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Seed predation, often referred to as granivory, is a type of plant-animal interaction in which granivores (seed predators) feed on the seeds of plants as a main or exclusive food source (Hulme and Benkman 2002), in many cases leaving the seeds damaged and not viable. Granivores are found across many families of vertebrates and invertebrates (especially mammals, birds and insects) (Janzen 1971); thus, seed predation occurs in virtually all ecosystems (see further readings). Seed predation is commonly divided into two distinctive temporal categories, pre-dispersal and post-dispersal predation, which may involve different strategies and requirements and have different implications at the individual and population level. Nevertheless, regardless of timing, seed predation has great implications for plant population dynamics (see below). To counterbalance effects of predation, plants have evolved defenses such as seed morphology (size, shape, toughness) and chemical defenses (secondary compounds such as tannins and alkaloids) to defend against their seed predators (see below). However, as plants have adapted defenses to seed predation (e.g., chemical compounds), so have seed predators adapted to plant defenses (e.g., ability to detoxify chemical compounds). Thus, many interesting examples of coevolution arise from this dynamic relationship.
Seeds and their defenses
Plant seeds are important sources of nutrition for animals across most ecosystems. Seeds contain food storage organs (e.g., endosperm) that provide nutrients to the developing plant embryo (cotyledon). This makes seeds an attractive food source for animals because they a highly concentrated and localized nutrient source in relation to other plant parts.
Seeds of many plants have evolved a variety of defenses to deter predation from seed predators. Plants may allocate resources toward physical or chemical defenses. Seeds are often contained inside protective structures or fruit pulp that encapsulate seeds until they are ripe. Other physical defenses include spines, hairs, fibrous seed coats and hard endosperm.
Some seeds have evolved strong anti-herbivore chemical compounds. In contrast to physical defenses, chemical defenses in seeds to deter consumption by seed predators by using chemicals that are toxic to granivores or inhibit the digestibility of the seed. These chemicals include non-protein amino acids, cyanogenic-gycosides, protease and amylase inhibitors, Phytohaemaglutinins (Herrera and Pellmyr 2002). Seeds face trade-offs between allocation toward defenses and the size and number of seeds produced. Additionally plants have trade-offs in seed defense allocation between deterring seed predators and attracting seed dispersers.
Plants may reduce the severity of seed predation by making seeds spatially or temporally scarce to granivores. Seed dispersal away from the parent plant is hypothesized to reduce the severity seed predation (see below for further details) (Janzen 1970, Connell 1971). Seed masting is an example of how plant populations are able to temporally regulate the severity of seed predation. Masting refers to a concerted abundance of seed production followed by a period of paucity. This strategy has the potential to regulate the size of the population of seed predators.
Pre- vs. post-dispersal seed predation
Seed predation can often occur at two different stages. Pre-dispersal seed predation occurs when seeds are consumed while still on the plant, thus being concentrated and very conspicuous. Conversely, post-dispersal seed predation occurs when seeds are consumed after being released by the parent plant (i.e., dispersed), thus being at low densities, scattered and/or hidden in the environment.
Both pre- and post-dispersal seed predation are extremely frequent in nature. However, fundamental differences occur between the two. Pre-dispersal predators are most often specialists, as they are more likely to adapt to clustered resources. Predator specialization requires that these animals utilize specific cues like plant chemistry (volatile compounds), color and size to attack seeds. In addition, specialization requires that the life cycle of the predator match the production of seeds by the host plant. Therefore, animals with shorter life span, like insects, are more common pre-dispersal predators. Common insect groups of pre-dispersal predators are Coleoptera, Hemiptera, Hymenoptera and Lepidoptera. For this reason, a greater diversity of predators feed on seeds prior to dispersal (Hulme and Benkman 2002). Nevertheless, there are important vertebrate pre-dispersal predators, especially birds and small mammals.
Post-dispersal seed predation is extremely common in virtually all ecosystems. Given the heterogeneity in both resource type (seeds from different species are available), quality (seeds of different ages and/or different status of integrity or decomposition) and location (seeds are scattered and hidden in the environment), most post-dispersal predators have generalist habits (Hulme and Benkman 2002). These predators usually belong to a diverse array of animals, such as ants, beetles, crabs, fish, rodents and birds. In addition, not only is the assemblage of post-dispersal seed predators diverse, but its composition varies considerably among ecosystems (Hulme and Benkman 2002).
The severity of predation rates also varies between pre- and post-dispersal predation. Although both types of predation can be extremely high, studies simultaneously evaluating both pre- and post-dispersal seed predation suggest that the latter is often more severe than the former (Hulme and Benkman 2002). However, seeds already dispersed and not consumed have a chance to germinate, and seeds consumed could die regardless of predation due to constraints imposed by unsuitable germination sites. On the other hand, all non-dispersed seeds predated would be no more than a waste of energy by the parent plant. Therefore, the demographic impacts of pre-dispersal predation on plant populations are likely to be stronger than the impacts of post-dispersal predation.
Effects on plant demography
The complex relationship between seed predation and plant demography is an important topic of plant-animal interactive studies. Plant population structure and size over time is closely associated with the effectiveness at which seed predators locate, consume, and disperse seeds. In many cases this relationship is dependent upon the type of seed predator (specialist vs. generalist) or the particular habitat in which the interaction is taking place. The role of seed predation on plant demography may be either detrimental or in particular cases actually beneficial to plant populations.
The Janzen-Connell model (Janzen 1970, Connell 1971) is one of the most important studies of how seed predation may influence plant demography. The Janzen-Connell model illustrates how seed density and survival respond to distance from the parent tree and differential rates of seed predation. Seed density is hypothesized to decrease as distance from the parent tree increases. Where seeds are most abundant under the parent tree, seed predation is predicted to be at its highest. As distance from the parent tree increases, seed abundance and thus seed predation are predicted to decrease as seed survival increases.
The degree to which seed predation influences plant populations may vary by whether a plant species is safe site limited or seed limited. If a population is safe site limited it is likely that seed predation will have little impact to the success of the population. In safe site limited populations increased seed abundance does not translate into increased seedling recruitment. However, if a population is seed limited, seed predation has a better chance of negatively affect the plant population by decreasing seedling recruitment. A study by Maron and Simms (1997) found both safe site limited and seed limited populations depending on the habitat in which the seed predation was taking place. In dune habitats seed predators (Deer mice) were limiting seedling recruitment in the population, thus negatively affecting the population. However, in grassland habitat the seed predator had little effect on the plant population because it was safe site limited.
In many cases seed predators support plant populations by dispersing seeds away from the parent plant. By dispersing seeds away from the parent plant the seed predator is in effect supporting gene flow between populations. Other seed predators collect seeds and then store or cache them for later consumption (Harper 1977). In the case that the seed predator is unable to locate the buried or hidden seed there is a chance that it will later germinate and grow, supporting the species dispersal.
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