An Experimental Separation of Interference and Exploitative Competition in Larval Damselfly

ABSTRACTDensity‐dependent processes are important for a fundamental understanding of population regulation, as well as for understanding responses to and recovery from stressors. While exploitative competition is well‐studied, interference competition is rather difficult to investigate, but it has been regularly observed to occur in many aquatic insect populations. We conducted laboratory experiments with the non‐biting midge Chironomus riparius (Diptera: Chironomidae) to investigate the impact of different combinations of food supply and larval densities on development and mortality at a constant temperature of 20°C. The chosen two‐factorial experimental design allowed a separate evaluation of exploitative (food) and interference (mortality) competition across a gradient of larval densities. The use of different vessel sizes between 50 cm2 and 600 cm2 made it possible to quantify the functional response at different food densities. To test mechanistic explanations for the statistically significant empirical relationships found in this study and to predict density‐dependent processes, we used a dynamic process‐oriented modeling approach. We extended a recently developed DEB‐IBM full life cycle model for C. riparius and successfully applied it under variable food conditions at the population level under laboratory conditions. Our study showed that chironomid development and reproduction are primarily dependent on food supply, whereas larval density drives the density‐dependent mortality rate. The interaction of food availability and interference competition determined the effective mortality over time. Killing by conspecifics was the most likely mechanism responsible for the intraspecific mortality of the larval stages. Combining data generated using a tailor‐made experimental design with a mechanistic model provided insights into and quantified regulation mechanisms of chironomid populations, allowing future uses of this information in the context of population‐level risk assessment from exposure to chemicals.

Cannibalism by larval damselflies late in larval development on larvae a few instars smaller has been widely documented. I examine here the survival of eggs oviposited near the end of the flight season of adult Enallagma boreale in the presence and absence of potential cannibals, individuals that hatched from eggs earlier in the season, over an extended part of the life-cycle. The role of competition as a modifier of cannibalism was examined by manipulating egg density, environmental productivity, and habitat complexity. Survival in the absence of potential cannibals ranged from 5% to nearly 50% but was only 0-3% in the presence of cannibals. Survival of small larvae was related to manipulations of habitat complexity but not initial density or resources. There were no significant interactions of the presence of large larvae with other experimental treatments on the survival of small larvae. The mean size of small larvae was greater in the presence of cannibals. This may be because the cannibalism treatment reduced the density of small larvae and reduced competition for resources, or that the cannibals preferentially fed on small larvae and only relatively large individuals remained. Fertilization of the habitat or manipulating the initial density of small larvae did not affect mass of small larvae at the end of the experiment, which would be expected if small larvae were affected by competition for resources. Potential cannibals, however, emerged at higher mass when small larvae were present at low density and when productivity of the habitat was increased. This suggests that the negative effect of competition by small larvae outweighs the positive effect of being potential prey for large larvae.

The potential connection between exploitation and interference competition was recognized long ago but has not been evaluated. We measured the levels of both forms of competition for the protist Didinium preying upon Paramecium. Across populations, exploitation intensity was tightly linked to interference intensity, and the form of this relationship follows from a simple model of interaction speeds. The variation in interference competition was as large across populations of Didinium as has been observed previously across species from a variety of taxa including birds, mammals, insects, crustaceans, flatworms and protists. The link between exploitation and interference competition alters our understanding of how interference competition influences population dynamics. Instead of simply stabilizing systems, variation in interference levels can shift population dynamics through qualitatively different regimes because of its association with exploitation competition. Strong interference competition pushes a system to a regime of deterministic extinction, but intermediate interference generates a system that is stable with a high competitive ability. This may help to explain why the distribution of interference values is unimodal and mostly intermediate in intensity.SynthesisExploitation and interference competition are typically viewed as separate processes. Exploitation is described with a functional response in which the inclusion of interference competition – the effect of predator density on foraging rates – is optional. Although recent work indicates that interference competition is widespread, there is little work linking the two forms of competition. In this article we present evidence that exploitation and interference competition are linked mechanistically through movement patterns that simultaneously generate beneficial interactions of consumers with resources and detrimental interactions with other consumers. This connection alters our view of the role that interference plays in ecological dynamics.

Interference competition for food is difficult to measure because feeding rates altered by behavioral interactions of competitors are confounded by exploitative competition, the depletion of food over time. We quantified the magnitude of interference competition between individuals using a combination of experiments and computer simulations, which allowed us to remove the effect of exploitative competition. We used this approach to test the hypothesis that interference competition reduces feeding rates of young red drum (Sciaenops ocellatus) and that the magnitude of interference competition is related to phenotypic traits, such as the sizes of competitors and behavioral interactions between them. In laboratory experiments, we measured prey consumption and behavioral interactions between 2 fish of different sizes (7- to 25-mm standard length). We used computer simulations to remove the effects of exploitative competition and found that prey consumed by focal individuals decreased in the presence of competitors due to interference competition and that the decrease was stronger when focal individuals were smaller than competitors and when the competing individuals were generally larger. We also found greater impairment of prey consumption by focal individuals when they were more vigilant (more reactive) to the presence of competitors. A better understanding of the magnitude of asymmetry in interference competition has important implications for understanding population dynamics, spatial distribution, and reproductive strategies of many organisms. This is the first study to quantify the asymmetry in interference competition when prey density changes over time. Key words: aggression, body size, exploitative competition, interference competition, red drum, Sciaenops ocellatus, vigilance. [Behav Ecol]

Competition between apex predators can alter the strength of top‐down forcing, yet we know little about the behavioral mechanisms that drive competition in multipredator ecosystems. Interactions between predators can be synergistic (facilitative) or antagonistic (inhibitive), both of which are widespread in nature, vary in strength between species and across space and time, and affect predation patterns and predator–prey dynamics. Recent research has suggested that gray wolf (Canis lupus) kill rates decrease where they are sympatric with brown bears (Ursus arctos), however, the mechanisms behind this pattern remain unknown. We used data from two long‐term research projects in Scandinavia (Europe) and Yellowstone National Park (North America) to test the role of interference and exploitation competition from bears on wolf predatory behavior, where altered wolf handling and search time of prey in the presence of bears are indicative of interference and exploitation competition, respectively. Our results suggest the mechanisms driving competition between bears and wolves were dependent on the season and study system. During spring in Scandinavia, interference competition was the primary mechanism driving decreased kill rates for wolves sympatric with bears; handling time increased, but search time did not. In summer, however, when both bear and wolf predation focused on neonate moose, the behavioral mechanism switched to exploitation competition; search time increased, but handling time did not. Alternartively, interference competition did affect wolf predation dynamics in Yellowstone during summer, where wolves prey more evenly on neonate and adult ungulates. Here, bear presence at a carcass increased the amount of time wolves spent at carcasses of all sizes and wolf handling time for small prey, but decreased handling time for the largest prey. Wolves facilitate scavenging opportunities for bears, however, bears alter wolf predatory behavior via multiple pathways and are primarily antagonistic to wolves. Our study helps to clarify the behavioral mechanisms driving competition between apex predators, illustrating how interspecific interactions can manifest into population‐level predation patterns.

Competition is a major regulatory factor in population and community dynamics. Its effects can be either direct in interference competition or indirect in exploitative competition. The impact of exploitative competition on population dynamics has been extensively studied from empirical and theoretical points of view, but the consequences of interference competition remain poorly understood. Here we study the effect of different levels of intraspecific interference competition on the dynamics of a size-structured population. We study a physiologically structured population model accounting for direct individual interactions, allowing for a gradient from exploitative competition to interference competition. We parameterize our model with data on experimental populations of the collembolan Folsomia candida. Our model predicts contrasting dynamics, depending on the level of interference competition. With low interference, our model predicts juvenile-driven generation cycles, but interference competition tends to dampen these cycles. With intermediate interference, giant individuals emerge and start dominating the population. Finally, strong interference competition causes a novel kind of adult-driven generation cycles referred to as interference-induced cycles. Our results shed new light on the interpretation of the size-structured dynamics of natural and experimental populations.

Understanding the functional significance of shelter for animal populations requires knowledge of the behavioural mechanisms that govern the dynamics of shelter use. Exploitation of shelters may be impeded by mutual interference, yet interference competition can be difficult to distinguish from exploitation competition. We used bullheads (Cottus gobio) as a model system of mobile fish to investigate the effect of intraspecific competition on shelter use. A series of field experiments was conducted under controlled conditions of shelter availability and population density. For each experiment, the location of each individual fish was observed over a period of 10 days. We then constructed a continuous-time Markov-chain model for the movement of fish between shelters and the open stream, which explicitly parameterised exploitation competition and interference competition for shelter and which accounted for two different size-classes of fish. By using a stochastic rather than a deterministic model, we were able to account for the distribution of fish across shelters, and not just the average occupation. Analysis of the model showed strong evidence of exploitation competition, which was highly dependent on body size, and an increased departure rate from shared shelters. Over and above exploitation, interference competition limited the ability of unsheltered fish to colonise vacant shelters at high population densities. Different formulations of the interference competition were compared using the Akaike Information Criterion. The formulation that best fitted the observations modelled interference competition as an increasing function of average shelter occupancy rather than population density per se.

Africa's apex predator, the lion, is limited by interference and exploitative competition with humans

The combined effects of food supply and larval density on survival, growth and metamorphosis of Chinese tiger frog (Hoplobatrachus rugulosa) tadpoles

It is well established that large Daphnia can markedly reduce rotifer populations in both natural communities and laboratory cultures, but the mechanism for this reduction has been debated. Both exploitative competition for resources and interference have been invoked, but experiments have not directly assessed the independent roles of these mechanisms. Using laboratory chambers divided with 53-µm Nitex screen, I have separated the effects of interference from exploitative competition by Daphnia pulex on Keratella cochlearis and Keratella crassa. At both high and low food levels, K. cochlearis is strongly affected by interference competition but not exploitative competition, and K. crassa shows no evidence of suppression by Daphnia through interference competition. These results suggest that laboratory studies examining the effects of Daphnia on rotifers are correct in attributing much of the reduction in rotifer numbers to direct interference competition.

IntroductionGlobal change is a multi-faceted issue putting many species at risk. The broad range of potentially interacting environmental stressors is problematic for effective and efficient conservation and management. In freshwater systems, habitat degradation and introduced species have been repeatedly recognized for their extensive impacts on native ecosystems. However, the simultaneous impacts of these environmental stressors on naturally depauperate and inherently vulnerable communities are poorly understood.MethodsIn southern New Zealand, the fish communities in 14 tributaries of three lowland lakes were surveyed to quantify the within- and between-community changes along gradients of habitat complexity and abundance of introduced species, specifically brown trout (Salmo trutta Linnaeus) and redfin perch (Perca fluviatilis Linnaeus).ResultsStable isotope analyses identified that trophic diversity increased with habitat complexity and an abundance of native eels (Anguilla spp.) but was unaffected by introduced species. Within each community, only perch exhibited distinct dietary shifts along all environmental gradients, whereas trout and the native fish had consistent, generalist diets. When supported with length–weight regressions, these impacts became increasingly size-dependent. For example, among the native fish, only the larger eels were unaffected by habitat and achieved greater body conditions with increased numbers of eels and perch; however, more trout were detrimental to eel body condition. In contrast, the smaller bodied natives, including elvers, all had improved body conditions from increased habitat complexity and reduced numbers of trout and perch. For the introduced species, perch weights were consistent regardless of the local environment due to their variable diet, but larger trout generally increased in weight with reduced habitat complexity and greater numbers of introduced fish, although high eel densities were detrimental.DiscussionOverall, our results highlight how the responses to environmental stressors, even in depauperate communities, are complicated and generally species-specific. Nonetheless, habitat degradation had the most wide-ranging negative impacts on native fish, with perch numbers only affecting the smaller bodied natives and trout only affecting one native species. We conclude that focusing on habitat restoration in conservation strategies will provide the most efficient and effective use of resources, although the realized benefits for native species will be limited if introduced species are overly abundant.

We examined the effect of flow regime on the occurrence of interference and exploitative competition in a salmonid species, white-spotted char ( Salvelinus leucomaenis ). In the lotic treatment, char showed typical consequences of interference competition (i.e., fish aggressively defended their foraging positions, and individuals occupying the most profitable positions grew fastest). In the lentic treatment, however, char behavior was consistent with exploitative competition; fish cruised over a larger area in search of food resources and showed limited aggression and no evidence of a dominance hierarchy. Our results indicate that white-spotted char showed interference competition more commonly than exploitative competition in the lotic habitat and vice versa in the lentic habitat.

The effect of competition among similarly aged larvae of the alfalfa blotch leaf miner, Agromyza frontella (Rondani), was investigated in laboratory studies over a range of larval densities (1–10 larvae per leaflet) commonly found on alfalfa in Quebec. In these studies A. frontella larvae were found to be resource limited when developing in small leaflets and (or) with other larvae. Larval mortality owing to interference (cannibalism) during the first two larval instars, and exploitation (starvation) competition during the third and final instar, increased in a density-dependent manner. Prepupal and pupal mortality increased and pupal weight decreased as larval density increased. However, pupal developmental rates and adult sex ratios of survivors were not significantly affected by larval density. Interference competition during the first two larval instars reduced larval density and thus diminished the probability that third-instar larvae would be subjected to exploitation competition. These findings were confirmed under field conditions.

The role of interference competition in a sustained population outbreak of the aspen leaf miner in Alaska

Biological invasions are a contemporary global threat because invasive species can have substantial negative economic and ecological impacts. Invasive species can outcompete native species through two main mechanisms: interference competition (direct, negative interactions like aggression) and/or exploitative competition (indirect, negative interactions resulting from species using the same, limited resources like food). The invasive Italian wall lizard (Podarcis siculus) was introduced into Lisbon, Portugal, 20 years ago, and is believed to be locally displacing the native green Iberian wall lizard (Podarcis virescens). We experimentally tested for competition between these two lizard species by establishing heterospecific (one pair of each species) and conspecific (two pairs of the same species; control) treatments in enclosures containing a high- and a low-quality refuge. Lizards were fed from food dishes every other day. We tested if species showed interference (aggressive behaviour, stealing food and shelter exclusion) or exploitative competition (tolerance between species but differences in food consumption efficiency). We found evidence for exploitative competition: the invasive species arrived first at food stations, consumed more food and gained more weight than the native species. We suggest that exploitative competition may, in part, explain the observed displacement of P. virescens from contact areas with the invasive P. siculus. Deciphering the competitive mechanisms between invasive and native species is vital for understanding the invasion process. To become successful invaders, alien species must often outcompete native species they encounter in a new location. But how can an alien species outcompete a resident with a long evolutionary history in an environment in which the resident is expected to be better adapted? We studied an invasive and native congeneric pair of sympatric lizards to understand how they interact and potentially compete in a controlled environment. The invasive Italian wall lizard and a native congener were very tolerant of each other; however, the invasive species was first to arrive at food, ate more and grew faster, suggesting exploitative competition. This contrasts with previous studies in other introduced locations where the Italian wall lizard was more aggressive towards native lizards, suggestive of interference competition. Our results help explain why the Italian wall lizard is so successful and suggests it may compete in different ways, possibly in response to local environmental conditions and which species it may be competing with, but with the same outcome: the displacement of native species.