Low Larval Density Research Articles

Density dependence in blowfly populations: experimental evaluation of non‐parametric time‐series modelling

Two approaches for describing density dependence in demographic rates of stage‐structured populations are compared in this study. Time‐series data from laboratory blowfly populations (Lucilia sericata) have been analysed in a separate study, with a statistical modelling approach that incorporated density dependences as unspecified (non‐parametric) functions. In this study, we assessed density‐dependent structures by manipulating densities of larvae and adults in cohorts of blowflies and measuring the demographic rates. We here compare the density‐dependent structures revealed by the cohort experiments with those estimated by the non‐parametric model. This model estimates the demographic rates to have the following density‐dependent structures: (i) larval survival was non‐linearly density‐dependent (a ‘humped’ function), (ii) adult survival was density‐independent, and (iii) reproductive rate decreased with adult density. In the cohort experiments reported here, (i) juvenile survival exhibited a positive density dependence in low densities (facilitation), which became negative at higher densities (competition). Pupal and adult size decreased with initial larval density. (ii) Adult survival was reduced by high initial larval density, but it was independent of adult density. (iii) Reproductive rate was reduced by high initial larval density, and by high adult density in populations of large individuals (from low larval density). Hence, the results from these experiments support the non‐parametric model estimates regarding density‐dependent structures of demographic rates in the blowfly populations. The mean demographic rates, however, were apparently underestimated by the model. We conclude that non‐parametric modelling is a useful first approach for exploratory analysis of ecological time‐series data.

Factors which affect the occurrence of cannibalism and the broad-headed "cannibal" morph in larvae of the salamander Hynobius retardatus

Cannibalism in amphibian larvae may be suppressed among siblings in comparison to distant relatives or nonkin, even when a "cannibal" morph that can consume conspecifics shows adaptive advantages. Two experimental studies were undertaken to analyze factors which affect the frequency of cannibalism and the occurrence of the broad-headed "cannibal" morph in larvae of the salamander Hynobius retardatus. The first experiment investigated whether or not the broad-headed "cannibal" morph is only induced after performing cannibalism. Because the broad-headed "cannibal" morph was observed in larval populations that showed no cannibalism, it was concluded that the "cannibal" morph could be induced without actual cannibalism. Second, possible factors affecting the occurrence of the broad-headed "cannibal" morph were analyzed with respect to the density of larvae, level of food supply, and kinship among larvae, alone or in combination. Appearance of the broad-headed "cannibal" morph was affected by interactive effects of density×kinship: although the morph was induced significantly more often at a higher density if the conspecifics were distantly or not related, it was strongly suppressed even at high density if larval kinship was very close or among siblings. In contrast, the frequency of cannibalism was independently affected by larval density, level of food supply, and kinship among larvae: it was significantly larger at high than at low larval densities, at low than at high food supply, and in mixed-sibling groups than in pure sibships. These results suggest that the "cannibal" morphs in H. retardatus are induced even without cannibalism at a high density of conspecifics in mixed-sibling groups, but the occurrence of this morph, which continuously consumes conspecifics, is suppressed among siblings.

Influence of larval diet and population density on pre-adult developmental rate of laboratory populations of <i>Culex quinquefasciatus</i> (Diptera: Culicidae

The influence of larval diet and population density on pre-adult developmental rate, adult population sizes, pre-adult survival and sex ratio was investigated. Mean developmental period varied significantly with low larval densities. Similarly, there were significant variations of developmental periods with different diets. A comparison of the survival rate of larvae fed with the three types of diets showed significant variations (P KEY WORDS: Larval diet, population density, Pre-adult developmental rate, Laboratory populations, Culex quinquefasciatus. Tropical Freshwater Biology, 10 (2001) 1 – 8

Population density of Plutella xylostella (Lepidoptera: Plutellidae) and its parasitoid Cotesia plutellae (Hymenoptera: Braconidae) on two varieties of cabbage in an urban environment.

Population density of Plutella xylostella on two cabbage varieties (Kinkei 201 and Shinsei) in an urban environment created different pest loads. A maximum larval density of 9.3 larvae per plant was recorded in Kinkei 201 field. In contrast, the corresponding value was 6.3 larvae per plant in Shinsei field. Shinsei cabbage showed lower larval density of pest but higher parasitism by Cotesia plutellae. Four major parasitoids and two hyperparasitoids were collected and identified during the two year period. No egg parasitoids of the DBM were observed. Comparison of C. plutellae parasitism rates between the two varieties in the field showed a higher parasitism rate on Shinsei cabbage. Parasitoid density peaked on July 13 (4.1 cocoons/plant) in Shinsei and on June 27 (2.3 cocoons/plant) in Kinkei 201. Parasitism of host larvae by C. plutellae was high on Shinsei cabbage in both years and ranged from 40 to 83.3%. Pest density, parasitism rates and rain impact on Kinkei 201 and Shinsei are discussed.

Quantitative trait loci for life span in Drosophila melanogaster: interactions with genetic background and larval density.

The genetic architecture of variation in adult life span was examined for a population of recombinant inbred lines, each of which had been crossed to both inbred parental strains from which the lines were derived, after emergence from both high and low larval density. QTL affecting life span were mapped within each sex and larval density treatment by linkage to highly polymorphic roo-transposable element markers, using a composite interval mapping method. We detected a total of six QTL affecting life span; the additive effects and degrees of dominance for all were highly sex- and larval environment-specific. There were significant epistatic interactions between five of the life span QTL, the effects of which also differed according to genetic background, sex, and larval density. Five additional QTL were identified that contributed to differences among lines in their sensitivity to variation in larval density. Further fine-scale mapping is necessary to determine whether candidate genes within the regions to which the QTL map are actually responsible for the observed variation in life span.

Differential Morphology and Jumping Performance of Newly Metamorphosed Frogs of the Hybridogenetic Rana esculenta Complex

Closely related clonal and sexual populations may coexist in spite of the theorized lower potential for the evolution of clonal genotypes. Water frogs of the Rana esculenta complex have hemiclonal inheritance but most populations coexist with one of the recombinant parental species. We examine whether hemiclonal lineages may counterbalance their limitations of genetic variability by the adoption of one or more non-exclusive mechanisms: the general-purpose genotype or the frozen niche-variation model. Three coexisting hemiclones of the hybrid R. esculenta (GUT1, GUT2, GUT3) and both parental species (syntopic R. lessonae and allopatric R. ridibunda) were raised at two larval densities to examine morphological traits affecting jumping performance at the time of metamorphosis and size-independent jumping ability tested at three temperatures. Hind leg length and body mass at metamorphosis, traits that explain most of the variance in jumping performance, differed across hemiclones of R. esculenta. Metamorphs of hemiclone GUT1 had the longest hindlimbs and were bigger than metamorphs of the other hemiclones at low larval density but not at high density. Size adjusted jumping performance exhibited a significant genotype by larval density interaction. Metamorphs of GUT1 showed maximal jumping performance when raised at low larval density but at high density metamorphs of GUT2 were the best jumpers. In addition, within particular traits, differences were found between hemiclones across densities. These results appear to be consistent with both frozen niche-variation model and the general-purpose genotype model. Comparison with parental species revealed syntopic R. lessonae was smaller than hemiclones at metamorphosis but conversely exhib- ited better size-adjusted jumping performance when raised at low larval density. Temperature affected size- adjusted jumping performance only for frogs raised at low larval density but not for those raised at high larval densities. There was no significant temperature by hemiclone interaction.

Covariation of Morphology and Jumping Performance in Newly Metamorphosed Water Frogs: Effects of Larval Growth History

Abstract We examined the functional prediction of covariation between relative hind limb morphology and jumping performance for newly metamorphosed frogs from the hybridogenetic Rana esculenta complex. For the two parental species, R. lessonae and R. ridibunda, and three coexisting hybrid hemiclones (GUT1, GUT2, GUT3), we varied larval densities to test the relationship of morphology to performance at both the intraspecific and interspecific levels. Across all genotypes, frogs raised at low density were larger and jumped farther than those raised at high densities. Size-independent maximum jumping performance was repeatable for all hemiclones and species. Comparisons across densities indicated that covariation between morphology and performance matched the biomechanical prediction except for R. ridibunda. Rana ridibunda frogs raised at low larval density jumped relatively farther than those raised at high larval density but, in spite of differing growth environments, did not show differences in relative l...

Density-dependent natural selection in Drosophila: correlations between feeding rate, development time and viability

Abstract We have previously hypothesized that density-dependent natural selection is responsible for a genetic polymorphism in crowded cultures of Drosophila. This genetic polymorphism entails two alternative phenotypes for dealing with crowded Drosophila larval cultures. The first phenotype is associated with rapid development, fast larval feeding rates but reduced absolute viability, especially in the presence of nitrogenous wastes like ammonia. The second phenotype has associated with it the opposite set of traits, slow development, slow feeding rates and higher viability. We suggested that these traits are associated due to genetic correlations and that an important selective agent in crowded larval cultures was high levels of ammonia. To test this hypothesis we have examined viability and larval feeding rates in populations kept at low larval densities but selected directly for (i) rapid egg-to-adult development, (ii) tolerance of ammonia in the larval environment and (iii) tolerance of urea in the larval environment. Consistent with our hypothesis we found that (i) larvae selected for rapid development exhibited increased feeding rates, and decreased viability in food laced with ammonia or urea relative to controls, and (ii) larvae selected to tolerate either ammonia or urea in their larval environment show reduced feeding rates but elevated survival in toxin-laced food relative to controls. It would appear that development time and larval feeding rate are important characters for larvae adapting to crowded cultures. The correlated fitness effects of these characters provide important insights into the nature of density-dependent natural selection.

Jumping performance and short-term repeatability of newly metamorphosed hybrid and parental leopard frogs ( Rana sphenocephala and Rana blairi )

Differential fitness between hybrid and parental genotypes plays a critical role in explaining the maintenance of natural hybrid zones as well as the production of novel genetic variation that may lead to diversification. Because locomotor performance is a reliable and practical measure of potential fitness related to morphological variation, we tested for differences in jumping performance among parental and hybrid genotypes of newly metamorphosed leopard frogs (Rana sphenocephala and Rana blairi). Tadpoles of the parental species and primary and backcross hybrid tadpoles, generated from artificial crosses (a total of five genotypes), were reared at two initial larval densities. Locomotor performance of newly metamorphosed frogs, as measured by jumping ability, was tested three times over 6 days in the laboratory at 24-25°C. Maximum and average jump lengths were greater for metamorphs reared at low larval density than for those reared at high density. Regression analyses indicated that 70-79% of the variation in jump length was due to body mass. When reared at low density, metamorphs of two F1 backcross genotypes (HB and HS) and one primary hybrid genotype (SB) jumped shorter distances than either parental species. When reared at high density, hybrid performance was indistinguishable from that of the parentals, except for one backcross hybrid (HB). Moderately high short-term repeatabilities (0.47-0.66) of metamorphs reared at the high density indicate that measures of performance in newly metamorphosed frogs can be predictive. We suggest that, owing to poor jumping performance, some hybrid frogs would be at a selective disadvantage relative to their parental species in the terrestrial environment and thus would partially reinforce mechanisms of reproductive isolation in this leopard frog system. Yet equivalent performance of some hybrids relative to the parentals, at least when reared at low density, suggests that hybrid lineages also have the potential to evolve independently in some environments.

Synergism Between Natural Enemies and Biopesticides: a Test Case Using the Stinkbug Perillus bioculatus (Hemiptera: Pentatomidae) and Bacillus thuringiensis tenebrionis Against Colorado Potato Beetle (Coleoptera: Chrysomelidae)

Sublethal effects of Bacillus thuingiensis on pests could synergize with natural enemies through extended larval development and reduced defense against predators. Mortality ranged from 18 to 76% for 4th instar Colorado potato beetles feeding on potato foliage dipped in M-Trak at 0.2-3.2 m1/liter H2O. Survivors resumed feeding in 48 h, but feeding was prolonged and time to pupation was extended 33-46%. Predation success by small nymphs of Perillus bioculatus was 40-77% on 4th instar potato beetle larvae that survived M-Trak exposure compared to only 3-40% on healthy 4th. instars. Thus, susceptibility to predation as well as extended development of larvae constitute mechanisms by which B. t. tenebrionis could synergize with predators in increasing mortality. This prediction was tested in 1994-1995 using field releases of P. bioculaturs followed by applications of low doses of B. t. tenebrionis. At rates of 1-4 per plant in both years, P. bioculatus destroyed 17-55% of potato beetle eggs compared to 1.6- 4.9% in control plots. In 1994, potato beetle larval mortality could not be related to rates of either control or their combination due to early P. bioculatus dispersal at low larval prey density. However, interaction between P. bioculatus and B. t. tenebrionis was significant in 1995 in combinations of P. bioculatus (1-2 per plant) and Novodor (2 liters/ha), the interactive effect causing 31% more larval mortality than expected from simple additive effects. The 2-yr field results indicate that synergism of B. tllllringiell8is with the egg-larval predator P. bioculatus in killing potato beetle larvae would critically depend on enough beetle eggs escaping predation to locally retain the predator during larval development..

SELECTION FOR KNOCKDOWN RESISTANCE TO HEAT IN DROSOPHILA MELANOGASTER AT HIGH AND LOW LARVAL DENSITIES.

Responses to short-term selection for knockdown resistance to heat (37°C) in Drosophila melanogaster reared under stressful (high larval density) and nonstressful (low larval density) conditions were compared. No difference in selection response between density treatments was found. A test of heat resistance (39°C) after pretreatment (37°C) did not reveal an increase in survival for selected lines as compared to controls. Flies reared at high density had higher knockdown resistance throughout the experiment. Resistance to heat was not associated with body size.

Selection for Knockdown Resistance to Heat in Drosophila melanogaster at High and Low Larval Densities

Selection for Knockdown Resistance to Heat in Drosophila melanogaster at High and Low Larval Densities

Density-dependent natural selection in Drosophila: Adaptation to adult crowding

The effects of adult crowding on two components of fitness were studied in three sets of Drosophila melanogaster populations, subjected to life-stage-specific, density-dependent natural selection in the laboratory for over 50 generations. Three days of crowding, early in adult life, were observed to increase mortality significantly during the episode of crowding, as well as decrease subsequent fecundity. Populations selected for adaptation to high adult densities suffered significantly lower mortality during episodes of adult crowding, as compared to populations selected specifically for adaptation to larval crowding, as well as control populations typically maintained at low larval and adult densities. Moreover, populations adapted to larval crowding seemed to be adversely affected by adult crowding to a greater extent than the controls, raising the possibility of trade-offs between adaptations to larval and adult crowding, respectively. Preliminary evidence suggests that the populations adapted to adult crowding may have evolved a propensity to stay away from the food medium, which is where most deaths occur when adults are crowded in culture vials.

Population dynamics of the Wolbachia infection causing cytoplasmic incompatibility in Drosophila melanogaster.

Field populations of Drosophila melanogaster are often infected with Wolbachia, a vertically transmitted microorganism. Under laboratory conditions the infection causes partial incompatibility in crosses between infected males and uninfected females. Here we examine factors influencing the distribution of the infection in natural populations. We show that the level of incompatibility under field conditions was much weaker than in the laboratory. The infection was not transmitted with complete fidelity under field conditions, while field males did not transmit the infection to uninfected females and Wolbachia did not influence sperm competition. There was no association between field fitness as measured by fluctuating asymmetry and the infection status of adults. Infected field females were smaller than uninfecteds in some collections from a subtropical location, but not in other collections from the same location. Laboratory cage studies showed that the infection did not change in frequency when populations were maintained at a low larval density, but it decreased in frequency at a high larval density. Monitoring of infection frequencies in natural populations indicated stable frequencies in some populations but marked fluctuations in others. Simple models suggest that the infection probably provides a fitness benefit for the host in order to persist in populations. The exact nature of this benefit remains elusive.

Seasonal dynamics and larval drift of Chironomus riparius (Diptera) in a metal contaminated lowland river

Chironomus riparius is one of the insect species which inhabit polluted rivers in large densities, indicating a high adaptive capacity. Previous studies showed that this capacity is expressed by the occurrence of adapted strains in metal polluted rivers. Differences in life history between metal-exposed and non-exposed midges have been demonstrated in laboratory experiments, and therefore a comparative field study of seasonal dynamics was carried out at two metal polluted sites and one reference site. Just downstream from a massive metal discharge, seasonal dynamics were almost identical to the upstream reference site. Circa four generations per year were produced. Further downstream, lower larval densities were recorded, especially during the second half of the sampling period. The influx of upstream C. riparius larvae into polluted sites was estimated by measuring larval drift just upstream from the point source of metal contamination and indicated a massive input to the standing stock downstream. It is concluded that drift of non-tolerant larvae is dominating the seasonal dynamics of midges downstream. Accordingly, genetic uniformity of chironomids inhabiting upstream and downstream sites is expected most of the time. However, research performed during the last decade, demonstrated that genetically adapted strains of C. riparius may develop at certain stages in the seasonal cycle. However, a stable metal-adapted C. riparius population at the first downstream site, is most likely present on rare occasions only.

LABORATORY EVOLUTION OF LIFE-HISTORY TRAITS IN THE BEAN WEEVIL (ACANTHOSCELIDES OBTECTUS): THE EFFECTS OF DENSITY-DEPENDENT AND AGE-SPECIFIC SELECTION.

Four types of laboratory populations of the bean weevil (Acanthoscelides obtectus) have been developed to study the effects of density-dependent and age-specific selection. These populations have been selected at high (K) and low larval densities (r) as well as for reproduction early (Y) and late (O) in life. The results presented here suggest that the r- and K-populations (density-dependent selection regimes) have differentiated from each other with respect to the following life-history traits: egg-to-adult viability at high larval density (K > r), preadult developmental time (r > K), body weight (r > K), late fecundity (K > r), total realized fecundity (r > K), and longevity of males (r > K). It was also found that the following traits responded in statistically significant manner in populations subjected to different age-specific selection regimes: egg-to-adult viability (O > Y), body weight (O > Y), early fecundity (Y > O), late fecundity (O > Y), and longevity of females and males (O > Y). Although several life-history traits (viability, body weight, late fecundity) responded in similar manner to both density-dependent and age-specific selection regimes, it appears that underlying genetic and physiological mechanisms responsible for differentiation of the r/K and Y/O populations are different. We have also tested quantitative genetic basis of the bean weevil life-history traits in the populations experiencing density-dependent and age-specific selection. Among the traits traded-off within age-specific selection regimes, only early fecundity showed directional dominance, whereas late fecundity and longevity data indicated additive inheritance. In contrast to age-specific selecton regimes, three life-history traits (developmental time, body size, total fecundity) in the density-sependent regimes exhibited significant dominance effects. Lastly, we have tested the congruence between short-term and long-term effects of larval densities. The comparisons of the outcomes of the r/K selection regimes and those obtained from the low- and high-larval densities revealed that there is no congruence between the selection results and phenotypic plasticity for the analyzed life-history traits in the bean weevil.

Reaction norms for metamorphic traits in natterjack toads to larval density and pond duration

AbstractThe evolution of environmentally‐induced changes in phenotype or reaction norm implies both the existence at some time of genetic variation within a population for that plasticity measured by the presence of genotype x environment interaction (G x E), and that phenotypic variation affects fitness. Otherwise, the genetic structure of polygenic traits may restrict the evolution of the reaction norm by the lack of independent evolution of a given trait in different environments or by genetic trade‐offs with other traits that affect fitness. In this paper, we analyze the existence of G x E in metamorphic traits to two environmental factors, larval density and pond duration in a factorial experiment with Bufo calamita tadpoles in semi‐natural conditions and in the laboratory.Results showed no plastic temporal response in metamorphosis to pond durability at low larval density. The rank of genotypes did not change across different hydroperiods, implying a high genetic correlation that may constrain the evolution of the reaction norm. At high larval density a significant G x E interaction was found, suggesting the potential for the evolution of the reaction norm. A sibship (#1) attained the presumed “optimal” reaction norm by accelerating developmental rate in short duration ponds and delaying it in longer ponds. This could be translated in fitness by an increment in metamorphic survival and size at metamorphosis in short and long ponds respectively with respect to non‐plastic sibships. However, genetic variability for plasticity suggests that optimal reaction norm for developmental rates may be variable and hard to achieve in the heterogeneous pond environment.Mass at metamorphosis was not plastic across different pond durations but decreased at high larval density. Significant adaptive plasticity for growth rates appeared in environments that differed drastically in level of crowding conditions, both in the field and in the laboratory. The fact that survival of juveniles metamorphosed at high density ponds was a monotonic function of metamorphic size, implies that response to selection may occur in this population of natterjacks and that genetic variability in plasticity may be a reliable mechanism maintaining adaptive genetic variation in growth rates in the highly variable pond environment.

Impact of balsam fir flowering on pollen and foliage biochemistry in relation to spruce budworm growth, development and food utilization

AbstractThe impact of balsam fir (Abies balsamea (L.) Miller) flowering on nutritional and allelochemical quality of pollen, current‐year and one‐year‐old foliage is studied in relation to spruce budworm (Choristoneura fumiferana Clem.) (Lepidoptera: Tortricidae) growth, development and utilization of food and nitrogen. In the laboratory, using fresh food from the field, we simulated conditions of low larval population density, in which there is no current‐year foliage depletion during the spruce budworm feeding period. Similarly, we simulated conditions of high larval population density when current‐year foliage depletion occurs.Because of the high nutritive value of pollen (high amounts of amino acids and minerals, especially nitrogen; low monoterpene content), insects from flowering trees reached the fifth instar five days earlier than those from non‐flowering trees, and had heavier dry‐ and nitrogen‐weights at the beginning of the fifth instar. At budbreak, switching from pollen to current‐year foliage negatively affected conversion efficiencies and digestibilities of food and nitrogen (AD; ADN; ECDN; ECI; ECIN). The switch from pollen to new foliage had a detrimental impact on fifth‐instar survival and on newly‐moulted sixth‐instar dry‐ and nitrogen‐weights. Moreover, during the fifth instar, balsam fir flowering reduced the nutritive value of current‐year foliage, which in turn, might have contributed to the reduced larval growth. Nevertheless, during the sixth instar, balsam fir flowering affected the biochemistry of current‐year foliage in ways that enabled larvae to compensate for their low fifth‐instar biological performance; larvae also managed to reach pupal dry weight similar to larvae reared on non‐flowering trees. Current‐year foliage from flowering trees contained less nitrogen, total soluble sugars and total monoterpenes. Those foliar characteristics enabled larvae to increase food and nitrogen consumption rates (RCR; RNCR), because of lower repellency and/or post‐ingestional feedback from monoterpenes.As for current‐year foliage, balsam fir flowering reduced nitrogen, total soluble sugar and total monoterpene contents in one‐year‐old foliage during the sixth‐instar feeding period. These characteristics enabled sixth‐instar larvae, fed on old foliage from flowering trees, to have high relative food and nitrogen consumption rates (RCR; RNCR). Larvae were also able to reach higher relative growth rates (RGR) and relative nitrogen accumulation rates (RNAR) compared to larvae reared on one‐year‐old foliage from non‐flowering trees. Finally, larvae on flowering trees had pupal dry weight similar to those from non‐flowering trees, but reached the adult stage nine days earlier.Regardless the foliage type consumed by spruce budworm larvae during the sixth instar, pollen consumption during early larval stages reduced total development time, and thus exposure time to natural enemies. This phenomenon might increase larval survival. Balsam fir flowering changed the biochemistry of one‐year‐old and current‐year foliages, but did not affect pupal dry weights of larvae reared on flowering trees compared to those reared on non‐flowering trees. Thus, at low population density, spruce budworm populations in balsam fir flowering stands might be favoured over those in balsam fir non‐flowering stands. In addition, when larvae consumed one‐year‐old foliage during the entire sixth instar, those on flowering trees are probably favoured over those on non‐flowering trees. However, because flowering trees produce less new foliage than non‐flowering trees, current‐year foliage depletion may occur earlier on flowering trees than on non‐flowering trees. Thus, at similar larval population density, larvae on flowering trees might have to feed on one‐year‐old foliage earlier than those on non‐flowering trees. In that case, spruce budworm populations on non‐flowering stands would be favoured over those on flowering stands.

Fish predation, cannibalism, and larval development in the dragonfly Epitheca cynosura

We manipulated the risk of fish predation and cannibalism in semi-field and laboratory experiments with larvae of the dragonfly Epitheca cynosura. Results of a 22-week semi-field experiment showed that larvae from different densities were indistinguishable in mean size by the end of the experiment, but that individuals from low densities were larger until week 12. Larvae from low densities varied less in size than did those from high densities. Larvae showed rapid convergence of densities in the absence of fish. Fish predation also generated rapid convergence of densities. The presence of fish initially reduced larval size variation. There was a significant trend for the presence of fish scent to increase larval survival at low larval densities over survival in the absence of fish. Behavioral observations in the laboratory supported the results of the semi-field study in that (i) dragonfly larvae reduced their activity under daytime conditions, (ii) the presence of large larvae caused small larvae to increase their hiding inside a refuge and inhibited their crawling while exposed, (iii) small larvae ate fewer prey in the presence of fish. The results of our study suggest that the interaction between intra- and inter-specific predation risk may prove to be important for survival and development of animals in natural systems.

Intraguild Predation and Cannibalism among Larvae of Detritivorous Caddisflies in Subalpine Wetlands

Comparative data from subalpine wetlands in Colorado indicate that larvae of the limnephilid caddisflies, Asynarchus nigriculus and Limnephilus externus, are reciprocally abundant among habitats–Limnephilus larvae dominate in permanent waters, whereas Asynarchus larvae dominate in temporary basins. The purpose of this paper is to report on field and laboratory experiments that link this pattern of abundance to biotic interactions among larvae. In the first field experiment, growth and survival were compared in single and mixed species treatments in littoral enclosures. Larvae, which eat mainly vascular plant detritus, grew at similar rates among treatments in both temporary and permanent habitats suggesting that exploitative competition is not important under natural food levels and caddisfly densities. However, the survival of Limnephilus larvae was reduced in the presence of Asynarchus larvae. Subsequent behavioral studies in laboratory arenas revealed that Asynarchus larvae are extremely aggressive predators on Limnephilus larvae. In a second field experiment we manipulated the relative sizes of larvae and found that Limnephilus larvae were preyed on only when Asynarchus larvae had the same size advantage observed in natural populations. Our data suggest that the dominance of Asynarchus larvae in temporary habitats is due to asymmetric intraguild predation (IGP) facilitated by a phenological head start in development. These data do not explain the dominance of Limnephilus larvae in permanent basins, which we show elsewhere to be an indirect effect of salamander predation. Behavioral observations also revealed that Asynarchus larvae are cannibalistic. In contrast to the IGP on Limnephilus larvae, Asynarchus cannibalism occurs among same—sized larvae and often involves the mobbing of one victim by several conspecifics. In a third field experiment, we found that Asynarchus cannibalism was not density—dependent and occurred even at low larval densities. We hypothesize that Asynarchus IGP and cannibalism provide a dietary supplement to detritus that may be necessary for the timely completion of development in these nutrient—poor, high—elevation wetlands.