Frequency‐dependent and density‐dependent larval competition between life‐history strains of a fly,Lucilia cuprina

1. Competition was created between the larvae of two life‐history strains of the blowfly Lucilia cuprina (Wiedemann) that have different requirements for larval resource acquisition. Adult females of one strain had the ability to mature eggs in the absence of adult feeding (autogeny) whereas the other strain lacked this ability. Autogeny shifts the burden of resource acquisition from adults to larvae, potentially leading to greater competition at this earlier life history stage. 2. A replacement series was used to determine the per‐capita competitive effect between strains relative to the intra‐strain effect, and density‐ and frequency‐dependent variation in this per‐capita effect was then evaluated. Evidence was found of competitive superiority of autogenous larvae when they occurred at a low frequency and low density, but their competitive ability was lost or reversed at higher frequencies and densities. 3. A dynamic competitive environment created by frequency and density dependence can account for the maintenance of genetic diversity for major life‐history traits. Such competition may explain why autogeny is rare in field populations of L. cuprina even although underlying genetic variation for the trait seems to be present.     

Sex‐specific foraging behavior in response to fishing activities in a threatened seabird

Some seabird species have learnt to efficiently exploit fishing discards from trawling activities. However, a discard ban has been proposed as necessary in Europe to ensure the sustainability of the seas. It is of crucial importance for the management and conservation purposes to study the potential consequences of a discard ban on the foraging ecology of threatened seabirds. We assessed the influence of fishing activities on the feeding habits of 22 male and 15 female Audouin's gulls (Larus audouinii) from the Ebro Delta (Mediterranean Sea) during the breeding period using GPS loggers together with Stable Isotope Analysis (SIA), which provided new insights into their foraging behavior and trophic ecology, respectively. GPS data revealed different sex‐specific foraging patterns between workdays and weekends. Females were highly consistent in that they foraged at sea throughout the week even though discarding stops at weekends. In contrast, males switched from foraging at sea during the week (when discards are produced) to an increased use of rice field habitats at weekends (when fishermen do not work). This sex‐specific foraging behavior could be related to specific nutritional requirements associated with previous egg production, an energetically demanding period for females. However, on a broader time scale integrated by the SIA, both sexes showed a high degree of individual specialization in their trophic ecology. The need to obtain detailed information on the dependence and response of seabirds to fishing activities is crucial in conservation sciences. In this regard, sex‐specific foraging behavior in relation to fisheries has been overlooked, despite the ecological and conservation implications. For instance, this situation may lead to sex differentiation in bycatch mortality in longlines when trawlers do not operate. Moreover, any new fisheries policy will need to be implemented gradually to facilitate the adaptation of a specialized species to a discard ban scenario.     

Parental care masks a density‐dependent shift from cooperation to competition among burying beetle larvae

Studies of siblings have focused mainly on their competitive interactions and to a lesser extent on their cooperation. However, competition and cooperation are at opposite ends on a continuum of possible interactions and the nature of these interactions may be flexible with ecological factors tipping the balance toward competition in some environments and cooperation in others. Here we show that the presence of parental care and the density of larvae on the breeding carcass change the outcome of sibling interactions in burying beetle broods. With full parental care there was a strong negative relationship between larval density and larval mass, consistent with sibling competition for resources. In the absence of care, initial increases in larval density had beneficial effects on larval mass but further increases in larval density reduced larval mass. This likely reflects a density‐dependent shift between cooperation and competition. In a second experiment, we manipulated larval density and removed parental care. We found that the ability of larvae to penetrate the breeding carcass increased with larval density and that feeding within the carcass resulted in heavier larvae than feeding outside the carcass. However, larval density did not influence carcass decay.     

Aboveground competition influences density‐dependent effects of cordgrass on sediment biogeochemistry

Interspecific interactions between plants influence plant phenotype, distribution, abundance, and community structure. Each of these can, in turn, impact sediment biogeochemistry. Although the population and community level impacts of these interactions have been extensively studied, less is known about their effect on sediment biogeochemistry. This is surprising given that many plants are categorized as foundation species that exert strong control on community structure. In southern California salt marshes, we used clipping experiments to manipulate aboveground neighbor presence to study interactions between two dominant plants, Pacific cordgrass (Spartina foliosa) and perennial pickleweed (Sarcocornia pacifica). We also measured how changes in cordgrass stem density influenced sediment biogeochemistry. Pickleweed suppressed cordgrass stem density but had no effect on aboveground biomass. For every cordgrass stem lost per square meter, porewater ammonium increased 0.3–1.0 µM. Thus, aboveground competition with pickleweed weakened the effects of cordgrass on sediment biogeochemistry. Predictions about plant–soil feedbacks, especially under future climate scenarios, will be improved when plant–plant interactions are considered, particularly those containing dominant and foundation species.     

Linking isotopic and migratory patterns in a pelagic seabird

The value of stable isotope analysis in tracking animal migrations in marine environments is poorly understood, mainly due to insufficient knowledge of isotopic integration into animal tissues within distinct water masses. We investigated isotopic and moult patterns in Cory’s shearwaters to assess the integration of different stable isotopes into feathers in relation to the birds’ transoceanic movements. Specimens of Mediterranean Cory’s shearwater Calonectris diomedea diomedea caught accidentally by Catalan longliners were collected and the signatures of stable isotopes of C (δ¹³C), N (δ¹⁵N) and S (δ³⁴S) were analysed in 11 wing and two tail feathers from 20 birds, and in some breast feathers. Based on isotopic signatures and moult patterns, the feathers segregated into two groups (breeding and wintering), corresponding to those grown in the Mediterranean or Atlantic regions, respectively. In addition, feathers grown during winter, i.e. moulted in Atlantic waters, were grouped into two isotopically distinct profiles, presumably corresponding to the two main wintering areas previously identified for Mediterranean Cory’s shearwater in tracking studies. N signatures mainly indicated the Mediterranean-to-Atlantic migration, whereas C and S signatures differed according to the Atlantic wintering area. Our results indicate that isotopic signatures from distant oceanic regions can integrate the feathers of a given bird and can indicate the region in which each feather was grown. This study thus underscores how stable isotope analysis can link marine animals to specific breeding and wintering areas, and thereby shed new light on studies involving assignment, migratory connectivity and carry-over effects in the marine environment. Xavier Ruiz deceased 27 April 2008.     

Effect of colony size on the competitive outcome of encrusting colonial organisms

The relative importance of colony size ratio of interacting species was studied in Tomioka Bay, Japan. Six encrusting colonial species belonging to the following three different taxonomic groups were tested: Ascidia (three species), Bryozoa (two) and Porifera (one). Colonies of these organisms were grown in the community of sessile organisms developed on plastic panels. Logistic regression analysis was carried out to determine the effect of size ratio on the competitive outcome of interacting colonies. The results between all possible combinations among these six species did not show a significant size effect in competitive outcome (i.e. a larger colony size did not always prove important in the success of a competitive interaction with smaller colonies of other species). On the contrary, competitive success depends on the types of species interacting. Certain species such asDidemnum moseleyi (ascidian) andHaliclona sp. (sponge), in spite of being smaller in colony size, won in competitive interactions with larger colonies of other species such asDiplosoma mitsukurii (ascidian) andWatersipora subovoidea (bryozoan). These results contradict the one reported earlier: that the larger the colony size, the more chance the colony will have to win in competitive interactions.     

Experimental evidence of long‐term reproductive costs in a colonial nesting seabird

Trade‐offs between current and future reproduction are central to the evolution of life histories. Experiments that manipulate brood size provide an effective approach to investigating future costs of current reproduction. Most manipulative studies to date, however, have addressed only the short‐term effects of brood size manipulation. Our goal was to determine whether survival or breeding costs of reproduction in a long‐lived species manifest beyond the subsequent breeding season. To this end, we investigated long‐term survival and breeding effects of a multi‐year reproductive cost experiment conducted on black‐legged kittiwakes Rissa tridactyla, a long‐lived colonial nesting seabird. We used multi‐state capture–recapture modeling to assess hypotheses regarding the role of experimentally reduced breeding effort and other factors, including climate phase and colony size and productivity, on future survival and breeding probabilities during the 16‐yr period following the experiment. We found that forced nest failures had a positive effect on breeding probability over time, but had no effect on long‐term survival. This apparent canalization of survival suggests that adult survival is the most important parameter influencing fitness in this long‐lived species, and that adults should pay reproductive costs in ways that do not compromise this critical life history parameter. When declines in adult survival rate are observed, they may indicate populations of conservation concern.     

Fungal chemical defence alters density‐dependent foraging behaviour and success in a fungivorous soil arthropod

1. Aggregative behaviour in fungivorous soil arthropods is widespread; its adaptive value, however, is largely unknown. In this study, the spatial foraging behaviour of a collembolan, Folsomia candida, and the fitness consequences of feeding at different densities on the filamentous fungus Aspergillus nidulans were investigated. The effect of two fungal strains were compared; a wild‐type (wt) and a transgenic strain that lacks the ability to express the global secondary metabolite regulator LaeA (ΔlaeA). 2. In laboratory foraging tests, F. candida exhibited aggregated distributions of individuals across four distinct fungal colonies that were arranged in short distances from each other. By quantifying the extent of the feeding damage at each single colony, a more evenly distributed feeding activity was found among wt colonies than among chemical‐deficient colonies. 3. In a fitness experiment, where collembolans at different densities were restricted to feed on single A. nidulans colonies, mean growth rate of F. candida was positively related to density on the wt A. nidulans strain, but negatively related to density on the chemical‐deficient strain. 4. Depending on the fungus' ability to express secondary chemicals and availability of fungal food sources, F. candida may employ different foraging strategies: (i) avoidance of prolonged feeding on single colonies in a rich habitat (travel costs low), and (ii) intensified group feeding on single colonies in a resource‐limited habitat (travel costs high). It was hypothesised that flexibility in fungivore foraging behaviour (clumping vs. spreading feeding activity) is adaptive because it allows avoidance/overcoming induced fungal chemical defence.     

Commercial fisheries,inter‐colony competition and sea depth affect foraging location of breeding Scopoli's Shearwaters Calonectris diomedea

The distribution of seabirds at sea is influenced by physical, ecological and anthropogenic factors such as sea depth, prey distribution, intra‐specific competition and commerical fishing activities. We quantified the foraging habitat preferences of Scopoli's Shearwater Calonectris diomedea in the Mediterranean Sea. We analysed habitat preferences in relation to a suite of physical and ecological variables including sea depth, net primary production and distance to other colonies (as a proxy of intra‐specific competition). Since the Mediterranean is heavily impacted by commercial fisheries, we also incorporated the distance to fishing harbours in our analyses as a proxy of the availability of discards which are a potential feeding source for Scopoli's Shearwater. Foraging birds preferred shallower waters and avoided areas close to other colonies, thereby reducing interactions with conspecifics. We also found that long‐distance trips were undertaken to areas close to fishing harbours, suggesting that these represented particularly profitable locations to compensate for the greater travelling costs involved. No differences in foraging between the sexes were recorded. This study improves our understanding of the at‐sea distribution and habitat preference of a seabird inhabiting the over‐exploited Mediterranean Sea. Our results support growing evidence that seabirds exhibit complex relationships with commerical fishing activities, which must be considered when planning conservation programmes.     

Cascading effects of belowground predators on plant communities are density‐dependent

Soil food webs comprise a multitude of trophic interactions that can affect the composition and productivity of plant communities. Belowground predators feeding on microbial grazers like Collembola could decelerate nutrient mineralization by reducing microbial turnover in the soil, which in turn could negatively influence plant growth. However, empirical evidences for the ecological significance of belowground predators on nutrient cycling and plant communities are scarce. Here, we manipulated predator density (Hypoaspis aculeifer: predatory mite) with equal densities of three Collembola species as a prey in four functionally dissimilar plant communities in experimental microcosms: grass monoculture (Poa pratensis), herb monoculture (Rumex acetosa), legume monoculture (Trifolium pratense), and all three species as a mixed plant community. Density manipulation of predators allowed us to test for density‐mediated effects of belowground predators on Collembola and lower trophic groups. We hypothesized that predator density will reduce Collembola population causing a decrease in nutrient mineralization and hence detrimentally affect plant growth. First, we found a density‐dependent population change in predators, that is, an increase in low‐density treatments, but a decrease in high‐density treatments. Second, prey suppression was lower at high predator density, which caused a shift in the soil microbial community by increasing the fungal: bacterial biomass ratio, and an increase of nitrification rates, particularly in legume monocultures. Despite the increase in nutrient mineralization, legume monocultures performed worse at high predator density. Further, individual grass shoot biomass decreased in monocultures, while it increased in mixed plant communities with increasing predator density, which coincided with elevated soil N uptake by grasses. As a consequence, high predator density significantly increased plant complementarity effects indicating a decrease in interspecific plant competition. These results highlight that belowground predators can relax interspecific plant competition by increasing nutrient mineralization through their density‐dependent cascading effects on detritivore and soil microbial communities.     

Effective size of density‐dependent populations in fluctuating environments

Reliable estimates of effective population size are of central importance in population genetics and evolutionary biology. For populations that fluctuate in size, harmonic mean population size is commonly used as a proxy for (multi‐) generational effective size. This assumes no effects of density dependence on the ratio between effective and actual population size, which limits its potential application. Here, we introduce density dependence on vital rates in a demographic model of variance effective size. We derive an expression for the ratio in a density‐regulated population in a fluctuating environment. We show by simulations that yearly genetic drift is accurately predicted by our model, and not proportional to as assumed by the harmonic mean model, where N is the total population size of mature individuals. We find a negative relationship between and N. For a given N, the ratio depends on variance in reproductive success and the degree of resource limitation acting on the population growth rate. Finally, our model indicate that environmental stochasticity may affect not only through fluctuations in N, but also for a given N at a given time. Our results show that estimates of effective population size must include effects of density dependence and environmental stochasticity.     

Ontogenetic variation in density‐dependent growth of brown trout through habitat competition

1. Density‐dependent growth has been widely reported in freshwater fishes, but the ontogenetic evolution of competition and its subsequent effects on growth through a life span remains unclear. 2. Patterns of competition can be described by integrating population abundance data with habitat‐modelling results. Weighted usable area (WUA; m² WUA ha^?1) curves are obtained for each flow value and are then coupled with demographic data to obtain the occupancy rates (trout m^?2 WUA, the density of a given age class related to its suitable habitat) of the WUA for every age class, year and site. 3. We examined a long‐term data series searching for temporal variation in the influence of habitat occupancy rate on the growth of brown trout Salmo trutta. We tested whether (i) mean cohort mass (mean mass of the cohort during the first 3 years of life) is affected by the occupancy rate experienced across a life span; and (ii) the occupancy rate experienced at different ages influenced mean body size. 4. We observed a consistent negative power relationship between average cohort mass and mean occupancy rate through a life span, indicating that stronger cohorts were related to a reduced growth, with likely consequences for individual fitness. 5. The effects of occupancy rate on size‐at‐age were mainly detected in the size attained at the second year of life, but they were because of the competition at different times. Thus, the level of competition varied through ontogeny, in some of the rivers affecting growth since the first year of life, whereas in most of the rivers the main effects on body size resulted from the competition during the second year of life. 6. Occupancy rate appears more appropriate than density for assessing the occurrence of habitat competition in freshwater fishes, since it encompasses the differences in quantity and quality of suitable habitat for each age class. 7. Our study highlights the importance of density‐dependent growth as a key process in the dynamics of brown trout populations, its temporal variation depending on the temporal changes of density and the variation of competition associated with the habitat capacity for each life stage.     

Nonlinear impacts of climatic variability on the density‐dependent regulation of an insect vector of disease

Aedes aegypti is one of the most common urban tropical mosquito species and an important vector of dengue, chikungunya, and yellow fever viruses. It is also an organism with a complex life history where larval stages are aquatic and adults are terrestrial. This ontogenetic niche shift could shape the density‐dependent regulation of this and other mosquito species, because events that occur during the larval stages impact adult densities. Herein, we present results from simple density‐dependent mathematical models fitted using maximum likelihood methods to weekly time series data from Puerto Rico and Thailand. Density‐dependent regulation was strong in both populations. Analysis of climatic forcing indicated that populations were more sensitive to climatic variables with low kurtosis, i.e., climatic factors highly variable around the median, rainfall in Puerto Rico, and temperature in Thailand. Changes in environmental variability appear to drive sharp changes in the abundance of mosquitoes. The identification of density‐independent (i.e., exogenous) variables forcing sharp changes in disease vector populations using the exogenous factors statistical properties, such as kurtosis, could be useful to assess the impacts of changing climate patterns on the transmission of vector‐borne diseases.     

Spatial segregation between immatures and adults in a pelagic seabird suggests age‐related competition

Individual competitiveness conditions access to resources when they are limited. Immature individuals that are less skilled than adults have to adapt their foraging strategies to survive. Among strategies to reduce competition, spatial segregation has been widely demonstrated. However, the use of spatial segregation by immatures to limit intra‐specific competition with adults has rarely been tested. In this study, we investigated and compared habitat preferences and distributions of free‐ranging immature and breeding adult northern gannets Morus bassanus in order to determine whether they compete for similar habitats during the year, and if this results in a spatial segregation between birds of different age groups. Based on > 66 000 km of aerial surveys conducted in the North‐East Atlantic Ocean during winter and summer 2012, habitats selected by immatures and adult birds were modelled independently, linking gannet density to a set of oceanographic and physiographic predictors. Their large‐scale seasonal distribution was then predicted. We found that gannets displayed a strong season‐dependent competition between immatures and adults, as a consequence of immatures and adults using similar habitats in both summer and winter. During summer, when adults are constrained by reproduction, both groups were spatially highly segregated despite similar habitat preferences (thermal fronts), with youngest individuals selecting habitats out of range of central‐place foragers, highlighting intra‐specific competition. Contrastingly during winter, when reproductive constraints disappear, immature and adult distributions largely overlapped. Our study provides new insights into the role played by age, foraging experience and reproductive constraints on the distribution of marine predators. More specifically, these results highlight in seabirds how the youngest fraction mitigates, through spatial segregation, the competition with experienced adults, and suggest a progressive strategy along the maturation process.     

Non‐additive effects of density‐dependent and density‐independent factors on brown trout vital rates

Interactions between density‐dependent and density‐independent processes can lead to variation in both growth and survival rates. Detecting such effects, however, will often require sampling on an individual level and at the appropriate spatial and temporal scale. This study documents substantial variation in survival and growth of stream‐dwelling brown trout Salmo trutta from a small Norwegian stream. The data is based on seasonal capture–recaptures of individually marked trout on fixed stations during eight years. The fish were small‐sized, rarely reaching sizes larger than 20 cm and ages older than seven years. Density varied between 0.2–0.8 fish m^?2. Variation in survival and recapture probabilities was analysed using program MARK. Apparent survival (the probability of being alive and present within the study area) generally decreased with increasing trout density and increasing drought level (measured as lowest observed water flow) during both winter and summer. Further, there was a significant interaction effect between density and water flow, indicating that density‐dependent effects on survival predominated when environmental conditions were benign (no drought), while density‐independent processes were most important under harsh environmental conditions (drought). Observed length‐at‐age during autumn indicated a more or less linear growth trajectory throughout life, and no effect of density, water flow or temperature was found. However, using the individual‐based capture–recapture data to estimated specific growth rate, significant positive effects of water flow and temperature and a negative effect of density were identified. Thus, the capture–recapture data suggest a strong potential for population regulation at the rather low densities found in this stream, and regulation may occur both through effects on survival and growth.     

An experimental test of density‐dependent selection on temperament traits of activity,boldness and sociability

Temperament traits are seen in many animal species, and recent evolutionary models predict that they could be maintained by heterogeneous selection. We tested this prediction by examining density‐dependent selection in juvenile common lizards Zootoca vivipara scored for activity, boldness and sociability at birth and at the age of 1 year. We measured three key life‐history traits (juvenile survival, body growth rate and reproduction) and quantified selection in experimental populations at five density levels ranging from low to high values. We observed consistent individual differences for all behaviours on the short term, but only for activity and one boldness measure across the first year of life. At low density, growth selection favoured more sociable lizards, whereas viability selection favoured less active individuals. A significant negative correlational selection on activity and boldness existed for body growth rate irrespective of density. Thus, behavioural traits were characterized by limited ontogenic consistency, and natural selection was heterogeneous between density treatments and fitness traits. This confirms that density‐dependent selection plays an important role in the maintenance of individual differences in exploration‐activity and sociability.     

Intraspecific and interspecific competition induces density‐dependent habitat niche shifts in an endangered steppe bird

Interspecific competition is a dominant force in animal communities that induces niche shifts in ecological and evolutionary time. If competition occurs, niche expansion can be expected when the competitor disappears because resources previously inaccessible due to competitive constraints can then be exploited (i.e., ecological release). Here, we aimed to determine the potential effects of interspecific competition between the little bustard (Tetrax tetrax) and the great bustard (Otis tarda) using a multidimensional niche approach with habitat distribution data. We explored whether the degree of niche overlap between the species was a density‐dependent function of interspecific competition. We then looked for evidences of ecological release by comparing measures of niche breadth and position of the little bustard between allopatric and sympatric situations. Furthermore, we evaluated whether niche shifts could depend not only on the presence of great bustard but also on the density of little and great bustards. The habitat niches of these bustard species partially overlapped when co‐occurring, but we found no relationship between degree of overlap and great bustard density. In the presence of the competitor, little bustard's niche was displaced toward increased use of the species' primary habitat. Little bustard's niche breadth decreased proportionally with great bustard density in sympatric sites, in consistence with theory. Overall, our results suggest that density‐dependent variation in little bustard's niche is the outcome of interspecific competition with the great bustard. The use of computational tools like kernel density estimators to obtain multidimensional niches should bring novel insights on how species' ecological niches behave under the effects of interspecific competition in ecological communities.