The cost of habitat selection in prairie voles: an empirical assessment using isodar analysis

The ideal free distribution assumes that habitat selection is without cost and predicts that fitness should be equal in different habitats. If habitat selection has a cost, then individuals should only move to another habitat when potential fitness in the new habitat exceeds that in the source habitat by an amount greater than the cost of habitat selection. We used isodar techniques to assess the cost of habitat selection. In an experimental landscape, we monitored density, movement, and reproductive success of adult female prairie voles, Microtus ochrogaster, in adjacent paired habitats with low and high cover. We tested the following hypotheses: (1) adult female prairie voles exhibited density-dependent habitat selection; (2) the cost of habitat selection was density-independent. Habitat quality based on population density and fitness of adult females was higher in high cover habitats. Net movement was from low cover to high cover habitats. The results indicated that adult female prairie voles exhibited density-dependent habitat selection. Furthermore, there was a significant cost of habitat selection, and the cost was density-independent.     

Spatial and temporal population structure of Brevicoryne brassicae

Allozymes from individual aphids were used as markers to examine the structure of Brevicoryne brassicae colonies collected from field experiments at Horticulture Research International, Wellesbourne, during the summers of 1991, 1992 and 1993. Eighteen enzyme systems were examined using cellulose acetate electrophoresis, but only 6-phosphogluconate dehydrogenase (6-PGDH) showed polymorphism (three alleles) in the samples. There were significant differences in gene frequencies between populations at some sites within 1 kilometre and between times of sampling, with an increase in the proportion of the most common genotype AA later in the season, possibly due to selection during the summer of the best adapted clones. These changes in gene frequency might produce biased results in trials screening brassica crops against aphids.     

Spatial structure of a leaf-removing crab population in a mangrove of North-Brazil

The leaf-removing decapod crab, Ucides cordatus plays a key role as ecological engineer in Brazilian mangrove ecosystems. We analyzed the spatial distribution of a specific population at two different scales to observe how individual behavior could alter spatial population structure. First, we conducted a spatial point pattern analysis of the burrow entrances and the Rhizophora mangle prop roots on the mangrove floor at a scale of few meters. Secondly, we analyzed at a large scale (10–100 m) the potential effects of surface elevation, light intensity, prop root coverage, species of neighboring tree (R. mangle, Laguncularia racemosa, Avicennia germinans) and pneumatophore density on the size and number of burrow entrances. At the same large scale, we conducted an analysis of clustering of the crabs around the R. mangle trees. At small scale, the burrow entrances, although aggregated around the prop roots, showed a regularly spaced distribution (∼25 cm) signaling an intraspecific competition among the crabs. At large scale, crabs preferred to install their burrows at an intermediate level of surface elevation and prop root coverage, and in R. mangle-dominated areas. At the same kind of habitats, the largest burrows, and thus potentially the largest crabs, were found in higher number than on other habitats. The R. mangle-dominated areas preference was confirmed by an aggregating around R. mangle trees in R. mangle-dominated forest, but only of large individuals in L. racemosa-dominated forest. These observations lead us to the definition of a preferred habitat for U. cordatus. Competition leading to the small-scale regular patterns was proposed as an explanation for exclusion of smaller crabs from preferred habitats seen at large scale. We hypothesize that this preferred habitat might explain at regional scale the variation of U. cordatus importance in Neotropical mangroves.     

The effect of acidification in enclosures on the biomass and population size structure of Bosmina longirostris

An in situ enclosure experiment was performed from June to September, 1982, in Lake O'Woods, West Virginia (pH7.0), to examine the effects of acidification on the Bosmina longirostris population. Treatments were two control enclosures and two enclosures acidified from an initial pH of 7.2 to pH 4.2 over a 44 day period, using H₂SO₄. B. longirostris was not adversely affected by the acidification. In fact, its abundance, biomass, and mean body size increased in the acid treatment, as compared withe control. This positive response was probably not a direct consequence of the altered water chemistry, but instead may have occured due to reduced competition for food, and also reduced predation pressure. Acidification resulted in a decreased abundance of other herbivores, and elimination of the dominant carnivore, Mesocyclops edax.     

Effects of Anisops sardea (Hemiptera: Notonectidae) on oviposition habitat selection by mosquitoes and other dipterans and on community structure in artificial pools

We investigated the effects of the predatory backswimmer, Anisops sardea, on oviposition habitat selection of mosquitoes and other dipterans, and on community structure in experimental pools. We predicted that those dipteran species whose larvae were shown to be highly vulnerable to predation by Anisops would avoid Anisops pools when choosing an oviposition site. We established the following treatments in plastic tubs: (1) Control (without Anisops); (2) Free Anisops (ten Anisops within pool but not caged); (3) Caged Anisops (ten Anisops in cage). The pools were open to colonization by insects. We added resting stages of crustaceans and first instar larvae of the mosquitoes Culiseta longiareolata and Culex laticinctus. Among the dipteran species, Culiseta longiareolata, Culex laticinctus, Chironomus riparius (Chironomidae) and Forcipomyia sp. (Ceratopogonidae), only Culisetalarvae were highly vulnerable to predation. As predicted, based on larval vulnerability, Culiseta, but not the other species, avoided Anisops pools when ovipositing. Free Anisops reduced taxon richness. This reduction resulted largely from the elimination of the cladoceran Ceriodaphniasp. and Culiseta in most free Anisops pools. Thus, Anisops sardea structures the community, both by a behavioral response of prey to its presence and by consumption of prey.     

Effects of early adult experience on host selection in insects: Some experimental and theoretical results

A field experiment with Drosophila melanogasterrevealed that when flies encounter a particular food type soon after emergence, the probability of their subsequently being attracted to such a resource is increased. In this experiment, the length of time flies experienced their postemergence environments was under the control of the flies themselves. The experiment thus realistically mimicked one form of experiential effect that may be important in nature. A theoretical model is developed which shows that enhanced adult preferences for the types of resources fed on as larvae can substantially increase the degree of host-based genetic subdivision within a polyphagous population.     

Genetic variation and population structure inOryza malampuzhaensis Krish.et Chand. endemic to Western Ghats, South India

Oryza malampuzhaensis Krish.et Chand. (2n = 4x = 48; Poaceae,Oryza) is endemic to Western Ghats, South India, and shows a highly localized distribution over a small geographical area in this region. This is the most poorly understood taxon in genusOryza and is often misidentified asO. officinalis owing to their close morphology. We assessed the nature and distribution of genetic variation among 11 populations ofO. malampuzhaensis using random amplified polymorphic DNA markers. The analysis revealed low genetic variation inO. malampuzhaensis. Cluster analysis of pairwise genetic distances of populations revealed three distinct clusters and the grouping of populations largely corresponded to their geographical proximity. Restricted gene flow and a geography-dependent differentiation were evident among populations. The altitude-influenced differences in ecological factors among the natural habitats of the populations seem to be the cause of the geography-dependent differentiation. Genetically isolated smaller populations and a narrow genetic base inO. malampuzhaensis point to its vulnerability to genetic drift and genetic depauperation. ThusO. malampuzhaensis appears to be under the threat of extinction and needs to be conserved by use of suitable methods. The present study also identified molecular markers diagnostic forO. malampuzhaensis.     

Allozyme variation and population genetic structure of Betula alnoides from Guangxi,China

Allozyme variation and population genetic structure of Betula alnoides Buch. Ham. ex D. Don in 11 natural populations from Guangxi Zhuang Autonomous Region, China, were investigated by starch gel electrophoresis. Variation at 15 loci from 10 enzyme systems was analyzed. Allozyme analysis revealed a high level of genetic variation in this species, with percentage of polymorphic loci (P _p ), the average number of alleles per locus (A _p ), and the expected heterozygosity (H _ep ) being 55.2%, 2.0, and 0.204, respectively, which exceeds the average level among out-crossing wind-pollinated woody species at the population level. At the species level, P _s , A _s , and H _es were 60.0%, 2.67, and 0.206, respectively.The observed heterozygosity (H _op ) was higher than H _ep , indicating the existence of natural selection against homozygotes. The negative fixation index (F = –0.216) implied a significant excess of heterozygosity at the population level. Among-population differentiation (F _ST ) accounted for 4.0% of the total variation. No significant correlation was detected between the genetic distance and geographic distance among populations. Extensive gene flow was inferred, based on the allozyme data )N _m = 6.000 from F _ST , N _m = 5.605 from the private allele method). The results demonstrated that the fragmentation status of B. alnoides had no remarkable effects on the population genetic structure of this species. Some populations are recommended for both in situ genetic conservation and germplasm collection for breeding programs.     

Mating biology and population structure of the ant, <Emphasis Type="Italic">Leptothorax gredleri</Emphasis>

Summary Female sexuals of the ant Leptothorax gredleri attract males by sexual calling. In an experimental set-up allowing for competition among males, both female and male sexuals copulated with up to four partners, with the median being one mate in both sexes. Neither male nor female sexuals invariably mated with the first partner they encountered, but we could not find any morphological difference between sexuals that succeeded in mating multiply and those that copulated only once. Males did not aggressively compete for access to the female sexuals. According to microsatellite genotyping, workers produced by multiply mated queens were all offspring of a single father, i.e. queens appear to use sperm from a single mate to fertilize their eggs. Population genetic studies revealed a strong population subdivision, suggesting that both male and female sexuals mate in the vicinity of their maternal nests and that gene flow is strongly restricted even between forest patches isolated only by a few meters of grassland.     

Genetic variation, genetic structure and effective population size in the tropical holoparasitic endophyteBdallophyton bambusarum (Rafflesiaceae)

The genetic population structure inBdallophyton bambusarum, an endoparasite, was studied in ten subpopulations from a subdeciduous tropical forest in Veracruz Mexico. The sample was analyzed using seven polymorphic loci in cellulose acetate electrophoresis. Isozyme data indicated that the subpopulations ofB. bambusarum contained high genetic variability (H_ep = 0.452 ± 0.045, S.E.). Our analysis suggests that almost each inflorescence ofB. bambusarum is an individual. The subpopulations studied were genetically similar (average Nei's genetic identity 0.941 ± 0.051 and F _st values 0.097 ± 0.026), suggesting that genetic differentiation among subpopulations was small. Direct estimates of effective population size was derived from observations of three fluorescent dyes, and from the genetic neighborhood area derived from these data. The neighborhood area, multiplied by the total density of individuals, gave an N_e = 124.84 plants, and when corrected to consider the proportion of males and females gave an N_e = 118.59 individuals. An indirect estimate of Nm was obtained from the F _st values (mean Nm=2.037), giving an indirect estimate of the effective population size N_b = 12.8 individuals. Both values are relatively high when compared to other plant studies. The gene flow and/or effective populations size of the studied subpopulations ofB. bambusarum are believed to be large enough to prevent differentiation among subpopulations due to genetic drift.     

Distribution pattern, population structure, and growth of Septifer bilocularis (Bivalvia: Mytilidae) on reefs of Southern Vietnam

The features of the spatial distribution, size and age structure of populations, and growth of the bivalve mollusk Septifer bilocularis (Linnaeus, 1758) were investigated for the first time in the waters of Vietnam. It is shown that in the subtidal area, on reefs well protected from wave effects, S. bilocularis occupies a corallogenous substrate constructed of dead branchy colonies of Acropora and Porites and forms settlements there with a population density and biomass exceeding those in populations located along the open coast and developing on massive coral colonies of the genera Porites, Favia, and Favites. The extensive accumulation of sediments in the inner part of Vanphong Bay (Khanh Hoa Province) correlated with a decrease in the size and age ranges for populations of S. bilocularis. It is found that in places with regularly active hydrodynamics the mollusks grew somewhat more slowly and formed a more convex shell than on reefs protected from surf. The longevity of S. bilocularis estimated both from the greatest observed age and from the growth equation of Bertalanffy was somewhat greater in the former case compared with the latter one, but in general did not exceed 11 years for both cases. The results of the studies are discussed from the position of morphophysiological adaptations of sessile bivalve mollusks to life in contrasting environmental conditions of the upper subtidal zone.     

Comparison between community structures of fishes in <Emphasis Type="Italic">Enhalus acoroides</Emphasis>- and <Emphasis Type="Italic">Thalassia hemprichii</Emphasis>-dominated seagrass beds on fringing coral reefs in the Ryukyu Islands,Japan

To clarify differences in community structures and habitat utilization patterns of fishes in Enhalus acoroides- and Thalassia hemprichii-dominated seagrass beds on fringing coral reefs, visual censuses were conducted at Iriomote and Ishigaki islands, southern Japan. The numbers of fish species and individuals were significantly higher in the E. acoroides bed than in the T. hemprichii bed, although the 15 most dominant fishes in each seagrass bed were similar. Cluster and ordination analyses based on the number of individuals of each fish species also demonstrated that fish community structures were similar in the two seagrass beds. Species and individual numbers of coral reef fishes which utilized the seagrass beds numbered less than about 15% of whole coral reef fish numbers, although they comprised about half of the seagrass bed fishes. Of the 15 most dominant species, 5 occurred only in the two seagrass beds, including seagrass feeders. Ten other species were reef species, their habitat utilization patterns not differing greatly between the two seagrass beds. Some reef species, such as Lethrinus atkinsoni and L. obsoletus, showed ontogenetic habitat shifts with growth, from the seagrass beds to the coral areas. These results indicate that community structures and habitat utilization patterns of fishes were similar between E. acoroides- and T. hemprichii-dominated seagrass beds, whereas many coral reef fishes hardly utilized the seagrass beds.     

Strong habitat selection and the development of population structure in a mosquito

Summary Remarkably simple and regular patterns of genic variation for two electrophoretically assayed loci are presented against a background of strong habitat selection, population isolation, and population structuring in the pitcher-plant mosquitoWyeomyia smithii. The genic variation assessed thus far comes from 29 local populations, all but one of which lie within the glaciated region of North America. Although the conclusions drawn from these patterns are still quite tentative, it appears that the fact that the same higher frequency electromorphs occurred everywhere for both loci may be due more to selection than drift. Drift or selection, acting alone or in concert may be responsible for the almost complete absence of electromorphs of intermediate frequency. A weak association was found between the two-locus genic variation and the well-studied diapause-development time variation in this species. It is suggested that a Wrightian interaction system and fluctuating-stabilizing selection might account for maintenance of the genic level polymorphism. The observations reported are circumstantially in accord with Wright's shifting balance process of evolution. Future studies in the older parts of the species range will put these hypotheses to a strong test.     

Genetic diversity and population structure of wolverine (Gulo gulo) populations at the southern edge of their current distribution in North America with implications for genetic viability

The current range of wolverines (Gulo gulo) within the lower 48 states includes small, remnant populations in Idaho, Washington, Wyoming and Montana. The size and trend of each of these populations and connectivity to adjacent populations in the contiguous United States and Canada are poorly understood. In this study, levels of genetic diversity and population genetic structure were examined in three states (Idaho, Wyoming, and Montana) and two Canadian provinces (Alberta and British Columbia) using both mitochondrial (mtDNA) and nuclear microsatellite DNA. Restricted levels of gene flow were detected among these populations with mitochondrial and nuclear DNA and our observations suggest a pattern of male-mediated gene flow. Populations in the United States appear to be receiving migrants from Canada, however, substantial genetic differentiation suggests that gene flow may not be high enough to prevent genetic drift. Our analyses suggest that at least 400 breeding pairs or 1–2 effective migrants per generation would be needed to ensure genetic viability in the long-term for each of the populations in the United States. Significant matrilineal structuring and restricted female gene flow indicates that demographic viability will depend upon the movement of female wolverines into new territories. Results from this study provide guidelines for conservation and management and indicate the need for more ecological data.     

Hide and seek on two spatial scales – vegetation structure effects herbivore oviposition and egg parasitism

Habitat parameters as well as host density and distribution play an important role in host-parasitoid interactions. Vegetation structure can affect both, herbivorous insects searching for places for egg deposition and foraging egg parasitoids. Herbivores might escape egg parasitism by depositing their eggs on sites with vegetation factors unfavourable for host searching parasitoids. The importance of a particular vegetation parameter for the host-parasitoid relationship, however, might depend on the spatial scale. In this study the influence of vegetation structure on oviposition of the polyphagous tansy leaf beetle Galeruca tanaceti L. (Coleoptera: Chrysomelidae) and on egg parasitism by the specialist egg parasitoid Oomyzus galerucivorus was investigated on two spatial scales in the field. Factors like vegetation height and coverage, presence of host plants and microclimate were measured and correlated with presence of herbivore eggs as well as incidence or rate of parasitism. On a larger scale 25 sites (patch size = 300–2000 m²) of suitable semi-arid grassland in different successional stages were studied. The probability that egg clutches of G. tanaceti were present at a site increased with a decreasing area covered by shrubs. Parasitism rates on the macro-site scale showed the same trend and were also higher at sites with a lower percentage of shrubs covering the area.On a smaller scale vegetation structure was investigated using 40 patches (patch size = 1 m²). While the best predictor for the presence of beetle egg masses on this scale was the complexity of vegetation, parasitism was influenced by the presence of a host plant, Achillea millefolium, and a sunny microclimate.In Wirt-Parasitoid Interaktionen spielen Wirtsdichte und -verteilung, aber auch Habitat-Parameter eine wichtige Rolle. Die Vegetationsstruktur kann sowohl herbivore Insekten auf der Suche nach Ei- ablageplätzen als auch ihre Eiparasitoide auf der Suche nach Wirten beeinflussen. Herbivore Insekten könnten einer Eiparasitierung dadurch entgehen, dass sie ihre Eier an Orte legen, die für Parasitoide ungünstige Vegetations-Parameter aufweisen. Die Bedeutung eines Vegetations-Parameters auf die Wirt-Parasitoid Beziehung kann von der räumlichen Skala abhängen, die untersucht wird. Diese Arbeit untersucht den Einfluss der Vegetationsstruktur auf die Eiablage des polyphagen Rainfarnblattkäfers Galeruca tanaceti L. (Coleoptera: Chrysomelidae) und auf die Parasitierung durch den spezialisierten Ei-Parasitoiden Oomyzus galerucivorus auf zwei räumlichen Skalen im Freiland. Verschiedene Vegetations-Parameter wie die Vegetationshöhe, der Deckungsgrad, das Wirtspflanzenvorkommen und das Mikroklima wurden gemessen und mit dem Vorhandensein von Eigelegen und Parasitierung korreliert. Auf der großflächigen Skala wurden 25 Flächen (Größe = 300–2000 m²) semiariden Grasslands in verschiedenen Sukzessionsstadien untersucht. Die Wahrscheinlichkeit, dass Eigelege von G. tanaceti auf einer Fläche gefunden wurden nahm mit sinkendem Verbuschungsgrad zu. Die Parasitierungsrate auf der großflächigen Skala nahm ebenfalls mit sinkendem Verbuschungsgrad zu.Kleinflächig wurde die Vegetationsstruktur in 40 Quadraten (Größe = 1 m²) untersucht. Während die Eiablage des Käfers am stärksten von der Komplexität der Vegetation beeinflusst wurde, wurde die Parasitierungsinzidenz durch das Vorkommen der Wirtspflanze Achillea millefolium und durch ein sonniges Mikroklima bestimmt.     

Population dynamics and population structure ofFerrissia wautieri (Mirolli, 1960) (Gastropoda,Ancylidae) in a pond near Nijmegen (The Netherlands)

Ferrissia wautieri, a freshwater limpet, is a widely distributed species in The Netherlands. In a pond near Nijmegen samples were taken twice a month over the year to study the population size and structure of this species in relation to the water temperature. Only ancyloids were found. Production of juveniles is temperature-dependent; peak numbers occurred in July and August. Just-hatched juveniles (shell length 0.6–1.0 mm) occurred over a lengthy period in the year, but were absent in March and April. During these months the collected numbers of specimens were very low. The largest specimens were collected during March, April and May.     

Colour polymorphism in relation to population dynamics of the leaf beetle, Chrysomela lapponica

We studied colour morph diversity and frequencies of light and dark morphs in non-fluctuating and fluctuating populations of willow feeding leaf beetle Chrysomela lapponica in the Kola Peninsula, NW Russia. Population-specific Shannon–Weaver diversity index positively correlated with dark morph frequencies, indicating that the larger part of colour polymorphism is related with numbers and diversity of dark morphs. Among-population variation in studied characters was not explained by pollution load or predation rates, but depended on the type of the population and the stage of density change in the fluctuating populations: both colour morph diversity and frequency of dark morphs were low in declining post-outbreak populations but equally high in non-fluctuating populations and in fluctuating populations at peak densities. In time-series, both diversity index and frequency of dark morphs decreased with post-outbreak density decline in the fluctuating population, but did not change in the non-fluctuating population. In the experiment, when adults received low quality food (plants from post-outbreak site), mortality of dark morphs during the hibernation was almost doubled relative to the mortality of light morphs, whereas on high quality food the colour morphs demonstrated similar mortality. This may indicate, that decrease in colour polymorphism extent and dark morph frequencies in the declining populations is due to selective mortality of dark morphs imposed by density dependent (induced by heavy herbivore damage during an outbreak) decrease in host-plant quality (delayed inducible resistance, DIR). DIR is known as one of the factors driving herbivore populations, but our result is the first evidence that DIR may act as a factor of natural selection. Dark morphs are not only susceptible to low food quality, but also have smaller size compared to light morphs, and therefore the dark females are presumably less fecund. Thus, decrease in frequency of low-fitness (dark) individuals in post-outbreak populations and accumulation of low-fitness phenotypes at the popu-lation peak may create feedbacks contributing to regulation of density fluctuations in Ch. lapponica.     

Effect of population spatial structure on pollination and seed set of a clonal distylous plant,Persicaria japonica (Polygonaceae)

In order to assess the mechanisms through which the spatial structure of the population influences female reproductive success, spatial distribution of clones, degrees of limitation of legitimate (inter-morph) pollination, type and abundance of pollen loaded on the stigmas, and seed set were measured for many clones of two natural populations of the distylous clonal plant,Persicaria japonica. Within the populations, according to the spatial relation to the nearest opposite morph clone, individual clones were assorted into two spatial types,i.e., clones that congregated with clones of the opposite morph (congregating clones), and clones that occurred singly at a considerable distance from the nearest opposite-morph clone (single clones). The pollination success,i.e., the proportion of legitimately pollinated flowers, and seed set were severely limited in the single clones compared to the congregating clones. Since artificial legitimate pollination improved the seed set in single clones, at least to some degree pollination failure was responsible for the reduced seed set in the single clones.     

RAPD variation and population genetic structure of Physalaemus cuvieri (Anura: Leptodactylidae) in Central Brazil

Studies about the organization of the genetic variability and population structure in natural populations are used either to understand microevolutionary processes or the effects of isolation by human-inducted landscape modifications. In this paper, we analyzed patterns of genetic population structure using 126 RAPD loci scored for 214 individuals of Physalaemus cuvieri, sampled from 18 local populations. Around 97% of these loci were polymorphic. The among-population variation component (Φ_ST) obtained by AMOVA was equal to 0.101 and θ ^B obtained using a Bayesian approach for dominant markers was 0.103. Genetic divergence, analyzed by Mantel spatial correlogram, revealed only a short-distance significant correlation between genetic and geographic distances. This is expected if low levels of population differentiation, due to high abundance buffering the effect of stochastic processes, are combined with low spatially restricted gene flow. Although this may be consistent with the current knowledge of species’ biology, the spatial distribution of local populations observed in this study also suggest that, at least in part, recent human occupation and habitat fragmentation may also explain part of the interpopulational component of the genetic variation.     

The effects of harvesting of the South African kelp (Ecklonia maxima) on kelp population structure, growth rate and recruitment

Ecklonia maxima is an economically important kelp in South Africa. The harvested kelp is used mainly as feed for cultured Haliotis midae (abalone) on farms all along the South African South and West Coast. The effects that different harvesting methods have on the growth of sub-canopy kelps, kelp population structure and kelp recruitment were tested in a kelp bed at Bordjies Rif near Cape Town. Two 30 × 10 m sites were set up, about 100 m apart, in near monoculture stands of E. maxima. Each 30×10 m area was subdivided into three treatments. In treatment 1 (T1) the whole ‘head’ of each kelp sporophyte that reached the surface was cut off between the bulb and the primary blade (‘lethal’ method). In treatment 2 (T2) (‘non-lethal’ method), the secondary fronds of all sporophytes that reached the surface were cut 20–30 cm from their junction with the primary blade, and removed. In the control plot, the kelp plants were not treated. Harvesting treatments were done approximately every four months, at low spring tide, from 3 March 2003 to 3 November 2003 (three treatments). The effects of harvesting on the kelps depend largely on the size of plant and the time the fronds were removed; however, no seasonal pattern could be observed. The different treatments had no effect on the growth rate, population structure or recruitment of the kelp. This means that factors other than light play an important role in the growth, structure and recruitment of the kelp beds in False Bay. Results are discussed in relation to current commercial harvesting practices.