diversity of herbivores 2. Herbivore–plant population dynamics 3. The impact of herbivores on plant populations 4. Plant responses to herbivory 5. The impact of plants on herbivore populations 6. Herbivore–plant interactions at the community level Herbivores are animals that feed on living plants. Herbivory is one of most common ecological interactions and is exhibited by species ranging from microscopic mites to giant pandas. Herbivore–plant interactions have features in common with all other consumer–resource interactions, although there are significant differences. Notably, plants do not necessarily die when they have been attacked by herbivores. Although there is no compulsory link between herbivore and plant dynamics, herbivores can affect the population dynamics of the plants on which they feed, and plants can affect herbivore population dynamics. Herbivore –plant interactions have been studied through a combination of observational time series data, mathematical modeling, and experimentation, and here a variety of examples are discussed. GLOSSARY functional response. Results from switching behavior when the herbivore alters the composition of its diet as a result of short-term changes in relative food availability herbivore. An animal that feeds solely on living plant tissue herbivory. The consumption of living plant material host plant. The plant on which an insect herbivore feeds numerical response. Acts by dispersal with mobile herbivores aggregating in regions of high food availability, or in the longer term by increasing reproductive success population cycles. Changes in the numbers of individuals in a population repeatedly oscillating between periods of high and low density population dynamics. The variation in time and space in the size and density of a population resource. An environmental factor that is directly used by an organism and that potentially influences individual fitness; plants are a resource for herbivores 1. THE DIVERSITY OF HERBIVORES Herbivores are animals that feed solely on living plant material. They are taxonomically and ecologically diverse and range from single-celled zooplankton to wildebeest, and from leaf-mining moths to marine iguanas. They can be found in terrestrial, marine, and freshwater ecosystems. Insects and mammals are the most well-known groups of herbivores and have been studied most intensively, but there are many other types of herbivore including some species of birds, fish, reptiles, crustaceans, and molluscs. Herbivores can feed on all the different types of living plant tissue including leaves, fruits, pollen, flowers, and seeds. Each herbivore, however, tends to specialize on a particular type of plant tissue. Herbivores exhibit a variety of feeding methods including chewing, sucking, boring, and galling. Folivores, which feed on leaves, are some of the most common herbivores and include mammals such as deer and insects such as grasshoppers. Frugivores are fruit eaters ranging from monkeys to wasps; and granivores are the seed eaters, or seed predators, including squirrels and weevils. Herbivores remove approximately 10% of net primary production, at least in terrestrial ecosystems. Herbivory typically does not kill the plant but influences the fitness of plants by reducing growth and reproduction and potentially increasing mortality. However, seed eaters (and some species that feed on seedlings) do have a significant effect on seed abundance and can directly influence plant populations, assuming that the plant is seed limited. There is such a wide variety of herbivore species that it is useful to consider the differences between the two most studied groups of herbivores: insects and mammals . Insect herbivores differ from mammalian herbivores in their size, metabolic rate, population density, numbers, and the kinds of damage they cause. Insects tend to be more specialized than mammalian herbivores and are more likely to have an intimate lifelong association with their host plant. Mammalian herbivores are likely to have a more immediate and, in the long term, more profound impact on plant populations than invertebrates because of their greater body size, polyphagy, individual bite size, mobility, and tolerance of starvation. A relatively high proportion of mammalian herbivore populations are food limited, whereas a comparatively high proportion of insect herbivore populations are regulated by predators, parasites, and diseases. 2. HERBIVORE–PLANT POPULATION DYNAMICS The population dynamics of herbivore–plant systems shares features exhibited by all consumer–resource interactions. Consequently, the mathematical models for these systems have the same logical foundations, largely based on the Lotka-Volterra model and its variations. However, herbivore–plant systems differ from other consumer–resource relationships, for example , predator–prey relationships, in several important ways. Classifying consumer–resource relationships according to the...