Seed Predator and Disperser Functional Groups

Fruits and seeds are highly nutritive food resources as a consequence of plant provision for germination and, often, attraction of dispersal agents. A wide variety of animals feed on fruits or seeds. For example, Turgeon et al. (1994) reported that more than 400 species of insects, representing seven orders, feed on conifer cones, seeds, or both. Some species are obligate fruit- or seed-feeders, whereas others feed primarily on other resources but exploit fruits, seeds, or both when available.

Seed dispersal can be accomplished through both abiotic and biotic mechanisms. Abiotic dispersal involves wind and water; biotic dispersal involves auto-genic mechanisms, such as explosive fruits, and various animal agents, including insects, fish, reptiles, birds, and mammals. Dispersal by animals usually is a consequence of frugivory or seed predation, but some species acquire seeds or spores through external attachment by various kinds of clinging devices (e.g., sticky material or barbed spines). Seeds of a majority of plant species are dispersed by animals in many ecosystems (Howe and Smallwood 1982).

Seed predator and seed disperser functional groups can be distinguished on the basis of consumption of fruits or seeds versus transport of seeds. Frugivores feed on fleshy fruits and may terminate fruit or seed development (Sallabanks and Courtney 1992), but many vertebrate frugivores (including fish, reptiles, birds, and mammals) consume entire fruits and disperse seeds that are adapted to survive passage through the digestive tract (Crawley 1989, de Souza-Stevaux et al. 1994, M. Horn 1997, Sallabanks and Courtney 1992,Temple 1977). Seed predators include a number of insect, bird, and rodent species that consume seeds where found. Some seed predators eat the entire seed (e.g., vertebrates and ants), but others penetrate the seed coat and consume only the endosperm (e.g., seed bugs, Lygaeidae and Coreidae, and weevils, Curculionidae) or develop and feed within the seed (e.g., seed wasps, Torymidae, and seed maggots, Anthomyi-idae) (J. Brown et al. 1979, Crawley 1989, Louda et al. 1990b, Schowalter 1993, Turgeon et al. 1994). Seed cachers eat some seeds and move others from their original location to storage locations. Although ants and rodents are best known for caching seeds (J. Brown et al. 1979), at least one carabid beetle, Synuchus impunctatous, caches seeds of Melampyrum in hiding places after consuming the caruncle at the end of the seed (Manley 1971). Seed vectors include primarily vertebrates that carry seeds adapted to stick to fur or feathers. Insects generally are too small to transport seeds in this way but often transmit spores of microorganisms adapted to adhere to insect exoskeletons or pass through insect digestive systems.

These functional groups can be subdivided on the basis of predispersal or postdispersal seed predation, seed size, etc. Predispersal frugivores and seed predators feed on the concentrated fruits and seeds developing on the parent plant, whereas postdispersal frugivores and seed predators must locate scattered fruits and seeds that have fallen to the ground. Rodents and birds usually exploit larger seeds than do insects, and species within taxonomic groups also partition seeds on the basis of size (e.g., J. Brown et al. 1979, Davidson et al. 1984, Whitford 1978). Vertebrates are more likely to disperse seeds from consumed fruits than are insects, which (because of their small size) usually feed on portions of fruits and on or in seeds. However, dung beetles and ants may be important secondary dispersers, redistributing seeds from animal dung (Andresen 2002, Martínez-Mota et al. 2004). Insects, especially ants, are more likely to disperse small seeds, particularly of plant species adapted for dispersal by ants (myrmecochory).

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