Spatial aggregation and association in different resource-patch distributions: experimental analysis with Drosophila

1. Laboratory experiments using homogeneous resources were conducted to examine intra- and interspecific spatial egg distribution of D. simulans, D. auraria and D. immigrans in three different resource-patch distribution patterns: patchy, even and clustered. 2. Individuals of each species were introduced separately or simultaneously into the cage, into which artificial substrates were placed and allowed to oviposit for 24 h. Spatial analyses were performed with indices of intraspecific aggregation (J), interspecific association (C) and L-function based on Ripley's K-function. 3. Eggs were always spatially aggregated irrespective of species and the resource-patch distribution patterns. Spatial egg aggregation was influenced significantly by the resource-patch distribution pattern and tended to be weaker in the clustered resource-patch distribution than in the patchy or even resource-patch distribution. 4. Spatial extent of egg aggregation was always beyond the single resource patch scale, indicating aggregation of ovipositing females. 5. Interspecific association of egg distribution was absent or very weak. Thus, these results present experimental evidence of independent egg aggregation among drosophilids.     

Predicting species distributions in poorly-studied landscapes

Conservationists are increasingly relying on distribution models to predict where species are likely to occur, especially in poorly-surveyed but biodiverse areas. Modeling is challenging in these cases because locality data necessary for model formation are often scarce and spatially imprecise. To identify methods best suited to modeling in these conditions, we compared the success of three algorithms (Maxent, Mahalanobis Typicalities and Random Forests) at predicting distributions of eight bird and eight mammal species endemic to the eastern slopes of the central Andes. We selected study species to have a range of locality sample sizes representative of the data available for endemic species of this region and also that vary in their distribution characteristics. We found that for species that are known from moderate numbers (N = 38–94) of localities, the three methods performed similarly for species with restricted distributions but Maxent and Random Forests yielded better results for species with wider distributions. For species with small numbers of sample localities (N = 5–21), Maxent produced the most consistently successful results, followed by Random Forests and then Mahalanobis Typicalities. Because evaluation statistics for models derived from few localities can be suspect due to the poor spatial representation of the evaluation data, we corroborated these results with review by scientists familiar with the species in the field. Overall, Maxent appears to be the most capable method for modeling distributions of Andean bird and mammal species because of the consistency of results in varying conditions, although the other methods have strengths in certain situations. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Spatiotemporal population distributions and their implications for species coexistence in a variable environment

A population experiences environmental variation both directly, through effects on life history parameters such as fecundity, and indirectly, through effects on the population distributions of competitors and thus on the distribution of competition. Which spatial and temporal scales of environmental variation most influence the coexistence of two species thus depends in part on the degree to which the resident population responds to different scales of variation. In this paper, I calculate an approximation for a spatiotemporal population distribution as the result of a filter function convolved with the environmental variation. I find that there is no straightforward connection between spatial or temporal scales inherent to an organism's life history, such as mean lifetime or dispersal distance, and the population's sensitivity to variation at different scales. Rather, life history traits interact sensitively with the way environmental variation affects the organism. I comment on the implications for variation-mediated coexistence.     

Spatial distribution of turtle barnacles on the green sea turtle, Chelonia mydas

Distribution patterns of epibiotic barnacles on green sea turtles were investigated in waters neighboring Okinawa, Japan. A number of barnacle species were found to coexist on the turtles and were classified into three genera: Chelonibia, Platylepas and Stomatolepas. Attachment sites on the turtles varied among the barnacle species, suggesting that there is niche partitioning with respect to their microhabitat selection. Turtle bodies offer a “patchy” environment for barnacles, so we also analyzed coexistence patterns in the context of an aggregation model. Within each genus, individual barnacles showed a clumped distribution. The different genera do not have mutually exclusive distribution patterns, but instead occur on the same turtle to various degrees. However, when turtles were divided into two size classes, both the level of aggregation and the degree of interspecific overlap among the barnacles was significantly lower on large turtles. We suggest that obtaining basic information on turtle epibionts will shed light on the biology of wild turtles, which is still largely unknown.     

Spatial and temporal patterns of coexistence between competing <Emphasis Type="Italic">Aedes</Emphasis> mosquitoes in urban Florida

Understanding mechanisms fostering coexistence between invasive and resident species is important in predicting ecological, economic, or health impacts of invasive species. The mosquito Aedes aegypti coexists at some urban sites in southeastern United States with invasive Aedes albopictus, which is often superior in interspecific competition. We tested predictions for three hypotheses of species coexistence: seasonal condition-specific competition, aggregation among individual water-filled containers, and colonization–competition tradeoff across spatially partitioned habitat patches (cemeteries) that have high densities of containers. We measured spatial and temporal patterns of abundance for both species among water-filled resident cemetery vases and experimentally positioned standard cemetery vases and ovitraps in metropolitan Tampa, Florida. Consistent with the seasonal condition-specific competition hypothesis, abundances of both species in resident and standard cemetery vases were higher early in the wet season (June) versus late in the wet season (September), but the proportional increase of A. albopictus was greater than that of A. aegypti, presumably due to higher dry-season egg mortality and strong wet-season competitive superiority of larval A. albopictus. Spatial partitioning was not evident among cemeteries, a result inconsistent with the colonization-competition tradeoff hypothesis, but both species were highly independently aggregated among standard cemetery vases and ovitraps, which is consistent with the aggregation hypothesis. Densities of A. aegypti but not A. albopictus differed among land use categories, with A. aegypti more abundant in ovitraps in residential areas compared to industrial and commercial areas. Spatial partitioning among land use types probably results from effects of land use on conditions in both terrestrial and aquatic-container environments. These results suggest that both temporal and spatial variation may contribute to local coexistence between these Aedes in urban areas.     

Spatial scale of aggregation in three acarine predator species with different degrees of polyphagy

Aggregative responses by the predatory mites, Phytoseiulus persimilis, Typhlodromus occidentalis, and Amblyseius andersoni (Acari: Phytoseiidae), to spatial variation in the density of mobile stages of Tetranychus urticae (Acari: Tetranychidae) were studied over different spatial scales on greenhouse roses. Significant spatial variations in prey numbers per leaflet, per leaf, per branch or per plant were present in all experimental plots. None of the predator species responded to prey numbers per plant, and all searched randomly among plants. Within a plant, the oligophagous P. persimilis searched randomly among branches, but aggregated strongly among leaves within a branch and among leaflets within a leaf. The narrowly polyphagous T. occidentalis searched randomly among leaflets within a leaf and amond leaves within a branch, but aggregated strongly among leaflets or leaves within a plant. The boradly polyphagous A. andersoni searched randomly among leaflets within a leaf, a branch or a plant, and among leaves within a branch or a plant, but distributed themselves more often on branches with lower prey densities. Thus, specialist predators aggregate strongly at lower spatial levels but show random search at higher spatial levels, whereas generalist predators show random search at lower spatial levels but aggregate at higher spatial levels. This is the first empirical evidence demonstrating the relation between the degree of polyphagy and the spatial scale of aggregation. It is also concluded that both the prey patch size (i.e. grain) and predator foraging range (i.e. extent) are important for analyzing spatial scales of predator aggregation. The importance of studying spatial scale of aggregation is also discussed in relation to predator-prey metapopulation dynamics.     

Analytical theory of species abundance distributions of a random community model

We review the history and recent progress of the analytical theories of a random community models. In particular, we focus on a global stability analysis of replicator equations with random interactions and species abundance distributions based on statistical mechanics.     

Spatial distribution of resting stages,rate of emergence from diapause and times to adulthood and to the appearance of the first clutch in 3 species of cyclopoid copepods

The spatial distribution of dormant copepodids of 3 species of cyclopoid copepods — Cyclops vicinus, Mesocyclops leuckarti and Thermocyclops crassus — was studied in 4 small lakes in South Germany. The rate of emergence from diapause and times from the resting stage to adulthood and from adulthood to the appearance of the first clutch was studied at 4 constant temperatures (5, 10, 15, 20 °C) in the laboratory. Resting stages of C. vicinus were always concentrated in the deepest parts of the lakes and were found relatively deep in the mud. M. leuckarti- and T. crassus-copepodids preferred shallow areas in deep lakes but were concentrated in the deep areas in shallow lakes. Copepodids of both species were always concentrated in the uppermost mud layers.Rate of emergence from diapause was strongly temperature-dependent. At high temperatures (20 °C) copepodids of all species under study emerged within 2 weeks. At lower temperatures C. vicinus copepodids showed the highest rate of emergence. At 5 to 10 °C only few M. leuckarti- and T. crassus-copepodids had emerged after the investigation period (7 weeks). Both C. vicinus and T. crassus showed the highest rate of emergence at the natural end of diapause but even at that time only few T. crassus-copepodids emerged at 5 °C. Times to adulthood at 5 °C were shortest in C. vicinus. At higher temperatures this species was passed by M. leuckarti. Times from adulthood to the appearance of the first clutch at 5–15 °C were shortest in C. vicinus. T. crassus produced no clutch at 5 and 10 °C.     

Power-law versus exponential distributions of animal group sizes

There has been some confusion concerning the animal group size: an exponential distribution was deduced by maximizing the entropy; lognormal distributions were practically used; as power-law decay with exponent 3/2 was proposed in physical analogy to aerosol condensation. Here I show that the animal group-size distribution follows a power-law decay with exponent 1, and is truncated at a cut-off size which is the expected size of the groups an arbitrary individual engages in. An elementary model of animal aggregation based on binary splitting and coalescing on contingent encounter is presented. The model predicted size distribution holds for various data from pelagic fishes and mammalian herbivores in the wild.     

禾谷缢管蚜的空间格局及其抽样技术

以抽穗后的小麦为材料,用各种聚集强度指标对禾谷缢管蚜种群在麦田的分布进行测定．结果表明,在小麦灌浆初期及近收获时该蚜的空间格局呈聚集分布,在小麦灌浆中期呈均匀分布．空间格局变化的原因与麦田有蚜茎率有关．当有蚜茎率大于９３％时,该蚜的空间格局就由聚集分布转向均匀分布．最后提出了该虫的最适理论抽样数和以无蚜茎率确定防治指标     

南亚热带常绿阔叶林不同大小和发育阶段林隙的树种多样性研究

分析了南亚热带常绿阔叶林不同大小和发育阶段林隙内树种多样性的变化规律.结果表明,在南亚热带常绿阔叶林中,多样性指数在<400m²的林隙中变化不大,但在400～500m²的林隙中达到最大,而在500～600m²的林隙中最小,在>600m²的林隙中又有所增大.树种多样性指数随林隙年龄的变化趋势是中间高两端低,即在20～50年期间的多样性最大,其次是20年以下的,50年以后的多样性相对最小.林隙更新层中树种多样性指数在500～600m²的林隙中达到最大,在>600m²和200～300m²的林隙中最小.林隙更新层树种多样性指数在林隙形成最初的10年内达到最大值,但随着林隙年龄的增加,总体上表现出下降趋势,在30～40年和50～60年左右又分别形成两个相对的峰值.物种丰富度的变化趋势总体上与树种多样性指数相一致.不同大小和发育阶段的林隙通过其生态因子的改变,对不同树种的更新起到了不同的作用,从而使得不同大小和发育阶段的林隙中树种的多样性特征不同.林隙是维持南亚热带常绿阔叶林树种多样性的一个重要机制.     

不同成熟度杨梅果实上果蝇的动态变化

【目的】果蝇类害虫是杨梅果实的主要害虫,本文旨在探讨果蝇对不同成熟度果实的选择倾向以及杨梅园果蝇动态变化规律。【方法】通过饲养获得黑腹果蝇Drosophila melanogaster种群,探讨果蝇在不同成熟度杨梅果实上的繁殖量,通过果园调查统计杨梅成熟过程中落果数量、落果和树上挂果中果蝇数量。【结果】杨梅成熟过程中,果色经历绿色、粉红色、红色和紫红色变化过程;果蝇分别在各种成熟度果实上繁殖时,绿色果实上果蝇后代数量最少,但接近成熟的粉红色、红色和紫红色果实上果蝇后代数量无显著差异;同时提供各种成熟度的果实时,随着果实成熟度增加,黑腹果蝇的后代繁殖数量增多。在果实成熟前后对果园果蝇数量的调查表明,在果实成熟前,落果中果蝇数量显著高于树上挂果,落果是果园果蝇的主要繁殖地;但当果园果实大量成熟时,挂果中果蝇数量显著多于落果,挂果成为果蝇主要繁殖场所。【结论】在杨梅园,果蝇存在从地面落果转移至树上挂果的动态变化,果蝇对不同成熟度果实的选择性差异可能是这种动态变化的主要原因。     

不同区域典型树木的空间分布格局及关联性

以温带长白山(CBS)、暖温带关帝山(GDS)、亚热带黑石顶(HSD)样地监测数据为基础,采用成对相关函数g(r)分析了3个样地共有的3科(松科、壳斗科、蔷薇科)树木的空间分布格局及其关联性.结果表明:不同样地3科树木分布数量及结构特征存在差异.松科在GDS分布数量较多,呈双峰型径级结构,在CBS和HSD分布数量少,径级结构近似正态分布;壳斗科在CBS和GDS分布数量少,分别呈双峰型和偏正态径级结构,在HSD分布数量多,呈倒"J"型径级分布;蔷薇科在GDS分布数量多,呈"L"型径级分布,在CBS和HSD分布相对较少,径级结构分别呈倒"J"型和"L"型分布.3科树木空间分布格局在不同样地表现也不同,松科的大径级个体在CBS和GDS小尺度上呈均匀分布,在HSD呈聚集分布,中、小径级在3个样地均呈聚集分布;壳斗科在CBS以大径级个体为主,呈近似随机分布,在GDS和HSD以中、小径级为主,呈聚集性分布格局;蔷薇科在3个样地均为聚集分布.3科树木的聚集程度均随尺度增加而降低.大径级的壳斗科个体在CBS和HSD与松科呈不相关或小尺度负相关关系,中、小径级的壳斗科在CBS和GDS与松科呈负相关关系,但在HSD与松科表现为正相关;松科与蔷薇科在3个样地均表现为负相关;中、小径级的壳斗科与蔷薇科在CBS和GDS呈正相关关系,但在HSD呈负相关关系.总之,3科树木空间分布格局及关联性随径级、尺度而变化且在不同样地内有不同表现.     

Microspectrophotometric comparison of the genome sizes of Drosophila hydei and some related species

The relative genome sizes and the proportions of X- and Y-chromosomal DNA in Drosophila hydei, D. neohydei and D. eohydei were measured by microspectrophotometry. Some implications of the results with respect to genome evolution in these species are discussed.     

Influences of population size and woodland area on bird species distributions in small woods

Distributions of individual bird species in 151 small woods (size range 0.02–30 ha) were investigated in 3 consecutive years during which the abundance of certain species varied markedly. Relationships between the probabilities of certain bird species breeding and woodland area were described using incidence functions derived from logistic regression analysis. In general, for species which were largely dependent on woodland and seldom occurred in other habitats (such as hedgerows and gardens), the probability of breeding approached 100% only for woods of 10 ha and more, whereas species with less stringent habitat requirements occurred in the majority of woods, including those of 1 ha and less. The sensitivity of incidence functions to changes in regional abundance and the size distribution of the study woods was examined. For some species, distribution patterns could not be distinguished from those expected if pairs had been distributed in proportion to woodland area (random placement), but the majority did not conform to random placement in at least 1 of the 3 years. This nonconformity was consistent across all 3 years for some species, such as wren (Troglodytes troglodytes), despite substantial fluctuations in population sizes between years, while for others, such as robin (Erithacus rubecula), distribution patterns changed with changes in regional abundance. The results suggested that some species, such as wren and blackbird (Turdus merula), preferred small woods, while other species, such as chiffchaff (Phylloscopus collybita), preferred large woods. For several other species, including robin, great tit (Parus major), long-tailed tit (Aegithalos caudatus) and marsh tit (P. palustris), small woods appeared to be sub-optimal under at least some conditions.     

Inconstancy of Taylor'sb: Simulated sampling with different quadrat sizes and spatial distributions

Taylor 's power law, s²=am^b, provides a precise summary of the relationship between sample variance (s²) and sample mean (m) for many organisms. The coefficient b has been interpreted as an index of aggregation, with a characteristic value for a given species in a particular environment, and has been thought to be independent of the sample unit. Simulation studies were conducted that demonstrate that the value of b may vary with the size of the sample unit in quadrat sampling, and this relationship, in turn, depends on the underlying spatial distribution of the population. For example, simulated populations with hierarchical aggregation on a large scale produced values of b that increased with the size of the sample unit. In contrast, for a simulated population with randomly distributed clusters of individuals, the value of b eventually decreased with increasing quadrat size, as sample counts became more uniform. A single value ofTaylor 's b, determined with a particular sample unit, provides neither a fixed index of aggregation nor a complete picture of a species' spatial distribution. Rather, it describes a consistent relationship between sample variance and sample mean over a range of densities, on a spatial scale related to the size of the sample unit. This relationship may reflect, but not uniquely define, density-dependent population and behavioral processes governing the spatial distribution of the organism. Interpretation ofTaylor 'sb for a particular organism should be qualified by reference to the sample unit, and comparisons should not be made between cases in which different sample units were used. Whenever possible, a range of sample units should be used to provide information about the pattern of distribution of a population on various spatial scales.     

Squares of different sizes: effect of geographical projection on model parameter estimates in species distribution modeling

In species distribution analyses, environmental predictors and distribution data for large spatial extents are often available in long‐lat format, such as degree raster grids. Long‐lat projections suffer from unequal cell sizes, as a degree of longitude decreases in length from approximately 110 km at the equator to 0 km at the poles. Here we investigate whether long‐lat and equal‐area projections yield similar model parameter estimates, or result in a consistent bias. We analyzed the environmental effects on the distribution of 12 ungulate species with a northern distribution, as models for these species should display the strongest effect of projectional distortion. Additionally we choose four species with entirely continental distributions to investigate the effect of incomplete cell coverage at the coast. We expected that including model weights proportional to the actual cell area should compensate for the observed bias in model coefficients, and similarly that using land coverage of a cell should decrease bias in species with coastal distribution. As anticipated, model coefficients were different between long‐lat and equal‐area projections. Having progressively smaller and a higher number of cells with increasing latitude influenced the importance of parameters in models, increased the sample size for the northernmost parts of species ranges, and reduced the subcell variability of those areas. However, this bias could be largely removed by weighting long‐lat cells by the area they cover, and marginally by correcting for land coverage. Overall we found little effect of using long‐lat rather than equal‐area projections in our analysis. The fitted relationship between environmental parameters and occurrence probability differed only very little between the two projection types. We still recommend using equal‐area projections to avoid possible bias. More importantly, our results suggest that the cell area and the proportion of a cell covered by land should be used as a weight when analyzing distribution of terrestrial species.     

Spatial distribution patterns of trees at different life stages in a warm temperate forest

We have investigated tree distributions in relation to topography between different tree life history stages, from the seed-dispersal stage to the adult stage in a warm temperate evergreen broadleaved forest on Yakushima Island, Japan, to clarify the critical stages in determining adult tree distributions. We conducted a census of all living trees > or =30 cm tall and collected seed falls over three years using 25 seed traps in a 50 m x 50 m quadrat. Four life stages were defined: stage 1, dispersed seed; stage 2, individuals taller than 30 cm and diameter at breast height (DBH) < 1 cm; stage 3, trunks 1 cm < or = DBH < 10 cm; stage 4, trunks with DBH > or = 10 cm. We classified 17 common tree species into three groups; group A was distributed mainly on the upper slope, group B on the lower slope, and group C on both. Most of group A and B trees at stages 2-4 showed an aggregated distribution along the topographical gradient. The densities at stage 1 showed weaker aggregations according to slope. Topography-specific tree distribution was probably determined at the regeneration stage, and later survival was less effective as a mechanism of vegetation differentiation.