Abiotic and biotic constraints across reptile and amphibian ranges

EDITOR'S CHOICE JANUARY

By Heather R. Cunningham

Does the relative strength of the abiotic and biotic factors limiting species distributions differ at poleward and equatorward boundaries?

It is well-known that abiotic and biotic factors can shape and limit species’ distributions (e.g. Parmesan et al. 2005, Gaston 2009, Sexton et al. 2009, Cahill et al. 2014). Yet, understanding variation in the relative strength of those factors across a single species range is less clear. One longstanding hypothesis, called here the North-South Hypothesis (NSH), posits abiotic factors (climate) primarily determine species’ poleward range limits while biotic factors (species interactions) sets equatorward limits (Dobzhansky 1950, MacArthur 1972, Brown et al. 1996, Gaston 2003, Parmesan et al. 2005, reviewed in Schemske et al. 2009, reviewed in Cahill et al. 2014). We tested this hypothesis for amphibian and reptile species endemic to the United States.

Using correlative environmental niche models (ENMs), we evaluated the relative role of climate in determining northern versus southern range limits of 214 amphibian and reptile species. We assayed correspondence between each species actual geographic range and a predicted geographic range. We downloaded range maps for all species from NatureServe (http://natureserve.org). Maxent version 3.2.1 (Phillips et al. 2004) was used to generate a predicted range for each species based on 19 WorldClim bioclimatic variables (Hijmans et al. 2005) and species locality data downloaded from HerpNET (www.herpnet.org) and GBIF (www.gbif.org). If climate primarily determines northern range limits (in the Northern Hemisphere), we expect a close alignment of actual and predicted northern range boundaries (Fig. 1). If biotic factors constrain southern range limits, more so than climate, then the predicted southern range limit should extend beyond the actual southern range limit (Fig. 1). Offset between predicted and actual ranges was determined by calculating the length and bearing of a vector extending from the geometric centroid of the actual range to that of the predicted range (Fig. 1).

Figure 1. Hypothetical observed range (hatched) and predicted range (shaded gray). If climate drives northern range limits, then the alignment between observed and predicted ranges should be close at the northern edge. If species interactions drive southern range limits, then a predicted range, based solely on climate, should predict beyond the southern edge of the observed range limit. Offset between observed and predicted ranges will result in northern or southern misalignment of the geographic centroids of the actual (closed circle) and predicted ranges (open circle). Bearing of vector (arrow), from centroid of the actual range to the centroid of the predicted range indicates the direction of offset.

Overall, we detected a southern shift of the predicted range center relative to the actual range center; compass bearing 150.14° (Fig. 2a). In general, suitable habitat was predicted south, rather than north, of the center of the actual range. The direction of offset between predicted and actual ranges of amphibians and reptiles was significantly different (Fig. 2b and 2c). The predicted range center was south of the actual range center for reptiles (bearing = 158.5°) but north (bearing = 7.58°) for amphibians. We did not detect bias in our results due to range size, biogeographic region (i.e. eastern versus western USA), potential non-equilibrium due to previous glaciation, or southern limits bordering the Gulf of Mexico.

Figure 2. Circular plots showing overall mean angular difference between actual and predicted centroids of ranges for (a) all species, (b) amphibians, and (c) reptiles. Individual mean angles binned in 10o intervals for all species are shown externally. The overall mean angle, 95% confidence intervals (shaded) and density line are shown. Inner rose diagram illustrating the frequency of angular offset for amphibians (b) and reptiles (c).

The NSH predicts that biotic interactions determine equatorial range boundaries and abiotic factors determine poleward boundaries. Overall, we find support for this general pattern. Yet, when amphibians and reptiles were analyzed separately, the prediction only held for reptiles. Our results suggests that factors other than climate limit distributions at southern boundaries for reptiles and at northern boundaries for amphibians. Hence, the NSH may not generally apply, across a set of ectotherms exhibiting many ecological similarities within the same biogeographic region. Understanding the proximate and ultimate mechanisms influencing macroecological patterns, like the NSH, will provide insight into the manner in which physiology, behavior, and biogeography intersect to shape species’ distributions.

References

Brown, J. H. et al. 1996. The geographic range: size, shape, boundaries and internal structure. – Annu. Rev. Ecol. Syst. 27: 597–623.

Dobzhansky, T. 1950. Evolution in the tropics. – Am. Sci. 38: 209–221.

Cahill, A. E. et al. 2014. Causes of warm-edge range limits: systematic review, proximate factors and implications for climate change. – J. Biogeogr. 41: 429–442.

Gaston, K. J. 2003. The structure and dynamics of geographic ranges. – Oxford Univ. Press.

Gaston, K. J. 2009. Geographic range limits of species. – Proc. R. Soc. B. 276: 1391–1393.

Hijmans, R. J. et al. 2005. Very high resolution interpolated climate surfaces for global land areas. – Int. J. Climatol. 25: 1965–1978.

MacArthur, R. H. 1972. Geographical ecology. – Harper and Row.

Parmesan, C. et al. 2005. Empirical perspectives on species borders: from traditional biogeography to global change. – Oikos 108: 58–75.

Phillips, S. J. et al. 2004. A maximum entropy approach to species distribution modeling. – ACM. Int. Conf. Proc. Ser. 69: 655–662.

Sexton, J. P. et al. 2009. Evolution and ecology of species range limits. – Annu. Rev. Ecol. Evol. Syst. 40: 415–436.

Schemske, D. W. et al. 2009. Is there a latitudinal gradient in the importance of biotic interactions? – Annu. Rev. Ecol. Evol. Syst. 40: 245–269.

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