What do you mean by “Dispersal Agent” in Ecosystem?

Dispersal agents constitute a resource which, in any given community or ecosystem, may be present in abundance, or may be scarce. According to McKey (1975), in those plants which produce large numbers of small seeds, the fruits tend to be broad niched, i.e., eaten by a wide variety of animals. Thus there are enough dispersal agents available.

Examples of such plants are Ficus and Miconia. In contrast, those plants which produce small num­bers of large, nutritious seeds, tend to evolve towards dispersal by a small number of subset of specialized animals. Unlike the fruiting seasons of Ficus and Miconia, the fruiting seasons of plants belonging to this second category are often spread out over a relatively long period.

Image Source: jeb.biologists.org

Many plants of the Lauraceae, Burseraceae, and Palmae, for instance, have quite long fruit­ing seasons extending over a few months. According to McKey (1975):

(1) The limited number of dispersal agents may be easily overlooked by a mass- ripened fruit crop;

(2) Fewer species would be sharing the specialized frugivores, hence there would be fewer fruiting seasons with which each species must avoid overlap;

(3) intra-crop fruit-bearing synchrony would not be required for escape from seed predators, because these plants are usually not predator satiators; and

(4) Since a specialized frugivore must need ex­tract a balanced and adequate diet from the fruits available at any given time, its existence would require that several species of fruits, each provid­ing different nutrients, should have broadly overlapping fruiting seasons.

The above situation contrasts with that in plants with broad-niched fruits each adapted for exploitation by a variety of different animals. Since each broad- niched species utilizes a large proportion of the available dis­persal agents when it is fruiling, one would expect strong selection for fruiting seasons of such species to be displaced one from another (McKey, 1975).

Conversely, in a species whose fruiting season is squeezed between those of other species, one would logically expect selection for a broad- niched fruit so that adequate dispersal may be obtained in the fairly short intervening period available for fruiting.

Many plants belonging to the Melastomaceae and Moraceae, and some wind-pollinated plants belonging to families Meliaceae, Bignoniaceae and Bombacaceae, have r-selected reproductive strategies. This strategy is based on many small seeds which are not very nutritious and which are dispersed in a quantitative sense (not in a high quality maimer) by a variety of oppor­tunistic animals or by wind.

These plants in effect get low quality dispers­al. In contrast, those plants which have coevolved with specialized frugivores, have A’-selected reproductive strategies; they produce small crops of large but nutritious seeds which attract the high quality reliable dispersal service of the specialized animals.

Good examples occur in the family’s Lauraceae, Palmae, Burseraceae, Myristicaceae and Sapindaceae. In the r-selected species, the dispersal costs, per seed/fruit, to the plant are quite low. In the selected species, such costs, per fruit or seed, are quite high (as the plant must spend much energy to produce the single, large, protein and fat-rich seed).