Seed predation
Seed predation includes any process inflicted on a plant’s seeds by an animal that results in the inviability of the seed. Generally this refers to the consumption and digestion of the seed, but also includes the parasitization of seeds by insect larvae (Janzen 1971). The high nutrient content of seeds makes them a valuable food source for many mammals, birds and insects. Seed predation is an important ecological process that can affect the reproductive success of individual plants, the dynamics of plant populations (Crawley 1992), and the evolution of defensive dispersal mechanisms and plant morphological traits (Schöning et al. 2004).
Seed predation may occur while seeds are still attached to the parent plant, or after they have been dispersed (Janzen 1971). General differences exist between pre- and post-dispersal seed predation in aspects such as the type of predator attending the seed (Crawley 1992) and the response of the plant to predation.
Pre-dispersal seed predation
Before dispersal, seeds are clustered in space and time, occurring in localised areas (i.e. on the plant) for relatively short periods of time (Crawley 1992). Additionally the presence of seeds on a plant may be advertised, intentionally or unintentionally, by the presence of flowers or fruits. Animals preying on undispersed seeds are typically small insects, such as flies, beetles, and moth larvae, with limited mobility (Crawley 1992). These predators are often specialist feeders, restricted to one or a few plant species. However, larger generalist species, such as birds and mammals, may also eat undispersed seed (Harper 1977).
In response to pre-dispersal seed predation, plants may produce far more flowers than they are capable of supporting to full fruit maturity. This enables them to selectively abort damaged fruits and seeds, to prevent the input of further resources towards seeds that will be unsuccessful (Crawley 1992).
Post-dispersal seed predation
Once seeds have been dispersed they present a very different resource to potential predators. Dispersed seeds occur at low density and are sparsely distributed, and may be very inconspicuous in the environment (Crawley 1992). Locating dispersed seeds is thus a very different process to locating undispersed seeds. Post-dispersal seed predators are usually larger and more mobile than pre-dispersal seed predators, and tend to be generalists, able to use most of the seed they encounter (Crawley 1992). Rodents, birds, and ants are all important post-dispersal seed predators.
Mast seeding may enable plants to avoid post-dispersal seed predation. This is where plants in a population all produce their seeds at the same time, and the timing of such seed production events varies between years, making it difficult for predators to predict (Kon et al. 2005). Mast seeding results in so much seed being produced at once that predators are unable to use all of it, so the remaining seed survives. Additionally, the periods of time when no seed is being produced may reduce predator population sizes due to the reduction in food available (Crawley 1992). The Japanese beech tree, Fagus crenata, is an example of a mast seeding species. In years when mast seeding does not occur, there is a high rate of seed predation (about 80% of seeds are damaged) but populations of seed predators are kept small by the reduced availability of food. In years where populations of Japanese beech produce 20 times more seed than in the previous season (i.e. mast seeding years), the reduced population of seed predators has a much smaller impact (about 30% of seeds are damaged) on the increased number of seeds produced (Kon et al. 2005).
Chemical defence of seeds
A common plant defence strategy is the production of chemical compounds toxic to herbivores (Janzen 1971). A wide variety of defensive compounds are used, and these may be present in any part of a plant. Plants may use the same compound in different structures, such as leaves and seeds. However, predators of leaves are likely to differ from seed predators, so different defensive compounds, specific to each type of predator, may be used in each type of structure (Janzen 1971). A chemical defence strategy may protect seeds both pre- and post-dispersal. The presence of toxic quinolizidine alkaloids in the seeds of Ormosia arborea did not prevent their being collected and hoarded by red-rumped agoutis (Dasyprocta leporina), but they were not eaten after hoarding due to their toxic content (Guimaraes Jr. et al. 2003). In this example, the plant achieves dispersal of its seeds by a potential predator.
Dispersal by seed predators
Many seed predators, particularly ants, birds and rodents, collect and store seed for later consumption (Harper 1977). If stores are surplus to the requirements of the predator, some of this seed may escape being eaten and be able to germinate. This will also depend on the type of store a seed finds itself in, as the stores of some predators are completely unsuitable for seed germination. For example the acorn woodpecker stores acorns in individual holes drilled in the trunks of trees (Harper 1977). However, seeds that are buried may have quite a good chance of germination, and may also enjoy benefits of dispersal, through being transported away from the parent plant by the predator (Harper 1977). Some plant species rely on seed predators for seed dispersal, and pay the cost of a certain level of seed mortality for the benefits gained through dispersal of just a few seeds (Crawley 1992).
See also
References
- Crawley, M.J. 1992. Seed Predators and Population Dynamics. In: Seeds: The Ecology of Regeneration in Plant Communities. Fenner, M. (ed.). C.A.B. International, Oxon, U.K.
- Guimaraes Jr., P.R., Jose, J., Galetti, M. and Trigo, J.R. 2003. Quinolizidine alkaloids in Ormosia arborea seeds inhibit predation but not hoarding by agoutis (Dasyprocta leporina). Journal of Chemical Ecology 29: 1065-1072
- Harper, J.L. 1977. Population Biology of Plants. Academic Press, New York, N.Y.
- Janzen, D.H. 1971. Seed Predation by Animals. Annual Review of Ecology and Systematics 2: 465-492.
- Kon, H., Noda, T., Terazawa, K., Koyama, H. and Yasaka, M. 2005. Evolutionary Advantages of Mast Seeding in Fagus crenata. Journal of Ecology 93: 1148-1155.
- Schöning, C., Espadaler, X., Hensen, I. and Roces, F. 2004. Seed Predation of the Tussock-grass Stipa tenacissima L. by Ants (Messor spp.) in South-eastern Spain: the Adaptive Value of Trypanocarpy. Journal of Arid Environments 56: 43-61.
