Seed predation

Seed predation, often also called granivory, is a type of plant-animal interaction in which seed predators granivores feed on the seeds of plants as a main or exclusive food source, in many cases leaving the seeds damaged and not viable. Granivores are found across many families of vertebrates (especially mammals and birds) as well as invertebrates (mainly insects); thus, seed predation occurs in virtually all terrestrial ecosystems.

Seed predation is commonly divided into pre-dispersal and post-dispersal predation, which affect the fitness of the parental plant and the dispersed offspring (the seed), respectively. The two types of predation may be mitigated by different strategies. To counter seed predation, plants have evolved both physical (e.g., shape and toughness of the seed coat) and chemical defenses (such as tannins and alkaloids). As plants have evolved seed defenses, seed predators have adapted in response (e.g., becoming able to detoxify chemical compounds). Thus, many examples of coevolution arise from this dynamic relationship.

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 are 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. 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. Seeds, especially in arid areas, may have a mucilaginous seed coat that can glue soil to seed hiding it from granivores.

Some seeds have evolved strong anti-herbivore chemical compounds. In contrast to physical defenses, chemical seed defenses deter consumption using chemicals that are toxic or distasteful to granivores or that inhibit the digestibility of the seed. These chemicals include toxic non-protein amino acids, cyanogenic glycosides, protease and amylase inhibitors, and phytohemagglutinins. Plants may face trade-offs between allocation toward defenses and the size and number of seeds produced.

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 of seed predation. 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 can regulate the size of the population of seed predators.

Adaptations to defend seeds against predation can impact seeds' ability to germinate and disperse. Thus anti-predator adaptations often occur in a suite of adaptations for a particular seed life history. For example, chili plants selectively deter mammal seed predators and fungi using capsaicin, which does not deter bird seed dispersers because bird taste receptors do not bind with capsaicin. Chili seeds in turn have higher survival if they pass through a bird's stomach than if they fall to the ground.

Seed predation can occur both before and after seed dispersal.