Isolation affects reproductive success in low-density but not high-density populations of two wind-pollinated <Emphasis Type="Italic">Thalictrum</Emphasis> species

Since pollen usually travels limited distances in wind-pollinated plant species, plants growing at low density may become pollen limited. We examined how local pollen availability and population density affect reproductive success in two wind-pollinated, dioecious species, Thalictrum fendleri and Thalictrum dioicum. Distance to the nearest flowering male, the number of flowering males within 2 m, and flower number on those males served as measures of local pollen availability. Increased distance from pollen donors reduced seed set in the lowest-density population of each species, but seed set in high-density populations was not correlated with local pollen availability. For plants in high- and low-density populations at similar distances from pollen donors, this distance only affected seed set in low-density populations. To ensure that differences in resource availability were not causing spurious correlations between seed set and plant density, we constructed low-density artificial arrays in populations of T. dioicum. In these, seed set decreased rapidly with increases in distance from pollen donors. Despite these effects, the density of males in a population was not correlated with average seed set in T. dioicum, and hand pollination in the T. dioicum populations also failed to increase seed set over natural levels. These results suggest that pollen receipt only limits seed set on isolated plants within low- density populations of T. dioicum and T. fendleri.     

Patch size effects are more important than genetic diversity for plant–herbivore interactions in Brassica crops

1. Considerable evidence suggests that the diversity within plant communities may strongly affect the strength of species interactions, but the majority of studies only considered interspecific diversity. 2. This paper examines the effect of intraspecific genetic diversity within Brassica fields on two Brassica specialists, cabbage root fly, and diamondback moth, and on a parasitoid attacking diamondback moths. Genetic diversity was manipulated both in a replacement and an additive design. 3. Both herbivore densities and parasitism rates were higher in smaller plots, with limited responses to increased within‐plot diversity. All species showed variable densities across genotypes, and preference hierarchies were species specific. 4. Responses to plot size in root flies scaled with the diameter‐to‐area ratio, suggesting that patch detectability affected local density, whereas responses by diamondback moths and parasitoids deviated from this ratio. These species differences could be traced to differences in the residence time within patches, where diamondback moths typically spend longer and more variable time periods in patches than root flies. 5. The lack of response to genetic diversity by both herbivores suggests that egg‐laying rates are affected by decisions on the plant and not by attraction from a distance, neither to the plant itself nor the patch. Patterns of differential attack may then be due to different acceptability for studied genotypes. 6. Future theories on insect responses to spatial heterogeneity should focus on species traits and how traits interact with information landscapes in the field.     

Synergistic effects of food shortage and an insecticide on a <Emphasis Type="Italic">Daphnia</Emphasis> population: rapid decline of food density at the peak of population density reduces tolerance to the chemical and induces a large population crash

Laboratory populations of cloned Daphnia magna were exposed at different population phases (growing phase, density peak, stable phase) to the insecticide carbaryl at 15 μg 1⁻¹, which was harmful to juveniles but not to adults, and their population dynamics were analyzed. The population declined most at the density peak, when not only juveniles but also many adult individuals died. To analyze the factors affecting population vulnerability to carbaryl, acute toxicity tests were conducted using Daphnia individuals of different body sizes under different food conditions. The test revealed that daphnid sensitivity to carbaryl increased greatly when food density was changed from a high food level to a low level. This food condition, of low availability, might be the condition to which the Daphnia populations were exposed at their density peak. The synergism of the negative impacts of anthropogenic and natural stresses such as insecticides and food shortage may control aquatic populations.     

An assessment of overwinter food limitation in a snowshoe hare population at a cyclic low

Snowshoe hares (Lepus americanus) undergo 8- to 11-year population cycles caused by direct and/or interactive effects of overwinter food shortage and predation. However, the demographic significance of food shortage during cyclic population lows remains unclear. I evaluated the importance of overwinter food limitation to the demography (numbers, age and sex ratios) of low-density hare populations during two winters in Manitoba. Also, I examined whether the hypothesized differences in demography of fed and unfed hare populations could be explained by altered movement patterns or social dynamics. Bimonthly live-trapping revealed that food failed to have a direct long-term effect on the number, or change in number, of hares estimated to be on the three supplemented areas, relative to three control areas. Modest numerical responses to supplementation tended to be short-term (i.e., restricted to winter) and related to pre-supplementation densities, with the study area characterized by the highest hare density displaying the strongest and most consistent response to added food. During winter the percentage of females was remarkably variable among study areas and time periods, but added food may have augmented slightly the proportion of females captured in traps. There tended to be slightly more juveniles on supplemented areas during winter periods, and this effect was strongest during the first winter (1991–1992). I found that immigration rates and percentage of hares that were considered to be transient animals were similar on supplemented and control areas, and that spatial distribution of radio-collared animals on versus off of study areas also was similar. Because the overall effect of food on hare populations was small and short-lived, and could be explained largely by small increases in survival and reproduction, I conclude that the study population was not subject to overwinter food limitation. Received: 22 February 1998 / Accepted: 12 February 1999     

The costs of colour: plasticity of melanin pigmentation in an outbreaking polymorphic forest moth

Outside the context of industrial melanism, little is known about the physiological and ecological importance of genetic melanic polymorphisms in moths. Melanin pigments are synthesized from amino acid precursors and should therefore be costly to produce in nitrogen‐limited insects. A genetic melanic polymorphism is present in adult Malacosoma disstria Hübner (Lepidoptera: Lasiocampidae), a widespread forest moth with outbreaking population dynamics. We test the hypotheses that melanin‐based colouration is physiologically costly in M. disstria, that expression of melanin‐based colouration is a plastic trait which varies with population density and nutrition, and that the genetically based melanic phenotype is disadvantaged under nutritionally poor conditions. Two experiments were used to test these hypotheses. A field study compared pigmentation and phenotypic frequencies in moths collected from high‐ and low‐density populations. A laboratory experiment investigated the effects of larval nitrogen availability on adult pigmentation and phenotypic frequencies. High population density and nitrogen limitation reduced pigmentation and size of all moths, but phenotypic frequencies were not affected in either experiment. The effects of diet on both pigmentation and size were stronger for melanic moths than for typical moths. Our results show that adult melanism in M. disstria is physiologically costly, that colour expression is plastic despite its genetic component, and that the melanic phenotype may be disadvantaged under poor conditions but favoured under good conditions. We suggest that temporal variation in selection and trait plasticity help maintain polymorphism stability.     

Ecological correlates of population density and behavior in the circumtropical black triggerfish Melichthys niger (Balistidae)

The remarkable swarming of black triggerfish Melichthys niger (Balistidae) on oceanic islands led us to undertake a comparative ecological study of high-density and low-density populations of this unusual circumtropical species. Abundance, distribution, and aggressive encounters were recorded in the field from the high-density Johnston Atoll (JA) population, the low-density Belize (BE) population, and a local high-density aggregation from Puerto Rico (PR). JA and PR populations of M. niger occurred at significantly higher densities and were more aggregated than in BE. Intraspecific aggression (chases/min/fish) was an order of magnitude higher in BE than in JA, while interspecific aggression was similar among sites. Size frequency, growth rate, diet composition, body condition (liver-somatic index LSI) were assessed from collected specimens. JA grew more slowly than BE as determined by back-calculations from dorsal spine rings, suggesting density-dependent growth limitation. Size frequency analysis reflected this difference with significantly more large fish in the BE sample. Maximum age of triggerfish in both populations was estimated at 11 years, and 35–40% of growth occurred in the first year. Diet composition was similar in all three locations and indicated broad omnivory. Percent organic composition of gut contents (food quality measure) and LSI (body condition assay) were both significantly higher in the PR population, but no difference was detected between JA and BE. An increase in consumption of algae on the degraded PR reef may explain this pattern.     

Implications of small scale variation in ecological conditions for the diet and density of red colobus monkeys

In this 3-year investigation we documented patterns of density, diet, and activity of red colobus monkeys (Procolobus tephrosceles) in six areas in or near Kibale National Park, Uganda and related these patterns to availability of food resources. There were large differences in the density and behavior of the red colobus among the sites. For example, the red colobus at one site with a diverse plant community of more than 61 tree species, had a diet that included at least 42 species. In contrast, at a second site red colobus spent 92% of their feeding time eating from one species that dominated the tree community. The density of important red colobus food trees varied among sites from 32 trees/ha to 204 trees/ha, and red colobus density ranged from 0.70 groups/km² to 7.41 groups/km². Among sites, red colobus density was related to the cumulative DBH of important food trees, when one apparently anomalous site was excluded, and populations with more plant species in their diets tended to be those that were found at higher densities. Activity budgets of the red colobus populations varied markedly among sites. For example, feeding time ranged among sites from 29 to 55%, and traveling varied from 5 to 20%. When faced with increased foraging demands, red colobus reduced the time spent resting, while the time spent socializing remained fairly constant. Comparative socioecological studies typically contrast species separated by large geographical distances to ensure there is sufficient variation in the environment to detect behavioral responses. The marked differences in ecological conditions and red colobus behavior we documented over short geographical distances, suggests that small-scale contrasts are a useful tool to examine ecological determinants of behavior and community structure.     

Limits to genetic bottlenecks and founder events imposed by the Allee effect

The Allee effect can result in a negative population growth rate at low population density. Consequently, populations below a minimum (critical) density are unlikely to persist. A lower limit on population size should constrain the loss of genetic variability due to genetic drift during population bottlenecks or founder events. We explored this phenomenon by modeling changes in genetic variability and differentiation during simulated bottlenecks of the alpine copepod, Hesperodiaptomus shoshone. Lake surveys, whole-lake re-introduction experiments and model calculations all indicate that H. shoshone should be unlikely to establish or persist at densities less than 0.5–5 individuals m⁻³. We estimated the corresponding range in minimum effective population size using the distribution of habitat (lake) sizes in nature and used these values to model the expected heterozygosity, allelic richness and genetic differentiation resulting from population bottlenecks. We found that during realistic bottlenecks or founder events, >90% of H. shoshone populations in the Sierra Nevada may be resistant to significant changes in heterozygosity or genetic distance, and 70–75% of populations may lose <10% of allelic richness. We suggest that ecological constraints on minimum population size be considered when using genetic markers to estimate historical population dynamics.     

Host plant selection behaviour of Chilo partellus and its implication for effectiveness of a trap crop

Female lepidopterans can display a hierarchy of preference among potential host species, a trait thought to arise from the balance between attractants and deterrents to which the insects respond. Host plant ranking by moths and larvae of Chilo partellus Swinhoe (Lepidoptera: Crambidae), an important pest of cereals in Africa, was investigated, and whether eggs deposited on specific host plants yield larvae of particular host preferences. Trap plants are used in management of this pest. However, any ‘disagreement’ in host ranking between moths and larvae could potentially reduce effectiveness of trap crops as larvae emigrate to the main crop from the parent’s preferred trap plant. We also investigated whether host plant preference is influenced by the diet upon which larvae fed as part of an integrated assessment of the relationship between host plant selection and learning in C. partellus. Five host plants (all Poaceae) were used: maize (Zea mays L.), sorghum (Sorghum bicolor Moench), Napier grass (Pennisetum purpureum Schumach), and two varieties of signal grass [Brachiaria brizantha (A. Rich.) Stapf], viz., local (henceforth signal grass) and improved (‘Mulato’). In multiple choice tests, C. partellus female moths preferentially oviposited on Napier grass, followed by sorghum, maize, and signal grass, and least preferred ‘Mulato’. Larvae however equally orientated and settled on leaf cuts of maize, sorghum, signal grass, and Napier grass, but least preferred ‘Mulato’. Moreover, eggs from specific host plants did not yield larvae of particular host preferences. Furthermore, oviposition preference was not altered by the larval food. These results imply only a slight ‘disagreement’ in host ranking behaviour between moths and larvae, which is beneficial for trap cropping as larvae would not ‘reject’ the trap plant and appreciably disperse to the neighboring plants. Moreover, absence of larval learning behaviour indicates that regardless of the larval food C. partellus moths would still be attracted to the selected trap plant.     

The food habits of arctic fox (<Emphasis Type="Italic">Alopex lagopus semenovi</Emphasis>) reproductive families on mednyi island (Commander Islands)

In total, 1406 samples of scat of 19 arctic fox families and 1755 prey remains collected near the dens of 32 families during the cub rearing period were analyzed. This is the first attempt to evaluate the food use in the population according to averaged data for a large number of families. After the population had passed through the bottleneck, the arctic fox diet changed considerably. Colonies of petrels—the northern fulmar Fulmarus glacialis rodgersii and the storm-petrels Oceanodroma furcata and O. leucorhoa—became the main food source, whereas the use of alternative resources (alcids, cormorants, marine invertebrates, and otarid rookery products) decreased considerably. The following factors are assumed to determine the arctic fox foraging strategy, i.e., the selectivity in food acquisition: (1) passing of the population through the bottleneck; (2) termination of anthropogenic influence due to the liquidation of human settlements on the island; (3) decrease in both the bird populations and abundance of marine invertebrates (sea urchins and mussels); and (4) stable low density of the current arctic fox population. A hypothetical scenario for formation of the foraging specialization in the island population is discussed.     

Diet variability among insular populations of Podarcis lizards reveals diverse strategies to face resource‐limited environments

Access to resources is a dynamic and multicausal process that determines the success and survival of a population. It is therefore often challenging to disentangle the factors affecting ecological traits like diet. Insular habitats provide a good opportunity to study how variation in diet originates, in particular in populations of mesopredators such as lizards. Indeed, high levels of population density associated with low food abundance and low predation are selection pressures typically observed on islands. In the present study, the diet of eighteen insular populations of two closely related species of lacertid lizards (Podarcis sicula and Podarcis melisellensis) was assessed. Our results reveal that despite dietary variability among populations, diet taxonomic diversity is not impacted by island area. In contrast, however, diet disparity metrics, based on the variability in the physical (hardness) and behavioral (evasiveness) properties of ingested food items, are correlated with island size. These findings suggest that an increase in intraspecific competition for access to resources may induce shifts in functional components of the diet. Additionally, the two species differed in the relation between diet disparity and island area suggesting that different strategies exist to deal with low food abundance in these two species. Finally, sexual dimorphism in diet and head dimensions is not greater on smaller islands, in contrast to our predictions.     

Density-dependent changes in individual foraging specialization of largemouth bass

 Individual foraging specializations are an important source of intraspecific variability in feeding strategies, but little is known about what ecological factors affect their intensity or development. We evaluated stomach contents in marked individual largemouth bass (Micropterus salmoides) and tested the hypothesis that diet specialization is most pronounced during periods with high conspecific densities. We collected diet data over 10 years from an unexploited population of largemouth bass that displayed a greater than threefold variation in density. Although diet composition of the aggregate bass population did not change during the study, bass body condition was inversely correlated with population size. Individual marked bass exhibited high diet consistency (diet overlap between successive captures) during years with high population densities. Diet overlap between randomly assigned pairs of bass was not correlated with population size. We did not detect the expected positive relationship between diet breadth and population size. Our analyses demonstrate that population responses to density changes may represent the sum of many unique individual foraging responses and would be obscured by pooled sampling programs. Behavioral flexibility of individuals may contribute to the ability of largemouth bass to function as a keystone predator in many aquatic communities. Received: 29 March 1996 / Accepted: 8 January 1997     

The impact of habitat fragmentation on trophic interactions of the monophagous butterfly <Emphasis Type="Italic">Polyommatus coridon</Emphasis>

Theory predicts that habitat fragmentation, including reduced area and connectivity of suitable habitat, changes multitrophic interactions. Species at the bottom of trophic cascades (host plants) are expected to be less negatively affected than higher trophic levels, such as herbivores and their parasitoids or predators. Here we test this hypothesis regarding the effects of habitat area and connectivity in a trophic system with three levels: first with the population size of the larval food plant Hippocrepis comosa, next with the population density of the monophagous butterfly species Polyommatus coridon and finally with its larval parasitism rate. Our results show no evidence for negative effects of habitat fragmentation on the food plant or on parasitism rates, but population density of adult P. coridon was reduced with decreasing connectivity. We conclude that the highly specialized butterfly species is more affected by habitat fragmentation than its larval food plant because of its higher trophic position. However, the butterfly host species was also more affected than its parasitoids, presumably because of lower resource specialization of local parasitoids which also frequently occur in alternative hosts. Therefore, conservation efforts should focus first on the most specialized species of interaction networks and second on higher trophic levels.     

Density-independent resource limitation and the transmission of an insect pathogen

The effects of resource limitation on the transmission of a pathogen were explored. Resource limitation was achieved by replacing part of the host’s diet with an indigestible bulking agent. Populations of the pyrallid moth, Plodia interpunctella, raised on high- and low-quality food regimes were exposed to a granulosis virus. Moths subjected to a lower food quality were more likely to become infected, despite the fact that in previous studies, individuals showed no increased susceptibility when exposed individually to the virus. This effect is suggested to be due to a higher exposure to the pathogen due to a faster feeding rate and longer developmental period. The implications of resource levels to the population dynamics of host-pathogen interactions are discussed. Received: 8 July 1999 / Accepted: 14 February 2000     

Differentiation and adaptation in <Emphasis Type="Italic">Brassica nigra</Emphasis> populations: interactions with related herbivores

Local adaptation and population differentiation of plants are well documented, but studies on interactions with natural enemies are rare. In particular, evidence for plant adaptation to the local biotic environment, such as herbivores remains poor. We used the black mustard Brassica nigra, an annual species of river valley and coastal habitats to (1) analyse population differentiation in plant traits and herbivory in a common garden experiment, (2) examine home versus away differences in a reciprocal transplant experiment and (3) test whether plants are adapted to local herbivores or vice versa under standard greenhouse conditions. In the common garden experiment, we found significant differentiation in plant traits, leaf damage and herbivore number among seven populations of B. nigra from France and Germany (distance 15–1,000 km). Differences were particularly strong among coastal and river valley populations and did not necessarily increase with geographical distance. A herbivore removal treatment did not change population differentiation when compared with the control allowing natural colonisation. The reciprocal transplant experiment at a scale of 15–30 km did not reveal local plant adaptation, whilst one dominant herbivore species (Meligethes aeneus) occurred in significantly higher numbers on local plants. A greenhouse experiment combining three aphid (Brevicoryne brassicae) and plant populations of the same provenance indicated herbivore adaptation to their local plants rather than plant adaptation, but overall contrasts between local and non-local combinations were not significant. The results suggest that herbivores may counteract local plant adaptation to other environmental factors. Our study has important implications for plant translocations in ecological restoration projects.     

Seasonality in neotropical populations of Plutella xylostella (Lepidoptera): resource availability and migration

The present study aimed to verify (1) whether seasonal increases in neotropical populations of Plutella xylostella are directly provoked by regular influxes of migrants, and (2) whether temporal variation in food availability is the ecological process behind such predictable events. Over 3 years, plants that P. xylostella prefers were cultivated and irrigated in order to provide a continuous and abundant supply of food. Nevertheless, seasonal oscillations in the population of the herbivore still occurred. The hypothesis of seasonal availability of host plants could not explain the population pattern. In April, when the insect was practically extinct from the area, an artificial infestation (immigration) with 10,000 pupae established a precocious population. Therefore, the start of the natural cycles of population growth, during July–August, seems to be due to external factors, rather than an improvement in local conditions for resident individuals. In the beginning of the natural cycles, the increase in the density of adults significantly preceded the increase in immatures. Plutella xylostella does not diapause, and therefore immigration is the proximate cause of the seasonal population increases. Hypotheses about local factors are suggested to explain the decreasing phase of the predictable population cycles.     

Comparison of quantitative genetic parameters between two natural populations of a selfing plant species, Medicago truncatula Gaertn.

 In this paper we compare mean values, heritability estimates, coefficient of genetic variation, and genetic correlations among several fitness components of two natural populations of a selfing plant species, Medicago truncatula L. It is shown that the population that had been found most polymorphic for molecular markers in a previous study was also the most variable for quantitative characters. Depending on the traits, the larger heritabilities in this population were due to either larger coefficients of genetic variances or smaller coefficients of environmental variances. Whereas genetic and phenotypic correlation matrices were very similar within each population, they were quite different between populations. In particular, although a positive correlation between age and size at maturity was found in both populations, the correlation between age at maturity and reproductive success was negative in the more variable population (late flowering plant, with a larger size at flowering, produced fewer pods), whereas no correlation was observed in the less variable population. We suggest that while in the less variable population all individuals have a high reproductive effort, several strategies coexist in the more variable population, with some early-flowering genotypes showing a high reproductive effort and other late-flowering genotypes showing a larger competitive ability through increased vegetative growth. Received: 25 June 1996 / Accepted: 11 October 1996     

Host‐associated divergence in sympatric host races of the leaf beetle Lochmaea capreae: implications for local adaptation and reproductive isolation

Ecological specialization is widely recognized as a major determinant of the emergence and maintenance of biodiversity. We studied two critical facets of specialization – local adaptation and habitat choice – in the host races of the leaf beetle Lochmaea capreae on willow and birch. Our results revealed that there is asymmetric disruptive selection for host use traits, and host races achieved different adaptive sets of life history traits through association with their host plant. Beetles from each host race exhibited food and oviposition preference for their own host plant. Reciprocal transplant displayed significant variation in host acceptance and performance: all families from the willow race rejected the alternative host plant before initiation of feeding and all died on this host plant. By contrast, all families from the birch race accepted willow for feeding, but they consumed less and performed less well. Intriguingly, families that performed well on birch also performed well on willow, suggesting positive genetic correlation rather than genetic trade‐offs. Our results suggest that the major proximal determinant of host specialization in the willow race is the behavioural acceptance of a plant rather than the toxicity of the food resource. However, in the birch race a combination of behavioural host acceptance and performance may play a role in specialization. Our study sheds light on the mechanisms by which divergent host adaptation might influence the evolution of reproductive isolation between herbivorous populations.     

Diamondback moth compensatory consumption of protease inhibitor-transformed plants

Prior study of the effect of protease inhibitors (PIs) on diamondback moths suggests that moths are resistant to them, so PIs represent an ineffective defence against moths. However, our data suggest that diamondback moths do suffer lower growth rates when they consume plants transformed with potato protease inhibitor (PI2), but that effect is hidden by compensatory consumption. Plants, instead of gaining an advantage by lowering the insect growth rate, suffer a disadvantage as moths consume more tissue to mitigate the effect. Furthermore, PI2, when used in conjunction with another transgenic pesticidal protein, Bt (from Bacillus thuringiensis) counteracts the effectiveness of Bt at protecting plant tissue. Thus, transgenic PIs are not only less effective than previously thought in protecting Brassica plants from diamondback moths, they may actually lead to increased plant damage by the moths.     

Interpopulation quality in gypsy moths with implications for success of two pupal parasitoids: Brachymeria intermedia (Nees) and Coccygomimus turionellae (L.)*

Abstract.

• 1 Gypsy moth egg masses were collected from innocuous, release and outbreak populations and reared in the laboratory on synthetic diet under identical conditions.
• 2 Outbreak population gypsy moths hatched sooner, were smaller and less fecund than innocuous or release gypsy moths, but had a higher concentration of total carbohydrates in their haemolymph.
• 3 Pupae from each population source were submitted to parasitization by two pupal parasitoids. Emerging B.intermedia, an established parasitoid of the gypsy moth associated with outbreak populations, were largest on outbreak source gypsy moths. C. turionellae, not a usual parasitoid of the gypsy moth, were largest when emerging from innocuous or release population gypsy moths. Implications for population dynamics of the gypsy moth are discussed.