Life-history variation in Crepis tectorum (Asteraceae)

Summary Populations of the monocarpic plant Crepis tectorum were grown in a series of uniform environments to test the hypothesis that weedy populations are more r-selected than populations from a more natural habitat. Weedy populations exhibited a combination of r- and K-selected traits. The relatively rapid growth, the potential for a summer annual habit, and the relatively high fecundity that characterized at least one of the two weed populations studied were considered as r-selected traits favored in habitats of unpredictable duration. However, high levels of competition from other weedy species or from the crop in arable fields may explain at least some presumably K-selected traits observed in the weedy populations, e.g. relatively large seeds and late flowering in the summer. Results indicated that stress due to abiotic factors (strong winds, desiccation and nutrient deficiency) has been a more important selective factor than r- or K-selection, in non-weedy populations from calcareous grasslands (alvars) on the Baltic islands.     

DENSITY-DEPENDENT EVOLUTION OF LIFE-HISTORY TRAITS IN DROSOPHILA MELANOGASTER

Populations of Drosophila melanogaster were maintained for 36 generations in r- and K-selected environments in order to test the life-history predictions of theories on density-dependent selection. In the r-selection environment, populations were reduced to low densities by density-independent adult mortality, whereas populations in the K-selection environment were maintained at their carrying capacity. Some of the experimental results support the predictions or r- and K-selection theory; relative to the r-selected populations, the K-selected populations evolved an increased larval-to-adult viability, larger body size, and longer development time at high larval densities. Mueller and Ayala (1981) found that K-selected populations also have a higher rate of population growth at high densities. Other predictions of the thoery are contradicted by the lack of differences between the r and K populations in adult longevity and fecundity and a slower rate of development for r-selected individuals at low densities. The differences between selected populations in larval survivorship, larval-to-adult development time, and adult body size are strongly dependent on larval density, and there is a significant interaction between populations and larval density for each trait. This manifests an inadequacy of the theory on r- and K-selection, which does not take into account such interactions between genotypes and environments. We describe mechanisms that may explain the evolution of preadult life-history traits in our experiment and discuss the need for changes in theories of density-dependent selection.     

Crowding-triggered phenotypic responses alleviate consequences of crowding inEpirrita autumnata (Lep., Geometridae)

Summary CrowdedEpirrita larvae had shorter larval periods than, and similar pupal masses to, their solitary siblings when reared on low quality diets. When fed on high quality diets, pupal masses of crowded larvae were lower than in singletons, and there was no difference in larval period. Because changes in food availability (absolute shortage, induced resistance in foliage) are caused by high larval densities in the field, crowding-triggered phenotypic changes may helpEpirrita to overcome detrimental consequences of high larval density. Pupal period was longer in crowded larvae than in singletons and crowded adults emerged later than their solitary siblings. Eggs of late emerging moths eclosed late in the ensuing spring, which coincides with delayed leaf flush in the year after defoliation. The reason for the faster growth of crowded individuals on poor diets was higher intake albeit less thorough processing of food in crowded, but not in solitary, larvae. On good diets solitary individuals tended to consume more than crowded larvae but there was no difference in processing. Predicted differences of host plant use between stealthy and opportunistic types of herbivores (sensu Rhoades 1985) were generally found between solitary and aggregated larvae on poor but not on good diets. The group response could not be explained by benefits to the group although the assumptions of Wilson's model of group selection were satisfied.     

Food preference and performance of the larvae of a specialist herbivore: variation among and within host-plant populations

Specialist herbivores are suggested to be unaffected by or attracted to the defense compounds of their host-plants, and can even prefer higher levels of certain chemicals. Abrostola asclepiadis is a specialist herbivore whose larvae feed on the leaves of Vincetoxicum hirundinaria, which contains toxic alkaloids and is unpalatable to most generalist herbivores. The food choice, leaf consumption and growth of A. asclepiadis larvae were studied to determine whether there is variation among and within host-plant populations in their suitability for this specialist herbivore. There was significant variation in food preference and leaf consumption among host-plant populations, but no differences were found in larval growth and feeding on different host-plant populations. A. asclepiadis larvae preferred host-plant populations with higher alkaloid concentrations, but did not consume more leaf material from plants originating from such populations in a no-choice experiment. There was also some variation in food preference of larvae among host-plant individuals belonging to the same population, suggesting that there was variability in leaf chemistry also within populations. Such variation in larval preference among host-plant genotypes and populations may create potential for coevolutionary dynamics in a spatial mosaic.     

Reproductive strategy in Drosophila melanogaster: Significance of a genetic divergence between temperate and tropical populations

Summary Reproductive capacities of tropical and temperate populations of D. melanogaster were compared using three complementary techniques: (1) measure of egg production by females grown in the laboratory under uncrowded conditions and provided as adults with abundant food; (2) study of egg production of flies of unknown ages, collected in nature and then kept in similar conditions; and (3) analysis of ovarian activity of wild females dissected just after their capture.Tropical populations showed a lower fecundity in the laboratory and this was also observed in laboratory reared adults. On the average, flies also appeared to be older in the tropics than in temperate countries. These data, together with ecological observations showing that tropical populations live in a more predictable and stable environment, suggest that temperature populations are r-selected, while tropical ones are K-selected. The study of ovarian activity of wild females failed however to confirm this expectation. Tropical flies, which have a lower genetic fecundity, generally appeared to produce more propagules than did temperate flies. Such a contradiction shows how the ideas of r- and K-selection are difficult to apply to natural populations of Drosophila. Population density and interindividual competition are probably not the main selective forces in nature. Attention must also be paid to the necessity of exploring the environment to find resources, to the role of predation and parasitism, and to the occurrence in temperate countries of seasonal fluctuations with different selective pressures on successive generations.     

Do pioneers have r-selected traits? Life history patterns among colonizing terrestrial gastropods

We examine whether pioneer species of terrestrial gastropods (snails and slugs) possess particular life history traits commonly associated with r-selection, using data on gastropod colonization in four areas in north-west Europe (the Kvarken and Tvärminne archipelagos in the Baltic, polder woods in IJsselmeer, and a rehabilitated quarry near Maastricht). Data on age at first reproduction, longevity, clutch size, egg size and lifetime fecundity were gathered from the literature. In order to control for potentially confounding effects of body size on life history traits, we compared the residuals from the allometric relations between life history traits and body size for pioneers and non-pioneers. In snails, all life history traits examined were related to body size. In slugs, all traits except age at first reproduction scaled with body size. Body sizes did not differ between pioneers and non-pioneers in any area. In all four areas, there were no significant differences between pioneers and non-pioneers in any of the life history traits examined, after body size had been taken into account. This indicates that pioneer terrestrial gastropods generally cannot be regarded as r-selected. Pioneer species may possess any of several life history strategies, and the combinations of traits shown by them may have little in common with the r-K selection concept.     

Coexistence of two competing corixid species (Heteroptera) in an archipelago of temporary rock pools

Summary Coexistence of Callicorixa producta and Arctocorisa carinata in an archipelago of rock pools appears to be facilitated by division of the environment on the time axis. To test this, a hypothesis is built on r-K theory. As A. carinata is twice the size of C. producta it is expected that the former species is more K-selected living in more permanent pools where food resources are limiting and competition strong. The smaller C. producta is expected to prefer newly refilled pools with abundant food, and thus to be more r-selected. Gathered test data on 17 life history and population ecological features agree with the prediction. It is also shown that the both rock-pool corixids are more r-selected than other species of the family.     

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

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.     

Site specific differentiation in metal tolerance in the midge Chironomus riparius (Diptera,Chironomidae)

Metal tolerance in Chironomus riparius (Diptera) populations from contaminated streams was studied by comparing the effects of cadmium, zinc and iron on first generation laboratory reared midges. First instar larvae were exposed for four days, after which surviving larvae were counted and their length measured. Larvae from two highly polluted sites, kept under control conditions, grew substantially slower than those from other populations. All populations showed the same growth responses to increased zinc concentrations, but differences were found in the responses to both cadmium and iron. Since population differentiation was demonstrated in first generation laboratory animals, it is suggested that the differences between populations of C. riparius have a genetic basis.     

Performance of pine looper Bupalus piniarius larvae under population build-up conditions

In the first year of an outbreak, Bupalus piniarius larvae, encounter intense crowding. In the later stages of larval development, they are forced to feed on the non-preferred current-year needles of Scots pine or even on alternative hosts. It was hypothesized that larval feeding on a non-preferred resource (current-year needles) will negatively affect B. piniarius performance. It was also hypothesized that larval mutual interference (crowding without competition for food) will have negative additive effects. These hypotheses were tested in laboratory and field experiments. Fourth instar larvae were reared singly and under crowded conditions in cohorts of ten. Larvae in both situations were reared on control branches (containing both mature and current-year shoots) and branches containing only current-year shoots. Crowded larvae were reared also on Norway spruce, an alternative host. Crowding and feeding on a non-preferred resource had opposite effects on B. piniarius larval performance. Crowding in the late larval instars enhanced larval performance while absence of the preferred resource had negative effects. Larval growth rate was higher and development time was shorter for larvae exposed to crowded conditions than for solitary larvae. There was, however, no difference between the groups in final pupal weights or survival. Survival was 25% lower for larvae feeding on non-preferred current-year needles and pupal weights 9% lower, compared with results for larvae feeding on mature needles. Larvae feeding on Norway spruce suffered greatly extended development time, 82% lower survival, and resulted in 60% lighter pupae compared with conspecifics on Scots pine. It was concluded that not only quantity but also quality of the available food resource is critical for B. piniarius development.     

Predator protection versus rapid growth in a montane leaf beetle

Summary Adults and larvae of Chrysomela aenicollis (Coleoptera: Chrysomelidae) feed on foliage of Salix species (Salicaceae) between 2,400–3,400 m above sea level in the eastcentral Sierra Nevada mountains of California. We predicted that (1) cold climatic conditions would be a more frequent source of mortality at higher elevations, (2) mildweather agents of mortality such as predation should be more severe at lower elevations, and (3) populations of C. aenicollis would be adapted to the local selective regime at each elevation. We tested these predictions in 1984 and 1985 by transferring over 6,000 eggs and larvae within and between two sites at 2,810 and 3,240 m elevation above sea level. During mild summer weather at both sites, survivorship on Salix branches isolated by a barrier of sticky resin was similar to that on control branches, and we concluded that aerial predators were the primary cause of mortality. At least one major predator, a solitary wasp (Symmorphus sp., Hymenoptera: Eumenidae), was specifically associated with C. aenicollis at the lower site, where beetle mortality was highest. At both sites in 1984 and 1985, larvae originating from the lower site remained in aggregations and survived more frequently than larvae from the upper site, suggesting that they are better defended against predators. During a storm with cold weather late in the 1984 season, larvae and pupae died more frequently at the upper site, and there was a marginally significant trend (P<0.1) for the lower site individuals to die more frequently than upper site larvae during the cold storm. Upper site larvae grew approximately 10% faster than lower site larvae at the lower site and under controlled conditions in the laboratory. These findings indicate that upper and lower site populations were adapted to the local selective regime, which suggest how populations of montane phytopagous insects may adapt to changing elevations.     

How do food quality and larval crowding affect performance of the autumnal moth,Epirrita autumnata?

Outbreak densities of autumnal moth, Epirrita autumnata (Borkhausen) (Lepidoptera: Geometridae), lead to high larval crowding. Phenotypic responses of E. autumnata to larval crowding and to food quality were studied by measuring growth and consumption as well as pupal weight and fecundity. Crowding may trigger increased consumption and faster development to avoid impending food shortage on good quality food. This is suggested by the result that on a good‐quality diet, the growth of crowded larvae was better than that of solitary larvae, though they did not consume more food than solitary larvae. Crowded larvae also completed the last instar earlier than solitary larvae. The fecundity of crowded autumnal moths was not lower than the fecundity of solitarily grown autumnal moths. This may provide conditions for extra rapid population build‐up of E. autumnata. During the population increase phase the crowding effect may facilitate larval performance; however, at peak density the crowding starts to have negative effects on the performance of larvae. On a poor‐quality diet, the performance of crowded and solitary larvae did not differ. The growth of larvae was better on a good‐quality diet than on a poor‐quality diet, due to higher efficiency in food utilization. Larvae feeding on low‐quality diet did not prolong their development time, but pupated at smaller size; this resulted in lower fecundity. A decrease in food quality can be seen as a cue of oncoming food shortage and resource depletion; it may be advantageous to pupate at a smaller size and ensure survival till reproduction, rather than risk prolonging development to achieve larger size and higher fecundity.     

Density-dependent physiological phase in a non-migratory grasshopperAiolopus thalassinus

Aiolopus thalassinus thalassinus (Fabricius) (Orthoptera: Acrididae) is a non-migratory grasshopper of widespread geographical distribution, also endemic in the Tel-Arad region of the Northern Negev of Israel, where it is liable to sporadically damage agricultural crops. Periodic sampling in uncultivated ‘batha’ and agricultural fields, conducted during 1990/1991, indicate thatA. thalassinus populations exhibit seasonal fluctuations in density. Local spatial and temporal distribution, within this region, are dependent on food availability. Field observations and laboratory studies suggest that the local population ofA. thalassinus exhibits genetic heterochromy unaffected by density. The duration of nymphal developmental is 34 days for nymphs reared in isolation, whereas crowded nymphs complete their development within only 21 days. No striking density related changes in gross morphometric features are evident, but, in adults from isolated culture, an increased abundance ofsensilla coeloconica, presumably involved in olfactory chemoreception, may be functionally related to enhanced (23.6-fold higher) activity of nymphs reared under crowded conditions. This higher level of activity is correlated to higher levels of energy reserves in the haemolymph — lipids and carbohydrates — and to increased respiration. Finally, the individual food consumption of nymphs from crowded culture is almost 5-fold higher than that of isolated nymphs and may increase the potential for crop damage. These results are similar to those obtained with the migratory locustLocusta migratoria and support the hypothesis that non-migratory grasshoppers exhibit some density-dependent physiological characteristics of locusts, but do not exhibit overt chromatic or morphometric phase characteristics.     

Niche width and variation within and between populations in colonizing species (Carex flava group)

Summary Taxa of the Carex flava group in Switzerland show a trend towards increased colonizing ability (r-selection). High colonizing ability is correlated with a large fundamental but a small and discontinuous realized niche. It is argued that r-selected species with wide niches should be monomorphic with generalists rather than polymorphic with specialists since they have small effective population sizes in which high genetic variability cannot be maintained. The most r-selected taxon of the group, C. viridula spp. viridula, has indeed the lowest genetic variability within populations but, in ecologically important characters, expresses the highest plasticity. The taxonomically important characters (inflorescences) have high heritability and differences between populations of C. viridula ssp. viridula are probably much affected by genetic drift.     

Populations of Predaceous Natural Enemies Developing on Insect-Resistant and Susceptible Tomato in North Carolina

Naturally occurring populations of immature and adultGeocoris punctipes,adultColeomegilla maculataand immature coccinellids were monitored on field-grown tomato lines susceptible and resistant toManduca sextaandHelicoverpa zea. Helicoverpa zeaandHeliothis virescenseggs and small larvae that serve as prey for these predators also were monitored. MoreH. zeaandH. virescenseggs and small larvae were found on resistant than on susceptible plant lines. However, similar populations of largeH. zeaandH. virescenslarvae were found on resistant and susceptible plants. The number of adultGeocoris punctipes,adultColeomegilla maculataand immature coccinellids on resistant plants was always as high or higher than the number on susceptible plants. The data demonstrate no incompatibility of host-plant resistance with biological control provided by these predaceous insects, but indicate that the number ofG. punctipesand coccinellids required to provide effective biological control may develop too late in the season to be of practical value. Large populations of stilt bugs (Jalysus wickhami,Hemiptera: Berytidae) and spiders were observed to occur earlier in the growing season than eitherG. punctipesor coccinellids and may be a significant source of mortality forH. zeaeggs and small larvae.     

Age-related changes in foraging in the German cockroach (Dictyoptera: Blattellidae)

The dynamics of exploitation of standard experimental food sources by the German cockroach, Blattella germanicaL. (Blattellidae), were analyzed in an urban habitat in relation to developmental stage. The data presented here stress differences in foraging capacities between small (first-and second-instar) larvae and animals of other developmental stages. The first animals to arrive in a food patch presented a developmental-stage distribution significantly different from that of the general population. Adults and large larvae (fifth and sixth instars) were the first to find food sources and, in particular, before small larvae. Significant differences appeared between developmental stages concerning givingup time and the time animals left a patch. Small larvae were significantly underrepresented in a patch just before food exhaustion but they were significantly more numerous than expected just after depletion. Small larvae remained in the vicinity of a depleted food dish longer than animals of other developmental stages. Adults left patches as soon as these were depleted, long before small larvae did. Developmental stage influenced rate of departure. These observations indicate that cockroaches improve their foraging performance as they grow larger.     

Short term experiments on calanoid-cyclopoid-phytoplankton interactions

To investigate their potential effects on each other in nature, calanoid (Diaptomus clavipes and D. siciloides) and cyclopoid (Acanthocyclops vernalis and Mesocyclops edax) copepod populations were manipulated in 5 liter aquaria in laboratory experiments of 20–60 days duration. Diaptomus generally had a strongly negative effect on both cyclopoid species. The cyclopoids established populations more successfully when introduced to aquaria before calanoids than they did when calanoids were already present. On the other hand, whether introduced earlier or later than the cyclopoids, Diaptomus populations were unaffected by Acanthocyclops and were strongly depressed by Mesocyclops. Diaptomus effects on the phytoplankton were often strong but varied markedly among experiments. They included reduction of populations of edible algae, such as Chlamydomonas, which are essential for both calanoid and cyclopoid nauplii, and large increases in inedible algae, such as Kirchneriella. Feeding experiments revealed that under conditions of food scarcity Acanthocyclops nauplii survived less well than did Diaptomus nauplii. Competition for edible phytoplankton seemed to be a key factor in the calanoid-cyclopoid interactions, since the survival of herbivorous cyclopoid larvae determined the abundance of the predaceous adults. This indicates that the competitive effects of calanoids on cyclopoids often may exceed the predative effects of cyclopoids on calanoids.     

Inspection and feeding of larvae by worker honey bees (Hymenoptera: Apidae): Effect of starvation and food quantity

Honey bee larvae are frequently inspected and, sometimes, provided with food by adult workers, but the stimuli that elicit the important task of food provisioning have never been investigated. Larvae with their food experimentally deprived received more frequent inspection and feeding visits from nurse bees than normally fed larvae, suggesting that there could be a hunger signal. Food-deprived larvae with artificially supplied larval food received the same rate of feeding visits from nurse bees as did normally fed larvae but still received more inspection visits. These results suggest that stimuli eliciting feeding are different from those for inspection. They also support the hypothesis that worker bees deposit food in a larval cell only when the quantity of food is below a certain minimum threshold that is perceived during larval inspections. A model is presented regarding the stimuli from larvae that result in worker feeding behavior.     

Polymorphism and geographic variation of flower color in chilean populations ofEschscholzia californica

In twelve populations ofEschscholzia californica from central Chile, where it was introduced in the latter part of the 19th century, there is a regular gradient in frequency of the allelesJ andj, which control orange vs. yellow flowers, ranging from a maximum value of 0.531 to a minimum of 0.160 forJ. Populations having higher values ofJ occupy more mesic sites. In addition, the great majority of the populations have significant excess of heterozygotes over those expected on the basis of theHardy-Weinberg equilibrium, and so exhibit balanced polymorphism for theJ-j pair of alleles, probably associated with heterosis ofJj genotypes. Evidence is presented which suggests that this locus serves as a marker for adaptative gene complexes rather than being itself associated with adaptation. In Chile, the association ofJJ genotypes with cooler, moister habitats is the opposite of the condition existing in California, and suggests that extensive reorganization of the gene pool, through changed linkage relationships, took place asE. californica populations were being established in Chile.     

Plant growth stage effects on the feeding and growth responses of the bertha armyworm,Mamestra configurata to canola and mustard foliage

Mamestra configurata (Walker) (Lep., Noctuidae) larvae were fed excisedBrassica juncea (commercial brown mustard) orB. rapa cv. Tobin (Canola) foliage of three plant growth stages-rosette (stage 2), stem elongation (stage 3) and flowering (stage 4). Relative consumption rates (RCRi) were not significantly different between the plant species. Within theB. juncea treatments, there were no significant growth stage differences in RCRi. However, withinB. rapa, RCRi increased with advancing plant growth stage. Larvae fedB. juncea foliage had significantly reduced relative growth rates (RGRi) compared to larvae fedB. rapa foliage. Within theB. juncea treatments, RGRi decreased with advancing plant growth stage. There were no significant growth stage differences in RGRi in theB. rapa treatments. RGRi was inversely proportional to the levels of isothiocyanate-releasing glucosinolates in theB. juncea treatments. RCRi was inversely proportional to the levels of indolyl glucosinolates in theB. rapa treatments. Levels of total phenols and catechols inB. juncea did not show any trend which could be related to growth stage effects in the insect nutritional indices. InB. rapa, levels of phenols and catechols in stage 3 and 4 foliage were lower than that of stage 2 foliage. Analyses of total nitrogen in field-grown plants showed reductions in percent nitrogen from rosette to flowering stage foliage. The response ofM. configurata to different growth stages of its host plants are discussed in relation to changing levels of allelochemicals and nitrogen.