Amphibian monitoring in the National Capital Region

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The National Capital Region Network has identified amphibians as a priority taxonomic group for its Inventory and Monitoring program. Amphibian monitoring was initiated in 2005, and is currently concentrated in Chesapeake and Ohio Canal National Historic Park (CHOH) and Rock Creek Park (ROCR), with stream sampling also occurring in Prince William Forest Park (PRWI).

The objectives of the monitoring program are to develop an efficient long-term sampling design to:

  1. describe the current distribution of amphibians and explore factors that may influence occupancy probabilities or distributional patterns,
  2. determine if amphibian distributions are changing annually, and if so, explore whether occupancy changes are relate to habitat quality and
  3. provide information to aid in the generating and testing of hypotheses that differentiate among possible causes of long-term changes in the proportion of area occupied among species, habitats, and park areas.

Wetland habitats

We detected 12 amphibian species or species complexes in each year of sampling 33 wetlands from the Potomac Gorge area within Chesapeake and Ohio Canal National Historic Park (CHOH). Results from our model indicated only a 23% probability that there were greater than 12 species in the region suggesting that most, if not all species were observed on at least one occasion during the period of sampling. We found that detection probabilities (the probability of detecting a species, given it is present at a site) were less than one and were strongly influenced by when sampling occurred (i.e., day of the year) but were only mildly variable between years.

We are investigating the relationships between wetland species richness and two important limiting variables in amphibian communities--habitat area and hydroperiod. Lentic species richness is generally highest in permanent, followed by semi-permanent, and lowest in temporary wetlands. Total richness (i.e., the average number of species at a wetland that were present in at least one year of sampling) ranges from 2.7 to 11.0 across the set of permanent wetlands (mean: 7.4; median: 9.0), from 3.2 to 10.0 in semi-permanent wetlands (mean: 6.8; median: 7.4) and from 0.3 to 10.0 in temporary wetlands (mean: 2.5; median: 1.4). The mean estimated richness for each year of the survey was variable in each of the wetlands, with some wetlands having fairly constant richness and others experiencing large annual changes. Annual wetland variability in richness varied independently of hydroperiod category. Increasing wetland area generally had a positive effect on species occurrence probabilities but the result was fairly minimal.

Stream habitats

We observed four species of stream salamanders in each year of our study: Desmognathus fuscus, Eurycea bislineata, E. longicauda, and Pseudotriton ruber. Detection probabilities varied among these species and years. In general, initial occupancy estimates were higher for stream transects in CHOH compared to ROCR and PRWI, except for E. bislineata which had high occupancy probabilities in all parks. D. fuscus and P. ruber had higher occupancy probabilities at transects near the stream headwaters, but our a priori hypothesis that proximity of the stream origin to the park boundary or road would result in lower occupancy probabilities was not well supported for any of the species. As with our lentic sampling, more years of data, encompassing both wet and dry years, are needed to further elucidate the potential relationship between spatial covariates and site occupancy and related rate parameters for stream salamander species. We are also investigating how dry years change the distribution of a population, which will inform the monitoring program design and future studies of stream salamander populations.

In addition to the annual monitoring, sampling in 2010-2011 included intensive effort to survey 3 complete second- and third-order stream networks in CHOH over 3 seasons each year to assess the effectiveness of the stream monitoring design for the National Capital Region. The completeness of these surveys allows us to evaluate seasonal dynamics in amphibian occupancy for four stream salamander species as well as design considerations such as transect length, placement within the network, sampling season, and sampling frequency.  We are currently investigating relationships between occupancy dynamics, habitat covariates, and stream network structure in order to determine the most cost-effective and informative monitoring design for streams in this study area.  Ideally, information from the monitoring program will provide information useful for discriminating among future management decisions.