Question

Discuss fire as a mechanism of secondary succession and how a natural fire can enhance species diversity of a forest. Use the Intermediate Disturbance Hypothesis.

Answer 1

The processes responsible for disturbance of existing vegetation are referred as secondary processes of secondary succession. It occurs on pre-existing soil.

Secondary succession always starts by an event such as forest fire, harvesting, hurricane or natural calamity. Loss of organic resources in ecosystem occurs due to fire.

Intermediate disturbance hypothesis advocates that local species diversity is maximized when ecological disturbance is neither too rare nor too frequent.

Due to fire many species are at risk of extinction. As per this theory, at intermediate levels of disturbance due to fire, diversity of species is increased at greater level since species that grow well at both early and late successional stages can coexist in that ecosystem.

This hypothesis describes the relationship between disturbance and species diversity.

Ecological disturbances like fire have most important effects on species richness in particular area of disturbance.

Due to competition between species, one species drive a competitor to extinction and become dominant in the ecosystem. Adequate ecological scale disturbances inhibit competition between species.

After fire in forests, carbonates produced from burnt plant material are mixed with soil. It results in initial increase in soil pH and has impact on rate of secondary succession, in addition to types of organisms will be able to succeed.

Fire also affects secondary succession by soil microorganisms. Some fungal species like Trichoderma and Penicillium have considerably reduced success rate in spore germination within fire-affected areas, declining their ability to recolonize.

After fire early successional species disperse and launch first. It is later followed by late successional species.

Species which are more flammable are considered as fire intolerant because they are desolated by fire. In this situation more tolerant species are able to survive or disperse in fire.

After fire incidence deadwood and snags are formed in forests which create habitat and resources for a variety of species. Fire also performs as a seed dispersing stimulant.

Numerous species need fire events to reproduce, disperse and establish like Pinus attenuate in which closed cones are open and seeds are dispersed when cones are exposed to heat produced by forest fires.

Fire diminishes previously established ecosystem to a smaller population of species. Fire not only burns existing species but also seeds of established species.

Due to natural fire dominant species in that ecosystem is reduced and now it is an opportunity for other species to grow on the same land where there is no competition with dominant species.

In the forest plots which are burned due to fire plant species like Melastoma, Eupatorium, Ficus etc. are very common as they strongly increase with the age of regeneration. By this way species diversity is enhanced.