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Range Matters

Updated: Mar 27, 2018

By Professor Elizabeth Jordan, Science Writer



As we observe species’ populations dwindle significantly in recent years, ecologists have become interested in the overall geographic distribution and abundance of species and species’ ranges, what impacts their dispersal and what affects their dispersal has on their chances of survival.


The area in which species can be found during all parts of their life cycle is considered a species’ range. Some species occupy relative narrow ranges and others occupy wider ranges. Typically, the larger the animal, the larger its range, as they need plenty of room for food, resources and mates. Large animals such as tigers have a large geographic range because they need a lot of room to roam, hunt for food, and find mates. There appears to be a positive correlation between distribution and size.


In other words, the more widespread a species range, then typically one will find more members of the species. If there is a narrower range, then there are typically less individuals in that species (1).


According Shelford’s Law of Tolerance a species’ distribution will be controlled mostly by the factor of which species has the lowest tolerance for. In other words, species are only as strong as their weakest trait.


Large mammal populations for the most part face their greatest threat from human populations; their weakest trait is their tolerance for the forces we put on them. From forming physical barriers such as building of roads, fences and settlements, to reduction of availability of food or depletion and fragmentation of population due to poaching and hunting. Many large mammal species like elephants now occupy a small percentage of their original range, and populations are discontinuous and fragmented.


Smaller more fragmentation ranges harm a species overall chances of survival resulting in a loss in species numbers (2). This is due to several reasons:


Genetic Diversity: Fragmenting a larger population into several smaller populations can result in a lack of genetic diversity within the smaller fragmented populations created, giving that population a lower chance of survival against disease and a higher chance of genetic inbreeding.


Natural and un-natural Disasters: If a species overall range is small then they have less chance of surviving un-natural disasters such as war break out and natural disasters like floods, fires and volcano eruptions. For instance, Angola and Democratic Republic Congo suffered from civil wars for nearly a decade, these countries are now almost completely lacking in elephant populations. If elephants were only found in these countries (had that small a range) the species could have gone extinct. It’s only because their range extends to areas outside these countries that they still exist. The larger the range of the species, the less likely they are to go extinct due to a disaster as the disaster is less likely to encompass their entire range.


Lower carrying capacity: A range has a maximum carrying capacity for a population of a species. A smaller range results in a lower maximum population number.


If we can increase the range of some of these species and connect the smaller now fragmented populations. They will stand a higher chance of survival in the long run. This can be done by re-introducing small populations to protected areas which were in the species original historic range and connecting present populations by forming wildlife corridors or extending protected areas.


References:

1. GASTON, K.J., 1990. Patterns in the geographical ranges of species. Biological Reviews, 65(2), pp.105-129.


2. Joshi, A., Vaidyanathan, S., Mondol, S., Edgaonkar, A. and Ramakrishnan, U., 2013. Connectivity of tiger (Panthera tigris) populations in the human-influenced forest mosaic of central India. PloS one, 8(11), p.e77980.

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