Very often, when the environmental factors limiting a species' distribution are discussed, climatic variables dominate the conversation. However, going as far back as Darwin's 'On the Origin of Species' it was recognized that the latitudinal extent (north or south) of a species' distribution being considered affected the importance of limiting factors. Darwin himself recognized that where environments were stressful (e.g. far northern latitudes) abiotic environmental conditions were almost exclusively the determining factors limiting that extent of the species distribution. Conversely, where abiotic conditions posed little to no challenges for most species (e.g. tropical/sub-tropical climates) it was recognized that biotic interactions were the dominant limiting factors for a species' distribution. In their 2015 paper 'Where and When do Species Interactions Set Range Limits?' Louthan et al. take on the long overdue task of formalizing a hypothesis to explain Darwin's (and many, many others since) observations on species range limitations.
The authors refer to their hypothesis as the 'Species Interactions- Abiotic Stress Hypothesis,' abbreviated as SIASH. SIASH in its simplest form predicts that the more stressful abiotic conditions are for species, the less important biotic interactions will be for determining range limitations and vice versa. Of course, there are known exceptions to SIASH. Many ecologists have adopted the mantra of 'it depends' as exceptional cases occur throughout many natural patterns. An example contrary to SIASH given in this paper are that high-latitude invasive species tend to have larger ranges compared to lower (but not tropical) latitude invasive species.
As mentioned earlier, a large portion of the literature has focused solely on the abiotic dimensions limiting distributions. However, according to SIASH, we may be making erroneous assumptions, particularly in the southern latitudes, if we are not also considering the biotic dimensions when determining the primary limiting factors of a distribution. Very little work has been done to consider the biotic and abiotic environment in tandem as limiting factors, and even fewer, if any, studies have considered both environmental dimensions at both latitudinal extents of a species. This is likely because it would be quite a task to conduct the necessary field work at two latitudinal range extents, which in many cases could be hundreds to thousands of miles apart! However, elevational as opposed to latitudinal range extents could be used to limit the distance between the abiotic-stressful and non-stressful extents of ranges. The University of Georgia sits at an ideal location where many temperate tree species find their southern (non-stressful) limits at or around the latitude of Athens, whilst being a short drive to high-stress elevations in the Southern Appalachians where the same tree species are also (but, due to abiotic stress) limited! Sounds like a research project to me!
Paper reference:
Louthan A.M., Doak D.F., Angert A.L. (2015). Where and When do Species Interactions Set Range Limits. Trends in Ecology and Evolution. 30(12): 780 - 792.
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