Читать книгу North American Agroforestry - Группа авторов - Страница 86

It is important to review the theoretical basis for species coexistence before discussing the biophysical interactions among them. The competitive exclusion principle (CEP), termed by Hardin (1960), also known as the competitive displacement principle, Grinnell’s axiom, the Volterra–Gause principle, or Gause’s law, has been a cornerstone in ecological thinking regarding species coexistence for decades. The CEP is based on Gause’s (1934) contention that two similar species competing for the same resources cannot stably coexist. Competition between species may lead to three outcomes: (a) if there is no trade‐off between competitive abilities, competition will lead to competitive exclusion; however, (b) if there is a trade‐off between competitive abilities for different resources, where the stronger competitor of one resource also obtains relatively more of another resource in which the other species is a stronger competitor and vice versa, competition will lead to stable coexistence; or (c) if there is a trade‐off between competitive ability for resources and each species is a better competitor for a specific resource, competition will lead to alternative stable states (Passarge, Hol, Escher, & Huisman, 2006). In agroforestry systems, we deliberately mix tree and crop species so that they exert minimal or weak competition among themselves.

Even though there are examples of violations to the fundamental conditions of the CEP, using the concept as the initial inquiry of ecological thought has elucidated many other mechanisms that contribute to our knowledge of how species coexist in nature. For example, the resource‐ratio hypothesis, proposed by Tilman (1980, 1982, 1990), has been used to explain species coexistence (for a review of examples in which the resource ratio theory has been tested, see Miller et al., 2004). According to this hypothesis, coexistence occurs where resource requirements differ among species. Greater capture of a limiting resource would be accompanied by an increased ability to utilize nonlimiting resources, which, by definition, are available but underutilized. In an agroforestry setting, based on the differences in physical or phenological characteristics of the component species, the interactions between tree and crop species may lead to an increased capture of a limiting growth resource. The system as a whole could then accrue greater total biomass than the cumulative production of those species if they were grown separately on an equivalent land area (Cannell et al., 1996).

Hubbell (2001) has challenged the notion that trade‐offs are necessary for understanding broad patterns of species diversity and relative abundance. In contrast to trade‐off‐based theories, Hubbell developed a neutral model (united neutral theory) that explains plant species coexistence without any trade‐offs. Neutral theories focus on “community drift” and explain the maintenance of biodiversity at large spatial and temporal scales by a balance between speciation and stochastic extinction events. These are caused by random drifts in population size in communities of ecologically identical (hence neutral) species, that is, without invoking any species‐specific traits or interspecific trade‐offs.

Finally, spatially explicit models of plant species coexistence have been developed (e.g., Gravel, Mouquet, Loreau, & Guichard, 2010; Isabelle, Damien, & Wilfried, 2014). They do not require trade‐off or neutrality assumptions to explain plant species coexistence, and they predict coexistence if interactions among conspecifics (individuals of the same species) occur across larger distances than interactions among heterospecifics (individuals of different species). Moreover, they lend themselves to more direct experimental tests than the more general trade‐off or neutral theories.

Analysis of ecological interactions has shown both competitive and facilitative (complementary) interactions in agroforestry systems (Jose et al., 2004), which occur both above‐ and belowground (Ong, Corlett, Singh, & Black, 1991; Singh, Ong, & Saharan, 1989). As stated by Shainsky and Radosevich (1992), mechanisms of competition for resources should at least include documentation of: (a) depletion of resources associated with the presence and abundance of plants; (b) changes in physiological and morphological growth responses associated with changes in the resource environment; and (c) correlations between the presence or abundance of neighbors, depression in resource availability, and physiological performance. In contrast, according to Kelty (2000), facilitative interactions are those in which one species benefits another and occur under four mechanisms: (a) increased nutrient cycling efficiency, e.g., increasing N availability by planting an N₂–fixing species with non‐N₂–fixing species; (b) increased water and nutrient retention through improved soil structure; (c) increased water availability for understory species because of reduced evaporative demand or “hydraulic lift” of moisture from the lower levels in the soil by overstory species; and (d) decreases in productivity losses from insect pests, pathogens, and weeds.

Competition and facilitation are not necessarily independent of each other (Holmgren, Scheffer, & Huston, 1997); the balance between these factors may vary along a resource gradient (Brooker & Callaghan, 1998). Proper management of an agroforestry system that increases facilitative interactions and limits competitive interactions requires an understanding of the possible interactions in these systems. Therefore, an examination of both the effect that plants have on the shared resources and their response to the changed environment must occur in order for proper management to take place (Casper & Jackson, 1997; Goldberg, 1990).