Spatial Distribution Of Individuals Research Articles

The impact of phenology, exposure and instar susceptibility on insecticide effects on a chrysomelid beetle population

Direct topical impact of an insecticide spray on a population of a non-target leaf-eating beetle, Gastrophysa polygoni, was studied, and the relative importance of phenology, instar susceptibility and instar specific exposure was evaluated. Two insecticides, cypermethrin and dimethoate, were investigated. In the laboratory, topical toxicity to eggs, second-instar larvae and adults was recorded in dose-response experiments. The spatial distribution of larvae and eggs were measured in the field. Deposition of insecticide onto eggs, second-instar larvae and adult specimens was measured at different positions within the crop canopy by use of a dye tracer technique. A temperature-driven population model was constructed to simulate population development of all life stages in the field. The model was based on laboratory measures of growth and development at various temperatures. Mortality due to direct insecticide exposure was calculated as a function of population demography, spatial distribution of individuals, spatial deposition of the insecticide, and stage-specific susceptibility. Cypermethrin had the greatest impact, reducing population size by 19–32%. The life stages most sensitive to cypermethrin were the larval instars. As the population developed from eggs to larvae and imagines, the impact of one spraying first increased and then decreased according to the proportion of larvae in the population. Dimethoate had less effect on the population, i.e. 1·9–7·6% reduction. Dimethoate was most toxic to the egg stage, and consequently the effect on the population decreased as the proportion of eggs decreased due to hatching. The direct effect of insecticide spraying was significantly affected by all three factors investigated, i.e. phenology, life stage susceptibility and stage-specific exposure. The latter factor is composed of both spray flux at various spatial positions in the canopy and the ability of different life stages to retain spray droplets. © 1998 SCI

Behavioral responses of the sea urchin Tetrapygus niger to predators and food

To test the importance of predators and food availability in determining patterns of aggregation and movement in the urchin Tetrapygus niger, we studied the behavioral responses of animals with different body sizes under laboratory conditions. Experiments were carried out on a square platform in tanks with circulating water. Twelve sea urchins of three size ranges were placed following a uniform spatial distribution on the platform and maintained in the presence or absence of: (a) one predator (the fish Pinguipes chilensis or the asteroid Meyenaster gelatinosus), or (b) food (fronds of the brown kelp Lessonia nigrescens). The experiments were followed with a video camera. The movement patterns of the sea urchins, the number and size of aggregations, and the dispersion patterns were quantified. To determine the importance of aggregation as a defensive mechanism, artificial aggregations of large sea urchins were created on the platform in the presence or absence of predators. Behavioral responses were followed and analyzed as described above. Individuals of T. niger recognize the presence of predators, and respond with increased mobility (e.g., number of sea urchins moving or average speed of movement; i.e., escape responses) but do not increase the number or size of aggregations. Moreover, the increased mobility of sea urchins would account for the random spatial distribution of individuals at the end of the experiments with predators. The escape responses were specially marked in the presence of the starfish M. gelatinosus. In this case, the aggregations were unstable, suggesting that the aggregations of T. niger would not have a protective function against this starfish. By contrast, sea urchins aggregated in the presence of food. It is concluded that the generation of aggregations in this species would be more related to feeding patterns (or to other factors related to spatial heterogeneity; e.g., substrate topography) than to responses to predators. The escape behavior of sea urchins to starfish is also described.

Response of Deer Mice (Peromyscus maniculatus) to shrubs in shortgrass prairie: linking small‐scale movements and the spatial distribution of individuals

1. Organisms respond to the abundance and spatial distribution of resources at both individual and population scales but there have been relatively few attempts to link insights from studies of these different phenomena, especially for wide‐ranging vertebrates. 2. Deer Mice (Peromyscus maniculatus) were live‐trapped and tracked across a gradient of shrub cover in shortgrass prairie to determine patterns of abundance, microhabitat use and movements. 3. In areas with few shrubs, mice preferred shrub microhabitats and their movement trails were relatively straight. Both trail tortuosity and population density increased with increasing shrub cover, so that mice tended to accumulate in areas where their trails were most convoluted. However, movements were also linear where shrubs were dense, presumably because mice could achieve the benefits of association with shrubs without travelling directly beneath them. Areas with dense shrubs also had high but variable population densities, suggesting that other factors such as intraspecific interactions may have affected movements on a larger, home‐range scale. 4. Apparent thresholds in the selective vs random use of shrubs, movement patterns and abundance occurred over a narrow range of shrub cover where shrubs were most aggregated, underscoring the importance of both shrub density and dispersion. Non‐linear relationships in the response to resources at both behavioural and population scales thus may complicate our ability to predict abundance from individual movements across a broad range of resource distributions.

Conservation in relation to mating system in Nesohedyotis arborea (Rubiaceae), a rare endemic tree from St Helena

A field survey of Nesohedyotis arborea reveals that there are 132 adult trees in the wild, making it one of the commoner endemic plants of St Helena. The trees exhibit leaky dioecy: females are entirely male-sterile, but ‘males’ do set a small amount of seed. The floral differences between the two morphs are marked: the female is long-styled with large, smooth stigmatic papillae and vestigial anthers, while the male is short-styled with small, rugose stigmatic papillae and large anthers overarching the style. The dioecy probably evolved autochthonously on the island from an ancestral heterostylous condition. The partial loss of female function in males is associated with the production of smaller ovules: approximately a third of the volume of the female ovules. The sex ratio of adult trees is 3:2 in favour of males, due to greater allocation of resources in vegetative survival by males. Pollination is by small syrphid flies and is highly efficient up to distances of 50 m, and most of the gene flow in the population is contributed by pollen transport rather than seed dispersal. Although the population has been fragmented by human land-use patterns, rather few individuals are completely reproductively isolated. An assessment is given of the implications for conservation of the patterns of gene flow and spatial distribution of individuals. The mating system of Nesohedyotis reduces the effective population size, but this is likely to be more than outweighed by the effective outbreeding that it promotes. However, subpopulations should be monitored to assure co-sexuality, with, if appropriate, supplementary planting of opposite sex individuals.

Patchy Landscapes and Animal Movements: Do Beetles Percolate?

Simple neutral percolation models of ecological landscapes predict a critical threshold of landscape connectivity at ca 60% coverage of cells that are permeable to movement. Using such models as an inspiration, we conducted a field experiment in which we employed an experimental model system of tenebrionid beetles (Eleodes obsoleta) moving through mosaics of grass and bare ground in which grass coverage was systematically varied from 0% to 80% in a random pattern. Three parameters of beetle movement pathways (mean step length, mean vector length, and net displacement rate) all showed a sharp reduction between 0% and 20% grass cover and no differences over further 20% coverage increments; the mean fractal dimension of pathways (which could not be derived for the 0% coverage treatment) also did not differ with increasing grass coverage from 20% to 80%. The proportion of time spent by beetles in grass patches did not increase with increasing grass coverage, although individuals spent significantly more time steps in grass in the 20% grass-cover treatment than would be expected by chance. The distance moved per time step was greater when beetles moved over bare ground than in grass, but the mean step length while on grass was significantly lower when grass constituted only 20% of the experimental landscape than when more grass was present, perhaps because beetles stopped more frequently on grass when little of it was present. The threshold in beetle movement through the experimental landscapes differed markedly from that predicted by simple neutral percolation models, probably because, in contrast to the models, both cover types were permeable to beetle movements and individuals moved nonrandomly. The results of this experiment indicate that definitions of landscape connectivity depend on both the spatial pattern of the landscape and how individuals move within and among patches. Moreover, it appears that movement patterns within a particular patch may be contingent on the characteristics of the surrounding landscape. As a consequence, a land-cover map alone may not predict whether, to a particular kind of organism, a landscape is fragmented or connected, and the spatial distribution of individuals in a population may not map closely onto the distribution of suitable habitat patches. This finding suggests that assessments of metapopulation structure must consider both overall landscape patterns and the nonlinear responses of organisms to such patterns.

Microhabitat selection by the Iberian rock lizard Lacerta monticola: Effects on density and spatial distribution of individuals

We describe microhabitat selection in the Iberian rock lizard Lacerta monticola, a species endemic to some high mountains of the Iberian peninsula. This species is potentially threatened, in particular, by changes in habitat conditions on the ski slopes and from associated buildings in resorts that proliferate precisely in areas it favours. We analyse the relationship between habitat physical structure and population densities, in order to establish a predictive habitat-population model. Lizards select large rock screes and Cytisus bushes, but scarcely use Juniperus bushes and grassy areas. Individuals have an aggregated spatial distribution. The application of this habitat model may allow predictions to be made about the response of L. monticola to artificial and natural habitat changes, and could help to manage suitable habitats for populations.

Competition in Foraging Groups

Individuals foraging in groups search for food patches and exploit the food found. Information sharing is used when the foragers search for food but also take benefit of prey in patches located by others (Ward and Zahavi 1973, Clark and Mangel 1984). This behaviour directly implies improved food-finding rate with group size by speeding up the patch discovery time (Caraco 1981), but it also means that the prey found have to be shared (Clark and Mangel 1984, 1986, Vickery et al. 1991). The simplest models on group foraging with information sharing assume that individuals share food in proportion to group size, 1/n (Clark and Mangel 1984, 1986), the food-intake rate thus being equal for all group members. Recent research has, however, emphasized that presence of unequal competitors greatly affects not only individual food intake rate (Milinski 1988, Milinski and Parker 1991) but also spatial distribution of individuals assigned to different phenotypes (Sutherland and Parker 1985, 1992, Parker and Sutherland 1986). This tradition largely builds on the theory of ideal free distribution (Fretwell and Lucas 1970). Our task here is to widen the scope to further explore consequences of individual differences in competitive ability to foraging group structure (Lindstr6m and Ranta 1993, Ranta 1993, Ranta et al. 1993, 1994, 1995). For this purpose we shall make use of a simple competition model.

Dynamic process, spatial pattern and species coexistence in plants

The significance of dynamic processes of individual genets/ramets for the spatial pattern of plant species and community structure is discussed. It is suggested that under a different mode of competition (symmetric vs. asymmetric), spatial distribution of individuals, initial size distribution at the establishment stage and boundary conditions as recruitment influence differently the species coexistence pattern. It is therefore important to consider the mode of competition for the study of community structure. To know the mode and degree of intra- and interspecific competition, the dynamic processes of individual genets/ramets must be studied by following the growth, mortality and recruitment of each genet/ramet of each component species in a plant community. The models and methods of plant population ecology are therefore useful also for plant community ecology.

The effects of density and spatial distribution on selection for emergence time in Prunella vulgaris (Lamiaceae)

We investigated the effects of both overall density and variation in local density on the relationship between emergence time and final biomass in Prunella vulgaris. The relationship between emergence time and final biomass was used to quantify the pattern of selection on emergence time. Seeds were planted in flats in three different spatial distributions (hexagonal, random, high variance) at each of three overall densities (308, 769, and 3,077 seeds/m2). Individual seedlings were marked upon emergence, and their final biomass was determined after 90 days of growth. With increasing overall density, mean plant biomass decreased, but the coefficient of variation in biomass and the magnitude of directional selection for early emergence increased. Increasing variation in the spatial distribution of the plants had no effect on mean plant biomass but did significantly increase the coefficient of variation in biomass at both low and medium densities. Both the magnitude of directional selection and the curvature in the relationship between emergence time and final biomass tended to increase with increased variation in the spatial distribution. Our results suggest that both overall plant density and the spatial distribution of individuals can affect the pattern of selection on plant traits.

The reflection of deciduous forest communities in leaf litter: implications for autochthonous litter assemblages from the fossil record

To assess the degree to which forest litter reflects the source forest, three 1-ha plots of temperate deciduous forest were mapped and litter accumulating in these forests was sampled. Identity, position, and diameter of all stems 2 cm or larger diameter at breast height are known for each forest. Composition of the leaf litter is governed by two key factors: (1) abscised leaves are deposited primarily on the forest floor directly underneath the canopy that produced them, and (2) the leaf mass of a species is highly correlated with its stem cross-sectional area. These factors produce autochthonous litter samples that correspond closely in composition to the forest within a circle of canopy-height radius or less. Even relatively small litter samples (350 leaves) consistently contained all the common species in the local area. However, the rarer tree species were seldom recovered in the litter samples. Correlation coefficients for litter mass and basal area by species are typically over .80.These observations have three important implications for interpreting autochthonous compression-fossil assemblages. First, approximate relative abundances of locally dominant and subdominant forest taxa can be obtained from relatively small samples of autochthonous compression-fossil assemblages. Second, representation of rare forest species, even in large fossil samples, will be fortuitous. For this reason, complete species lists and consistent estimates of richness cannot be derived directly from most existing samples of autochthonous compression-fossil assemblages. Third, the strong tendency for leaves to fall beneath the canopy of the tree that sheds them suggests that properly sampled autochthonous fossil leaf assemblages may yield information on crown size of individual trees and the spatial distribution of individuals and species, aspects of vegetational structure that have been thought accessible only in well-preserved “fossil forests” with standing trunks.

EFFECTS OF POPULATION STRUCTURE AND CONE PRODUCTION ON OUTCROSSING RATES IN ENGELMANN SPRUCE AND SUBALPINE FIR.

Variation in the mating systems of Engelmann spruce and subalpine fir was examined in two adjacent subpopulations in the Colorado Front Range. Multilocus outcrossing rates were estimated from analysis of allozyme variation in embryo and megagametophyte tissue. The overall multilocus outcrossing rate of 0.93 for spruce was significantly higher than the outcrossing rate of 0.89 for fir. Outcrossing rates varied from 0.73 to 0.97 for spruce and from 0.65 to 0.94 for fir when trees of each species were grouped according to age, size, spatial distribution, maternal heterozygosity, cone production, and year of sampling. In both spruce and fir, trees with higher levels of male-cone production and/or more clumped spatial distribution had lower outcrossing rates. Temporal variation in outcrossing rates within subpopulations, examined only in spruce, was less than variation between subpopulations. Male-cone production was significantly correlated with tree size, and regression analyses revealed that the highest outcrossing rates are expected on trees that are medium to large in size, rather than the largest trees in the forest. The results show that, while there is some temporal variation, patterns of male cone production and spatial distribution of individuals are the most important factors influencing outcrossing levels in these wind-pollinated forest trees.

Neighborhood models of plant population dynamics. 2. Multi-species models of annuals

Models are developed for the dynamics of multi-species communities of annual plants that lack seed dormancy. These models explicitly include plastic plant growth, the spatial distribution of individuals, and the fact that individuals interact primarily with nearby individuals. Because the models are based on submodels of individual plants (fecundity, survivorship and dispersal, and how these are affected by inter-individual interactions), they provide explanations of community-level phenomena in terms of the biology of individuals. All model parameters and functional forms may be estimated from data obtained in simple experiments of a single years's duration. The models are used to examine the community-level consequences of some types of inter-individual interactions that have been reported in the ecological literature. In addition, the models are used to demonstrate that dispersal may markedly influence the outcome of competition among plant species, even in a physically homogeneous environment, due to an effect of dispersal on the spatial distribution of individuals.

Ecology of the terrestrial snail (Brephulopsis bidens): age composition, population density and spatial distribution of individuals

Studies were carried out during the spring, summer and autumn seasons of 1974–1976 on a natural population of the terrestrial snail Brephulopsis bidens (Pulmonata: Enidae), located on a mountain in Crimea (USSR). The overall population was characterized by a high density (50.2–124.4 ind/m2 in 1974, 61.3–162.5 ind/m2 in 1975 and 27.8–157.3 ind/m2 in 1976) and a strongly pronounced congregational tendency of snails. There was a significant correlation (r=087, P <O.O1) between the mean number and size of snail colonies on random 10 × 10 m areas during active season. The proportion of sexually mature snails reached maximum (70–80%) in autumn before the hibernation, and was minimal (about 25%) in the spring. The better winter survival of the pre‐adolescent snails (81.7%vs 56.3%) and their earlier emergence from the ground following hibernation, both explain the preponderance of young molluscs observed in the spring. Density and spatial distribution of individuals were subject to considerable seasonal and spatial variability. Seasonal variation of the active snails density in dry years seemed to depend on the amount of rainfall (r=0.93). Spatial distribution of the molluscs significantly correlated with humus as well as with the nitrogen and magnesium ion concentration.

Feeding Ecology of Hummingbirds in the Highlands of the Chisos Mountains, Texas

-The feeding habits of three species of hummingbirds were examined during the summer of 1975 in the highlands of the Chisos Mountains, Big Bend National Park, Texas. The Blue-throated Hummingbird (Lampornis clemenciae), the Broad-tailed Hummingbird (Selasphorus platycercus) and the Rufous Hummingbird (S. rufus) were common in the study area, the first occupying the mesic Cypress-Pine-Oak Association and the other two species inhabiting the Pinyon-Juniper-Oak Woodland. Blue-throated Hummingbirds were almost exclusively insectivorous prior to the flowering of mountain sage (Salvia regla), after which their diet began to include its nectar. Males fed outside their territories in the morning at nectar sources in the woodland. Selasphorus hummingbirds overlapped extensively in diet. S. platycercus was common prior to the full flowering of sage, and migrating S. rufus entered the area later, when the sage was in bloom. Males of both species typically established territories around large patches of the sage and open areas while females were relegated to smaller Salvia stands in the woodlands. Such a pattern is apparently maintained by the aggressiveness of the dominant males. Recent examinations of tropical nectarivorous guilds (Feinsinger 1976, Feinsinger and Colwell 1978) have shown that such communities are organized about the distribution of nectar resources. Niche differences among tropical hummingbirds may result from aggression (Wolf 1970, Lyon 1976, Wolf et al. 1976), different foraging patterns (Linhart 1973, Feinsinger 1976), microhabitat selection (Stiles and Wolf 1970), or the exploitation of specific resources by highly specialized species (Snow and Snow 1972). Far fewer hummingbird species inhabit temperate areas than the tropics, and most species are migratory. North American hummingbirds differ from one another ecologically in the same ways as their tropical counterparts. These differences include habitat selection (Pitelka 1951, Lyon 1973, Yeaton and Laughrin 1976), spatial distribution of individuals through territoriality (Pitelka 1942, Cody 1968), and variations in foraging patterns (Feinsinger and Chaplin 1975, Yeaton and Laughrin 1976). We examined the feeding ecology of three interacting hummingbird species with emphasis on their habitat partitioning and timing. We hypothesized that these characteristics would be important to the organization of a nectarivore guild where nectar sources are uncommon. Such conditions appear to occur in the highlands of the Chisos Mountains during the summer. STUDY AREA AND METHODS Data were obtained in the Chisos Mountains (29'15', 103'18') of Big Bend National Park, Brewster Co., Texas during the summer of 1975. The study area was located in Boot Canyon (2,072 m) and centered in two highland plant associations: the Cypress-Pine-Oak Association and the Pinyon-Juniper-Oak Woodland (Wauer 1973). We also examined open areas located within Boot Canyon because they contained patches of nectar-producing flowers and were often visited by the hummingbirds. The Cypress-Pine-Oak Association (henceforth woodland) occurs in the most mesic sections of the Chisos highlands, Pine and Boot canyons, where annual rainfall may exceed 500 mm (Wauer 1973). In the study area, this association occupies a narrow band along Boot Creek and is less diverse than the other woodland association. Arizona cypress (Cupressus Arizonica), pifion pine (Pinus cembroides) and Graves oak (Quercus gravesii) predominate. The Pinyon-JuniperOak Woodland (henceforth juniper woodland) is the most common plant association in the Chisos Mountains and in our study area. Typically, it contains pifion pine, emory oak (Q. emoryi), Graves oak and Mexican drooping (Juniperus flaccida), which can be found on north-facing slopes from 1,650 m to 2,300 m. Within Boot Canyon, the woodland adjoins the cypress woodland. Plant species diversity is greatest in the few open or exposed areas along upper Boot Creek, where herbaceous growth is abundant. Nectar sources were most plentiful in these areas.

Social Interaction among Chorusing Pacific Tree Frogs, Hyla regilla

A shift of call timing by Hyla regilla males in response to playback of recorded calls with various fixed repetition rates supports the hypothesis that alternate calling by chorusing males is true social interaction and not an artifact of body temperature. The observed patterns of phase shifting, inhibition of simultaneous calling and spatial distribution of individuals within the chorus support the hypothesis that social interaction serves to minimize call interference among neighboring males. This appears to be a strategy to maximize each male's chances of attracting a mate.