Effects Of Conspecific Density Research Articles

Spatial, social, and environmental factors influencing natal dispersal in the colonial griffon vulture

Abstract Natal dispersal is a critical trait for individual fitness and the viability, structure, and genetic identity of populations. However, there is a pronounced information gap for large and long-lived species due to the difficulty of monitoring individuals at appropriate spatio-temporal scales. Here we study how individual traits and social and environmental characteristics influence natal dispersal decisions of griffon vultures (Gyps fulvus) using long-term (30 years) monitoring of a large number of individuals marked as nestlings in Spain. Our results show a strong philopatry in both sexes, with some individuals recruiting as breeders on the same cliffs, and even the same nests, where they were born. This philopatric tendency was modulated by the effect of conspecific density on individual parameters and emphasized the importance of conspecific attraction and changes in food availability that may have influenced the increment in colony size and the colonization of new areas. Although further research is needed considering smaller colonies and more isolated population nuclei, our results highlight the importance of long-term studies on long-lived species to understand the factors that determine their population dynamics and their relationship with anthropogenic activities, whose effects should be predicted and managed using conservation criteria.

Effects of conspecific density on tadpole risk assessment and defensive behaviour

Abstract Prey species assess predation risk by using either direct and indirect cues and both may contribute to a proper evaluation of the actual risk. As postulated by the risk assessment hypothesis, conspecific density may also provide useful information for tuning defensive responses. We tested this hypothesis using a combination of five density levels (1, 2, 4, 8 and 16 individuals) of Italian agile frog Rana latastei tadpoles and three treatments (control, predatory cues of common backswimmer Notonecta glauca and a waterjet of tap water as mechanical disturbance). Tadpole activity decreased in response to all stimuli but, as expected, backswimmer cues induced a stronger and lasting response. However, tadpole activity level did not vary with group size, thus providing no support for the risk assessment hypothesis and confirming that conspecific density might have less consistent effects on short-term behavioural responses than morphological and life history traits.

Do local conspecific density and floral display size influence fruit set via pollinator visitation in Orchis militaris?

Plant density varies naturally, from isolated plants to clumped individuals, and this can influence pollinator foraging behaviour and plant reproductive success. In addition, the effect of conspecific density on reproduction may depend on the pollination system, and deceptive species differ from rewarding ones in this regard, a high density being often associated with low fruit set in deceptive plants. In our study, we aimed to determine how local conspecific density and floral display size (i.e. number of flowers per plant) affect fruit set in a deceptive orchid (Orchis militaris) through changes in pollinator visitation. We measured fruit set in a natural population and recorded pollinator abundance and foraging behaviour within plots of different O. militaris densities. Detailed data were recorded for the most abundant potential pollinators of O. militaris, i.e. solitary bees. Floral display size was negatively correlated to fruit set in medium‐density plots, but uncorrelated in low‐ and high‐density plots. Plot density had no effect on solitary bee abundance and visitation, which may be due to low pollinator abundance within the study site. The proportion of visited flowers per inflorescence was negatively influenced by floral display size, which is in line with previous studies. In addition, solitary bees spent decreasing time in successive flowers within an inflorescence, and the time spent per flower was negatively affected by ambient temperature. Our results suggest that pollinator behaviour during visitation is poorly linked to pollen deposition and reproductive success in O. militaris.

A time-lagged effect of conspecific density on habitat selection by snowshoe hare.

Ideal free distribution theory predicts that increased conspecific density redistributes individuals to low-density, suboptimal habitat. However, possible lags in response to population density remain poorly documented. Snowshoe hares (Lepus americanus) may exhibit density-dependent habitat selection due to its marked variation in population density. Based on 11 years (2004–2014) of snow tracking in Quebec (Canada), we investigated snowshoe hares’ short-term and delayed habitat selection responses to population density. We predicted that at high densities, hare distribution expands into low-density habitat, thus weakening the association between hares and high-density habitat. We surveyed hare tracks along 95 km of transects on average each year and georeferenced 14,240 tracks. We used Generalized Estimating Equations for track count per 100 m transect segment as a function of the proportion of different forest age classes (0–20 y, 20–40 y and 40–80 y) within 50 m of the segments. We used model coefficients for each age class as a measure of habitat preference, and modeled those coefficients as a function of a population density index in current and previous winters. Coefficients for 20- to 40-y-old forests were positive each year, indicating that this habitat was preferred. The association between track counts and 20- to 40-y-old forest significantly declined with density during the previous winter, suggesting that hare spread from preferred forest with a lagged response to density. To our knowledge, no previous empirical studies have documented a lagged habitat selection response to population density. Time lags offer possible explanation for documented deviations from ideal free distribution models.

Testing foraging arena theory: The effects of conspecific density and habitat type on time and energy budgets of juvenile cunner

Density-dependent settlement, growth and mortality are often the major factors controlling recruitment success of recently-settled marine fishes. During this stage, juvenile fishes generally have spatial refuges from predation, and forage in limited but risky areas near refuges. Little is known about the mechanisms by which the tradeoff between feeding and refuge use lead to density dependent mortality. Foraging arena theory predicts that feeding activity should depend strongly on juvenile density and predation risk. Selection should act on the time that juveniles spend foraging, so as to strike a balance between growth and mortality. Because the risk of predation also varies with habitat, it is expected that variation in foraging times and resulting growth and mortality rates will be habitat-specific and density-dependent. This study tested these concepts by respirometric measurement of the metabolic cost of feeding and shelter site defense in young-of-year cunner (Tautogolabrus adspersus) in the northwest Atlantic. Metabolic costs were applied to time budgets measured in the field to estimate in-situ energy budgets. Contrary to expectation, time and energy spent on foraging increased as habitat complexity or conspecific density decreased. Time and energy spent on refuge defense increased with increasing predation risk (as mediated by habitat complexity) or conspecific density, highlighting the importance of refuge for a species that enters torpor at night and during the winter. Future recruitment studies should include examination of spatial habitat use by juveniles, and the behavioral and physiological mechanisms for adjusting behavior to varying food density and predation risk.

The Interactive Effects of Fish Predation and Conspecific Density on Survival and Growth of Tadpoles ofRana sylvaticain a Subarctic Wetland

In nature, prey are often simultaneously exposed to predators and competitors, leading to conflicting demands between survival and growth. Larval amphibians are model organisms to investigate the balancing of life history trade-offs because individuals must metamorphose before a wetland dries. Therefore, we deployed enclosures in a natural wetland to investigate the interactive effects of Wood Frog (Rana sylvatica) density (50 or 100 tadpoles) and Brook Stickleback (Culaea inconstans) presence on survival and growth of tadpoles. We hypothesized that the effects of conspecific density on tadpole survival and growth would be dependent on the presence of fish predators. Stickleback predation significantly decreased Wood Frog survival; however, a trend of higher tadpole survival with fish was detected at high conspecific densities. Wood Frog time to and mass at metamorphosis was not significantly affected by either conspecific density or stickleback presence. Our results demonstrate that Brook Sticklebacks ca...

Influence of Intra- and Interspecific Interactions on Short-term Movement of the Amphipod Corophium volutator in Varying Environmental Conditions

Movement of animals on intertidal mudflats can have profound consequences for population and community dynamics. Such movement is often influenced by biotic interactions, but the abiotic environment can also be important, affecting the strength of species’ interactions and even changing their outcome. Here, we focused on two biotic interactions and examined their effect on movement of the burrow-dwelling amphipod Corophium volutator: intraspecific interactions (specifically, the effect of conspecific density) and interspecific interactions with a competitor–predator, the mud snail Ilyanassa obsoleta (=Nassarius obsoletus). We assessed the magnitude of the temporal variation and examined the influence of pertinent abiotic variables (lunar cycle, presence of tide pools). Despite substantial temporal variation in movement levels, density of conspecifics did not influence emigration or immigration rates of C. volutator. Mud snails negatively affected immigration of C. volutator, and this was consistent across all lunar phases. However, mud snails influenced mortality and emigration of C. volutator in a contrasting way relative to lunar phases; sometimes amphipods evaded snails and survived, but at other times did not emigrate and were killed. The presence of tide pools did not change the nature or strength of the snail–amphipod interaction. Our results further clarify the complex interactions between C. volutator and I. obsoleta and suggest that resources on the mudflat are sufficient to sustain high densities of amphipods in a density-independent manner. Further, they highlight the importance of considering multiple environmental variables when examining interspecific interactions.

Natal dispersal of European hare in a high-density population

Despite the importance of conservation and management of the European hare, a game species that has declined throughout Europe, little is known about its natal dispersal process. To date, only one radio-tracking study in a mixed cropping-farmed landscape provides a fine description of the dispersal pattern of this species. The study shows in particular a negative density dependence dispersal, juvenile hares dispersing more in a low-density hunting zone rather than in a high density non-hunting zone. Unfortunately, the effect of conspecific density on dispersal remains ambiguous due to the confounding effect of hunting. To provide more insight into the influence of conspecific numbers on the dispersal propensity in hares, we renewed the study in a harvested high density population. Our results confirmed the known sex-biased and age-related dispersal in this species but we did not find any effect of the birth site density on either the propensity to disperse or the distance moved in juvenile hares. Overall, the dispersal rate and bias were equal to those previously found in the high density non-hunting zone, where density was almost the same as in our population even though hunting pressure differed. Our results emphasize that natal dispersal is a ubiquitous feature in this species, which may respond more to density than hunting.

High population density in bank voles stimulates food hoarding after breeding

The effects of conspecific density (i.e. risk of intraspecific competition) on hoarding behaviour were studied in breeding pairs of bank voles,Clethrionomys glareolus.I simulated high population density by using odours of conspecifics, to exclude the direct effects of exploitation or interference competition for food. The pairs of bank voles hoarded only at the end of their breeding season. Hoarding was not correlated with whether the voles had a litter during the experiment or their litter size, but was more likely at the high population density and for pairs with small females. This may indicate that smaller females especially make caches in the autumn, possibly to ensure better winter survival under high population density. I discuss the hoarding behaviour of bank voles in relation to their territorial behaviour and food resources during the breeding season and predictable population changes during winter.

Effects of conspecific density on settlement, growth and post-settlement survival of a temperate reef fish

Settlement, growth and post-settlement mortality of the cunner ( Tautogolabrus adspersus Walbaum) was investigated on isolated boulder reefs in St. Margaret's Bay, Nova Scotia. Populations of cunner on these reefs were subjected to experimental manipulations of adult and recruit population density. Settlement of cunner was unaffected by the density of prior resident adults. Elevation of adult densities to approximately twice natural levels resulted in complete mortality of newly settled fish, while removal of adults resulted in enhanced growth and recruitment success over natural populations. This clearly demonstrates that natural populations of cunner in St. Margaret's Bay are sufficiently high to result in density-dependent growth and post-settlement mortality. Growth and mortality of newly settled fish were also suppressed by high recruit densities and enhanced by low densities of conspecific recruits. On all study reefs, newly settled cunner formed social dominance hierarchies, and both growth rate and post-settlement survival were positively correlated with fish size. Recruitment success of species such as cunner, for which post-settlement mortality is size-dependent, may be strongly influenced by processes affecting growth rate.

Density-Dependence in Salvia Lyrata: Experimental Alteration of Densities of Established Plants

(1) Two experiments were conducted in a natural population of Salvia lyrata to investigate the effects of conspecific density on survival, fecundity and growth of established individuals. In both experiments, the density around monitored Salvia individuals was altered, either by removal of surrounding Salvia individuals within a specified distance, or by planting Salvia in as neighbours. (2) In both experiments, survival, fecundity and growth showed small responses to the density manipulations. The responses varied with individual size. In large individuals these traits tended to decrease with increasing density, whereas small plants were in several respects favoured by more crowded conditions. (3) The weakness of the responses to neighbours suggests that the population of mature Salvia is not strongly self-regulating at typical natural densities or that marked effects of crowding appear only after a delay of more than two years. Mortality at the seedling stage as well as density-independent determinants of growth appear to maintain the adult population below the threshold at which neighbour interactions have strong effects.