Analysis of Foraging Behaviour of the Whitefly Parasitoid Encarsia formosa on a plant: A Simulation Study

The foraging behaviour of Encarsia formosa was analyzed using a stochastic simulation model of the parasitoid's behaviour. Parasitoids were allowed to search during a day on a tomato plant infested with immatures of the greenhouse whitefly, Trialeurodes vaporariorum. The model simulates searching, host selection, host handling and patch leaving behaviour, and the physiological state of the parasitoid. The outputs of the model are the number of visited leaflets and the number of hosts encountered, parasitized or killed by host feeding. The simulation results agreed well with observations of parasitoids foraging on tomato plants. The number of encounters and ovipositions on the plant increased with host density according to a type II functional response. At a clustered host distribution over leaflets and low host densities, the most important parameters affecting the number of ovipositions were the leaf area, the parasitoid's walking speed and walking activity, the probability of oviposition after encountering a host, the initial egg load and the ratio of search times on both leaf sides. At high densities, the maximum egg load and the giving-up time on a leaflet since latest host encounter were the most essential parameters.     

Interpreting phenotypic variation in plant allelochemistry: problems with the use of concentrations

Ecologists often use concentrations of defensive compounds as measures of plant allocation to defence and/or allelochemical production. I demonstrate that this practice may lead to erroneous conclusions because plants produce and allocate molecules (quantities) of compounds whereas concentrations reflect the distribution of these quantities in plant tissues and are, therefore, functions of plant biomass. As a tool for distinguishing between shifts in allelochemical production versus changes in plant biomass in determining allelochemical concentrations, I suggest using a technique known as graphical vector analysis (GVA) which has been developed for diagnosing nutrient limitations in forest stands, but has seldom been applied by researchers studying plant allelochemicals. I used data from several published studies to demonstrate how GVA can be applied to interpret ontogenetic and environmental effects on allelochemical levels and to compare the results obtained for different allelochemical types, plant species, treatments and experiments. These examples show that changes in plant biomass per se are an important source of variation in allelochemical concentrations and, therefore, concentration data can be easily misinterpreted if changes in absolute content and plant biomass are not considered simultaneously. Because studies reporting variation in allelochemical concentrations have been considered as tests for general theories of plant chemical defence, evidence in support of or against these theories should be re-examined using multivariate techniques such as analysis of covariance, allometric analysis and GVA. Received: 16 September 1998 / Accepted: 4 March 1999     

Latitudinal differences in somatic energy storage: adaptive responses to seasonality in an estuarine fish (Atherinidae: Menidia menidia)

This study focuses on the seasonal accumulation and depletion of somatic energy in the Atlantic silverside (Menidia menidia), an annual estuarine fish. Previous research revealed that northern silversides are subject to strong size-dependent winter mortality, while southern fish suffer no appreciable winter mortality. To examine whether there was geographic differentiation in allocation strategies, we compared temporal patterns of energy storage and utilization among three populations along this gradient in seasonality. The comparative design used monthly or biweekly samples of fish collected in the wild, as well as samples of fish from each population reared in a common environment, where genetic differences can be clarified. Somatic energy stores were quantified via gravimetric analysis of neutral storage lipids and lean tissue. Analysis revealed that small individuals maintained relatively low levels of lipid reserves, which may account for their lower survival in winter. Wild fish in the north rapidly accumulated large somatic reserves, which were depleted over the winter and then increased again during the subsequent spring breeding season. In wild southern fish, relatively small reserves accumulated slowly until breeding commenced in the spring. The common-environment comparison of somatic storage patterns revealed a genetic basis for among-population differences in reserve accumulation rates, but no differences in the amount of reserves stored. We conclude that the overwinter depletion of somatic reserves has a significant selective impact on energy accumulation and allocation strategies in seasonal environments. Received: 1 November 1995 / Accepted: 13 September 1996     

Compensatory growth responses to clipping defoliation in Leymus chinensis (Poaceae) under nutrient addition and water deficiency conditions

Compensatory growth responses of Leymus chinensis, a dominant species in Inner Mongolia steppe, to clipping defoliation were evaluated in a pot-cultivated experiment under different nutrient (N and P) and water availability conditions. Leymus chinensis exhibited over-compensatory growth at the light and moderate clipping intensities (20% and 40% aerial mass removed) with a greater accumulated aboveground biomass, higher relative growth rate (RGR), more rhizomatic tillers and a stimulation of compensatory photosynthesis to the remnant leaves as compared with those of the unclipped plants. Intense clipping (80% aerial mass removed), which removed most of the aboveground tissues, greatly reduced the growth of aboveground biomass in comparison with that of the unclipped plants. Nitrogen addition only slightly improved the biomass production and RGR in light and moderately clipped plants, and it did not allow plants in the intense clipping condition to over-compensate. Phosphorus addition had no obvious influences on the growth and physiological responses to clipping defoliation. These results indicated that nutrient addition could not compensate for the negative effects of severe clipping on the defoliated grass. On the other hand, there were no distinct positive responses under water deficiency condition for L. chinensis at all clipping intensities with a significant reduction of aboveground and belowground biomass, lower RGR, fewer rhizomatic tillers, and a lower net photosynthetic rate than other wet treatments. Additionally, the chlorophyll contents of remnant leaves gradually increased with the increase of clipping intensities in each treatment. In conclusion, although L. chinensis could compensate for tissues removal by some morphological and physiological responses, intense clipping and drought can result in a significant decrease of biomass and growth rate, even under enriched nutrition conditions.     

From species to communities: the signature of recreational use on a tropical river ecosystem

Disturbance can impact natural communities in multiple ways. However, there has been a tendency to focus on single indicators of change when examining the effects of disturbance. This is problematic as classical diversity measures, such as Shannon and Simpson indices, do not always detect the effects of disturbance. Here, we instead take a multilevel, hierarchical approach, looking for signatures of disturbance in the capacity and diversity of the community, and also in allocation and demography at the population level. Using recreational use as an example of disturbance, and the freshwater streams of Trinidad as a model ecosystem, we repeatedly sampled the fish communities and physical parameters of eight pairs of recreational and nonrecreational sites every 3 months over a 28‐month period. We also chose the Trinidadian guppy (Poecilia reticulata) as the subject of our population‐level analyses. Regression tree analysis, together with analysis of deviance, revealed that community capacity and community species richness were greater at sites with higher levels of recreational use. Interestingly, measures of community diversity that took into account the proportional abundance of each species were not significantly associated with recreational use. Neither did we find any direct association between recreational use and proportion of guppy biomass in the community. However, population‐level differences were detected in the guppy: Sex ratio was significantly more female‐biased at more disturbed sites. Our findings emphasize the importance of considering multiple levels when asking how disturbance impacts a community. We advocate the use of a multilevel approach when monitoring the effects of disturbance, and highlight gaps in our knowledge when it comes to interpreting these effects.     

An ED-based Protocol for Optimal Sampling of Biodiversity

While conservation planning requires good biodiversity data, our knowledge of most living groups is scarce and patchy even in well-sampled regions. Therefore, we need methodologies for rapid assessments for particular groups and regions. Maps of any biodiversity surrogate can be interpolated from even a few well-known sites, but such places are usually lacking. We therefore propose a protocol for designing field surveys to obtain good coverage of pattern variations of biodiversity in a given region. To represent biodiversity patterns comprehensively, we use a rule step site-allocation procedure, partially based on Faith and Walker's ED criterion that takes environmental and spatial variation into account, together with other criteria such as survey costs. A preliminary assessment of the adequacy of this site sampling strategy is made. Then a set of complementary sites is selected for further sampling. Using the ED criterion, during the stepwise process a p-median analysis is applied both to an environmental distance matrix and to a spatial distance matrix, to maximize the amount of variation covered by our survey planning. This rule-set allocation procedure is integrated into a continuous sampling design protocol directed to ensure we can sample all biodiversity of a region. This protocol requires the gathering of both biological and environmental information, an assessment of previously available information, the choice of sampling methods and dates, and a continuous assessment of the success of the survey being carried out. An example of the application of this protocol to the survey design of dung beetle (Coleoptera, Scarabaeoidea) diversity in the Comunidad de Madrid (Spain) is included.     

Interactive effect of flooding and grazing on the growth of Serengeti grasses

Summary Grazing and flooding may potentially interact in particular habitats of many grassland regions around the world. We tested the hypothesis that grazing and flooding induce different and largely opposed allocation responses in individual plants. As a result, their combined effect on plant growth would be negative. We studied the response of three grass species from the Serengeti ecosystem (Tanzania) to the effects of flooding and clipping. Plants under the combined effect of flooding and clipping had lower growth rates than plants growing under the effect of either of the two factors individually. Plants under flooding grew taller and allocated more resources to stem growth than controls; for two of the three species, flooded plants also generated a new root system above soil level. All these morphological and physiological responses conflict with the ability of a plant to respond to defoliation with minimum reduction in growth rates. The three species showed a response to flooding reflecting their distribution ranges in the field: the species from the most flood-prone habitat showed a positive effect of flooding on growth, whereas the species from dry uplands showed a strong negative effect of flooding. Flood-tolerant species were taller and less tolerant of clipping than flooding sensitive species. Our results suggest that, in ecological time, individuals subjected to both flooding and grazing have their growth reduced to a greater extent than by either of the two factors acting individually, whereas in evolutionary time, species adapted to flooding are poor grazing tolerators and species adapted to grazing are poor flooding tolerators.     

Seasonal change in offspring sex and size in Dawson's burrowing bees (Amegilla dawsoni) (Hymenoptera: Anthophorini)

Abstract.  1. Nesting females of Dawson's burrowing bees, Amegilla dawsoni , produce a large size class of offspring, which includes daughters and major sons, and a small size class, which consists entirely of minor sons averaging half the weight of their larger siblings. Female allocation patterns change over the flight season such that the initial pattern of producing daughters shifts toward the production of both daughters and major sons in the middle of the season, and then the production of primarily minor sons in the latter part of the nesting season.
2. In Dawson's burrowing bees, this pattern is correlated with declines in pollen and nectar availability as the nesting season progresses as well as a heightened risk of dying before the final brood cell is completed. Here, the relation between these factors and the provisioning tactics of nesting Dawson's burrowing bees is discussed.     

Growth and production characters of elymus cylindricus in different shortening heading time

Ramet modules in a certain population differ in terms of functions, which accounts for different contributions of the same ramets. Shortening heading time brings about different contributions of such modules. Ramets heading one after another were treated as a continuum in respective cohorts of Elymus cylindricus aged two. The reproductive ramets that head earlier were marked with tags every four days during the whole heading stage from the beginning to the end, after which all the labeled ramets at the maturity period were gathered. The results showed that, the height and biomass of ramets, the length and biomass of inflorescences, percentage of inflorescence length to ramet height, percentage of inflorescence biomass to ramet biomass, the number and biomass of seeds, seed-setting rate, and percentage of seed biomass to ramet biomass declined with the increasing intensity of heading time shortening as displayed with linear or quadratic function. Ramet characteristics weakened remarkably when shortened heading time added up to 17 days. The biomass distribution in relation to inflorescence and seed maintain a stable rate at the early heading stage and dwindled quickly at the near-end stage, but the biomass of ramets remain constant throughout the entire heading stage. The ramets with earlier heading time make greater contribution to the survival of population than the shortened heading time in this species of bunchgrass.