Ecological Mechanisms that Contributes to Biodiversity

A number of simple and complex ecological mechanisms contribute to biodiversity. According to one view, the so – called local or deterministic view, the biodiversity is determined principally by biological interactions such as competition and predation.

The other view considers the importance of other environmental factors such as soil type, moisture, temperature, gradient, productivity, niche and habitat diversity and some other factors like stability, disturbance, immigration, extinction and species differentiation and movement at the regional level and the interaction between local and regional processes.

Thus, there are a variety of determinants of species richness, some of which are discussed below.

1. Competition:

The ecological effects of inter specific competition are too many. These effects, however, depend on the scale of competition. On a small scale we can observe the co-occurrence of only a few species with complementary ecological niches; but on a broad scale the community will contain more species, occurring within a patchwork, with each patch supporting only a few. In nature, competition is often avoided by differential resource utilization, e.g., different species of fish feeding at different depths.

When exotic animal or plant species are introduced to a new habitat, they sometimes prove to be better competitors and many indigenous species suffer. For example, introduction of common carp to some reservoirs of Punjab and elsewhere has adversely affected the native populations of major carps.

In brief, the significance of inter specific competition depends on how widespread are its evolutionary and ecological consequences. Inter specific competition tends to affect communities and their biodiversity in many ways some of which are less understood even today.

2. Predation:

Predation affects both prey populations and whole ecological communities. When predation promotes the coexistence of species among which there would otherwise be competitive exclusion, this is called predator- mediated coexistence. The effect of predation on a group of competing species depends on which species suffer most. If it is subordinate species, then these may be driven to extinction and therefore the total number of species in the community will decrease.

However, if dominant species suffer most, heavy predation will create space and resources for other species, consequently species numbers may then increase. The numbers of species in a community are usually maximum at intermediate levels of predation.

The effects of predation on biodiversity have been extensively studied in aquatic systems, in which the introduction of a predatory fish, starfish or salamander can greatly change the community structure of primary producers and consumers (carpenter et al. 1987,1988). The effect of herbivore on plant species diversity has also been studied.

The pest pressure hypothesis suggests that seedlings are most dense close to the parent tree, but their survival is maximum at a distance from the parent, because herbivores will be more common among the dense seedlings adjacent to parent tree. Several authors suggested that herbivore promotes more diversity in tropical forests (Clark and Clark, 1984).

3. Productivity:

Nutrient input and productivity affect biodiversity. Plant productivity often depends on the nutrient or condition which is most limiting to growth. Animal productivity also depends on resource levels and some other key factors such as temperature and moisture (for terrestrial environments) and temperature, dissolved oxygen and depth for aquatic systems. However, these are not the only factors that affect productivity.

It was suggested by Connell and Orias (1964) that biodiversity should be highest in relatively stable habitats having high productivity. This is called productivity-stability hypothesis (Tilman,1982). Tilman and Pacala (1993) suggested that biodiversity does not increase monotonically with productivity for any group of species, but that species richness varies depending on what environmental factor is used as a measure of productivity and which species are being taken into consideration.

It has been also suggested that predator-prey ratios increase with the increase in productivity. However, at high productivity levels, predators consume a disproportionate share of the available production, thereby causing a decline in community biodiversity. The intertaxon competition hypothesis (Rosenweig and Abramsk 1993) holds that the peaks of species diversity for different multispecies taxa should occur in areas showing different productivity levels. But, Tilman’s (1982) hypothesis suggested that habitat heterogeneity increases with productivity to a certain point only after which it decreases. However, increase in biodiversity with productivity is not a universal phenomenon.

4. Spatial Heterogeneity:

Spatially heterogeneous habitats offer a wide spectrum of resources and food chains. Accordingly, they are expected to support more species as they provide a greater variety of microhabitats (spatial niches), a greater range of microclimates, more types of places to hide from predators and a greater variety of trophic niches.

Gould and Walker (1997) have shown a positive relationship between number of vascular plant species and index of spatial heterogeneity (ranging from 0 to 1), based on a number of things including soil pH, slope, drainage pattern and substrate types. There are several studies indicating a positive relationship between animal species richness and plant spatial heterogeneity.

5. Climate:

The effects of climatic variation on species richness depend on whether the variation is unpredictable or predictable. In seasonally changing environment, different species may occur at different times of the year. Therefore, more species may be expected living together in a seasonal environment than a completely constant one.

For example, in temperate regions different annual plants germinate, grow, flower and produce seeds at different times during a seasonal cycle. However, there is no firm relationship between species richness and climatic instability. Stable climate is likely to support more species. Tropics often showing better climatic regulation are richer in species than temperate regions.

6. Harshness of Environment:

An environment may be called harsh or extreme if organisms are unable to live there. However, some organisms do occur in very cold and very hot environments and grossly polluted rivers and lakes. But the distribution pattern of organisms generally indicates that species richness is quite lower in harsh environments. Many studies have indicated that diversity of benthic macro-invertebrates and fish was quite low in streams and lakes having low pH.

Most caves and hot springs also exhibit low biodiversity. The deepest parts of the oceans (200 m to 8000 m) also have few species of fish such as tripod fish and lizard fish adapted to living in complete darkness, low temperature and very high pressures, some of them showing interesting specialization of eyes and luminous organs.

7. Disturbance:

Many communities experience periodic physical disturbance. Anthropogenic activities that alter habitat characteristics also affect local species diversity. The intermediate disturbance hypothesis (Connell, 1978) suggested that communities are expected to have more species when the frequency of disturbance is neither too high nor too low. This hypothesis was proposed to account for patterns of species richness in tropical rain forests and coral reefs.

In upland streams disturbances are created by a number of factors including water diversion for fishing or other purposes and quarrying. These activities affect indigenous fish species, benthic invertebrates and riparian vegetation. Sometimes flash floods with enormous gushing waters carrying tons of silt load play havoc with the natural communities as the entire river bed is destroyed and boulders and fish species are washed off by speedily flowing water (Singh and Badola, 1980, Sharma and Singh 1980). Disturbance by municipal sewage also affects biological diversity (Nautiyal et al 1996,2000).

Townsend et al (1997) observed that the pattern of richness of macro-invertebrate species conformed with the intermediate disturbance hypothesis. Disturbances often keep the community in early stages of succession and therefore poor diversity.

8. Other Factors:

Other factors and mechanisms that affect biodiversity are community succession, latitudinal gradient, altitudinal gradient and depth, immigration, emigration, extinction, and evolutionary age (time) and evolutionary adaptations.

The communities may differ in species diversity because some are closer to ecological or evolutionary equilibrium and others are still evolving. For example, tropics are richer in species than temperate regions because, among other things, tropics have existed over long periods of evolutionary time, whereas the temperate regions are still recovering from the Pleistocene glaciations.

Latitudinal gradients of diversity are quite obvious, species of plants, and animals increasing toward the equator. For example, within a small region at 60° north latitude one might found 10 species of ants; at 40°, there may be 50 to 100 species; and in a similar area within 20° of the equator, between 100 to 200 species (Ricklefs and Miller, 2000).

Diversity in marine environments follows a similar trend. This increase in diversity from the poles to the tropics has been attributed to a number of factors including greater predation, productivity, light, temperature and water regimes.

Altitudinal gradients (Singh and Nautiyal 1990) and depth also affect species richness. In hillstreams, low diversity of macro-invertebrates was observed at higher altitudes. In terrestrial environments, a decrease in species richness with altitude is a widespread phenomenon.

Bird, mammal and vascular plant species richness declined with the increase in altitude in Himalayan mountains of Nepal (Hunter and Yonzon, 1992; Whittaker,1977). Singh and Kumar (2003) found no fish species in Garhwal hillstreams at high altitudes (2400 to 3600 m). Number of species of phytoplankton, zooplankton and fish also tend to decline with increasing depth in the ocean.

Isolation and extinctions have also played their role in affecting species richness in different parts of this earth. The extinctions of many large animals in the Pleistocene may reflect the role of human migration. It is well known that over the past 30,000 to 40,000 years, a major loss of animal biodiversity has occurred over Australia, North America, New Zealand and Madagascar.