CAN NUTRIENT ADDITIONS MEDIATE GAPE LIMITED PREDATION IN LARVAL RANA SPHENOCEPHALA?

Abstract

Some predators are limited in the size prey item that they can catch and consume, and this is referred to as "gape limited". Prey found with these predators may have the ability to respond by growing rapidly to reach a size too large to be preyed upon, and thus reach a size refuge from predation. Due to decreased competition, these prey may have the additional benefit of experiencing an environment with more consumable resources available. One factor that may influence the ability of prey to outgrow the gape limitation of their predators is nutrient availability. Providing an influx of limiting nutrients can result in an increase in abundance of primary producers, and allow herbivore prey living in nutrient enriched environments to grow more quickly than those that live in a nutrient-limited environment. Thus, prey living in nutrient rich environments may be more likely to be able to outgrow the gape limitation of a predator, and reach a predator-free size refuge, than those prey living in areas with less nutrient availability. I was interested in whether nutrient additions can mediate the effects of gape limited predation in an aquatic food web. A mesocosm experiment was performed to evaluate the effects of nutrient influx on Rana sphenocephala tadpoles in the presence of two groups of bluegill fish predators: one that is likely to become gape limited and one that is not. The nutrients added were crystalline NaNO₃ and K₂HPO₄, in a 16:1 N:P ratio, in six different amounts ranging from 0.0 to 4.0 mg/l. The experiment ran for four months to evaluate the larval anuran response (survival, mass, time to metamorphosis) after one pulse of nutrient influx. Algal biomass and zooplankton abundance were evaluated at two times during the experiment as zooplankton feed on algae and bluegill can also feed on zooplankton. The results of this experiment indicate that although nutrient addition was found to significantly impact larval anuran survival, there was no evidence of an interaction between nutrient addition and predation. Thus, there is no strong support for the hypothesis that bottom up forces created by nutrient addition can mediate the top down forces created by predation. Higher abundances of leopard frogs in more nutrient rich environments, as a result of higher survivorship, may change the structure of aquatic communities and have cascading effects on the rest of the food web. Currently we are experiencing an influx of inorganic nutrients reaching our waterways due to anthropomorphic sources. Worldwide human activities, such as agriculture and development, have increased the amount of nitrogen and phosphorus reaching our aquatic systems. This experiment reinforces the idea that increases in these nutrient levels can cause changes in life history characteristics of populations and in community structure of aquatic and terrestrial systems impacted by nutrient additions.