The blue lizard spandrel and the island syndrome

Background  

Many small vertebrates on islands grow larger, mature later, lay smaller clutches/litters, and are less sexually dimorphic and aggressive than their mainland relatives. This set of observations is referred to as the 'Island Syndrome'. The syndrome is linked to high population density on islands. We predicted that when population density is low and/or fluctuating insular vertebrates may evolve correlated trait shifts running opposite to the Island Syndrome, which we collectively refer to as the 'reversed island syndrome' (RIS) hypothesis. On the proximate level, we hypothesized that RIS is caused by increased activity levels in melanocortin receptors. Melanocortins are postranslational products of the proopiomelanocortin gene, which controls pleiotropically pigmentation, aggressiveness, sexual activity, and food intake in vertebrates.     

Top‐down control by an aquatic invertebrate predator increases with temperature but does not depend on individual behavioral type

Variation in behavioral traits among individuals within a population can have implications for food webs and ecosystems. Temperature change also alters food web structure and function, but potential interactions between warming and intraspecific behavioral variation are largely unexplored. We aimed to test how increased temperature, individual activity level of a predatory backswimmer (Anisops assimilis), and their interaction influenced the strength of top‐down control of zooplankton and phytoplankton. We used stable isotopes to support our assumption that the study population of A. assimilis is zooplanktivorous, and behavioral trials to confirm that activity level is a repeatable trait. We established freshwater microcosms to test for effects of warming, backswimmer presence, and backswimmer behavioral type on zooplankton density, zooplankton composition, and phytoplankton chlorophyll a. Top‐down control was present and was generally stronger at increased temperature. There was no indication that predator behavioral type influenced the strength of top‐down control either on its own or interactively with temperature. Predator behavioral type may not be associated with ecologically important function in this species at the temporal and spatial scales addressed in this study, but the links between behavior, temperature, and food web processes are worthy of broader exploration.     

Sex,Age, and Population Density Affect Aggressive Behaviors in Island Lizards Promoting Cannibalism

Island populations may evolve distinct behavioral repertoires as a response to the conditions of insular life. Strong intraspecific competition is typical in insular lizards and may include cannibalism. In this study, we investigated sexual and age patterns of aggression in two populations of the Skyros wall lizard (Podarcis gaigeae), one from the main island of Skyros (Aegean Sea, Greece) and another from the satellite islet Diavates. The latter is terrestrial predator‐free biotope, hosting a dense population of large‐bodied lizards that have been reported to exert cannibalism. In staged encounters, we examined the aggressive propensities of adult male and female lizards against their age‐peers and juveniles. Males from both populations were much more aggressive than females toward juveniles and other adults. Males from Diavates were more frequently aggressive to juveniles and other male lizards than males from Skyros. Diavates cannibals also captured their targets at shorter latency. We ascribe this distinct behavioral pattern to the high population density. Infanticide and intramale aggressiveness confer two great advantages to cannibals: food and elimination of future rivals.     

Temperature Mediates Shifts in Individual Aggressiveness,Activity Level,and Social Behavior in a Spider

Although in recent years behavioral syndromes have received a wealth of attention, how traits within syndromes respond to changing environments is not well resolved. Here, we test the effects of temperature on a suite of behavioral traits in the spider Anelosimus studiosus to determine (1) whether there are shifts in individuals’ social tendency, activity level, and foraging behavior in response to temperature, (2) if these traits shift are in the direction predicted by within‐population axes of trait covariance, and (3) whether the effects of temperature differ among individuals. In previous work, we documented a behavioral syndrome in A. studiosus where increased tolerance of conspecifics is correlated with decreased activity level and aggressiveness toward prey. Furthermore, there are distinct among‐population differences in behavior, where individuals from warm sites tend to be more aggressive and active than individuals from cold sites. Our data here reveal that at warmer temperatures A. studiosus exhibit diminished tolerance of conspecifics, increased activity levels, shorter latencies of attack, and increased tendencies to attack multiple prey items. Furthermore, we found that individual differences in behavior were consistent across temperature regimes for the majority of behavioral traits considered here: social tendency, activity level, and latency of attack. These findings are consistent with the hypothesis that these behaviors are linked together by shared genetic underpinnings (e.g., metabolic differences) and shift non‐independently in response to contemporary abiotic environment (i.e., temperature). Furthermore, our data suggest that temperature itself could be responsible for the among‐population variation in social structure in A. studiosus.     

The interaction between predation risk and food ration on behavior and morphology of Eurasian perch

The risk of both predation and food level has been shown to affect phenotypic development of organisms. However, these two factors also influence animal behavior that in turn may influence phenotypic development. Hence, it might be difficult to disentangle the behavioral effect from the predator or resource‐level effects. This is because the presence of predators and high resource levels usually results in a lower activity, which in turn affects energy expenditure that is used for development and growth. It is therefore necessary to study how behavior interacts with changes in body shape with regard to resource density and predators. Here, we use the classic predator‐induced morphological defense in fish to study the interaction between predator cues, resource availability, and behavioral activity with the aim to determine their relative contribution to changes in body shape. We show that all three variables, the presence of a predator, food level, and activity, both additively and interactively, affected the body shape of perch. In general, the presence of predators, lower swimming activity, and higher food levels induced a deep body shape, with predation and behavior having similar effect and food treatment the smallest effect. The shape changes seemed to be mediated by changes in growth rate as body condition showed a similar effect as shape with regard to food‐level and predator treatments. Our results suggests that shape changes in animals to one environmental factor, for example, predation risk, can be context dependent, and depend on food levels or behavioral responses. Theoretical and empirical studies should further explore how this context dependence affects fitness components such as resource gain and mortality and their implications for population dynamics.     

Talkin' 'bout my generation: environmental variability and cohort effects

In variable environments, it is probable that environmental conditions in the past can influence demographic performance now. Cohort effects occur when these delayed life-history effects are synchronized among groups of individuals in a population. Here we show how plasticity in density-dependent demographic traits throughout the life cycle can lead to cohort effects and that there can be substantial population dynamic consequences of these effects. We show experimentally that density and food conditions early in development can influence subsequent juvenile life-history traits. We also show that conditions early in development can interact with conditions at maturity to shape future adult performance. In fact, conditions such as food availability and density at maturity, like conditions early in development, can generate cohort effects in mature stages. Based on these data, and on current theory about the effects of plasticity generated by historical environments, we make predictions about the consequences of such changes on density-dependent demography and on mite population dynamics. We use a stochastic cohort effects model to generate a range of population dynamics. In accordance with the theory, we find the predicted changes in the strength of density dependence and associated changes in population dynamics and population variability.     

The effects of food level and social density on reproduction in the Least Killifish,Heterandria formosa

The feedbacks from population density to demographic parameters, which drive population regulation, are the accumulated results of several ecological processes. The compensatory feedback from increased population density to fertility includes at least two distinct factors, the effects of decreases in per capita food level and increases in the social density (the number of interacting individuals). Because these effects have been studied separately, their relative importance is unknown. It is also unclear whether food limitation and social density combine additively to influence fertility. We investigated these questions with two factorial experiments on reproduction in the Least Killifish, Heterandria formosa. In one experiment, we crossed two levels of density with two levels of a total food ration that was distributed to all individuals. In the other experiment, we crossed two levels of density with two levels of per capita food. Whereas the first experiment suggested that the effects of variation in food level and density were synergistic, the second experiment indicated that they were not. The apparent synergism—the statistical interaction of food and density levels—was the result of confounding per capita food with social density in that design. In the second experiment, the effects of social density on reproductive rate were stronger than the effects of food level, whereas the effects of food level were stronger on offspring size at parturition than those of social density. The results suggest that the social stresses that emerge at higher densities play an important role in the compensatory response of fertility to density, a role, that is, at least as important as that of decreased per capita food levels.     

Temporal changes in kin structure through a population cycle in a territorial bird, the red grouse Lagopus lagopus scoticus

Populations of red grouse ( Lagopus lagopus scoticus ) undergo regular multiannual cycles in abundance. The 'kinship hypothesis' posits that such cycles are caused by changes in kin structure among territorial males producing delayed density-dependent changes in aggressiveness, which in turn influence recruitment and regulate density. The kinship hypothesis makes several specific predictions about the levels of kinship, aggressiveness and recruitment through a population cycle: (i) kin structure will build up during the increase phase of a cycle, but break down prior to peak density; (ii) kin structure influences aggressiveness, such that there will be a negative relationship between kinship and aggressiveness over the years; (iii) as aggressiveness regulates recruitment and density, there will be a negative relationship between aggressiveness in one year and both recruitment and density in the next; (iv) as kin structure influences recruitment via an affect on aggressiveness, there will be a positive relationship between kinship in one year and recruitment the next. Here we test these predictions through the course of an 8-year cycle in a natural population of red grouse in northeast Scotland, using microsatellite DNA markers to resolve changing patterns of kin structure, and supra-orbital comb height of grouse as an index of aggressiveness. Both kin structure and aggressiveness were dynamic through the course of the cycle, and changing patterns were entirely consistent with the expectations of the kinship hypothesis. Results are discussed in relation to potential drivers of population regulation and implications of dynamic kin structure for population genetics.     

Small-scale processes in desert locust swarm formation: how vegetation patterns influence gregarization

Desert locusts ( Schistocerca gregaria ) change phase in response to population density: 'solitarious' insects avoid one another, but when crowded they shift to the gregarious phase and aggregate. This individual-level process is the basis for population-level responses that may ultimately include swarm formation. We have recently developed an individual-based model of locust behavior in which contagious resource distribution leads to phase change. This model shows how population gregarization can result from simple processes operating at the individual level. In the present study, we performed a series of laboratory experiments in which vegetation pattern and locust phase state were assigned quantitative, measurable indices. The pattern of distribution of the resource was represented via fractal dimension; the phase state was evaluated using a behavioral assay based on logistic regression analysis. Locusts were exposed to different patterns of food resource in an artificial arena, after which their behavioral phase state was assayed. These experiments showed that when the distribution of the vegetation was patchy, locusts were more active, experienced higher levels of crowding, and became more gregarious. These results are consistent with simulation predictions and field observations, and demonstrate that small-scale vegetation distribution influences individual behavior and phase state and plays a role in population-level responses.     

Population dynamics of thrips prey and their mite predators in a refuge

Prey refuges are expected to affect population dynamics, but direct experimental tests of this hypothesis are scarce. Larvae of western flower thrips Frankliniella occidentalis use the web produced by spider mites as a refuge from predation by the predatory mite Neoseiulus cucumeris. Thrips incur a cost of using the refuge through reduced food quality within the web due to spider mite herbivory, resulting in a reduction of thrips developmental rate. These individual costs and benefits of refuge use were incorporated in a stage-structured predator-prey model developed for this system. The model predicted higher thrips numbers in presence than in absence of the refuge during the initial phase. A greenhouse experiment was carried out to test this prediction: the dynamics of thrips and their predators was followed on plants damaged by spider mites, either with or without web. Thrips densities in presence of predators were higher on plants with web than on unwebbed plants after 3 weeks. Experimental data fitted model predictions, indicating that individual-level measurements of refuge costs and benefits can be extrapolated to the level of interacting populations. Model-derived calculations of thrips population growth rate enable the estimation of the minimum predator density at which thrips benefit from using the web as a refuge. The model also predicted a minor effect of the refuge on the prey density at equilibrium, indicating that the effect of refuges on population dynamics hinges on the temporal scale considered.     

Scaling from optimal behavior to population dynamics and ecosystem function

While behavioral responses of individual organisms can be predicted with optimal foraging theory, the theory of how individual behavior feeds back to population and ecosystem dynamics has not been fully explored. Ecological models of trophic interactions incorporating behavior of entire populations commonly assume either that populations act as one when making decisions, that behavior is slowly varying or that non-linear effects are negligible in behavioral choices at the population scale. Here, we scale from individual optimal behavior to ecosystem structure in a classic tri-trophic chain where both prey and predators adapt their behavior in response to food availability and predation risk. Behavior is modeled as playing the field, with both consumers and predators behaving optimally at every instant basing their choices on the average population behavior. We establish uniqueness of the Nash equilibrium, and find it numerically. By modeling the interactions as playing the field, we can perform instantaneous optimization at the individual level while taking the entire population into account. We find that optimal behavior essentially removes the effect of top-down forcing at the population level, while drastically changing the behavior. Bottom-up forcing is found to increase populations at all trophic levels. These phenomena both appear to be driven by an emerging constant consumption rate, corresponding to a partial satiation. In addition, we find that a Type III functional response arises from a Type II response for both predators and consumers when their behavior follows the Nash equilibrium, showing that this is a general phenomenon. Our approach is general and computationally efficient and can be used to account for behavior in population dynamics with fast behavioral responses.     

Density‐related effects on the infectivity and aggressiveness of a sterilising smut in a wild population of Digitaria sanguinalis

Understanding host–pathogen evolutionary dynamics needs characterisation and quantification of processes occurring at many spatiotemporal scales. With this aim, the effects of smut on a naturally infected population of the summer annual Digitaria sanguinalis were followed for 4 years in an uncropped field. The main purpose of the study was to quantify the effects of within‐population density on the infectivity and the aggressiveness of the pathogen in a range of densities that occurred naturally. The infectivity‐related variable measured was the proportion of smutted plants at the end of each growing season; proportions were analysed using a generalised linear model with a binomial distribution considering the year, the density and their interaction as effects. The aggressiveness‐related variables chosen were the number of smutted inflorescences per plant and per area, obtained over the last 2 years; they were analysed by means of ancova considering disease status (seeded or smutted), year, density and all the interactions between them. Although the disease is monocyclic, results showed clearly that infectivity increased with plant density. The number of inflorescences per plant was 1.5 times higher in smutted plants than in healthy plants throughout the range of densities. This variable declined when density increased, but as the infectivity increased at a higher rate, the aggressiveness also increased with density. The surprising results on infectivity are discussed in the context of current knowledge of plant–pathogen interaction dynamics, as well as neighbour effects on pathogen aggressiveness. Moreover, the results could be useful to develop weed biological control strategies.     

Predation-mediated coexistence of large- and small-bodied Daphnia at different food levels

Using an individual-based age-structured population model (a combination of O'Brien's apparent-prey-size approach, Eggers's reactive-field-volume model, and Holling's disk equation), we could predict that (1) a Daphnia population could be kept at low density by fish predation irrespective of food level, with greater recruitment at higher food being instantly compensated for by raised mortality reflecting increased predation, and (2) Daphnia density levels are species specific and inversely related to both body size at first reproduction and the reaction distance at which a foraging fish sees its Daphnia prey. These two hypotheses were experimentally tested in outdoor mesocosms with two Daphnia species of different body sizes grown in the absence or presence of fish that were allowed to feed for 2-3 h each evening. While each Daphnia quickly reached high density with reproduction halted by food limitation in the absence of fish, the populations stayed at much lower species-specific density levels, similar in low and high food concentrations, in the presence of fish. This suggests that our model offers a reasonable mechanistic explanation for the coexistence of large- and small-bodied zooplankton in proportions reflecting their body sizes throughout habitats comprising a wide productivity spectrum, with each species at a density level at which it becomes included in a predator's diet.     

Components of Grizzly Bear Habitat Selection: Density,Habitats, Roads,and Mortality Risk

Abstract: We used resource selection functions (RSF) to estimate the relative probability of use for grizzly bears (Ursus arctos) adjacent to the Parsnip River, British Columbia, Canada, 1998-2003. We collected data from 30 radiocollared bears on a rolling plateau where a large portion of the landscape had been modified by human activities, primarily forestry. We also monitored 24 radiocollared bears in mountain areas largely inaccessible to humans. Bears that lived on the plateau existed at less than one-quarter the density of bears in the mountains. Plateau bears ate more high-quality food items, such as meat and berries, leading us to conclude that food limitation was not responsible for the differences in densities. We hypothesized that plateau bears were limited by human-caused mortality associated with roads constructed for forestry activities. Independent estimates of bear population size from DNA-based mark-recapture techniques allowed us to link populations to habitats using RSF models to scale habitat use patterns to population density. To evaluate whether differences in land-cover type, roads, or mortality risk could account for the disparity in density we used the mountain RSF model to predict habitat use and number of bears on the plateau and vice versa. We predicted increases ranging from 34 bears to 96 bears on the plateau when switching model coefficients, excluding land-cover types; when exchanging land-cover coefficients, the model predicted that the plateau population would be 9 bears lower than was observed. Large reductions in the numbers of mountain bears were predicted by habitat-selection models of bears using the plateau landscape. Although RSF models estimated in mountain and plateau landscapes could not predict bear use and abundance in the other areas, contrasts in models between areas provided a useful tool for examining the effects of human activities on grizzly bears.     

Discovery–dominance trade‐off among widespread invasive ant species

Ants are among the most problematic invasive species. They displace numerous native species, alter ecosystem processes, and can have negative impacts on agriculture and human health. In part, their success might stem from a departure from the discovery–dominance trade‐off that can promote co‐existence in native ant communities, that is, invasive ants are thought to be at the same time behaviorally dominant and faster discoverers of resources, compared to native species. However, it has not yet been tested whether similar asymmetries in behavioral dominance, exploration, and recruitment abilities also exist among invasive species. Here, we establish a dominance hierarchy among four of the most problematic invasive ants (Linepithema humile, Lasius neglectus, Wasmannia auropunctata, Pheidole megacephala) that may be able to arrive and establish in the same areas in the future. To assess behavioral dominance, we used confrontation experiments, testing the aggressiveness in individual and group interactions between all species pairs. In addition, to compare discovery efficiency, we tested the species' capacity to locate a food resource in a maze, and the capacity to recruit nestmates to exploit a food resource. The four species differed greatly in their capacity to discover resources and to recruit nestmates and to dominate the other species. Our results are consistent with a discovery–dominance trade‐off. The species that showed the highest level of interspecific aggressiveness and dominance during dyadic interactions.     

Diet and activity pattern of howler monkeys (Alouatta palliata) in Los Tuxtlas, Mexico: effects of habitat fragmentation and implications for conservation

Accelerated deforestation is causing the rapid loss and fragmentation of primary habitat for primates. Although the genus Alouatta is one of the most studied primate taxa under these circumstances, some results are contradictory and responses of howlers to habitat fragmentation are not yet clear. In this paper, we conduct a cross-study of the available researches on mantled howlers (Alouatta palliata) in forest fragments in Los Tuxtlas, Mexico, to (1) describe the diet and activity pattern of howlers; (2) analyze the similarity in the diet across studies; and (3) relate both fragment size and howler population density with different characteristics of their diet, home range size, and activity pattern. Howlers consumed 181 plant species belonging to 54 families. Ficus was the most important taxa in the howlers' diet, followed by primary species such as Pterocarpus rohrii, Nectandra ambigens, Poulsenia armata, and Brosimum alicastrum. Secondary and non-secondary light-demanding plant species, which are representatives of disturbed habitat, contributed with a high percentage of their feeding time. Only 23% of the species consumed were the same across all the studies, suggesting that howlers adapt their diet to the food availability of their respective habitats. Population density is the best predictor of howlers' ecological and behavioral changes in response to forest fragmentation, probably owing to its relationship with food availability. Howlers respond to the increase in population densities by increasing the (1) diversity of food species in the diet; (2) consumption of non-tree growth forms; and (3) consumption of new plant items. Home range size is also predicted by population density, but fragment size is a better predictor, probably owing to the fact that howler groups can overlap their home ranges. Our results emphasize the importance of conserving the larger fragments and increasing the size of small and medium-sized ones.     

Acute and chronic toxicity of the pesticide methyl parathion to the rotifer Brachionus angularis (Rotifera) at different algal (Chlorella vulgaris) food densities

Methyl parathion is a commonly used insecticide in Mexico to eradicate insect pests. We evaluated the effects of this insecticide on rotifer B. angularis using both acute and chronic toxicity tests. Median lethal concentration (LC₅₀) of methyl parathion for B. angularis for a 24-h bioassay in the presence and absence of an algal diet was derived. Elevated LC₅₀ due to the survival of a greater number of test individuals in the presence of food was observed. Regardless of the toxicant concentration, population growth curves of the animals maintained at the low food level (0.75×10⁶ cells ml^-1) had a longer lag phase than those at the high food level (1.5×10⁶ cells ml^-1). Regardless of food level, an increase in the toxicant concentration in the medium resulted in decreased population growth. The lowest peak population density (50 ind. ml^-1) was observed at the highest toxicant concentration and the lower food level. The highest population density (200 ind. ml^-1) was observed in the controls at high food level. The rates of population increase per day (r) in the controls were higher (from 0.14 to 0.37 depending on the food level). Irrespective of food level, there was a decrease in the r values with increasing pesticide concentration in the medium. In order to detect the effect of population density on the growth rates in relation to the toxicant stress, we plotted the daily growth rate against initial density for the entire duration of the experiment. We observed the existence of a significantly inverse relation at all treatments except at the low food level and high toxicant concentrations (0.625 and 1.25 mg l^-1). We discuss the role of algae in the toxicity of methyl parathion to zooplankton.     

The intensity of interference varies with food density: support for behaviour-based models of interference

Dolman (1995) measured the intake rates of snow buntings feeding on seed patches at difference bird densities, for each of two different seed densities. Interference occurred in the low food density treatment, with intake rate declining at high bird densities, but did not occur in the high food density treatment. Dolman states that existing models of the interference process assume that the intensity of interference is independent of food density. While this statement is true for the model of Hassell and Varley (1969), we show that behaviour-based models of interference do not assume that interference is independent of food density. We examine two simple analytic behaviour-based models of interference and show that, in agreement with Dolman's observations, intensity of interference is predicted to decrease with increasing food density. Dolman plotted log (intake rate) against log (bird density). Rather than obtaining a linear relationship predicted by the Hassell and Varley model, the results from the low seed density treatment indicated a curvilinear relationship consistent with those produced by our behaviour-based models. These results provide support for the use of behaviour-based models of interference over the model of Hassell and Varley.     

Metabolic rate, growth and aggressiveness in three Atlantic salmon Salmo salar populations

Standard metabolic rate ( R _S), specific growth rate ( G ) and aggressiveness were investigated in three Finnish populations of Atlantic salmon Salmo salar (Neva, Saimaa and Teno), which were reared in identical hatchery conditions. The populations differed in their geographical origin and native habitat. There was a significant difference between populations in R _S: the southernmost Neva population had higher values in R _S than the northernmost Teno population. No difference was found in G or aggressiveness between the populations. G was found to have a significant positive association with aggressiveness and R _S among the three populations, however, these results were not statistically significant after correction for multiple tests. There was no significant association between R _S and aggressiveness. Higher metabolic rate of the most southern population Neva is suggested to be an adaptation to the more abundant food sources of the southern stream.     

An energy-based model of optimal feeding-territory size

An energy-based model of feeding territoriality is described. The model predicts an optimal territory size where the territory holder's net energy intake is maximized, on a daily or seasonal time scale. Simulated effects on optimal territory size of animal size, food availability, and competitor density are in general agreement with observed relationships in a wide variety of animals. When salmonid data are used to estimate the model parameter values, predicted territory sizes are similar to those actually observed. When emigration is allowed, the model predicts that territoriality, through individual selection alone, can regulate population size, but that this regulation breaks down when initial densities exceed some threshold value. Sensitivity analysis shows optimal territory size to be most affected by those parameters influencing food intake and energy expenditure. Some alternative criteria for optimization are also discussed. An animal maximizing net foraging efficiency has a smaller territory than one maximizing net energy, but the effects of animal size, food availability, and competitor density on territory size are the same in either case.