Thursday, October 14, 2010

Discussion Question 4 (Primary Threats)

Why is habitat degradation the greatest threat to biodiversity?


Brennan Ko-Madden said...

Habitat Degradation is possibly the greatest threat to biodiversity because of its incredibly capability to cause a major disturbance in the long term natural processes and the obvious immediate destruction of habitat. Degrading or changing a natural habitat has been a facet of human expansion and enterprise today resulting in the loss of healthy forests, extinctions, and polluted waters.
The organisms that reside in these changing habitats are either displaced, reduced in numbers, or altogether destroyed. This can result from fragmentation of the habitat, urbanization, induced climate change, or the increasingly problematic introduction of invasive species.
In short, when an organisms habitat is reduced the carrying capacity of that population is likewise reduced leaving behind the threat of having a small population that has a greater chance of becoming extinct.
Finally, some habitats are not created equal. Biodiversity hotspots throughout the world contain some of the rarest organisms left on this planet. Nearly all of them are endemic and are not found anywhere else, but there specific habitat. It is important that as a species, humans recognize their impact on the other organisms and the value of their existence before permanently altering a habitat or ecosystem.

Tim said...

Habitat degradation is the greatest threat to biodiversity because of its direct negative impact on species diversity through niche destruction. From an evolutionary standpoint, species can be defined by their specific niche. As different habitats are destroyed, the amount of unique niches available are continuously destroyed, causing there to be an overall lower amount of species available.

Habitat degradation is being caused by a large variety of factors, the most serious which is human activity. Human activity has resulted in the continuous urbanization of different habitats, resulting in a few select niches to be available on a global scale. Animal and plant species that can survive in the niches available in urban and suburban areas become more and more common, while the majority of biota which cannot survive with such specific niches are disappearing.

In her study "Effects of Habitat Fragmentation on Biodiversity," Fahrig defines habitat fragmentation as a landscape-scale process involving habitat loss and the breaking apart of habitat. Using empirical studies, Fahrig suggests that habitat loss has large, consistently negative effects on biodiversity. However, Fahrig brings up an interesting point stating that habitat fragmentation could also have positive effects upon biodiversity. Therefore, Fahrig concludes that the term fragmentation should be reserved for the breaking apart of habitat, independent of habitat loss since habitat loss has a consistently negative impact on biodiversity.

Sonya Vargas said...

Habitat degradation affects communities in more ways than we can imagine. Some examples we discussed in class talked about fragmentation. This simple aspect of degradation is known to not only decrease the total area of that habitat, but it increases the proportion of edges (leaving communities susceptible to “edge effects”), and it increases the isolation of patches (often creating barriers to movement for the organisms living in those individual islands.

In addition to all of these consequences of habitat degradation, it has also been understood that the creation of these small populations can also be very dangerous. As we learned in lecture, small populations tend to go extinct. Many factors contribute to this such as environmental and demographic stochasticity, allee effects, genetic drift, and inbreeding. It is the combination of all of these factors that leads to the extinction vortex which basically means that small populations would only tend to get smaller and smaller as these factors play a bigger and bigger role in the deduction of individual fitness and population adaptability.

Also, a very interesting factor that comes into play is that rare species are not the only organisms being threatened by habitat loss due to their already low numbers as we would expect, but common species are just as threatened especially in cases such as the ones we saw in lecture where the best competitors of a community went extinct because other species which were better colonizers and could adapt to that sudden change in habitat rather quickly in order to persist and attain new equilibrium abundances.
In a paper that was published last month called Averting biodiversity collapse in tropical protected areas, the authors formed a study which gathered expert knowledge from 262 interviews (veteran field biologists and environmental scientists ~2 decades of experience). The questionnaires focused on longer-term changes in abundance of 31 animal and plant guilds. It is interesting because they point out that even though there are protected areas intended to preserve biodiversity, it does not mean that the “protected areas” are then somehow immune to the threats and disturbances that happen outside of those protected areas. Thus, they assessed environmental drivers both inside and immediately outside each protected area by using a reserve health index that they generated.

The authors recognize that an important predictor of reserve health is improving reserve management, however, they also find that protecting biodiversity involves more than just safeguarding the reserves themselves. They show that often times, landscapes and habitats surrounding the reserves are under imminent threat and that degradation outside of the designated protected areas predisposes the reservations themselves to similar kinds of degradation. The authors mention what we were discussing in lecture about edge effects and isolation of small chunks, except that they describe how it’s not just edges that are affected, but the entire reserve. As expected, there is still not much known about the likelihood of biodiversity persisting in these protected areas, but the authors suggest that there should be a larger target on not just the internal threats, but the external threats as well in order to increase the resilience of biodiversity for the future.

Laurance, W., Useche, C., Rendeiro, J., Kalka, M., Bradshaw, C., Sloan, S., … Zamzani, F. (2012). “Averting biodiversity collapse in tropical protected areas.” Nature, 489.7415: 290-294. Web of Science. Web. 16 Oct. 2012. doi:10.1038/nature11318

Brennan Ko-Madden said...

In addition to my previous comment here some relavant evidence of habitat degradation. "Disturbances that alter the availability of habitat resources will affect bottom-up processes. Over-exploitation and habitat loss account for >90% of all known marine population and species extinctions..." This refers mainly to coral reef ecosystems where climate change and rising levels of acidification in the ocean are threatening the survival of coral and all other species that depend on coral for habitat.

"Exploitation and habitat degradation as agents of change within coral reef communities"
Article first published online: 20 OCT 2008

Narek Ohanian said...

Habitat degradation or habitat loss can create a wide array of issues in the population dynamic of certain areas. The obvious effect would be the loss of inhabitable area for species to thrive, however, it is not only loss of terrain that causes a decline in biodiversity. Most of the species of a given habitat are known to congregate more towards the center of the inhabitable area, leaving the edges less populated. This is because the center of the habitat differs from the outer edges drastically. As the size of the habitat is reduced, the percentage of area taken up by the edge increases, thus lowering the survival rates of species that are accustomed to living in the center. This phenomenon is known as the edge effect. When a habitat is fragmented, reduced in size, certain populations are no longer able to contribute to individuals to one another. A demographically closed population is created, one in which the total number of individuals is decided from the number of deaths and births each year. Recruitment of new species becomes uncommon, so the population of species is limited to its principal amount. This would not be a problem if the habitat loss was minimal and the number of individuals per species was in high numbers. Often this is not the case, population size decreases so much that inbreeding insists. Through inbreeding the numbers of viable young will decrease overtime as recessive genes become more prominent. The gene pool of a certain species will decrease overtime allowing the effects of genetic drift to transpire, further weakening the viability of the population. Habitat degradation kick starts a domino effect which eventually leads a species into a downward spiral known as the extinction vortex.

Chrissy Roualdes said...

Ecological issues can often be traced back to habitat destruction; one such example involves invasive species. An article published in “Ecological Informatics” expresses a similar idea that “accelerating habitat of the leading causes of species loss, will change the native community structure and the susceptibility of native species to invasion” (Liu 69). Species that reside near destroyed habitats are often the victims of refugee species that can increase competition for space and resources. Although the refugee species may not survive if the new habitat does not fulfill its niche, the native species may also suffer if it is a weaker competitor. Speaking of niches, this is another issue to be considered in regard to habitat destruction. Other habitats may not be up to par with such specific niches; and for those niches that are not so widespread, habitat destruction can be responsible for extinction of a rare species.

Besides invasive and rare species, small populations are also a concern when it comes to habitat destruction. Such populations are perhaps a greater concern than rare species due to the fact that it is difficult to recover the population once it has reached such low levels. What was once a sufficient population could become an endangered small population because of habitat destruction. Merely considering these few examples of habitat destruction as the precursor to other issues is enough to recognize why it ranks top spot as the greatest threat to biodiversity.

Huiyu Liu, Zhenshan Lin, Xiangzhen Qi, Mingyang Zhang, Hao Yang, Interactive effects of habitat destruction and competition on exotic invasion, Ecological Informatics, Volume 9, May 2012, Pages 69-75, ISSN 1574-9541, 10.1016/j.ecoinf.2012.03.006.

Alexander Phillips said...

As mentioned in previous comments and lecture 6, habitat degradation is almost always involved in threats to biodiversity, whether due to natural causes (volcanoes, geological processes, etc.) or human causes (introduced species, overexploitation, etc.). While the immediate effect of habitat loss due to humans is visible - less habitat - we also find that human driven habitat degradation has larger effects.

Fragmentation results in edge effects, patch isolation, and smaller populations, all of which negatively affect biodiversity. When a landscape becomes fragmented, the changing of the habitat increases the proportion of edges to total area, thereby increasing the magnitude of "edge effects". Furthermore, we know that as size of habitats decrease, extinction rates increase and immigration rates decrease (a patch of land becomes more isolated, creating barriers of movement for many organisms). And as we also learned in lecture, smaller populations tend to go extinct due to factors such as demographic stochasticity, environmental stochasticity, allee effects, genetic drift, and inbreeding.

"Ecology Letters" created a review of studies, within the last two decades, on plants and the effects that fragmentation has had on them. Because plants are not mobile (seeds and pollen are transported through different means), they are highly susceptible to isolation effects. In studies, it was shown that pollinators and seed dispersers proved to be significantly less effective in heavily fragmented environments. The greater hostility of surrounding matrices decreased the possibility of arrival of both outcross pollen from other populations and of seeds which may eventually germinate and establish in the fragment.

Habitat degradation is the greatest threat to biodiversity. Fragmentation, in particular, can have enormous effects on the biodiversity of habitats, through both effects we understand, and through processes and connections between organisms which we do not fully comprehend.

Aguilar R, Ashworth L, Galetto L, Aizen MA (2006) Plant reproductive susceptibility to habitat fragmentation: review and synthesis through a meta-analysis. Ecol Lett 9:968–980

Jenna Escobedo said...

The threats to biodiversity involve introduced predators and new species, also habitat destruction. The greatest threat to biodiversity overall is habitat destruction whether the cause be natural or induced by humans. In a study done by Stendera et al., the stressors of freshwater biodiversity patterns were analyzed. In this study it was concluded that "land use, eutrophication and habitat destruction were identified as most important stressors".
Habitat destruction can lead to fragmentation which first reduces the total habitat area. This ultimately leads to the edge effect where eventually the habitat area begins to shrink into smaller and smaller patches. The decrease in area leads to a decrease in species immigration which allows for the increase in extinction rates. This leads to a decrease in biodiversity. What causes biodiversity directly to be decreased when habitat is destroyed is the lack of sufficient resources. The carrying capacity of the habitat decreases as the resources such as food, space, shelter are decreased.
An alternative threat to biodiversity are invasive species. Species that are not native to the habitat may cause the extinction of other species. The introduction of these species may be natural. If a habitat is destroyed to only be contained within a patch area, species that can survive in the edges of the habitat may become inhabitants of the habitat. This can affect the ecosystem substantially.
Most causes of habitat destruction however are unnatural and caused by human hands. We as humans often believe in Manifest Destiny which will in the end cause resource shortages. In the end we need to do plan more for our generation and future generation's futures considering our affect on this planet is catching up with us and will continue so unless we start conservation efforts.

Stendera, S., Adrian, R., Bonada, N., Canedo-Arguelles, M., Hugueny, B., Januschke, K., Pletterbauer, F., Hering, D. "Drivers and Stressors of Freshwater Biodiversity Patterns Across Different Ecosystems and Scales: A Review" Hydrobiologia, 696:1 p 1-28. Web of Science. Web. 18 Oct 2012. DOI 10.1007/s

ethan schwartz said...

Habitat degradation and loss pose the greatest threat to biodiversity because their influence is experienced on many different levels whose combined influence is catastrophic.

A loss in habitat typically leads to a loss in population size (lack of resources). Small populations are particularly susceptible to genetic drift and inbreeding depression. The former is often associated with the loss or fixation of certain alleles within a population, and the latter the expression of deleterious homozygote recessives. This loss in genetic diversity makes individuals and the population as a whole less capable of surviving environmental change. Such populations may enter into an extinction vortex where population reduction and genetic loss enhance one another in a feedback loop.

Reduction in habitat area, and most specifically the fragmentation of habitat into many smaller parcels, leads to an increase in edge habitat. Habitat edges are typically more dynamic environments than a habitat's interior, as they are more susceptible to weather and the influences of the neighboring area. While some species are well adapted to edge-type environments, most are not. Habitat edges are often affected by human activity (agriculture, pollution, recreation) and present opportunities for highly-competitive invasive species.

Yet another aspect of habitat loss and fragmentation is the destruction of food webs; a predator population and its associated prey population may become separated into isolated parcels. The predator population may die off for lack of resources, and the prey population may experience a "boom and bust" cycle having been removed predation pressures.

Cumming GS, Southworth J, Rondon XJ, Marsik M. 2012. "Spatial complexity in fragmenting Amazonian rainforests: do feedbacks from edge effects push forests towards an ecological threshold?"Ecological Complexity 11;67-74.