Competition for two resources

Summary Tadpoles of the tree frog, Hyla chrysoscelis, were raised at low and high levels of two independent, growth-limiting resources: solid food substrates and ingestible particles. The effect of resource level on intraspecific competition was measured as the relative importance of interference versus exploitative competitive mechanisms. The levels of both solid food substrates and particles affected the outcome of competition among the tadpoles, despite their independent effects on growth. Current models of competition generally allow only one resource to limit population growth. This result implies that more than one resource may influence the outcome of intraspecific competition.     

Food partitioning among Malagasy primates

Summary Food partitioning among lemurs was studied in relation to food patch size and plant chemistry in the eastern rainforest and a western deciduous forest of Madagascar. Patch size (i.e. crown diameter of food trees) is significantly correlated with group body weight of different lemur species. But intraspecific variability is high and prevents effective species separation. Chemical analyses of more than 400 plant parts eaten by seven different lemur species revealed major differences in their food choice with respect to protein concentrations, condensed tannins and alkaloids. Among the leaf eating lemurs discriminant analysis segregates three groups à two species based on chemical characteristics of their food. Whereas differences in food chemistry are pronounced between groups they are lacking between the two species within each group. The two species of each group avoid interference competition by different activity rhythms. Actual competition for slowly renewable resources such as leaves and fruit, however, can not be reduced by different activities. Here interspecific differences in gross categories of food, food species composition and different habitat utilization due to other constraints may contribute to the possible coexistence of species. Thus interspecific differences in food selection in relation to primary and secondary plant chemicals is an integrated part of the mechanisms allowing several primate species to coexist in sympatry.     

Competition between Bufo larvae in a eutrophic pond

Competition between larvae of two anuran species (Bufo bufo and B. calamita) was investigated under field conditions likely to disfavour cell-mediated interference mechanisms. The experiment used triplicated cage treatments in an unshaded farm pond, a poor habitat for the unicellular pathogen Anurofeca richardsi implicated in interference competition between these anurans in sand dune ponds. The farm pond experienced lower maximum temperatures than a nearby dune pond but sustained larger numbers of eukaryotic algae and therefore had higher primary productivity. Survival and growth of B. calamita larvae were inversely related to density in all treatments but interspecific effects were much more severe than intraspecific ones. There was no evidence of A. richardsi in any treatment and competition between the Bufo larvae was therefore intense in the absence of Anurofeca-mediated interference effects. Anuran larvae reduced the standing crop and altered the community composition of algae in the treatment cages but larval growth rates were not simply related to food availability. Algal cell numbers in larval guts, a measure of food acquisition, were however inversely related to tadpole density in both species. Feeding niche overlap was high but decreased as larval density increased. Resource competition was implicated as the most probable major mechanism. Received: 1 September 1999 / Accepted: 20 January 2000     

Cooccurrence of prey species alters the impact of predators on prey performance through multiple mechanisms

When prey are differentially affected by intra and interspecific competition, the cooccurrence of multiple prey species alters the per capita availability of food for a particular prey species which could alter how prey respond to the threat of predation, and hence the overall‐effect of predators. We conducted an experiment to examine the extent to which the nonconsumptive and overall effect of predatory water bugs on snail and tadpole traits (performance and morphology) depended on whether tadpoles and snails cooccurred. Tadpoles and snails differed in their relative susceptibility to intraspecific and interspecific competition, and predators affected both prey species via consumptive and nonconsumptive mechanisms. Furthermore, the overall effect of predators often depended on whether another prey species was present. The reasoning for why the overall effect of predators depended on whether prey species cooccurred, however, differed for each of the response variables. Predators affected snail body growth via nonconsumptive mechanisms, but the change in the overall effect of predators on snail body growth was attributable to how snails responded to competition in the absence of predators, rather than a change in how snails responded to the threat of predation. Predators did not affect tadpole body growth via nonconsumptive mechanisms, but the greater vulnerability of competitively superior prey (snails) to predators increased the strength of consumptive mechanisms (and hence the overall effect) through which predators affected tadpole growth. Predators affected tadpole morphology via nonconsumptive mechanisms, but the greater propensity for predators to kill competitively superior prey (snails) enhanced the ability of tadpoles to alter their morphology in response to the threat of predation by creating an environment where tadpoles had a higher per capita supply of food available to invest in the development of morphological defenses. Our work indicates that the mechanisms through which predators affect prey depends on the other members of the community.     

Interspecific interference and the functional response of four species of carnivorous stoneflies

1. A previous study compared the functional responses to their prey and intraspecific interference in mature larvae of Perlodes microcephalus, Isoperla grammatica, Dinocras cephalotes and Perla bipunctata. The present study extends this work by assessing interspecific interference between pairs of these species in equal numbers (one, two or three larvae per species) to provide total predator densities of two, four or six larvae. Baetis larvae as prey were replaced as they were eaten, and their density per predator was varied between 20 and 200 larvae. 2. The number of prey eaten by each competing species increased curvilinearly with prey density, the relationship being well described by a Type II model. Of the two constants in the model, handling time varied considerably between species, mean values being shortest for Perlodes, slightly higher for Isoperla, and much higher for Dinocras and Perla. It was not affected significantly either by predator density or the identity of the competing species. 3. Attack rate also varied between species and decreased with predator density. This decrease was slight for Perlodes, and also for Dinocras and Perla in competition with Isoperla. The decrease in Dinocras and Perla was similar to that for intraspecific interference. 4. The decrease in attack rate was described by a convex curve for Perlodes with the other three species and for Dinocras/Perla with Isoperla, but by a concave curve (negative power function) for Isoperla competing with the other three species, and for both Dinocras and Perla in competition with Perlodes. Prey consumption also decreased with predator density, the severity of competition with different species reflecting that for attack rate. 5. A comparison with previous results for intraspecific interference showed that the latter was dominant for Perlodes in all contests and for Dinocras or Perla competing with Isoperla, whilst interspecific interference dominated for Isoperla in all contests and for Dinocras and Perla competing with Perlodes. Both types of interference were applicable to competition between Dinocras and Perla. Isoperla was the least, and Perlodes the most, aggressive of the four species with Dinocras and Perla intermediate.     

Density and intercohort priority effects on larval <Emphasis Type="Italic">Salamandra salamandra</Emphasis> in temporary pools

Priority effects, i.e., effects of an early cohort on the performance of a later cohort, are generally studied between, and not within, species. The paucity of intraspecific assessments does not reflect a lack of ecological importance, but the technical problem associated with differentiating between conspecific cohorts. Here, we examine priority and density-dependent effects on larval Salamandra salamandra infraimmaculata. Larvae deposited by their mother early in the season have increased risk of desiccation, as rains at the beginning of the season are less frequent and unpredictable. However, breeding later may incur a high cost through conspecific priority effects, including cannibalism and competition. In an outdoor artificial pool experiment, we established densities of 0, 1, 2, 4 or 6 newly born larvae per pool (∼30 l), and 40 days later, added a second cohort of three newly born larvae to each pool. We differentiated between cohorts using natural individual-specific markings. For the early cohort, increasing density decreased survival and size at metamorphosis, and increased time to metamorphosis. For the late cohort, survival was 100% in pools without early-cohort larvae, but ranged between 13 and 33% in the presence of early-cohort larvae. Time to metamorphosis was significantly longer in the presence of low vs high densities of early-cohort larvae. Results suggest that early-cohort larvae are mainly subjected to exploitative competition and cannibalism mediated by food limitation, and that late-cohort larvae are subjected to cannibalism and interference due to size asymmetry between cohorts. The strong priority effects suggest that Salamandra females could increase their fitness by adjusting the number of larvae they deposit in specific pools to avoid cannibalism and intraspecific competition.     

Cladistic analysis and osteological descriptions of the frog species in the Leptodactylus fuscus species group (Anura, Leptodactylidae)

The genus Leptodactylus is predominantly Neotropical (a few species have colonized the southern Neartic region) and is distributed from Texas to Argentina and on certain Caribbean islands. Leptodactylus was divided into five groups of species: Leptodactylus melanonotus , Leptodactylus ocellatus, Leptodactylus fuscus , Leptodactylus pentadactylus and Leptodactylus marmoratus . Among these, the L. fuscus group is the one with most species, with 27 taxa. Characters unverified in most of the species are used to define the L. fuscus group. However, the monophyly of the group has never been tested rigorously in a quantitative phylogenetic context. Thus, the main goal of this study was to test such monophyly and to construct a phylogeny of the L. fuscus group. A matrix of 114 characters scored across 43 taxa was constructed, with 31 characters taken from external morphology, 58 from adult skeletons, 16 from larval chondrocranium, 5 from ethology and 4 from morphometric data were included. Out of all the species examined, 23 belonged to the ingroup and 20 to the outgroup. The data set was analysed with implied weights, by using TNT software. The monophyly of the group was strongly supported in the fittest cladogram obtained. The optimizations of some characters on this hypothesis support traditional evolutionary hypotheses. The optimizations also suggest the presence of paedomorphic character states in some species, which is also discussed.     

Competition between tadpoles and mosquito larvae

Tadpoles and mosquito larvae often co-occur, and may compete for scarce resources. However, competition between such distantly related organisms has attracted less scientific attention than have interactions among closely related taxa. We examined ecological interactions in two tadpole-mosquito systems in southeastern Australia, one from freshwater ponds (Limnodynastes peronii and Culex quinquefasciatus) and one from brackish-water habitats (Crinia signifera and Ochlerotatus australis). Diets of these tadpoles and mosquito larvae overlap considerably, potentially leading to competition for food. Laboratory experiments show that, in both study systems, mosquitoes reduced the growth rates of tadpoles, and tadpoles reduced the growth rates and survival of mosquito larvae. These negative effects were seen even at high food levels. Thus, our study suggests that tadpoles and mosquito larvae affect each other strongly, and do so via pathways other than simple consumptive competition. Because mosquitoes are important vectors for human diseases, the global decline in amphibian populations may have more impact on human health than has generally been anticipated.     

Chromosomal studies on five species of the genus Leptodactylus Fitzinger, 1826 (Amphibia, Anura) using differential staining

Cytogenetic studies were carried out on five species of Leptodactylus, namely L. fuscus, L. notoaktites, L. labyrinthicus, L. ocellatus, and L. podicipinus, after standard staining, Ag-NOR and C-banding as well as BrdU incorporation for three of them. The species had 2n = 22 chromosomes and two basic karyotype patterns. Chromosome 8 was a marker bearing a secondary constriction. In all species, this secondary constriction corresponded to the Ag-NOR site. The species had centromeric C-bands in all chromosomes of the complement, but some interstitial or telomeric bands seemed to differentiate some karyotypes, either at the species or the population level. In L. ocellatus, the C-banding pattern confirmed the occurrence of a heteromorphic pericentric inversion in chromosome 8 in specimens from one of the populations. The BrdU incorporation technique showed no detectable difference in the replication patterns of the major bands in the chromosomes of L. notoaktites, L. labyrinthicus, and L. ocellatus.     

Warming effects on consumption and intraspecific interference competition depend on predator metabolism

1.?Model analyses show that the stability of population dynamics and food web persistence increase with the strength of interference competition. Despite this critical importance for community stability, little is known about how external factors such as the environmental temperature affect intraspecific interference competition. 2.?We aimed to fill this void by studying the functional responses of two ground beetle species of different body size, Pterostichus melanarius and Poecilus versicolor. These functional response experiments were replicated across four predator densities and two temperatures to address the impact of temperature on intraspecific interference competition. 3.?We generally expected that warming should increase the speed of movement, encounter rates and in consequence interference among predator individuals. In our experiment, this expectation was supported by the results obtained for the larger predator, P.?melanarius, whereas the opposite pattern characterized the interference behaviour of the smaller predator P.?versicolor. 4.?These results suggest potentially nontrivial implications for the effects of environmental temperature on intraspecific interference competition, for which we propose an explanation based on the different sensitivity to warming of metabolic rates of both species. As expected, increasing temperature led to stronger interference competition of the larger species, P.?melanarius, which exhibited a weaker increase in metabolic rate with increasing temperature. The stronger increase in the metabolic rate of the smaller predator, P.?versicolor, had to be compensated by increasing searching activity for prey, which did not leave time for increasing interference. 5.?Together, these results suggest that any generalization how interference competition responds to warming should also take the species' metabolic response to temperature increases into account.     

How variation between individuals affects species coexistence

Although the effects of variation between individuals within species are traditionally ignored in studies of species coexistence, the magnitude of intraspecific variation in nature is forcing ecologists to reconsider. Compelling intuitive arguments suggest that individual variation may provide a previously unrecognised route to diversity maintenance by blurring species‐level competitive differences or substituting for species‐level niche differences. These arguments, which are motivating a large body of empirical work, have rarely been evaluated with quantitative theory. Here we incorporate intraspecific variation into a common model of competition and identify three pathways by which this variation affects coexistence: (1) changes in competitive dynamics because of nonlinear averaging, (2) changes in species’ mean interaction strengths because of variation in underlying traits (also via nonlinear averaging) and (3) effects on stochastic demography. As a consequence of the first two mechanisms, we find that intraspecific variation in competitive ability increases the dominance of superior competitors, and intraspecific niche variation reduces species‐level niche differentiation, both of which make coexistence more difficult. In addition, individual variation can exacerbate the effects of demographic stochasticity, and this further destabilises coexistence. Our work provides a theoretical foundation for emerging empirical interests in the effects of intraspecific variation on species diversity.     

Cell mediated immune response of Bufo arenarum and Leptodactylus ocellatus (Amphibia, Anura) to mycobacterial antigens

The lymphocyte transformation test was applied to the study of cell mediated immune (CMI) response in vitro to mycobacterial antigens in Bufo arenarum and Leptodactylus ocellatus previously inoculated with a suspension of Mycobacterium tuberculosis. The stimulated samples of blood both in B. arenarum and L. ocellatus showed a significant increase in the percent of blastic dedifferentiation (almost 100% with respect to controls) suggesting the presence of a positive CMI response to PPD. The differences were observed on the fifth day of incubation and lasted until the seventh day.     

Kin preference behaviour is present after metamorphosis in Rana cascadae frogs

Kin recognition was investigated in newly metamorphosed Rana cascadae frogs. Previous work has shown that larvae of this species prefer to associated with siblings over non-siblings. Juvenile frogs from three clutches were reared with siblings and tested for sibling preference as larvae and at 4–12 days and 39–47 days after metamorphosis. Tadpoles and froglets of the three clutches displayed a significant preference to associate with siblings.     

Competition among the adults of three grasshoppers (Orthop., Acrididae) on an alpine grassland

Abstract:  The intraspecific and interspecific competition among the adults of three common grasshoppers, Angaracris rhodopa (Fischer-Waldheim), Chorthippus dubius (Zubovsky) and Chorthippus fallax (Zubovsky) was investigated on Ganjia High Mountain Grassland in north-west China. The results indicated a strong intraspecific competition within species at high densities. The interspecific competition between the larger-sized species, A. rhodopa , and the smaller-sized species, C. dubius or C. fallax , was weak, while competition between C. dubius and C. fallax was strong. An asymmetric competitive interaction in mortality and fecundity associated with the diet and size was observed among the three species. Competition was strong between the two species that had a similar diet. The large-sized species had higher mortality rates and lower fecundity than the smaller-sized species. Intraspecific competition between the three species was mainly affected by food limitation, while interspecific competition was mainly associated with food limitation as well as the grasshoppers' adaptation to the environment.     

Cytogenetic and random ampliied polymorphic DNA analysis of Leptodactylus species from rural and urban environments (Anura, Amphibia)

Cytogenetic and random amplified polymorphic DNA analyses carried out in the species Leptodactylus podicipinus, L. ocellatus, L. labyrinthicus, and L. fuscus from rural and urban habitats of the northwest region of S?o Paulo State, Brazil, showed that the karyotypes (2n = 22), constitutive heterochromatin distribution and nucleolus organizer region (NOR) location did not differ between the populations from the two environments. The in situ hybridization with an rDNA probe confirmed the location of the NORs on chromosome 8 revealing an in tandem duplication of that region in one of the chromosomes of L. fuscus. DAPI showed that part of the C-band-positive heterochromatin is rich in AT, including that in the proximity the NORs in L. podicipinus and L. ocellatus. The molecular analyses showed that the two populations (urban and rural) of L. podicipinus and L. fuscus are similar from a genetic point of view. The urban and rural populations of species L. ocellatus and L. labyrinthicus showed differences in genetic structures, probably due to urbanization which interferes with the dispersion of those frogs. The marked differences observed between the two populations of L. ocellatus can be representing the cryptic condition of the species. Unweighted pair-group method of analysis and genetic distance analysis detected the genetic proximity between L. ocellatus and L. fuscus. The results indicate that there was no reduction in the genetic diversity in the populations from the urban environment; however, the survival of these frogs would not be guaranteed in the case of an increase in human impact especially for populations of L. labyrinthicus and L. ocellatus.     

Resource exploitation of larvae ofGastrophysa atrocyanea Motschulsky andgalerucella vittaticollis Baly (Coleoptera: Chrisomelidae) under a limited resource condition

Resource exploitation by and intraspecific competition in larvae of Gastrophysa atrocyanea and Galerucella vittaticollis were investigated in field and laboratory experiments. Larvae of both species frequently suffered from food shortages in the field. When G. atrocyanea larvae suffered from a food shortage, severe intraspecific competition occurred because of lack of predation and parasitism. This exploitive competition was caused by a local food shortage of the host plant. Individuals survived by fast exploitation when food became abundant (contest type competition). the G. atrocyanea larvae were wasteful of the food resource, and no mechanism by which to economize on the utilization of the resource was acquired because of their exploitation of the abundant resource. In contrast, the G. vittaticollis population probably is regulated by extrinsic factors such as predation and parasitism. Those larvae grew into smaller adults than those of G. atrocyanea under a food shortage, so that their wasted food consumption was lower than that of G. atrocyanea. Although intraspecific competition was similar to that for G. atrocyanea, it was not as severe. The food for G. vittaticollis was apt to be appropriated by other wasteful exploitators such as G. atrocyanea, which was superior in resource exploitation; therefore G. vittaticollis frequently suffered a food shortage. Consequently selection in relation to tolerance to starvation became more acute for G. vittaticollis than for G. atrocyanea, and individuals of G. vittaticollis that could endure starvation better may have been selected.     

Patchiness, aggregation, and the coexistence of competitors for ephemeral resources

Abstract. 1. A laboratory experiment was conducted to determine the outcome of competition between two species of carrion-breeding Diptera for food at two levels of resource patchiness.
2. Adults of Fannia howardi Malloch (Muscidae) and Megaselia scalaris Loew (Phoridae) were released into a large population cage and were given access to three 2g sections of pork kidney in one treatment (low patchiness) and to twelve 0.5g sections in the other (high patchiness).
3. Increasing the patchiness of the resource resulted in reduced overlap in the distribution of the two species, and therefore less interspecific competition. However, this did not result in improved survival of the poorer competitor, F.howardi. Both species aggregated their eggs at high patchiness, and therefore suffered from increased levels of intraspecific competition.
4. The experiment shows that increasing patchiness in this way could lead to prolonged coexistence, as has been predicted in other studies.     

荒漠草原三种蝗虫成虫种内和种间竞争的研究

荒漠草原三种蝗虫成虫种内和种间竞争的研究贺达汉方成郑哲民（宁夏农学院农学系,永宁７５０１０５）（陕西师范大学动物研究所,西安７１００６２）Ｉｎｔｒａ＿ａｎｄＩｎｔｅｒｓｐｅｃｉｆｉｃＣｏｍｐｅｔｉｔｉｏｎａｍｏｎｇｔｈｅＡｄｕｌｔｓｏｆＴｈｒｅｅＧｒ...     

Competitive ability of neotropical Drosophila predicted from larval development times

Interspecific competition is a major aspect of the ecology of insect communities exploiting ephemeral and fragmented resources. To generate testable predictions on the role of competition in such communities, information is required about the competitive abilities of the species involved. In this study we test whether larval development time, an important life-history trait, can be used to predict interspecific competitive ability in neotropical Drosophila species. Based on earlier work, we predicted a negative relationship between larval development time and competitive ability. Pairwise competition experiments were conducted with seven Drosophila species (in all possible combinations) that are known to coexist in central Panama and represent a wide range of larval development times. We used a novel approach to determine experimental densities, standardising total larval food requirements, and thereby increasing the resolution for detecting general traits involved in exploitation competition. The effects of competition were measured by comparing values for three fitness-related parameters (thorax length, larval survival, and total mass of emerged adults) against their values at two levels of intraspecific competition. The outcomes of the pairwise competition were significantly explained by the differences in larval development times: within species pairs, species with a longer development suffered more from interspecific competition than those with shorter development. It is argued that larval development time is a major determinant of competitive rank order among drosophilid species, allowing testable community-wide predictions. In addition, this relationship provides a basis for studying the role of life-history trade-offs in community-level processes. Given the generality of the Drosophila biology, development time is expected to play a similar role in other insect communities exploiting ephemeral and fragmented resources.     

Competition between perch (Perca fluviatilis) and ruffe (Gymnocephalus cernuus): the advantage of turning night into day

1. The outcome of interspecific competition for food resources depends both on the competitors’ sensory abilities and on environmental conditions. In laboratory experiments we tested the influence of daylight and darkness on feeding behaviour and specific growth rate (SGR) of two species with different sensory abilities. 2. We used perch (Perca fluviatilis) as a visually orientated, and ruffe (Gymnocephalus cernuus) as a mechano‐sensory oriented predator and tested their growth rates and behaviour under conditions of interspecific and intraspecific competition. Three different foraging conditions were used: food supplied (i) only during the day, (ii) only during the night or (iii) during both day and night. 3. In perch neither SGR nor feeding behaviour were influenced substantially by interspecific competition during daylight. During darkness their foraging behaviour changed markedly and their access to the food source as well as their SGR were negatively affected by the presence of ruffe. 4. Ruffe's foraging behaviour did not change during either day or night with interspecific competition. During the night ruffe's SGR was higher with interspecific competition, probably because of a release from intraspecific competition and the competitive inferiority of perch during the night. 5. Because of its seonsory abilities ruffe feeds predominantly at night, thereby reducing competitive interference from perch.