Quantitative and Qualitative Components of Seed Dispersal Effectiveness

The effectiveness of a disperser is defined as the contribution that it makes to plant fitness, or in other words, the number of new adult plants produced by the activities of a disperser relative to the number produced by other dispersers. This term has both quantitative and qualitative components, and both need to be considered to assess how effective a disperser is for a given plant; it can be considered at a variety of scales from individuals to communities. The quantity component is dependent upon the number of visits made to the plant by the disperser, upon the number of seeds dispersed in each visit, and is influenced by factors that are either intrinsic to the plant (e.g., size, fruit crop size, pulp/seed ratio) or extrinsic to it (e.g., neighborhood of conspecifics, surrounding vegetation, fruit crops of neighbors) and also by the biology of the dispersers (fruit-handling methods, degree of generalism in feeding, etc.). By contrast, the quality of seed dispersal, usually more difficult to evaluate, is a function of (1) the quality of the dispersed seed (often associated with fruit and/or seed size and rather variable within an individual plant, and influenced by factors such as number of seeds/fruit); (2) the quality of seed treatment in the digestive tract of the disperser, in turn dependent upon traits like seed coat thickness, chemical composition of pulp, gut passage time, morphology and physiology of the digestive tract, type of food ingested along with seeds, etc.; and (3) the quality of the microhabitat where the seed is deposited, which will ultimately determine the probability of germination and establishment; the sites where the seeds are deposited will be determined by factors such as frugivore movements after fruit removal, frugivore habitat preferences, etc., while the quality of the microsite will depend on abiotic (light levels, soil texture and humidity, etc.) and biotic conditions (levels of predation, competition, her-bivory, etc.).

The successfulness of recruitment resulting from frugivore activity, thus, depends on what type of fruit is selected, how it is processed, and the movements of the dispersers, and is further determined by the biotic and abiotic factors prevailing in the recipient microhabitat where the seed is dropped. Considering all this is crucial if we are to assess the demographic and evolutionary consequences of frugivore activity. With the available information, we know that the effects of the qualitative components of dispersal may erase the initial differences among dispersers in their quantity component, but more studies are needed to know which is more important determining the final pattern of plant recruitment.

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