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Population Variability in Species

When looking at the different scenarios that can lead to species extinction, the variation in population is an extremely important topic. The amount of genetic diversity, coupled with a series of other factors can have a dramatic effect on the survival rates or chances of a species. When this is coupled with outside influences, it can be seen how fragile some species and the ecosystems around them are. This article will look at the processes and events that can threaten species, especially species that have small populations and are at risk of extinction, and how scientists can measure population in order to avoid the possibility of further species extinctions.

Population Size

The population size of a species can be affected by a number of different factors, both internal and external. The reason for these factors can range from naturally occurring events to human induced events. Some of the factors can include:

  • Life History: The specific history of the species that is living in that area, for example the species previously had no predators, but now a predator is introduced, changing the dynamics.
  • Environmental Variation: Where the dynamics of the outside environment are changed, resulting in different rates of survival and reproduction.
  • Disease: Diseases can affect populations of any size and at any time. Some diseases my sweep through, kill animals and then leave, or there may be diseases that stay within the population and slowly move through the gene pool, such as specific cancers and tumors.
  • Human Impact: This is the most damaging factor to all species. Land clearing, over exploitation, farming, pollution and loss of habitat due to human activities have caused the extinction of many species and will continue to do so in the future.
  • Inbreeding Depression: When small populations are isolated, they can lose genetic variability due to inbreeding, or the lack of choice in reproductive partners. This can cause an overall weakening of the gene pool, leading to lower rates of survival.
  • Catastrophes: Naturally occurring events such as fires, floods, heat waves, droughts and storms can have a severe impact on population numbers.
  • Habitat Quality: The quality of the environment in which the species lives can also have an impact on populations. The amount of food sources available, specific areas suitable for breeding and areas in which animals can hide from predators can all impact survival rates.

When a species population is large, it is resilient to certain factors due to fact of sheer numbers and this is called demographic variation. For example if there is a disease that sweeps through a species in an area, the larger the population the more survivors there are. Smaller populations do not have this luxury, and often events such as disease outbreaks can be enough to wipe them out, or leave them extremely vulnerable. The fortunes of large populations can be governed by the law of averages, however small populations depend on the fortunes of a few individuals within the group. Demographic variation is the variation in the birth and death rates of the population, along with the sex ration within the species. Small populations can be affected by this for example if there are a large number of males born in one breeding season, which can disrupt the balance of the species causing a further drop in numbers due to the absence of breeding females 1). Environmental variation occurs due to a series of external factors, such as fires, floods, heatwaves and drought. These events pose a great risk to all species regardless of population size, however it affects smaller populations more harshly due to the law of averages as discussed above. This variation can affect survival and reproduction rates, two factors that are inherently crucial to species longevity 2).

ocelot.jpg

The North American Ocelot is an endangered species 3)

Population Viability Analysis

The Population Viability Analysis (PVA) is a tool used for measuring the populations of certain species, and more specifically, how susceptible they may be to extinction. It measures this by working out the identifiable threats to the species, such as loss of habitat, increase in predation or competitors, parasites, diseases and over exploitation. This model cannot predict future unknown events such as catastrophes or new diseases. To determine whether the particular species is at risk, scientists will look factors such as birth rates, death rates, species dispersal, sex ratio and gene diversity. Smaller populations will naturally have greater variations in these rates, and thus these species are prone to what scientists call the 'Extinction Vortex'. This is caused by a number of negative feedbacks, for example the genetic diversity is low, and due to this it will continue to get lower, thus causing a runaway negative effect within the population. This continues to decline until eventually the species is extinct. When a population is prone to the extinction vortex, it means that is is below what is called the 'Minimum Viable Population' (MVP). This is the threshold at which the vortex will start to occur 4).

Like most tools of this sort, the PVA can have some weaknesses. As aforementioned, environmental catastrophes cannot be predicted, but also there are weaknesses when it comes to certain assumptions. For example, when scientists first measure the number of species in a population, past history is not taken into account, such as has there been a recent outbreak in disease or predation that has significantly altered the population size, or if the last breeding season was particularly successful leading to a bigger than usual population, or whether a predator has recently been introduced or removed from the area. There are also unknowns in regards to usual reproduction rates, size of litter and reproduction frequency, for example some species may reproduce twice a year when conditions are favourable, but sometimes due to lack of food or other factors this is reduced to once a year or not at all. Whilst the PVA is a good estimation tool, it must be remembered that it can only measure what is currently occurring at a location unless there have been previous studies conducted on the same population 5).

numbatumbat.jpg

The Australian Numbat is also an endangered species 6)

It must also be remembered that the PVA is only a measuring and estimation tool. It can only give raw data by the means of measurements, and is not a diagnostic tool. It cannot tell you how the population numbers got to where they are, or how that it can be fixed in the future. The PVA should be used in conjunction with other methods in order to find solutions to solving extinction risks and concentrating on reducing controllable factors such as human induced extinctions that have occurred far too frequently in the past.

Diagnosis of Declines

In order to conduct a remedial response to population declines, it is important that a scientific approach is adhered to. This includes the developing of a hypothesis as to why the specific population is in decline, and not just automatically assume that a certain factor is the cause of the decline. Firstly it must be proven that the population is in decline and that it was more widely distributed and abundant previously. This can be achieved by previous studies such as population counts. Secondly a study must be conducted into the general dynamics of the species, for example has there been a history of population surges and declines, and could this decline merely be a part of the regular cycle for this species. Once this information is gained, it will become easier to eliminate factors that aren't responsible for the decline. The next step is to look at where the species once existed and no longer exists and work out what the possible factors for this could be, for example land clearing, loss of food sources, introduction of a predator etc, and then examine the area in which the species now resides and see if any of those factors are present in their current habitat. Once these factors have been examined, there should be a clearer picture of the cause of decline. Where possible studies can be conducted under controlled environments to test the theory, for example a small concentrated area study where you replicate the probable cause on a small population and test the results 7).

Treatment of Declines

The most effective way of treating population declines is to remove the factors that are causing the decline where possible. This may not always be a viable option, for example where the population decline is occurring due to land clearing and development, in all likelihood you will not be able to remove the development. In this case it may be possible to increase food supply in the smaller habitat area in order to help boost numbers. On the other hand, if there has been an introduced predator, it could be possible to remove the predator from the habitat of the target species and this method has proven to be successful in the past. Another way in which to combat population decline is controlled breeding programs, where the species is bred elsewhere under controlled conditions and then released into it's natural habitat afterwards. This method is used when the species is having trouble reproducing due to environmental or human factors but survival rates after birth are good 8).

Populations can also be increased by supplementing resources, such as increasing nesting sites for birds, creating artificial ponds for amphibians or planting suitable host plants for insects. Food supplementation can also be an effective method, however this is only viable as a short term option and the best practice here would be habitat management such as improving the conditions for the food source to become viable in the long term. Another method is to provide areas in which species can hide from predation, however this can only be successful if you can achieve this with the minimum of destruction and impact on other natural habitats and systems. Education and legislation is another tool which can be used to reduce population decline. Educating people about the impacts of their activities can increase awareness and protect habitats from further destruction, and legislation such as declarations of national parks and protected areas can protect species from habitat loss due to human activities. The eradication and control of pests and predators can be a useful method in treating population decline, however unfortunately it is a very difficult, costly and time intensive process. The risks of such a process is that the removal of one predator may cause the increase of the animal which it hunted, causing a whole new set of problems and an imbalance on the ecosystem 9).

Ecology

1) Frederiksen, M., Harris, M. P., & Wanless, S., 2005, 'Inter‐population variation in demographic parameters: A neglected subject?' Oikos, vol.111, no.2, pp.209-214
2) Nikinmaa, M., & Tjeerdema, R., 2013, 'Environmental variations and toxicological responses', Aquatic Toxicology, vol.127, no.1
3) Ocelot by Tom Smylie, Available: http://commons.wikimedia.org/wiki/File:Ocelot.jpg
4) Boyce, M. S., 1992, 'Population viability analysis. Annual Review of Ecology and Systematics', vol.23, no.1, pp.481-497
5) Cuthbert, R., 2003, 'Population viability analysis', Biological Conservation, vol.114, no.1, p.153
6) Numbat by martybugs, licence CC 3.0, Available: http://commons.wikimedia.org/wiki/File:Numbat.jpg
7) Mace, G. M., Collar, N. J., Gaston, K. J., Hilton-Taylor, C., Akçakaya, H. R., Leader-Williams, N., Stuart, S. N., 2008, 'Quantification of extinction risk: IUCN's system for classifying threatened species' Conservation Biology, vol.22, no.6, pp.1424-1442
8) Miller, S., 2007, 'Endangered species', JAMA: The Journal of the American Medical Association, vol.297, no.11, p.1171
9) Clark, C. W., & Munro, G. R., 1978, 'Renewable resource management and extinction;, Journal of Environmental Economics and Management, vol.5, no.2, pp.198-205

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