Impact of forest fragmentation on biodiversity
The biodiversity hotspots of the globe contain a high degree of endemism and are undergoing gradual loss of habitats. Maximum portion of these hotspots are located in tropical forests, which are considered as the most endangered. Habitat fragmentation is one of the major causes of the biodiversity loss. Habitats can either disappear completely or they may become degraded and/or fragmented, both causing serious impact on biodiversity as well as ecosystem processes. Loss of natural forests and the fragmentation of remaining areas into progressively smaller patches is a significant global trend. The habitat fragmentation occurs in different ways, like in patches (e.g.), in waves (e.g. by urbanization) or linear (e.g. by construction of roads).
Tropical deforestation involves the conversion of continuous forest to the remnant of forest patches set in a matrix of non-forest vegetation. This manipulation of ecosystems has consequences for biodiversity at both landscape and fragment levels. The altered microclimate becomes unsuitable for certain species by reducing the fragment size further, increasing mortality rates near the edge and reducing recruitment to their populations. The tropical forest ecosystem is often characterized by a heavy dependency on mutualistic species interactions for its stability. Many plant species in the tropical forests are reliant on animals as agents of dispersal for either pollen or seeds or both. If habitat fragmentation causes the extinction of certain important pollinating or seed-dispersing animals, this severely limits regeneration of rare plant species and hence initiates an extinction vortex.
Both population size and species richness decreases as the habitat abundance decreases. Rare and patchily distributed species requiring a large range or specialist habitats seem particularly susceptible to fragmentation. With the decrease of habitat proportion, patch size decreases and between patches increases. Larger patches contain more species than do small patches. This occurs because small patches experience more extinctions (small populations are more vulnerable to chance events) and receive fewer immigrants. Patches that are more remote from the mainland or source population have fewer species because the extinction rate is the same but the immigration rate is lower.
Larger species may have trouble finding habitat in not sufficient density to support a home range in heavily fragmented forests. Factors such as fragment size, degree of isolation and time since excision from the continuous forest directly influence the biodiversity of a fragment. Species distribution patterns are usually patchy in the tropical forest landscape and this increases the likelihood of certain species being exterminated by fragmentation. As a fragment gets very small, populations fall below specific levels and extinction ensue. Small populations are more liable to fluctuations which inevitably include local extinctions; as they also tend to suffer from genetic drift and inbreeding.
The failure of many animals to move between fragments can also restrict the immigration of plant species when these animals include seed dispersers; gene flow is restricted if they are pollinators. If they do not cross open areas, they are unlikely to utilize fragmented habitats so the conservation value of isolated forest patches will diminish. Immigration is an important phenomenon for the maintenance of high local levels of diversity in tropical forests. In isolated fragments the rare species will die out relatively rapidly and not be replaced by other species because of a failure of immigration.
Edge phenomenon in the physical environment may have direct effects on the forest community. Fragment edges are inhospitable to a majority of forest species. If certain animal or plant groups are more susceptible to local extinction through fragmentation than others, a change in community structure within the fragment is highly likely, which may ultimately lead to further changes and more extinctions, producing second and higher order effects. The deforested matrix of a fragmented landscape is often dominated by alien species, because few of the native species are tolerant of the extremely exposed conditions in the cleared areas.
Habitat fragmentation, introduction of exotic species, and management of exploitable systems tend to decrease species richness and heterogeneity. The alteration of land use pattern results in fragmentation of habitats, ecosystems and landscapes in most parts of the world. Different studies show that all our natural old forests have become critically fragmented to the point where they are considered unlikely to maintain rich level of biodiversity, nor support viable populations of natural and native species of flora and fauna. Encroachment, clear felling, illegal logging, lopping, shifting cultivation, zhum cultivation, urbanization, industrialization, agroforestation, land use change and agricultural expansions are the major causes of forest fragmentations.
Abundant species has become occasional, occasional become rare, rare become very rare and very rare become extinct. Once upon a time, Sal forests of Bangladesh were sweet home of beautiful Capped Langur (“Sonamukhi Bandor” in local language). It lives in group and the home ranges are large. The Capped Langur group wakes with the dawn, but they remain in their sleeping trees until the sun has fully risen. They need undisturbed and continuous natural habitats to live. Now it is threatened by habitat loss and fragmentation, and has become an IUCN- red listed species.
Large-scale habitat fragmentation and decimation of prey populations are the major long-term threats to the existence of the Royal Bengal Tiger population in the Sundarbans. Less than a hundred years ago, tigers prowled all across the Indian sub-continent. But increasing habitat loss and fragmentation have contracted the tiger's former range. Tigers need large territories for roaming and preying.
The Population Viability Analysis (PVA) predicts a 95% decline in the population of Western Hoolock Gibbons in Bangladesh over the next two decades based on the current effects of human impacts and habitat fragmentations. Asian Elephant once roamed across Sylhet to Chittagong Hill Tract. Now it is the largest critically endangered terrestrial animal in Bangladesh. The major causes of the decline in the wild elephant population in Bangladesh are habitat fragmentation and destruction, expansion of agriculture and human settlement.
There has been rampant habitat loss of Marbled and Fishing Cat throughout the Sal forests over the past 20 years. They are secretive, elusive and arboreal in nature, relying on the treetop canopy for both shelter and food. Marbled Cat prefers to prey squirrels, fruit bat, mice and rats living on tree canopy. These cats are also intolerant of other human disturbances and abandon a forest that is even moderately disturbed. Fragmentation of forest throughout Southeast Asia is occurring at an alarming rate to the peril of biodiversity.
The core of 'The Convention on Biological Diversity (CBD)' is the promotion of an integrated approach to natural resource management on large landscapes and to biodiversity conservation through enhancing wildlife habitat and reducing habitat fragmentation. In this context, the writer likes to propose an integrated approach which will include: a) introducing biological corridors; b) maintaining buffer zones in between core and peripheral zones; c) preventing fragmentation of land blocks and ecosystems; d) developing restoration programme in conjunction with local communities; e) involving indigenous peoples and traditional communities in conservation programme; f) strengthening forest monitoring, research and development, education, and capacity building to maintain a “cradle” of biodiversity within the core areas of each protected forests; g) halting the continued introduction of alien invasive species; h) gap filling by rare tree species; i) aforestation and reforestation by native and natural species; j) facilitating natural regeneration in degraded forests; k) leaving denuded forest lands as untouched for 20 years to promote natural succession of forests; l) stopping further clear felling and illegal logging; m) protecting natural regenerations (seedling, sapling and juvenile trees) from cutting; n) introducing pioneer and early successional species in the degraded forests; o) taking effective actions against the encroachers and land grabbers; p) establishing gene banks to conserve the gene pool of endangered species; and p) bringing endangered animals in captivity for breeding.
Dr. Md. Mizanur Rahman is a biodiversity specialist, NDC, Jhalakathi Collectorate.
E-mail: mizan_peroj @yahoo.com