Effects of agricultural landscape and pesticides on parasitism in native bullfrogs

Native amphibian populations are shrinking worldwide, and both parasitic infections and environmental stress from agriculture have been implicated. We investigated the principal hypothesis that environmental by-products of agricultural activity mediate parasitism in native frogs. Bullfrogs were collected from wetlands with variable landscape disturbance and water quality and examined for helminth parasites. We predicted that pesticide pollution and landscape development would be significant factors shaping the parasite communities and populations. Parasite diversity and species richness were lower in wetlands impacted by both pesticides and land use. Two parasite groups, direct life-cycle nematodes and echinostomes, were common in polluted habitats, potentially increasing frog pathology and mortality risk. In areas with agricultural landscape and reduced forest cover, parasite diversity and species richness were low, perhaps because of less parasite transmission from birds and mammals. This result suggests that land development limits terrestrial vertebrate access to wetlands. Our results indicate that parasite abundance and community structure in wetlands are influenced by factors operating locally within the wetland and more broadly in the surrounding landscape. We suggest that parasite communities in amphibians are effective indicators of ecosystem health and animal biodiversity, and thus useful tools for conservation biology.     

A meta-analysis of fauna and flora species richness and abundance in plantations and pasture lands

We conducted a systematic global review of differences between timber plantations and pasture lands in terms of animal and plant species richness and abundance, and assessed the results using meta-analysis techniques. Our principal aim was to test the hypothesis that plantations contain higher species richness or abundance than pasture. Of the 1967 studies of potential relevance, 66 provided biological monitoring information and 36 met the requirements for meta-analyses. Sufficient data were available for meta-analyses to be conducted comparing the species richness and abundance of plantations and pasture lands for five taxonomic groups: plants, invertebrates, reptiles/amphibians, mammals, and birds. Within each taxon there was considerable variation in the difference between species richness and abundance between plantations and pasture lands. Birds and reptile/amphibians exhibited significantly higher species richness, and mammals exhibited significantly higher abundance, in plantations than in pasture lands which lacked remnant vegetation. Reptile/amphibian species richness was significantly higher in plantations in general. No significant differences in species richness were found for mammals, plants, or invertebrates, and no significant differences in abundance were found for birds, reptiles/amphibians, invertebrates, or plants. It is only within the presence of taxonomic caveats (i.e. reptiles/amphibians), or specific landscape features (i.e. absence of remnant vegetation within pasture), that it can be concluded that plantations support higher species richness or abundance than pasture land. We emphasize that caution is warranted when making general statements about the inherent biodiversity value of diverse and broadly-defined land-uses.     

Detrimental effects of peat mining on amphibian abundance and species richness in bogs

Peat bogs of eastern North America are increasingly being mined for peat, which results in the extensive draining of these systems. First, I quantified the effects of peat mining on amphibian species richness and abundance in bog remnants adjacent to mining activity and unmined bogs. I then investigated amphibian occurrence patterns in bog remnants relative to the distance to the mined edge, after accounting for microhabitat and distance to landscape elements. The response of amphibians to peat mining varied across taxa. For instance, amphibian species richness, total amphibian captures (all spp. combined), as well as those of green frogs (Rana clamitans) were greater in unmined bogs than in bog remnants. Wood frogs (Rana sylvatica) were most abundant in unmined bogs in areas far from bog ponds, as well as in unmined bogs in areas near shrub patches. After accounting for microhabitat and landscape variables, American toads (Bufo americanus) in bog remnants were most abundant close to mined edges, whereas the other species did not respond to the proximity of mined edges. Peat mining negatively impacts amphibians occurring in bog remnants. The response of amphibians to peat mining is similar to the one of birds and plants, but differs with small mammals. Maintaining a complex mosaic of bog ponds, shrub and forest patches might help mitigate peat mining effects on amphibian patterns of abundance.     

Recovery of amphibian species richness and composition in a chronosequence of secondary forests,northeastern Costa Rica

In some tropical regions, following the abandonment of agriculture and pastures, secondary forests can recover plant species richness and forest structure (e.g. canopy cover, biomass); however, the importance of these secondary forests for fauna is not clear. Secondary forests can benefit fauna by providing suitable habitats, connecting forests fragments, and increasing gene flow. Previous studies of forest regeneration have showed different levels of amphibian recovery. In Puerto Rico, 1–5 years old secondary forests achieved similar amphibian species richness and composition in comparison with old-growth forests, while in Brazil secondary forests from 14 to 19 years of recovery only recovered 60% of the species of old-growth forests. We evaluated amphibian recovery in secondary forests in northeastern Costa Rica, by assessing amphibian recovery in 12 secondary forests that vary in age of recovery and in three old-growth forests using visual and acoustic surveys. Our sites varied in terms of their landscape (e.g. amount of surrounding forest) and forest characteristics (e.g. forest age, aboveground biomass, basal area, number of tree species, number of stems, leaf-litter depth), but there was no relationship between these characteristics and amphibian species richness or species composition. We found that amphibians are recovering rapidly in secondary forests in Costa Rica, and even young forests (10–16 years) had similar species richness and composition in comparison with old-growth forests. These forests are providing suitable microhabitats conditions for amphibians. In addition, this study highlights the importance of landscape characteristics. The abundance of amphibian species sources (e.g. forest patches) and connections between forests appear to be helping the species colonize these sites. Worldwide, the area of secondary forests is increasing, and our results show that these habitats are suitable for a diversity of amphibian species, suggesting that these forests can help reduce amphibian population and species decline.     

Temporal variation of soil compaction and deterioration of soil quality in pasture areas of Colombian Amazonia

In the Amazon basin, tropical rainforest is being slashed and burned at accelerated rates for annual crops over a couple of years, followed by forage grasses. Because of poor management, the productivity of established pastures declines in a few years so that grazing plots are abandoned and new areas are deforested. Previous studies in the region report higher bulk density in soils under pasture than in similar soils under forest. The objective of this study was to detect changes in the physical quality of the topsoil of nutrient-poor Typic Paleudults in the colonisation area of Guaviare, Colombian Amazonia, and analyse the effect of soil deterioration on pasture performance. Temporal variation of soil compaction under pasture was analysed by comparing natural forest taken as control and pasture plots of Brachiaria decumbens (Stapf) grouped into three age ranges (<3, 3–9, >9 years). Evidence of soil compaction through cattle trampling, after clearing the primary forest, included the formation of an Ap horizon with platy structure and dominant greyish or olive colours, reflecting impaired surface drainage, the increase of bulk density and penetration resistance, and the decrease of porosity and infiltration rate. From primary forest to pastures older than 9 years, bulk density of the 5–10 cm layer increase was 42% in fine-textured soils and 30% in coarse-textured soils. Penetration resistance ranged from 0.45 MPa under forest to 4.25 MPa in old pastures, with maximum values occurring at 3–12 cm depth in pastures older than 9 years. Average total soil porosity was 58–62% under forest and 46–49% under pasture. Basic infiltration dropped from 15 cm h⁻¹ in the original forest conditions to less than 1 cm h⁻¹ in old pastures. Crude protein content and dry matter yield of the forage grass steadily decreased over time. No clear relationship between declining protein content as a function of pasture age and changes in chemical soil properties was found, but there was a high negative correlation (r=−0.81) between protein content and bulk density, reflecting the effect of soil compaction on pasture performance. After about 9–10 years of use, established grass did no longer compete successfully with invading weeds and grazing plots were abandoned. As land is not yet a scarcity in this colonisation area, degraded pastures are seldom rehabilitated.     

Differential responses of amphibians and reptiles in riparian and stream habitats to land use disturbances in western Georgia, USA

Urban and agricultural land uses have caused documented declines in diversity of many organisms. However, responses of stream- and riparian-dwelling amphibians and reptiles to anthropogenic land development are collectively understudied and, when studied, are often grouped together as though these two taxa respond in a similar fashion. We surveyed watersheds in four land use categories (reference, pasture, developing, and urban) for amphibian and reptile species richness over a two-year period in the southeastern United States. Total herpetofauna species richness was equivalent among all watershed types, but amphibians and reptiles responded differently to urbanization when analyzed separately. Urban watersheds had significantly fewer amphibian species than all other watershed types, but these losses were counterbalanced by significant increases in reptile species richness in these same watersheds. We also found that local, riparian-scale, habitat differences were strongly correlated with species composition differences between pasture and developing watersheds. While the difference in species composition between pasture and developing sites may have multiple, species-specific explanations, the dramatic differences between amphibian and reptile species richness in urban watersheds suggest broad trends that may be important to conservation planning. Specifically, our observations suggest that urbanization in the studied watersheds alters small streams from closed-canopy, shallow-water features of the forested landscape likely present before settlement by Europeans and favored by many salamanders and frogs to features associated with open vegetation and deeper, warmer waters favored by riverine turtles and snakes. We conclude that amphibians and reptiles, despite some physiological similarities, are not equivalent for monitoring purposes. Additionally, if future development in the Piedmont mirrors current urbanization, then significant faunal shifts seem assured.     

Switch of tropical Amazon forest to pasture affects taxonomic composition but not species abundance and diversity of arbuscular mycorrhizal fungal community

Arbuscular mycorrhizal fungi (AMF) community composition and species richness are affected by several factors including soil attributes and plant host. In this paper we tested the hypothesis that conversion of tropical Amazon forest to pasture changes taxonomic composition of AMF community but not community species abundance and richness. Soil samples were obtained in 300 m × 300 m plots from forest (n = 11) and pasture (n = 13) and fungal spores extracted, counted and identified. A total of 36 species were recovered from both systems, with 83% of them pertaining to Acaulosporaceae and Glomeraceae. Only 12 species were shared between systems and spore abundance of the majority of fungal species did not differ between pasture and forest. Spore abundance was significantly higher in pasture compared to forest but both systems did not differ on mean species richness, Shannon diversity and Pielou equitability. Species abundance distribution depicted by species rank log abundance plots was not statistically different between both systems. We concluded that conversion of pristine tropical forest to pasture influences the taxonomic composition of AMF communities while not affecting species richness and abundance distribution.     

Plant influences on native and exotic earthworms during secondary succession in old tropical pastures

Land-use changes can drastically alter earthworm communities. Native species are often lost and few exotic species, such as Pontoscolex corethrurus, rapidly prevail when tropical forests are converted to pastures. However, this process can be reversed when forests recover from abandoned pastures through secondary succession. We hypothesized (1) that the formation of forest floor mass during secondary succession in pastures promotes the recovery of native, anecic earthworms and (2) that the shift from grass vegetation in pastures to woody plants in secondary forests decreases the abundance and biomass of the exotic, endogeic P. corethrurus. To test the first hypothesis, we developed a litter manipulation experiment by removing and adding plant litter in plots of mature secondary forests in the Cayey Mountains, Puerto Rico. To test the second hypothesis we performed a greenhouse experiment to examine the influence of a pasture grass species Axonopus compressus and a dominating woody species Miconia prasina of the secondary forests on the number and biomass of the earthworm P. corethrurus. We found in the litter manipulation experiment that earthworm diversity, density and fresh weight were not affected by litter input. However, in the greenhouse experiment, A. compressus increased the number and biomass of P. corethrurus, whereas M. prasina decreased the exotic, endogeic earthworm. Our results suggest that the quantity of litter does not promote rapid changes (<1 year) in native, anecic earthworm diversity, and that the exotic, endogeic P. corethrurus is favored by grass A. compressus compared to the woody plant M. prasina. The shift in vegetation from grass to woody plants promotes the decrease in the density and biomass of the exotic, endogeic P. corethrurus during secondary succession in old tropical pastures.     

Herpetofauna diversity and microenvironment correlates across a pasture-edge-interior ecotone in tropical rainforest fragments in the Los Tuxtlas Biosphere Reserve of Veracruz, Mexico

We evaluated the relationship between amphibian and reptile diversity and microhabitat dynamics along pasture-edge-interior ecotones in a tropical rainforest in Veracruz, Mexico. To evaluate the main correlation patterns among microhabitat variables and species composition and richness, 14 ecotones were each divided into three habitats (pasture, forest edge and forest interior) with three transects per habitat, and sampled four times between June 2003 and May 2004 using equal day and night efforts. We measured 12 environmental variables describing the microclimate, vegetation structure, topography and distance to forest edge and streams.After sampling 126 transects (672 man-hours effort) we recorded 1256 amphibians belonging to 21 species (pasture: 12, edge: 14, and interior: 13 species), and 623 reptiles belonging to 33 species (pasture: 11, edge: 25, and interior: 22 species). There was a difference in species composition between pasture and both forest edge and interior habitats. A high correlation between distance to forest edge and temperature, understorey density, canopy cover, leaf litter cover, and leaf litter depth was found. There was also a strong relationship between the composition of amphibian and reptile ensembles and the measured environmental variables. The most important variables related to amphibian and reptile ensembles were canopy cover, understorey density, leaf litter cover and temperature.Based on amphibian and reptile affinity for the habitats along the ecotone, species were classified into five ensembles (generalist, pasture, forest, forest edge and forest interior species). We detected six species that could indicate good habitat quality of forest interior and their disappearance may be an indication of habitat degradation within a fragment, or that a fragment is not large enough to exclude edge effects. Different responses to spatial and environmental gradients and different degrees of tolerance to microclimatic changes indicated that each ensemble requires a different conservation strategy. We propose to maintain in the Los Tuxtlas Biosphere Reserve the forest remnants in the lowlands that have gentler slopes and a deep cover of leaf litter, a dense understorey, and high relative humidity and low temperature, to buffer the effects of edge related environmental changes and the invasion of species from the matrix.     

Presence of an emerging pathogen of amphibians in introduced bullfrogs Rana catesbeiana in Venezuela

Chytridiomycosis is an emerging fungal disease of amphibians responsible for mass mortalities and population declines globally. One hypothesis for its recent emergence is anthropogenic introduction of the causative agent Batrachochytrium dendrobatidis through trade in amphibians for pets, food and biocontrol. In this study, we examined histological samples from apparently healthy American bullfrogs Rana catesbeiana that have been introduced into the Venezuelan Andes. B. dendrobatidis was present in 96% (46/48) of the individuals examined. In contrast to cases of chytridiomycosis outbreaks, the majority (44/46) of frogs had few, small lesions consistent with little or no clinical disease and no unusual mortality was observed. These findings have implications for amphibian declines in Venezuela and elsewhere. First, the high prevalence of Batrachochytrium but lack of clinical signs or chytridiomycosis-related mortality suggests that R. catesbeiana may be a good reservoir for this parasite in Venezuela. Second, the presence of this emerging pathogen in an introduced amphibian species suggests that trade and introduction of amphibians should be monitored and controlled to halt the spread of this pathogen nationally and internationally.     

Earthworm composition,diversity and biomass under three land use systems in south-eastern Australia

In south-eastern Australia, strips of planted native trees and shrubs (shelterbelts) are frequently established to restore ecosystem services altered by agriculture. Despite their wide use, little is known about the effects of establishing shelterbelts on soil macro invertebrates, especially earthworms, which are of major importance in soil processes. We assessed earthworm composition, diversity and biomass in three land use systems: native shelterbelts dominated by Acacia and Eucalyptus species, agricultural pastures and native remnant woodland fragments dominated by Eucalyptus blakelyi and/or Eucalyptus melliodora. Earthworm communities differed significantly among systems, with abundance, biomass and diversity greatest under pasture. Within shelterbelts we saw a shift from high earthworm biomass and density to low with increasing time after establishment. Soil edaphic variables did not correlate strongly with earthworm biomass or density, but were correlated with earthworm community composition. Overall the introduction of native woody vegetation was associated with a decline in density and biomass of earthworms, including a decrease in the relative abundance of exotic species. As such shelterbelts can be used to promote native earthworm relative abundance, which may be important for local diversity, soil function and landscape connectivity.     

Habitats of the British amphibians (3): river valley marshes

An attempt to assess the distribution and abundance of the common British amphibians in low-lying river valley marshes in part of Sussex is reported. These areas contain large amounts of freshwater habitat, mainly in the form of drainage dykes which intersect otherwise continuous tracts of (predominantly) pasture land. Indigenous amphibians were, however, rare: Triturus vulgaris was well-spread across some of the areas examined but population densities were very low; Rana temporaria was very infrequent, and a single specimen of Triturus helveticus was seen. T. cristatus and Bufo bufo were not encountered at all. On the other hand, the introduced frog Rana ridibunda was common and widespread in one sector of the marshes. Possible explanations for these observations are discussed.     

Pasture and forest soil microbial communities show distinct patterns in their catabolic respiration responses at a landscape scale

Catabolic responses to specific substrates can be used to differentiate soil microbial communities. We hypothesized that the catabolic respiration responses of microbial communities from pastures would differ from those of forest soils, and that the differences would be consistent at a landscape scale, due to inherent differences in litter quality and management regimes. We analysed respiration responses to 25 different substrates of 20 pasture soils (dominated by rye grass and white clover) and 20 forest soils (indigenous forest species or the plantation species Pinus radiata) over a wide geographical range in New Zealand.Within each pasture or forest category, the catabolic responses showed a similar pattern, suggesting similarities in functional catabolic capability and microbial community Indigenous forests and pine forests microbial communities did not differ in their responses. Pasture soil communities had significantly higher relative responses to carbohydrate and amino-acid substrates and significantly lower relative response to carboxylic acid substrates, than microbial communities from forest soils. Forest soils had relatively greater responses to carboxylic acids as a group, as well as citric acid, α-ketobutyric acid, α-ketoglutaric acid, and α-ketovaleric acid, than did the pasture soils. A subset of 6 substrates was equally as effective at differentiating the microbial catabolic response of pasture soils from forest soils as the entire set of 25 substrates. The results demonstrated distinct differences in the respiration responses of the soil microbial communities of pastures and forests, but showed strong similarities within each vegetation class, despite the wide geographical spread, different soils and plant species.     

Nitrogen dynamics in different types of pasture in the Austrian Alps

 Soil N dynamics were compared in Alpine pastures on two mountains. N-pool sizes and N fluxes were measured relative to N losses via leaching and denitrification in summer. On each mountain, four types of pasture were studied: (1) forest pastures, (2) recently developed pastures formed by forest clearance ("new pastures"), (3) older established pastures, and (4) pastures planted with clover. At both study sites (Scheuchegg and Teufelstein) we obtained similar results. Compared with forest pasture soils, open pasture soils were found to have greater microbial biomass and faster mineralisation potentials, but net field mineralisation rates were slower. In the forest pastures, highest N losses via denitrification were found. Higher potential leaching of NO₃ ^–, estimated by accumulation of NO₃ ^– on ion-exchange resins, in the forest pasture soils suggests lower N uptake by microbes and herbaceous plants compared with open pastures. N₂O-production rates of the forest pasture soils at the Scheuchegg site (11.54 μg N₂O-N m^–2 h^–1) were of similar magnitude to those reported for spruce forests without pastures, but at Teufelstein (53.75 μg N₂O-N m^–2 h^–1) they were higher. However, if forest pastures are not overgrazed, no elevated N loss through N₂O production and leaching of NO₃ ^– is expected. Denitrification rates in the open pastures (0.83–7.50 μg N₂O-N m^–2 h^–1) were low compared with reports on lowland pastures. In soils of the new pastures, rates of microbial N processes were similar to those in the established pastures, indicating a high capacity of soils to restore their internal N cycle after forest clearance. Received: 19 August 1999     

Seasonal and land-use induced variations of soil macrofauna composition in the Western Ghats, southern India

Soil macrofauna was surveyed in six sites characterised by different vegetation types on five occasions in the Western Ghats, India. Sampling sites included a primary forest, a weakly disturbed forest (slightly logged in the past), a highly disturbed forest (intensively logged), an Acacia auriculiformis plantation (8 years old), a pasture with high density of Phoenix humilis and a pasture without P. humilis. We showed that both land management and temporal variability induced significant changes in the soil macrofauna. Forest sites hosted larger densities of soil macroorganisms. The effect of seasons was apparent as some clear modifications in the fauna composition occurred. Some groups like earthworms mainly exhibited temporal variability whereas others like millipedes were chiefly affected by land management options. The seasonal rhythms of soil macrofauna were poorly expressed in the pasture plots and the Acacia plantation, but were particularly clear in the forest sites. This interaction between land management and temporal patterns may be explained by some changes in the species composition associated with certain land-uses. Our approach was based on a between-within classes PCA that proved particularly useful by providing statistical tests and a hierarchy of land management and temporal rhythm effects.     

Parasite infection rates of impala (Aepyceros melampus) in fenced game reserves in relation to reserve characteristics

Under certain conditions reserves can pose a threat to wildlife conservation by increasing the transmission of parasites and pathogens. In this study, I investigated associations between reserve characteristics including area, density and species richness and parasite infection rates in impala (Aepyceros melampus). Using coprological methods to measure gastrointestinal parasitism rates of impala inhabiting five fully or partially fenced game reserves in central Kenya, I found that bovid species richness was correlated with parasite taxa richness across reserves, and that prevalence rates of multi-host strongyle nematodes were higher in reserves with more species. In addition, reserve size was also implicated as a potential predictor of infection risk. Overall, these results suggest that wildlife inhabiting highly diverse and small reserves may suffer from higher than normal rates of infection. Given the potential debilitating effects increases in parasitism can have on wildlife, these results underscore the importance of considering parasite transmission dynamics in the management of small, fenced protected areas.     

Patch and landscape characteristics associated with the distribution of woodland amphibians in an agricultural fragmented landscape: an information-theoretic approach

In the Midwestern United States, agricultural landscapes with scattered patches of fragmented forest are common. To investigate the relationship between amphibian distributions and wetland, woodlot, and landscape characteristics, we studied the pond-breeding amphibians within a 15,450-ha plot in rural north-central Ohio. We surveyed 25 woodlots and one area of continuous riparian forest for amphibians, and each surveyed woodland contained at least one temporary wetland. We used Akaike's Information Criterion (AIC) to evaluate the effectiveness of 13 a priori models in predicting total amphibian species richness, anuran richness, caudate richness, and the presence of individual species in woodlots. We identified 13 species of amphibians within the study plot, and every woodlot contained at least one amphibian species. The most important variable in predicting total amphibian and anuran species richness was hydroperiod. For caudates, woodlot edge-to-area ratio, hydroperiod, pH, and ammonia were important characteristics in predicting species richness. Woodlots within agricultural landscapes are important refuges for amphibians.     

Effects of the introduced parasite Philornis downsi on nestling growth and mortality in the medium ground finch (Geospiza fortis)

Invasive species have the potential to detrimentally affect native ecosystems by out competing or directly preying upon native organisms, and have been implicated in the extinction of endemic populations. One potentially devastating introduced species in the Galápagos Islands is the parasitic fly Philornis downsi. As larvae, P. downsi parasitize nestling birds and have been associated with high nestling mortality and reduced growth rates. Here I document nestling growth and mortality in a bimodal population of the medium ground finch, Geospiza fortis. Observations were conducted over three years, and under variable ecological conditions. Annual parasite prevalence in nests ranged from 64% to 98%, and nestling mortality in nests with parasites ranged from 16% to 37%. Parasite load and parasite load per nestling follow a skewed distribution with many nests having relatively few parasites, and few nest having many. Parasite load, however, was not correlated with onset of breeding, clutch size, the number of nestlings, nestling survival or fledgling success. Parasite load per nestling, on the other hand, was correlated with clutch initiation date and the proportion of nestlings that died in parasitized nests. Neither nestling size nor growth rate differed between parasitized and unparasitized nests. In addition, male and female beak morphology was not correlated with parasite load, breeding variables or nestling survival. Thus, while overall mortality due to parasitism is high, ecological conditions and possible host defenses may potentially counter some of the detrimental affects of P. downsi on nestling size and growth. These results taken together suggest that parasitism of P. downsi larvae on nestling G. fortis has the potential to lead to large population declines.     

Soil degradation by land use change in an agropastoral area in Sardinia (Italy)

In recent decades the clearing of Mediterranean maquis along with the creation of new pastures has been a major factor of land degradation in Sardinia (Italy). This was due to an inadequate implementation of agricultural policies. Consequently, tillage and water erosion intensified over a wide area. The present work assesses the impacts of land use change on soil properties in a representative area of central-eastern Sardinia. Paired forest and pasture sampling sites were selected in relation to present land use, land suitability, and land use history. Different soil properties were considered: physical (sand, silt, clay, soil thickness, bulk density, and penetration resistance), chemical (pH, OC, N, C/N, Ca, Mg, Na, K, CEC, and BS), biological (BQI), and micromorphological (microporosity and microstructure). The comparison of forest and pasture soils showed a significant soil loss (in terms of soil thickness, −22%), and a clear decrease in organic carbon storage (−64% on average). An increase in bulk density (+ 44%) and a change in microporosity and its vertical distribution were also observed, respectively by field measurements and micromorphological quantification on digital images.