Dietary resource utilization among three sympatric watersnake species in western Kentucky

Background/Question/Methods

Understanding how similar species coexist is a fundamental ecological question, with theory predicting the most highly competitive species driving others to extinction.  For similar species, coexistence may be facilitated by divergence in dietary resource utilization patterns, which can occur across a variety of factors, including among species and age classes.  Our study focused on three sympatric, congeneric watersnake species in northwestern Kentucky.  The diamondback (Nerodia rhombifer), northern (N. sipedon) and plain-bellied (N. erythrogaster) watersnakes inhabit similar wetland habitats and appear to have considerable dietary overlap, with all three species feeding on fish and amphibians. In order to determine differences in dietary resource utilization patterns, we used a combination of stomach content analysis and stable isotope techniques to determine trophic resource position, trophic niche width and trophic niche overlap across snake species and snake body sizes.

Results/Conclusions

Carbon isotopic analysis showed that northern watersnakes had a greater niche width than the other species (Levene’s test for homogeneity of variance: δ¹³C, F_2,72 = 6.43, p <0.01), suggesting that this species fed across a wider range of terrestrial to aquatic habitats and thus preyed on a greater diversity of amphibians and fishes.  Diamondback watersnakes fed over a larger range of trophic levels (Levene’s test for homogeneity of variance:δ¹⁵N, F_2,72 =3.88, p=0.03) than did northern and plain-bellied watersnakes. Stomach analysis showed that northern watersnakes (Simpson’s diversity index D =4.51) and diamondback watersnakes (D =4.48) had diverse diets.  Plain-bellied watersnakes had narrow diets (D =1.40), feeding primarily on anurans (88.5% gut content), with stable isotope mixing models indicating 0.85 for anurans. 

Snout-vent length of snakes was related to bothδ¹³C (r²=0.19, F_1,146 =35.07, p<0.01) and δ¹⁵N (r²=0.55, F_1,146 =183.57, p<0.01) with longer snakes taking prey from differing habitat types and higher trophic levels than shorter snakes.  While there were overlaps in dietary resource utilization, watersnakes species had either generalist or specialist diets, which may allow coexistence.  Snake diet also varied with size, which is an intraspecific diet variation that can promote species coexistence.