Alligator-engineered impacts on consumer nutrient dynamics and isotopic niche

Background/Question/Methods

Empirical evidence indicates bottom-up and top-down effects are acting simultaneously in most food webs. Nutrient addition stimulates plant biomass and enhances consumer biomass (bottom-up), while predation decreases herbivore biomass reducing grazing pressure on primary producers (top-down). However, little attention has been given to animals that may exert both forces through roles as both predators and ecosystem engineers, such as American alligators (Alligator mississippiensis) in the Florida Everglades. Alligators create “alligator ponds” (hereafter “ponds”) by removing vegetation and redistributing soil. This results in dense vegetation surrounding the pond (hereafter “near pond”). The adjacent marsh, near pond, and pond have distinct plant and consumer communities. We hypothesize that among habitats alligator engineering creates a nutrient gradient, decreases stoichiometric ratios (X:P) for basal resources along the gradient, while consumers maintain homeostasis, and increases consumer’s isotopic niche (measured using stable isotopes of carbon and nitrogen). To test these hypotheses, we collected flocculent matter (hereafter “floc”) and two consumers (Eastern Mosquitofish - Gambusia holbrooki and Bluefin Killifish – Lucania goodei) for stoichiometric and isotopic analysis in each habitat at 10 ponds. Stoichiometric data were analyzed using profile analysis. Additionally, isotopic niches were modelled using the SIBER package in R followed by pairwise comparisons of posterior distributions.

Results/Conclusions

We found that floc C:N, C:P, and N:P all differed along the marsh to pond gradient. C:P and N:P both decreased (by a factor of 3-4 in many cases), while C:N increased (marginally relative to changes in C:P and N:P). Conversely, consumers demonstrated homeostasis, as predicted by stoichiometric theory, but the variance around mean stoichiometric ratios differed between consumers. Meanwhile, consumer isotopic niches varied along this gradient. However, the direction and magnitude of change depended on the consumer. Eastern Mosquitofish had their smallest isotopic niche area in the marsh and largest in the near pond, while their pond isotopic niche differed from neither marsh nor near pond. In contrast, Bluefin Killifish had their largest niche in the marsh, a slight, not statistically notable decrease in the near pond, followed by a sharp contraction of isotopic niche area in the pond. This may be driven by species-specific variation in diet selection. In addition to top-down predation effects, we demonstrated that alligators simultaneously generate bottom-up effects by inducing a gradient of increasing P from adjacent marsh to engineered habitats. Furthermore, we showed that these effects may extend throughout the aquatic food web, altering consumer isotopic niches from the bottom-up.