Thomas A. Gorman

Quantitative evidence for the effects of multiple drivers on continental-scale amphibian declines

Since amphibian declines were first proposed as a global phenomenon over a quarter century ago, the conservation community has made little progress in halting or reversing these trends. The early search for a “smoking gun” was replaced with the expectation that declines are caused by multiple drivers. While field observations and experiments have identified factors leading to increased local extinction risk, evidence for effects of these drivers is lacking at large spatial scales. Here, we use observations of 389 time-series of 83 species and complexes from 61 study areas across North America to test the effects of 4 of the major hypothesized drivers of declines. While we find that local amphibian populations are being lost from metapopulations at an average rate of 3.79% per year, these declines are not related to any particular threat at the continental scale; likewise the effect of each stressor is variable at regional scales. This result - that exposure to threats varies spatially, and populations vary in their response - provides little generality in the development of conservation strategies. Greater emphasis on local solutions to this globally shared phenomenon is needed.

SPACE USE AND SOCIALITY OF RIVER OTTERS (LONTRA CANADENSIS) IN MINNESOTA

Abstract The river otter (Lontra canadensis) has a complex social system, which varies widely across its range. We examined patterns of space use and social interactions for a native population of river otters in southeastern Minnesota. We radiomarked 28 river otters and monitored annual home ranges and core areas, static and dynamic interactions, and site fidelity. We compared these characteristics and interactions between sexes and age classes. Annual home ranges of male river otters were 3.2 times greater than those of females (P = 0.042), and annual core areas of males were 2.9 times greater than those of females (P = 0.083). The static interactions among river otters were extensive, with 69% of the individuals exhibiting core-area overlap. Overall dynamic interactions were positive (i.e., animals were closer together than expected). Males used 74% of their year 1 home range during year 2, and females used 75%. Females on average used only 39% of their core area from year 1 during year 2, whereas males used 65%. In general, conspecifics were not excluded from home ranges or core areas and signs of cooperation were evident, suggesting that river otters in southeastern Minnesota were social rather than territorial.

Factors Related to Occupancy of Breeding Wetlands by Flatwoods Salamander Larvae

The flatwoods salamander (Ambystoma cingulatum) was listed as federally threatened in 1999. Alteration of habitat was considered the main threat to the species, especially the loss of habitat for larval flatwoods salamanders that develop in isolated, seasonally flooded wetlands. Our objectives were to evaluate a suite of within-pool factors (i.e., vegetation structure, water level, and an index to presence of fish) that could influence occupancy of breeding wetlands by larval flatwoods salamanders on Eglin Air Force Base in Florida, USA. We dip-netted for larval salamanders from January through March 2003–2006 and we measured a suite of vegetation characteristics in 2006–2007. Further, in 2006 we measured the level of water and relative presence of fish over the salamander breeding season. Site occupancy over the four year period was best described by a model that incorporated high herbaceous vegetation cover and open canopy cover. Detection probability was assessed, but it varied among years and was not included in the model. Our study suggests that managing the breeding habitat of flatwoods salamander for open canopies and dense herbaceous vegetation may contribute to this species’ recovery.

Site Characteristics of River Otter (Lontra canadensis) Natal Dens in Minnesota

Abstract Knowledge of the habitat and spatial characteristics of natal dens used by female river otters (Lontra canadensis) is limited to a few observations. Documentation of these characteristics in human-altered landscapes may improve management opportunities for this species. We monitored 8 adult (>2 y old) radio-marked female otters during the natal denning season (March–May) in southeastern Minnesota during 2003–2004 and quantified 10 micro- and 2 macro-habitat characteristics of dens. Females began denning in March, with a mean initiation date of 31 March, and used natal dens for a mean of 49 d (se = 3). Two females used man-made brush piles as dens, four used small limestone caves, one used a cavity in the roots of a big-toothed aspen (Populus grandidentata) and one used a beaver (Castor canadensis) bank-den. Dens were located a mean of 316 m (se = 79) from the nearest body of water and averaged 61 m (se = 15) of elevation higher than the nearest body of water. Seven of eight females placed dens outside of their normal activity areas, and all females appeared to select den sites that were protected from flood events. Our results suggest that to promote successful reproduction of river otter populations, managers should protect potential denning sites in upland habitats adjacent to bodies of water.

Diel activity patterns of river otters (Lontra canadensis) in southeastern Minnesota

Abstract An understanding of activity patterns of wildlife in relation to abiotic and biotic factors enables biologists to better understand the ecology of species, manage resources, standardize survey methods, and serve as an index of the relative density of a species. River otters (Lontra canadensis) were radiotracked between June 2002 and October 2003. Using radiotracking data, we conducted exploratory analyses to determine relative influence of abiotic and biotic factors on 2 measures of activity of otters. Abiotic factors included air temperature, barometric pressure, lunar phase, biological season, and time of day; the biotic factor was sex. Activity was measured indirectly via movement rates and directly as the proportion of location attempts recorded as active (PLA). Movement rate was defined as the distance traveled by an otter between consecutive location estimates. Generalized linear mixed models were used to explore the influence of covariates on both measures of otter activity. The model best explaining variation in movement rate included biological season, sex, a season*sex interaction, and time of day. Males moved at greater rates than females during breeding and winter seasons but moved at similar rates to females during summer. Covariates found to account for most variation in the PLA of otters included time of day, season, and temperature. Otters were active throughout the day but with bimodal peaks in the PLA during late evening and early morning hours. The PLA of otters was highest during breeding season, lowest during winter, and intermediate in summer months. In addition, the PLA of otters decreased slightly with increasing temperature. Overall, the PLA of otters in our study area was influenced by abiotic factors, and movement rates of otters were influenced by abiotic and biotic factors.

Spatial Interactions between Two Species of Frogs: Rana okaloosae and R. clamitans clamitans

Abstract Rana okaloosae is endemic to northwestern Florida and is sympatric with R. clamitans clamitans, its closest relative and possibly its main interspecific competitor. We evaluated the movements, dynamic interactions, and nearest neighbor distances of male R. okaloosae and R. c. clamitans in Okaloosa County, Florida. Mean minimum daily distances moved and maximum daily distances moved were greater for R. c. clamitans than R. okaloosae. Dynamic interactions tended to be positive and suggested that there were positive associations between individuals of the two species. Further, nearest neighbor analyses suggested that calling males of both species were positioned in a clumped distribution on any single night. The intraspecific and interspecific interactions of these species were complex and suggested that these species may have selected similar calling locations and appeared not to exclude either conspecifics or heterospecifics.

Survival and Cause-specific Mortality of a Protected Population of River Otters In Minnesota

ABSTRACT Determining causes of mortality and estimating survival rates can provide insight into the status of species for which population trends are not well understood. From Apr. 2002–May 2004 we radio-marked and monitored 39 (13 adult males; 6 subadult males; 8 adult females; 12 subadult females) river otters (Lontra canadensis) in the upper Mississippi River watershed to document causes of mortality, and to evaluate the effects of season, age and sex on survival of river otters in southeastern Minnesota. Further, we assessed the relative importance of demographic parameters to population growth using a projection matrix, which incorporated reproductive data with our observed survival estimates. Human induced mortalities, including accidental captures by fur-harvesters targeting other species (n = 6) and vehicle collisions (n = 1), accounted for the majority of deaths while natural mortality was low (n = 1). Annual survival of females was 0.680 (SE = 0.099) and was 0.946 (SE = 0.052) for adult males. Elasticity of adult female survival was 3.1 times higher than subadult survival, 2.7 times higher than juvenile survival and 2.7 times higher than the sum of elasticity for subadult and adult female reproduction. River otters and other furbearers need to be monitored to assess population status, and management should be responsive to ensure persistence of populations experiencing intentional and/or accidental harvest.

Hindcasting Historical Breeding Conditions for an Endangered Salamander in Ephemeral Wetlands of the Southeastern USA: Implications of Climate Change

The hydroperiod of ephemeral wetlands is often the most important characteristic determining amphibian breeding success, especially for species with long development times. In mesic and wet pine flatwoods of the southeastern United States, ephemeral wetlands were a common landscape feature. Reticulated flatwoods salamanders (Ambystoma bishopi), a federally endangered species, depend exclusively on ephemeral wetlands and require at least 11 weeks to successfully metamorphose into terrestrial adults. We empirically modeled hydroperiod of 17 A. bishopi breeding wetlands by combining downscaled historical climate-model data with a recent 9-year record (2006–2014) of observed water levels. Empirical models were subsequently used to reconstruct wetland hydrologic conditions from 1896–2014 using the downscaled historical climate datasets. Reconstructed hydroperiods for the 17 wetlands were highly variable through time but were frequently unfavorable for A. bishopi reproduction (e.g., only 61% of years, using a conservative estimate of development time [12 weeks], were conducive to larval development and metamorphosis). Using change-point analysis, we identified significant shifts in average hydroperiod over the last century in all 17 wetlands. Mean hydroperiods were shorter in recent years than at any other point since 1896, and thus less suitable for A. bishopi reproduction. We suggest that climate change will continue to impact the reproductive success of flatwoods salamanders and other ephemeral wetland breeders by reducing the number of years these wetlands have suitable hydroperiods. Consequently, we emphasize the importance of conservation and management for mitigating other forms of habitat degradation, especially maintenance of high quality breeding sites where reproduction can occur during appropriate environmental conditions.

Climbing Behavior of Flatwoods Salamanders (Ambystoma bishopi /A. cingulatum)

Abstract As a result of declining populations and reduced availability of suitable habitat, Ambystoma bishopi (Reticulated Flatwoods Salamander) and A. cingulatum (Frosted Flatwoods Salamander) are federally listed as endangered and threatened, respectively, by the United States Fish and Wildlife Service under the Endangered Species Act. Recovery efforts are hindered by a lack of basic natural history information. Although the fossorial life history of ambystomatid salamanders often precludes direct observation of adults and juveniles, an incidental encounter of A. bishopi climbing on Aristida stricta (Wiregrass) sparked a new search effort. On 13 occasions from 29 April 2010 to 23 November 2011, we examined herbaceous ground cover in and around six known breeding wetlands in the Florida panhandle, documenting 36 observations of flatwoods salamanders (10 adults and 26 juveniles), 30 of which were climbing up to 0.5 m above the ground in Wiregrass. These observations led us to hypothesize that Wiregrass, a fire-dependent grass species associated with flatwoods salamander breeding habitat, may be used by juvenile and adult flatwoods salamanders as a foraging substrate (foliage) and/or as refugia (foliage and root base). Although these observations are interesting, further research is needed to determine if the salamanders are selectively using wiregrass and if a foraging and/or refugial advantage is gained by climbing.

The Effects of Habitat Structure on Winter Aquatic Invertebrate and Amphibian Communities in Pine Flatwoods Wetlands

Natural disturbances play a critical role in structuring many ecosystems. In the southeastern United States, fire suppression and exclusion have removed the natural disturbance regime from many ecosystems, including ephemeral wetlands embedded within longleaf pine forests. We sampled aquatic invertebrate and amphibian communities in 21 pine flatwoods wetlands in northwest Florida from 2012 to 2014. Our objectives were to quantify amphibian and invertebrate community structure, identify differences in amphibian communities across an environmental gradient, and identify how invertebrate communities responded to wetland habitat characteristics. Amphibian communities were more diverse in wetlands with longer hydroperiods but were similar across wetlands with different vegetation structures. To examine the effects of wetland characteristics on aquatic invertebrate communities, we created a set of a priori models relating the abundance of isopods, chironomids, and damselflies to wetland characteristics. The best-approximating models indicated that isopods and damselflies were more abundant in wetlands that were not fire-suppressed. Similarly, total invertebrate abundance was higher in sections of wetlands with low canopy cover when compared to sections of the same wetlands with high canopy cover. Restoration of vegetation structure in wetlands that have experienced long-term fire suppression and wetlands that support longer hydroperiods should be a management priority.