The roles of abiotic and biotic factors in driving range shifts: An invasive Pomacea snail facilitates Rostrhamus sociabilis (Snail Kite) northward range expansion

Abstract

Rostrhamus sociabilis (Snail Kite) have recently expanded their range in Florida, tracking the invasion of a Pomacea snail (P. maculata), and exhibiting considerable changes in bill size and feeding niche. This range expansion is not aligned with changes in climatic conditions or the distribution of their historic prey (P. paludosa). The Eltonian Noise Hypothesis (ENH), which posits that interactive (biotic) factors have stronger effects on species’ distributions at local scales, predicts that non-interactive (abiotic) factors are generally more relevant at geographic extents. However, in this study, we explore the R. sociabilis range shift as a potential counterexample of the ENH. Under the biotic-abiotic-mobility (BAM) framework, we explore the role of biotic and abiotic factors in the northward range expansion of this endangered species. Over the past 15 years, R. sociabilis have begun consuming the more-abundant invasive snails more often, while increasing in bill size, expanding ~175 km northward from previous range limits in the Kissimmee River Valley. We developed ecological niche models using 3 algorithms (Maxent, GLM, ellipsoids) and found stability in climatic suitability between past and present models. Moreover, although native snails occur in northern Florida, R. sociabilis have had a historically patchy northern distribution due in part to the availability of appropriate wetland conditions. We found a strong latitudinal cline, with bill length increasing with latitude at least through 2020, suggesting that this morphological change broadened the species’ biotic suitable area and distributional potential. The interplay between changes in phenotype and biotic interactions has been poorly documented in distributional ecology, given a lack of rich occurrence datasets. Here, we highlight a case in which a biological invasion and subsequent changes in morphology and diet have facilitated expansion of a specialized predator into areas that were unsuitable until recently.