A Cross-System Comparison of Bacterial and Fungal Biomass in Detritus Pools of Headwater Streams

The absolute amount of microbial biomass and relative contribution of fungi and bacteria are expected to vary among types of organic matter (OM) within a stream and will vary among streams because of differences in organic matter quality and quantity. Common types of benthic detritus [leaves, small wood, and fine benthic organic matter (FBOM)] were sampled in 9 small (1st-3rd order) streams selected to represent a range of important controlling factors such as surrounding vegetation, detritus standing stocks, and water chemistry. Direct counts of bacteria and measurements of ergosterol (a fungal sterol) were used to describe variation in bacterial and fungal biomass. There were significant differences in bacterial abundance among types of organic matter with higher densities per unit mass of organic matter on fine particles relative to either leaves or wood surfaces. In contrast, ergosterol concentrations were significantly greater on leaves and wood, confirming the predominance of fungal biomass in these larger size classes. In general, bacterial abundance per unit organic matter was less variable than fungal biomass, suggesting bacteria will be a more predictable component of stream microbial communities. For 7 of the 9 streams, the standing stock of fine benthic organic matter was large enough that habitat-weighted reach-scale bacterial biomass was equal to or greater than fungal biomass. The quantities of leaves and small wood varied among streams such that the relative contribution of reach-scale fungal biomass ranged from 10% to as much as 90% of microbial biomass. Ergosterol concentrations were positively associated with substrate C:N ratio while bacterial abundance was negatively correlated with C:N. Both these relationships are confounded by particle size, i.e., leaves and wood had higher C:N than fine benthic organic matter. There was a weak positive relationship between bacterial abundance and streamwater soluble reactive phosphorus concentration, but no apparent pattern between either bacteria or fungi and streamwater dissolved inorganic nitrogen. The variation in microbial biomass per unit organic matter and the relative abundance of different types of organic matter contributed equally to driving differences in total microbial biomass at the reach scale.     

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.     

Impact and management of purple loosestrife (Lythrum salicaria) in North America

The invasion of non-indigenous plants is considered a primary threat to integrity and function of ecosystems. However, there is little quantitative or experimental evidence for ecosystem impacts of invasive species. Justifications for control are often based on potential, but not presently realized, recognized or quantified, negative impacts. Should lack of scientific certainty about impacts of non-indigenous species result in postponing measures to prevent degradation? Recently, management of purple loosestrife (Lythrum salicaria), has been criticized for (1) lack of evidence demonstrating negative impacts of L. salicaria, and (2) management using biocontrol for lack of evidence documenting the failure of conventional control methods. Although little quantitative evidence on negative impacts on native wetland biota and wetland function was available at the onset of the control program in 1985, recent work has demonstrated that the invasion of purple loosestrife into North American freshwater wetlands alters decomposition rates and nutrient cycling, leads to reductions in wetland plant diversity, reduces pollination and seed output of the native Lythrum alatum, and reduces habitat suitability for specialized wetland bird species such as black terns, least bitterns, pied-billed grebes, and marsh wrens. Conventional methods (physical, mechanical or chemical), have continuously failed to curb the spread of purple loosestrife or to provide satisfactory control. Although a number of generalist insect and bird species utilize purple loosestrife, wetland habitat specialists are excluded by encroachment of L. salicaria. We conclude that (1) negative ecosystem impacts of purple loosestrife in North America justify control of the species and that (2) detrimental effects of purple loosestrife on wetland systems and biota and the potential benefits of control outweigh potential risks associated with the introduction of biocontrol agents. Long-term experiments and monitoring programs that are in place will evaluate the impact of these insects on purple loosestrife, on wetland plant succession and other wetland biota.     

Reciprocal subsidies in ponds: does leaf input increase frog biomass export?

Reciprocal subsidies occur when ecosystems are paired, both importing and exporting resources to each other. The input of subsidies increases reciprocal subsidy export, but it is unclear how this changes with other important factors, such as ambient resources. We provide a conceptual framework for reciprocal subsidies and empirical data testing this framework using a pond–forest system in Missouri, USA. Our experiment used in situ pond mesocosms and three species of anurans: wood frogs, American toads, and southern leopard frogs. We predicted that increases in ambient resources (primary productivity) and detrital subsidy input (deciduous tree leaves) into pond mesocosms would increase reciprocal export (frog biomass) to the surrounding terrestrial ecosystem. In contrast, we found that increases in primary productivity consistently decreased frog biomass, except with leaf litter inputs. With leaf inputs, primary productivity did not affect the export of frogs, indicating that leaf detritus and associated microbial communities may be more important than algae for frog production. We found that subsidy inputs tended to increase reciprocal exports, and thus partial concordance with our conceptual framework.     

Phenotypic plasticity of introduced versus native purple loosestrife: univariate and multivariate reaction norm approaches

The plastic responses to environmental change by Lythrum salicaria (purple loosestrife) were compared between native plants derived from seeds collected in Europe and those introduced into North America. Plants from nine populations each were grown under two levels of water and nutrient conditions. At the end of the growing season, samples were evaluated for eight traits related to their life history, plant size/architecture, and reproduction. Genetic (G), environmental (E), and G × E interactions were assessed by restricted maximum likelihood (REML) analysis of covariance (ANCOVA) and multivariate analysis of covariance (MANCOVA). Both univariate and multivariate reaction norm analyses were used to test for differences in the magnitude and direction of phenotypic plasticity between introduced and native plants. Under high-nutrient conditions, introduced plants were taller and had more branches and greater aboveground biomass. They also exhibited significantly greater amounts of phenotypic plasticity for aboveground biomass than did the natives in response to changing nutrient levels in standing water. This difference in univariate plasticity contributed to the general contrast in multivariate plasticity between introduced and native plants. These results support the idea that introduced plants may successfully invade a habitat and grow better than native plants in response to increased resources.     

An invasive species imposes selection on life‐history traits of a native frog

As well as their direct ecological impacts on native taxa, invasive species can impose selection on phenotypic attributes (morphology, physiology, behaviour, etc.) of the native fauna. In anurans, body size at metamorphosis is a critical life‐history trait: for most challenges faced by post‐metamorphic anurans, larger size at metamorphosis probably enhances survival. However, our studies on Australian frogs (Limnodynastes convexiusculus) show that this pattern can be reversed by the arrival of an invasive species. When metamorph frogs first encounter invasive cane toads (Bufo marinus), they try to eat the toxic invader and, if they are able to do so, are likely to die from poisoning. Because frogs are gape‐limited predators, small metamorphs cannot ingest a toad and thus survive long enough to disperse away from the natal pond (and thus from potentially deadly toads). These data show that larger size at metamorphosis can reduce rather than increase anuran survival rates, because larger metamorphs are more easily able to ingest (and thus be poisoned by) metamorph cane toads. Our results suggest that patterns of selection on life‐history traits of native taxa (such as size and age at metamorphosis, seasonal timing of breeding and duration of pondside aggregation prior to dispersal) can be modified by the arrival of an invasive species. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 329–336.     

How Diverse Detrital Environments Influence Nutrient Stoichiometry between Males and Females of the Co-Occurring Container Mosquitoes Aedes albopictus,Ae. aegypti,and Culex quinquefasciatus

Allocation patterns of carbon and nitrogen in animals are influenced by food quality and quantity, as well as by inherent metabolic and physiological constraints within organisms. Whole body stoichiometry also may vary between the sexes who differ in development rates and reproductive allocation patterns. In aquatic containers, such as tree holes and tires, detrital inputs, which vary in amounts of carbon and nitrogen, form the basis of the mosquito-dominated food web. Differences in development times and mass between male and female mosquitoes may be the result of different reproductive constraints, which could also influence patterns of nutrient allocation. We examined development time, survival, and adult mass for males and females of three co-occurring species, Aedes albopictus, Ae. aegypti, and Culex quinquefasciatus, across environments with different ratios of animal and leaf detritus. We quantified the contribution of detritus to biomass using stable isotope analysis and measured tissue carbon and nitrogen concentrations among species and between the sexes. Development times were shorter and adults were heavier for Aedes in animal versus leaf-only environments, whereas Culex development times were invariant across detritus types. Aedes displayed similar survival across detritus types whereas C. quinquefasciatus showed decreased survival with increasing leaf detritus. All species had lower values of ¹⁵N and ¹³C in leaf-only detritus compared to animal, however, Aedes generally had lower tissue nitrogen compared to C. quinquefasciatus. There were no differences in the C:N ratio between male and female Aedes, however, Aedes were different than C. quinquefasciatus adults, with male C. quinquefasciatus significantly higher than females. Culex quinquefasciatus was homeostatic across detrital environments. These results allow us to hypothesize an underlying stoichiometric explanation for the variation in performance of different container species under similar detrital environments, and if supported may assist in explaining the production of vector populations in nature.     

Canopy Closure,Competition, and the Endangered Dusky Gopher Frog

Abstract: A major challenge facing wildlife biologists is understanding why some species go extinct while others persist in the same habitat. To address this question, we investigated whether tree canopy closure over ponds affects growth and survival of rare and common tadpoles within ponds and mediates competitive interactions among species. We conducted 2 experiments to test whether canopy closure and competition may have contributed to the decline of the endangered dusky gopher frog (Rana sevosa), but allowed the persistence of the southern leopard frog (R. sphenocephala). We explored the response of both species to canopy closure in single-species and mixed- (1:1) species treatments of identical total tadpole density. An experiment using aquatic enclosures in temporary ponds showed that canopy closure reduced tadpole growth approximately 20% for both species. Survival of dusky gopher frog tadpoles was higher in mixed-species enclosures than in single-species enclosures. In a complementary experiment using artificial ponds, dusky gopher frogs had lower survival to metamorphosis, reduced size at metamorphosis, and produced a lower total biomass of metamorphosed juveniles in shaded ponds. Southern leopard frogs exhibited reduced body size at metamorphosis only when shaded. These studies suggest that pond canopy closure, not larval competition, may be contributing to the decline of the dusky gopher frog. The different responses to canopy closure suggest a potential mechanism for the loss of dusky gopher frogs and the persistence of southern leopard frogs. Removal of trees from historically open-canopy ponds may help facilitate the recovery of dusky gopher frogs and benefit similar species.     

Comparison of life history traits between invasive and native populations of purple loosestrife (Lythrum salicaria) using nonlinear mixed effects model

We compared growth patterns of invasive and native populations of purple loosestrife (Lythrum salicaria) while varying water and nutrient levels. We examined three life-history traits (height, number of branches, and the size of largest leaf) during the growth period adopting a nonlinear mixed effects model. Invasive populations were found to be slower in shoot elongation but grew to be taller than native populations. Invasive populations produced more branches than natives only in the high water, high nutrient treatment. Invasive populations had a similar increase in the size of the largest leaf compared to natives, but ultimately produced a greater size of largest leaf than natives. Invasive populations were found to display a greater vegetative expansion, but this was not strongly affected by our treatments.     

Post-metamorphic change in activity metabolism of anurans in relation to life history

Summary Newly-metamorphosed individuals of some species of frogs and toads differ from adults in behavior, ecology, and physiology. These differences may be related to broader patterns of the life histories of different species of frogs. In particular, the length of larval life and the size of a frog at metamorphosis appear to be significant factors in post-metamorphic ontogenetic change. These changes in performance are associated with rapid post-metamorphic increases in oxygen transport capacity. Bufo americanus (American toads) and Rana sylvatica (wood frogs) spend only 2–3 months as tadpoles and metamorphose at body masses of 0.25 g or less. Individuals of these species improve endurance and aerobic capacity rapidly during the predispersal period immediately following metamorphosis. Increases in hematocrit, hemoglobin concentration, and heart mass relative to body mass are associated with this improvement in organismal performance. Rana clamitans (green frogs) spend from 3 to 10 months as larvae and weigh 3 g at metamorphosis. Green frogs did not show immediate post-metamorphic increases in performance. Rana palustris (pickerel frogs) are intermediate to wood frogs and green frogs in length of larval life and in size at metamorphosis, and they are intermediate also in their post-metamorphic physiological changes.American toads and wood frogs appear to delay dispersal from their natal ponds while they undergo rapid post-metamorphic growth and development, whereas green frogs disperse as soon as they leave the water, even before they have fully absorbed their tails. The very small body sizes of newly metamorphosed toads and wood frogs appear to limit the scope of their behaviors. The brief larval periods of these species permit them to exploit transient aquatic habitats, but impose costs in the form of a period of post-metamorphic life in which their activities are restricted in time and space compared to those of adults.     

Comparison of life history traits between invasive and native populations of purple loosestrife (Lythrum salicaria) using nonlinear mixed effects model

We compared growth patterns of invasive and native populations of purple loosestrife (Lythrum salicaria) while varying water and nutrient levels. We examined three life-history traits (height, number of branches, and the size of largest leaf) during the growth period adopting a nonlinear mixed effects model. Invasive populations were found to be slower in shoot elongation but grew to be taller than native populations. Invasive populations produced more branches than natives only in the high water, high nutrient treatment. Invasive populations had a similar increase in the size of the largest leaf compared to natives, but ultimately produced a greater size of largest leaf than natives. Invasive populations were found to display a greater vegetative expansion, but this was not strongly affected by our treatments.     

The influence of below ground herbivory and plant competition on growth and biomass allocation of purple loosestrife

Experiments investigating plant-herbivore interactions have primarily focused on above-ground herbivory, with occasional studies evaluating the effect of below-ground herbivores on plant performance. This study investigated the growth of the wetland perennial Lythrum salicaria (purple loosestrife) under three levels of root herbivory by the weevil Hylobiustransversovittatus and three levels of plant competition by the grass Phleumpratense in a common garden. Plant growth, flowering phenology, and biomass allocation patterns of purple loosestrife were recorded for two growing seasons. During the first year, root herbivory reduced plant height; plant competition delayed flowering; and the interaction of root herbivory and plant competition resulted in reductions in plant height, shoot weight and total dry biomass. Plant competition or larval feeding did not affect the biomass allocation pattern in the first year. These results indicate the importance of interactions of plant competition and herbivory in reducing plant performance – at least during the establishment period of purple loosestrife. In the second growing season, root herbivory reduced plant height, biomass of all plant parts, delayed and shortened the flowering period, and changed the biomass allocation patterns. Plant competition delayed flowering and reduced the dry weight of fine roots. The interaction of root herbivory and plant competition delayed flowering. Root herbivory was more important than plant competition in reducing the performance of established purple loosestrife plants. This was due, in part, to intense intraspecific competition among the grass individuals effectively preventing shoot elongation of P. pratense and resulting in a carpet like growth. Received: 3 April 1997 / Accepted: 27 July 1997     

Reproductive strategy and carry-over effects for species with complex life histories

We investigated the influences of natal-pool and metamorph characteristics on juvenile survival, age-specific breeding probabilities, and dispersal of wood frogs (Lithobates sylvatica) and used this information to infer how life history strategies of short-lived species may offset risks associated with breeding in highly variable habitats. We used multistate mark-recapture data from eight wood frog populations in Maryland, USA, to investigate the influences of natal-pond and metamorph characteristics on post-metamorphic demographics. We found that post-metamorphic juvenile survival was highly variable and negatively influenced by abiotic conditions experienced during development but showed little relationship to larval density or size at metamorphosis. Estimates of recruitment and dispersal probabilities indicated that males mature earlier than females, and a small percentage of each sex disperse to non-natal pools. Survival probabilities for adults during the non-breeding season were less variable than juvenile rates, lower for females, and negatively related to mean monthly precipitation. Survival of adults during the breeding season was generally very high. We provide the first robust estimates of post-metamorphic vital rates of wood frogs that allow for variation in capture probabilities. We found little evidence for an effect of metamorph traits on juvenile survival, suggesting that wood frogs may be able to overcome initial disadvantages to have similar post-metamorphic performance. Our study suggests that variation in the age of maturity for wood frogs may mitigate risks associated with breeding in a highly variable habitat to maximize lifetime fitness without increasing lifespan, and this strategy is minimally affected by carry-over effects from the larval stage.     

Urea is not a universal cryoprotectant among hibernating anurans: Evidence from the freeze-tolerant boreal chorus frog (Pseudacris maculata)

Freeze-tolerant organisms accumulate a diversity of low molecular weight compounds to combat negative effects of ice formation. Previous studies of anuran freeze tolerance have implicated urea as a cryoprotectant in the wood frog (Lithobates sylvatica). However, a cryoprotective role for urea has been identified only for wood frogs, though urea accumulation is an evolutionarily conserved mechanism for coping with osmotic stress in amphibians. To identify whether multiple solutes are involved in freezing tolerance in the boreal chorus frog (Pseudacris maculata), we examined seasonal and freezing-induced variation in several potential cryoprotectants. We further tested for a cryoprotective role for urea by comparing survival and recovery from freezing in control and urea-loaded chorus frogs. Tissue levels of glucose, urea, and glycerol did not vary significantly among seasons for heart, liver, or leg muscle. Furthermore, no changes in urea or glycerol levels were detected with exposure to freezing temperatures in these tissues. Urea-loading increased tissue urea concentrations, but failed to enhance freezing survival or facilitate recovery from freezing in chorus frogs in this study, suggesting little role for urea as a natural cryoprotectant in this species. These data suggest that urea may not universally serve as a primary cryoprotectant among freeze-tolerant, terrestrially hibernating anurans.     

Benthic invertebrate communities in six Danish forest streams: Impact of forest type on structure and function

I investigated the impact of riparian vegetation type on stream invertebrate communities in SIX Danish forest streams during the period 1992–1993 Two of the streams ran through beech forest Fagus sylvatica , two through mixed, mainly deciduous forest, and two through conifer plantations Algal biomass and detritus standing stock differed significantly depending on forest type In mixed deciduous forest streams algal biomass was higher and detritus standing stock lower than in beech and conifer forest streams Benthic invertebrate community composition was functionally similar in all streams, with detritivores comprising >80% by numbers of the community Grazer abundance was low in all streams Shredder abundance in the four deciduous streams was significantly correlated to coarse detritus standing stock When taking into account both quantitative and qualitative POM parameters for all six streams combined, shredder abundance correlated significantly to the amount of CPOM There were significantly more invertebrates with a ≥ 2 yr life cycle in the conifer and beech forest streams than m the mixed forest streams, probably reflecting the larger and more stable food resource For all streams combined there was a significant correlation between predator abundance and the abundance of potential prey My findings suggest that Danish forest streams are regulated by "bottom-up control" at all trophic levels within the invertebrate community, and hence that forest type can structure benthic communities in forest streams     

Comparing amphibian habitat quality and functional success among natural,restored, and created vernal pools

The fact that several vernal pool restoration and creation attempts in eastern Pennsylvania and New Jersey have been paired with conservation of natural pools in the same area provided a valuable research opportunity to compare amphibian habitat quality between project sites and natural reference pools. To measure desired outcomes, we used successful reproduction and metamorphosis of two vernal pool indicator species, the wood frog and spotted salamander. Although many previous studies indicate that restored and created pools rarely replace function lost in the destruction of natural pools, success of vernal pool indicator species was not necessarily related to pool type in this study. Results indicate a strong correlation between reproductive success for both species and vernal pool size (i.e. mean depth and volume), regardless of pool type. Although overall survival rates of wood frog larvae were significantly higher in natural pools with hydroperiods between 12 and 35 weeks, wood frogs were also successful in one restored and one created vernal pool. Salamander survival rates were highest in two natural and two created pools, which had in common both greater volumes and higher proportions of forest land cover in the surrounding 1,000 m. The documented success of vernal pool indicator species in two well‐established created pools demonstrates that pool creation can sometimes restore communities and ecological functions lost, especially when nearby natural pools are degraded or destroyed.     

Liver glycogen,glucose mobilization and freezing survival in chorus frogs,Pseudacris triseriata

We compared liver glycogen stores and glucose mobilization during freezing among winters in chorus frogs, Pseudacris triseriata, where populations varied in freezing survival. We also characterized tissue glycogen levels across the annual cycle. Frogs with low liver glycogen stores mobilized low amounts of glucose during freezing, and these were correlated with population variation in freezing survival. Moreover, liver glycogen stores were significantly and positively related to body mass. These data suggest that chorus frogs store liver glycogen in preparation for hibernation and that body size and glycogen stores must reach threshold levels for successful survival of freezing bouts during the winter.     

Nutrient dynamics of vegetation and detritus following two intensities of fire in the New Jersey pine barrens

The nutrient dynamics of upland forest sites in the New Jersey Pine Barrens exposed to different fire intensities were determined. Nutrient concentrations and inventories in biomass, litter, humus, and standing dead wood were determined for an unburned site, two sites burned by severe wildfire and two sites burned by light prescribed burning. Humus nutrient levels were similar among sites despite differing fire histories. Amounts of calcium, magnesium, and potassium in biomass and in litter were lower in wildfire sites than in prescribed burn sites, all of which were lower than the control. Standing dead wood nutrient levels were much higher in the wildfire sites than in the other three sites. Output of nutrients to groudwater correlated poorly with the amounts of nutrients retained in humus or standing dead wood; however strong inverse correlations were found between nutrient output and nutrient storage in biomass or biomass plus litter and humus. These results emphasize the central role of nutrient immobilization in regrowing biomass in after fire on nutrient poor soils.     

Anthropogenic changes to leaf litter input affect the fitness of a larval amphibian

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The effects of flooding,plant traits,and predation on purple loosestrife leaf‐beetles

Classical biological weed control is based on the premise that introducing specialized natural enemies from the native range re‐establishes herbivore control of plant invaders, ultimately leading to negative population growth rates. Evidence from past biocontrol programs suggests that herbivores are not solely responsible for shaping plant demography. Diverse environmental conditions in the introduced range may not only affect demography, but also influence top‐down control of target plants. We investigated how flooding affects impacts of predators (top‐down) and plant quality (bottom‐up) on performance of two leaf‐beetles, Galerucella calmariensis L. and Galerucella pusilla Duftschmid (Coleoptera: Chrysomelidae: Galerucini), released in North America as biocontrol agents of purple loosestrife, Lythrum salicaria L. (Lythraceae). Predation and flooding regime have been linked to low leaf‐beetle recruitment at sites where insects failed to attain outbreak populations. Predator exclusion experiments at adjacent flooded and non‐flooded sites indicated a positive effect of flooding on leaf‐beetle survival for all developmental stages, whereas predator exposure had little effect. There was no difference in predation rates at sites with successful or failed purple loosestrife control, questioning the importance of predation in limiting growth and impact of these biocontrol agents’ populations. Effect of flooding on purple loosestrife quality was evaluated in a common garden study where plants were grown under different flooding treatments. Plants grown in flooded soil had higher water content and lower tannic acid concentration than plants grown in well‐drained soil. Consistent with field observations, leaf‐beetle oviposition rate and survival were higher on flooded plants. Results indicate that both bottom‐up and top‐down forces operate on Galerucella populations, yet their relative strength is mediated by flooding regime. Ignoring intricacies of plant‐herbivore and trophic interactions in the introduced range appears to be a major handicap for the improvement of weed biocontrol programs.