Risk Of Desiccation Research Articles

Movement behaviour of woodland salamanders is repeatable and varies with forest age in a fragmented landscape

Landscape modification (loss, fragmentation and degradation of habitat) is a universal form of human-induced rapid environmental change that creates strong spatial variation in environmental conditions. Behaviourally mediated responses to landscape modification may generate behavioural divergence among populations along environmental gradients. We tested whether movement behaviour of red-backed salamanders, Plethodon cinereus, is repeatable and whether it varies within and between forest stands of different ages in a fragmented landscape in New York, U.S.A. Water loss is a major constraint on P. cinereus mobility, and forest fragmentation creates spatial variation in desiccation risk. We quantified multiple components of individual movement behaviour in an unfamiliar environment in the laboratory and used ordination to identify behavioural variation among individuals in multivariate space. We tested for repeatability of individual behaviour and then examined behavioural differences between individuals captured from populations at two spatial scales where desiccation risk was expected to vary: (1) between edge and interior locations within forest stands and (2) between forest stands varying in time since agricultural abandonment. Variation in movement behaviour among individuals was characterized by three components: total movement activity (e.g. surface activity, distance moved, boundary crossings), substrate use (time on moist soil versus dry sand) and latency to initial movement. Each behavioural component was significantly repeatable. At the population level, individuals from old forest stands (≥77 years) showed greater movement activity than individuals from young stands. Movement behaviour did not differ between individuals from edge and interior locations within forest stands. Our results demonstrate that P. cinereus individuals show consistent differences in movement behaviour in an unfamiliar environment. Moreover, we show that movement behaviour can be spatially structured within fragmented landscapes, but that behavioural divergence among populations depends on spatial scale.

Growth strategies of tadpoles along the pond permanency gradient

The preference for particular features of water bodies for reproduction is one of the most important aspects of anuran ecology, affecting key aspects of both tadpole and adult life. The use by species of different habitats along the pond permanency gradient has been already studied, noting conflicting selective pressures from predation and desiccation risk. Here, we aim to discover physiological patterns related with this gradient. As a study system, we used the full anuran community of the NE Iberian Peninsula. We quantified growth rate, consumption rate, food assimilation and the proportion of energy allocated to growth, as well as gut length, for all species. Food consumption rate and growth allocation were the variables that defined tadpole growth, while food assimilation abilities and gut length seem to have a secondary or cryptic role in growth. More interestingly however, our data suggests a labile continuum of consumption-based versus allocation/assimilation-based growth strategies differentiating species. Differences among species follow predictions of adaptation to the pond permanency gradient selective pressures. Species from ephemeral ponds are more prone to use consumption-related growth tactics while species inhabiting ponds with longer hydroperiods are more efficient retaining and allocating energy into growth, although results seem partly shaped by strong interspecific competition. Physiological differences in growth and the usage of the assimilated energy could be an additional factor to understand how tadpoles adapt to the features of ponds they inhabit, as well as how they compete and coexist.

Root architecture might account for contrasting establishment success of Pseudotsuga menziesii var. menziesii and Pinus sylvestris in Central Europe under dry conditions

Pinus sylvestris seedlings quickly expand their roots to deeper soil layers while Pseudotsuga menziesii concentrates its root system in the topsoil, thereby running the risk of desiccation during long dry spells, as indicated by lower survival after simulated summer drought. Pseudotsuga menziesii (Douglas-fir) is regarded as a promising species to maintain the productivity of Central European lowland forests given the projected increase of long dry spells. Will the species be able to regenerate from seed and spread outside plantations in a drier temperate Europe? We measured the relative growth rate, biomass allocation, root architecture, and phenotypic plasticity of Pseudotsuga menziesii seedlings sown in a common garden and grown under current precipitation and prolonged drought, respectively. The species’ competitive ability with respect to Pinus sylvestris L., the most drought-tolerant native conifer in Central Europe, was assessed during three growing seasons. Pinus sylvestris seedlings had higher relative growth rates than did Pseudotsuga menziesii seedlings, first in terms of aboveground biomass and later in terms of shoot height. This resulted in heavier and taller seedlings after three growing seasons under both moist and dry conditions. Shorter vertical roots corresponded with lower survival of Pseudotsuga menziesii seedlings under dry conditions. Fast root proliferation allows Pinus sylvestris seedlings to reach deeper water pools that are less rapidly depleted during transient drought. By contrast, the shallow root system might put Pseudotsuga menziesii seedlings at the risk of desiccation during prolonged dry spells.

Desiccation resistance in tropical insects: causes and mechanisms underlying variability in a Panama ant community.

Desiccation resistance, the ability of an organism to reduce water loss, is an essential trait in arid habitats. Drought frequency in tropical regions is predicted to increase with climate change, and small ectotherms are often under a strong desiccation risk. We tested hypotheses regarding the underexplored desiccation potential of tropical insects. We measured desiccation resistance in 82 ant species from a Panama rainforest by recording the time ants can survive desiccation stress. Species' desiccation resistance ranged from 0.7 h to 97.9 h. We tested the desiccation adaptation hypothesis, which predicts higher desiccation resistance in habitats with higher vapor pressure deficit (VPD) – the drying power of the air. In a Panama rainforest, canopy microclimates averaged a VPD of 0.43 kPa, compared to a VPD of 0.05 kPa in the understory. Canopy ants averaged desiccation resistances 2.8 times higher than the understory ants. We tested a number of mechanisms to account for desiccation resistance. Smaller insects should desiccate faster given their higher surface area to volume ratio. Desiccation resistance increased with ant mass, and canopy ants averaged 16% heavier than the understory ants. A second way to increase desiccation resistance is to carry more water. Water content was on average 2.5% higher in canopy ants, but total water content was not a good predictor of ant desiccation resistance or critical thermal maximum (CT max), a measure of an ant's thermal tolerance. In canopy ants, desiccation resistance and CT max were inversely related, suggesting a tradeoff, while the two were positively correlated in understory ants. This is the first community level test of desiccation adaptation hypothesis in tropical insects. Tropical forests do contain desiccation‐resistant species, and while we cannot predict those simply based on their body size, high levels of desiccation resistance are always associated with the tropical canopy.

‘On the rocks’: reproductive biology of the endemic toad Xanthophryne (Anura: Bufonidae) from the Western Ghats, India

ABSTRACTXanthophryne is a toad genus endemic to the northern Western Ghats of India, with two extant sister species – Xanthophryne koynayensis and Xanthophryne tigerina. Both species are local endemics and endangered. We studied reproductive biology of these toads and found that they are specialized to the lateritic rocky outcrops at mid-elevations in high rainfall areas. Xanthophryne toads have sporadic, multiple spawning bouts lasting 2–4 days during early monsoon. In this explosive breeding behaviour, we observed male toads to engage in ‘pelvic thrusts’, a unique and novel behaviour among anurans. Females oviposit in shallow pools in depressions of lateritic boulders where their tadpoles metamorphose. These ephemeral rocky pools have limited resources and they desiccate rapidly with a break in the rains. To mitigate the stochastic risk of desiccation and subsequent large-scale egg/tadpole mortality, females may disperse their reproductive investment spatially and temporally in multiple clutches, and tadpoles metamorphose rapidly. Here, we describe the amplexus, spawning and male advertisement call, and provide a comparative account of the life history traits of the two Xanthophryne species. These toads face numerous threats and are in need of urgent conservation action. These toads, seemingly well adapted to the isolated rocky outcrops, offer an excellent opportunity to understand endemism, mating systems, anuran ecology and behaviour.

Regulation of seedling recruitment and survival in diverse ecotonal temperate forest understories

Fire is an important factor driving the position and stability of ecotones between fire-prone and less flammable forest types. To better understand, the recruitment processes that mediate plant persistence in ecotonal systems, we used manipulative field experiments in Australian wet eucalypt forests to investigate factors that may be regulating seedling recruitment and survival in burnt and unburnt forest. We found no evidence that safe sites from predators or exposure regulated seedling recruitment in three of the four study species, but one species associated with less flammable rainforest systems had increased germination rates in response to soil scarification. Seedlings of all four study species experienced higher rates of mortality resulting from herbivory or water stress in the post-fire environment. We found that intact ground cover increased seedling survival by reducing exposure to dessicating conditions and providing refuge from vertebrate herbivores. Our results suggest that shrubs associated with mesic, low-flammability forests have fewer opportunities for recruitment with desiccation risks in post-fire environments limiting seedling survival. In contrast, shrubs associated with high-flammability forests were capable of recruiting and surviving in both burnt and unburnt forests. Post-fire recruitment is likely to be more abundant after fire than at other times, but we found that survival can be highly constrained by post-fire herbivory. The ability of these species to recruit in undisturbed conditions provides some insurance against recruitment failure under stochastic variation in herbivory and desiccating conditions. The capacity to spread risks could be important in maintaining boundary position and diversity in these ecotonal systems.

Oviposition responses of two mosquito species to pool size and predator presence: varying trade-offs between desiccation and predation risks

Natural selection is predicted to favor females that can detect risks of desiccation and predation when choosing among temporary pools for oviposition. Pool size may serve both as a cue for desiccation risk and as a predictor for future colonization by predators or for the probability of present, undetected predators. Therefore, oviposition responses to pool size are expected to interact with the presence of predators that can be detected. We measured oviposition by two mosquito species, Culiseta longiareolata and Culex laticinctus, in a mesocosm experiment, crossing two pool surface sizes with presence or absence of the hemipteran predator, Notonecta maculata, which is chemically detectable by mosquitoes. Both mosquito species strongly avoided Notonecta pools. Using a mechanistic statistical model, we accounted for the higher encounter rate of females with larger pools, and determined their true oviposition preferences for pool size. C. laticinctus showed a clear preference for larger pools, but C. longiareolata, a species with larvae more vulnerable to predation, showed no significant preference for pool size. This study confirms the importance of risk of predation in explaining oviposition patterns, and suggests a possible inter-specific variation in the trade-off between predation and desiccation risks.

Invasive Asian Earthworms Negatively Impact Keystone Terrestrial Salamanders.

Asian pheretimoid earthworms (e.g. Amynthas and Metaphire spp.) are invading North American forests and consuming the vital detrital layer that forest floor biota [including the keystone species Plethodon cinereus (Eastern Red-backed Salamander)], rely on for protection, food, and habitat. Plethodon cinereus population declines have been associated with leaf litter loss following the invasion of several exotic earthworm species, but there have been few studies on the specific interactions between pheretimoid earthworms and P. cinereus. Since some species of large and active pheretimoids spatially overlap with salamanders beneath natural cover objects and in detritus, they may distinctively compound the negative consequences of earthworm-mediated resource degradation by physically disturbing important salamander activities (foraging, mating, and egg brooding). We predicted that earthworms would exclude salamanders from high quality microhabitat, reduce foraging efficiency, and negatively affect salamander fitness. In laboratory trials, salamanders used lower quality microhabitat and consumed fewer flies in the presence of earthworms. In a natural field experiment, conducted on salamander populations from “non-invaded” and “pheretimoid invaded” sites in Ohio, salamanders and earthworms shared cover objects ~60% less than expected. Earthworm abundance was negatively associated with juvenile and male salamander abundance, but had no relationship with female salamander abundance. There was no effect of pheretimoid invasion on salamander body condition. Juvenile and non-resident male salamanders do not hold stable territories centered beneath cover objects such as rocks or logs, which results in reduced access to prey, greater risk of desiccation, and dispersal pressure. Habitat degradation and physical exclusion of salamanders from cover objects may hinder juvenile and male salamander performance, ultimately reducing recruitment and salamander abundance following Asian earthworm invasion.

A large‐scale latitudinal pattern of life‐history traits in a strictly univoltine damselfly

1. Variation in thermal conditions and season length along latitudinal gradients affect body size‐related traits over different life stages. Selection is expected to optimise these size traits in response to the costs and benefits.2. Egg, hatchling, larval and adult size in males and females were estimated along a latitudinal gradient of 2730 km across Europe in the univoltine damselfly Lestes sponsa, using a combination of field‐collection and laboratory‐rearing experiments. In the laboratory, individuals were grown in temperatures and photoperiod simulating those at the latitude of origin, and in common‐garden conditions.3. The size of adults sampled in nature was negatively correlated with latitude. In all populations the females were larger than the males. Results from simulated and common‐garden rearing experiments supported this pattern of size difference across latitudes and between sexes, suggesting a genetic component for the latitudinal size trend and female‐biased size dimorphism. In contrast, hatchling size showed a positive relationship with latitude, but egg size, although differing between latitudes, showed no such relationship.4. The results support a converse Bergmann cline, i.e. a negative body size cline towards the north. This negative cline in body size is probably driven by progressively stronger seasonal time and temperature constraints towards the higher latitudes and by the obligate univoltine life cycle of L. sponsa. As egg size showed no relationship with latitude, other environmental factors besides temperature, such as desiccation risk, probably affect this trait.

Do biomass harvesting guidelines influence herpetofauna following harvests of logging residues for renewable energy?

Forests are a major supplier of renewable energy; however, gleaning logging residues for use as woody biomass feedstock could negatively alter habitat for species dependent on downed wood. Biomass Harvesting Guidelines (BHGs) recommend retaining a portion of woody biomass on the forest floor following harvest. Despite BHGs being developed to help ensure ecological sustainability, their contribution to biodiversity has not been evaluated experimentally at operational scales. We compared herpetofauanal evenness, diversity, and richness and abundance of Anaxyrus terrestris and Gastrophryne carolinensis among six treatments that varied in volume and spatial arrangement of woody biomass retained after clearcutting loblolly pine (Pinus taeda) plantations in North Carolina, USA (n = 4), 2011-2014 and Georgia (n = 4), USA 2011-2013. Treatments were: (1) biomass harvest with no BHGs, (2) 15% retention with biomass clustered, (3) 15% retention with biomass dispersed, (4) 30% retention with biomass clustered, (5) 30% retention with biomass dispersed, and (6) no biomass harvest. We captured individuals with drift fence arrays and compared evenness, diversity, and richness metrics among treatments with repeated-measure, linear mixed-effects models. We determined predictors of A. terrestris and G. carolinensis abundances using a priori candidate N-mixture models with woody biomass volume, vegetation structure, and groundcover composition as covariates. We had 206 captures of 25 reptile species and 8710 captures of 17 amphibian species during 53690 trap nights. Herpetofauna diversity, evenness, and richness were similar among treatments. A. terrestris abundance was negatively related to volume of retained woody biomass in treatment units in North Carolina in 2013. G. carolinensis abundance was positively related with volume of retained woody debris in treatment units in Georgia in 2012. Other relationships between A. terrestris and G. carolinensis abundances and habitat metrics were weak or absent. The lack of consistent community or population responses suggests the addition of a woody biomass harvest to a clearcut in pine plantations does not impact herpetofauna use of Coastal Plain loblolly plantations in the southeastern United States. We recommend additional research to examine relationships between woody biomass harvesting and rarer species or amphibians with high desiccation risk, particularly in other regions and harvesting systems.

Does the Cave Environment Reduce Functional Diversity?

Caves are not colonised by all taxa present in the surface species pool, due to absence of light and the tendency to food limitation when compared to surface communities. Under strong species sorting during colonisation and later by the restrictive environmental filter, traits that are not adaptive in subterranean habitats may be filtered out. We tested whether cave communities were assembled by the restrictive regime propitiated by permanent darkness or by competitive exclusion due to resource scarcity. When compared to surface communities, the restrictive subterranean regime would lead to lower functional diversity and phenotypic clustering inside the caves, and the opposite should be expected in the case of competitive exclusion. Using isopods (Oniscidea) as model taxa, we measured several niche descriptors of taxa from surface and cave habitats, used a multivariate measure of functional diversity, and compared their widths. We found phenotypic overdispersion and higher functional diversity in cave taxa when compared to surface taxa. On the one hand, the dry climate outside of caves hampered the survival of several taxa and their ecological strategies, not viable under severe desiccation risk, culminating in the clustering of functional traits. In contrast, this restriction does not occur inside of caves, where isopods find favourable conditions under lower predation pressures and more amenable environmental parameters that allow occupation and subsequent diversification. Our results showed that, at least for some taxa, caves may not be such a harsh environment as previously thought. The high functional diversity we found inside caves adds an additional reason for the conservation of these sensitive environments.

Trade-off between Desiccation and Predation Risk in the Blue Ridge Two-lined Salamander (Eurycea wilderae)

There has been increased attention on amphibian declines over the past two decades, and many researchers agree that the primary factor responsible for this decline is habitat fragmentation and degradation. A number of studies have begun to address the impacts of forest management practices on amphibian populations; however, information about behavioral responses of amphibians to forestry practices such as logging is still needed. Using laboratory experiments, I investigated the behavioral choices of Blue Ridge Two-lined Salamanders (Eurycea wilderae) when presented with dry and moist environments (representing logged and unlogged forests) both with and without predators. When no predator was present, E. wilderae preferred a moist environment. When a predator was present in the moist environment, the adult E. wilderae preferred to remain in the less suitable dry environment, while juveniles displayed no clear choice. After logging, E. wilderae are faced with a decision between two adverse environments, wit...

Fish embryos on land: terrestrial embryo deposition lowers oxygen uptake without altering growth or survival in the amphibious fish Kryptolebias marmoratus.

Few teleost fishes incubate embryos out of water, but the oxygen-rich terrestrial environment could provide advantages for early growth and development. We tested the hypothesis that embryonic oxygen uptake is limited in aquatic environments relative to air using the self-fertilizing amphibious mangrove rivulus, Kryptolebias marmoratus, which typically inhabits hypoxic, water-filled crab burrows. We found that adult mangrove rivulus released twice as many embryos in terrestrial versus aquatic environments and that air-reared embryos had accelerated developmental rates. Surprisingly, air-reared embryos consumed 44% less oxygen and possessed larger yolk reserves, but attained the same mass, length and chorion thickness. Water-reared embryos moved their opercula ∼2.5 more times per minute compared with air-reared embryos at 7 days post-release, which probably contributed to the higher rates of oxygen uptake and yolk utilization we observed. Genetically identical air- and water-reared embryos from the same parent were raised to maturity, but the embryonic environment did not affect growth, reproduction or emersion ability in adults. Therefore, although aspects of early development were plastic, these early differences were not sustained into adulthood. Kryptolebias marmoratus embryos hatched out of water when exposed to aerial hypoxia. We conclude that exposure to a terrestrial environment reduces the energetic costs of development partly by reducing the necessity of embryonic movements to dispel stagnant boundary layers. Terrestrial incubation of young would be especially beneficial to amphibious fishes that occupy aquatic habitats of poor water quality, assuming low terrestrial predation and desiccation risks.

Effects of stock type, irrigation and effluent dispersal on earthworm species composition, densities and biomasses in New Zealand pastures

We investigated the effects of grazing stock, irrigation and effluent dispersal on earthworm species compositions, densities and biomasses in 615 locations across 41 farms in the Waitaki Basin, New Zealand, between April and September 2012. No native megascolecid earthworms were found, but four introduced European species were encountered. Among earthworms collected, Aporrectodea caliginosa accounted for 70% of the total, 23% were Lumbricus rubellus and 4% Aporrectodea longa. When compared with untreated locations, total earthworm density was higher by 42% in effluent only locations and 72% in irrigated only locations. Maximum densities and biomasses occurred where both effluent and irrigation were applied. L. rubellus density was 32% higher in effluent only locations, 123% higher in irrigated only locations and 180% higher in effluent and irrigated locations than untreated locations. A. longa occurred in 24% of the sampled locations and appeared to be suppressed in irrigated locations. When equivalent treatments were applied, earthworm densities were 15.4% to 36.6% higher on sheep farms than on dairy farms; earthworm biomasses differed by –3.3% to 55.8% between these two kinds of stock animal farms. Treatment effects on earthworms were evident only in the upper 10cm soil layer. Effluent and water application may have reduced the risk of desiccation and increased the availability of resources for earthworms. However, local absence of the deep burrowing species (e.g. A. longa) raises concerns about ecosystem functioning. This is a topic that should be explored further.

Plant preference during oviposition in the endangered dragonfly Lestes macrostigma (Odonata: Zygoptera) and consequences for its conservation

Biotic and abiotic features impact the breeding success of animals and thereby induce selection pressures for habitat selection. Little is known about the plant selection by predatory insects which lay their eggs within plants. In previous work, we have highlighted that during oviposition males of Lestes macrostigma—an endangered dragonfly species—prefer to land on Bolboschoenus maritimus and dead shoots of Juncus maritimus but disfavour living shoots in that species, and that females seem to prefer dead material during substrate examination. In this study we assessed behavioural preference in females during substrate examination, substrates suitability for oviposition, the effort females had to make to lay their eggs and their resulting oviposition rate. We show L. macrostigma has a preference for B. maritimus and, albeit to a lesser extent, for dead substrates. No clear trend appeared regarding substrate suitability. Females had to make a greater effort to lay an egg within living shoots of J. maritimus. By contrast, this effort was less in B. maritimus and dead shoots of J. maritimus and the oviposition rates were higher for these two types of substrate. We hypothesize that these preferences are relevant in the selection of oviposition substrates which are more likely to be flooded earlier by rainfall, reducing risk of egg desiccation and increasing hatching success. With regard to conservation, B. maritimus and J. maritimus should be encouraged by wildlife managers especially in habitat restoration programs which aim to increase the number of suitable breeding sites for the species.

Melanin waterproofs mountain-dwelling weta

It can be hard to get your head around just how big some weta are if you're used to working on the Drosophila scale, but giant weta routinely tip the scales at up to 70 g and even the more diminutive tree weta tower over other more familiar insects. While some offshoots of the family make their homes in low-lying areas, others have pushed the boundaries into the mountains, where they are exposed to high winds, low oxygen, the searing sun and dry air. ‘Together, these environmental challenges…mean montane insects are at a significantly higher risk of desiccation’, say Keith King from the University of Otago, New Zealand, and Brent Sinclair from the University of Western Ontario, Canada. So how do these high-altitude insects cope with these conditions? Suspecting that they might reduce their water-loss rates, King and Sinclair set about measuring water-loss across the insect's cuticle in weta ranging from sea-level species (the Wellington tree weta, the tokoriro and the Canterbury tree weta) to the mountain dwellers (the alpine tree weta and the Banks Peninsula tree weta).The duo was impressed to see that the low-level dwellers’ cuticular water loss rate was 0.444 μmol–1 g–1 h–1 while the mountain species’ cuticular water loss rate was 45% lower at 0.245 μmol–1 g–1 h–1. They also found that the mountain dwellers had cut their respiratory water loss by 55%. And when they compared how much water the species lost relative to their metabolic rate, the mountain species were much more conservative than their low-altitude cousins, losing 64% less water.King and Sinclair then explain that the amount of melanin pigment in the cuticle of Drosophila is known to affect insect desiccation rates, with lighter flies losing water across the cuticle faster than darker species. Knowing that there are two forms of the alpine tree weta – one with a melanin-tinted cuticle and a lighter version – the duo decided to test whether the degree of melanisation affected the insects’ cuticular water loss rates. Through a direct comparison of cuticular water loss rates King and Sinclair could see that the darker form lost 46% less water than the lighter form. And when they stripped the waterproofing hydrocarbons from the surface of the cuticles of both insects and measured their cuticular water loss rates, the duo found that although the insects’ water loss rates rocketed, the darker weta still lost water 57% more slowly than the lighter insects. The high melanin content of the cuticle in the darker weta offers more protection from desiccation than the cuticle of the lighter weta.So, mountain-dwelling weta have developed various strategies to protect themselves from desiccation in their harsh mountain homes and some have probably benefited from the waterproofing effects of melanin as an unintended consequence when they became darker for camouflage.

Water loss in tree weta (Hemideina): adaptation to the montane environment and a test of the melanisation-desiccation resistance hypothesis.

Montane insects are at a higher risk of desiccation than their lowland counterparts and are expected to have evolved reduced water loss. Hemideina spp. (tree weta; Orthoptera: Anostostomatidae) have both lowland (Hemideina femorata, Hemideina crassidens and Hemideina thoracica) and montane (Hemideina maori and Hemideina ricta) species. H. maori has both melanic and yellow morphs. We use these weta to test two hypotheses: that montane insects lose water more slowly than lowland species, and that cuticular water loss rates are lower in darker insects than lighter morphs, because of incorporation of melanin in the cuticle. We used flow-through respirometry to compare water loss rates among Hemideina species and found that montane weta have reduced cuticular water loss by 45%, reduced respiratory water loss by 55% and reduced the molar ratio of V̇H2 O:V̇CO2 by 64% compared with lowland species. Within H. maori, cuticular water loss was reduced by 46% when compared with yellow morphs. Removal of cuticular hydrocarbons significantly increased total water loss in both melanic and yellow morphs, highlighting the role that cuticular hydrocarbons play in limiting water loss; however, the dark morph still lost water more slowly after removal of cuticular hydrocarbons (57% less), supporting the melanisation-desiccation resistance hypothesis.

Oviposition site choice under conflicting risks demonstrates that aquatic predators drive terrestrial egg-laying.

Laying eggs out of water was crucial to the transition to land and has evolved repeatedly in multiple animal phyla. However, testing hypotheses about this transition has been difficult because extant species only breed in one environment. The pantless treefrog, Dendropsophus ebraccatus, makes such tests possible because they lay both aquatic and arboreal eggs. Here, we test the oviposition site choices of D. ebraccatus under conflicting risks of arboreal egg desiccation and aquatic egg predation, thereby estimating the relative importance of each selective agent on reproduction. We also measured discrimination between habitats with and without predators and development of naturally laid aquatic and arboreal eggs. Aquatic embryos in nature developed faster than arboreal embryos, implying no cost to aquatic egg laying. In choice tests, D. ebraccatus avoided habitats with fish, showing that they can detect aquatic egg predators. Most importantly, D. ebraccatus laid most eggs in the water when faced with only desiccation risk, but switched to laying eggs arboreally when desiccation risk and aquatic predators were both present. This provides the first experimental evidence to our knowledge that aquatic predation risk influences non-aquatic oviposition and strongly supports the hypothesis that it was a driver of the evolution of terrestrial reproduction.

Evaluating connectivity for tropical amphibians using empirically derived resistance surfaces.

Agricultural expansion continues to drive forest loss in species-rich tropical systems and often disrupts movement and distributions of organisms. The ability of species to occupy and move through altered habitats likely depends on the level of contrast between natural forest and surrounding land uses. Connectivity models, such as circuit theory models, are widely used in conservation biology, and their primary input consists of resistance surfaces representing movement costs associated with landscape features. Cost values are most frequently determined by expert opinion, which may not capture relevant levels of contrast among features. We developed resistance surfaces using experiments that represent different local mechanisms hypothesized to affect connectivity for two Neotropical amphibian species. Response ratios were calculated to translate experimental results to cost values used in connectivity modeling. We used relative abundance data in three land-cover types to generate resistance surfaces for evaluating independent support of models derived from experiments. Finally, we analyzed agreement among movement pathways predicted for each species and among three commonly used connectivity measures: Euclidean, least cost, and resistance distances. Experiments showed that extreme microclimates associated with altered habitats significantly increased desiccation and mortality risk for both species. Resistances estimated from microclimate experiments were concordant with those from survey data for both species. For one focal species, resistance estimates derived from predator encounter rates were also highly correlated with abundance-derived resistances. There was generally low agreement among the three alternative distance measures, which underscores the importance of choosing connectivity models that are most appropriate for the study objectives. Overall, similarity among linkages modeled for each species was high, but decreased with declining forest cover. Our results highlight the value of experiments for drawing inferences about processes in resistance modeling, as well as the need to consider model differences and species-specific responses when developing strategies to maintain connectivity.

Desiccation resistance explains amphibian distributions in a fragmented tropical forest landscape

Context Although amphibian distributions are associated with environmental moisture at global and local scales, less is known about how desiccation tolerance influences landscape distributions of amphibians. Objectives We evaluated two hypotheses linking amphibian distributions in a fragmented tropical forest landscape to desiccation risk. The patch quality hypothesis predicts that desiccation-prone species are absent on small forest patches, which are generally warmer and drier than large patches. Alternatively, the matrix effects hypothesis suggests that desiccationprone species are absent on isolated forest patches surrounded by open savanna because they will be unable to traverse the matrix in which patches occur. Methods We quantified interspecific variation in desiccation proneness using field-based desiccation trials, and tested for associations between desiccation proneness and distributions of amphibians in fragmented forest in northeastern Bolivia. Results Rates of evaporative water loss were negatively associated with an index of dispersal limitation, but unrelated to species’ area requirements. Conclusions By demonstrating that desiccationprone species do not occur on isolated forest patches, we provide clear support for the matrix effects hypothesis. We suggest that desiccation proneness is a key trait that may determine amphibian responses to a range of global change drivers, including habitat loss and fragmentation, invasive species, and climate change.