Intense Intraspecific Competition Research Articles

Competition Relationship of Populations of Natural Secondary Acer mono Forest

The intraspecific and interspecific competition intensities in the secondary Acer mono forest were quantitatively analyzed by Hegyi single-tree competition index model.The results showed that the intraspecific competition intensity in A.mono was bigger,accounting for 26 percents of total competition,and the competition intensity gradually increased with the increasing tree diameter.However,the competition intensity decreased after diameter at breast height DBH was greater than 30 cm.The interspecific competition(89.061) in A.mono was more intensive than the intraspecific competition(31.487).The competition intensity of interspecific competition with A.mono followed the order of A.monoTilia mandshuricaQuecus mongolicaPhellodendron amurenseJuglans mandshuricaTilia amurensis.The competition intensity of A.mono declined with the increasing DBH,and the stress was more intense for trees less than 15cm in DBH.The relationship between competition intensity and DBH of target trees followed the exponential function.The tending management should be actualized to increase the timber utilization quotiety when DBH of A.mono individual near 15cm in natural conditions.

Widespread disruptive selection in the wild is associated with intense resource competition

BackgroundDisruptive selection has been documented in a growing number of natural populations. Yet, its prevalence within individual systems remains unclear. Furthermore, few studies have sought to identify the ecological factors that promote disruptive selection in the wild. To address these issues, we surveyed 15 populations of Mexican spadefoot toad tadpoles, Spea multiplicata, and measured the prevalence of disruptive selection acting on resource-use phenotypes. We also evaluated the relationship between the strength of disruptive selection and the intensity of intraspecific competition—an ecological agent hypothesized to be an important driver of disruptive selection.ResultsDisruptive selection was the predominant mode of quadratic selection across all populations. However, a directional component of selection favoring an extreme ecomorph—a distinctive carnivore morph—was also common. Disruptive selection was strongest in populations experiencing the most intense intraspecific competition, whereas stabilizing selection was only found in populations experiencing relatively weak intraspecific competition.ConclusionsDisruptive selection can be common in natural populations. Intraspecific competition for resources may be a key driver of such selection.

Individual variation in response to intraspecific competition: problems with inference from growth variation measures

Summary1. The intensity of intraspecific competition may influence the growth of differently sized individuals to different extents. Measures of among‐individual variation in growth and body size are commonly applied to assess such effects. The measure chosen is then supposed to control for the effect of mean (also influenced by competition) on variance [e.g. coefficient of variation (CV)].2. Although there is an appealing simplicity to this approach, interpretations of such data have some apparently unrecognised underlying complexities. Here we combine empirical data from salmonid fishes and simulations to illustrate this.3. Simulations show that even if all individuals are equally affected by competition, CV of body mass will not be constant. In fact, the CV of body mass may either decrease or increase with increasing competition.4. Using empirical data from growth experiments, where differences among replicates in mean growth was controlled by temperature rather than competitive intensity, we show that two salmonid fish species differ with respect to how SD growth scales with mean growth. Because this slope represents a possible null‐hypothesis, and will rarely be known, observed relations between competitive intensity and SD or CV growth are unlikely to yield information regarding size‐specific effects of competition.5. Based on the empirical data, simulation results with respect to body mass variation showed that the relation between mean and CV final mass varied from positive to negative across species and scenarios.6. Insights regarding size‐specific competitive effects appear unlikely to be obtained from measures of variation in either growth or body size. If attempts to do so are made, a positive correlation between SD body size and measures of competitive intensity (e.g. population density) does suggest that smaller individuals are more influenced by increased competition. However, this will be an extremely conservative test, because the appropriate null‐hypothesis is a negative correlation of unknown strength.7. Future attempts to quantify relationships between body size and impacts of competitive intensity should employ more traditional analyses of variance on individual data (i.e. interactions between body size and competition on growth rates).

Competition among loblolly pine trees: Does genetic variability of the trees in a stand matter?

The intensity of intraspecific competition among trees in a stand was compared for seed origin versus clonal origin stands of loblolly pine using tree size-based competition indices. A method that uses maximum likelihood with simulated annealing (MLSA) and one that uses discrete radii with simple correlation coefficients (DRSC) were applied and their results compared to stand level observable effects of competition. DRSC was found to give estimates of competitor search radii that were reasonable and in line with observable stand level effects of intraspecific competition. MLSA appeared to overestimate the optimum competitor search radius, had the potential to give estimates of the competitor search radius that were not unique, and was generally not conclusive on which genetic variety had the most intense level of competition among trees in its stand. The DRSC approach provides a simple alternative to MLSA where the assumptions behind MLSA do not hold or where it may not be easy for them to be checked. A comparison of competitor search radii in the different genetic variety stands and the observable stand level effects of competition did not find evidence to support the hypothesis that greater genetic uniformity among trees in loblolly pine stands would result in more intense intraspecific competition among the trees in the stand.

Stand Development and Population Dynamics of Curlleaf Mountain Mahogany (Cercocarpus ledifolius Nutt.) Woodlands in Utah's Bear River Mountains

Abstract Curlleaf mountain mahogany (Cercocarpus ledifolius Nutt.) is a little-studied woodland tree that occurs in pure stands throughout the Intermountain West. Stand development and population dynamics of this species are poorly understood, despite their relevance to management. We describe here the development of stand age structures and population dynamics of mahogany woodlands in northern Utah using tree ages and measurements representing five structurally diverse stands. Establishment periods in all stands lasted decades, and regeneration continued intermittently in the absence of stand-replacing disturbance, eventually creating multiaged structure. Height, crown size, and basal area varied among older mahogany, which may reflect more intense intraspecific competition or increased likelihood of crown damage in older stands. Mahogany woodland managers face significant challenges. It is difficult to characterize historic stand conditions using current age structures because aging mature trees is generally unfeasible. Furthermore, there is little precedent for regenerating mahogany using silvicultural methods. Results suggest that the biggest risk in regenerating mahogany woodlands is low seedling survival, leading to the prolonged absence of mature trees. Consequently, multiaged methods that retain mature trees are recommended.

Evaluating the effects of trophic complexity on a keystone predator by disassembling a partial intraguild predation food web

1. Many taxa can be found in food webs that differ in trophic complexity, but it is unclear how trophic complexity affects the performance of particular taxa. In pond food webs, larvae of the salamander Ambystoma opacum occupy the intermediate predator trophic position in a partial intraguild predation (IGP) food web and can function as keystone predators. Larval A. opacum are also found in simpler food webs lacking either top predators or shared prey. 2. We conducted an experiment where a partial IGP food web was simplified, and we measured the growth and survival of larval A. opacum in each set of food webs. Partial IGP food webs that had either a low abundance or high abundance of total prey were also simplified by independently removing top predators and/or shared prey. 3. Removing top predators always increased A. opacum survival, but removal of shared prey had no effect on A. opacum survival, regardless of total prey abundance. 4. Surprisingly, food web simplification had no effect on the growth of A. opacum when present in food webs with a low abundance of prey but had important effects on A. opacum growth in food webs with a high abundance of prey. Simplifying a partial IGP food web with a high abundance of prey reduced A. opacum growth when either top predators or shared prey were removed from the food web and the loss of top predators and shared prey influenced A. opacum growth in a non-additive fashion. 5. The non-additive response in A. opacum growth appears to be the result of supplemental prey availability augmenting the beneficial effects of top predators. Top predators had a beneficial effect on A. opacum populations by reducing the abundance of A. opacum present and thereby reducing the intensity of intraspecific competition. 6. Our study indicates that the effects of food web simplification on the performance of A. opacum are complex and depend on both how a partial IGP food web is simplified and how abundant prey are in the food web. These findings are important because they demonstrate how trophic complexity can create variation in the performance of intermediate predators that play important roles in temporary pond food webs.

Density-dependent growth and reproduction of Microstegium vimineum in contrasting light environments1

The invasive grass Microstegium vimineum (Trin.) A. Camus grows in dense monocultures beneath the canopy of woodlands in eastern North America and along sunny forest edges. This annual matures seeds in chasmogamous (CH) spikelets on terminal racemes and cleistogamous (CL) spikelets on axillary racemes. The objectives were to determine whether density-dependent reductions in growth and reproduction occur, what patterns they show across densities in a sunny vs. shady environment, and whether the intensity of intraspecific competition depends on light environment. Seedlings of M. vimineum were grown at densities corresponding to 105, 526, 947, and 1368 individuals m−2 in 11 cm diameter pots in a greenhouse exposed to full sun or under deep shade (5.8% of full sun). Mature, partially senesced target plants were harvested prior to seed drop. Shoot dry mass, CH and CL spikelet, and seed production, were significantly reduced under shade and at all densities relative to the control (no competition). However, in shade there were no density-dependent effects on reproduction via CL or CH. There was also no density-dependent mortality. Relative competition intensity, based on the density-dependent reduction in shoot mass of target plants increased with increasing mass of competitors and was greatest in the sunny environment. Reproductive mass (i.e., dry mass of CH or CL spikelets and seeds per target plant) was positively correlated with shoot mass across density treatments, suggesting that density-dependent effects on reproduction in competing monocultures were indirectly due to decreased shoot mass at higher densities. Density-yield curves were highest in the sunny environment where they quickly reached saturation, revealing that a solitary individual can produce the same biomass as a group of competing individuals at higher densities. The success of this invasive species in woodlands may be due to high tolerance to a range of intraspecific densities and to an ability to set seed under shady conditions even when densities are high.

When climate change affects where birds sing

Recent changes in temperature and precipitation have implications for transmission and excess attenuation of sounds, with important consequences for the choice of vocal display sites by animals. Birds typically sing from within or at the top of the vegetation, and the relative height of such song posts varies consistently among species. I estimated relative height of positions in the vegetation used by singing birds in 1986–1989 and again in 2010 after spring and summer temperatures had increased by 20% and precipitation by 30%, predicting that these changes would increase the height of song post positions. Average song post height increased by 18% or 1.2 m during the study. Because the increase in song post height should depend on relative costs and benefits of such change, I predicted that sexually dichromatic species and species with increasing populations and hence intense intraspecific competition for mates should cause increases in song post height, whereas high predation risk by the sparrowhawk Accipiter nisus should prevent increases in song post height because sparrowhawks preferentially prey on birds high in the vegetation. That was indeed the case. These results suggest that display sites for singing birds can change rapidly, with potential consequences for optimal design of songs, variance in mating success, and predator–prey interactions.

Gregarious desert locusts have substantially larger brains with altered proportions compared with the solitarious phase

The behavioural demands of group living and foraging have been implicated in both evolutionary and plastic changes in brain size. Desert locusts show extreme phenotypic plasticity, allowing brain morphology to be related to very different lifestyles in one species. At low population densities, locusts occur in a solitarious phase that avoids other locusts and is cryptic in appearance and behaviour. Crowding triggers the transformation into the highly active gregarious phase, which aggregates into dense migratory swarms. We found that the brains of gregarious locusts have very different proportions and are also 30 per cent larger overall than in solitarious locusts. To address whether brain proportions change with size through nonlinear scaling (allometry), we conducted the first comprehensive major axis regression analysis of scaling relations in an insect brain. This revealed that phase differences in brain proportions arise from a combination of allometric effects and deviations from the allometric expectation (grade shifts). In consequence, gregarious locusts had a larger midbrainoptic lobe ratio, a larger central complex and a 50 per cent larger ratio of the olfactory primary calyx to the first olfactory neuropile. Solitarious locusts invest more in low-level sensory processing, having disproportionally larger primary visual and olfactory neuropiles, possibly to gain sensitivity. The larger brains of gregarious locusts prioritize higher integration, which may support the behavioural demands of generalist foraging and living in dense and highly mobile swarms dominated by intense intraspecific competition.

Effects of temporal heterogeneity of water supply on the growth of Perilla frutescens depend on plant density

Plants respond to the spatial and temporal heterogeneity of a resource supply. However, their responses will depend on intraspecific competition for resource acquisition. Although plants are subject to various intensities of intraspecific competition, most studies of resource heterogeneity have been carried out under a single density so that the effects of intraspecific competition on plant responses to resource heterogeneity are largely unknown. A growth experiment was performed to investigate plant responses to the temporal heterogeneity of water supply and nutrient levels under multiple plant densities. The annual plant Perilla frutescens was grown using different combinations of frequency of water supply, nutrient level and density, while providing the same total amount of water under all conditions. The effects of the treatments on biomass, allocation to roots and intensity of competition were analysed after 48 d. Biomass and allocation to roots were larger under homogeneous than under heterogeneous water supply, and the effects of water heterogeneity were greater at high density than at low density. The effects of water heterogeneity were greater at high nutrient level than at low level for biomass, while the effects were greater at low nutrient level than high level for allocation to roots. Competition was severer under homogeneous than under heterogeneous water supply. Competition for water probably makes plants more sensitive to the water heterogeneity. In addition, the intensity of intraspecific competition can be affected by the temporal patterns of water supply. Because both resource heterogeneity and intraspecific competition affect resource acquisition and growth of plants, their interactive effects should be evaluated more carefully under future studies.

How Will Species Respond to Climate Change? Examining the Effects of Temperature and Population Density on an Herbivorous Insect

An important challenge facing ecologists is to understand how climate change may affect species performance and species interactions. However, predicting how changes in abiotic variables associated with climate change may affect species performance also depends on the biotic context, which can mediate species responses to climatic change. We conducted a 3-yr field experiment to determine how the herbivorous grasshopper Camnula pellucida (Scudder) responds to manipulations of temperature and population density. Grasshopper survival and fecundity decreased with density, indicating the importance of intraspecific competition. Female fecundity tended to increase with temperature, whereas grasshopper survival exhibited a unimodal response to temperature, with highest survival at intermediate temperatures. Grasshopper performance responses to temperature also depended on density. Peak survival in the low-density treatment occurred in warmer conditions than for the high-density treatment, indicating that the intensity of intraspecific competition varies with temperature. Our data show that changes to the temperature regimen can alter grasshopper performance and determine the intensity of intraspecific competition. However, the effects of temperature on grasshopper performance varied with density. Our data indicate the importance of the biotic context in mediating species responses to climatic factors associated with global change.

Body Size Evolution in Insular Speckled Rattlesnakes (Viperidae: Crotalus mitchellii)

BackgroundSpeckled rattlesnakes (Crotalus mitchellii) inhabit multiple islands off the coast of Baja California, Mexico. Two of the 14 known insular populations have been recognized as subspecies based primarily on body size divergence from putative mainland ancestral populations; however, a survey of body size variation from other islands occupied by these snakes has not been previously reported. We examined body size variation between island and mainland speckled rattlesnakes, and the relationship between body size and various island physical variables among 12 island populations. We also examined relative head size among giant, dwarfed, and mainland speckled rattlesnakes to determine whether allometric differences conformed to predictions of gape size (and indirectly body size) evolving in response to shifts in prey size.Methodology/Principal FindingsInsular speckled rattlesnakes show considerable variation in body size when compared to mainland source subspecies. In addition to previously known instances of gigantism on Ángel de la Guarda and dwarfism on El Muerto, various degrees of body size decrease have occurred frequently in this taxon, with dwarfed rattlesnakes occurring mostly on small, recently isolated, land-bridge islands. Regression models using the Akaike information criterion (AIC) showed that mean SVL of insular populations was most strongly correlated with island area, suggesting the influence of selection for different body size optima for islands of different size. Allometric differences in head size of giant and dwarf rattlesnakes revealed patterns consistent with shifts to larger and smaller prey, respectively.Conclusions/SignificanceOur data provide the first example of a clear relationship between body size and island area in a squamate reptile species; among vertebrates this pattern has been previously documented in few insular mammals. This finding suggests that selection for body size is influenced by changes in community dynamics that are related to graded differences in area over what are otherwise similar bioclimatic conditions. We hypothesize that in this system shifts to larger prey, episodic saturation and depression of primary prey density, and predator release may have led to insular gigantism, and that shifts to smaller prey and increased reproductive efficiency in the presence of intense intraspecific competition may have led to insular dwarfism.

Consumptive and nonconsumptive effects of cannibalism in fluctuating age‐structured populations

Theory and empirical studies suggest that cannibalism in age-structured populations can regulate recruitment depending on the intensity of intraspecific competition between cannibals and victims and the nature of the cannibalism window, i.e., which size classes interact as cannibals and victims. Here we report on a series of experiments that quantify that window for age-structured populations of salamander larvae and paedomorphic adults. We determined body size limits on cannibalism in microcosms and then the consumptive and nonconsumptive (injuries, foraging and activity, diet, growth) effects on victims in mesocosms with seminatural levels of habitat complexity and alternative prey. We found that cannibalism by the largest size classes (paedomorphs and > or = age 3+ yr larvae) occurs mainly on young-of-the-year (YOY) victims. Surviving YOY and other small larvae had increased injuries, reduced activity levels, and reduced growth rates in the presence of cannibals. Data on YOY survival in an experiment in which we manipulated the density of paedomorphs combined with historical data on the number of cannibals in natural populations indicate that dominant cohorts of paedomorphs can cause observed recruitment failures. Dietary data indicate that ontogenetic shifts in diet should preclude strong intraspecific competition between YOY and cannibals in this species. Thus our results are consistent with previous empirical and theoretical work that suggests that recruitment regulation by cannibalism is most likely when YOY are vulnerable to cannibalism but have low dietary overlap with cannibals. Understanding the role of cannibalism in regulating recruitment in salamander populations is timely, given the widespread occurrences of amphibian decline. Previous studies have focused on extrinsic (including anthropogenic) factors that affect amphibian population dynamics, whereas the data presented here combined with long-term field observations suggest the potential for intrinsically driven population cycles.

Using classification tree analysis to reveal causes of mortality in an insect population

Invasive species pose significant threats to native and managed ecosystems. How- ever, it may not always be possible to perform rigorous, long-term studies on invaders to determine the factors that influence their population dynamics, particu- larly when time and resources are limited. We applied a novel approach to determine factors associated with mortality in larvae of the sawfly Profenusa thomsoni Konow, a leafminer of birch, and a relatively recent invader of urban and rural birch forests in Alaska. Classification tree analysis was applied to reveal relationships between qualitative and quantitative predictor variables and categorical response variables in a large data set of larval mortality observations. 2 We determined the state (living or dead) of sawfly larvae in samples of individual leaves. Each leaf was scored for variables reflecting the intensity of intra-specific competition and leaf quality for leafminers, year of collection and degree-days accumulated were recorded for each sample. We explored the association of these variables with larval state using classification tree analysis. 3 Leafminer mortality was best explained by a combination of competition and resource exhaustion and our analysis revealed a possible advantage to group feeding in young larvae that may explain previously observed patterns of resource over- exploitation in this species. Dead larvae were disproportionately found in smaller leaves, which highlights the potential effect of competition on mortality and suggests that smaller-leaved species of birch will better able to resist leafminer damage. 4 We show that classification tree analysis may be useful in situations where urgency and/or limited resources prohibit traditional life-table studies.

The mystery of the missing toes: extreme levels of natural mutilation in island lizard populations

Summary 1. Ecological interactions that involve aggressive confrontations between animals are important in shaping the evolution of morphology, behaviour and life history. However, as such confrontations are rarely witnessed, direct quantification of the intensity of these processes in natural populations is notoriously difficult. While the utilization of the frequency of non‐lethal injuries is fraught with difficulties, it may provide information concerning different types of interaction, such as predation, intraspecific aggression and interspecific interference competition. 2. In this paper, we report on an exceptionally large difference in toe loss incidence between two populations of Podarcis sicula lizards living on small, neighbouring islands in the Adriatic Sea. We caught 900 lizards and recorded the number and location of missing toes. Subsequently, we investigated five non‐mutually exclusive hypotheses concerning differences in bite force capacity, bone strength, predation intensity, average age and intraspecific aggression that may provide proximate explanations for the observed differences in injury frequencies. 3. Bite force differences differed considerably between the populations, but bone strength was found to be stronger in the populations with a higher frequency of natural scars. Predation pressure clearly differed between the populations, but we found higher injury rates under predation relaxation. 4. Our results indicate that density and consequently an increased intraspecific competition is the most likely explanation for the observed high frequencies of injuries. We suggest that the intensity of toe amputation between lizard populations may be a useable indirect indication for the intensity of intraspecific competition. 5. This study shows how a combination of morphological, physiological, behavioural and ecological measurements can be used to test assumptions implicit to alternative explanations of an observed phenomenon. Such tests can reveal how likely each of these explanations is, even if the processes leading to the phenomenon are difficult to observe directly.

Bulk leaf δ18O and δ13C reflect the intensity of intraspecific competition for water in a semi‐arid tussock grassland

We investigated the extent to which plant water and nutrient status are affected by intraspecific competition intensity and microsite quality in a monodominant tussock grassland. Leaf gas exchange and stable isotope measurements were used to assess the water relations of Stipa tenacissima tussocks growing along a gradient of plant cover and soil depth in a semi-arid catchment of Southeast Spain. Stomatal conductance and photosynthetic rate decreased with increasing intensity of competition during the wet growing season, leading to foliar delta(18)O and delta(13)C enrichment. A high potential for runoff interception by upslope neighbours exerted strong detrimental effects on the water and phosphorus status of downslope S. tenacissima tussocks. Foliar delta(15)N values became more enriched with increasing soil depth. Multiple stepwise regression showed that competition potential and/or rhizosphere soil depth accounted for large proportions of variance in foliar delta(13)C, delta(18)O and delta(15)N among target tussocks (57, 37 and 64%, respectively). The results presented here highlight the key role that spatial redistribution of resources (water and nutrients) by runoff plays in semi-arid ecosystems. It is concluded that combined measurement of delta(13)C, delta(18)O and nutrient concentrations in bulk leaf tissue can provide insight into the intensity of competitive interactions occurring in natural plant communities.

Reproductive biology of a pride of lions on Karongwe Game Reserve, South Africa

The reproductive biology of a pride of lions (Panthera leo) was studied on the 8500 ha Karongwe Game Reserve from 1999 to 2005. Over this period, the pride consisted of between four and 11 lions with a paired coalition of adult males during the first three years and a single adult male for the next three years. We recorded shorter than normal interbirth intervals, high birth rates of 1.3 cubs/lioness/year, very high cub survival rates, and subadults leaving the pride at young ages. This translated into substantially faster growth rates than are typical in large lion populations in ecologically similar circumstances such as Kruger National Park, but are similar to those of lions in Serengeti National Park. These demographic characteristics were probably induced initially by a lack of intense intraspecific competition and high prey availability, but population stability was maintained through the removal of young subadults by management. Interestingly, variability in conception rates between lionesses resulted in lower growth rates than have been found in other similar reintroduced lion populations. These demographic traits characterize many of the small reintroduced lion populations, and call for appropriate management to avoid the consequences of high predator densities.

Reproductive biology of a pride of lions on Karongwe Game Reserve, South Africa

The reproductive biology of a pride of lions (Panthera leo) was studied on the 8500 ha Karongwe Game Reserve from 1999 to 2005. Over this period, the pride consisted of between four and 11 lions with a paired coalition of adult males during the first three years and a single adult male for the next three years. We recorded shorter than normal interbirth intervals, high birth rates of 1.3 cubs/lioness/year, very high cub survival rates, and subadults leaving the pride at young ages. This translated into substantially faster growth rates than are typical in large lion populations in ecologically similar circumstances such as Kruger National Park, but are similar to those of lions in Serengeti National Park. These demographic characteristics were probably induced initially by a lack of intense intraspecific competition and high prey availability, but population stability was maintained through the removal of young subadults by management. Interestingly, variability in conception rates between lionesses resulted in lower growth rates than have been found in other similar reintroduced lion populations. These demographic traits characterize many of the small reintroduced lion populations, and call for appropriate management to avoid the consequences of high predator densities.

CHIHUAHUAN DESERT KANGAROO RATS: NONLINEAR EFFECTS OF POPULATION DYNAMICS, COMPETITION, AND RAINFALL

Using long-term data on two kangaroo rats in the Chihuahuan Desert of North America, we fitted logistic models including the exogenous effects of seasonal rainfall patterns. Our aim was to test the effects of intraspecific interactions and seasonal rainfall in explaining and predicting the numerical fluctuations of these two kangaroo rats. We found that logistic models fit both data sets quite well; Dipodomys merriami showed lower maximum per capita growth rates than Dipodomys ordii, and in both cases logistic models were nonlinear. Summer rainfall appears to be the most important exogenous effect for both rodent populations; models including this variable were able to predict independent data better than models including winter rainfall. D. merriami was also negatively affected by another kangaroo rat (Dipodomys spectabilis), consistent with previous experimental evidence. We hypothesized that summer rainfall influences the carrying capacity of the environment by affecting seed availability and the intensity of intraspecific competition.

Intra- and interspecific competition among helminth parasites: Effects on Coitocaecum parvum life history strategy, size and fecundity

Larval helminths often share intermediate hosts with other individuals of the same or different species. Competition for resources and/or conflicts over transmission routes are likely to influence both the association patterns between species and the life history strategies of each individual. Parasites sharing common intermediate hosts may have evolved ways to avoid or associate with other species depending on their definitive host. If not, individual parasites could develop alternative life history strategies in response to association with particular species. Three sympatric species of helminths exploit the amphipod Paracalliope fluviatilis as an intermediate host in New Zealand: the acanthocephalan Acanthocephalus galaxii, the trematode Microphallus sp. and the progenetic trematode Coitocaecum parvum. Adult A. galaxii and C. parvum are both fish parasites whereas Microphallus sp. infects birds. We found no association, either positive or negative, among the three parasite species. The effects of intra- and interspecific interactions were also measured in the trematode C. parvum. Both intra- and interspecific competition seemed to affect both the life history strategy and the size and fecundity of C. parvum. Firstly, the proportion of progenesis was higher in metacercariae sharing their host with Microphallus sp., the bird parasite, than in any other situation. Second, the intensity of intraspecific competition apparently constrained the ability of metacercariae to adopt progenesis and limited both the growth and egg production of progenetic individuals. These results show that the life history strategy adopted by a parasite may be influenced by other parasites sharing the same host.