Slow Isotope Turnover Rates and Low Discrimination Values in the American Alligator: Implications for Interpretation of Ectotherm Stable Isotope Data

Stable isotope analysis has become a standard ecological tool for elucidating feeding relationships of organisms and determining food web structure and connectivity. There remain important questions concerning rates at which stable isotope values are incorporated into tissues (turnover rates) and the change in isotope value between a tissue and a food source (discrimination values). These gaps in our understanding necessitate experimental studies to adequately interpret field data. Tissue turnover rates and discrimination values vary among species and have been investigated in a broad array of taxa. However, little attention has been paid to ectothermic top predators in this regard. We quantified the turnover rates and discrimination values for three tissues (scutes, red blood cells, and plasma) in American alligators (Alligator mississippiensis). Plasma turned over faster than scutes or red blood cells, but turnover rates of all three tissues were very slow in comparison to those in endothermic species. Alligator δ15N discrimination values were surprisingly low in comparison to those of other top predators and varied between experimental and control alligators. The variability of δ15N discrimination values highlights the difficulties in using δ15N to assign absolute and possibly even relative trophic levels in field studies. Our results suggest that interpreting stable isotope data based on parameter estimates from other species can be problematic and that large ectothermic tetrapod tissues may be characterized by unique stable isotope dynamics relative to species occupying lower trophic levels and endothermic tetrapods

What do alligators eat on golf courses?

Urbanization is an ever-increasing threat to wildlife and their natural habitats, yet research has been limited to a small number of taxa and very few large predator species. The American alligator (Alligator mississippiensis) is an apex predator across the southeast U.S. and has surprisingly received minimal attention within urban areas. To investigate the potential effects of land development on alligator trophic ecology, we performed gut content analysis on golf course alligators found on Jekyll Island, Georgia. We then made comparisons with alligators found in more natural areas on Sapelo Island, Georgia. In total, we collected stomach content samples from 25 alligators on Jekyll Island golf courses, of which only one had an empty stomach. Data provided from Sapelo Island consisted of 93 alligators within our alligator size range, of which only one had an empty stomach. While analysis of similarity, non-metric multidimensional scaling, and simplified Morisita index analyses show no significant difference in diets between the two areas (possibly because of a low sample size from Jekyll Island), %IRI values for prey items reveal that there may be functional differences in prey choice or availability. Further land development and increasing human activity may therefore shift diets toward reliance on prey items usually of lesser importance. These trophic effects could possibly lead to local population declines, if paired with habitat degradation or other stressors

The Roles of Large Top Predators in Coastal Ecosystems: New Insights from Long Term Ecological Research

During recent human history, human activities such as overhunting and habitat destruction have severely impacted many large top predator populations around the world. Studies from a variety of ecosystems show that loss or diminishment of top predator populations can have serious consequences for population and community dynamics and ecosystem stability. However, there are relatively few studies of the roles of large top predators in coastal ecosystems, so that we do not yet completely understand what could happen to coastal areas if large top predators are extirpated or significantly reduced in number. This lack of knowledge is surprising given that coastal areas around the globe are highly valued and densely populated by humans, and thus coastal large top predator populations frequently come into conflict with coastal human populations. This paper reviews what is known about the ecological roles of large top predators in coastal systems and presents a synthesis of recent work from three coastal eastern US Long Term Ecological Research (LTER) sites where long-term studies reveal what appear to be common themes relating to the roles of large top predators in coastal systems. We discuss three specific themes: (1) large top predators acting as mobile links between disparate habitats, (2) large top predators potentially affecting nutrient and biogeochemical dynamics through localized behaviors, and (3) individual specialization of large top predator behaviors. We also discuss how research within the LTER network has led to enhanced understanding of the ecological roles of coastal large top predators. Highlighting this work is intended to encourage further investigation of the roles of large top predators across diverse coastal aquatic habitats and to better inform researchers and ecosystem managers about the importance of large top predators for coastal ecosystem health and stability

The Roles of Large Top Predators in Coastal Ecosystems: New Insights from long Term Ecological Research

Abstract: During recent human history, human activities such as overhunting and habitat destruction have severely impacted many large top predator populations around the world. Studies from a variety of ecosystems show that loss or diminishment of top predator populations can have serious consequences for population and community dynamics and ecosystem stability. However, there are relatively few studies of the roles of large top predators in coastal ecosystems, so that we do not yet completely understand what could happen to coastal areas if large top predators are extirpated or significantly reduced in number. This lack of knowledge is surprising given that coastal areas around the globe are highly valued and densely populated by humans, and thus coastal large top predator populations frequently come into conflict with coastal human populations. This paper reviews what is known about the ecological roles of large top predators in coastal systems and presents a synthesis of recent work from three coastal eastern US Long Term Ecological Research (LTER) sites where long-term studies reveal what appear to be common themes relating to the roles of large top predators in coastal systems. We discuss three specific themes: (1) large top predators acting as mobile links between disparate habitats, (2) large top predators potentially affecting nutrient and biogeochemical dynamics through localized behaviors, and (3) individual specialization of large top predator behaviors. We also discuss how research within the LTER network has led to enhanced understanding of the ecological roles of coastal large top predators. Highlighting this work is intended to encourage further investigation of the roles of large top predators across diverse coastal aquatic habitats and to better inform researchers and ecosystem managers about the importance of large top predators for coastal ecosystem health and stability.Keywords: Individual specialization, Morone saxatilis, Migratory striped bass, Sea otters, Bottoom up control, Alligator holes, Trophic cascades, Food webs, Kelp forests, EvergladesThis is the publisher’s final pdf. The published article is copyrighted by the Oceanography Society and can be found at: http://www.tos.org/oceanography/

Factors affecting individual foraging specialization and temporal diet stability across the range of a large “generalist” apex predator

Individual niche specialization (INS) is increasingly recognized as an important component of ecological and evolutionary dynamics. However, most studies that have investigated INS have focused on the effects of niche width and inter- and intraspecific competition on INS in small-bodied species for short time periods, with less attention paid to INS in large-bodied reptilian predators and the effects of available prey types on INS. We investigated the prevalence, causes, and consequences of INS in foraging behaviors across different populations of American alligators (Alligator mississippiensis), the dominant aquatic apex predator across the southeast US, using stomach contents and stable isotopes. Gut contents revealed that, over the short term, although alligator populations occupied wide ranges of the INS spectrum, general patterns were apparent. Alligator populations inhabiting lakes exhibited lower INS than coastal populations, likely driven by variation in habitat type and available prey types. Stable isotopes revealed that over longer time spans alligators exhibited remarkably consistent use of variable mixtures of carbon pools (e.g., marine and freshwater food webs). We conclude that INS in large-bodied reptilian predator populations is likely affected by variation in available prey types and habitat heterogeneity, and that INS should be incorporated into management strategies to efficiently meet intended goals. Also, ecological models, which typically do not consider behavioral variability, should include INS to increase model realism and applicability

Shifts in plant nutrient content in combined warming and drought scenarios may alter reproductive fitness across trophic levels

Ecological effects of climate change are difficult to predict because climate change is a multi-variable phenomenon that can impact ecosystems through diverse pathways. Despite this fact, climate change ecology research has been dominated by relatively simplistic experiments and models. To test the importance of assessing more realistic climate and ecological scenarios an experiment was conducted to assess the interactive effects of multiple climate change variables (warming, drought) on survival and reproductive fitness across three trophic levels within a well-studied terrestrial food web. The effects of warming and drought on the nutrient content of plants and how such changes may affect consumers was also examined. Results showed that warming and drought in combination can significantly alter the nutrient profiles of plants relative to climate variables in isolation and that multi-variable climate change can severely impact plant reproductive fitness. Also, consumer nutrient profiles did not shift in accordance with their resources, but reproductive fitness of grasshoppers was nonetheless severely affected by warming and drought in combination. Predator survival rates decreased by more than 45% under all climate change scenarios, highlighting the variable responses of different trophic levels. The impacts across the entire food web were likely caused by a combination of metabolic changes due to warming, dehydration, altered nutrient availability and resource quality, and behavioral shifts. The results suggest that realistic climate change scenarios could dramatically affect long-term demographic patterns as well as food web dynamics, and that single variable studies may fail to reveal the true impacts of climate change