The sugarcane borer Diatraea saccharalis (Fabr.) (Lep., Crambidae) and its parasitoids: a synchrony approach to spatial and temporal dynamics

Abstract:  A data set on Diatraea saccharalis and its parasitoids, Cotesia flavipes and tachinid flies, was analysed at five spatial scales – sugarcane mill, region, intermediary, farm and zone – to determine the role of spatial scale in synchrony patterns, and on temporal population variability. To analyse synchrony patterns, only the three highest spatial scales were considered, but for temporal population variability, all spatial scales were adopted. The synchrony–distance relationship revealed complex spatial structures depending on both species and spatial scale. Temporal population variability [SD log( x  + 1)] levels were highest at the smallest spatial scales although, in the majority of the cases, temporal variability was inversely dependent on sample size. All the species studied, with a few exceptions, presented spatial synchrony independent of spatial scale. The tachinid flies exhibited stronger synchrony dynamics than D. saccharalis and C. flavipes in all spatial scales with the latter displaying the weakest synchrony levels, except when mill spatial scales were compared. In some cases spatial synchrony may at first decay and then increase with distance, but the presence of such patterns can change depending on the spatial scale adopted.     

Nonmetric multidimensional scaling analyses of temporal variation in the structure of limnetic zooplankton communities

Nonmetric multidimensional scaling analysis was used to graphically summarize and analyze seasonal changes in the structure of limnetic zooplankton communities in Blelham Tarn, English Lake District and two large experimental enclosures (tubes A and B) therein. Species abundances in weekly samples taken from June–December, 1976 from the three sites were ordinated. Paths through species space describing temporal changes in community structure indicated that the tube B community oscillated around some average composition and was dominated by small zooplankters whereas those from tube A and the tarn changed quite distinctly through the season and had higher predator densities. Examination of other characteristics of the sites indicated that the observed differences in seasonal cycles could be related to variations in the intensity of predation, the quality of food for herbivorous zooplankters and the dynamics of nutrient input. The importance of considering time variation in lake properties, particularly in assessing lake responses to stress and in typing lakes for experimentation or management, is emphasized.     

Testing a biological mechanism of the insurance hypothesis in experimental aquatic communities

1.  The insurance hypothesis predicts a stabilizing effect of increasing species richness on community and ecosystem properties. Difference among species' responses to environmental fluctuations provides a general mechanism for the hypothesis. Previous experimental investigations of the insurance hypothesis have not examined this mechanism directly.
2.  First, responses to temperature of four protist species were measured in laboratory microcosms. For each species, we measured the response of intrinsic rate of increase ( r ) and carrying capacity ( K ) to temperature.
3.  Next, communities containing pairs of species were exposed to temperature fluctuations. Community biomass varied less when correlation in K between species (but not r ) was more negative, and this resulted from more negative covariances in population sizes, as predicted. Results were contingent on species identity, with findings differing between analyses including or not including communities containing one particular species.
4.  These findings provide the clearest support to date for this mechanism of the insurance hypothesis. Biodiversity, in terms of differences in species' responses to environmental fluctuations (i.e. functional response diversity) stabilizes community dynamics.     

Community structure and temporal variability of ichthyoplankton in North Brazilian mangrove creeks

The species composition and dynamics of fish larvae in three mangrove creeks located in the Caeté Estuary (north Brazil) were studied monthly using a trap net during diurnal ebb tides. A total of 109 954 larvae, representing 25 families and 54 species, were collected from October 1996 to October 1997. The community was dominated numerically by a few species, a feature common for other estuarine fish populations. The most abundant taxa were estuarine species, namely the eleotrid Guavina guavina (46·7%) and the engraulid Anchovia clupeoides (14·9%). The sciaenid Cynoscion acoupa was the only marine species that used the mangroves extensively as a nursery site, occurring mainly at the postflexion stage. The size distribution of G. guavina did not produce shifting modes, indicating continuous transport out of the mangroves by tidal currents. Significantly lower species richness was observed in the late rainy season, primarily due to the emigration of marine species. Intermediate seasons were characterized by more complex larval fish assemblages. The temporal trends of the dominant species was influenced to a great degree by their life history strategy.     

Large-scale synchrony and inter-annual variability in roach recruitment in the Rhône River: the relative role of climatic factors and density-dependent processes

1. We report patterns of temporal variation in the recruitment of roach ( Rutilus rutilus ). The data consist of the annual abundance of the first 2 year-classes, 0+ and 1+ fish, at four sites in the Rhône River (France) between 1987 and 1997. Over this 11-year period both 0+ and 1+ fish fluctuated strongly.
2. Cross-correlation indicated high spatial synchrony in 0+ dynamics, although correlations among sites in 1+ dynamics were weaker. No clear pattern was apparent in the relationship between the level of synchrony and distance between pairs of sites.
3. The spatial synchrony in 0+ fish could be attributable to large-scale variations in weather, influencing water temperature. Total body length of 0+ roach was correlated with water temperature (expressed in degree-days over 12 °C), and water temperature was the main factor explaining inter-annual variation in 0+ cohort size. Monthly variation in abiotic factors (measured by standard deviation in water temperature and discharge) did not influence 0+ fluctuations. Correlations with June water temperature suggest that year-class strength was mainly determined by abiotic factors during the first few months of life.
4. The absence of spatial synchrony in 1+ fluctuations suggests little correlation between survival and abiotic conditions during the first year of life, other factors influencing survival.
5. Survival in the first year was density-dependent. Intraspecific competition within the 0+ cohort could thus influence the fluctuations in recruitment to older age-classes.
6. The implications of age- or stage-dependent synchrony in temporal variation for species with complex life histories are discussed. Studying spatial synchrony for the different life history stages could enhance our understanding of the population dynamics of spatially structured species.     

Small-scale spatial variability in intertidal and subtidal turfing algal assemblages and the temporal generality of these patterns

Spatial and temporal variation in patterns of distribution and abundance of algal assemblages is large and often occurs at extremely small spatial and temporal scales. Despite this, few studies investigate interactions between these scales, that is, how patterns of spatial variation change through time. This study investigated a number of scales of spatial variation (from tens of centimetres to kilometres) in assemblages of intertidal and subtidal turfing algae. Significant differences were found in the composition and abundances of species in assemblages of turf at all spatial scales tested. Much of the variation among assemblages could, however, be explained at the scale of quadrats (tens of centimetres apart) (27±1.4 (SE)% of dissimilarity) with an additional 7±1.2% explained at the scale of sites (tens of metres apart) and 10±1.5% at the scale of locations (kilometres apart). Although the greatest dissimilarity in assemblages occurred at the scale of habitats, this accounted for a relatively small proportion of the overall variation in assemblages. These patterns were consistent through time, that is, at each sampling time the spatial scale explaining the greatest proportion of variation in assemblages was replicate quadrats separated by tens of centimetres. These patterns appear to be due to small-scale variation in patterns of distribution and abundances of the individual species that comprise turfing algal assemblages. The results of this experiment suggest that large scale processes have less effect on patterns of variability of algal assemblages than those occurring on relatively smaller spatial scales and that small-scale spatial variation should not be considered as simply “noise”.     

Rainfall and flowering synchrony in a tropical shrub: Variable selection on the flowering time ofErythroxylum havanense

Summary We tested the adaptive significance of flowering synchrony by means of a quantitative analysis of selection and by flowering induction experiments with the deciduous shrubErythroxylum havanense. Temporal schedules of flower and fruit production were determined for a local population (in three sites) in a Mexican seasonal forest for 2 years (1987–1988). The consequences of natural variation in flowering time (flowering initiation day) on maternal reproductive success (fecundity) were evaluated. We observed high levels of inter- and intraindividual flowering synchrony in 1987, but not in 1988 and this contrast was related to differences in rainfall patterns between the two years. A significant proportion (15.4%) of the phenotypic variation in flowering initiation day was accounted for by environmental variance. The expression of phenotypic variance of flowering time and, consequently, the opportunity for selection to act, are controlled by annual variation in rainfall. Despite the between-year difference in flowering synchrony, we detected a relatively intense directional selection on flowering initiation day in both years, but selection coefficients were of opposite sign (standardized directional gradients were –0.326 and 0.333 for 1987 and 1988, respectively). For both years there was a significant relationship between individual relative fitness and the number of neighbouring flowering plants in a given day, suggesting positive frequency-dependent selection.     

Stable isotope analysis of aquatic invertebrate communities in irrigated rice fields cultivated under different management regimes

In this study we have used stable isotope analysis to identify major food resources driving food webs in commercial rice agroecosystems and to examine the effects of agricultural management practices on the trophic structure of these food webs. Potential carbon sources and aquatic macroinvertebrate consumers were collected from large-scale rice farms in south-eastern Australia cultivated under three different crop management regimes conventional-aerial (agrochemicals applied, aerially sown), conventional-sod (agrochemicals applied, directly sown) and organic-sod (agrochemical-free, directly sown). Evidence from stable isotope analysis demonstrated the importance of food sources, such as biofilm and detritus, as the principal energy sources driving aquatic food webs in rice agroecosystems. Despite the greater diversity of potential food sources collected from the organic-sod regime across all sampling occasions, the range of food resources directly assimilated by macroinvertebrate consumers did not differ substantially across management regimes. Trophic complexity of aquatic food webs, as evidenced by the number of trophic levels identified using δ¹⁵N data, differed across management regimes at the early season sampling. Sites with low or no agrochemical applications contained more than two trophic levels, but at the site with the highest pesticide application no primary or secondary consumers were found. Our data demonstrates that the choice of agricultural management regime has a season-long influence on aquatic food webs in rice crops, and highlights the importance of conserving non-rice food resources that drive these trophic networks.     

The importance of urban reserves for conserving beetle communities: a case study from New Zealand

Urban reserves provide a major opportunity for conservation of indigenous biodiversity in the heavily urbanised landscape of Waitakere City (Auckland), New Zealand. However, there is little documented information on what indigenous fauna survives in these reserves. Beetle (Coleoptera) communities associated with two small, isolated urban reserves and two sites in a larger forest area within the Waitakere Ranges were sampled using pitfall traps and analysed. A total of 887 beetles, from 23 families and 89 recognisable taxonomic units were caught. The urban reserves had a marked reduction in species richness and abundance of beetles compared with the sites within the larger forest areas. Various environmental factors influencing the distribution of beetles across the sites were investigated. The most important factors were size of fragment, local forest cover and soil moisture. Common species (>5 specimens in the total dataset) found in the Waitakere Ranges and small urban reserves, were either endemic or indigenous to New Zealand. Therefore, even though these reserves may be isolated from a larger, more continuous forest tract, they have considerable potential as reservoirs of beetle diversity in highly modified landscapes and the contribution of urban reserves to the local sustainability of beetle assemblages emphasises the importance of maintaining green areas in and around cities.     

Diversity and temporal stability of bacterial communities in a model passerine bird, the zebra finch

The composition and dynamics of the gastrointestinal bacterial communities in birds is determined by both host-specific and environmental exposure factors yet these are poorly understood. We selected the zebra finch, Taeniopygia guttata, as the host species to examine the diversity and temporal stability of the faecal microflora in a bird, owing to its importance as a model organism in avian ecology, neuroscience and evolution studies. The stability of the gut bacterial community of individual male and female zebra finches was assessed through repeat faecal sampling via culture and temperature gradient gel electrophoresis and partial sequencing of PCR-amplified eubacterial 16S rRNA gene products. Nineteen bacterial genera were detected across all samples (n = 99), with each bird carrying on average six operational taxonomic units. Using a novel statistical approach, we showed that bacterial assemblages and community richness varied between individual birds but remained stable over time within individuals. Neither the composition nor richness of bacterial communities differed significantly between the sexes. Our results show that zebra finches housed together under controlled conditions show consistent variation between individuals in their gut microflora that is not attributable to differences in host exposure to environmental microbial sources. Future studies could usefully explore the origin of this individual-specific variation and its consequences for host fitness and sexual selection.     

Assessment of the spatial variability of intertidal benthic communities by axial tomodensitometry: importance of fine-scale heterogeneity

At the water-sediment interface of aquatic ecosystems, the presence of biogenic structures produced by benthic invertebrates strongly affects biogeochemical processes. The quantification of these structures and the assessment of the vertical distribution of fauna are essential for determining the impact of communities in sediments. In the present study, computer axial tomodensitometry (CAT-scan) was used to measure the space occupied by an intertidal community of the St. Lawrence estuary. Three cores were sampled at a site that was considered homogeneous according to surface sediments. The vertical distribution of biogenic structures and gravel were measured in the three cores using CAT-scan; the vertical distribution of fauna was also analysed for each core. The biogenic structures were highest at the water-sediment interface and decreased with depth in the three cores. The number of invertebrates also decreased with depth. We observed similar distributions of biogenic structures in cores 1 and 2. However, fewer biogenic structures were observed below 90 mm in core 3. This result was correlated with a high quantity of gravel from 90 to 140 mm in core 3 whereas the other cores had lower quantities of coarse material. We found relationships among the distributions of biogenic structures, fauna, and sediment characteristics (gravel quantity) that can affect species distribution. The vertical distributions of Macoma balthica, Mya arenaria, Nereis virens, and small-sized gallery-producing species (nematodes and oligochaetes) could also be recorded with the CAT-scan method. Thus, CAT-scan is an excellent tool to determine the fine-scale heterogeneity in the space occupied by benthic invertebrates in sediments.     

Structure and temporal dynamics of macroinfaunal communities of a sandy reef flat in the northwestern Philippines

Temporal variation in macroinfaunal community structure wasassessed from monthly monitoring of the sandy substrate of theLucero reef flat in Bolinao, Pangasinan (northwesternPhilippines)between November 1990 and November 1991. The community wascomposedof 98 taxa from 10 phyla and was consistently dominated by thepolychaete family Syllidae (19–33% of monthly totalabundance).Five polychaete and crustacean taxa co-dominated with thesyllidsand exhibited monthly shifts in dominance ranks. Overall,there wasno significant change in the composition of the communityafter ayear.Results of the study show that the temporal dynamics of thecommunity was a function of the seasonality in salinity and oftherelatively consistent nature of the substrate. Abundances ofmacroinfaunal crustaceans, chaetognaths, and molluscsfluctuatedsignificantly over 12 months, and corresponded to the seasonalfluctuation in salinity. Total and polychaete abundances didnotshow significant monthly variation, but tended to beinfluenced bysalinity changes. Densities of turbellarians, nemerteans, andechinoderms appeared homogeneous across 12 months, andcorrelatedwith the consistency of substrate structure in the reef flatoverthe experimental period. In addition, sipunculid numbersseemed tobe supported by the stable amount of organic matter in thesandysubstrate.     

Clusterv: a tool for assessing the reliability of clusters discovered in DNA microarray data

We present a new R package for the assessment of the reliability of clusters discovered in high-dimensional DNA microarray data. The package implements methods based on random projections that approximately preserve distances between examples in the projected subspaces.     

Fractal geometry: a tool for describing spatial patterns of plant communities

Vegetation is a fractal because it exhibits variation over a continuum of scales. The spatial structure of sandrim, bryophyte, pocosin, suburban lawn, forest tree, and forest understory communities was analyzed with a combination of ordination and geostatistical methods. The results either suggest appropriate quadrat sizes and spacings for vegetation research, or they reveal that a sampling design compatible with classical statistics is impossible. The fractal dimensions obtained from these analyses are generally close to 2, implying weak spatial dependence. The fractal dimension is not a constant function of scale, implying that patterns of spatial variation at one scale cannot be extrapolated to other scales.     

Temporally stable genetic variability and dynamic kinship structure in a fluctuating population of the root vole Microtus oeconomus

Genetic variability, kin structure and demography of a population are mutually dependent. Population genetic theory predicts that under demographically stable conditions, neutral genetic variability reaches equilibrium between gene flow and drift. However, density fluctuations and non‐random mating, resulting e.g. from kin clustering, may lead to changes in genetic composition over time. Theoretical models also predict that changes in kin structure may affect aggression level and recruitment, leading to density fluctuations. These predictions have been rarely tested in natural populations. The aim of this study was to analyse changes in genetic variability and kin structure in a local population of the root vole (Microtus oeconomus) that underwent a fourfold change in mean density over a 6‐year period. Intensive live‐trapping resulted in sampling 88% of individuals present in the study area, as estimated from mark–recapture data. Based on 642 individual genotypes at 20 microsatellite loci, we compared genetic variability and kin structure of this population between consecutive years. We found that immigration was negatively correlated with density, while the number of kin groups was positively correlated with density. This is consistent with theoretical predictions that changes in kin structure play an important role in population fluctuations. Despite the changes in density and kin structure, there was no genetic differentiation between years. Population‐level genetic diversity measures did not significantly vary in time and remained relatively high (H_E range: 0.72–0.78). These results show that a population that undergoes significant demographic and social changes may maintain high genetic variability and stable genetic composition.