Habitat and biodiversity loss One of the major ways that habitat fragmentation affects
biodiversity is by reducing the amount of suitable habitat available for organisms. Habitat fragmentation often involves both
habitat destruction and the subdivision of previously continuous habitat. Plants and other
sessile organisms are disproportionately affected by some types of habitat fragmentation because they cannot respond quickly to the altered spatial configuration of the habitat. Habitat fragmentation consistently reduces biodiversity by 13 to 75% and impairs key ecosystem functions by decreasing biomass and altering
nutrient cycles. This underscores the severe and lasting ecological impacts of fragmentation, which could be highlighted in the sections discussing the consequences of fragmentation. Habitat loss, which can occur through the process of habitat fragmentation, is considered to be the greatest threat to species. But, the effect of the configuration of habitat patches within the landscape, independent of the effect of the amount of habitat within the landscape (referred to as fragmentation per se), has been suggested to be small. A review of empirical studies found that, of the 381 reported significant effect of habitat fragmentation per se on species occurrences, abundances or diversity in the scientific literature, 76% were positive whereas 24% were negative. Despite these results, the scientific literature tends to emphasize negative effects more than positive effects. Positive effects of habitat fragmentation per se imply that several small patches of habitat can have higher conservation value than a single large patch of equivalent size.. Area is the primary determinant of the number of species in a fragment and the relative contributions of demographic and genetic processes to the risk of global population extinction depend on habitat configuration, stochastic environmental variation and species features. Minor fluctuations in climate, resources, or other factors that would be unremarkable and quickly corrected in large populations can be catastrophic in small, isolated populations. Thus fragmentation of habitat is an important cause of species extinction. Additionally, habitat fragmentation leads to
edge effects. Microclimatic changes in light, temperature, and wind can alter the ecology around the fragment, and in the interior and exterior portions of the fragment.
Fires become more likely in the area as humidity drops and temperature and wind levels rise. Exotic and pest species may establish themselves easily in such disturbed environments, and the proximity of domestic animals often upsets the natural ecology. Also, habitat along the edge of a fragment has a different climate and favours different species from the interior habitat. Small fragments are therefore unfavourable for species that require interior habitat. The percentage preservation of contiguous habitats is closely related to both genetic and species biodiversity preservation. Generally a 10% remnant contiguous habitat will result in a 50%
biodiversity loss. Much of the remaining terrestrial
wildlife habitat in many third world countries has experienced fragmentation through the development of
urban expansion such as roads interfering with
habitat loss. Aquatic species' habitats have been fragmented by
dams and
water diversions. These fragments of habitat may not be large or connected enough to support species that need a large territory where they can find mates and food. The loss and fragmentation of habitats makes it difficult for migratory species to find places to rest and feed along their migration routes. The existence of viable habitat is critical to the survival of any species, and in many cases, the fragmentation of any remaining habitat can lead to difficult decisions for conservation biologists. Given a limited amount of resources available for conservation is it preferable to protect the existing isolated patches of habitat or to buy back land to get the largest possible contiguous piece of land. In rare cases, a
conservation reliant species may gain some measure of disease protection by being distributed in isolated habitats, and when controlled for overall habitat loss some studies have shown a positive relationship between species richness and fragmentation; this phenomenon has been called the habitat amount hypothesis, though the validity of this claim has been disputed. The ongoing debate of what size fragments are most relevant for conservation is often referred to as
SLOSS (Single Large or Several Small). Habitat loss in a biodiversity hotspot can result in a localized extinction crisis, generally speaking habitat loss in a hotspot location can be a good indicator or predictor of the number of threatened and extinct endemic species. One solution to the problem of habitat fragmentation is to link the fragments by preserving or planting
corridors of native vegetation. In some cases, a bridge or underpass may be enough to join two fragments. This has the potential to mitigate the problem of isolation but not the loss of interior habitat. Wildlife corridors can help animals to move and occupy new areas when food sources or other natural resources are lacking in their core habitat, and animals can find new mates in neighbouring regions so that
genetic diversity can increase. Species that relocate seasonally can do so more safely and effectively when it does not interfere with human development barriers. Due to the continuous expansion of urban landscapes, current research is looking at
green roofs being possible vectors of habitat corridors. A recent study has found that green roofs are beneficial in connecting the habitats of arthropods, specifically bees and weevils. Another mitigation measure is the enlargement of small remnants to increase the amount of interior habitat. This may be impractical since developed land is often more expensive and could require significant time and effort to restore. The best solution is generally dependent on the particular species or ecosystem that is being considered. More mobile species, like most birds, do not need connected habitat while some smaller animals, like rodents, may be more exposed to predation in open land. These questions generally fall under the headings of
metapopulations
island biogeography.
Genetic risks As the remaining habitat patches are smaller, they tend to support smaller populations of fewer species. Small populations are at an increased risk of a variety of genetic consequences that influence their long-term survival. Remnant populations often contain only a subset of the genetic diversity found in the previously continuous habitat. In these cases, processes that act upon underlying genetic diversity, such as
adaptation, have a smaller pool of fitness-maintaining alleles to survive in the face of environmental change. However, in some scenarios, where subsets of genetic diversity are partitioned among multiple habitat fragments, almost all original genetic diversity can be maintained despite each individual fragment displaying a reduced subset of diversity.
Gene Flow and Inbreeding Gene flow occurs when individuals of the same species exchange genetic information through reproduction. Populations can maintain genetic diversity through
migration. When a habitat becomes fragmented and reduced in area, gene flow and migration are typically reduced. Fewer individuals will migrate into the remaining fragments, and small disconnected populations that may have once been part of a single large population will become reproductively isolated. Scientific evidence that gene flow is reduced due to fragmentation depends on the study species. While trees that have long-range pollination and dispersal mechanisms may not experience reduced gene flow following fragmentation, most species are at risk of reduced gene flow following habitat fragmentation. Inbreeding depression is associated with conservation risks, like local extinction.
Genetic drift Small populations are more susceptible to
genetic drift. Genetic drift is random changes to the genetic makeup of populations and leads to reductions in genetic diversity. The smaller the population is, the more likely genetic drift will be a driving force of evolution rather than natural selection. Because genetic drift is a random process, it does not allow species to become more adapted to their environment. Habitat fragmentation is associated with increases to genetic drift in small populations which can have negative consequences for the genetic diversity of the populations.
Adaptation In order for populations to evolve in response to natural selection, they must be large enough that natural selection is a stronger evolutionary force than genetic drift. Recent studies on the impacts of habitat fragmentation on adaptation in some plant species have suggested that organisms in fragmented landscapes may be able to adapt to fragmentation. However, there are also many cases where fragmentation reduces adaptation capacity because of small population size.
Examples of impacted species Some species that have experienced genetic consequences due to habitat fragmentation are listed below: •
Macquaria australasica •
Fagus sylvatica •
Betula nana •
Ochotona princeps •
Uta stansburiana •
Plestiodon skiltonianus In addition, when animals happen to venture into unknown areas in between fragmented forests or landscapes, they can supposedly come into contact with humans which puts them at a great risk and further decreases their chances of survival.
Predation behaviours Habitat fragmentation due to anthropogenic activities has been shown to greatly affect the predator-prey dynamics of many species by altering the number of species and the members of those species. The way in which fragmentation changes and re-shapes these interactions can occur in many different forms. Most prey species have patches of land that are a refuge from their predators, allowing them the safety to reproduce and raise their young. Human introduced structures such as roads and pipelines alter these areas by facilitating predator activity in these refuges, increasing predator-prey overlap. These features have allowed their natural predators, the wolf, and the black bear to more efficiently travel over landscapes and between patches of land. Occasionally it is used as a threat signal to signify an impending attack on territory. A large song repertoire can enhance a male's ability to survive and reproduce as he has a greater ability to defend his territory from other males, and a larger number of males in the species means a larger variety of songs being transmitted. Although there are some species which are able to survive these kinds of harsh conditions, such as, cutting down wood in the forests for
pulp and paper industries, there are animals which can survive this change but some that cannot. An example includes, varying
aquatic insects are able to identify appropriate ponds to lay their eggs with the aid of
polarized light to guide them, however, due to
ecosystem modifications caused by humans they are led onto artificial structures which emit artificial light which are induced by dry asphalt dry roads for an example.
Effect on microorganisms While habitat fragmentation is often associated with its effects on large plant and animal populations and biodiversity, due to the interconnectedness of ecosystems there are also significant effects that it has on the
microbiota of an environment. Increased fragmentation has been linked to reduced populations and diversity of fungi responsible for decomposition, as well as the insects they are host to. This has been linked to simplified food webs in highly fragmented areas compared to old growth forests. Furthermore, edge effects have been shown to result in significantly varied
microenvironments compared to interior forest due to variations in light availability, presence of wind, changes in precipitation, and overall moisture content of leaf litter. These microenvironments are often not conducive to overall forest health as they enable
generalist species to thrive at the expense of
specialists that depend on specific environments. For example, the mutualistic relationship between
Mesogyne insignis and
Megachile. A study has found greater
pollination and increased fruit production of
M. insignis in unfragmented forests verses fragmented forests. As for an example of an antagonistic relationship of nest predation, a study found that there is no increase in nest predation on fragmented forests - thus not supporting the
edge effect hypothesis.
Effect on ecosystem services Habitat fragmentation has profound effects on
ecosystem services, impacting nutrient retention, species richness, and local biophysical conditions. Fragmentation-mediated processes cause generalizable responses at the
population,
community, and
ecosystem levels, resulting in decreased nutrient retention. Furthermore, habitat fragmentation alters relationships between biodiversity and ecosystem functioning across multiple scales, affecting both the local loss of
biodiversity and the local loss of function. Overall, habitat fragmentation significantly disrupts ecosystem services by altering nutrient retention, biodiversity, and ecosystem functioning at various spatial and temporal scales. == Forest fragmentation ==