Overview

The short-term foci of ARMI activities in the Lower Mississippi Region have been two-fold. First, we are documenting the distributions and abundances of anuran species on key DOI lands within the Lower Mississippi Alluvial Valley (LMAV) of Louisiana and in Big Bend National Park, Texas. Second, for both the Louisiana and Texas sites, we are evaluating whether various factors (both biotic and abiotic) play a role in the observed patterns of distribution, species richness and abundances of individual species. These factors include water chemistry, the hydrology of amphibian breeding sites, the influence of local and landscape-level habitat features, and the role of climatic factors.

Monitoring at "mid-level" sites in Louisiana - For the past two years, we have conducted call surveys to monitor the status of anuran amphibian populations at 40 replicate sites located in the Atchafalaya National Wildlife Refuge and Sherburne and Indian Bayou Wildlife Management Areas (our "mid-level" monitoring sites). To accomplish our second objective (listed above), we have deployed water level recorders at a subset of sites to monitor the hydrology of these breeding sites. We also installed weather stations (alongside automated recording devices or "frogloggers"; see below) at selected sites to gather climatic data (to accompany data on anuran calling activity) at regular intervals. Last, we collaborated with Bruce Moring, WRD, Austin, TX, to collect water samples at 20 amphibian breeding sites located in these same areas. All of these samples were analyzed for nutrients and major ions; a subset of 4 samples were additionally analyzed for pesticides. Our goal is to discern which factors are most closely associated with anuran species richness and the abundance of each species detected at our sites.

To enable us to meet the national ARMI goals, our long-term objective is to conduct an analysis of the proportion of area occupied (PAO) for these sites within the Atchafalaya Basin. To perform this analysis, at each site we have collected data on the species composition of trees, basal area, tree density, and vertical structure of the forest understory; these data are to be used as our site covariates in this analysis. Completion of this analysis awaits further clarification from ARMI (see below).

In 2003, we initiated two novel studies. First, we examined the mouth parts (oral disc) of larval anurans collected from natural to semi-natural sites in Louisiana and Mississippi, where breeding and metamorphosis of local species are known to be successful, where no apparent declines in local populations are known, and no previous examples of oral deformities have been noted. This study was undertaken because previous studies have focused on sites known to be contaminated/polluted, and comparative data on the frequency of larval defects are lacking from more natural sites. Recent studies conducted elsewhere have indicated that examination of tadpoles can provide early detection of potential problems (e.g. chytrid infection in populations and defects due to contaminants). By focusing on natural sites without a known history of contamination, our study may provide a baseline for future studies of larval anuran defects.

Second, we advanced the technology of automated call recorders ("frogloggers"), which are commonly used to monitor calling activity of anuran amphibians. Current models use automated tape recorders and analog cassette tapes to record frog calls throughout the evening and over several nights before field maintenance, such as tape retrieval, is necessary. However, our experience has been that these models can be a challenge to construct, unpredictable in their recording activities in the field, and they require high levels of maintenance (by personnel with expertise in electronics) to keep them operational. Alternatively, we used a personal digital assistant (PDA) equipped with a digital voice recorder and a proprietary software program to function as a digital, automated froglogger. This prototype has numerous benefits compared to the more traditional froglogger model; i.e. maintenance needs are low and digital recordings allow for direct downloading to a computer, as well as precision in the timing of recordings. This advancement promises to enhance researchers' capability to monitor remote populations of anurans for longer periods of time, thus expanding ARMI's repertoire of monitoring tools.

Monitoring activities in Big Bend National Park, Texas (an ARMI "apex" site) - From 2000-October, 2003, ARMI has supported monitoring and research activities in Big Bend National Park by means of a cooperative agreement with Texas A & M University. The focus of our efforts in Big Bend has been on monitoring the hydrology and water quality of amphibian breeding sites, along with determining the distribution and abundance of anurans in this region. There are several reasons for this focus. First, for desert ecosystems, water is the principal factor affecting anuran distributions; amphibian breeding is tightly synchronized with the timing of rainfall events, and successful metamorphosis ultimately depends upon the length of time a breeding site retains water (i.e. its hydroperiod). Second, the quality of the aquatic environment (i.e. chemistry of the water) may also strongly impact larval populations, and changes in water chemistry, particularly that of acidification, are a potential concern for the Big Bend region. This concern is due to increasing air pollution in the park, especially sulfur dioxide emissions originating from sources such as coal-burning power plants in Mexico. Air quality monitors in Big Bend have recorded the highest concentration of sulfates of any western park. At present, it is unknown how water chemistry (including pH) varies among amphibian breeding sites in the Big Bend region, especially after rainfall events (when egg deposition will occur). It is also unclear whether this variation may be associated with amphibian distributions and, if so, whether its potential effects might be exacerbated by other factors.

       Since 2002, water level data loggers and rain gauges have been installed at 10 amphibian breeding sites to correlate anuran breeding activity with rain fall and hydroperiod in Big Bend. We also have evaluated water and sediment samples for possible contamination (Bruce Moring, WRD) and have monitored pH of amphibian breeding sites. Long-term monitoring of these factors at selected sites will continue in 2004.

Results for the past year.

Call surveys in the Atchafalaya Basin, Louisiana - The number of anuran species detected at each site was significantly lower in 2003 than in 2002. The reason for this difference is not clear, but could possibly be due to differences in the amount of rainfall received during the six month study period (January - June) of each year (76.7 cm in 2002 vs. 65.7 cm in 2003). The difference between the two years in mean species richness was greatest for the month of April, although the total amount of rainfall reported for that month was similar for both years. The amount of rainfall reported within 24 h of each call survey was significantly correlated with the number of species heard calling, suggesting that recent precipitation history may be the more important influence on the number of species that are active on a given night.

There was no overall difference among the 12 species we detected in terms of the percentage of wetlands each occupied in the two years, although four species exhibited a decrease in this percentage (Hyla chrysoscelis, Hyla cinerea, Rana clamitans and Bufo woodhousii) . In contrast, only one species (Bufo nebulifer) showed a marked increase in its occupancy of different sites. The mean abundance code for each species was similar in both years, suggesting that whatever factor(s) influenced species richness did not affect the abundance of each individual species.

Occurrence of visually-detectable mouthpart defects in larval anurans. - We made a total of 11 collections of 100 tadpoles each from three families (Ranidae, [3], Hylidae [4], Bufonidae [4]) from a total of nine natural to semi-natural sites in Louisiana. We examined the oral disc of the tadpoles and quantified and categorized the defects in 19 zones. Defects were observed in 20-98% of each collection, with the percent varying significantly among collections. All 11 samples differed significantly in observed vs. expected number of oral defects. Tooth rows contained 75.6% of all observed defects whereas jaw sheaths and marginal papillae contained 15.8% and 8.3%, respectively. Absent teeth with disrupted adjacent tissue was the most common defect (52.7%). Further studies are necessary to eludicate the sources(s) of the defects and their potential long-term impacts.

Manuscripts submitted/in draft:

Walls, S.C., R.C. Taylor, J.S. Lichtenberg, and J.L. Doherty. The effect of conspecifics on growth and foraging of juvenile spadefoot toads (Scaphiopus holbrooki): the consequences of variation in body size at metamorphosis. Submitted to Behavioral Ecology and Sociobiology.

Lichtenberg, J.S., S. King, J. Grace, and S.C. Walls. Habitat associations of chorusing anurans in the Lower Mississippi River Alluvial Valley. In draft.

Walls, S.C., J.L. Doherty, D.L. Drake, and K.A. Roberts. Monitoring the status of amphibian populations and possible environmental correlates in the Atchafalaya Basin, Louisiana. In draft.

Walls, S.C., J.S. Lichtenberg, R.J. Leger and M.M. Dettling. A comparison of methods for capturing amphibians in small wetlands. In draft.

Presentations:

Assisted in tadpole identification workshop (organized/presented by Dana Drake) for USFWS (Louisiana and Mississippi refuge personnel), held 3-25-03 at Lacassine National Wildlife Refuge, Louisiana.

Plans for the coming year

Continue long-term monitoring of amphibian populations, hydroperiod, and water quality in Big Bend National Park.

Continue call surveys for anuran amphibians in the Atchafalaya Basin, Louisiana.

Resolve issues with PAO analysis regarding its use with our data sets.

Improve use of digital frogloggers (PDAs) in the field.

Conduct a "common garden" mesocosm experiment to test the effects of environmental vs. taxonomic effects on the occurrence of mouthpart defects in larval anurans. Determine whether defects in tadpoles have consequences to survival, growth and the timing of metamorphosis. We also plan to examine the extent to which surgically-modified tadpoles experience regeneration in the oral disc and, if so, the time required for regeneration to occur.

Manuscripts to be prepared from work completed in 2003.

Drake, D.L., S.C. Walls and R. Altig. Occurrence of visually-detectable mouthpart defects in larval anurans.

Walls, S.C., J. Doherty, K.A. Roberts, D. Drake, and K.L. Wharton. The utility of personal digital assistants (PDAs) as automated recording devices: the future of frogloggers?

Walls, S., K. Wharton, J. Doherty, D. Drake and K. Roberts. Monitoring anuran calling activity and associated climatic factors at different wetland types.

Walls, S. C. The importance of temporal and environmental variation for species' coexistence in a guild of pond-breeding salamanders.