A number of different hypotheses exist to explain why we see species distribution limits where we do. A straightforward explanation is the species interaction abiotic stress hypothesis (SIASH), which states that as abiotic conditions become less stressful, biotic factors become more important for determining range limits. However, counter-examples to SIASH abound, showing instances where distributions do not follow the predictions of this hypothesis. Enter interactive range limit theory (iRLT). Proposed by Siren and Morelli (2020), iRLT builds extensions onto the predictions made by SIASH. Namely, iRLT allows for abiotic factors to enhance or lessen biotic stress at the low-abiotic-stress end of a distribution and for biotic factors to enhance or lessen abiotic stress at the high-abiotic-stress end of a distribution (Figure 1). This expansion on SIASH covers the most of the cases that have been noted to violate SIASH predictions. An interesting question which emerges is why do some species fall under the simpler framework of SIASH as opposed to the sometimes counter-intuitive iRLT? Is there a way we can group the species that conform to SIASH vs iRLT?
One important limitation of iRLT is that the evidence provided for it relies nearly exclusively on animal systems. While Siren and Morelli (2020) suggest iRLT should also apply to plants, evidence provided in their article is sparse. There are, however, plant examples given in other papers addressing range limit hypotheses which seem to suggest some species may conform to the patterns predicted by iRLT. A more direct test of iRLT in plants would be necessary to draw strong conclusions, though.
Citation:
Siren, A.P.K. and Morelli, T.L. (2020) Interactive range-limit theory (iRLT): An extension for predicting range shifts. Journal of Animal Ecology, 89(4), 940-954. https://doi.org/10.1111/1365-2656.13150
Figure 1 Highlighting the differences in the predictions made by SIASH versus iRLT. Adapted from Siren and Morelli 2020.