摘要:

Recent in situ fertilization studies of free spawning and brooding marine organisms have focused almost exclusively on the yield of fertilized ova (female reproductive success). As a consequence, we known little about the factors that determine male reproductive success. If marine organisms compete for fertilizations (as do many terrestrial organisms), then a male's reproductive success should be reduced by the presence of other males. We tested this hypothesis via in situ experiments employing allozyme markers for both a colonial ascidian (Botryllus schlosseri) and a bryozoan (Celleporella hyalina). Under moderate density conditions, the presence of closer male—functioning colonies reduced the fertilization success of more distant males in both species. In C. hyalina, male fertilization success also increased with allocation to sperm production. In addition, selfing rates in this species were negatively correlated with the abundance of outcross sperm. These results suggest that male reproductive success in sessile marine invertebrates must be assessed as a function of the gamete output and spatial distribution of other males in a population, and that the performance of isolated males may yield overestimates of male fertilization success in natural populations.

ISSN:0012-9658    

DOI:10.2307/1940874

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

The high tertiary production of estuaries is largely supported by photoplankton primary production. An important question thus concerns how much phytoplankton production enters the food web through planktonic grazing and what physical, chemical, or biological factors influence this trophic transfer. We conducted a 2—yr, diel investigation of planktonic trophic transfer and the factors influencing in the Neuse River Estuary, North Carolina. Zooplankton community grazing rates were generally lowest in winter and highest spring through late summer, ranging from 0.1 to 310 mL°L—1°h—1. There were few significant diel differences in community grazing rates. The overall daytime mean (± 1 SE) rate was 3.30 ± 0.62 mL°L—1°h—1while the night mean rate was 3.07 ± 0.60 mL°L—1°h—1. Post—naupliar copepods were usually more abundant at night than day, but tintinnids were often more abundant by day, while total zooplankton, copepod nauplii, and rotifers displayed no significant diel abundance differences. Community grazing was positively correlated with primary productivity and the abundance of total phytoplankton, centric diatoms, dinoflagellates, and the small diatom Thalassiosira. Community grazing was also positively correlated with upstream river flow and negatively correlated with salinity. However, there were no significant correlations with water temperature, nutrient concentrations, or grazer abundance variables. On an annual basis, the zooplankton community grazed °38—45% of daily phytoplankton production. Planktonic trophic transfer was coincidentally greatest in late spring through summer, during the period when anadromous fish larvae migrating from the open ocean reach their estuarine primary nursery areas. The fish arrive when planktonic trophic coupling is strongest and depart in fall, when planktonic trophic transfer, zooplankton abundance, and phytoplankton productivity all decrease.

ISSN:0012-9658    

DOI:10.2307/1940875

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Synchronous release of gametes or larvae by marine animals may be controlled as many as four environmental cycles thereby enabling an array of reproductive timing patterns. Cohesive scenarios that account for the diversity of reproductive patterns are rare and tests of their adaptive significance have been even rarer. By exploiting plasticity in the timing of larval release, we isolated proximate factors regulating reproductive synchrony and provided evidence that predation ultimately may best explain the diversity of hatching patterns by brachyuran crabs. Tides entrain reproductive rhythms by many crabs, and therefore spatial and temporal variation in tides produces intraspecific variation in reproductive patterns. This was demonstrated by determining the timing of larval release of the same or sibling species of intertidal crabs in Pacific semidiurnal and Caribbean mixed semidiurnal tidal regimes on the two coasts of the Republic of Panama. The time of larval release varied during the year in the Caribbean, where entraining physical cycles exhibited complicated changes in phase, but not along the Pacific coast, where the phase relationships among physical cycles varied little year—round. Crabs timed larval release relative to the light—dark, tidal phase, and tidal amplitude cycles, but not the lunar cycle, suggesting that three rhythms determined when larvae were released. For each species we ranked these rhythms by the degree to which larval release kept phase with their entraining physical cycles. The species—specific hierarchies of rhythms we observed match those expected if the time of larval release minimizes predation on females, embryos, and newly hatched larvae. Such hierarchies enable crabs to track phase shifts of cycles in variable tidal environments and may enhance reproductive success across tidal regimes. However, larval release may be timed better in some tidal regimes than others due to differences in the phasing of environmental cycles. In some tidal regimes, larval release cannot be synchronized with all three physical during the year, and hierarchies of rhythms may determine the timing and duration of breeding.

ISSN:0012-9658    

DOI:10.2307/1940876

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

One of the major questions in ecology is, what controls the structure of communities? We used projection matrix models to examine community dynamics and patterns of succession. The inputs of the model are transition probabilities of species replacements that were measured repeatedly during a long—term (1962—1989) study of diverse coral assemblages on Heron Island, Great Barrier Reef. Transitions varied strikingly among species and sites, reflecting differences in recruitment, growth, longevity (persistence), and the rate of replacement of one species by another. Species that had a poor ability to persist (e.g., algae and Pocilloporid corals) were generally good colonists. The observed number of transitions expressed as a proportion of the maximum number possible provides an index of the complexity of interactions in an assemblage, analogous to the concept of connectance in food—web analysis. Transitions occurred to and from nearly every species group, indicating that there was no competitive dominant in this system. We use the models in simulations to track transitory changes in species abundance and community composition following a major disturbance (e.g., due to a cyclone or outbreak of crown—of—thorns starfish). Some species showed a rapid initial increase followed by a decline to lower equilibrium levels, while others increased smoothly to a generally higher equilibrial abundance. The length of time required to reach a climax assemblage using the same matrix recurrently (°20 yr) is far greater than the observed interval between major disturbances, supporting nonequilibrium theories of coral reef communities. Climax assemblages were highly diverse and varied in composition from site to site. The intermediate disturbance “hypothesis” does not fully predict successional changes in these shallow—water coral assemblages since diversity remained very high at equilibrium (i.e., long after major disturbance). Competitively inferior species were not eliminated because routine mortality ensured that some space always remained available for colonization. We also present a novel method for quantifying the relative importance of each species interaction to community composition and the rate of succession, based on a sensitivity analysis of the transition matrix. The analysis shows that the importance of a species to the dynamics of a community may be unrelated to its abundance at equilibrium, with some rare species groups having a greater impact than more common ones. Sensitivity analysis of this type will provide a powerful means of identifying “keystone” species in complex assemblages where experimental manipulation of each species is impossible.

ISSN:0012-9658    

DOI:10.2307/1940877

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Previous studies of the effects of predation risk on patch choice have assumed the distribution of foragers to be Ideal Free. However, recent investigations have revealed systematic departures from the Ideal Free Distribution. In this study we do not assume that the distribution of foragers fits an Ideal Free Distribution. Instead we use a method that separates the effects on patch choice of predator avoidance (site bias) from effects due to the resource ratio. Our experiment examines the distribution of foraging upland bullies (Gobiomorphus breviceps) between two resource sites over a series of five different food input ratios. We had two conditions, a predator—free and a predator condition. The bullies exhibited a strong bias (preference) toward Site 1 when there was no predator (quinnat salmon, Oncorhynchus tshawystcha). When a salmon was present at Site 1 they exhibited a strong bias towards the alternative resource site (Site 2). We estimated that it would be necessary to change the relative food availabilities by a factor of °28 to remove the effect of the presence of the predator. The slope of the distribution of bullies (their sensitivity to resource profitability) deviates from the Ideal Free prediction for both conditions. A trend in the slope of the distribution of the bullies between the predator—free and predator conditions suggested that the presence of a predator may alter the behavior of the bullies. By acknowledging that the distribution of foragers may deviate from Ideal Free, we demonstrate that it is possible to separate the effects of predator avoidance from a change in their ability to discriminate between resource sites caused by the presence of a predator.

ISSN:0012-9658    

DOI:10.2307/1940878

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

A general tritrophic model of intermediate complexity representing the dynamics of trophic level biomass and numbers is presented. The rudiments of the behavior and physiology of resource acquisition and conversion are incorporated as functional and numerical response models. The tritrophic model is used to examine the effects of trophic position on bottom—up—top—down regulation of populations in theory and in practice. The zero growth isoclines of the interacting populations are used to examine the dynamics of the tritrophic system. The herbivore (M2) and predator (M3) but not the plant (M1) isoclines can be solved explicitly. The plant and herbivore isoclines have two forms that depend on whether the proportion of the trophic level available to its consumer (i.e., its apparency) is greater than or less than its potential per unit biomass population growth rate. Rough estimates of the parameters of these inequalities may be deduced from our knowledge of the search biology of the species and known size to growth rate relationships. The model shows clearly that bottom—up regulation sets the upper limit for trophic—level growth and top—down regulation determines the level of realized growth. The model explains the paradoxes of enrichment and of biological control that arise from the standard Lotka—Volterra models, and its qualitative predictions compare well to the general conclusions of intensive studies on biological control of the cassava mealybug on cassava by an exotic parasitoid. However, discrepancies that were found caution against unconsidered extrapolation of theoretical predictions to specific situations. The model qualitatively defines the dynamics required of a successful weed biological control agent, of a stable fresh water algal—arthropod herbivore—vertebrate predator system, and of a marine phytoplankton—krill—whale system. The utility of the model is its generality and its basis in quantifiable biology.

ISSN:0012-9658    

DOI:10.2307/1940879

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Freshwater zooplankton communities typically undergo pronounced seasonal succession and often show dramatic responses to external factors such as changes in zooplanktivore abundance. For this reason it is important to assess how common zooplankton species differ in their grazing impacts on planktonic ecosystems. To accomplish this we used single—species treatments with Diacyclops bicuspidatus thomasi, Daphnia rosea, Diaptomus novamexicanus, and Holopedium gibberum in situ in Castle Lake, California. These taxonomically diverse zooplankters differ markedly in feeding modes and typical seasonal population dynamics. We measured the response of nutrient concentrations, bacterioplankton abundance, phytoplankton species composition and biomass, primary production, a grazing index (phaeophytin/chlorophyll a), and microzooplankton to our single species treatments. The filter—feeding cladocerans Daphnia and Holopedium and the raptorial filter—feeding calanoid copepod Diaptomus showed several effects typical of herbivorous zooplankton. These included increasing dissolved nutrient concentrations, decreasing algal biomass and the abundance of several common algae, increasing a grazing index, increasing the ratio of bacterial to algal biomass, as well as depressing ciliate microzooplankton abundance. The raptorial cyclopoid copepod Diacyclops was apparently exclusively predaceous as it decimated the ciliate and rotifer microzooplankton, but had no notable effect on the other measured parameters relative to zooplankton—free controls. Diacyclops had the greatest effect on the microzooplankton and Daphnia and Diaptomus had the greatest effect on inorganic nutrients and characteristics of the phytoplankton. Holopedium had qualitatively similar but weaker impacts compared to Daphnia and Diaptomus. None of the zooplankton treatments had an effect on bacterioplankton abundance, nor did grazing by any of these zooplankters increase total algal primary production. Our results suggest differences in the grazing effects of common freshwater zooplankton can be pronounced and indicate that both seasonal succession and long—terms shifts in the zooplankton community structure should have marked effects on microzooplankton competitors and prey, the phytoplankton, and nutrient cycling.

ISSN:0012-9658    

DOI:10.2307/1940880

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Diel vertical migration (DVM) of zooplankton is a behavioral antipredator defense that is shaped by the trade—off between higher predation risk in surface waters and reduced growth in deeper waters. We conducted two laboratory experiments to quantify the costs connected with DVM. In the first experiment, Daphnia magna were kept individually in thermally stratified flow—through tubes and exposed to seven different concentrations of fish—exuded kairomones. Above a threshold concentration, the strength of migration increased with increases in the concentration of fish exudates. Enhancement of migration resulted in a lower mean ambient temperature experienced by the animals and marked decreases in individual growth and reproduction rates. In order to separate costs due to low hypolimnetic temperatures in a stratified system from costs due to reduced food concentrations in deeper waters, we conducted an experiment with a 2 ° 2 factorial design (fish presence vs. absence and high vs. low food conditions). Differences in mean ambient temperature between Daphnia that performed DVM and nonmigrating animals were found to have a much stronger impact on life history parameters than food effects. A reevaluation of field data on DVM in Daphnia further supports the view that vertical gradients are more important than food gradients.

ISSN:0012-9658    

DOI:10.2307/1940881

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

I examined growth and survival of the European toad, Bufo bufo, from hatching to the approximate time of first hibernation. I varied tadpole density in experimental ponds such that individuals from low—density ponds emerged 48.5% larger than those from high—density ponds. In the laboratory, metamorphs from both pond densities were maintained in containers at densities of one or six. Nine weeks after metamorphosis they were exposed to 0 or 80 larvae of the lungworm, Rhabdias bufonis. A factorial experiment aimed to determine (1) the extent to which conditions experienced by larvae carried over to the terrestrial stage (2) the effects of resource limitation and past history on a host's response to a potential pathogen. The density of metamorphs had the strongest effect on growth and survival: 18 wk after metamorphosis, those raised alone were °80% heavier than those in groups and they had 31% higher survival. However, larval history also affected growth and survival and affected how metamorphs responded to density. First, single toads emerging from low—density ponds were 14.5% larger at the time of hibernation than those from high—density ponds. The mechanism for this growth advantage probably lies in the consistently higher growth rates of single toads from low—density ponds, especially during the first 3 wk after metamorphosis. Second, survival in group containers was higher for toads from low—density ponds, especially during the first few weeks after metamorphosis when most deaths involved toads from high—density ponds. These results support a carry—over effect between larval history and subsequent performance in an organism with a complex life cycle. Infection with lungworms had no detectable effect on metamorph growth or survival. This result contrasts earlier studies on this system, possibly due to the relatively low worm burdens or low statistical power. Yet, the results may also indicate that the predicted impact of infection on hosts, especially those limited by resources, is not as straightforward as theoretical studies suggest.

ISSN:0012-9658    

DOI:10.2307/1940882

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

We can compare natural communities with null models of communities to indicate how they differ from random assemblages of species (i.e., how much structure is present). However, because null models draw on observed values of species' prevalences, whatever structure already exists in natural communities affects the composition of a null model and weakens its comparative power. To address this, we developed formulae to estimate "pre—interactive" species prevalences permitting a more sensitive quantification of community structure. Nonetheless, if a null model deviates from the community that we base it on, it is difficult to separate the effects of heterogeneity in recruitment from competition. We have developed a method to test for each independently. Applying our analytical techniques to a well—studied guild of larval trematodes in the salt marsh snail Cerithidea californica revealed that competitive interactions among species were the most significant structuring force. Interestingly, spatial heterogeneity acted to significantly intensify species co—occurrences. This differs from previous studies, which argued that the isolating effects of spatial heterogeneity, not competition, structure these communities by reducing co—occurrences.

ISSN:0012-9658    

DOI:10.2307/1940883

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY