Steve A. Johnson

The Fungicide Chlorothalonil Is Nonlinearly Associated with Corticosterone Levels, Immunity, and Mortality in Amphibians

Background: Contaminants have been implicated in declines of amphibians, a taxon with vital systems similar to those of humans. However, many chemicals have not been thoroughly tested on amphibians or do not directly kill them. Objective: Our goal in this study was to quantify amphibian responses to chlorothalonil, the most commonly used synthetic fungicide in the United States. Methods: We reared Rana sphenocephala (southern leopard frog) and Osteopilus septentrionalis (Cuban treefrog) in outdoor mesocosms with or without 1 time (1×) and 2 times (2×) the expected environmental concentration (EEC) of chlorothalonil (~ 164 μg/L). We also conducted two dose–response experiments on O. septentrionalis, Hyla squirella (squirrel treefrog), Hyla cinerea (green treefrog), and R. sphenocephala and evaluated the effects of chlorothalonil on the stress hormone corticosterone. Results: For both species in the mesocosm experiment, the 1× and 2× EEC treatments were associated with > 87% and 100% mortality, respectively. In the laboratory experiments, the approximate EEC caused 100% mortality of all species within 24 hr; 82 μg/L killed 100% of R. sphenocephala, and 0.0164 μg/L caused significant tadpole mortality of R. sphenocephala and H. cinerea. Three species 
showed a nonmonotonic dose response, with low and high concentrations causing significantly greater mortality than did intermediate concentrations or control treatments. For O. septentrionalis, corticosterone exhibited a similar nonmonotonic dose response and chlorothalonil concentration was inversely associated with liver tissue and immune cell densities (< 16.4 μg/L). Conclusions: Chlorothalonil killed nearly every amphibian at the approximate EEC; at concentrations to which humans are commonly exposed, it increased mortality and was associated with elevated corticosterone levels and changes in immune cells. Future studies should directly quantify the effects of chlorothalonil on amphibian populations and human health.

Community ecology theory predicts the effects of agrochemical mixtures on aquatic biodiversity and ecosystem properties

Ecosystems are often exposed to mixtures of chemical contaminants, but the scientific community lacks a theoretical framework to predict the effects of mixtures on biodiversity and ecosystem properties. We conducted a freshwater mesocosm experiment to examine the effects of pairwise agrochemical mixtures [fertiliser, herbicide (atrazine), insecticide (malathion) and fungicide (chlorothalonil)] on 24 species- and seven ecosystem-level responses. As postulated, the responses of biodiversity and ecosystem properties to agrochemicals alone and in mixtures was predictable by integrating information on each functional groups (1) sensitivity to the chemicals (direct effects), (2) reproductive rates (recovery rates), (3) interaction strength with other functional groups (indirect effects) and (4) links to ecosystem properties. These results show that community ecology theory holds promise for predicting the effects of contaminant mixtures on biodiversity and ecosystem services and yields recommendations on which types of agrochemicals to apply together and separately to reduce their impacts on aquatic ecosystems.

Early-life exposure to a herbicide has enduring effects on pathogen-induced mortality

Exposure to stressors at formative stages in the development of wildlife and humans can have enduring effects on health. Understanding which, when and how stressors cause enduring health effects is crucial because these stressors might then be avoided or mitigated during formative stages to prevent lasting increases in disease susceptibility. Nevertheless, the impact of early-life exposure to stressors on the ability of hosts to resist and tolerate infections has yet to be thoroughly investigated. Here, we show that early-life, 6-day exposure to the herbicide atrazine (mean ± s.e.: 65.9±3.48 µg l−1) increased frog mortality 46 days after atrazine exposure (post-metamorphosis), but only when frogs were challenged with a chytrid fungus implicated in global amphibian declines. Previous atrazine exposure did not affect resistance of infection (fungal load). Rather, early-life exposure to atrazine altered growth and development, which resulted in exposure to chytrid at more susceptible developmental stages and sizes, and reduced tolerance of infection, elevating mortality risk at an equivalent fungal burden to frogs unexposed to atrazine. Moreover, there was no evidence of recovery from atrazine exposure. Hence, reducing early-life exposure of amphibians to atrazine could reduce lasting increases in the risk of mortality from a disease associated with worldwide amphibian declines. More generally, these findings highlight that a better understanding of how stressors cause enduring effects on disease susceptibility could facilitate disease prevention in wildlife and humans, an approach that is often more cost-effective and efficient than reactive medicine.

A Simple Technique for Trapping Siren lacertina, Amphiuma means, and Other Aquatic Vertebrates

ABSTRACT We describe a commercially-available funnel trap for sampling aquatic vertebrates. The traps can be used in heavily vegetated wetlands and can be set in water up to 60 cm deep without concern for drowning the animals. They were especially useful for capturing the aquatic salamanders Siren lacertina and Amphiuma means, which have been difficult to capture with traditional sampling methods. They also were effective for sampling small fishes, particularly centrarchids, and larval anurans. In total, 14 species of amphibians, nine species of aquatic reptiles, and at least 32 fish species were captured. The trap we describe differs significantly from traditional funnel traps (e.g., minnow traps) and holds great promise for studies of small, aquatic vertebrates, in particular Siren and Amphiuma species.

Canopy closure and emigration by juvenile gopher frogs.

Abstract Although studies have addressed effects of abrupt transitions in habitat type (e.g., forest–clear-cut or forest–field edges) on amphibian movements, little is known about effects of more subtle habitat transitions on patterns of migration and habitat use in amphibians. We used radiotelemetry to study movement patterns of juvenile gopher frogs (Rana capito) emigrating from ponds that were surrounded by longleaf pine (Pinus palustris) forest that varied in structure as a result of fire suppression. Our primary purpose was to determine if frogs emigrate directionally from their natal ponds and select habitat at random during their first month following metamorphosis. We found that frogs emigrated in nonrandom directions from ponds that were surrounded by heterogeneous habitat and selected fire-maintained habitat that was associated with an open canopy, few hardwood trees, small amounts of leaf litter, and large amounts of wiregrass (Aristida beyrichiana). Fire-maintained habitat contained higher densities of burrows excavated by gopher tortoises (Gopherus polyphemus) and small mammals, which are the primary refuge sites for both juvenile and adult gopher frogs. Frogs moved up to 691 m from their natal ponds, frequently crossed dirt roads, and even seemed to use these roads as migration corridors. To maintain suitable terrestrial habitat for gopher frogs, including habitat used by migrating individuals, it is important to apply frequent prescribed fire to uplands surrounding breeding ponds that lead all the way to the edges of breeding ponds, as well as through ponds during periodic droughts.

Evaluation of a new technique for marking anurans

Amphibians can be very difficult to mark because of their extraordinary powers of regeneration. Although many amphibian marking techniques have been developed, few meet the rigorous assumptions of capture-mark-recapture models. Also, excessive toe-clipping may affect frog survivorship adversely. We tested the efficacy of a new hybrid marking technique (VIE-C) that combines Visible Implant Elastomer (VIE) and toe-clipping on four species of treefrogs in West-Central Florida. Of the 840 treefrogs recaptured over a 15-month period, only one mark was unreadable. A significantly higher percentage of VIE marks (80%) than toe-clips (55%) remained viable for the duration of the study. On average, toe-clips remained readable for 100 days, and VIE marks remained readable for 112 days. There were no significant species differences in the length of time that either type of mark lasted. The hybrid VIE-C method represents an improvement over either method used alone, but the VIE mark will be more helpful in correctly reading and clarifying toe-clipping errors than will toe-clips be helpful in reading and clarifying VIE marks.

Technology meets tradition: A combined VIE-C technique for individually marking anurans

We report on the use of a hybrid marking technique (VIE-C) combining Visible Implant Elastomer (VIE) marks with toe-clipping (C) to mark individuals of several species of treefrogs (Hylidae). Our marking strategy entailed injecting elastomer into the plantar surface of the digits and clipping only one toe. This method allows large numbers of frogs to be individually marked, reduces the potential for negative effects due to clipping multiple toes, and minimizes the frequency of elastomer migration from the injection site, a common problem with VIE marks on the body or limbs. We found retention rate of VIE marks in the digits to be similar to that of toe-clips, indicating that VIE provides a satisfactory alternative to multiple toe-clips. In addition, cost of materials, frog handling time, and ill effects were minimal. This VIE-C marking scheme is highly recommended when considering techniques for marking anurans, as it reduces potential negative effects of clipping multiple toes, and provides a large number of inexpensive and long-lasting individual marks that can be easily applied and quickly read in the field by trained observers.

Changes in a Northwestern Florida Gulf Coast Herpetofaunal Community Over a 28-y Period

ABSTRACT Population declines of amphibians and reptiles throughout the world have led to the initiation of projects to monitor their status and trends. Historical collections give an indication of which species occurred in an area at one time, although the ambiguity surrounding locations and environmental conditions associated with collection decreases the value of this information source. Resampling using the same general protocols can give valuable insights to changes in community structure. However, this is only feasible when sampling methodology and exact site locations are known. From 2002–2005 we resampled 12 sites in St. Marks National Wildlife Refuge in Floridas panhandle, an area in which intensive herpetological surveys were conducted in 1977–1979. We documented a general decrease in species richness among the diversely managed sites, changes in dominant species and diversity and an increasing trend toward homogeneity of the herpetofaunal community among habitats. Changes were attributed to four causes: 28-y of forest community succession, changes in management practices, non-detection of species due to variation in sampling conditions and a decrease in occupancy by four amphibians and three reptiles. The use of population and habitat-related indexes helped define possible influences on community change and can be used to target species for monitoring. Declines of these seven species are of concern, especially considering the protected status of the refuge and its increasing isolation as surrounding landscapes are converted to urbanized settings.

LIFE HISTORY OF THE STRIPED NEWT AT A NORTH-CENTRAL FLORIDA BREEDING POND

Abstract I studied the life history of Striped Newts (Notophthalmus perstriatus) at a breeding pond in north-central Florida. Newts were captured in pitfall traps at a drift-fence as they migrated into and out of the pond basin. During the 2-year study, I recorded 10,290 captures (8,127 individuals) of newts at the drift-fence. Newts were active during each month of the study, but there were four peak activity periods, each of which included immigration and emigration events. Immigration events were almost exclusively comprised of adults, whereas emigration events were comprised of adults and recently transformed larvae. I documented 5,296 recently transformed, immature larvae (efts) and 435 recently transformed mature larvae (paedomorphs) during four distinct periods of emigration. Efts matured in the uplands before returning to the pond to breed. In the uplands, male efts (n = 16) grew 0.0183 mm/day on average, whereas average female (n = 24) growth was 0.0167 mm/day. Immigrating adults of both sexes were significantly smaller than emigrating adults. Emigrating efts were smallest, followed by emigrating paedomorphs, immigrating adults, then emigrating adults. The overall adult sex ratio was 1:1.25 (m:f). Sex ratio of emigrating paedomorphs was highly skewed towards females, with one male for every 4.43 females. Newts tended to move during wetter periods, and captures were significantly correlated with rainfall, but rainfall was a poor predictor of the magnitude of newt movements.

Home Range Size and Habitat Selection in the Florida Pine Snake (Pituophis melanoleucus mugitus)

Recent reports of declines in terrestrial snake populations in the southeastern U.S. have highlighted the need for life history information for these often cryptic animals. In this study, we used radio-telemetry to describe home range size and habitat use of the Florida Pine Snake (Pituophis melanoleucus mugitus), a large fossorial species associated with the endangered Longleaf Pine (Pinus palustris) ecosystem. Florida Pine Snakes had large home ranges that varied in size among individuals (mean  =  59.2 ± 50.8 ha, minimum convex polygon). There was no significant difference between mean annual home range size of males and females, but home range size differed significantly among seasons and between males and females by season. At the home range scale, Florida Pine Snakes selected natural pine forests and mature slash pine plantations, and they tended to use sites with shrubs and vines and very little bare ground at a local scale. As has been observed in other studies, Florida Pine Snakes were highly fossorial and they frequently sought shelter in Southeastern Pocket Gopher (Geomys pinetus) burrows. Snakes in our study rarely crossed paved or graded dirt roads; however, they frequently crossed low-use harrowed dirt roads. Collectively, our findings indicate that Florida Pine Snakes require large contiguous pine forests and that paved and graded roads may be significant barriers to their movement.