Life at the edge: an experimental study of a poleward range boundary

Experimental studies of biogeographic processes are important, but rarely attempted because of the logistical challenges of research at large spatial scales. I used a series of large-scale transplant experiments to investigate the mechanisms controlling species abundance near a poleward range boundary. The intertidal limpet Collisella scabra experiences a 100-fold decline in abundance over the northernmost 300 km of its range. Temperature and food supply both strongly influenced individual survival, growth, and maturation. Regression analysis also revealed significant interactions among these conditions: the effect of one could not be predicted without knowing the level of the other. But these relationships could not explain geographic abundance patterns. Instead, individual limpets were highly successful at sites with relatively low abundance. These results suggest that, even though temperature is important to the success of individual C. scabra populations, the primary effect of warming temperatures under climate change may not be a shift in geographic distribution.     

Algal species composition and phosphorus recycling at contrasting grazing pressure: an experimental study in sub-Antarctic lakes with two trophic levels

1. To test the response of algal communities to altered grazer abundance in lakes lacking efficient predators on herbivores, we performed field and experimental studies in two sub-Antarctic lakes (South Georgia).
2. The number of algal species in these high latitude lakes is low, and all dominant species have grazer-resistant adaptations, including spines in three dimensions ( Staurastrum sp.), large size ( Tribonema sp.), a mucus sheet allowing viable passage through the gut ( Chlamydocapsa sp., Elakatothrix sp.) or ability to recruit individuals from the sediment surface ( Mallomonas sp.).
3. Algal community composition was only slightly changed by experimentally altered grazer abundance, indicating that it was already adapted for a high grazing pressure. Hence, the diets of herbivores were restricted to vulnerable food organisms such as Mallomonas sp. and heterotrophic flagellates in the water column, and to benthic food sources.
4. At high grazer abundance, the concentration of available phosphorus (PO₄-P) in the water was lower than at low grazer abundances, due to inefficient nutrient regeneration by the copepod herbivores. Hence, in lakes where copepods are dominant grazers, algae suffer both directly from grazing and indirectly from reduced nutrient availability.     

Algal species composition and phosphorus recycling at contrasting grazing pressure: an experimental study in sub-Antarctic lakes with two trophic levels

1. To test the response of algal communities to altered grazer abundance in lakes lacking efficient predators on herbivores, we performed field and experimental studies in two sub-Antarctic lakes (South Georgia).
2. The number of algal species in these high latitude lakes is low, and all dominant species have grazer-resistant adaptations, including spines in three dimensions ( Staurastrum sp.), large size ( Tribonema sp.), a mucus sheet allowing viable passage through the gut ( Chlamydocapsa sp., Elakatothrix sp.) or ability to recruit individuals from the sediment surface ( Mallomonas sp.).
3. Algal community composition was only slightly changed by experimentally altered grazer abundance, indicating that it was already adapted for a high grazing pressure. Hence, the diets of herbivores were restricted to vulnerable food organisms such as Mallomonas sp. and heterotrophic flagellates in the water column, and to benthic food sources.
4. At high grazer abundance, the concentration of available phosphorus (PO₄-P) in the water was lower than at low grazer abundances, due to inefficient nutrient regeneration by the copepod herbivores. Hence, in lakes where copepods are dominant grazers, algae suffer both directly from grazing and indirectly from reduced nutrient availability.     

Life at the dry edge: Microorganisms of the Atacama Desert

The Atacama Desert, located in northern Chile, is the driest and oldest Desert on Earth. Research aimed at the understanding of this unique habitat and its diverse microbial ecosystems begun only a few decades ago, mainly driven by NASA's astrobiology program. A milestone in these efforts was a paper published in 2003, when the Atacama was shown to be a proper model of Mars. From then on, studies have been focused to examine every possible niche suitable for microbial life in this extreme environment. Habitats as different as the underside of quartz rocks, fumaroles at the Andes Mountains, the inside of halite evaporates and caves of the Coastal Range, among others, have shown that life has found ingenious ways to adapt to extreme conditions such as low water availability, high salt concentration and intense UV radiation.     

Behaviour of parents and children in two contrasting urban neighbourhoods: an observational study

Studies based on self-report questionnaires suggest that there are social gradients in parental investment within developed societies, with reduced investment occurring in families of lower socioeconomic position. However, these results have not been investigated by direct observation of behaviour in its natural setting. Here, I report the results of an observational study of behaviour of parents and children on the streets of two neighbourhoods of the same city, one affluent and one deprived. In the deprived neighbourhood relative to the affluent one, children were more likely to be on the streets in the evenings, were less likely to be accompanied by adults, infants were more likely to be crying, and babies were more likely to be in the care of children. Where mothers were present, though, they were less likely to be the sole adult with a brood. In particular, there was a greater prevalence of groups consisting of multiple females plus children. Although only two sites have been studied here, the method could be extended to other areas, and the initial results confirm that, under harsh environmental conditions, children receive less care by adults, alloparenting of infants by children becomes more important, and women mitigate the costs of caring for children by cooperating with other adults, particularly other women.     

The role of edge contrast and forest structure in edge influence: vegetation and microclimate at edges in the Brazilian cerrado

The effect of the adjacent non-forested environment on the forest near the edge, edge influence (EI), is an important impact in fragmented landscapes and is believed to vary with factors such as forest structure and edge contrast. In order to improve our understanding of the factors governing the variability in EI, we studied microclimate and vegetation at cerrado edges surrounded by variable land uses in southeastern Brazil, a system with both forest and savanna fragments. We determined the significance, magnitude and distance of EI on microclimate, vegetation structure and grass biomass which we measured along five transects perpendicular to fourteen edges in forest or savanna next to different land uses. We introduce a quantitative measure of edge contrast that considers land uses at different distances from the same edge (e.g., a firebreak between a forest edge and a plantation) and verified whether edge contrast is correlated with EI in this system. Notwithstanding the large variation in EI among variables and study sites, there were some similarities in the patterns of EI between forest and savanna edges. Edge contrast was successfully quantified by our measure but was only correlated with EI on moisture and grass biomass. Our results point to the high variability in EI within a region. Our quantitative measure of edge contrast may be useful in explaining variability in EI. However, much unexplained variation remains in the highly fragmented cerrado system which is affected by EI in both forest and savanna fragments.     

Biodiversity surrogate effectiveness in two habitat types of contrasting gradient complexity

Enormous and increasing loss of biodiversity requires evaluation of surrogate taxa as a tool for conservation biology and new reserve selection, in spite of the fact that this approach has become questionable. The aim of this study was to assess the effect of gradient complexity on species richness and community composition among three taxonomic groups. We compared efficiency of vascular plants to indicate diversity of cryptogams (bryophytes, lichens) and snails in two contrasting habitat types (treeless fens and forests) within the same geographic region. We examined correlation of their species richness (Spearman rank correlation), community composition (Bray–Curtis similarity, Mantel test) and their responses to environmental variables (detrended and canonical correspondence analysis). We also focused on Red List species. We found that spatial congruence among studied taxa was affected by habitat type, however vascular plants were good indicator of snail biodiversity in both habitats. Nevertheless, all significant positive correlations of species richness were associated with the congruence in main environmental gradients. Although there was a consistency in significantly positive cross-taxon correlation in community similarity, the congruence was insufficient for conservation purposes. Furthermore we confirmed the necessity of integration of at-risk species in conservation planning as Red List species were poor indicators for total species richness and vice versa. We suggest the complementation of existing reserve network with small-scale protected areas focused on conservation of at-risk ecosystems, communities or species. In this study vascular plants were not found as a sufficient indicator for fine-filter conservation of other taxa.     

Pollination and phenology of flowers in the canopy of two contrasting rain forest types in Amazonia,Colombia

The main objective of this investigation was to study the pollination characteristics of two types of Amazonian rain forest at plant community level. Seasonally inundated forest was compared with upland (tierra firme) forest. The study focused on plant species in the canopy. The pollination spectra show that in both forests most canopy trees and lianas are pollinated by small bees, large bees, butterflies or by small, relatively unspecialized insects. In the upland forest small bees are the most important pollinators (32% of all species of trees and lianas are pollinated by them), whereas large bees are predominant in the floodplain (22%). Other pollinators, like hummingbirds, bats, moths, and beetles are less common (>10%), but always somewhat more important in the flood plain than in the upland forest. Bees are the most common pollinators of epiphytes. In the flood plain forest, flies are also important as epiphyte pollinators (19%), whereas in the upland forest hummingbirds pollinate more epiphytes. The phenological patterns are quite similar in both the upland and the flood plain. We found a peak in flowering in the transition period between the wet and the dry season. Flowering activity was lowest during the wet season. Differentiation in sexual systems was correlated with life form. Dioecy and monoecy were found mostly among tree species. Most species of all life forms though were hermaphroditic. No difference with respect to the relative importance of sexual systems was found between the two forest types.     

Scorpion diversity in two different habitats in the Arid Chaco,Argentina

Scorpions are one of the most important taxa of predators in terms of density, biomass, and diversity in various areas of the world. In this study, we compared population- and community-level data between a mature and a secondary forest in the Chancaní Reserve (Córdoba, Argentina). Scorpions were collected using pitfall traps (54 nights per site), and their nocturnal activity was observed by means of UV light (26 nights per site) over 7 months. Seven species of scorpions (1964 individuals) were observed in the study area (Bothriuridae and Buthidae). Brachistosternus ferrugineus composed >74 % of all individuals and was numerically dominant in most months. It was the most common species sampled with UV light method in all months (85.73 % in mature and 81.80 % in secondary forest). Timogenes elegans was the most common species sampled with the pitfall traps method in secondary forest (48.58 %). General sex ratio (males:females) for B. ferrugineus was 1:1.24 and for T. elegans was 1:0.53. The Shannon index was not significantly different between sites. Species richness was similar, and the Jaccard index was Cs = 0.86, indicating that both sites share 86 % of the species. Tityus confluens was the only species not shared between sites. Our results indicate that species composition in regenerating forest resembles that of primary forest after c. 15 years, but the relative abundances of these species differ.     

The zooplankton-phytoplankton interface in lakes of contrasting trophic status: an experimental comparison

We report here the results of an experimental study designed to compare algal responses to short-term manipulations of zooplankton in three California lakes which encompass a broad range of productivity (ultra-oligotrophic Lake Tahoe, mesotrophic Castle Lake, and strongly eutrophic Clear Lake). To assess the potential strength of grazing in each lake, we evaluated algal responses to a 16-fold range of zooplankton biomass. To better compare algal responses among lakes, we determined algal responses to grazing by a common grazer (Daphnia sp.) over a range ofDaphnia densities from 1 to 16 animals per liter. Effects of both ambient grazers andDaphnia were strong in Castle Lake. However, neither ambient zooplankton norDaphnia had much impact on phytoplankton in Clear Lake. In Lake Tahoe, no grazing impacts could be demonstrated for the ambient zooplankton butDaphnia grazing had dramatic effects. These results indicate weak coupling between phytoplankton and zooplankton in Clear Lake and Lake Tahoe, two lakes which lie near opposite extremes of lake trophic status for most lakes. These observations, along with work reported by other researchers, suggest that linkages between zooplankton and phytoplankton may be weak in lakes with either extremely low or high productivity. Biomanipulation approaches to recover hypereutrophic lakes which aim only to alter zooplankton size structure may be less effective if algal communities are dominated by large, inedible phytoplankton taxa.     

Variation of cytosine methylation in response to water availability in two contrasting growth types of an amphibious plant Alternanthera philoxeroides

We used methylation-sensitive amplified polymorphisms (MSAP) to examine the alterations of cytosine methylation in two contrasting growth types of an amphibious plant Alternanthera philoxeroides in response to change of water availability. Using 34 pairs of selective primer combinations, we amplified 1026 and 1128 clear and reproducible bands in root and leaf of A. philoxeroides, respectively. When the aquatic types of plants were transplanted into drought culture, we found a decrease in the overall DNA methylation. When the terrestrial types of plants were transferred into flood culture, we detected a higher frequency of methylation than demethylation events. Alterations of DNA methylation were more evident in root than in leaf in response to change of water availability. When the confounding effects of variable environmental factors were removed, differences of cytosine methylation profiles were observed between two growth types of plants under common growth conditions.     

Changes to the N cycle following bark beetle outbreaks in two contrasting conifer forest types

Outbreaks of Dendroctonus beetles are causing extensive mortality in conifer forests throughout North America. However, nitrogen (N) cycling impacts among forest types are not well known. We quantified beetle-induced changes in forest structure, soil temperature, and N cycling in Douglas-fir (Pseudotsuga menziesii) forests of Greater Yellowstone (WY, USA), and compared them to published lodgepole pine (Pinus contorta var. latifolia) data. Five undisturbed stands were compared to five beetle-killed stands (4–5 years post-outbreak). We hypothesized greater N cycling responses in Douglas-fir due to higher overall N stocks. Undisturbed Douglas-fir stands had greater litter N pools, soil N, and net N mineralization than lodgepole pine. Several responses to disturbance were similar between forest types, including a pulse of N-enriched litter, doubling of soil N availability, 30–50 % increase in understory cover, and 20 % increase in foliar N concentration of unattacked trees. However, the response of some ecosystem properties notably varied by host forest type. Soil temperature was unaffected in Douglas-fir, but lowered in lodgepole pine. Fresh foliar %N was uncorrelated with net N mineralization in Douglas-fir, but positively correlated in lodgepole pine. Though soil ammonium and nitrate, net N mineralization, and net nitrification all doubled, they remained low in both forest types (<6 μg N g soil^?1 NH₄ ⁺or NO₃ ^?; <25 μg N g soil^?1 year^?1 net N mineralization; <8 μg N g soil^?1 year^?1 net nitrification). Results suggest that beetle disturbance affected litter and soil N cycling similarly in each forest type, despite substantial differences in pre-disturbance biogeochemistry. In contrast, soil temperature and soil N–foliar N linkages differed between host forest types. This result suggests that disturbance type may be a better predictor of litter and soil N responses than forest type due to similar disturbance mechanisms and disturbance legacies across both host–beetle systems.     

Relationships between livestock grazing intensity and mammal predator-prey: A study case in Copo National Park in the dry Chaco forests

Extensive livestock ranching is one of the main drivers of habitat degradation in terrestrial communities in the dry Chaco forest (Argentina). Grazing intensity could differentially affect native mammals and their interactions, which could impact both, native mammal communities and livestock production systems. Here, we determined how the activity index of grey foxes Lycalopex gymnocercus and capture abundance and richness of small mammals vary along a grazing intensity gradient in a particular region of the dry Chaco forest (Copo National Park, Argentina). Track plots were used to estimate the activity index of foxes and cattle, and Sherman traps for small mammals. Fresh scats were collected to analyse the diet of foxes and to assess possible changes in predator-prey dynamics. Fruit availability and shrub density were measured in 6 plots of 2 m × 50 m. We used generalized linear mixed models, Spearman’s nonparametric rank correlation, Chi-squared test, and Spearman’s partial correlation coefficient to analyse the potential effects of grazing intensity. We found that the activity index of foxes increased (0.06 ± 0.018) while small mammal abundance (-0.08 ± 0.024) and species richness decreased (r_s = -0.94) with increasing grazing intensity. However, the proportion of scats with mammalian remains decreased with increasing grazing intensity. Also, we did not find a strong partial correlation between foxes and small mammals when we controlled for grazing intensity. This suggests that the abundance and diversity of small mammals in the study area are determined more by grazing intensity than by predator-prey interactions. Grazing intensity could negatively affect small mammals, but not through changes in fruit availability or shrub density, but possibly by affecting grass cover. Consequently, foxes’ activity could increase to meet caloric intake requirements. Our results suggest that specific cattle management recommendations depend on the wildlife species that serves as a conservation target. We recommend testing whether reducing cattle load can make this productive activity compatible with wildlife conservation in dry Chaco forests.     

Additive impacts of experimental climate change increase risk to an ectotherm at the Arctic's edge

Globally, Arctic and Subarctic regions have experienced the greatest temperature increases during the last 30 years. These extreme changes have amplified threats to the freshwater ecosystems that dominate the landscape in many areas by altering water budgets. Several studies in temperate environments have examined the adaptive capacity of organisms to enhance our understanding of the potential repercussions of warming and associated accelerated drying for freshwater ecosystems. However, few experiments have examined these impacts in Arctic or Subarctic freshwater ecosystems, where the climate is changing most rapidly. To evaluate the capacity of a widespread ectotherm to anticipated environmental changes, we conducted a mesocosm experiment with wood frogs (Rana sylvatica) in the Canadian Subarctic. Three warming treatments were fully crossed with three drying treatments to simulate a range of predicted changes in wetland environments. We predicted wetland warming and drying would act synergistically, with water temperature partially compensating for some of the negative effects of accelerated drying. Across all drying regimes, a 1 °C increase in water temperature increased the odds of survival by 1.79, and tadpoles in 52‐day and 64‐day hydroperiod mesocosms were 4.1–4.3 times more likely to survive to metamorphosis than tadpoles in 45‐day mesocosms. For individuals who survived to metamorphosis, there was only a weak negative effect of temperature on size. As expected, increased temperatures accelerated tadpole growth through day 30 of the experiment. Our results reveal that one of the dominant herbivores in Subarctic wetlands, wood frog tadpoles, are capable of increasing their developmental rates in response to increased temperature and accelerated drying, but only in an additive manner. The strong negative effects of drying on survival, combined with lack of compensation between these two environmental drivers, suggest changes in the aquatic environment that are expected in this ecosystem will reduce mean fitness of populations across the landscape.     

Climate limitation at the cold edge: contrasting perspectives from species distribution modelling and a transplant experiment

The role of climate in determining range margins is often studied using species distribution models (SDMs), which are easily applied but have well-known limitations, e.g. due to their correlative nature and colonization and extinction time lags. Transplant experiments can give more direct information on environmental effects, but often cover small spatial and temporal scales. We simultaneously applied a SDM using high-resolution spatial predictors and an integral projection (demographic) model based on a transplant experiment at 58 sites to examine the effects of microclimate, light and soil conditions on the distribution and performance of a forest herb, Lathyrus vernus, at its cold range margin in central Sweden. In the SDM, occurrences were strongly associated with warmer climates. In contrast, only weak effects of climate were detected in the transplant experiment, whereas effects of soil conditions and light dominated. The higher contribution of climate in the SDM is likely a result from its correlation with soil quality, forest type and potentially historic land use, which were unaccounted for in the model. Predicted habitat suitability and population growth rate, yielded by the two approaches, were not correlated across the transplant sites. We argue that the ranking of site habitat suitability is probably more reliable in the transplant experiment than in the SDM because predictors in the former better describe understory conditions, but that ranking might vary among years, e.g. due to differences in climate. Our results suggest that L. vernus is limited by soil and light rather than directly by climate at its northern range edge, where conifers dominate forests and create suboptimal conditions of soil and canopy-penetrating light. A general implication of our study is that to better understand how climate change influences range dynamics, we should not only strive to improve existing approaches but also to use multiple approaches in concert.