Chin J Plan Ecolo ›› 2002, Vol. 26 ›› Issue (增刊): 1-8.

### ON MECHANISMS OF SPECIES COEXISTENCE IN PLANT COMMUNITIES

HOU Ji-Hua, MA Ke-Ping

1. Laboratory of Quantitative Vegetation Ecology, Institute of Botany, the Chinese Academy of Sciences, Beijing 100093, China
• Online:2015-03-09 Published:2002-09-01

Abstract:

Abstract Species coexistence, a key question in plant community ecology, means the abundance or the number of species occurring in a certain time at a given community. Zobel emphasized that species coexistence should be determined by the processes of the evolutionary, historical and ecological scale. So far, many theories explaining the mechanisms of species coexistence at certain levels and under particular conditions exist. This review outlines some important theories, such as species pool hypothesis, regeneration niche theory, and competition theory, to elucidate species coexistence.
The term “species pool” was introduced to indicate that a set of species is potentially capable of coexisting in a particular community. Species pool plays an important role in species coexistence and abundance. The species pool theory proposed that the number of species occupying a certain habitat should be determined by characteristics of the particular habitat. The larger the area that a habitat type occupies, the greater the opportunity for speciation, and hence the larger the number of available species adapted to that particular habitat. Further, the availability of species also depends on historical processes at a bio-geographical scale, which determine the migration of species among regions, and the dispersal of species between and within local populations. Species pool theory explains the origination of species coexistence on evolutionary scale, but itself works in ecological scale.
Since traditional niche theory cannot effectively explain the numerous species coexisting in tropical forest and temperate grasslands, alternative niche theories are blooming. The regeneration niche theory is, in principle, a compromise between the niche differentiation and balanced competition, thus the species differing in their requirements for seed production, dispersal, germination and so on can coexist. Resource ratio/heterogeneity hypothesis, a promising theory, gives the possible solution by considering ratios of limiting nutrients to their absolute amounts.
Competitive species can coexist. If the species have similar competitive abilities, competitive exclusion might not occur at all, or occur at such a low frequency that it takes a long time for compensatory processes (eg. evolutionary change) to operate. Some factors, such as disturbance and herbivory, can mainly influence the dominant species, thus shifting competitive advantage to inferior species, and there are more chances for the superior and inferior species to coexist. The storage effect, where reproductive potential can be “stored” through unfavorable periods, promotes the coexistence of competing species under fluctuating environmental conditions. The distribution pattern of species, patchiness of environment and resource, and harsh and fluctuating conditions can also favor the competing coexistence of species.
Under non-equilibrium circumstance, species coexistence can be mediated by two main factors. One is disturbance, which interrupts the development of communities, prevents resource exploitation by overgrowth species, and avoids competitive exclusion. In the meantime, disturbances can create temporal and spatial heterogeneity, and provide new resource axes for niche differentiation. The other is the biotic agent. Biotic interactions among and within nutritional ranks can affect the coexistence of competing species. Janzen-Connell suggests that host-specific pests reduce recruitment of conspecific adults where conspecific seed density is greatest, thus free space is occupied by other plant species, which is density- or distance-dependent. The herbivore’s action can change the growth rate, growth form and growth rhythm of plants so that plant species produce the capture ability of resources and competing ability (exploiting resources or enduring resource stress). In addition, microorganisms play some role in plant species coexistence.
The unified neutral theory, seeking to unify both the number of species in a community and the distribution of the relative abundances of those species, assumes that every individual in every species in a biological community is identical, and that the total abundance of all species is fixed. All changes in distribution and abundance occur because of purely random variation in births, deaths, migration and speciation. This theory can accurately predict many attributes of ecological communities — particularly the distribution of abundances of tree species.
All the above mechanisms for plant species coexistence are not incompatible, but complementary, and it seems that plant species coexist within a community for different reasons. More investigation should be carried out on these reasons.

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