Relative Importance Of Competition Research Articles

Out of the frying pan into the fire: an ecological perspective on evolutionary reversal in life history in plethodontid salamanders (Amphibia: Plethodontidae)

I review and analyze interaction webs involving competition and predation in ensembles of plethodontid salamanders in the southern Appalachian Mountains, USA. The objective is to evaluate hypotheses on interaction webs and population regulation in these ensembles in the context of traditional and recent models of phylogeny and life-history evolution of plethodontids. Two ecological hypotheses evaluated are (1) Hairston’s conjecture on differences in the kinds of interactions prevalent among biphasic species with an aquatic larval stage, versus terrestrial species with direct development; and (2) Grover’s hypothesis on differences in the factors that limit the distribution of species along the moisture gradient from stream to forest. An evolutionary question concerns the supposition derived from recent phylogenetic reconstructions that the history of one major clade, the supergenus Desmognathus, involved an early transition or reversal from a simple life cycle (direct development) to a biphasic life cycle (larval development, metamorphosis), and that this transition promoted the subsequent adaptive radiation of the taxon. Chippindale et al. proposed that selection for life-history reversal in the genus Desmognathus was promoted by competition between direct-developing ancestral desmognathans and other terrestrial plethodontids. Experimental studies of interaction webs in extant ensembles of plethodontids in the southern Appalachians, in concert with data from descriptive studies of feeding and habitat use, demonstrate variation in the relative importance of competition and predation in population regulation of species in the several codistributed lineages of these salamanders. Whereas the existing ecological data do not refute the Chippindale et al. hypothesis, many unresolved questions require additional investigation.

Competition as a factor structuring species zonation in riparian fens ‐ a transplantation experiment

AbstractQuestion: What is the relative importance of competition and physical factors in restricting the occurrence of different fen species to certain zones of the riparian landscape?Location: Biebrza National Park, NE Poland.Methods: We carried out a two‐season reciprocal transplantation experiment: sod‐blocks were replaced between a low‐productive groundwater‐fed small‐sedge fen and a high‐productive fluviogenous tall‐sedge fen. Five treatments were applied to the transplanted sods: (1) no manipulation; (2) fertilization with NPK to exclude the effect of nutrient limitation; (3) clipping vegetation around the transplant to exclude competition for light; (4) clipping + fertilizing; (5) sods were also transplanted within their own environment.Results: After two seasons, the small‐sedge sods transplanted into the high‐productive zone had a biomass two times lower than that of the control transplants, mainly due to a decrease in small sedges and bryophytes. Tall sedges of the high‐productive zone did not respond significantly to transplantation in the low‐productive zone. Removal of the surrounding vegetation largely enhanced the growth of small sedges and bryophytes and, to a lesser degree, of tall sedges. Fertilization, on the other hand, resulted in increased growth of tall sedges, grasses and non‐graminoid herbs.Conclusion: Species characteristic of low‐productive fen communities are competitively excluded from the high‐productive zone through light competition. In contrast, the performance of tall sedges in the low‐productive zone is lowered by nutrient limitation. In the long run this may lead to a complete disappearance of these species from this zone. We did not find evidence that the physical stress of flooding has a direct effect on the performance and distribution of species. Results from the experiment suggest that productivity gradients and their influence on competition intensity are of primary importance for structuring vegetation patterns in lowland riparian fens.

THE IMPORTANCE OF COMPETITION IN REGULATING PLANT SPECIES ABUNDANCE ALONG A SALINITY GRADIENT

Current theories differ in their predictions concerning the effects of interspecific interactions on species growth and distribution along environmental gradients. In this study, we examined the influence of competition on species composition across a salinity gradient. This work involved three common fresh and brackish marsh species. The three species, Spartina patens, Sagittaria lancifolia, and Panicum hemitomon, differ in salt tolerances but are commonly found in overlapping zones across coastal marsh gradients in the southeastern United States. Plants were grown in a greenhouse under four salinity treatments (0, 2, 4, and 8 g/kg), in monocultures and in three-species mixtures. Data from the monocultures and mixtures were used to examine the importance of competition in regulating community composition and species' growth. Results for individual species were significantly different when grown in monoculture vs. the three-species mixture. While with increasing salinity community dominance shifted toward the most salt-tolerant species, Spartina patens, competition was found to alter community composition equally over all salinities. However, the responses of individual species demonstrated a very different pattern; the relative importance of competition in influencing individual growth differed substantially depending on the salinity tolerances of species. These results support the contention that, as abiotic stress increases, competition becomes less of a limiting factor in regulating the abundance of a species. However, the role of competition at the community level is the sum of the individual species' responses and may or may not vary with the level of abiotic stress.

The Relative Importance of Competition and Predation Varies with Productivity in a Model Community.

Recent theory predicts that productivity can influence the relative importance of predation and competition in determining patterns in abundance, diversity, and community structure. In low-productivity systems, competition is predicted to be the major influence on community patterns, while at high productivity, the major influence is predicted to be predation. We directly tested this theory using a laboratory model community. Our model community consisted of the bacteriophage T2 (a virus that feeds on Escherichia coli) and two populations of E. coli, in glucose-limited chemostats. One E. coli population consisted of individuals that were sensitive to predation by T2 ("vulnerable" E. coli), and the other population consisted of individuals that were partially resistant to predation by T2 ("less vulnerable" E. coli). We manipulated productivity in this experiment by running replicate chemostats with different input concentrations of glucose. Our observations were consistent with theoretical predictions. We observed the decline of the more vulnerable prey population at higher productivity but not at lower productivity, and the decline of the less vulnerable prey population at lower productivity but not at higher productivity. However, the rate of decline in some replicates was slower than predicted, and extinctions were not observed during the experiments, contrary to theoretical predictions. We present some testable hypotheses that might explain the slow rate of decline observed.

Relative importance of competition with Daphnia (Cladocera) and nutrient limitation on Anuraeopsis (Rotifera) population dynamics in a laboratory study

Summary 1 In the laboratory, the growth and reproduction of Anuraeopsis fissa were measured when fed on Scenedesmus species grown in nutrient-sufficient, nitrogen-limited and phosphorus-limited media and in the presence or absence of one adult Daphnia longispina per vial. 2 Poor food quality may reduce the effect of competition on rotifers. Competition from Daphnia was stronger with nutrient sufficient algae than with nutrient limited algae. P-limitation significantly reduced Anuraeopsis population growth rate and fecundity. The effect of nutrient limitation on Anuraeopsis was stronger than that of competition with Daphnia. The Anuraeopsis population declined with P-limited food in both the presence and absence of Daphnia. 3 Exploitative competition by Daphnia on Anuraeopsis was stronger in the nutrient-sufficient treatment than in the N-limited one. Density, fecundity and population growth rate of Anuraeopsis were negatively affected by Daphnia in the nutrient-sufficient treatment, while only fecundity was reduced by Daphnia in the N-limited treatment. Consequently, in the N-limited treatment, mortality should be lower in the presence of Daphnia. This result could suggest that Anuraeopsis lives longer when short of nitrogen. 4 Nutrient limitation may affect to the competitive interactions between zooplankton species. P-limitation decreased the quality of algae as food for Anuraeopsis while N-limitation decreased the susceptibility of this rotifer species to exploitative competition by Daphnia.

Indirect effects in a desert plant community: is competition among annuals more intense under shrub canopies?

An unresolved discussion in contemporary ecology deals with the relative importance of competition along environmental gradients. In deserts, local-scale differences in environmental productivity may be caused by the presence of shrubs, which represent a favorable habitat for annual populations within a nutrient-poor matrix. In this study, we attempted to test the hypothesis that facilitation of desert annuals by shrubs increase the intensity of competition among the annual plants. Such negative indirect effects have so far been ignored in studies about plant-plant interactions. We tested our hypothesis by measuring seedling survival and fecundity of four abundant annual plant species with and without neighbors in open areas and under shrub canopies in a sandy desert area. Our findings did not indicate indirect negative effects of shrubs on their understory annuals. Sensitivity to the presence of neighbors varied between species and surprisingly, the species with the smallest seeds was the only one which was not negatively affected by the presence of neighbors. In contrast to our hypothesis, there was no difference between the habitat types shrubs and openings in absolute and relative competition intensity. Our overall results suggest that negative indirect effects of shrubs are unimportant in determining demographic response of understory annual plants.

The Relative Importance of Competition and Predation Varies with Productivity in a Model Community

The Relative Importance of Competition and Predation Varies with Productivity in a Model Community

The Relative Importance of Competition and Contagion in Intra-Industry Information Transfers: An Investigation of Dividend Announcements

Large revisions in dividends are accompanied by stock price reactions for industry rivals of the announcing firm. Though these effects are near-zero on average, their magnitude differs systematically across the firms in the industry. Rivals that are unlikely to be affected by competitive realignments within the industry tend to experience stock price effects like those of the announcing firm. Those that are likely to be affected tend to experience statistically insignificant reactions of the opposite sign. Thus, for some rivals, competitive effects apparently offset contagion effects. We find supporting results for changes in rival's dividends over a longer period.

Competition and Herbivory in Establishing Grassland Communities: Implications for Plant Biomass, Species Diversity and Soil Microbial Activity

The two main biotic factors affecting grassland plant species are herbivory and competition. We investigated the significance of both these factors in establishing grassland communities through manipulation experiments conducted in both winter and summer in glasshouse conditions. Manipulations consisted of addition of above-ground and below-ground herbivores, reduction of herbivory through use of an insecticide, and varying of competitive pressure by removing either dicotyledonous or monocotyledonous seedlings as they appeared. In the winter experiment, the total biomass of dicotyledonous plants was reduced by both above-ground and below-ground herbivory, while monocotyledonous plant biomass was unaffected. However, in the summer experiment, only those treatments in which competition was manipulated had any detectable effect. This suggests that the relative importance of competition and herbivory can vary considerably. The above-ground herbivory treatment sometimes stimulated and sometimes reduced plant species diversity, and sometimes also induced significantly higher dicotyledonous species diversity than the root herbivory treatment. Those treatments in which competitive pressure was manipulated had no detectable effects on diversity. There were many strong effects detected when plant species were considered individually, with different species responding to different treatments, and with all treatments affecting at least some species. Different species therefore respond quite differently to competition and herbivory. None of the treatments affected soil microbial activity or active soil microbial biomass in the winter experiment, although active biomass was significantly positively correlated with dicotyledonous plant biomass. In the summer experiment, soil activity was positively affected by monocotyledonous plants, while active biomass was the same in all treatments except for the treatment with all plants removed, where biomass was less. None of the soil biotic variables showed any relationship with plant species richness. Our study shows that competition and herbivory can both exert important influences at both the community and ecosystem levels of resolution.

On the Relative Importance of Competition in Unproductive Environments

I Whether or not competition intensity increases or is similar along productivity gradients has been highly controversial for a number of years, but empirical results bearing on this question are quite variable and no consistent answer has yet emerged. We have developed a more general hypothesis that includes these contradictory predictions as special cases that apply under different types of resource dynamics and different types of interactions between the growth and survival components of fitness. 2 The two-phase resource dynamics hypothesis of plant interactions along productivity gradients is based on the fact that soil resources are usually supplied in pulses rather than continuously, as assumed by most formal theory for community dynamics of terrestrial plants. When soil resource supply is temporally variable, individual plants will experience two distinct phases of resource availability: pulse periods when resources are high and most growth and resource accumulation occurs, and interpulse periods when resources are too low for most plants to take up and most mortality due to resource deficits takes place. 3 Competitive effects on growth should occur during pulses at both high and low productivity. In productive environments, interpulse intervals should be relatively mild and infrequent and therefore competitive effects during pulses will usually be important for individual and population persistence. However, as productivity decreases, the frequency of pulses (as well as or in place of their magnitude) often decreases and the duration of interpulse periods increases. Therefore, we suggest that processes occurring during interpulse intervals become increasingly important for individual and population persistence as interpulse intervals become longer. Whether or not competition occurs under low productivity will then depend on (i) the extent to which the asymptotically low resource availability during interpulse periods is determined by plant uptake or by abiotic factors such as leaching, drainage, evaporation and volatization, and (ii) the extent to which decreased growth due to competition during pulses results in decreased survival during interpulse periods. 4 According to the two-phase resource dynamics hypothesis, Grime's hypothesis that competition is unimportant at low productivity will hold when soil resource availability between pulses in unproductive environments is controlled by abiotic factors and when survival during interpulse intervals is independent of or even negatively correlated with growth during pulse periods. In contrast, Newman's and Tilman's hypothesis that competition is equally important along productivity gradients will apply when either of these conditions is not true. We predict that the conditions for Grime's hypothesis to apply are more likely for productivity gradients driven by water than by mineral nutrients and when response to competition is measured for community structure or individual survival rather than for individual growth. 5 We tested the predictions about water vs. nutrient gradients and growth vs. survival or community structure responses using data from a literature survey on field and

Applications of the Menge‐Sutherland Model to Acid‐Stressed Lake Communities

The predictions of the Menge‐Sutherland model with respect to the effects of environmental stress on species richness, food web complexity, food chain length, and the relative importance of predation, competition, and disturbance in structuring communities were examined using midsummer zooplankton communities of 46 lakes near Sudbury, Ontario with pHs of 3.8‐7.2. The lakes were sampled in the 1970s and in 1990 yielding a total of 92 food webs. Species richness increased linearly with pH rather than peaking at intermediate levels of stress as predicted by the model. Food web complexity and food chain length increased with pH, as predicted. The importance of competition in food web structure was variable at low pH and consistently high at moderate pH, a result that does not support model predictions that disturbance, competition, and predation should sequentially predominate with increasing pH. High variability in the relative importance of competition in acidic lakes was explained by fish presence or absence; in lakes without fish, a larger proportion of links in the zooplankton food web was due to predation by invertebrates. Persistence of predators (fish) in some acidic lakes reduces the applicability of the Menge‐Sutherland model.

Zooplankton Assemblages in Fishless Bog Lakes: Influence of Biotic and Abiotic Factors

Some bog lakes in northern Wisconsin and Michigan lack fish because of low pH and low winter oxygen concentrations. The lakes are characterized by a dominance of large zooplankton species. Small zooplankton species are scarce, whereas in nearby lakes with fish they are common. The importance of biotic and abiotic factors in preventing the successful invasion of small zooplankton species into these fishless lakes was determined using a series of field manipulations. The relative importance of competition with large herbivores (the cladoceran Daphnia pulex), predation by large invertebrates (the dipteran Chaoborus americanus and the copepod Diaptomus leptopus), and abiotic conditions including pH were examined experimentally. Abiotic and/or resource conditions suppressed the population growth rate of 70% of the small zooplankton species introduced into enclosures in the fishless lakes relative to those introduced into enclosures in their resident lake (a lake with fish). Fewer small zooplankton species were suppressed in a similar experiment when pH was experimentally raised to 7, suggesting that pH is one factor that inhibits the successful invasion of small zooplankton into these lakes. Manipulations of large herbivores and invertebrate predators showed that predation was more important than competition in restricting the distribution of small species. Predation by Chaoborus primarily reduced the densities of crustaceans while predation by Diaptomus reduced the densities of rotifers. Competitive suppression of small zooplankton by Daphnia was not detected. The results suggest that both predation by Chaoborus and Diaptomus and abiotic conditions, including pH, are important factors determining zooplankton community structure in these fishless bog lakes.

The relative importance of competition, microsite, and climate in controlling the stem taper and profile shape in jack pine

The purpose of this study was to identify the relative importance that environmental factors have on the taper and profile shape of mature jack pine (Pinusbanksiana Lamb.) stems. A total of 60 dominant trees from six different stands (10 per stand) were felled and measured to determine stem size, taper, and profile shape. Total height, basal age, and diameters at three locations (breast height, 20% total height, and 70% total height) were collected. The last two diameters were then used to calculate stem taper and profile shape. Data on competition (4 variables) and microsite (10 variables) were collected. Climatic variables (6) were interpolated from nearby weather station data. From the regression results, several environmental factors from all three categories (competition, microsite, and climate) proved to be important in determining taper and shape. The most relevant measures from each category, respectively, were percent canopy opening (taper) or total basal area (shape), slope, and growing degree-days. When combined in a multiple linear regression, these environmental variables, along with stand age, explained 61.9% of the variation in stem taper and 38.6% in stem shape. Although these values represent a significant portion of the variation in stem profile characteristics, further exploration into additional factors (e.g., bending and stress caused by wind action, nutrition, genetics) affecting stem taper and shape is required.

Salt Marsh Plant Zonation: The Relative Importance of Competition and Physical Factors

In Carpinteria Salt Marsh, Salicornia virginica (pickleweed) grows at lower marsh elevations than does Arthrocnemum subterminalis (Parish's glasswort). Standing biomass of both species was greatest immediately adjacent to their abrupt border, suggesting that conditions for plant growth were best here. We utilized field experiments, in which growth rates of naturally occurring and transplanted individuals of both species were measured in four marsh zones, to investigate the role of edaphic factors and competition in maintaining this zonation pattern. The frequency of flooding, and hence soil waterlogging, was greatest at lower marsh elevations, whereas salinity was highest at higher marsh elevations. Consequently, it was not clear, a priori, which part of the marsh had the most severe physical conditions. In our field experiments, both Salicornia and Arthrocnemum grew better in the two middle marsh zones (high Salicornia zone and Arthrocnemum zone) than in either the low marsh (low Salicornia zone), where flooding was frequent and soils were waterlogged, or the high marsh (transition zone), where soil salinity was extremely high during much of the year and plant water potentials very low. However, Salicornia appeared better able to tolerate flooding, and so persisted in the low Salicornia zone, whereas Arthrocnemum appeared better able to tolerate high salinities, and so persisted in the transition zone. Interspecific competition was most important in the relatively benign middle marsh zones, where each species excluded the other from a portion of this prime habitat. In this marsh, flooding, soil salinity, and competition all interacted to determine plant zonation patterns, but the relative importance of these factors varied at different elevations.

Disturbance and Variation in Competition Between Two Stream Insects

This study examined temporal and spatial variability in competition as well as the relative importance of competition and natural disturbance to Hydropsyche oslari and Simulium virgatum in a southern California stream. These two filter—feeding insects dominated the community studied, accounting for 99% of all individuals. I maintained four field treatments every 2 wk for a year: (1) removal of Simulium; (2) removal of Hydropsyche; (3) removal of both taxa and subsequent scrubbing of the rock surfaces; and (4) unmanipulated controls. Competition from Hydropsyche oslari restricted the spatial distribution of Simulium virgatum in all seasons except late winter in upstream but not in downstream sections of the study site. Downstream abundance of Simulium was probably limited by food availability, not competition from Hydropsyche. During the winter, when floods occurred and densities were low, competition between these insects had no significant impact on spatial distributions. Although competition was seasonally variable, it occurred frequently and was not rare. Although disturbance mediated competition, competition was more important in determining the abundance of Simulium in 1983—1984, a drought year. Disturbance caused a 13% increase, while competition caused a 34% decline in the simuliid population over the year. It was not possible, however, to rank the importance of competition and disturbance when considering both species together. The relative importances were taxon specific. The abilities of Simulium and Hydropsyche to recover quickly after disturbance were inversely related to their ability to withstand competition. In addition, the interaction between the two factors was extremely important in determining the relative abundance of both taxa through the year. Competition and disturbance both played an important role in regulating this community, at times accounting for up to 60% reductions in the numbers of individuals present in the stream. The relative importance of competition and disturbance in Refugio Creek should vary from year to year depending on the timing and intensity of winter storms.

Assessing components of competition indices for young boreal plantations

Important components of interspecific competition affecting apical and cambial growth in young plantations of black spruce (Piceamariana (Mill.) B.S.P.) and jack pine (Pinusbanksiana Lamb.) in northern Ontario were identified. The relative importance of competition, initial seedling status, and some soil factors was also assessed. Measures of perennial competition were calculated using hemispherical photographs, stand maps, and mensurational data from 360 seedling-centred plots (1.4 m radius) for each of the two species. Fractal geometry was used to quantify the textures of the competing canopies. Reliable competition indices should include horizontal and vertical dimensions and express the amount of competition relative to the size of the seedling. The total canopy fraction on the photographs and some simple field measurements of competition correlated significantly with seedling growth. Complex expressions of competition were unjustified, and measures of dispersion and canopy texture correlated weakly with growth. The consistently good correlations between growth and some components of competition revealed that an index applicable to all species, stock types, and growth responses could be developed.

Habitat Selection by Breeding American Redstarts in Response to a Dominant Competitor, the Least Flycatcher

-Through natural population changes and experimental field removals, we tested the hypothesis that Least Flycatchers (Empidonax minimus) restrict habitat use by socially subordinate American Redstarts (Setophaga ruticilla). On a 10-ha site 2-yr-and-older (ASY), but not yearling (SY), male redstarts avoided the sector occupied by flycatchers from 1975 to 1980, but preferred this sector from 1981 to 1985 when flycatchers were absent. Vegetation changed subtly on the site but could not account for the sudden shift in redstart settlement pattern. On 6 4-ha sites ASY male redstarts were most abundant in years of Least Flycatcher absence. On the 5 4-ha sites from which Least Flycatchers either disappeared independently or were removed experimentally between 1981 and 1984, redstart abundance increased on four and remained constant on the fifth; on three control areas redstart numbers declined during the same period. Least Flycatchers recolonized one removal site, and ASY redstart abundance subsequently declined. SY male redstart abundance varied inversely with that of ASY male redstarts. We conclude that flycatchers influenced the distribution of ASY male redstarts directly, and that of SY males indirectly, more than either vegetation structure or other habitat characteristics. At no spatial scale examined, however, did total redstart abundance (ASY + SY) vary inversely with that of Least Flycatchers; in fact, their total abundances correlated positively at a regional scale. These findings, combined with a model for asymmetric competition for mutually preferred habitat (Pimm et al. 1985, Rosenzweig 1985), illustrate how a socially dominant competitor could lead to a broadening rather than a narrowing of the habitat breadth of a subordinate species. We show that competitor species abundances need not vary inversely and that age classes may be affected differentially. This species interaction illustrates subtleties and complexities of how competition can modify avian habitat selection. Received 2 September 1987, accepted 22 January 1988. INTERSPECIFIC competition is widely believed to restrict the range of habitats or resources exploited by many species (e.g. Svardson 1949, Connell 1983). If each species reduces the other's abundance under different circumstances, two species should vary inversely in abundance in one habitat over time, or across an array of habitats at a particular time. Avian biologists in particular have frequently interpreted negative correlations in abundance and replacements along habitat gradients as evidence for interspecific competition (Svardson 1949, MacArthur 1972, Cody 1974, Terborgh and Weske 1975, Diamond 1978, Noon 1981, Mountainspring and Scott 1985, Grant 1986). Recent experimental studies, especially those involving the removal of individuals of one or more putatively competing species, have confirmed that interspecific competition for habitat occurs regularly in a variety of bird species (Mewaldt 1964, Davis 1973, Williams and Batzli 1979, Dhondt and Eyckerman 1980, Hogstedt 1980, Reed 1982, Garcia 1983, Alatalo et al. 1985). Other investigators recorded no negative correlations between densities, and concluded that species often respond independently to structural and floristic features of their environment (Wiens 1977, Rotenberry and Wiens 1980, Wiens and Rotenberry 1981, Collins et al. 1982, James and Boecklen 1984, James et al. 1984). These findings have been interpreted as evidence that species are distributed independently and are not strongly influenced by interspecific competition. The contrasting results between these two types of studies have contributed to the recent controversy concerning the relative importance of competition in structuring communities (Strong et al. 1984, Connor and Sim-

The experience of express coach deregulation in Great Britain

This paper reviews the effect of express coach deregulation in Britain following the Transport Act 1980. The authors have monitored the range of services offered before and after the Act, using operators' literature, roadside surveys, on-vehicle passenger interviews and interviews with operators as sources of information. These are used to present an overall picture of express deregulation, with particular reference to changes in real fare levels, and also the relative importance of competition within the coach industry compared with that of inter-modal competition (that between coach and rail). The latter has emerged as more significant. The greater part of the paper is devoted to analysis of the long-distance market but the development of "commuter coach" services into central London is also examined, together with the reasons why independent operators have secured a larger share of this market.

The Effects of Suspended Silt on Feeding and Reproduction of Daphnia pulex

This paper reports the effect of suspended silt on the feeding and reproduction of Daphnia pulex and the impact of suspended silt and clay on freshwater zooplankton community structure. The effects of suspended silt and clay on the filtering and assimilation rates of Daphnia pulex were determined using a 14C radiotracer method. Both filtering and assimilation rates are severely depressed at even low concentrations of suspended silt and clay. Life table studies also showed population growth rate of zooplankton was significantly diminished by suspended silts and clays. The relative abundance of zooplankton varied markedly between two lakes of differing turbidity levels, the more turbid lake having a higher relative abundance of large zooplankton species. Suspended silt and clay reduced zooplankton feeding and production but probably influenced zooplankton community structure by impairing the ability of visually feeding planktivorous fish to locate their prey. INTRODUCTION Zooplankton play a pivotal role in the transfer of energy in food chains of ponds, lakes and oceans. These small herbivores feed on diffuse plant resources, thereby repackaging energy into a much more concentrated form which can be used by carnivores. There have been many studies of the feeding ecology of zooplankton (Ryther, 1954; Rigler, 1961; Burns and Rigler, 1967; Hall et al., 1976; Porter et al., 1982) and the forces which limit zooplankton populations. Still, there is debate on the relative importance of competition among zooplankton for limited food resources vs. planktivory as the dominant force limiting zooplankton population densities and structuring zooplankton communities (Brooks and Dodson, 1965; Hall et al., 1970; Lynch, 1979; Neill and Peacock, 1980; Zaret, 1980). This debate arises anew when considering turbid lakes characteristic of midwestern and southeastern North America. Such lakes often have large zooplankton in relatively high densities and such large zooplankton could result from interaction with suspended silts and clays in at least two ways. First, dissolved organic matter adsorbed to silt particles ingested by filter-feeding zooplankton could supply added nutrition sources (Marzolf, 1965). Second, suspended silt particles may serve as a refuge for zooplankton from planktivorous fish which locate prey visually (Vinyard and O'Brien, 1976; Gardner, 1981). Vinyard and O'Brien (1976) and Wright (1981) showed that increasing turbidity impairs the ability of planktivorous fish to locate prey. Buck (1956) reported production of redear sunfish (Lepomis microlophus) in low turbidity ponds was 300 times greater than in ponds with high turbidity. While the effects of turbidity on predaceous fish are documented, similar data for herbivorous zooplankton are not available. The purpose of this study is to determine the effects of suspended silt and clay on filtering rates, assimilation efficiencies, population growth and community structure of zooplankton. METHODS Algal cultures used in filtering rate and assimilation efficiency determinations were 1 Current address: Department of Environmental Science and Engineering, University of California at Los Angeles, Los Angeles, California.

Coexistence of two species of Binella Motchulsky (Coleoptera: Ptiliidae) and the significance of their adaptation to different temperature ranges

Abstract. Ptinella aptera and Ptinella errabunda often coexist where their distributions overlap. The ecological requirements of P.aptera and P.errabunda are similar. Both are generalized mycetophages restricted to the same subcortical habitat of trees in a particular state of decay. Relative humidity appears important in determining habitat suitability for both species, neither of which survived in laboratory regimes below approximately 100% relative humidity. In laboratory studies the beetles differed in their response to temperature. Recorded maxima for activity and rate of increase were at higher temperatures for P.aptera than for Perrabunda. Temperatures which ‘Ptinella were exposed to in the laboratory occur in natural conditions under bark. Their effects on the species’ distributions are considered. The differential temperature adaptations of P.aptera and P.errabunda were probably evolved in allopatry, in response to different environmental conditions. In view of this, it is unreasonable to infer past interspecies competition from these niche differences. The niche difference between coexisting Ptinella species is discussed in the light of current ideas concerning the relative importance of competition and other factors in natural communities.