Patterns of species co-occurrence and density compensation: a test for interspecific competition in bat ectoparasite infracommunities

Parasites constitute an ideal system with which to investigate patterns and mechanisms of community structure and dynamics. Nevertheless, despite their prevalence in natural systems, parasites have been examined less often than other organisms traditionally used for testing hypotheses of community assembly. In the present study, we investigate possible effects of competitive interactions on patterns of distribution (co‐occurrence) and density among a group of streblid bat flies parasitic on short‐tailed fruit bats, Carollia perspicillata. Using null model analyses of species co‐occurrence, we did not find evidence that competition affects the distribution of bat fly species across hosts. Moreover, when non‐infested hosts were included, analyses showed evidence for interspecific aggregation, rather than for the segregation predicted by competition theory. Partial Pearson correlations among bat fly species densities showed no evidence of negative covariation in two of three cases. In the species pair for which a significant negative correlation was found, a visual analysis of plotted covariation indicated a constraint line, suggesting that competition between these two species might become operational only in some infracommunities when abundances of bat flies approach a maximum set by one or more limiting resources. Moreover, when a community‐wide estimation of the significance of density compensation was calculated, the result was not significant. Overall, we find no evidence that competition influences the distribution of bat flies on their hosts, and mixed support for effects of competition on the densities of species. These results are consistent with the idea that competition plays a role in structuring natural communities, but in many systems its effects are context‐dependent and might not be important relative to other factors. Wider analyses across taxonomic and environmental gradients and a detailed consideration of the different hypothesized effects of competition are necessary to fully understand the importance of competition on natural communities.     

On detecting and measuring competition in spatially structured plant communities

Recent models have shown that the development of spatial structure in plant mixtures may make strong competitive interactions between species hard to detect owing to spatial segregation of the competing species. Here we address the issue of measuring interspecific competition using a simulation based on a neighbourhood population model which assumes that both dispersal and competitive interactions are localized. Using known parameter combinations we use the model to test the power and efficiency of two approaches for detecting and measuring competition. The first approach is based on measuring the response of communities to the removal of neighbours. Measures of interspecific competition based on this approach are extremely biased by spatial segregation of species, although this bias may be partially overcome by altering the spatial scale at which the effects of removals are recorded. The second approach is based on multiple regression of per capita population growth rates on local densities of the interacting species. When dispersal is restricted, the regression approach provides accurate estimates of interspecific competition coefficients when the scale of the sampling unit (i.e. the quadrats within which plants are counted) is large compared to the scale at which interactions and dispersal occur. When seeds disperse globally the removal method performs best; the regression method fails because sampling units do not form closed dynamic systems. Our results highlight the importance of tailoring methods for detecting competition to the characteristics of the species in question. They also indicate that rapid nonmanipulative estimates of competition coefficients may be the best approach in communities where dispersal is restricted and competitive interactions localized, which is likely to be the case for the majority of plants.     

The spatial scale of competition from recruits on an older cohort in Atlantic salmon

Competitive effects of younger cohorts on older ones are frequently assumed to be negligible in species where older, larger individuals dominate in pairwise behavioural interactions. Here, we provide field estimates of such competition by recruits on an older age class in Atlantic salmon (Salmo salar), a species where observational studies have documented strong body size advantages which should favour older individuals in direct interactions. By creating realistic levels of spatial variation in the density of underyearling (YOY) recruits over a 1-km stretch of a stream, and obtaining accurate measurements of individual growth rates of overyearlings (parr) from capture–mark–recapture data on a fine spatial scale, we demonstrate that high YOY density can substantially decrease parr growth. Models integrating multiple spatial scales indicated that parr were influenced by YOY density within 16 m. The preferred model suggested parr daily mass increase to be reduced by 39% when increasing YOY density from 0.0 to 1.0 m⁻², which is well within the range of naturally occurring densities. Reduced juvenile growth rates will in general be expected to reduce juvenile survival (via increased length of exposure to freshwater mortality) and increase generation times (via increased age at seaward migrations). Thus, increased recruitment can significantly affect the performance of older cohorts, with important implications for population dynamics. Our results highlight that, even for the wide range of organisms that rely on defendable resources, the direction of competition among age classes cannot be assumed a priori or be inferred from behavioural observations alone.     

Seasonal dynamics and interspecific competition in Oneida Lake Daphnia

I investigated the population dynamics and competitive interactions of two species of the suspension-feeding crustacean Daphnia in Oneida Lake, N.Y. Both species have persisted in the lake for decades, but their water-column densities are negatively correlated. The larger Daphnia pulicaria dominates in some years, the smaller D. galeata mendotae in others, and in some years one species replaces the other seasonally. Although this pattern results in part from annual variation in vertebrate predation pressure, predation alone cannot explain the irregular daphnid dynamics. In 1992–1995, I examined the water-column abundances, birth and death rates of both species. D. pulicaria dominated in two years, D. galeata mendotae was replaced by D. pulicaria in one year and in 1994, both species persisted in low numbers. To test the effect of temporal changes on the strength of intra- and interspecific competition on both juvenile and adult daphnids, I manipulated a series of field enclosures in 1994 and 1995. The outcome of competition varied within and between years, and its effects were most evident at the highest densities and lowest resource levels. For adults of both species, the effects of interspecific competition were detected more often than those of intraspecific competition. Lipid reserves (a metric of fitness) among juveniles were generally low, with those of D. galeata mendotae often being less than those of D. pulicaria. Contrary to the results of other studies examining competition in daphnids, spatial segregation and predictable within-year reversals in competitive dominance most likely do not play a large role in fostering coexistence of the Oneida Lake daphnids. Instead, coexistence of these competitors is promoted by interannual variation and long-lived diapausing eggs. Received: 20 July 1997 / Accepted: 21 November 1997     

The genetic effects of competition in seaweed flies

In spite of abundant evidence that intra- and inter-specific competition occurs in natural communities, there is surprisingly little to suggest it is a major force promoting genetic change. This report assesses the genetic effects of competition in two species of seaweed fly, Coelopa frigida and C. pilipes. In laboratory cultures of C. frigida the relative survival of heterozygotes at the Adh locus, which was being used as a marker for the large αβ chromosomal inversion, was greater than that of homozygotes. In monocultures of C. frigida this competitive superiority was dependent on larval density. At low densities facilitation was seen, whereas at high larval densities there was competition. In mixed cultures of the two species, interspecific competition contributed to the differential mortality of C. frigida , and observations of natural populations suggested that competition may have similar effects to those described in laboratory culture. A possible mechanism involving the supply of nutritive microorganisms is proposed to underly both intra- and inter-specific competition. In seaweed flies, competition and the consequent differential mortality appear to be forces maintaining rather than reducing genetic variation.     

Convergence in a resource-based competition system

A resource-based competition model of two consumer species and one resource species is formulated in the form of a Lotka-Volterra system. The competition involves both exploitation and interference. By a method of asymptotic estimates, sufficient conditions are derived for the three species system to converge ast→∞ to an equilibrium point with all three species present; a generalization of the result forn≥2 and single resource species is indicated. The strong form of equilibrium perisistence of the three species consumer-resource system is achieved by the ability of each of the consumer species to exploit the resource and interfere with others in such a way which will avoid exclusion by the other.     

Qualitative geographical variation in interspecific interactions

We explore geographical variation in the density relationship between potential competitor forest bird groups, resident Parus spp. and migrant Fringilla spp., across Europe using published bird census results. In addition, we summarized results from three experimental studies from northern Europe on their density associations. Based on anticipated changes in the relative intensity of positive and competitive interactions we predicted a unimodal density association between Parus and Fringilla : at low and intermediate densities the two groups are positively associated (positive interaction), whereas high densities promote interspecific competition. In central Europe where densities are high, densities were unimodally related to each other. In northern and southern Europe linear and positive associations appeared. Experimental studies provided consistent support for positive interspecific interactions in the north. The results suggest that species interactions may indeed vary in relation to the density of potential competitor and switch from positive to negative along environmental gradients.     

Density dependence and temporal plasticity of competitive interactions during utilisation of resources by a community of lepidopteran stemborer species

Interactions within and between species sharing the same resources are characterised by competition or facilitation, and can be influenced by factors such as larval numbers and phenotypic plasticity of the interactions. The effect of larval density on the survival and relative growth rate of the stemborers Busseola fusca (Fuller) and Sesamia calamistis Hampson (both Lepidoptera: Noctuidae), and Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) were studied, as well as the temporal plasticity of their competitive interactions. These stemborers attack maize crops (Zea mays L.) (Poaceae) in sub‐Saharan Africa. Experiments were conducted in the laboratory under controlled conditions at the optimum development temperature (25 °C) for the three species. Surrogate stems filled with artificial diet were intra‐ and interspecifically infested with larvae of each species. The effect of larval density on competition was studied at low (six larvae) and high (12 larvae) levels of infestation, whereas the temporal plasticity of competition was evaluated at 7, 14, 21, or 28 days after infestation. The two experiments involved single‐ and multi‐species infestation treatments. Larval numbers and wet mass in each artificial stem were recorded in each experiment. Survival and relative growth rate of the three species were significantly higher at low‐infestation levels when facing either intra‐ or interspecific competition. The intensity of competition was also temporally plastic among the species and increased as the duration of competition increased. These results are discussed in terms of general infestations of cereal crops by borers.     

Larval competition within and between insecticide resistant and susceptible individuals in the oriental fruit fly,Bactrocera dorsalis

Insecticide resistance is a serious issue in agriculture. Ecological interactions may be used to manage the spread of resistance, but little information is currently available. In particular, virtually none is known about interactions between resistant and susceptible individuals. This study investigated competition between resistant and susceptible oriental fruit fly larvae (Bactrocera dorsalis). Competition in the larval stage was examined with an explicit consideration of individual identity (resistant or susceptible). Guava fruits were inoculated with eggs of susceptible and/or resistant flies, and their development and survival were monitored. Egg density influenced the time that larvae stayed in fruits as well as their survival. In addition, susceptible flies survived better when interacting with resistant flies than with other susceptible flies, indicating that susceptible flies are competitively superior to resistant flies. The results suggest that artificially creating environments that induce competition between susceptible and resistant flies can be useful for the management of insecticide resistance.     

Size-asymmetric competition and size-asymmetric growth in a spatially explicit zone-of-influence model of plant competition

Size-asymmetric competition among plants is usually defined as resource pre-emption by larger individuals, but it is usually observed and measured as a disproportionate size advantage in the growth of larger individuals in crowded populations (“size-asymmetric growth”). We investigated the relationship between size-asymmetric competition and size-asymmetric growth in a spatially explicit, individual-based plant competition model based on overlapping zones of influence (ZOI). The ZOI of each plant is modeled as a circle, growing in two dimensions. The size asymmetry of competition is reflected in the rules for dividing up the overlapping areas. We grew simulated populations with different degrees of size-asymmetric competition and at different densities and analyzed the size dependency of individual growth by fitting coupled growth functions to individuals. The relationship between size and growth within the populations was summarized with a parameter that measures the size asymmetry of growth. Complete competitive symmetry (equal division of contested resources) at the local level results in a very slight size asymmetry in growth. This slight size asymmetry of growth did not increase with increasing density. Increased density resulted in increased growth asymmetry when resource competition at the local level was size asymmetric to any degree. Size-asymmetric growth can be strong evidence that competitive mechanisms are at least partially size asymmetric, but the degree of size-asymmetric growth is influenced by the intensity as well as the mode of competition. Intuitive concepts of size-asymmetric competition among individuals in spatial and nonspatial contexts are very different.     

Predicting competition coefficients for plant mixtures: reciprocity, transitivity and correlations with life-history traits

There are few empirical or theoretical predictions of how per capita or per individual competition coefficients for pairs of plant species should relate to each other. In contrast, there are a considerable number of general hypotheses that predict competitive ability as a function of a range of ecological traits, together with a suite of increasingly sophisticated models for competitive interactions between plant species. We re-analyse a data set on competition between all pairwise combinations of seven species and show that competition coefficients relate strongly to differences between the maximum sizes, root allocation, emergence time and seed size of species. Regressions suggest that the best predictor of competition coefficients is the difference in the maximum size of species and that correlations of the other traits with the competition coefficients occur through effects on the maximum size. We also explore the patterns of association between coefficients across the competition matrix. We find significant evidence for coefficient reciprocity (inverse relationships between the interspecific coefficients for species pairs) and transitivity (numerically predictable hierarchies of competition between species) across competition matrices. These results therefore suggest simple null models for plant community structure when there is competition for resources.     

Two-species asymmetric competition: effects of age structure on intra- and interspecific interactions

1. The patterns of density-dependent resource competition and the mechanisms leading to competitive exclusion in an experimental two-species insect age-structured interaction were investigated. 2. The modes of competition (scramble or contest) and strength of competition (under- to overcompensatory) operating within and between the stages of the two species was found to be influenced by total competitor density, the age structure of the competitor community and whether competition is between stages of single or two species. 3. The effect of imposed resource limitation on survival was found to be asymmetric between stages and species. Environments supporting both dominant and subordinate competitors were found to increase survival of subordinate competitors at lower total competitor densities. Competitive environments during development within individual stage cohorts (i.e. small or large larvae), differed from the competitive environment in lumped age classes (i.e. development from egg-->pupae). 4. Competition within mixed-age, stage or species cohorts, when compared with uniform-aged or species cohorts, altered the position of a competitive environment on the scramble-contest spectrum. In some cases the competitive environment switched from undercompensatory contest to overcompensatory scramble competition. 5. Such switching modes of competition suggest that the relative importance of the mechanisms regulating single-species population dynamics (i.e. resource competition) may change when organisms are embedded within a wider community.     

Spatially disjunct effects of co-occurring competition and facilitation

Little is known of the co‐occurrence and implications of competitive and facilitative interactions within sites. Here we show spatially disjunct competition and facilitation at forest edges, with beneficial influences of trees on seedling growth via increased ectomycorrhizal infection apparent from 12 to 20 m while closer to trees seedling growth is negatively correlated with canopy closure. As a result, seedling growth is maximized at intermediate distances. Facilitative interactions were nonlinear: being within 15.7 m of a tree maximized seedling mycorrhizal infection; while competitive effects were correlated with canopy closure, which was related to distance and generally scales with density. These patterns result in a positive correlation of tree density and seedling growth at low densities of trees, and negative correlation at higher densities because of competition. A spatial model suggests that plant communities are a mosaic of positive and negative interactions, which may contribute to population homeostasis and plant diversity.     

Competitive exclusion and persistence in models of resource and sexual competition

 We study a combined mathematical model of resource and sexual competition. The population dynamics in this model is analyzed through a coupled system of reaction-diffusion equations. It is shown that strong sexual competition and low birth rate lead to competitive exclusion of the biological species. If sexual competition is weak, then the persistence of the species is possible, depending on the initial density functions and the growth rates of the species. When sexual competition affects both species, persistence and competitive exclusion results are also obtained in terms of the ecological data in the model. Received 1 November 1995; received in revised form 13 January 1996     

Host–parasitoid interaction as affected by interkingdom competition

Although still underrepresented in ecological research, competitive interactions between distantly related organisms (so-called “interkingdom competition”) are expected to be widespread in various ecosystems, with yet unknown consequences for, e.g. trophic interactions. In the model host–parasitoid system Drosophila melanogaster–Asobara tabida, toxic filamentous fungi have been shown to be serious competitors that critically affect the density-dependent survival of host Drosophila larvae. This study investigates the extent to which the competing mould Aspergillus niger affects key properties of the well-studied Drosophila–parasitoid system and how the host–parasitoid interaction influences the microbial competitor. In contrast to slightly positive density-dependent host mortality under mould-free conditions, competing A. niger mediated a strong Allee effect for parasitised larvae, i.e. mortality decreased with increasing larval density. It was found that the common toxic fungal metabolite kojic acid is not responsible for higher death rates in parasitised larvae. Single parasitised Drosophila larvae were less harmful to fungal reproduction than unparasitised larvae, but this effect vanished with an increase in larval density. As predicted from the negative effect of fungi on host survival and thus on parasitoid fitness at low larval densities, A. tabida females spent less time foraging in fungus-infested patches. Interestingly, even though high host larval densities increased host survival, parasitoids still reduced their search efforts in fungus-infested patches, indicating a benefit for host larvae from feeding in the presence of noxious mould. Thus, this experimental study provides evidence of the potentially important role of interkingdom competition in determining trophic interactions in saprophagous animal communities and the dynamics of both host–parasitoid and microbial populations.     

Spatial distribution, resource utilisation and intraspecific competition in the dung beetle Aphodius ater

Competitive interactions in northern temperate dung beetles are poorly understood. This investigation therefore comprises a series of field and experimental work on a dung beetle species common in northern Europe, Aphodius ater, with special focus on intraspecific competitive interactions. The between-pat distribution of adult A. ater in relation to the age of sheep dung pats was studied in the field. The distribution of both sexes was contagious in the fresh pats but became more regular with increasing pat age. The successional occurrence of males and females did not differ, but immature females tended to occur in fresh pats while mature females were mainly found in older pats. With increasing age of pats, the egg load of females also increased. Egg-laying behaviour of the beetles was studied in laboratory experiments. The mean number of eggs laid per female per dung pat decreased with increasing beetle density. Thus, density-dependent processes seem to regulate resource utilisation with regard to breeding behaviour, resulting in equal exploitation of the available pats. Survival and weight of recently hatched beetles decreased with increasing initial density of eggs. Hence, in A. ater, competition between larvae for food within pats does occur. Received: 4 February 1998 / Accepted: 20 April 1998     

Consequences of larval intraspecific competition to stonefly growth and fecundity

Summary We conducted experiments in replicated circular streams to measure the effect of intraspecific larval density on growth rates, size at emergence, timing of emergence, and fecundity of two species of predatory stoneflies (Megarcys signata and Kogotus modestus, Perlodidae). Early instars of both species showed no significant effect of intraspecific larval density on mean growth rates, despite the observation that in the absence of competitors stoneflies ate on average, significantly more prey (Baetis bicaudatus, Ephemeroptera, Baetidae) than in the presence of competitors. However, larval size of stoneflies held at higher densities (two per chamber) diverged over time, resulting in a greater size variability (coefficient of variation) among Kogotus than in treatments with low densities of stoneflies (one per chamber). The effect of doubling the density of early-instar Megarcys larvae was also asymmetrical, resulting in one larger and one smaller individual. In contrast, doubling the density of last-instar stoneflies whose feeding rates declined significantly prior to emergence had few measurable consequences, except that male Megarcys, which continued to feed throughout the last instar, had lower average feeding rates in high-density than in low-density chambers, and emerged at a significantly smaller mean size. We conclude that competition between early-instar stonefly larvae results in an asymmetry of body sizes, but that competitive effects are reduced as larvae slow or cease feeding before emergence. Since larger females of both stonefly species produced more eggs, the probable cost to females of early-instar larval competition was a reduction in their potential contribution of offspring to the next generation. The cost of attaining a smaller body size for male stoneflies is unknown; but if, as in many other insects, larger males have greater reproductive success, larval competition may increase the opportunity for sexual selection among males. This hypothesis remains to be tested experimentally.     

The role of competition in invertebrate community development in a recently formed stream in Glacier Bay National Park,Alaska

New streams formed following ice recession in Glacier Bay National Park, southeastern Alaska allow insights into the role of abiotic and biotic interactions in the assemblage of benthic communities. Reductions in abundance of a pioneer chironomid colonizer, Diamesa alpina/lupus, in one new stream, Wolf Point Creek, is considered to be a result of competitive interactions with subsequent colonizers, as opposed to intolerance of warmer water temperature as previously suggested. Reduced densities of potential competitors (25–50 larvae per 500 cm²) in a cobble transplant experiment between streams, allowed persistence of D. alpina/lupus at low densities. In addition, significantly more D. alpina/lupus larvae emigrated from artificial stream channels containing other chironomid taxa than channels without potential competitors while there was no significant correlation of emigration with water temperature. A small number of D. alpina/lupus transplanted from a cold stream (4–6 °C) survived at water temperatures of 10–15 °C for 1 week. These results infer that interference competition is the causal mechanism in the decline of D. alpina/lupus. Complete exclusion of D. alpina/lupus from the stream has not occurred and water temperature may play a role in partitioning D. alpina/lupus from other taxa on a temporal or a spatial basis. This revised version was published online in August 2006 with corrections to the Cover Date.     

Quantifying the community-level consequences of competition

Changes in plant community structure after changes in some aspect of the environment such as nutrients or grazing is often ascribed to changes in competitive relationships among the plants. However, very rarely is competition measured directly in such experiments. To distinguish between the direct effects of environmental treatments and changes in competitive relationships, it is necessary to quantify the influence of competition on community structure and compare the magnitude and direction of this influence between environments. We describe an experimental approach to accomplish this that is based on the classic yield-density experiment of agronomy. The approach is called the community-density experiment and requires experimental establishment of a gradient in total initial community density such that absolute densities of each species increase but initial relative abundances of each species stay constant along the gradient. We define various indices of the magnitude of community-level consequences of increasing density that can be compared among environments such as different fertilizer or grazing treatments. We also discuss various practical ways of achieving the experimental density gradient that are suitable for different kinds of communities. *** DIRECT SUPPORT *** A02DO006 00011     

Habitat-dependent competition and the coexistence of Australian heathland rodents

When competing species depress one another's fitness in the habitats that they occupy, their competitive effects will emerge in each species' pattern of density-dependent habitat choice. Thus, a regression analysis of joint densities, corrected by the habitat effect, should reveal the magnitude of interspecific competition. We tested this idea by 1) demonstrating the connection between removal experiments and regression estimates of competition with those obtained from isodars (regressions that implicitly incorporate evolutionarily stable strategies of habitat selection) and 2) evaluating whether interspecific competition inferred from isodars corresponded with the inferences emerging from regression and field experiments. Previous removal experiments on two herbivorous rodents occupying coastal wet heathlands in eastern Australia documented that competition between Rattus lutreolus and Pseudomys gracilicaudatus is asymmetrically biased in favor of the much larger Rattus . The asymmetry in competition was also revealed by regression estimates of competition. Isodar analyses illustrate a habitat-dependent mechanism for the asymmetry. Rattus compete effectively with Pseudomys in both 'wetter' and 'drier' patches of heath whereas Pseudomys appear to exert a competitive effect in only the drier sites. The magnitude of competition measured by a removal experiment in an area with more-or-less equal amounts of both habitats will be biased in favor of Rattus . More generally, one can use the isodar estimates to draw isolegs and isoclines of competitive coexistence. Isoclines for the two Australian rodents imply dynamic equilibria of stable competitive coexistence that vary with plant succession in fire-dominated heathland ecosystems.