Interference competition between alien invasive gammaridean species

The relative abundances of gammaridean species in the river Rhine have profoundly changed since the invasion of Dikerogammarus villosus in 1994/1995. This study tested whether these changes in gammaridean dominance could have been determined by interspecific competition and unequal mortality, for example by intraguild predation (IGP). Single and two species tests have been carried out in aquariums provided with all substrata present in the main channel of the Rhine. Changes in substratum choice, increased swimming activity and increased mortality of a species were used as indicators of interspecific competition during interaction between gammaridean species. Interspecific competition and mortality between the most abundant invasive gammaridean species in the Rhine, viz. Gammarus tigrinus, Echinogammarus ischnus and Dikerogammarus villosus were tested. In single-species experiments, G. tigrinus and D. villosus showed similar preferences for a stony substratum, whereas E. ischnus mostly occupied the water column. The two-species aquarium experiments indicated direct interference competition for substratum and unequal mortality between G. tigrinus and D. villosus, with D. villosus being the stronger competitor. Competitive stress was influenced by population density, was size-dependent and varied between the different types of substratum due to substratum choice. G. tigrinus did not show any behaviour indicative of interference competition in the presence of E. ischnus, and neither did E. ischnus or D. villosus in the presence of any of the other gammarideans. Swimming in the water layer may already enable E. ischnus to minimise its encounters with the stone-dwelling D. villosus and G. tigrinus. To maximise the encounters between E. ischnus and D. villosus, a fish (Lepomis gibbosus) was added to occupy the water layer during the aquarium experiments. E. ischnus showed a higher mortality in the presence of both D. villosus and fish, probably due to increased stress, as shelter opportunities to escape the predators had been minimised. The study shows that interference competition between gammaridean species can explain the replacement of the North American invader G. tigrinus by D. villosus in the river Rhine. E. ischnus and D. villosus both Ponto-Caspian invaders did not show interference competition in our experiments and co-exist in the Rhine.     

Impacts of seedling herbivory on plant competition and implications for species coexistence

Background and Aims

Although the causes and consequences of seedling herbivory for plant community composition are well understood, the mechanisms by which herbivores influence plant species recruitment to the established phase remain less clear. The aim was to examine how variation in the intensity of seedling herbivory interacts with growth-defence trade-offs and herbivore feeding preferences to affect plant community development.

Methods

Using 14-d-old seedlings of Trifolium pratense and T. repens, relative growth and susceptibility to herbivory by the snail Helix aspersa was quantified to elucidate putative growth-defence trade-offs for these species. Then mixed assemblages of 14-d-old Trifolium seedlings were exposed to herbivory by zero, two, five or ten snails and determined how variation in the intensity of herbivory affected competitive interactions into the mature phase (as measured by total plant biomass at 120 d old).

Key Results

In the absence of herbivory, communities were dominated by T. pratense; a result expected on the basis that it yielded larger and presumably more competitive seedlings. However, when seedlings were exposed to herbivory, the balance of competition shifted. At low levels of herbivory (two snails), both Trifolium species contributed equally to total plant biomass. More intense herbivory (five snails) resulted in almost total mortality of T. pratense and dominance of the mature community by T. repens. The most intense herbivory (ten snails) effectively removed all seedlings from the experimental community.

Conclusions

The study illustrates a mechanism whereby spatio-temporal fluctuations in seedling herbivory, when coupled with species-specific variation in competitive ability and sensitivity to herbivore attack, can differentially influence plant recruitment into the mature phase. This mechanism may be a key element in our attempts to understand plant species coexistence, since fluctuations in plant recruitment are fundamental to the many theories that view coexistence as a consequence of a spatio-temporal lottery for dominance over regeneration micro-sites.Key words: Growth-defence trade-off, lottery models, plant–animal interactions, plant size variability, seedling acceptability, seedling defence, spatio-temporal niches, Trifolium pratense, Trifolium repens     

Interference and exploitation competition of three nectar-thieving invasive ant species

Summary. Plant and insect exudates are known to play a key role in structuring tropical ant communities, but less is known about the utilization of these resources in communities dominated by invasive ants. Invasive ants are thought to require large amounts of carbohydrates such as honeydew or nectar to maintain their high abundances. Invasive ants that consume floral nectar may compete with legitimate floral visitors through interference or exploitation competition. I compared the nectar-thieving behavior of three widespread invasive ant species: long-legged ants (Anoplolepis gracilipes), Argentine ants (Linepithema humile), and big-headed ants (Pheidole megacephala) in inflorescences of the native Hawaiian ‘ōhi’a tree, an important food source for native fauna. A. gracilipes was least likely to leave inflorescences unvisited and visited inflorescences in higher numbers than both L. humile and P. megacephala. A. gracilipes and L. humile visited more flowers in an inflorescence and were less likely to retreat from a flower with a competitor than P. megacephala. A. gracilipes was able to take 5.5 and 11.3 times the amount of nectar than L. humile and P. megacephala, respectively. Thus, A. gracilipes may be effective at both interference and exploitation competition against other nectarivores, L. humile may be effective at interference competition, and P. megacephala may be relatively weak at both types of competition against other nectarivores. Ascertaining the competitive abilities of invasive ants against legitimate floral visitors will be especially important in agricultural and other systems that are nectar or pollinator limited.Received 6 December 2004; revised 13 January 2005; accepted 14 January 2005.     

Reduction of species coexistence through mixing in a spatial competition model

Many ecological systems exhibit self-organized spatial patterns due to local interactions. Such patterns can promote species diversity and therefore serve as an important mechanism for biodiversity maintenance. Previous work has shown that when species interactions occurred at local spatial scales, species diversity was greatest when robust mosaic spatial patterns formed. Also, intransitive interactions led to the emergence of spiral patterns, frequently resulting in multispecies coexistence. In some instances, intransitive interactions reduced species diversity as the consequence of competitive hierarchies. Here, we extend and broaden this line of investigation and examine the role of global competition along a continuum ranging from spatial mosaics to spiral patterns. While previous models have predicted that species diversity is reduced when interactions occur over larger spatial scales, our model considers the effects of various levels of mixing on species diversity, in the context of various network structures as measured by the covariance of row and column sums of the competition matrix. First, we compare local competition (unmixed system) versus global competition (mixed systems) and show that greater species diversity is maintained under a positive covariance. Second, we show that under various levels of mixing, species diversity declines more rapidly under a negative covariance. Lastly, we demonstrate that time to extinction in our model occurs much more rapidly under a negative covariance.     

Age-specific coexistence in two-species competition

Sufficient conditions are obtained for the existence and linear stability of stationary age distributions in a two-species competition model with age-dependent mortality and fertility functions.     

Tree competition and species coexistence in a warm-temperate old-growth evergreen broad-leaved forest in Japan

The growth dynamics and mode of competition between adult trees 5.0cm in diameter at breast height (DBH) of nine abundant treespeciesoccupying ca. 85% of the total basal area were investigated in a 4ha study plot (200 m × 200 m) of awarm-temperate old-growth evergreen broad-leaved forest in the Tatera ForestReserve of Tsushima Island, southwestern Japan. In the plot, adult trees 5.0 cm DBH co-occurred with 35 woody plant species (except forwoody vine species). The most dominant and largest species,Castanopsis cuspidata var. sieboldiiexhibited a bimodal DBH distribution; it was found in both the upper and lowervertical layers. Other tree species had unimodal DBH distributionscorrespondingmostly to the lower vertical layer. We developed a model for individual growthincorporating both intra- and interspecific competition and degree ofcompetitive asymmetry. One-sided interspecific competition was detected in 17cases out of the 66 possible combinations on the scale of the 4 hastudy plot. The direction of interspecific competition was generally one-sidedfrom layer-I species to layer-II and III ones. The effects of two-sidedcompetition were detected only in layer-II and III species. OnlyDistylium racemosum exhibited one-sided intraspecificcompetition. We also found 11 cases of positive interspecific relationships.Generally, competitive relationships prevailed over positive relationshipsbetween adult trees in this warm-temperate evergreen broad-leaved forest.Competition between adult trees 5.0 cm in DBH did not occurinthe same vertical layer, but occurred only between trees in different verticallayers. This suggests that competition between adult trees 5.0cm in DBH plays a key role in the variation in species coexistencebetween different vertical layers on the 4 ha scale of thewarm-temperate evergreen broad-leaved forests. Moreover, it was found bycomparing with three different forest types that interspecific competition ismore intense in warm-temperate forests than in cool-temperate or sub-borealforests. We conclude that, compared to cool-temperate or sub-boreal forests(which have little interspecific competition), warm-temperate forests supportmore complex interspecific relationships and species-specific habitatpreferences that result in higher species diversity.     

Interactions among mutualism,competition, and predation foster species coexistence in diverse communities

In natural systems, organisms are simultaneously engaged in mutualistic, competitive, and predatory interactions. Theory predicts that species persistence and community stability are feasible when the beneficial effects of mutualisms are balanced by density-dependent negative feedbacks. Enemy-mediated negative feedbacks can foster plant species coexistence in diverse communities, but empirical evidence remains mixed. Disparity between theoretical expectations and empirical results may arise from the effects of mutualistic mycorrhizal fungi. Here, we build a multiprey species/predator model combined with a bidirectional resource exchange system, which simulates mutualistic interactions between plants and fungi. To reach population persistence, (1) the per capita rate of increase of all plant population must exceed the sum of the negative per capita effects of predation, interspecific competition, and costs of mycorrhizal association, and (2) the per capita numerical response of enemies to mycorrhizal plants must exceed the magnitude of the per capita enemy rate of mortality. These conditions reflect the balance between regulation and facilitation in the system. Interactions between plant natural enemies and mycorrhizal fungi lead to shifts in the strength and direction of net mycorrhizal effects on plants over time, with common plant species deriving greater benefits from mycorrhizal associations than rare plant species.     

Interspecific competition and arbuscular mycorrhiza: Importance for the coexistence of two calcareous grassland species

Two naturally coexisting grassland species—Centaurea jacea (often predominating) andFragaria vesca (subordinate) were grown together in a pot experiment of factorial design, where competition and arbuscular mycorrhiza (AM) inoculation were used as treatments. The effects of competition were one-sided, i.e. the mass ofF. vesca decreased relatively more than that ofC. jacea as a result of competition. The root and total mass ofC. jacea increased with AM inoculation whileF. vesca did not respond. The mass difference betweenC. jacea andF. vesca in a particular pot increased when plants were mycorrhizal, mostly due to the mass increase ofC. jaceae. This can be explained by the differential response of these species to AM. We did not find any indirect indication of the interplant transfer of resources. The results of this experiment show the more unbalanced competition (larger differences in biomass) resulting from AM infection of plants. *** DIRECT SUPPORT *** A02DO006 00010     

Interference competition and parasite virulence

Within-host competition between parasites, a consequence of infection by multiple strains, is predicted to favour rapid host exploitation and greater damage to hosts (virulence). However, the inclusion of biological variables can drastically change this relationship. For example, if competing parasite strains produce toxins that kill each other (interference competition), their growth rates and virulence may be reduced relative to single-strain infections. Bacteriocins are antimicrobial toxins produced by bacteria that target closely related strains and species, and to which the producing strain is immune. We investigated competition between bacteriocin-producing, insect-killing bacteria (Photorhabdus and Xenorhabdus) and how this competition affected virulence in caterpillars. Where one strain could kill the other, and not vice versa, the non-killing strain was competitively excluded, and insect mortality was the same as that of the killing strain alone. However, when caterpillars were multiply infected by strains that could kill each other, we did not observe competitive exclusion and their virulence was less than single-strain infections. The ubiquity and diversity of bacteriocins among pathogenic bacteria suggest mixed infections will be, on average, less virulent than single infections.     

Clutch-size behavior and coexistence in ephemeral-patch competition models

Systems of patchy, ephemeral resources often support surprisingly diverse assemblages of consumer insects. Aggregation of consumer individuals over the landscape of patches has been suggested as one mechanism that can stabilize competition among consumer species. One mechanism for larval aggregation is the laying of eggs in clutches by females traveling among patches to distribute their total fecundity. We use simulation models to explore the consequences, for coexistence of competitors, of larval aggregation that arises from clutch laying. Contrary to some previous treatments, we find that clutch laying can be strongly stabilizing and under certain conditions can be sufficient to allow competitors to coexist stably. We extend these models by considering clutch size as a variable that responds to the abundance of resource patches. Such a relationship might be expected because females should lay their eggs in fewer but larger clutches when the cost of travel among patches is high (because patches are rare). When females adjust clutch size in response to resource abundance, coexistence can be easiest when resource patches are scarce and most difficult when resources are abundant.     

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.     

Multiple resource limitation: nonequilibrium coexistence of species in a competition model using a synthesizing unit

During the last two decades, the simple view of resource limitation by a single resource has been changed due to the realization that co-limitation by multiple resources is often an important determinant of species growth. Hence, the multiple resource limitation hypothesis needs to be taken into account, when communities of species competing for resources are considered. We present a multiple species–multiple resource competition model which is based on the concept of synthesizing unit to formulate the growth rates of species competing for interactive essential resources. Using this model, we demonstrate that a more mechanistic explanation of interactive effects of co-limitation may lead to the known complex dynamics including nonequilibrium states as oscillations and chaos. We compare our findings with earlier investigations on biological mechanisms that can predict the outcome of multispecies competition. Moreover, we show that this model yields a periodic state where more species than limiting complementary resources can coexist (supersaturation) in a homogeneous environment. We identify two novel mechanisms, how such a state can emerge: a transcritical bifurcation of a limit cycle and a transition from a heteroclinic cycle. Furthermore, we demonstrate the robustness of the phenomenon of supersaturation when the environmental conditions are varied.     

Species coexistence under resource competition with intraspecific and interspecific direct competition in a chemostat

Competition theory has developed separately for direct competition and for exploitative competition. However, the combined effects of the two types of competition on species coexistence remain unclear. To examine how intraspecific and interspecific direct competition contributes to the coexistence of species competing for a single resource, we constructed a chemostat-type resource competition model. With general functions for intraspecific and interspecific direct competition, we derived necessary and sufficient conditions (except for a critical case that rarely occurs in a biological sense) that determine the number of stably coexisting species. From these conditions, we found that the number of coexisting species is determined just by the invasibility of each species into subcommunities with a smaller number of species. In addition, using a combination of rigorous mathematical theory and a simple graphical method, we can demonstrate how the stronger intraspecific direct competition facilitates species invasion, leading to a larger number of coexisting species.     

Competitive intransitivity promotes species coexistence

Using a spatially explicit cellular automaton model with local competition, we investigate the potential for varied levels of competitive intransitivity (i.e., nonhierarchical competition) to promote species coexistence. As predicted, on average, increased levels of intransitivity result in more sustained coexistence within simulated communities, although the outcome of competition also becomes increasingly unpredictable. Interestingly, even a moderate degree of intransitivity within a community can promote coexistence, in terms of both the length of time until the first competitive exclusion and the number of species remaining in the community after 500 simulated generations. These results suggest that modest levels of intransitivity in nature, such as those that are thought to be characteristic of plant communities, can contribute to coexistence and, therefore, community-scale biodiversity. We explore a potential connection between competitive intransitivity and neutral theory, whereby competitive intransitivity may represent an important mechanism for ecological equivalence.     

Stoichiometrically explicit competition between grazers: species replacement, coexistence, and priority effects along resource supply gradients

Assuming key trade-offs among interactors, several models (resource ratio, keystone predation, intraguild predation) predict changes in species composition over resource supply gradients. Ecological stoichiometry could also predict compositional shifts of grazers over gradients of nutrient and light supply through a mechanism involving (mis)matches between elemental body composition of grazers and plants. This hypothesis is explored here using a suite of two-grazer, one-plant models that incorporate three key components: plant production depends on light and nutrients, nutrient content of plants can vary, and homeostatic grazers can be carbon or nutrient limited. The results from this suite closely resemble the classical resource ratio model describing plant competition for two resources. Here, the models predict shifts of grazer composition along resource supply gradients if species trade off competitive abilities for plant carbon and nutrients. Given this trade-off, superior nutrient competitors should dominate low nutrient environments, and superior carbon competitors should dominate high nutrient environments. At intermediate nutrient supply, species can coexist at a stable equilibrium, or alternative stable states emerge, depending on how grazers impact their resources. These results depend on food web architecture, however. For instance, predators can alter or reduce possibilities for stoichiometry-mediated coexistence of grazers.     

Interference competition among native and invader amphipods

Aquarium experiments were used to study indications of interference competition, such as substratum choice shifts, swimming activities and mortality of invasive and indigenous gammarids in each other's presence. The more recent invaders Gammarus tigrinus and Dikerogammarus villosus were more likely to prefer stone substratum, whereas the native Gammarus pulex and an earlier invader Gammarus roeseli were found more frequently in the water layer. Sand was the least likely substratum to be chosen by any of the species. G. pulex and G. roeseli did not alter their substratum preference in each other's presence. In the presence of D. villosus, G. pulex shifted towards smaller stones and increased its swimming activities, whereas D. villosus did not change its behaviour in the presence of G. pulex. These shifts may indicate interference competition, with D. villosus being the stronger competitor. The greatest shifts in substratum preference arose when one species had occupied a substratum before the other one was introduced, especially when D. villosus was already present before G. pulex was introduced, possibly indicating pre-emptive competition. Swimming activities of G. pulex increased in the presence of D. villosus, whereas D. villosus spent little time swimming. Mortality was comparable between the different experiments without any indication of predation. The effect of Intra Guild Predation (IGP) may not be reflected adequately by short-time experiments as moults occurred seldom during the experiments. Although no IGP was observed during our experiments, habitat shifts occurred, which may indicate that competitive interactions are apparent before IGP starts. Such shifts may serve to avoid intraguild competition.     

Effects of life history strategies and tree competition on species coexistence in a sub-boreal coniferous forest of Japan

We examined the effects of different life history strategies and tree competition on species coexistence in a northern coniferous forest. We investigated the growth and demography of trees with stems ≥1 cm dbh in a 2-ha study plot in the Taisetsu Mountains of northern Japan. Three species, Abies sachalinensis, Picea jezoensis, and Picea glehnii, were found to be dominant in the forest. A. sachalinensis was the most dominant species in the understory, while the two Picea spp. were more abundant in the larger dbh size classes. The turnover rate of A. sachalinensis was about twice that of the Picea spp. The relative growth rate of understory trees in each species did not differ between different canopy conditions (closed canopy or canopy gap). The competitive advantage between A. sachalinensis and P. glehnii switched as they grew from understory (A. sachalinensis superior competitor) to canopy trees (P. glehnii superior competitor). Meanwhile, A. sachalinensis and P. jezoensis exhibited different environmental preferences. We propose that reversal in competitive superiority between different growth stages and trade-off between longevity and turnover are more important factors to promote their coexistence than regeneration niche differentiation related to canopy gaps in this sub-boreal coniferous forest.     

Disturbance, coexistence, history, and competition for space

A simple model to elucidate the effect of disturbance on a large number of competitors that compete for space and exhibit a competitive hierarchy is developed. Conditions are derived that determine presence of species, and diversity is calculated as a function of percentage cover. The model is compared to data from coral reefs collected by J. W. Porter (1974, Science 186, 543–545). Using parameter values in the model that allow a fit to Porter's data, the response of an individual species to changes in disturbance becomes quite complex, depending on the position (odd or even) of the species in the competitive hierarchy. For these same parameter values, the system is interactive: the existence of a particular species may effect the presence of another. Different parameter values would lead to a noninteractive system.     

Population biology of multispecies helminth infection: competition and coexistence

The role that interspecific interactions play in shaping parasite communities is uncertain. To date, models of competition between helminth species have assumed that interaction occurs through parasite-induced host death. To our knowledge, there has been no theoretical exploration of other forms of competition. We examine models in which competition acts at the point of establishment within the host, and at the time of egg production by the adult worm. The models used are stochastic and we allow hosts to vary in their rate of exposure to infective larvae. We derive the Lotka-Volterra model of competition when exposure is homogenous and thus demonstrate that two helminth species cannot coexist on a single limiting resource. We show that coexistence of species is promoted by heterogeneity in host exposure provided that the rates of exposure to the two species are not perfectly correlated, and, if they are positively correlated, provided that the degree of heterogeneity in host exposure is similar for the two competing helminth species. These results are robust to the mechanism of competition.