In a given ecological setting, some species dominate and influence the abundance of the other species in that ecology. Two pathways, which lead to dominance, exist: competition where the dominance species has superior access to the limited resources and low susceptibility to other processes that are not competitive. In the second pathway, the non-dominant species are normally very susceptible to the non-competitive processes. The former pathway is highly interactive and the dominant species suppresses its subordinates by limiting the resources available to them. Sustained dominance reduces the diversity of species that ecology. The second pathway is weakly interactive and the dominant species is not affected by the processes that are highly restrictive to the other species. The dominance occurs naturally and, therefore, if the restrictive processes are removed from the environment, diversity will increase.
Both pathways share similar factors: the species produce structurally similar communities with a few dominants and numerous subordinates; the interaction between the species is invariable in that competition limits reproduction which reduces dispersal and subsequently further competition, and in the two pathways, the dominant species acquires majority of the available resources. Dominance over the limited resources can occur in two ways: the dominant species exhibits superior competition on other species and the dominant species may be so abundant that it overshadows other species through non-interactive factors. In order to fully comprehend dominance by species, it is important to understand the correlation between factors that facilitate dominance and those that restrict the propagation of other species.
The knowledge of dominance is important to understand the causes and effects of exotic species invasion. Numerous cases of exotic species dominating over native species have been reported. Biological invasions may occur in two ways: the invasive species act as drivers where they dominate over the native species through competition or ecosystem function changes like nutrient cycling. As a result, they decrease biodiversity by preventing recolonization by native species. The invasive species subsequently alter how the ecosystem functions in their new location. The second way is whereby invasive species act as passengers. They exploit the gaps left after ecological disturbances have eliminated the native species. The invasive species do not result to the loss of biodiversity of alter the function of the ecosystem. With time the species either become subordinate or are lost from the communities. It is important to distinguish between passengers and drivers since the knowledge helps to predict the long-term effects of invasion. It may also be used to predict the restoration potential of the native ecosystems. As drivers, invasive species form permanent communities as seen in grassland species such as Agropyron cristatum. They suppress the native species and only allow for the propagation of the invader. On the other hand, invasive species that act as passengers may be replaced by the diversity of the native species through ecological restoration.
The grassland species Agropyron cristatum, which acts as a driver, inhibits the restoration of native species in Oregon. Grassland restorations were reviewed and it was noted that the invasive species excluded the native species that were planted. Grasslands dominated by invasive species are different from the native vegetation in certain aspects like productivity, nutrient cycling and disturbance regimes. The grassland species, Bromus inermis, is an example of an invasive species that acts as a passenger in Kansas. This species allows for the restoration of the native species in Kansas and Minnesota. Other examples include Dactylis glomerata and Poa pratensis on Vancouver Island. The removal of these species did not affect the on the native species re-introduced.
Whether invasive species dominate by competition or by non-interactive factors is still debatable. Competition alone cannot explain why some species remain rare. Non-interactive factors such as limited seed dispersal and demographic constrains offered a better understanding. For instance, dominant species of grasses in the savannah suppress the invasion of woody plants and facilitated the survival of other minor species following disturbances. Study reviews have also indicated that the relative species abundance is greatly influenced by environmental conditions such as fire suppression, than competition for resources. It, therefore, points out that the passenger model offers a better explanation to exotic dominance. However, competition still occurs and greatly influences the species abundance in the community.
Different species exhibit different modes of competitive interactions to increase their dominance and abundance. For instance, in grasslands, availability of light is a key limiting resource when no disturbances appear in the environment. Poa produces a thick layer of litter and foliage which limits the amount of light reaching other species. It is also able to regenerate through tillering. Dactylis grows at a higher rate than other species of grasses. It is therefore able to thrive quickly in case of the slightest disturbance. Mature Dactylis have a long life span and can tolerate competition because they have dense foliage and are quite tall.
Study reviews also indicated that limited seed dispersal restricted the distribution of the dominated species. Poor dispersal causes exist; however, they are poorly understood. Different species express different patterns of dispersal, which when restricted, limits the abundance of such species. For instance, long distance dispersal may be crucial for some species, e.g. black-tailed deer. The movement of their seeds in the savanna may be limited because of the suburban sprawl. Slow rates of dispersal may slow down the rate of establishment of a species in an already existing population.
Dominant invasive species may have a positive impact on the survival of the subordinate species and the sustainment of the community. In case of an imbalance the dominant species may facilitate the survival of seedlings in the environment. For instance, removal of the thick foliage of dominant species may increase the amount of light penetrating to the soil causing heat stress and other factors. This increases the mortality of both the exotic and the native species. Other dominant species suppress the invasion by other species. For instance, dominant species of grass in the savannah suppress the invasion of woodlands in their environment. This helps to preserve the ecosystem especially in cases where its loss would lead to adverse effects.
It is important to know the causes and effects of dominance by exotic species since they have the potential to invade and dominate native flora and change the function of the ecosystem. Exotic invasion as a driver (competitive) alone cannot be used to explain the dominance and abundance of exotic species in invaded communities. Invasion as a passenger (non-interactive) offers a more relevant explanation to dominance by exotic species. The invasive species utilize the gaps left by the native species and propagate in case of a disturbance. The species increase in abundance and dominate for some time. However, repeated disturbances displace these species and they become subordinate. The restoration of the native species is also possible.
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