Seed predation, often referred to as granivory, is a type of plant-animal interaction in which granivores (seed predators) feed on the
seeds
A seed is an Plant embryogenesis, embryonic plant enclosed in a testa (botany), protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, includ ...
of
plants
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
as a main or exclusive food source,
[Hulme, P.E. and Benkman, C.W. (2002) "Granivory", pp. 132–154 in ''Plant animal Interactions: An Evolutionary Approach'', ed. C.M. Herrera and O. Pellmyr. Oxford: Blackwell. .] in many cases leaving the seeds damaged and not viable. Granivores are found across many families of
vertebrates
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, wi ...
(especially
mammals
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fu ...
and
birds
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
) as well as
invertebrates
Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
(mainly
insects
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs ...
); thus, seed predation occurs in virtually all terrestrial ecosystems. Seed predation is commonly divided into two distinctive temporal categories, pre-dispersal and post-dispersal predation, which affect the fitness of the parental plant and the dispersed offspring (the seed), respectively. Mitigating pre- and post-dispersal predation may involve different strategies. To counter seed predation, plants have evolved both physical defenses (e.g. shape and toughness of the seed coat) and chemical defenses (
secondary compounds such as
tannins
Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids.
The term ''tannin'' (from Anglo-Norman ''tanner' ...
and
alkaloids
Alkaloids are a class of basic, naturally occurring organic compounds that contain at least one nitrogen atom. This group also includes some related compounds with neutral and even weakly acidic properties. Some synthetic compounds of similar ...
). However, as plants have evolved seed defenses, seed predators have adapted to plant defenses (e.g., ability to detoxify chemical compounds). Thus, many interesting examples of
coevolution
In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well ...
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
The endosperm is a tissue produced inside the seeds of most of the flowering plants following double fertilization. It is triploid (meaning three chromosome sets per nucleus) in most species, which may be auxin-driven. It surrounds the embryo an ...
) that provide nutrients to the developing plant embryo (
cotyledon
A cotyledon (; ; ; , gen. (), ) is a significant part of the embryo within the seed of a plant, and is defined as "the embryonic leaf in seed-bearing plants, one or more of which are the first to appear from a germinating seed." The num ...
). 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
Mucilage is a thick, gluey substance produced by nearly all plants and some microorganisms. These microorganisms include protists which use it for their locomotion. The direction of their movement is always opposite to that of the secretion of ...
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 acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...
s,
cyanogenic glycoside
In chemistry, a glycoside is a molecule in which a sugar is bound to another functional group via a glycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides ...
s,
protease
A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the form ...
and amylase inhibitors, and
phytohemaglutinins.
[ 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 has the potential to regulate the size of the population of seed predators.
]
Seed predation vs. seed dispersal
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
Capsaicin (8-methyl-''N''-vanillyl-6-nonenamide) ( or ) is an active component of chili peppers, which are plants belonging to the genus ''Capsicum''. It is a chemical irritant for mammals, including humans, and produces a sensation of burning ...
, which does not deter bird seed dispersers because bird taste receptors do not bind with capsaicin
Capsaicin (8-methyl-''N''-vanillyl-6-nonenamide) ( or ) is an active component of chili peppers, which are plants belonging to the genus ''Capsicum''. It is a chemical irritant for mammals, including humans, and produces a sensation of burning ...
. Chili seeds in turn have higher survival if they pass through a bird's stomach than if they fall to the ground.
Pre- and post-dispersal
Seed predation can occur both before and after seed dispersal.
Pre-dispersal
Pre-dispersal seed predation takes place when seeds are removed from the parent plant before dispersal, and it has been most often reported in invertebrates, birds, and in granivorous rodents that clip fruits directly from trees and herbaceous plants. Post-dispersal seed predation arises once seeds have been released from the parent plant. Birds, rodents, and ants are known to be among the most pervasive postdispersal seed predators. Furthermore, postdispersal seed predation can take place at two contrasting stages: predation on the "seed rain" and predation on the "seed bank". Whereas predation on the seed rain occurs when animals prey on released seeds usually flush with the ground surface, predation on the seed bank takes place after seeds have been incorporated deeply into the soil.[ Nevertheless, there are important vertebrate pre-dispersal predators, especially birds and small mammals.
]
Post-dispersal
Post-dispersal seed predation is extremely common in virtually all ecosystems. Given the heterogeneity in both resource type (seeds from different species), 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.[ These predators belong to a diverse array of animals, such as ants, beetles, crabs, fish, rodents and birds. The assemblage of post-dispersal seed predators varies considerably among ecosystems.][ A dispersed seed is the first independent life stage of a plant, thus post-dispersal seed predation is the first potential mortality event and one of the first biotic interactions in a plant's life cycle.
]
Differences
Both pre- and post-dispersal seed predation are common. Pre-dispersal predators differ from post-dispersal predators in most often being specialists, adapted to clustered resources (on the plant). They use specific cues like plant chemistry (volatile compounds), color, and size to locate seeds, and their short life cycles often match the production of seeds by the host plant. Insect groups containing many pre-dispersal seed predators are Coleoptera
Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describ ...
, Hemiptera, Hymenoptera and Lepidoptera.[
]
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 depends on 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 concerns 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.[Connell, J.H. (1971) "On the role of natural enemies in preventing competitive excusion in some marine animals and in rainforest trees", pp. 298–312 in ''Dynamics of Populations'', ed. P.J. den Boer and G.R. Gradwell. Wageningen: Center for Agricultural Publishing and Documentation.]
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 affecting the plant population by decreasing seedling recruitment. Maron and Simms 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, in effect supporting gene flow between populations. Other seed predators collect seeds and then store or cache them for later consumption.[Harper, J. L. (1977) ''Population Biology of Plants'', New York: Academic Press.] 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. Generalist (vertebrate) seed predators may also aid the plant in other indirect ways, for instance by inducing top-down control on host-specific seed predators (termed "intra-guild predation"), and as such negating Janzen-Connell type effects and so benefiting the plant in competition with other plant species.
See also
* Consumer-resource systems
* Egg predation
Egg predation is a feeding strategy in many groups of animals (ovivores) in which they consume eggs. Since an egg represents a complete organism at one stage of its life cycle, eating an egg is a form of predation, the killing of another org ...
* Harvester ant
Harvester ant, also known as harvesting ant, is a common name for any of the species or genera of ants that collect seeds (called seed predation), or mushrooms as in the case of '' Euprenolepis procera'', which are stored in the nest in commun ...
* Herbivory
A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthp ...
* Seed dispersal
In Spermatophyte plants, seed dispersal is the movement, spread or transport of seeds away from the parent plant.
Plants have limited mobility and rely upon a variety of dispersal vectors to transport their seeds, including both abiotic vecto ...
References
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{{Feeding
Herbivory
Plant reproduction
Animals by eating behaviors