Habitat fragmentation involves alteration of habitat resulting in spatial separation of habitat units from a previous state of greater continuity.
|Figure 1. Aerial photograph of dry forest scrub in southern Zambia, fragmented by agricultural land conversion. 2008. Source: C.Michael Hogan|
This phenomenon occurs naturally on a geologic time-scale or in unusual and catastrophic events: however, since the Holocene era, humans have produced dramatic and swift transformation of landscapes throughout the world, resulting in a level of habitat fragmentation that has induced worldwide reduction in biodiversity and interuption of sustainable yields of natural resources.
Humans produce habitat fragmentation chiefly from agricultural land conversion, urbanization, pollution, deforestation and introduction of alien species; ironically, both human caused wildfires as well as the systematic practice of fire suppression can also create habitat fragmentation. Prior to the dominance of mankind, long term changes engendered by geologic processes or climate oscillations contributed to habitat fragmentation.
|Figure 2. The Great Wall of China has existed for two millennia as one of the largest man-made habitat fragmentation constructs. Source: C.Michael Hogan|
Habitat fragmentation can manifest in an endless array of geometries, depending on the shape and extent of the separation zone.
It is important to note that the separation distance required for effecting fragmentation may vary considerably depending upon the dynamics of reproduction of key species involved. These factors include such spatially related parameters as distance typically traveled for faunal mating, seed dispersal radii, seasonal migration patterns and diurnal faunal foraging. In general, the smallest of these characteristic distances must be regarded as the controlling factor in order to respect the integrity of the ecosystem.
There is a sizeable suite of geometric measures that can be useful in describing the patch geometry of a fragmented habitat; some of these major factors are patch area, number of patches, ratio of patch size distribution and the patch edge length to area ratio. Fundamentally, the risk of extinction from habitat fragmentation generally increases with terrestrial animal size, since home range and migration needs are largest; however, small terrestrial fauna and plants with compact seed dispersal patterns are vulnerable to very small separations of habitat patches.
Natural processes of fragmentation
|Figure 3. Colorado River viewed from Dead Horse Point, Utah. The canyon depth here is approximately 600 meters, where the river has gradually cut a wide separation of the original continuous habitat of the Colorado Plateau. Source: C. Michael Hogan|
The chief natural phenomena that have driven fragmentation are glacial advances, volcanic activity, geologic faulting, tectonic movement, mass land slumping, serpentinization, major sea level rise and climate oscillation. Each of these actions has the potential to create irreversible effective isolation of previously connected habitat units; note that, for example, climate oscillations or minor glacial advances lasting only a few centuries have a reasonable probability that the landscape will revert, since mass extinctions are not necessarily produced from natural oscillatory functions having an effective time scale this small, especially since regional refugia can mitigate losses of such scale.
Major glacial advances may have taken tens or hundreds of thousands of years, such that the resulting habitat fragmentation is likely to have translated into new speciation as well as extinction of populations that were driven below minimum viable population size. One notable example of long timescale fragmentation on a large scale is the Andean uplift in the Amazon Basin. In this pre-Pleistocene epoch topographic change occurred so slowly that the uplift engendered further speciation and actually enhanced biodiversity.
Habitat fragmentation is a significant cause of biodiversity destruction. Research has demonstrated that fragmentation characteristically reduces species richness and taxon diversity, and may reduce the efficacy of ecosystem functioning. Fragmentation not only reduces the amount of functional habitat, but it may isolate a species population into subpopulations, that may be sufficiently near the minimum viable population size to risk local extinction from successive demographic processes or catastrophic events. The mechanics of these impacts often relate to the alteration of relationships among species. In some cases the population of certain species may actually increase within the fragmented habitat complex; however, these few increases are typically already dominant or keystone species, and such increases are usually at the expense of reducing populations of (if not elimination of) other species. With an original habitat becoming fragmented, some species have insufficient dispersal robustness to travel among the fragmented patches. In these cases such taxa may suffer from genetic drift or inbreeding due to restricted gene flow, and may have difficulty in re-colonizing or rescuing a subpopulation from local extirpation. Even if a given species has dispersal strength, it may suffer from insufficient dispersal and survival of taxa with which it interacts.
Considerable research has been conducted on species impacts to vertebrates, being macroscopically observable in the landscape, and on flora, since they are somewhat stationary whilst being analyzed. Notably there is a lack of data on arthropods, which comprise most of the extant biomass of our planet. Furthermore, the position of arthropods within an ecosystem place them in a role of considerable influence on the entirety of ecosystem services. Conservation biologists have developed the concept of habitat corridors as partial mitigation for the adverse impacts of habitat fragmentation.
The adverse biodiversity impacts can be extended in time consequence. For, example Lovejoy and Hannah note that the massive pre-1850 New Zealand and Australian deforestation by aborigines and Europeans continues to express and magnify its adverse manifestation on the landscape. The same authors make the interesting conjecture that the presently extant species which have successfully survived the dramatic climate fluctuations of the Quaternary (with attendant large swings in species populations) may be somewhat immunized to future climate oscillations.
One of the most widely studied examples of habitat fragmentation is in the Central Amazon Conservation Complex of Brazil, where a number of controlled studies were conducted in the 1980s. Due to pressures of an expanding human population and associated economic pressures, the Brazilian government embarked on a permissive policy of systematic and large scale forest destruction. In one study north of Manaus these residual patches were variously created in one, ten and 100 hectare sizes. Resident bird species, both frugivores and insectivores, were studied over a seven year period post-clearing. Results showed that bird densities declined in residual patches, with the smaller patches suffering the greatest species loss, even though initial response of small patches showed fewer initial losses as measured by bird density. An even larger long time scale loss of species richness was evident from the century long trend of deforestation in the area, since many of the plots cleared from 100 years ago were enjoying no economic use, but did not effectively rebound with complete forest cover; in a certain number of cases reasonable regrowth of secondary forest occurred. The conclusion drawn is that the ecological damage from long term deforestation and resulting habitat fragmentation has been disproportionately related to the actual economic taking of forest.
A small scale example of grassland fragmentation has yielded considerable evidence of biodiversity change. While dominant grass species were not severely altered, there were significant changes in arthropod and other faunal characteristics. The target location were calcareous grasslands in central Europe, that are high in species richness. This class of European grasslands often include considerable hectarage that have been partially cultivated by humans, are which are, in fact, effective refugia for grass and forb taxa that might otherwise have become extinct. Ongoing threats to further fragmentation are over-fertilisation and ironically reforestation and abandonment. That is to say, these human induced constructs contain many species that are now dependent on man's continued tending as these units co-evolved with the advent of agriculture in the early Holocene. Braschler found that butterfly species richness declined in the face of habitat fragmentation. While populations of certain dominant ant species (notably Lasius paralienus) as well as certain aphid taxa increased, these increases came at the expense of population losses of numerous rarer taxa. The implications of abetting an already dominant arthropod taxon at the diminution of a plethora of other species suggests adverse impacts of habitat fragmentation upon biodiversity..
The Three Gorges Dam on China's Yangtze River represents the largest scale anthropomorphic intrusion into freshwater habitat in history. This aquatic barrier is a threat to the survival of numerous fish species and other aquatic biota. Besides the obvious impact to migratory species that utilize river reaches above and below the dam, there are extensive impacts to turbidity and hydrological characteristics that alter the natural habitat of hundreds of species. There were 162 endemic fish species recorded prior to dam development, 44 of which are endemic to the Yangtze Basin. Severing the upstream and downstream portions of the river by dam construction is expected to threaten the survival of 20 fish species, with six of them having a high probability of extinction. In addition to severing the aquatic habitat, the dam construction also severs the riparian zone on both sides of the river with dam anchorages and other industrial infrastructure, leading to fragmentation of that terrestrial habitat.
- ^Virginia H. Dale and Scott Pearson. 1997. Quantifying Habitat Fragmentation Due to Land Use Change in Amazonia. In "Tropical Forest Remnants: Ecology, Management, and Conservation of Fragmented Communities", eds William F. Laurance, Richard O. Bierregaard, University of Chicago Press, 1997 ISBN: 0226468992
- ^J.L. Patton, M.N.F. Da Silva and J.R. Malcolm. 2000. Mammals of the Rio Jurua and the evolutionary and ecological diversication of Amazonia. Bulletin of the American Museum of Natural History 244
- ^D. Simberloff. 2000. What do we really know about habitat fragmentation. Tex. J. Sci. 52 (Suppl.):5-22
- ^A. Kruess and T. Tscharntke. 1994. Habitat fragmentation, species loss and biological control, Science 264:1581-1584
- ^Thomas E. Lovejoy and Lee Jay Hannah. Climate Change and Biodiversity,Yale University Press, 2006 ISBN: 0300119801
- ^Richard O. Bierregaard, Jr. and Philip C. Stouffer. 1997. Understory Birds and Dynamic Habitat Mosaics in Amazonian Rainforests. In "Tropical Forest Remnants: Ecology, Management, and Conservation of Fragmented Communities", eds William F. Laurance, Richard O. Bierregaard, University of Chicago Press, 1997 ISBN: 0226468992
- ^Brigitte Michele Braschler. 2005. Effects of experimental small-scale grassland fragmentation on the population dynamics of invertebrates. Doctoral Dissertation. Universitat Basel
- ^Young-Seuk Park, Jianbo Chang, Sovan Lek, Wenxuan Cao and Sebastien Brosse. 2003. Conservation Strategies for Endemic Fish Species Threatened by the Three Gorges Dam, Interscience-Wiley