Mate choice tactics and swarm size: a model and a test in a dance fly

In the dance flyEmpis borealis (L.) (Diptera: Empididae) females gather to swarm and males visit swarms for mating. A model was constructed, based on previously published data, simulating how males may choose among females of different sizes in swarms of different sizes. The focal question was, what influences the number of individuals in the swarm in this and possibly other swarming insects? The relationships between original swarm size and both the number of males arriving per minute and the proportion of males mating are both logarithmic. The model predicted that if these relationships were linear, or if males were able to judge absolute female size, the mean swarm size should increase and be at least four times as large as those found in the field. The only type of male mate choice strategy that gave rise to very large swarms (>25) was size-related choice (if males are able to assess the size of a female in relation to the entire population and not merely to the swarm). Furthermore, no swarming behaviour would occur if males mate independently of swarm size. Thus, the numbers of females attending a given swarm site are influenced by male arrival pattern, male preference for larger swarms, the inability of males to judge the absolute body size of females, and female polyandry. Males searching for mates seem to prefer larger swarms than females searching for a swarm to join, but the mean swarm size is primarily set by the swarm size preference of females. Optimal swarm size predicted from the model was 4.68±0.53 females. In order to test model predictions, 69 natural swarm sites were studied during one season. The mean swarm size was 4.85±4.54 females (median 4.03), and about 90% of swarms consisted of 11 females or fewer. Predicted and observed swarm size did not differ significantly.     

Primary production and heterotrophy of a pennate and a centric salt marsh diatom

Radionuclide labeled sodium bicarbonate and glucose were used to measure the rates of primary production and glucose uptake by Nitzschia obtusa var. undulata and Cymbella pusilla. The presence of glucose in the medium enhanced primary production in both diatom species. Uptake of glucose in C. pusilla was photoinhibited. A maximum of 9.3 nmol C mg dry wt^-1 h^-1 of glucose was taken up in the light without the presence of NaHCO₃ in the medium and with NaHCO₃ present the value was 18 nmol C mg dry wt^-1 h^-1. The ratio of inorganic carbon fixed to the total carbon budget (autotrophy and heterotrophy) was 0.51 for N. obtusa var. undulata and 0.62 for C. pusilla.     

Growth and Cd accumulation of two halophytes and a non-halophyte grown in a non-saline and a saline soil with different Cd levels

Two halophytes, Salicornia europaea and Atriplex verucifera, and the non-halophyte Chenopodium album were grown in pots on sodic and non-sodic Iranian soils spiked with up to 100 mg Cd kg^?1. The halophytes grew best on the sodic soil in the absence of Cd spiking, while C. album performed better on non-sodic soil. Cadmium spiking reduced the growth of all plants, with Cd tolerance decreasing in the order S. europaea>A. verucifera>C. album. The observed order of Cd tolerance corresponds to the abilities of the plants to tolerate sodic soil properties. The variation of Cd concentration in shoots in response to sodicity was related to different mechanism, including dilution (C. album) Cd solubilisation by Ca²⁺ (S. europea) and Ca²⁺ competition for uptake and translocation of Cd (A. verucifera). Improved understanding of these complex interactions will help to design phytoextraction technology for Cd-polluted soils in arid regions.     

Cohabitation of a coral reef sponge and a colonial scyphozoan

A sensitive experimental protocol using cloned corals (hereafter microcolonies) of the branching scleractinian coral Stylophora pistillata and ⁴⁵Ca has been developed to enable reproducible measurements of physiological and biochemical mechanisms involved in calcium transport and compartmentalization during coral calcification. Cloned S. pistillata microcolonies were propagated in the laboratory from small fragments of parent colonies collected in 1990 in the Gulf of Aqaba, Jordan. Cloned microcolonies have several intrinsic properties that help to reduce unwanted biological variability: (1) same genotype; (2) similar sizes and shapes; and (3) absence of macroscopic boring organisms. Errors specifically associated with long-standing problems to do with isotopic exchange were further reduced by producing microcolonies with no skeletal surfaces exposed to the radioisotope-labelled incubation medium. The value of the technique resides principally in its superior ability to elucidate transportation pathways and processes and not in its ability to quantitatively estimate calcium deposition by corals in nature. We describe here a rapidly exchangeable calcium pool in which up to 90% of the radioactive label taken up during incubations is located. This pool (72.9±1.4 nmol Ca mg^-1 protein) is presumably located within the coelenteric cavity as suggested by the following: (1) it has 4-min half-time saturation kinetics; (2) the accumulation of calcium is linearly correlated with the calcium concentration of sea-water; and (3) its insensitivity to metabolic and ion transport inhibitors indicate that membranes do not isolate this compartment. Washout of this large extracellular pool greatly improved estimates of calcium deposition as evidenced by 10 to 40% reduction in coefficients of variation when compared with previous ⁴⁵Ca²⁺ methods described in the literature. Comparisons of calcification measurements simultaneously carried out using the alkalinity anomaly technique and the ⁴⁵Ca protocol described here show that the correlation coefficient of both techniques is close to 1. Unlike previous reports, our ⁴⁵Ca²⁺-derived measurements are slightly lower than those computed from the alkalinity depletion technique.     

Measurement of Odor-Plume Structure in a Wind Tunnel Using a Photoionization Detector and a Tracer Gas

The patterns of stimulus available to moths flying along pheromone plumes in a 3-m-long wind tunnel were characterized using a high frequency photoionization detector in conjunction with an inert tracer gas. Four contrasting flow regimes and source conditions were produced: odor released in pulses from a vertical and horizontal array of four sources, odor released continuously from a point source, and odor released continuously from a point source into an oscillatory wake. Although the four flow regimes produced plumes of intermittent and fluctuating concentration, there were considerable differences in the structure of the signal presented to the sensor. Pulses of tracer gas released at 10 Hz retained most of their longitudinal and lateral separation. The plume growing in the disturbed flow (`oscillatory'), was broader in its lateral extent than the plume growing in an undisturbed flow (`continuous'), and the concentrations in the former were lower at each downstream position. The signal recorded in the disturbed flow had higher intermittency, but the ratio between the peak concentration and the signal mean was lower than in the continuous plume. Time scales were typically longer in the tunnel than in a field setting, but length scales and the main features of intermittency and fluctuation were similar. Moths flying along plumes of pheromone in this and similar wind tunnels typically slow their velocity and narrow the lateral excursions of their flight track as they approach a pheromone source. Which features of the plumes measured in this study account for these behavioral reactions remains to be determined.     

Building a Regionally Connected Reserve Network in a Changing and Uncertain World

Abstract: Habitat connectivity is required at large spatial scales to facilitate movement of biota in response to climatic changes and to maintain viable populations of wide‐ranging species. Nevertheless, it may require decades to acquire habitat linkages at such scales, and areas that could provide linkages are often developed before they can be reserved. Reserve scheduling methods usually consider only current threats, but threats change over time as development spreads and reaches presently secure areas. We investigated the importance of considering future threats when implementing projects to maintain habitat connectivity at a regional scale. To do so, we compared forward‐looking scheduling strategies with strategies that consider only current threats. The strategies were applied to a Costa Rican case study, where many reserves face imminent isolation and other reserves will probably become isolated in the more distant future. We evaluated strategies in terms of two landscape‐scale connectivity metrics, a pure connectivity metric and a metric of connected habitat diversity. Those strategies that considered only current threats were unreliable because they often failed to complete planned habitat linkage projects. The most reliable and effective strategies considered the future spread of development and its impact on the likelihood of completing planned habitat linkage projects. Our analyses highlight the critical need to consider future threats when building connected reserve networks over time.     

Developing a Monitoring Program for Invertebrates: Guidelines and a Case Study

Abstract:  Invertebrates provide the majority of ecosystem services; thus, it is important that they be inventoried, monitored, and protected. Nevertheless, inventories, monitoring, and management generally focus on vertebrates and flowering plants. Consequently, there are few guidelines or case studies for invertebrates. We present a procedure for developing a monitoring program for species-rich invertebrates that entails (1) characterizing the community; (2) identifying surrogates for biodiversity; and (3) establishing efficient methods to monitor surrogates and any ecologically important or sensitive taxa. We used these procedures, biodiversity-based statistical advances, and a survey of arthropods to develop a monitoring plan for the forests of Shenandoah National Park, Virginia (U.S.A.). Our case study revealed that mixed hardwood and hemlock forests had significantly different compositions of arthropods in their soil and understory strata. Of the 10 orders tested Coleoptera and Hymenoptera were the only two to pass most of the five surrogate tests, and their combination improved predictions of overall arthropod diversity. Because arthropods represent the majority of macroscopic species in most ecosystems, the ability of this assemblage to predict overall arthropod diversity makes it a powerful surrogate. Of the 11 collecting methods used, the beat-sheet method was the most efficient for monitoring this surrogate assemblage. To complement this coarse-filter approach to monitoring at-risk, invasive, or other important taxa (fine filter), we used ordination analyses to match 66 taxa with the methods that most effectively sampled them. Our methods serve as a model for developing an invertebrate monitoring plan and should facilitate linking such monitoring with ecosystem functions and management.     

Shell preference in a hermit crab: comparison between a matrix of paired comparisons and a multiple-alternative experiment

The preference of the hermit crab, Calcinus californiensis, among six species of shells, was tested by two different experiments. The first experiment used pair-wise trials, analyzing the preference by Chi-square tests using two different constructions of the null hypothesis. One hypothesis was based on a no-preference among shell species, the second on comparing the number of crabs changing for a particular shell species when two options were given versus the changing when no options were offered. The second experiment was a multiple-alternative test based on a rank ordering of the shell preference. This method has both statistical and resource-saving advantages over the traditional pair-wise comparisons. The sequence of shell preference was similarly independent of the procedure used. The preferred shell species are heavy and might be associated with hydrodynamic advantages and with the protection against predation. The shell preference matches with the pattern of shell occupancy indicating that the shell use in nature is determined by the crab’s preference. The information generated may be used for further research on shell preference as a methodological alternative.     

Length of feeding day and body weight of great tits in a single- and a two-predator environment

The risk of predation may influence the acquisition of energy and the feeding activity of animals. Feeding activity and body reserves of wintering great tits Parus major in response to the priority to food access were studied in two areas differing in incidence of predators. The one-predator area contained sparrowhawks Accipiter nisus only, whereas the two-predator area contained both sparrowhawks and pygmy owls Glaucidium passerinum, whose hunting periods overlap at dawn and dusk. In the two-predator area dominant great tits arrived at feeders significantly later in the morning, and left earlier in the evening, than their subordinate flock-mates. Hence, feeding day length of dominants was found to be significantly shorter. The reverse was true for the one-predator area. In addition, dominants carried significantly greater reserves than subordinates in the area inhabited by two predators. Factors constraining subcutaneous energy reserves were also studied in removal experiments. After the removal of dominant individuals, subordinate great tits did not reduce their body reserves in the two predator area. In contrast, subordinate great tits significantly reduced evening body reserves in the single-predator area. I concluded that the presence of the two predators increases unpredictability in feeding conditions for great tits. Dominant individuals responded to this by shortening their feeding day and increasing body reserves at dusk. Received: 8 December 1999 / Received in revised form: 15 March 2000 / Accepted: 31 March 2000     

Feeding biology of a predatory and a facultatively predatory nematode (Enoploides longispiculosus and Adoncholaimus fuscus)

This paper reports on the feeding biology of a predatory and of a facultatively predatory nematode, Enoploides longispiculosus and Adoncholaimus fuscus, respectively. Both species represent genera which are common and abundant in the littoral of the North Sea and in adjacent estuaries. Observations on the foraging behaviour of both species are given, and for the former species, a range of prey from its natural habitat is identified. Respiration was determined using a polarographic oxygen electrode technique and compared to consumption determined as predation rates on the monhysterid nematode Diplolaimelloides meyli. The daily C-loss due to respiration accounted for 15% of the measured C-consumption in E. longispiculosus and for 111% in A. fuscus, proving the observed feeding rates in the latter species to have been inadequate for the maintenance of its aerobic metabolism. Daily respiration rates at an average environmental temperature were 219 ng C ind⁻¹d⁻¹ for adults of A. fuscus and 21.9 ng C ind⁻¹ d⁻¹ for adults of E. longispiculosus. Using radiotracer techniques, no uptake of bacterial cells or of organic matter in the dissolved phase was demonstrated for E. longispiculosus. In A. fuscus, however, a significant drinking of label in the dissolved or volatile fraction occurred; bacterial cells were taken up at a level insignificant to the nematode's daily C-ration. It is concluded that E. longispiculosus has a fairly strict predatory feeding strategy, while A. fuscus gains a majority of C from additional foraging strategies, among which the uptake of dissolved material and scavenging on macrofauna carcasses (as reported in the literature) may be of particular importance. Received: 28 August 1998 / Accepted: 8 March 1999     

Simultaneous nitrogen and silicate deficiency of a phytoplankton community in a coastal jet-front

The nutritional status of a phytoplankton community was investigated in a coastal jet-front located in the Gulf of St. Lawrence, Canada, in 1987. During the sampling period, the frontal community was mainly composed of the diatomsChaetoceros debilis, Skeletonema costatum, Thalassiosira gravida andC. pelagicus. As previously reported for the St. Lawrence, some frontal stations were depleted both in nitrate and silicate. At stations impoverished in nitrate, internal nitrate pool concentrations were low or undetectable, suggesting that cells had not, recently, been exposed to a nitrate flux which exceeded the nitrate assimilation rate. At these impoverished stations, however, ambient and intracellular concentrations of ammonium and urea were high, suggesting that the community was not nitrogen-deficient. The comparison between the ambient silicate concentrations and the silicate requirement (K _s ) of the dominant diatoms suggests thatC. debilis andS. costatum were Si-deficient. This is further supported by the low silicate uptake rates and intracellular concentrations measured at the silicate impoverished stations. The silicate deficiency also resulted in a decrease in the seston and phytoplankton N:C ratios.Please address all correspondence and requests for reprints to Dr Levasseur at his new address: Maurice Lamontagne Institute, 850 Route de la Mer, Mont-Joli, Québec G5H 3Z4, Canada     

Asymmetry in a visual signaling character and sexual selection in a wolf spider

Fluctuating asymmetry (FA) can indicate developmental instability in bilaterally symmetric organisms, and studies have shown that the degree of asymmetry in male secondary sexual characters influences female mate choice in a number of taxa. In male Schizocosa ocreata wolf spiders, conspicuous tufts of bristles on the forelegs are a critical component of visual courtship signals, which appear to play a role in female mate choice. Previous studies have shown that females exhibit reduced receptivity to males with regenerative asymmetry, a consequence of leg loss and regeneration that causes males to be grossly asymmetric with respect to this important signaling character. We provide data on the occurrence of FA in the tufts of S. ocreata, and examine further the influence of asymmetry on female mate choice. The distribution of tuft area asymmetry values from a sample of field-collected males was normal, with a mean value of zero, indicating true FA. For a subset of males measured directly after field collection and prior to feeding, tuft asymmetry was significantly negatively correlated with measures of body size (body length) and condition (abdomen volume/cephalothorax width). Receptivity responses of females to visual signals from live males of similar size varied with the degree of asymmetry in male tufts. Since FA covaries with male body size and condition, which may also influence behavioral vigor, we used video image manipulation to alter the degree of asymmetry in tufts of a courting male while holding size and condition constant. Asymmetry treatments represented values within the range of natural FA variation as well as more extreme values characteristic of regenerative asymmetry. With the confounding effects of male size, condition, and behavior held constant, female spiders exhibited reduced receptivity responses to all experimental asymmetric video images relative to a control video stimulus. There were no differences in the frequency of female receptivity among the various asymmetry treatments, suggesting that discrimination against asymmetry in conspecific male signal characters is not simply a rejection of extreme phenotypes. Results suggest that asymmetry in a key male secondary character used in visual signaling, independent of any concomitant behavioral or size factor, is an important criterion in mate choice. Received: 26 February 1998 / Accepted after revision: 12 September 1998     

Trail following,speed and fractal dimension of movement in a marine prosobranch,Littorina littorea,during a mating and a non-mating season

We quantitatively studied movement behaviour in the gastropod Littorina littorea in laboratory experiments during periods of their non-mating season (November 1992) and their mating season (April–May 1993). Snails were collected in 1992 and 1993 from one boulder shore on the north west coast of Sweden. In a comparison between the two seasons (one non-mating and one mating) we measured trail complexity of unsexed snails using fractal dimesion, the degree of mucus trail following (coincidence index of marker and tracker trails) and average movement speed of marker and tracker snails. We found no differences in fractal dimension and coincidence index of trails between the two seasons. Tracker snails moved, however, significantly faster than marker snails during both seasons. This could not be explained by trackers, when locomoting, using the mucus trail deposited by the marker to increase their speed, since there was no correlation between coincidence index and tracker speed. During the mating season we also conducted trail complexity, trail following and speed experiments comparing the behaviour of males and females. There was no difference between males and females in the fractal dimension of movement, nor was there any difference between the mean speed of male and female snails, although male marker snails tended to move faster than female marker snails. Males tracking other males, females tracking other females and females tracking males followed trails about equally long distances (i.e., coincidence indices did not differ). In contrast, males following female mucus trails showed a significantly higher degree of trail following than the other sex combinations. This new finding may suggest that females of L. littorea release pheromones in their mucus trails and that males are able to identify them.     

Linking species- and ecosystem-level impacts of climate change in lakes with a complex and a minimal model

To study the interaction between species- and ecosystem-level impacts of climate change, we focus on the question of how climate-induced shifts in key species affect the positive feedback loops that lock shallow lakes either in a transparent, macrophyte-dominated state or, alternatively, in a turbid, phytoplankton-dominated state. We hypothesize that climate warming will weaken the resilience of the macrophyte-dominated clear state. For the turbid state, we hypothesize that climate warming and climate-induced eutrophication will increase the dominance of cyanobacteria. Climate change will also affect shallow lakes through a changing hydrology and through climate change-induced eutrophication. We study these phenomena using two models, the full ecosystem model PCLake and a minimal dynamic model of lake phosphorus dynamics. Quantitative predictions with the complex model show that changes in nutrient loading, hydraulic loading and climate warming can all lead to shifts in ecosystem state. The minimal model helped in interpreting the non-linear behaviour of the complex model. The main output parameters of interest for water quality managers are the critical nutrient loading at which the system will switch from clear to turbid and the much lower critical nutrient loading – due to hysteresis – at which the system switches back. Another important output parameter is the chlorophyll-a level in the turbid state. For each of these three output parameters we performed a sensitivity analysis to further understand the dynamics of the complex model PCLake. This analysis showed that our model results are most sensitive to changes in temperature-dependence of cyanobacteria, planktivorous fish and zooplankton. We argue that by combining models at various levels of complexity and looking at multiple aspects of climate changes simultaneously we can develop an integrated view of the potential impact of climate change on freshwater ecosystems.     

Coupling a land use model and an ecosystem model for a crop-pasture zone

This paper describes the development of a land use model coupling ecosystem processes. For a given land use pattern in a region, a built-in regional ecosystem model (TESim) simulates leaf physiology of plants, carbon and nitrogen dynamics, and hydrological processes including runoff generation and run-on re-absorption, as well as runoff-induced soil erosion and carbon and nitrogen loss from ecosystems. The simulation results for a certain period from 1976 to 1999 were then used to support land use decisions and to assess the impacts of land use changes on environment. In the coupling model, the land use type for a land unit was determined by optimization of a weighted suitability derived from expert knowledge about the ecosystem state and site conditions. The model was applied to the temperate crop-pasture band in northern China (CCPB) to analyze the interactions between land use and major ecosystem processes and functions and to indicate the added value of the feedbacks by comparing simulations with and without the coupling and feedbacks between land use module and ecosystem processes. The results indicated that the current land use in CCPB is neither economical nor ecologically judicious. The scenario with feedbacks increased NPP by 46.78 g C m⁻² a⁻¹, or 32.23% of the scenario without feedbacks, also decreased soil erosion by 0.65 kg m⁻² a⁻¹, or 23.13%. Without altering the regional land use structure (proportions of each land use type). The system developed in this study potentially benefits both land managers and researchers.     

Isolation and identification of a bioflocculant-producing strain and optimisation of cultural conditions via a response surface model

A bioflocculant-producing strain named MY6-2 was isolated from a mixed activated sludge by a nitrogen-free medium. Based on 16S rDNA sequence, biochemical and morphological characteristics, the strain MY6-2 was identified as Bacillus mucilaginosus. The chemical analysis indicated that the flocculant MY6-2 was mainly composed of extracellular polysaccharide. The result of the composition of the medium showed that MY6-2 was able to generate bioflocculant in a nutrient-poor medium that consisted of 5?g?L^?1 sucrose and no nitrogen source. Response surface methodology (RSM) was used to optimise the fermentation condition of MY6-2 for maximum flocculating activity by using central composite design (CCD). Based on the result, the optimum conditions were as follows: 100?mL of broth in a 250?mL Erlenmeyer flask, initial pH 8.0 and inoculum concentration 10%, respectively. The highest flocculating rate of 90% was achieved under these conditions by adding 0.5?mL fermentation supernatant to 95?mL of Kaolin suspension.