Roads built through or near wetlands cause significant mortality of reptiles and amphibians and create barriers to migration and dispersal. I investigated the number of times turtles and other herpetofauna attempted to cross a 4-lane highway at Lake Jackson, Florida, USA, during a period of severe drought (Feb-Apr 2000). Levels of road mortality were so high that I designed and installed a temporary drift fence system to work with an existing drainage culvert and for the next 2.5 years I evaluated its effectiveness at reducing road mortality and facilitating migration. I monitored roads and fences several times per day for 44 months, during both drought and non-drought conditions. A total of 10,229 reptiles and amphibians of 44 species were found either road killed or alive behind drift fences: 8,842 turtles, 838 frogs, 363 snakes, 152 lizards, 32 alligators, and 2 salamanders. Drift fences combined with intensive monitoring greatly reduced turtle road kills and facilitated the use of an under-highway culvert. Along a 0.7-km section of the highway, turtle mortality before installation of the fence (11.9/km/day) was significantly greater than post-fence mortality (0.09/km/day) and only 84 of 8,475 turtles climbed or penetrated the drift fences. Pre-fence data provided strong evidence that turtles cannot successfully cross all 4 lanes of U.S. Highway 27, as 95% of 343 turtles were killed as they first entered the highway adjacent to the shoulder and the remaining 5% were killed in the first two traffic lanes. According to a probability model, the likelihood of a turtle successfully crossing U.S. Highway 27 decreased from 32% in 1977 to only 2% in 2001 due to a 162% increase in traffic volume. Therefore, at least 98% of turtles diverted by the fences probably would have been killed if fences were not in place. The results of this study represent the highest attempted road-crossing rate ever published for turtles (1,263/km/year). Because of demographic and life history constraints, turtle populations may incur irreversible declines in areas where road mortality is high, especially when mass migrations are triggered by periods of drought. JOURNAL OF WILDLIFE MANAGEMENT 69(2):549-560; 2005
Recent studies suggest that freshwater turtle populations are becoming increasingly male-biased. A hypothesized cause is a greater vulnerability of female turtles to road mortality. We evaluated this hypothesis by comparing sex ratios from published and unpublished population surveys of turtles conducted on-versus offroads. Among 38 166 turtles from 157 studies reporting sex ratios, we found a consistently larger female fraction in samples from on-roads (61%) than off-roads (41%). We conclude that female turtles are indeed more likely to cross roadways than are males, which may explain recently reported skewed sex ratios near roadways and signify eventual population declines as females are differentially eliminated.
Omnivory is ubiquitous in aquatic and terrestrial ecosystems and may increase stability of food webs. Turtles are longlived and among the most abundant organisms in lentic aquatic habitats. Many species are omnivores and thus may be especially important in regulating ecosystem dynamics in these systems. Turtles are also one of the most threatened vertebrate groups in the world; if they indeed play pivotal roles in lentic ecosystems, their declines may disrupt or even destabilize those ecosystems. In order to assess the role of turtles in an aquatic ecosystem, we examined sources of primary production, trophic structure, and ontogenetic diet shifts of five species of turtles, other herpetofauna, and fish in a large shallow lake in northern Florida. Stable isotope analyses (d 13 C and d 15 N) of 42 species of animals and five sources of primary productivity revealed that macroalgae and/or particulate organic matter were the foundation of the food web. Trophic structure of the herpetofauna and fish indicated a high degree of omnivory (TPs of 3-4), few specialists at high trophic levels (TP $ 4.0), and few strict primary consumers (TP = 2.0). Several predatory fish (Micropterus salmoides, Lepomis gulosus, and Pomoxis nigromaculatus) and herpetological predators (e.g., Alligator, Nerodia) were functionally trophic omnivores with trophic positions lowered by predation on primary and low trophic position secondary consumers. We found two major divisions in the trophic structure of turtles-an herbivore (Pseudemys floridana) and four omnivores (Apalone ferox, Chelydra serpentina, Sternotherus odoratus, and Trachemys scripta). There were at least three types of omnivores, species that are generalist feeders throughout their life (e.g., C. serpentina, S. odoratus), species that change their trophic position during ontogeny (e.g., A. ferox), and species with the same trophic position through ontogeny but resulting from different diet composition as juveniles and adults (e.g., T. scripta). Trophic position of some species indicated that even when turtles are primarily predatory, they also function as vegetative consumers and facultative scavengers (e.g., T. scripta, S. odoratus, A. ferox). Given that omnivory is a critical process that can stabilize food webs, there will certainly be dramatic shifts in how energy and nutrients flow through these lentic ecosystems if turtle populations decline.
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