Consequences and causes of geographic variation in the body size of a keystone predator,Notophthalmus viridescens

Two subspecies of the predatory aquatic salamanderNotophthalmus, N. viridescens viridescens andN. v. dorsalis, differ in adult body size and geographic distribution. We tested whether experimental populations of the two predator subspecies differed in their effects on prey populations ofB. americanus, and whether observed differences in predator body size were genetic and/or environmentally induced. We compared the effects of predation by bothNotophthalmus subspecies on larvalBufo americanus by experimentally manipulating the densities (0, 2, or 4 newts/m³) and subspecies ofNotophthalmus (N. v. viridescens orN. v. dorsalis) added to artificial ponds. BothNotophthalmus subspecies significantly reducedB. americanus survival, but differed significantly in this effect. FewerBufo survived with the larger subspecies,N. v. viridescens, than with the smallerNotophthalmus subspecies,N. v. dorsalis. TheNotophthalmus subspecies differed in their patterns of adult and larval growth. Adults of the smaller subspecies,N. v. dorsalis, had a significantly higher growth rate than the larger subspecies,N. v. viridescens, under common environmental conditions, suggesting that differences in predator size were partly genetic, rather than entirely environmentally induced. LarvalN. v. dorsalis metamorphosed significantly later in the season than larvae ofN. v. viridescens, suggesting that larvalN. v. dorsalis had a lower growth rate than larvalN. v. viridescens. Differences in adult and larval growth, together with differences in the minimum adult size observed in natural populations, suggest that differences in the rate or duration of pre-adult growth may contribute substantially to observed differences in size.     

On the role of sexual selection in ecological divergence: a test of body‐size assortative mating in the eastern newt Notophthalmus viridescens

Speciation processes initiated by divergent selection often fail to complete; yet, how sexual selection is involved in the progress of ecological speciation is rarely understood. Intraspecific body‐size variation affects mate preference and male–male competition, which can consequently lead to assortative mating based on body size. In the present study, we tested the importance of body size difference in the potential of assortative mating between the two eastern newt subspecies, larger Notophthalmus viridescens viridescens and smaller Notophthalmus viridescens dorsalis. Through differential expression of life‐cycle polyphenism, these two subspecies are adapted to contrasting environments, which has likely led to the subspecific body‐size difference. We found that males of both subspecies preferred larger females of N. v. viridescens as mates presumably because of the fecundity advantage of larger females. On the other hand, no evidence of female choice was found. Larger males of N. v. viridescens exhibited greater competitive ability and gained primary access to larger females of their own kind. However, smaller males were able to overcome their inferior competitive ability by interfering with larger males' spermatophore transfer and sneakily mating with larger females. Thus, the subspecific body‐size difference importantly affected sexual selection processes, resulting in nonrandom but not completely assortative mating patterns between the larger and smaller subspecies. Although life‐cycle polyphenism facilitates the intraspecific ecological divergence within N. v. viridescens sexual selection processes, namely smaller males' mate preference for larger females and sexual interference during spermatophore transfer, may be halting completion of the ecological speciation. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 884–897.     

Impacts of hydroperiod on growth and survival of larval amphibians in temporary ponds of Central Pennsylvania,USA

The effects of variable hydroperiod (three levels) and initial density of amphibians (two levels) on survival, growth rate, and time to and mass at metamorphosis were studied for wood frogs (Rana sylvatica), Jefferson salamanders (Ambystoma jeffersonianum), and spotted salamanders (A. maculatum). Experiments were carried out in 260-1 mesocosms set up outdoors in a forest. These pond simulations were designed to mimic conditions that occur in palustrine temporary wetlands in central Pennsylvania. No animals reached metamorphosis in the short hydroperiod (56 days). However a greater proportion (66%) of tadpoles of R. sylvatica survived to the end of the 56-day, treatment than the 84- or 158-day treatments (29 and 14%, respectively), from which all survivors metamorphosed. In contrast, neither of the salamanders metamorphosed by 84 days; survival to metamorphosis at 158 days was 15% for A. jeffersonianum and 10% for A. maculatum. Average instantaneous growth rates for A. jeffersonianum decreased with each increase in hydroperiod. Growth of R. sylvatica was greater in the 56-day hydroperiod than in hydroperiods of 84 or 158 days. Initial amphibian density had no effect on growth or survival of any species. It appears that salamander larvae were predatory on tadpoles, since survival of R. sylvatica was negatively correlated with survival of A. jeffersonianum in 84-day treatments and with growth of A. maculatum in 158-day treatments.     

Conservation genetics of the central newt (Notophthalmus viridescens) in Iowa: the importance of a biogeographic framework

In light of global declines in amphibian populations, genetic data have become increasingly important for understanding population structure and for revealing hidden diversity. At the species level, Notophthalmus viridescens is an IUCN species of “least concern”, but the subspecies N. v. louisianensis (central newt) is listed as “threatened” in Iowa, a state on the western periphery of the species range. Genetic data were collected from 282 N. v. louisianensis from 14 sites in Iowa. Sequences from 1,054 nucleotides of mitochondrial DNA from Iowa newts revealed unexpected diversity in the form of two major haplotype groups that are not sister clades, with southern Iowa N. v. louisianensis being more closely related to N. v. piaropicola (peninsula newt) from Florida than to consubspecifics in Iowa. Sequence differentiation indicates that the two lineages of newts present in Iowa diverged near the beginning of the Pleistocene. Northern and southern Iowa haplotypes were found together at one site, indicating an opportunity for hybridization near Remington’s biogeographic suture zone 1, a hotspot for hybridization in other species. Three microsatellite loci provided additional evidence for distinctness of northern and southern Iowa newt populations. This study highlights the relevance of historical biogeography to conservation, as management strategies for N. v. louisianensis in Iowa must reflect previously unrecognized diversity in this species. Nuclear and mitochondrial data indicate genetic isolation of nearby populations on the same drainage, and field data suggest the decline of one study population, emphasizing the need for identification and protection of newt breeding sites in Iowa.     

Are direct density cues,not resource competition,driving life history trajectories in a polyphenic salamander?

Polyphenisms, where multiple, discrete, environmentally-cued phenotypes can arise from a single genotype, are extreme forms of phenotypic plasticity. Cue acquisition and interpretation are vital for matching phenotypes to varying environments, but can be difficult if cues are unreliable indicators or if multiple cues are present simultaneously. Facultative paedomorphosis, where juvenile traits are retained at sexual maturity, is a density-dependent polyphenism exhibited by many salamanders. Favorable conditions such as low larval densities and stable hydroperiod delay metamorphosis and promote a paedomorphic strategy. We investigated proximate cues affecting facultative paedomorphosis in order to understand how larval newts (Notophthalmus viridescens louisianensis) assess conspecific density. To isolate the effects of density cues from the effects of resources and agonistic behavior, we caged larval newts in mesocosms in a 2?×?2 factorial design that manipulated both background larval newt densities (high or low) and food levels (ambient or supplemented). We found strong effects of both food and density on caged individuals. Under high densities, caged larvae were more likely to become efts, a long-lasting juvenile terrestrial stage, across both food levels, while paedomorphs were more common under low densities. Though food levels increased growth rates, density had strong independent effects on metamorphic timing and phenotype. Competition for food and space are classical density-dependent processes, but density cues themselves may be a mediator of density-dependent effects on polyphenisms and life history responses.     

The potential impact of simulated ground-water withdrawals on the oviposition,larval development,and metamorphosis of pond-breeding frogs

Wetland hydroperiod is a key factor for the reproductive success of pond-breeding amphibians. Ground-water withdrawals may cause intermittent ponds to dry prematurely, potentially affecting amphibian development. In three intermittent ponds, we monitored hydrology and tracked oviposition, larval development, and metamorphosis for three frog species that represented a range of breeding phenologies. The three species were the southern leopard frog (Lithobates sphenocephalus), spring peeper (Pseudacris crucifer), and Pine Barrens treefrog (Hyla andersonii). We simulated ground-water withdrawals by subtracting from 5 to 50 cm (in 5-cm increments) from the measured water-depth values at the ponds over a short-term (2-year) period and a long-term (10-year) period to estimate the potential impact of hydroperiod alterations on frog development. Short-term simulations indicated that 5 and 10 cm water-depth reductions would have resulted in little or no impact to hydroperiod or larval development and metamorphosis of any of the species. Noticeable impacts were estimated to occur for reductions ≥15 cm. Long-term simulations showed that impacts to the appearance of the first pre-metamorphs and metamorphs would have occurred at reductions ≥10 cm and impacts to initial egg deposition would have occurred at reductions ≥20 cm. For all simulations, successively greater reductions would have caused increasing impacts that varied by species and pond, with the 50-cm reductions shortening hydroperiods enough to practically eliminate the possibility of larval development and metamorphosis for all three species. Compared to the spring peeper and southern leopard frog, the estimated impacts of the simulations on the various life stages were the greatest for the Pine Barrens treefrog.     

Estimating hydroperiod suitability for breeding amphibians in southern Rhode Island seasonal forest ponds

In New England, seasonal forest ponds provide primary breeding habitat for several amphibian species, including Rana sylvatica (LeConte) and Ambystoma maculatum (Shaw). Because each species requires a minimum duration of inundation to complete its breeding cycle, one of the most important factors determining habitat suitability is a pond’s hydroperiod. The objective of this research was to develop a method for estimating pond hydroperiod from site characteristics such as pond morphology, geology, chemistry, and vegetation structure, and to use the estimates to assess the suitability of individual ponds for breeding amphibians. We monitored the duration of surface inundation in 65 ponds in the Pawcatuck River watershed of southern Rhode Island during 2001 and 2002. Pond hydroperiods, measured from 1 March, ranged from 19 to 44 weeks in 2001 and from 2 to 44 weeks in 2002; mean values were 30 and 21 weeks, respectively. Akaike’s Information Criterion was used to select a multivariate hydroperiod estimation model (R² = 0.59, p < 0.0001) that permitted identification of ponds with hydroperiods suitable for breeding by R. sylvatica (95.4% correct classification rate [CCR]) and A. maculatum (75.4% CCR). Canopy cover, open basin depth, and specific conductance of surface water were among the most useful site characteristics for estimating hydroperiod, while surficial geology and the texture of soil parent material made smaller contributions. The CCR using open basin depth alone was 95.4 and 73.8%, respectively. These findings indicate that it is possible to estimate the hydroperiod of seasonal ponds – and to assess their suitability for individual species of breeding amphibians – without prolonged periods of hydrologic monitoring. Such techniques could have considerable value to wetland regulatory agencies and for planning amphibian habitat management and acquisition at the landscape scale.     

Partitioning beta‐diversity through different pond hydroperiod lengths reveals predominance of nestedness in assemblages of immature odonates

Patterns of freshwater invertebrate assemblage structure in the transition from permanent to non‐permanent lentic habitats are well described in the literature. However, the effects of small changes in the hydroperiod of non‐permanent ponds on invertebrate assemblage structure remain less studied, especially on β‐diversity. Thus, we tested the effects of different pond hydroperiod lengths on the assemblage structure of immature odonates, in terms of both α‐ and β‐diversity. Small high‐altitude ponds with different hydroperiod lengths (assigned to ‘short’, ‘medium’ and ‘long’ hydroperiods) were sampled in southern Brazil between 2013 and 2014. Based on the hypothesis that shorter hydroperiods filter constituents of lentic fauna, i.e. that long‐living species cannot inhabit shorter‐hydroperiod ponds, we expected to find higher α‐ and β‐diversity in longer hydroperiods, as well as predominance of the nestedness component in β‐diversity. Restricted occurrence of some genera and higher α‐diversity of immature odonate assemblages was detected in long‐hydroperiod ponds. Within‐hydroperiod β‐diversity values did not vary among hydroperiods, because the occasional occurrence of some genera with high dispersal ability of adults in short‐hydroperiod ponds yielded similar values of the β‐diversity among hydroperiods. Partitioning of β‐diversity among hydroperiods revealed a significant higher contribution of the nestedness component rather than turnover. This pattern is explained by the occurrence of some generalist genera across the whole gradient of hydroperiod, as a subset of fauna in longer‐hydroperiod ponds. Thus, our results suggest that reduction in hydroperiod length, if occurring in the future climate change, would favor habitat‐generalist taxa in lentic ecosystems.     

Reaction norms for metamorphic traits in natterjack toads to larval density and pond duration

The evolution of environmentally-induced changes in phenotype or reaction norm implies both the existence at some time of genetic variation within a population for that plasticity measured by the presence of genotype x environment interaction (G x E), and that phenotypic variation affects fitness. Otherwise, the genetic structure of polygenic traits may restrict the evolution of the reaction norm by the lack of independent evolution of a given trait in different environments or by genetic trade-offs with other traits that affect fitness. In this paper, we analyze the existence of G x E in metamorphic traits to two environmental factors, larval density and pond duration in a factorial experiment with Bufo calamita tadpoles in semi-natural conditions and in the laboratory. Results showed no plastic temporal response in metamorphosis to pond durability at low larval density. The rank of genotypes did not change across different hydroperiods, implying a high genetic correlation that may constrain the evolution of the reaction norm. At high larval density a significant G x E interaction was found, suggesting the potential for the evolution of the reaction norm. A sibship (#1) attained the presumed “optimal” reaction norm by accelerating developmental rate in short duration ponds and delaying it in longer ponds. This could be translated in fitness by an increment in metamorphic survival and size at metamorphosis in short and long ponds respectively with respect to non-plastic sibships. However, genetic variability for plasticity suggests that optimal reaction norm for developmental rates may be variable and hard to achieve in the heterogeneous pond environment. Mass at metamorphosis was not plastic across different pond durations but decreased at high larval density. Significant adaptive plasticity for growth rates appeared in environments that differed drastically in level of crowding conditions, both in the field and in the laboratory. The fact that survival of juveniles metamorphosed at high density ponds was a monotonic function of metamorphic size, implies that response to selection may occur in this population of natterjacks and that genetic variability in plasticity may be a reliable mechanism maintaining adaptive genetic variation in growth rates in the highly variable pond environment.     

Differential growth of crayfish Procambarus alleni in relation to hydrological conditions in marl prairie wetlands of Everglades National Park, USA

Environmental conditions influence crustacean growth by affecting molt intervals and incremental increases in length and weight. In the seasonally-flooded marl prairie wetlands of eastern Everglades National Park, U.S.A., hydropattern exerts considerable influence on aquatic primary productivity, and so may influence the availability of food resources for higher trophic levels. The seasonal hydroperiod has been drastically altered by anthropogenic factors, but the impacts on the aquatic community are not well known. We studied whether differences in growth of crayfish Procambarus alleni could be detected in habitats with different hydroperiods. We first described growth patterns based on incremental increases in length and weight of crayfish on a high protein diet in the laboratory. Regression analyses indicated that growth patterns in males and females were similar. Although the intermolt period increased with age, the proportional increases in length and weight were similar through successive molts. The relationship between length and weight of crayfish was best described by a power equation for allometric growth. We then compared growth curves for crayfish subpopulations from different areas of the marl prairie. In habitats with the longest hydroperiods, crayfish weight-at-size was not significantly different from that in laboratory crayfish on the high protein diet. However, weight gain per unit increase in length in short hydroperiod sites was significantly less than in long hydroperiod sites or in the laboratory. These results indicate that crayfish productivity may be associated with hydroperiod in these stressed wetlands, and this may contribute to observed source-sink population regulation.     

Macroinvertebrate community structure across a wetland hydroperiod gradient in southern New Hampshire,USA

We conducted a field study to examine the influence of hydroperiod and concomitant changes in abiotic (wetland size, pH, conductivity, dissolved oxygen and water temperature) and biotic (predatory fish presence) characteristics on macroinvertebrate communities in isolated wetlands in southern New Hampshire. Invertebrates were sampled using dipnet sweeps in 42 wetlands with short (<4 months), intermediate (4–11 months) or long (permanent) hydroperiods in 1998 and 1999. We found that invertebrate genera richness, and to a lesser degree abundance, increased linearly along the hydrological gradient, and in response to temperature and dissolved oxygen. Relative abundance of genera also differed markedly with respect to hydroperiod. Most notably, invertebrate communities changed from Acilius-dominated communities to Notonecta-dominated communities. Invertebrate relative abundances in permanent wetlands also differed with respect to the occurrence of predatory fish. Some genera (e.g., Libellula, and Dytiscus) were more likely to occur in permanent wetlands without fish, whereas other genera (e.g., Buena, and Basiaeshna) were more likely to occur in wetlands with predatory fish. Because aquatic invertebrate communities differed markedly with respect to wetland hydroperiod, and in relation to the occurrence of predatory fish, it is essential to retain a diversity of wetlands in the landscape to ensure the long-term persistence of aquatic invertebrate biodiversity.     

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.     

The role of pollutant accumulation in determining the use of stormwater ponds by amphibians

Urbanization often results in the creation of habitats such as stormwater management ponds. Although stormwater ponds are designed to retain runoff and associated pollutants, they are frequently colonized by wildlife including pond-breeding amphibians. Understanding of the ecological function of these created habitats is limited. This study investigated the role of pollutants in shaping use of stormwater ponds by amphibians. A survey of 68 stormwater ponds in Baltimore County, Maryland, and statistical modeling found wood frogs (Rana [= Lithobates] sylvatica) were more likely to breed in ponds with longer hydroperiods and Cl^? concentrations below approximately 250 mg/L. American toad (Bufo [= Anaxyrus] americanus) use of ponds was primarily influenced by hydroperiod; toads were more likely to use longer hydroperiod ponds. To confirm use was a result of toxicity and differential sensitivity among species, wood frog and American toad embryos and larvae were exposed to sediment from six stormwater ponds spanning the range of pollutant conditions documented in the field. Survival of wood frogs through metamorphosis was related to metal and salt levels of pond sediments, but survival of American toads was not. In agreement with the field study, no wood frog larvae survived to metamorphosis when Cl^? levels were above 260 mg/L. The results suggest that pollutants that accumulate in stormwater ponds, specifically road deicing salts, are acting as local filters capable of creating unique assemblages of anuran larvae in urban areas.     

Inter- and intra-annual variations of macroinvertebrate assemblages are related to the hydroperiod in Mediterranean temporary ponds

Macroinvertebrate assemblages of 22 temporary ponds with different hydroperiod were sampled monthly during a dry year (2005–2006) and a wet year (2006–2007). Coleopteran and Heteropteran adults were most abundant at the end of the hydroperiod, while Coleopteran larvae, mainly Dytiscidae, were mostly recorded in spring. Macroinvertebrate assemblages differed between study years. The shorter hydroperiod of ponds in the dry year constrained the length of the aquatic period for macroinvertebrates, and three distinct wet phases of community composition could be distinguished: filling phase, aquatic phase and drying phase. In the wet year, with a longer pond hydroperiod, five phases could be identified, with the aquatic phase differentiated into winter, early spring and late spring phases. Dispersers such as Anisops sardeus, Berosus guttalis or Anacaena lutescens were typical during the filling phase and Corixa affinis or Enochrus fuscipennis during the drying phase. The ponds with intermediate hydroperiod showed a similar composition (mainly dispersers) at the beginning and end of their wet period; this is not being seen in early drying or long hydroperiod ponds. A general pattern was detected, with similar variation between both years, which may be associated with the life histories of the macroinvertebrate taxa recorded. Guest editors: B. Oertli, R. Cereghino, A. Hull & R. Miracle Pond Conservation: From Science to Practice. 3rd Conference of the European Pond Conservation Network, Valencia, Spain, 14–16 May 2008     

Emergence and dynamics of cyclopoid copepods in an unpredictable environment

SUMMARY. 1. We studied species composition, abundance and population dynamics of cyclopoid copepods emerging from dormancy from the sediments of a temporary pond in South Carolina in 1985, 1988 and 1989. During a drought in 1988–89, the maximum hydroperiod was 19 days; in 1985 and late 1989-early 1990, the hydroperiods were 57 and 118 days. We also report on species present in 1984 and 1987, two years that had longer hydroperiods, and on abundance of cyclopoids in 1984. 2. Within a day after standing water appeared, fourth-instar copepodids of Diacydops haueri and D. crassicaudis brachycercus became active. These two species appeared every time the pond filled in winter, spring, or autumn. Other species, including Acanthocyclops vernalis, were usually not collected until weeks or months after the pond filled. Because the times and durations of fillings were extremely variable, species composition differed among years, with the most species (eleven) appearing in 1984, the year with the longest hydroperiod. 3. The abundances of emerging Diacydops were much lower in 1988 and 1989 (range of means from seven fillings in spring and autumn: 675–7382 animals m^?2) than 1985 (range of means from three fillings in winter: 26,037–107,418 animals m^?2). Low abundances of emerging animals could have been caused by poor survival of dormant animals, poor production during preceding seasons, or incomplete emergence of the dormant populations. 4. Substrate samples from the dry pond were collected in spring, summer, and autumn 1988 and winter 1989 to measure emergence of the cyclopoids in laboratory incubations. Population densities of emerging animals were much lower in 1988 and 1989 (range of means from seven experiments: 0–120 animals m^?2 over the first 3 days) than in similar experiments in 1984 (Taylor & Mahoney, 1990, means from two experiments: 3630 and 6890 animals m^?2). 5. Despite the low abundance of animals emerging from dormancy in late 1989, the cyclopoid populations in 1990 reached similar densities of copepodids (10^4?-10⁵ animals m^?2) to those reached in 1984 and 1985. These results suggest that short generation time and high reproductive capacity permit rapid recovery from population reductions.     

The impacts of overwintered green frog larvae on wood frog embryo mortality under a wetter climate

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Connecting carbon and nitrogen storage in rural wetland soil to groundwater abstraction for urban water supply

We investigated whether groundwater abstraction for urban water supply diminishes the storage of carbon (C), nitrogen (N), and organic matter in the soil of rural wetlands. Wetland soil organic matter (SOM) benefits air and water quality by sequestering large masses of C and N. Yet, the accumulation of wetland SOM depends on soil inundation, so we hypothesized that groundwater abstraction would diminish stocks of SOM, C, and N in wetland soils. Predictions of this hypothesis were tested in two types of subtropical, depressional‐basin wetland: forested swamps and herbaceous‐vegetation marshes. In west‐central Florida, >650 ML groundwater day^?1 are abstracted for use primarily in the Tampa Bay metropolis. At higher abstraction volumes, water tables were lower and wetlands had shorter hydroperiods (less time inundated). In turn, wetlands with shorter hydroperiods had 50–60% less SOM, C, and N per kg soil. In swamps, SOM loss caused soil bulk density to double, so areal soil C and N storage per m² through 30.5 cm depth was diminished by 25–30% in short‐hydroperiod swamps. In herbaceous‐vegetation marshes, short hydroperiods caused a sharper decline in N than in C. Soil organic matter, C, and N pools were not correlated with soil texture or with wetland draining‐reflooding frequency. Many years of shortened hydroperiod were probably required to diminish soil organic matter, C, and N pools by the magnitudes we observed. This diminution might have occurred decades ago, but could be maintained contemporarily by the failure each year of chronically drained soils to retain new organic matter inputs. In sum, our study attributes the contraction of hydroperiod and loss of soil organic matter, C, and N from rural wetlands to groundwater abstraction performed largely for urban water supply, revealing teleconnections between rural ecosystem change and urban resource demand.     

The relative importance of wetland size and hydroperiod for amphibians in southern New Hampshire,USA

In the United States, the regulatory approach to wetland protection has a traditional focus on size as a primary criterion, with large wetlands gaining significantly more protection. Small, isolated wetlands have received less protection; however, these wetlands play a significant role in the maintenance of biodiversity of many taxonomic groups, including amphibians. An important question for directing conservation and management efforts for amphibians is whether size is a useful criterion for regulatory decisions. Because hydroperiod has an important influence on amphibian composition in wetlands, I conducted a study to examine the relative influence of wetland size and hydroperiod on amphibian occurrence. I sampled 103 wetlands in southern New Hampshire in 1998 and 1999 using dipnet sampling to document the presence of larval amphibians. Wetlands were placed into one of three hydroperiod categories; short (<4 months), intermediate (4–11 months), or long (permanent) based on field observations of drying pattern. Wetland size was determined from digitized national wetland inventory (NWI) maps (most wetlands) or measured in the field. I examined patterns of amphibian species richness and individual species occurrence using generalized linear models. Wetland size ranged from 0.01 to 3.27 ha. Overall, species richness was significantly influenced by hydroperiod (χ² = 18.6, p <0.001), but not size (χ² = 1.4, p = 0.24). Examination within hydroperiod categories revealed several significant relationships with wetland size. Species richness was related to wetland size in wetlands with short and intermediate hydroperiods, but not wetlands with long hydroperiods. Wetland size does not appear to be a useful sole criterion for determining wetland functional value for amphibians; assessments of functions of seasonally inundated wetlands for amphibians would benefit from examination of hydroperiod.     

Invasive bullfrog larvae lack developmental plasticity to changing hydroperiod

Determining the mechanisms responsible for the success of invasive species is critical for developing effective management strategies. Artificially draining managed wetlands to maintain natural ephemeral conditions is a common practice in the Pacific Northwest and is assumed to kill invasive American bullfrog (Lithobates catesbeianus) larvae, which typically overwinter in permanent wetlands before metamorphosis. Bullfrogs in the Willamette Valley, Oregon, however, have invaded ephemeral wetland sites with confirmed metamorphosis within 4 months after hatching at 1 site. We hypothesized that plasticity in growth and development rates in response to hydroperiod facilitated bullfrog invasion in Oregon. We tested this hypothesis by quantifying larval bullfrog development and growth in response to 3 hydroperiod conditions in a mesocosm setting. We tested clutches collected from both ephemeral (n = 3) and permanent (n = 3) wetlands. We found no differences in development or growth due to hydroperiod treatments (body length, P = 0.48; mass, P = 0.27), but we found differences in growth among clutches (P ≤ 0.001). These differences likely represent natural variation in growth rates because clutches collected from the same wetland type did not respond with similar growth and geographic barriers between collection sites did not account for the differences. These results indicate a lack of plasticity to hydroperiod and suggest that artificial hydroperiod manipulation in the Pacific Northwest will not induce rapid metamorphosis by invasive bullfrog larvae, although some genotypes may be capable of rapid growth and metamorphosis. © 2013 The Wildlife Society.