Habitat Selection in Plants

Habitat-selection concepts have rarely been explicitly used for plants, perhaps because the majority of them are immobile. For plants, habitat selection results from evolutionary adjustment of species to environmental factors so that the species functions better in some habitats than in others. Habitat choice refers to the ability of a plant to disperse, in space or time, to preferred patches. Habitat specialization means that a species performs best in a small subset of patches in a given location. The modular structure of plants and their growth response to the patterns of resource availability allows them to occupy large areas, exposing them to spatial and temporal heterogeneity. Modular structure, to a large extent, determines the options available, as well as the constraints, for plants choosing habitats. Choice, however, may be made by the habitat rather than by the plant. That is, the characteristics of the habitat determine which species of the plants that disperse into the habitat become established in that particular habitat. For plants, it is likely that choice is made proximally by the habitat but ultimately by the plants through natural selection. "Choice" habitats for plants (1) supply resources (e.g., light, water, nutrients, etc.) in sufficient quantities for growth and reproduction; (2) provide pollinators, dispersers, and other symbionts; and (3) have few herbivores, predators, and pathogens, except those that afflict their competitors. Because similar individuals prefer these sites, there can be intense competition among neighbors. Thus, the forces of preference for these choice habitats and habitat selection to reduce competition may oppose one another. Plants have several mechanisms that improve their ability to reach appropriate habitats and have evolved several attributes that allow them to function in a changing local environment despite their immobility. These mechanisms include (1) wide dispersal of many propagules, (2) targeted dispersal to favorable habitats or away from unfavorable ones, (3) dispersal with a supply of some required resource(s), (4) location of foraging organs in habitat patches where resources are less contested by neighbors, (5) morphological and physiological integration of several patch types, and (6) actual movement to choice habitats by clonal spread and fragmentation. The above mechanisms are forms of spatial habitat choice. Habitat choice occurs temporally also. Plant life-history events are timed to coincide with the availability of necessary resources for growth. For example, separation of seasonal and daily flowering phenology between species that require, and may compete for, specific pollinators may be a form of habitat selection. Other mechanisms of temporal habitat choice include (1) the controls on seed dormancy and germination, (2) the evolution of secondary dormancy in plants of unpredictable habitats, (3) the germination of certain desert species only when enough rain for the completion of the life cycle has fallen, and (4) the timing of fruit ripening to coincide with the arrival of their dispersers. Lacking mobility, and therefore being unable to move to a more hospitable location, many plants have evolved high acclimation potential and phenotypic plasticity to deal with local environmental variability. Their attributes, together with ontogenetic response shifts, lessen the need for precise habitat choice. Habitat selection is intimately tied to niche differentiation and coexistence in plant communities, a controversial area of ecology and evolutionary biology. Therefore, progress in one area will invariably lead to a better understanding of the other.