Egg-laying reptiles in cold climates: determinants and consequences of nest temperatures in montane lizards

Studies on reptilian life-history evolution have emphasized the role of cold climates as a selective force for the evolution of viviparity, but have tended to neglect the many cold-climate reptiles that retain oviparity. Many of these species avoid low incubation temperatures by selecting warm nest-sites, and the evolution of viviparity (by uterine retention and maternal thermoregulation) is an extension of this strategy. However, an alternative pathway exists: reptiles may adapt to low-temperature incubation rather than avoid it. The scincid lizard Nannoscincus maccoyi from high-elevation areas of south-eastern Australia follows this alternative strategy. Field studies show that Nannoscincus selects cooler oviposition sites than does the sympatric heliothermic skink Bassiana duperreyi, owing to differences in the type of cover object (log vs. rock), the size of cover object and the depth to which eggs are buried. These interspecific differences in natural incubation regimes are reflected in the responses of laboratory-incubated eggs to incubation temperature. Bassiana eggs tolerate higher temperatures than do Nannoscincus eggs, but do not develop as rapidly at low temperatures. Incubation at lower temperatures produces a larger, faster hatchling in Nannoscincus, whereas the reverse is true in Bassiana. Thus, Nannoscincus and Bassiana follow different pathways to overcome the difficulties of reproducing in cold climates. Only the Bassiana pathway is likely to be compatible with the evolution of viviparity.     

Variation of biomass and morphology of the cushion plant Androsace tapete along an elevational gradient in the Tibetan Plateau

Alpine ecosystems are among those biomes that are most vulnerable to climate change. Cushion plants are an important life form of alpine ecosystems and will likely play a critical role for the resilience of these habitats to climate change. We studied cushion size distribution and different measures of the compactness of cushions (biomass and rosette density, leaf area index) of the cushion plant, Androsace tapete along an elevational gradient from 4500 to 5200 m a.s.l. in the Nyainqentanglha Mountains of the central Tibetan Plateau. Cushion size distribution, total cover, and compactness of cushions varied substantially along the elevational gradient. At the driest site at low elevation we found the lowest total cushion cover, a particularly high proportion of very small cushions, and the most compact cushions (highest rosette and biomass densities, and leaf area index (LAI) per cushion). Our results indicate that in the semi‐arid Tibetan Plateau water availability is the more important climate factor than temperature affecting cushion plant traits and morphology.     

Variation in home range size along an elevational gradient in the iguanid lizard Sceloporus merriami

Summary Variation in home range size among three populations of the saxicolous iguanid lizard Sceloporus merriami was studied in relation to density, food availability, behavioral patterns, and daily activity periods. These populations, separated by a maximum distance of 50 km, occur along an elevational gradient which spans the elevational range of the species in west Texas. The population with the smallest home ranges had the highest density, the lowest food availability, and the most restricted daily activity period. Within populations, males consistently had larger home ranges than females. No significant between-year variation in home range size was noted within a given area or sex. Intraspecific variation in home range size is not explained by any single factor, such as food availability or density, but appears to be complexly determined by the interaction of energetic, breeding, and activity constraints.     

Nest site choice compensates for climate effects on sex ratios in a lizard with environmental sex determination

Theoretical models suggest that in changing environments natural selection on two traits, maternal nesting behaviour and pivotal temperatures (those that divide the sexes) is important for maintaining viable offspring sex ratios in species with environmental sex determination (ESD). Empirical evidence, however, is lacking. In this paper, we provide such evidence from a study of clinal variation in four sex-determining traits (maternal nesting behaviour, pivotal temperatures, nesting phenology, and nest depth) in Physignathus lesueurii, a wide-ranging ESD lizard inhabiting eastern Australia. Despite marked differences in air and soil temperatures across our five study sites spanning 19° latitude and 1200 m in elevation, nest temperatures did not differ significantly among sites. Lizards compensated for climatic differences chiefly by selecting more open nest sites with higher incident radiation at cooler sites. Clinal variation in the onset of nesting also compensated for climatic differences, but to a lesser extent. There was no evidence of compensation through pivotal temperatures or nest depth. More broadly, our results extend to the egg stage the life history prediction that behaviour is the chief compensatory mechanism for climatic differences experienced by species spanning environmental extremes. Furthermore, our study was unique in revealing that nest site choice influenced mainly the daily range in nest temperatures, rather than mean temperatures, in a shallow-nesting reptile. Finally, indirect evidence suggests that the cue used by nesting lizards was radiation or temperature (through basking or assessing substrate temperatures), not visual detection of canopy openness. We conclude that maternal nesting behaviour and nesting phenology are traits subject to sex ratio selection in P. lesueurii, and thus, must be considered among the repertoire of ESD species for responding to climate change.     

Atmospheric deposition and watershed nitrogen export along an elevational gradient in the Catskill Mountains, New York

Cumulative effects of atmospheric N deposition mayincrease N export from watersheds and contribute tothe acidification of surface waters, but naturalfactors (such as forest productivity and soildrainage) that affect forest N cycling can alsocontrol watershed N export. To identify factors thatare related to stream-water export of N, elevationalgradients in atmospheric deposition and naturalprocesses were evaluated in a steep, first-orderwatershed in the Catskill Mountains of New York, from1991 to 1994.Atmospheric deposition of SO ₄ ^2– , andprobably N, increased with increasing elevation withinthis watershed. Stream-water concentrations ofSO ₄ ^2– increased with increasing elevationthroughout the year, whereas stream-waterconcentrations of NO ₃ ^– decreased withincreasing elevation during the winter and springsnowmelt period, and showed no relation with elevationduring the growing season or the fall. Annual exportof N in stream water for the overall watershed equaled12% to 17% of the total atmospheric input on thebasis of two methods of estimation. This percentagedecreased with increasing elevation, from about 25%in the lowest subwatershed to 7% in the highestsubwatershed; a probable result of an upslope increasein the thickness of the surface organic horizon,attributable to an elevational gradient in temperaturethat slows decomposition rates at upper elevations. Balsam fir stands, more prevalent at upper elevationsthan lower elevations, may also affect the gradient ofsubwatershed N export by altering nitrification ratesin the soil. Variations in climate and vegetationmust be considered to determine how future trends inatmospheric deposition will effect watershed export ofnitrogen.     

Flowering range changes across an elevation gradient in response to warming summer temperatures

Many studies have demonstrated plant response to warming temperatures, both as advancement in the timing of phenological events and in range shifts. Mountain gradients are ideal laboratories for studying species range changes. In this study of 363 plant species in bloom collected in five segments across a 1200 m (4158 ft) elevation gradient, we look for changes in species flowering ranges over a 20-year period. Ninety-three species (25.6%) exhibited a significant change in the elevation at which they flowered from the first half to the second half of the record, with many of these changes occurring at higher elevations. Most of the species exhibiting the changes were perennial plants. Interestingly, though many changes in flowering range were specific to higher elevations, range changes occurred all across the gradient. The changes reported in this study are concurrent with significant increases in summer temperatures across the region and are consistent with observed changes around the globe.     

Evolution of viviparity in warm-climate lizards: an experimental test of the maternal manipulation hypothesis

The maternal manipulation hypothesis for the evolution of reptilian viviparity has been claimed to apply to any situation where gravid females are able to maintain body temperatures different from those available in external nests, but empirical data that support this hypothesis are very limited. Here, we tested this hypothesis using gravid females of a warm-climate lizard, Mabuya multifasciata, by subjecting them to five thermal regimes for the whole gestation period. We found gravid females selected lower body temperatures and thermoregulated more precisely than did nongravid females. Offspring produced in different treatments differed in head size, limb length and sprint speed, but not in overall body size or mass. Variation in morphological traits of offspring was induced primarily by extreme temperatures. Sprint speed of offspring was more likely affected by the mean but not by the variance of gestation temperatures. Gravid females maintained more stable body temperatures than did nongravid females not because these temperatures resulted in the optimization of offspring phenotypes but because the range of temperatures optimal for embryonic development was relatively narrow. Our data conform to the main predictions from the maternal manipulation hypothesis that females should adjust thermoregulation during pregnancy to provide optimal thermal conditions for developing embryos and that phenotypic traits forged by maternal thermoregulation should enhance offspring fitness.     

Evolvability of between‐year seed dormancy in populations along an aridity gradient

Under global climate change, adaptation to new conditions is crucial for plant species persistence. This requires the ability to evolve in traits that are correlated with changing climatic variables. We studied between‐year seed dormancy, which correlates with environmental variability, and tested for clinal trends in its evolvability along an aridity gradient in Israel. We conducted a germination experiment under five irrigation levels with two dryland winter annuals (Biscutella didyma, Bromus fasciculatus) from four sites along the gradient. Species differed in means and evolvability of dormancy. Biscutella had high dormancy, which significantly increased with aridity but decreased with higher irrigation. In Bromus, dormancy was low, similar among populations, and only marginally affected by irrigation. Evolvability in Biscutella was high and varied among populations, without a clinal trend along the gradient. Conversely, in Bromus, trait evolvability was low and declined with increasing aridity. We argue that changes in evolvability along climatic gradients depend on the relative intensity of stabilizing selection. This may be high in Bromus and not only depends on environmental stress, but also on variability. Our findings point to the importance of measuring evolvability of climate‐related traits across different natural and artificial environments and for many coexisting species. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 924–934.     

Intraspecific variability of leaf cuticle alkanes in Sedum lanceolatum along an elevational gradient

Characterization of variability patterns in leaf n-alkanes isolated from Sedum lanceolatum field populations indicates little correlation with elevation gradients. Inherent variability is better explained by genetic constitution than phenotypic plasticity, thus suggesting that leaf wax samples isolated from field-collected specimens are suitable for use in elucidation of systematic relationships.     

Seasonal and annual variation in nitrogen mineralization and nitrification along an elevational gradient in New Mexico

Patterns and amounts of nitrogen loss from disturbed ecosystems vary widely. The mineralization of organic nitrogen to ammonium and then nitrification to nitrate are important processes regulating nitrogen cycling rates and nitrogen losses. Nitrification is a significant process because of the production of the nitrate anion which is easily leached or denitrified. Most studies of these processes do not evaluate their seasonal and yearly variations. This study demonstrates that marked seasonal and yearly variations can occur in these processes in different ecosystems and suggests that nitrogen loss or other system properties correlated with one arbitrarily selected collection can be misleading. Spruce-fir and ponderosa pine ecosystems demonstrated little actual orpotential nitrification. Aspen and mixed conifer ecosystems demonstrated distinct seasonal patterns with increased rates of mineralization and nitrification during spring and summer months and a precipitous decline in both rates coincident with autumn foliage litterfall.The relative availability of soil nitrogen along with the amount of nitrogen circulating annually in litterfall prior to disturbance are useful predictors of the potential for nitrate production and loss following disturbance. However, other controls, including regulation by organic compounds, appear important in determining seasonal and annual variation in actual nitrification rates.     

Variation in radial growth of Pinus cooperi in response to climatic signals across an elevational gradient

Climate change could modify the biogeography of many forest species. Elevational gradients have been documented as strategic sites to better understand tree growth response to regional climate variables. Pinus cooperi Blanco is one of the most important species in Northern Mexico. However, little is known concerning effects of climate responses on growth of this species. We used tree data records to compare the influence of precipitation and temperature on radial growth among P. cooperi populations across a mountain landscape at elevation gradient. Correlation and regression analysis of the regional growth–climate relationships showed that radial growth was correlated with previous winter conditions at most sites along the gradient. Wet and cold winters were positively associated with radial growth. Although our results showed significant climate influences on tree radial growth, other site factors also may have affected growth–climate responses. The results support the idea that climate change influences P. cooperi growth.     

Effects of oxygen on responses to heating in two lizard species sampled along an elevational gradient

Thermal tolerance is an important variable in predictive models about the effects of global climate change on species distributions, yet the physiological mechanisms responsible for reduced performance at high temperatures in air-breathing vertebrates are not clear. We conducted an experiment to examine how oxygen affects three variables exhibited by ectotherms as they heat—gaping threshold, panting threshold, and loss of righting response (the latter indicating the critical thermal maximum)—in two lizard species along an elevational (and therefore environmental oxygen partial pressure) gradient. Oxygen partial pressure did not impact these variables in either species. We also exposed lizards at each elevation to severely hypoxic gas to evaluate their responses to hypoxia. Severely low oxygen partial pressure treatments significantly reduced the gaping threshold, panting threshold, and critical thermal maximum. Further, under these extreme hypoxic conditions, these variables were strongly and positively related to partial pressure of oxygen. In an elevation where both species overlapped, the thermal tolerance of the high elevation species was less affected by hypoxia than that of the low elevation species, suggesting the high elevation species may be adapted to lower oxygen partial pressures. In the high elevation species, female lizards had higher thermal tolerance than males. Our data suggest that oxygen impacts the thermal tolerance of lizards, but only under severely hypoxic conditions, possibly as a result of hypoxia-induced anapyrexia.     

Effects of drought and phenology on GPP in Pinus sylvestris: a simulation study along a geographical gradient

1. A simple canopy model was developed for Scots Pine ( Pinus sylvestris L.) and applied to a transect of six meteorological stations in Europe. The model accounts for possible genetic adaptation of phenology of photosynthesis to the local climate and to decreases of gas exchange owing to drought.
2. Simulations accounting for adaptation of phenology to the local climate differed up to 20% from simulations using the same phenology parameter values for all locations.
3. A temperature increase of 3°C and a doubling of the CO₂ concentration, while adjusting the photosynthesis parameters to give approximately the observed changed photosynthesis of +30%, also increased the length of the growing season by 23–42%. Combination of increases in the rate of photosynthesis and the length of the growing season resulted in increases of yearly Gross Primary Productivity (GPP) from 72 to 101%. Increases in transpiration were smaller.
4. A decrease of the precipitation by 25% reduced this increase to 54–64%.
5. The relative magnitude of the simulated increases in GPP was similar for locations representing boreal, temperate and mediterranean climates.     

Diversity of rodents and shrews along an elevational gradient in Bwindi Impenetrable National Park, south-western Uganda

Small mammal species diversity in the major vegetation zones of Bwindi Impenetrable National Park is discussed in relation to altitude. Species richness of the small mammals was found to decrease with an increase in altitude. The main factors accounting for the observed diversity are the wide altitudinal variation and a complex array of vegetation types. Sixty‐seven species of rodents and shrews were found to exist in the Park; 47 of which were rodents and 20 shrews. Of these, 26 species are new to the Bwindi Park list. Three species have probably not been described before. The study found 10 species of small mammals to be Albertine Rift endemics. Three genera are recorded in Uganda for the first time: Rwenzorisorex, Suncus and Paracrocidura. Five species are new records for East Africa. These are Crocidura stenocephala, Lophuromys rahmi, L. medicaudatus, Paracrocidura maxima and Hylomyscus aeta. Because of the high endemism of plants, butterflies, birds and now of small mammal species, Bwindi forest is a unique biodiversity hotspot and is among the highest conservation priorities in the Albertine Rift.     

Fecundity and sexual size dimorphism of wolf spiders (Araneae: Lycosidae) along an elevational gradient in the Arctic

Fecundity and body size are central fitness-related traits, and their intra-specific responses to environmental variation are receiving increasing attention in the context of climate change. Recent results from Greenland indicate that temporal and spatial variation in body size differences between sexes (sexual size dimorphism) may be widespread among wolf spider species and could be related to climate. Here, we tested whether variation in elevation affected body size of three wolf spider (Araneae: Lycosidae) species in low-Arctic Canada, whether the sexes differed in their response to the cline, and whether changes in local density influenced this relationship. We also tested whether fecundity changed with elevation in two of the species, independent of body size variation. We found a significant sex–elevation interaction for Pardosa lapponica: female size decreased more in response to elevation than that of males. Males and females of Pardosa uintana decreased significantly in size with elevation at a similar rate. Alopecosa aculeata males increased in body size along the gradient while females did not. Pardosa lapponica females, but not P. uintana females, showed significant reduction in fecundity in response to elevation. P. uintana showed significant decreases in body size with increases in its population density. Changes in temperature and potential resource availability along the elevational gradient are probably causing these species- and sex-specific responses. Further summer warming of the region may alleviate current constraints on growth and reproduction of these species although sex-specific responses may affect their population dynamics.     

Leaf thickness and UV-B absorbing pigments of plants in relation to an elevational gradient along the Blue Mountains,Jamaica

Terrestrial plant species vary widely in their adaptation to (increasing) solar UV-B radiation. Among the various responses of higher plants to enhanced UV-B are increasing leaf thickness and increasing concentrations of UV-B absorbing compounds. In some (UV-B resistant) plant species increased leaf thickness and UV-B absorbance may form part of mechanisms protecting plants from UV-B damage. However, in UV-B sensitive plant species leaf thickness and UV-B absorbance may increase as well with enhanced UV-B radiation. In the latter case however, this response cannot prevent plant damage and disturbance. In the present field study the relationship between these plant parameters and a natural elevational UV-B gradient on the tropical island of Jamaica was described. Four plant species of the Blue Mountain Tropical Montane Forest, occurring on open forest sites along the roadside and paths were studied along an elevational gradient. Plant species studied are Redbush (Polygonum chinense), Wild ginger (Hedychium gardneranum), John Crow Bush (Bocconia frutescens) and White clover (Trifolium repens). The elevational sites were at 800, 1000, 1200, 1400 and 1600 m above sea level. Leaf thickness was measured of leaves of intact plants around midday in the field. Leaf disks (5 mm) were sampled and extracted with a methanol/HCl mixture. UV-B absorption of these leaf extracts was measured spectrophotometrically. For all species leaves from higher elevations were thicker than those from lower elevations. In addition, the absorption of UV-B of leaf extracts increased with increasing elevations. It is assumed that the calculated gradient of the UV-B_BE from 800 m above sea level: 9.45 kJ m^-2 day^-1 to 9.75 kJ m^-2 day^-1 at 1600 m is related to the measured increase of leaf thickness and UV-B absorbing compounds. The responsiveness of these plant parameters to the elevational gradient does not necessarily imply that the plant species are UV-B resistant. One possibility is that the species studied, which are growing on open, disturbed sites on the forest floor and along mountain-roads, are relatively sensitive to UV-B. In addition to clear sky conditions, mist and clouds occur frequently in this tropical mountane forest at Jamaica. Also, the low nutrient status of the soil (low pH, nutrient deficiency) and the high content of polyphenols in leaves of many plant species of the tropical montane rain forest may relate to the marked response of the species studied with increasing elevation. Abbreviations: asl – above sealevel, UV-B – ultraviolet-B radiation (280–320 nm), TMCF – Tropical Montane Cloud Forest.     

Survival of honeybees in cold climates: the critical timing of colony growth and reproduction

Abstract. 1. The adaptive significance of the timing of growth and reproduction by honeybee, Apis mellifera L., colonies in cold climates was studied by describing the seasonal patterns of food storage, brood rearing, and swarming, and then observing the consequences of experimentally perturbing the seasonal cycles of brood rearing and swarming.
2. Colonies consume large amounts of food over winter (20+ kg of honey), but have only a brief period (about 14 weeks) for food collection each year.
3. The honeybee's striking habits of starting brood rearing in midwinter and swarming in late spring evidently help colonies achieve maximum use of the short summer season. Colonies whose onset of-brood rearing was experimentally postponed until early spring showed greatly retarded colony growth and swarming. Other experiments demonstrated that late swarms starve more often during winter than do early swarms.
4. We conclude that the timings of colony growth and reproduction are essential elements in the honeybee's suite of adaptations for winter survival.     

The response of forest plant regeneration to temperature variation along a latitudinal gradient

Background and Aims

The response of forest herb regeneration from seed to temperature variations across latitudes was experimentally assessed in order to forecast the likely response of understorey community dynamics to climate warming.

Methods

Seeds of two characteristic forest plants (Anemone nemorosa and Milium effusum) were collected in natural populations along a latitudinal gradient from northern France to northern Sweden and exposed to three temperature regimes in growth chambers (first experiment). To test the importance of local adaptation, reciprocal transplants were also made of adult individuals that originated from the same populations in three common gardens located in southern, central and northern sites along the same gradient, and the resulting seeds were germinated (second experiment). Seedling establishment was quantified by measuring the timing and percentage of seedling emergence, and seedling biomass in both experiments.

Key Results

Spring warming increased emergence rates and seedling growth in the early-flowering forb A. nemorosa. Seedlings of the summer-flowering grass M. effusum originating from northern populations responded more strongly in terms of biomass growth to temperature than southern populations. The above-ground biomass of the seedlings of both species decreased with increasing latitude of origin, irrespective of whether seeds were collected from natural populations or from the common gardens. The emergence percentage decreased with increasing home-away distance in seeds from the transplant experiment, suggesting that the maternal plants were locally adapted.

Conclusions

Decreasing seedling emergence and growth were found from the centre to the northern edge of the distribution range for both species. Stronger responses to temperature variation in seedling growth of the grass M. effusum in the north may offer a way to cope with environmental change. The results further suggest that climate warming might differentially affect seedling establishment of understorey plants across their distribution range and thus alter future understorey plant dynamics.     

The input of foraminiferal infaunal populations to sub-fossil assemblages along an elevational gradient in a salt marsh: application to sea-level studies in the mid-Atlantic coast of North America

The suitability of marsh sites for sea-level studies was examined based on a field study along a transect from high to low marsh. Living foraminifera at Bombay Hook (Delaware, USA) are considered to be shallow infaunal (i.e., uppermost 10 cm). Peak concentrations were found at 1–10 cm in the high marsh, 1–5 cm in the intermediate marsh, and 3–5 cm in the low marsh. However, sporadic deep infaunal inputs in the low marsh could significantly contribute to the sub-fossil assemblage. In the upper 10 cm buried (death + sub-fossil) and living assemblages showed a strong correlation, and the seasonal pattern of the buried assemblage paralleled the living one, suggesting that the buried assemblage reflected the most recent reproductive inputs. The cumulative standing crop of each dominant species was used to estimate their contribution to the buried assemblage in order to assess if the community is vertically homogeneous and, therefore, if the infaunal production causes differential sub-surface enrichment. The results showed that a “shallow” (0–5 cm) infaunal contribution is able to explain much of the sub-fossil assemblage beneath the surface in the high and intermediate marsh plots. However, in the low marsh plot, the deep infaunal contribution was greatest and significantly affected the sub-fossil assemblage. Therefore, modern analogues for sea-level studies in mid-Atlantic North American marshes should include the uppermost 5 cm. The interval proposed for the high and intermediate marsh is thin enough that epifaunal species are not underrepresented and encompasses only ~8 years (based on burial rates) providing a high temporal resolution. Handling editor: J. Saros