Long-term variation in the effect of lizards on spider density is linked to rainfall

The objective of this study was to ascertain whether the impact of lizards on spiders varies temporally, and if so, whether this variability is related to rainfall. We compared annual censuses of orb-spider populations on 24 islands with diurnal lizards present, and on 20 islands with diurnal lizards absent, to rainfall over a 10-year period. A strong positive correlation (Pearson r=0.877) was found between mean spider density on nolizard islands and the number of days of rain that occurred 2 months prior to spider censuses; correlation coefficients declined with longer and shorter time periods prior to censuses. Correlation coefficients between mean spider density on lizard islands and rainfall showed a similar pattern but were generally lower than those for no-lizard islands. The strength of the impact of lizards on spiders, measured as the ratio of mean spider density on no-lizard islands to mean spider density on lizard islands, varied considerably and was positively correlated with rainfall; the correlation was highest with number of rain days 6 months prior to spider censuses (r=0.741). Repeated-measures analysis of variance on the time series of spider densities showed that the lizard factor (present versus absent) varied significantly among years. Split-plot ANOVA with rainfall as a covariate indicated that spider density and the impact of lizards on spider density were both significantly correlated with rainfall. We discuss several hypotheses on the causal mechanisms that integrate abiotic and biotic factors.     

Maternal body-volume as a constraint on reproductive output in lizards: evidence from the evolution of viviparity

A few species of squamate reptiles contain both oviparous (egg-laying) and viviparous (live-bearing) populations, and thus offer exceptional opportunities to test adaptationist hypotheses on the determinants of reproductive output. We focus on the hypothesis that maternal body-volume constrains reproductive output in squamate reptiles. If females are “full” of eggs, what happens when viviparity evolves within a lineage? Eggs increase in volume and mass during development, primarily due to the uptake of water, so how can they be accommodated within the mother's abdomen? We predict that the resultant increase in relative clutch mass (RCM) will be lessened by (1) a decrease in reproductive output (by reducing the number or size of offspring), and/or (2) an increase in maternal body-volume (via modifications of size or shape of adult females). Our comparisons of conspecific oviparous and viviparous lizards (Lerista bougainvillii) confirm that live-bearers carry heavier clutches (in both absolute and relative terms) and show the predicted shifts in body size and shape of reproductive females. However, offspring size and number were unaffected by the evolution of viviparity, and the shifts in maternal morphology were too small to fully offset the increase in clutch mass. Thus, RCMs increased by 50%, indicating that viviparous females produced clutches which more completely filled the space available in the abdominal cavity. We conclude that maternal body-volume does play a role in determining reproductive output, but that the observed clutch masses may be optimized, rather than maximized, with respect to the abdominal space available.     

Life-history strategies of Australian lizards: a comparison between the tropics and the temperate zone

Summary Previous studies have suggested that tropical and temperate-zone lizards may differ fundamentally in life histories. We tested the applicability of this idea to Australian species by comparing temperate-zone species of agamid and scincid lizards with their congeners from the seasonal tropics. Data were derived from dissection of 1,941 specimens and from published information. Clutch size and egg size were positively correlated with mean maternal body size in most lizard species from both climatic zones. Mean body size of the lizards studies did not differ between the tropics and the temperate zone, nor did egg or hatchling size. However, tropical skinks showed considerably (approximately 20%) lower clutch size and relative clutch mass than did temperate-zone skinks. This difference was partly due to the higher incidence of species with low, invariant clutch size in the tropical lizard fauna (as seen in other continents as well), but primarily due to a trend for lineages (especially genera) with relatively high fecundity to be more common in the temperate zone than in the tropics. In contrast to studies on African lizards, our data suggested that modification of clutch size between areas has not occurred within genera: congeneric species from the tropics and temperate zone did not differ in clutch size. Production of more than one clutch per annum by individual females was common in both climatic zones. Tropical lizards may differ from temperate-zone species in showing higher reproductive frequencies, more rapid growth and earlier maturation. However, most of these effects may be due to phenotypic responses to environmental conditions (especially longer annual activity season), rather than to genetically based lifehistory adaptations.     

Weather variation and trophic interaction strength: sorting the signal from the noise

Weather can have important consequences for the structure and function of ecological communities by substantially altering the nature and strength of species interactions. We examined the role of intra- and inter-annual weather variability on species interactions in a seasonal old-field community consisting of spider predators, grasshopper herbivores, and grass and herb plants. We experimentally varied the number of trophic levels for 2 consecutive years and tested for inter-annual variation in trophic abundances. Grasshopper emergence varied between years to the extent that the second growing season was 20% shorter than the first one. However, the damage grasshoppers inflicted on plants was greater in the second, shorter growing season. This inter-annual variation in plant abundance could be explained using the foraging-predation risk trade-off displayed by grasshoppers combined with their survival trajectory. Decreased grasshopper survival not only reduced the damage inflicted on plants, it weakened the strength of indirect effects of spiders on grass and herb plants. The most influential abiotic factor affecting grasshopper survival was precipitation. We found a negative association between grasshopper survival and the total yearly precipitation. A finer scale analysis, however, showed that different precipitation modalities, namely, number of rainy days and average precipitation per day, had opposing effects on grasshopper survival, which were inconsistent between years. Furthermore, our results suggest that small changes in these factors should result in changes of up to several orders of magnitude in the mortality rate of grasshoppers. We thus conclude that in this system the foraging-predation risk trade-off displayed by grasshoppers combined with their survival trajectory and relevant weather variability should be incorporated in analytical theory, whose goal is to predict community dynamics.     

Population dynamics of two species of dragon lizards in arid Australia: the effects of rainfall

The population dynamics of two species of agamid (dragon) lizards were studied in the Simpson Desert, central Australia, over a period of 7 years, and modelled in relation to rainfall. Both species have annual life cycles, with adults predominating during the breeding season in spring and summer and juveniles predominating in other seasons. Within years, juvenile abundance in both species in autumn and winter was related most strongly to rainfall in the preceding summer and autumn. This pattern suggests that rainfall enhances survival, growth and possibly clutch size and hatching success. Between years, however, rainfall drove successional change in the dominant plant species in the study area, spinifex Triodia basedowii, causing in turn a shift in the relative abundance of the two species. Thus, the central netted dragon Ctenophorus nuchalis was most numerous in 1990 when vegetation cover was <10%, but declined dramatically in abundance after heavy rainfall at the end of that year. In contrast, the military dragon C. isolepis achieved greatest abundance following heavy rains in the summers of 1990 and 1994, when spinifex cover increased to >20%, and remained numerically dominant for much of the study. We suggest that drought-wet cycles periodically reverse the dominance of the two species of Ctenophorus, and perhaps of other lizard species also, thus enhancing local species diversity over time. Further long-term studies are needed to document the population dynamics of other species, and to identify the factors that influence them. Received: 11 September 1998 / Accepted: 10 February 1999     

The effect of predation on the structure and invasibility of assembled communities

Summary Fifty four microcosmic communities were assembled over 4 months from a 28-species source pool of phytoplankton using nine different invasion patterns each replicated six times. Three communities from each set of replicates then were invaded with a cladoceran that feeds on phytoplankton. All communities were then treated identically for an additional 4 months. In all nine invasion categories species richness was greater in predated communities. Predation opened communities to invasion by increasing the representation of infrequently sampled species at the expense of more common species. Invasion rate was four times more influential than predation and over eleven times more important than either invasion order or the timing pattern of interspecific arrivals in determining species richness in this system of communitites.     

Small island biogeography in the Gulf of California: lizards, the subsidized island biogeography hypothesis, and the small island effect

Aim We used insular lizard communities to test the predictions of two hypotheses that attempt to explain patterns of species richness on small islands. We first address the subsidized island biogeography (SIB) hypothesis, which predicts that spatial subsidies may cause insular species richness to deviate from species–area predictions, especially on small islands. Next, we examine the small island effect (SIE), which suggests small islands may not fit the traditional log‐linear species–area curve. Location Islands with arthropodivorous lizard communities throughout the Gulf of California. Methods To evaluate the SIB hypothesis, we first identified subsidized and unsubsidized islands based on surrogate measures of allochthonous productivity (i.e. island size and bird presence). Subsequently, we created species–area curves from previously published lizard species richness and island area data. We used the residuals and slopes from these analyses to compare species richness on subsidized and unsubsidized islands. To test for an SIE, we used breakpoint regression to model the relationship between lizard species richness and island area. We compared results from this model to results from the log‐linear regression model. Results Subsidized islands had a lower slope than unsubsidized islands, and the difference between these groups was significant when small islands were defined as < 1 km². In addition to comparing slopes, we tested for differences in the magnitude of the residuals (from the species–area regression of all islands) for subsidized vs. unsubsidized islands. We found no significant patterns in the residual values for small vs. large islands, or between islands with and without seabirds. The SIE was found to be a slightly better predictor of lizard species richness than the traditional log‐linear model. Main conclusions Predictions of the SIB hypothesis were partially supported by the data. The absence of a significant SIE may be a result of spatial subsidies as explained by the SIB hypothesis and data presented here. We conclude by suggesting potential scenarios to test for interactions between these two small island hypotheses. Future studies considering factors affecting species richness should examine the possible role of spatial subsidies, an SIE, or a synergistic effect of the two in data sets with small islands.     

Growth responses of Pinus sylvestris L. to climate from the southeastern limit of its natural distribution area,Turkey

Pinus sylvestris, the most widely distributed pine species, is commonly used in dendrochronological studies. Based on a lack of studies at its southeastern distribution, we analysed the growth responses of P. sylvestris to temperature and precipitation. We selected 13 sites to study the effects of climate on the growth of Scots pine stands throughout a geographic gradient over time. Trees were sampled from pure stands at different elevations and landscape conditions. The linear and non-linear associations between tree-ring widths and climate variables were calculated with locally specific linear correlation analysis and a mixed generalised additive model. Moving window correlation function was also performed to understand the temporal stability of limiting factors on growth from 1930 to 2013. Our findings showed that early spring temperature (March-April) and late spring-early summer precipitation (May-June) are the major drivers of growth at all sites, where high temperature constraints and high precipitation enhances the growth. Moving window correlation analysis highlighted that the response to precipitation was stationary while temperature changed over time. Our non-linear analysis provided a threshold for March-April temperature. The threshold indicates that the relative additional increment sharply increases up to 7 °C and then slightly decreases.     

Effect of rainfall exclusion on ant assemblages in montane rainforests of Ecuador

Models of global climate change generally predict higher rainfall variability, with more intense rainfall events separated by extended dry periods. We experimentally studied the effects of prolonged droughts on diverse ant assemblages found at three elevations in an Ecuadorian montane rainforest. We hypothesized that most species will be negatively affected directly (physiological stress) or indirectly (less food or suitable nesting resources) by drought. We thus expected a decrease in species richness as well as changes in assemblage composition. As the decrease of moisture content was expected to be more marked above ground than in soil, its impact on ants should differ according to their nesting site (dead wood, leaf litter, soil). After 6-month treatments, dead wood, leaf litter and soil samples were on average 53%, 46% and 4% drier under tents than controls, respectively. The drought did not alter overall species richness in the three nesting situations. However, species composition was significantly modified, mostly in dead wood and in leaf litter. Camponotus in dead wood, Strumigenys in leaf litter and Solenopsis in all three microhabitats were more abundant under tents. In contrast, Pheidole in dead wood, and Pachycondyla in leaf litter, were less abundant than in controls. Similar trends were observed at the three study sites. To conclude, after 6-month droughts dominant ant genera in the assemblages were affected differently. In the longer term, alteration of the rainfall regime is therefore expected to have an important impact on ants, as well as on their ecosystem functions and services, through shifts in species dominance and composition.     

Grazing of protozoa and its effect on populations of aquatic bacteria

Predation by bacterivorous protists in aquatic habitats can influence the morphological structure, taxonomic composition and physiological status of bacterial communities. The protistan grazing can result in bacterial responses at the community and the species level. At the community level, grazing-induced morphological shifts have been observed, which were directed towards either larger or smaller bacterial sizes or in both directions. Morphological changes have been accompanied by changes in taxonomic community structure and bacterial activity. Responses at the species level vary from species to species. Some taxa have shown a pronounced morphological plasticity and demonstrated complete or partial shifts in size distribution to larger growth forms (filaments, microcolonies). However, other taxa with weak plasticity have shown no ability to reduce grazing mortality through changes in size. The impact of protistan grazing on bacterial communities is based on the complex interplay of several parameters. These include grazing selectivity (by size and other features), differences in sensitivity of bacterial species to grazing, differences in responses of single bacterial populations to grazing (size and physiology), as well as the direct and indirect influence of grazing on bacterial growth conditions (substrate supply) and bacterial competition (elimination of competitors).     

The language of lizards: interpreting the function of visual displays of the Indian rock lizard, Psammophilus dorsalis (Agamidae)

The first step in understanding any communication system is to document signal diversity relative to the context of signalling (e.g. sex of the signaller and audience). Observation of 30 free-ranging rock lizards (Psammophilus dorsalis) on rock outcrops in southern India over a period of 18 months revealed that these lizards produce a complex array of ritualized signals involving push-ups (head-bobbing), dorsal flattening, extension of the legs or gular region, and tail-raising. Push-ups were performed by both sexes, usually after moving from one location to another. Push-ups were rarely accompanied by other postural modifications, and seem to function as non-directed signals. Dorsal flattening was elicited by birds flying overhead, and seems to make the lizard less conspicuous to predators. There was, nonetheless, a strong sex difference in the frequency of this behaviour, because the habitats used by males (open rocks) exposed them to more birds. Males displayed to females by extending their gular folds and arching their backs; other animals (e.g. squirrels, monkeys) also elicited the latter posture from both sexes. Leg extension was observed for both males and females, but in different contexts—males in response to conspecifics, females in response to other animals. Females raised their tails in response to encountering a male. Thus, these lizards have a complex repertoire of postures for predator evasion, for interaction with other species and with conspecifics, and for communicating sex-specific social information about gender (tail-raise) or dominance status (gular extension, leg extension).     

Habitat use and ecological interactions of an introduced and a native species of Anolis lizard on Grand Cayman,with a review of the outcomes of anole introductions

Since its introduction ten years ago, Anolis sagrei has spread over much of Grand Cayman and is now more common in some habitats than the native anole, A. conspersus. Interspecific differences in body size, perch height, and microclimatic preference may have facilitated the colonization. Nonetheless, competition may be occurring between the species; comparisons with studies of habitat use prior to the arrival of A. sagrei indicate that in open habitats, where A. sagrei is now abundant, A. conspersus perches higher, but in closed habitats, where A. sagrei is absent, no change in perch height is evident. Review of data concerning 23 Anolis introductions indicates that the presence or absence of an ecologically similar native species may be an important determinant of colonization success or failure.     

Increased seed consumption by biological control weevil tempers positive CO2 effect on invasive plant (Centaurea diffusa) fitness

Predicted increases in atmospheric CO₂ and temperature may benefit some invasive plants, increasing the need for effective invasive plant management. Biological control can be an effective means of managing invasive plants, but the anticipated range in responses of plant–insect interactions to climate change make it difficult to predict how effective biological control will be in the future. Field experiments that manipulate climate within biological control systems could improve predictive power, but are challenging to implement and therefore rare to date. Here, we show that free air CO₂ enrichment in the field increased the fitness of Centaurea diffusa Lam., a problematic invader in much of the western United States. However, CO₂ enrichment also increased the impact of the biological control agent Larinus minutus (Coleoptera: Curculionidae) on C. diffusa fitness. C. diffusa plants flowered earlier and seed heads developed faster with both elevated CO₂ and increased temperature. Natural dispersal of L. minutus into the experimental plots provided a unique opportunity to examine weevil preference for and effects on C. diffusa grown under the different climate change treatments. Elevated CO₂ increased both the proportion of seed heads infested by L. minutus and, correspondingly, the amount of seed removed by weevils. Warming had no detectable effect on weevil utilization of plants. Higher weevil densities on elevated CO₂ plants reduced, but did not eliminate, the positive effects of CO₂ on C. diffusa fitness. Correlations between plant development time and weevil infestation suggest that climate change increased weevil infestation by hastening plant phenology. Phenological mismatches are anticipated with climate change in many plant–insect systems, but in the case of L. minutus and C. diffusa in mixed-grass prairie, a better phenological match may make the biological control agent more effective as CO₂ levels rise.     

Moisture-driven changes in the sensitivity of the radial growth of Picea crassifolia to temperature,northeastern Tibetan Plateau

Precipitation is one of the most important climate factors controlling tree growth, yet it is not fully understood how changes in precipitation affect the relationship between growth and temperature. On the northeastern edge of the Tibetan Plateau, nine tree-ring chronologies of Picea crassifolia were developed along a precipitation gradient from semi-arid (mean annual precipitation, 255 mm) to semi-humid (710 mm). We analyze the growth-climate relationships along this precipitation gradient and assess whether these associations are regulated by local precipitation. From 1960 to 2014, temperature increased significantly while precipitation remained stable at the nine sampling sites. The radial growth of P. crassifolia decreased at the semi-arid sites but increased at the semi-humid sites. Growth-temperature relationships gradually changed from negative to positive along the precipitation gradient (from dry to wet sites), particularly during summer. The moist P. crassifolia sites are also characterized by positive correlations with the Palmer Drought Severity Index. The temporal growth-temperature relationships varied significantly among the different spruce sites over the last five decades. Although temperature remains the main factor controlling the growth of P. crassifolia, local precipitation variability is becoming increasingly important. Our findings indicate that considering species distribution areas supports the analyses of the impact of climate change on tree growth.     

CLIMBER: Climatic niche characteristics of the butterflies in Europe

Detailed information on species’ ecological niche characteristics that can be related to declines and extinctions is indispensable for a better understanding of the relationship between the occurrence and performance of wild species and their environment and, moreover, for an improved assessment of the impacts of global change. Knowledge on species characteristics such as habitat requirements is already available in the ecological literature for butterflies, but information about their climatic requirements is still lacking. Here we present a unique dataset on the climatic niche characteristics of 397 European butterflies representing 91% of the European species (see Appendix). These characteristics were obtained by combining detailed information on butterfly distributions in Europe (which also led to the ‘Distribution Atlas of Butterflies in Europe’) and the corresponding climatic conditions. The presented dataset comprises information for the position and breadth of the following climatic niche characteristics: mean annual temperature, range in annual temperature, growing degree days, annual precipitation sum, range in annual precipitation and soil water content. The climatic niche position is indicated by the median and mean value for each climate variable across a species’ range, accompanied by the 95% confidence interval for the mean and the number of grid cells used for calculations. Climatic niche breadth is indicated by the standard deviation and the minimum and maximum values for each climatic variable across a species’ range. Database compilation was based on high quality standards and the data are ready to use for a broad range of applications.It is already evident that the information provided in this dataset is of great relevance for basic and applied ecology. Based on the species temperature index (STI, i.e. the mean temperature value per species), the community temperature index (CTI, i.e. the average STI value across the species in a community) was recently adopted as an indicator of climate change impact on biodiversity by the pan-European framework supporting the Convention on Biological Diversity (Streamlining European Biodiversity Indicators 2010) and has already been used in several scientific publications. The application potential of this database ranges from theoretical aspects such as assessments of past niche evolution or analyses of trait interdependencies to the very applied aspects of measuring, monitoring and projecting historical, ongoing and potential future responses to climate change using butterflies as an indicator.     

Impact of predation by spiders on a territorial damselfly (Odonata: Calopterygidae)

Summary Predation by orb-weaving spiders and crab spiders on the damselfly Calopteryx haemorrhoidalis was studied at a small stream in Southern France. One species of orb-weaving spider, Larinioides folium, caught 76% of the damselflies which fell prey to spiders. Displacement experiments on spiders on sections of bank and the positioning of webs in male territories show that the density and distribution of damselflies is not influenced by orbweb density or by the position of webs. Predation rates corresponded to orb-web density, but neither for sex nor for stage was there a relationship with damselfly density. Mean daily predation rates ranged between 0.9% for females and 4.1% for adult males. Predation risk to adult damselflies by orb-weaving spiders was male biased, whereas among tenerals there was no bias. Males were captured more frequently at territories near the water. Captures show a maximum at noon when territorial disputes of adult males were most frequent. After orbwebs were placed within territories predation rate of males was strongly increased. Predation risk to adult females in the direct vicinity of the stream was less than in the bank vegetation where they perch close to orbwebs. The risk of predation by crab spiders, which catch damselflies at their perching sites, was not sex-biased.     

Thermoregulation of two sympatric species of horned lizards in the Chihuahuan Desert and their local extinction risk

Thermoregulatory studies of ectothermic organisms are an important tool for ecological physiology, evolutionary ecology and behavior, and recently have become central for evaluating and predicting global climate change impacts. Here, we present a novel combination of field, laboratory, and modeling approaches to examine body temperature regulation, habitat thermal quality, and hours of thermal restriction on the activity of two sympatric, aridlands horned lizards (Phrynosoma cornutum and Phrynosoma modestum) at three contrasting Chihuahuan Desert sites in Mexico. Using these physiological data, we estimate local extinction risk under predicted climate change within their current geographical distribution. We followed the Hertz et al. (1993, Am. Nat., 142, 796–818) protocol for evaluating thermoregulation and the Sinervo et al. (2010, Science, 328, 894–899) eco-physiological model of extinction under climatic warming. Thermoregulatory indices suggest that both species thermoregulate effectively despite living in habitats of low thermal quality, although high environmental temperatures restrict the activity period of both species. Based on our measurements, if air temperature rises as predicted by climate models, the extinction model projects that P. cornutum will become locally extinct at 6% of sites by 2050 and 18% by 2080 and P. modestum will become extinct at 32% of sites by 2050 and 60% by 2080. The method we apply, using widely available or readily acquired thermal data, along with the modeling, appeared to identify several unique ecological traits that seemingly exacerbate climate sensitivity of P. modestum.     

Zonation of ground beetles (Coleoptera: Carabidae) and spiders (Araneida) in salt marshes at the North and the Baltic Sea and the impact of the predicted sea level increase

The ground beetles and spiders of two salt marshes at the German Northand Baltic Sea coast were investigated by pitfall traps in 1997 and 1998. Whilethe sites at the North Sea coast are tidal salt marshes, the salt marshes at theBaltic Sea are not influenced by tides. Pitfall traps were installed in agradient from 20 to 150 cm above MHT (mean high tide: 157cm + NN, NN: 500 cm above 0 at Amsterdam gauge) atthe North Sea coast or NN at the Baltic Sea coast at six or sevensampling elevations, each with five replicates. Conductivity, water content, organic substance,frequency or duration of floodings, sand content and pH of the soil weredetermined. The flooding regime is the major factor controlling the zonation ofinvertebrates. Two and three invertebrate assemblages at the North and BalticSea, respectively, were distinguished. These corresponded well with thevegetational zones. The border between the two zones was at 60–80cm above MHT at the North Sea. The three zones at the Baltic Seaextended between 20 and 30 cm, 40 to 80 cm and 100 to150 cm above NN. The elevation of the mean abundance of speciesabove MHT or NN was calculated. A tide simulation experiment resulted in ashifting population and in an increasing activity under a tidal regime aspredicted for the global climate change conditions in 2050. From the actualelevation of the mean abundance, the habitat size of salt marsh species wascalculated for a moderate and worse scenario of global climate change. Habitatreduction becomes highest for species of the lower salt marsh zone. Under worseconditions the gradiental length of habitat will only amount to a maximum of 20m at the slopes of the dikes.