A quantitative test of the size efficiency hypothesis by means of a physiologically structured model

According to the size‐efficiency hypothesis (SEH) larger bodied cladocerans are better competitors for food than small bodied species. In environments with fish, however, the higher losses of the large bodied species due to size‐selective predation may shift the balance in favor of the small bodied species. Here we present a theoretical framework for the analysis of the competitive abilities of zooplankton species that takes both competition and predation into account in one coherent analysis. By applying the conceptually well‐understood framework of physiologically structured population models we were able to predict the relative difference in predation rates necessary to cause a shift in dominance of the large‐bodied species (Daphnia pulicaria) to the small‐bodied species (D. galeata). These predictions depend only on seven easily interpretable parameters per species: size at birth, size at maturity and maximum size, age at maturity, maximal clutch size, egg development time and finally the half‐saturation constant for food. The critical equilibrium mortality of D. pulicaria was 0.16 d^?1 at food concentrations close to the critical food concentration of D. galeata, i.e. D. pulicaria will win the competition as long as its mortality rate is below 0.16 d^?1. At higher food concentrations the differential mortality curve (plotting equilibrium mortalities of both species against each other) approached a linear function with a slope of one and an intercept equal to the difference in maximal population birth rates. The prediction of critical predation rates was independent of the ingestion rate of the cladocerans and the algal carrying capacity and food regeneration rate of the environment although the mechanism works through competition for a shared algal food resource. We interpret these findings in terms of the relative predation risk large and small‐bodied cladocerans will face in various freshwater ecosystems.     

Life history strategies of cladocerans: comparisons of tropical and temperate taxa

We review recent works on different life history variables of cladoceran taxa in tropical and temperate freshwater bodies, comparing the strategies that cladocerans have evolved to adapt to contrasting environmental conditions in the two geographical regions. These life-history parameters relate to age and size at maturity, survival, fecundity, life-expectancy at birth, lifespan, gross, and net reproductive rates, generation time, the rate of population increase, peak population density and day of peak abundance. We also discuss the role of photoperiod and temperature on some of these life history parameters. We found a general paucity of experimental work and field data in tropics on cladocerans. There is very limited information on the few Daphnia species found in the tropics. The misconception of low species diversity of cladocerans in the tropics arose due to several reasons including lack of extensive and intensive field collections. Higher water temperatures apparently promote permanent infestation of tropical waters with toxic cyanobacteria, which reduce the zooplankton diversity. In addition to higher temperatures in the tropics, the year-round high predation pressure of planktivorous fish probably causes the tropical species, particularly in pelagic habitats, to reach maturity earlier (< 3 days) than in temperate regions. Species of Daphnia in temperate regions are particularly adapted to living at food concentrations that are much lower and seasonably more variable than those for tropical genera such as Diaphanosoma. This is further corroborated by the more than an order of magnitude higher threshold food concentration (TFC) for tropical Cladocera than for their temperate counterparts. Fecundity patterns differ between tropical and temperate cladoceran taxa: cultured under optimal temperature regimes, tropical taxa have fewer eggs than temperate species of a comparable body size. Predation pressure may act differently depending on the size of the cladoceran neonates and thus on their population size structure. Global warming and climate changes seem to affect the behaviour (migration), distribution, and abundance of cladocerans. Apparently, in direct response to these changes, the possibility of encountering the tropical cladocerans in the northern, temperate hemisphere (bioinvasions) is on the rise.     

Effects of food limitation and temperature on cladocerans from a tropical Brazilian lake

Food limitation was tested in the laboratory by individual growth and reproduction of two cladoceran species, Ceriodaphnia richardi and Daphnia gessneri, from the shallow tropical Brazilian Lake Monte Alegre. The cladocerans were fed cultivated green alga Scenedesmus spinosus in concentrations of 0.20, 0.10, 0.05, and 0.025 mg C l⁻¹. Higher biomass and growth rates occurred in the two highest-food concentrations; the two lowest ones negatively affected clutch size and first reproduction. The threshold food concentration is lower than 0.025 mg C l⁻¹ and the incipient limiting level is a value between 0.10 and 0.20 mg C l⁻¹. The largest species, D. gessneri, was more sensitive to low food concentrations. The effects of low and high temperatures (19 and 27°C) were evaluated by life table experiments with three cladocerans from the lake—Daphnia ambigua, D. gessneri, and Moina micrura—with no food limitation (1 mg C l⁻¹ of S. spinosus). Higher population growth rates for the three species were found at 27°C; better performance in most life table parameters was observed for the former two species at the highest temperature, D. gessneri being the most sensitive to the lowest temperature. There are indications that temperature is an important abiotic factor that constrains populations of cladocerans for a short period in winter in the lake, when temperature decreases to 18–19°C. However, its influence cannot be separated from a biotic factor such as food, whose effect is stronger in the cool season, when concentrations are lower and contribution of inedible algae is relatively higher.     

Food limitation and body size in the life cycles of planktonic rotifers and cladocerans

This review considers what is known about the effects of food limitation upon the life cycle characteristics of rotifers and planktonic cladocerans. The characteristics considered in rotifers are the size of eggs, juveniles and adults and the durations of the juvenile phase and period of egg production. In cladocerans, the life history features dealt with are their length-weight relationships, the body size, instar stage, age and fecundity of the primiparous female and their fecundity-adult size relationship. The influence of limiting food conditions is demonstrated for these characteristics by comparison with the situation in non-limiting circumstances; the comparison is confined to experiments where food concentrations are quantified. A direct comparison is made between rotifers and cladocerans in conditions of defined food resource availability in terms of their length-weight relationships, the daily allocation of adults or near-adults to growth and reproduction and their threshold food concentrations. These comparisons are discussed in relation to the following topics: the high cost of cumulated respiration resulting from prolongation of the juvenile phase of body growth; the fundamentally different nature of growth in the two taxonomic groups; the body size of species and the size that must be attained for reproduction; the ecological implications of the very different threshold food concentrations.     

Effects of competitors and Chaoborus predation on the cladocerans of a eutrophic lake: an enclosure study

The role of large laboratory grown food competitors ofthe genus Daphnia as well as the predationimpact of Chaoborus on the cladoceran communityof an eutrophic lake was assessed in five insitu enclosure experiments. The hypothesis tested wasthat the outcome of competition and gape-limitedpredation on cladocerans is size dependent. Accordingto the generally accepted assumptions on competitionand invertebrate predation, large-bodied cladocerantaxa were expected to be less affected by competingcongeners and by Chaoborus than weresmall-bodied taxa. Effects of the predator upon anassemblage of differently sized cladoceran taxa weremuch more pronounced than effects of competition.There was a tendency of predation and competitionimpact to decrease with cladoceran size, but predationpressure was also low for some small cladocerans andhigh for some large cladocerans. The general trendswere further obscured by factors not or indirectlylinked to body size.     

Ecological constraints on sensory systems: compound eye size in Daphnia is reduced by resource limitation

Eye size is an indicator of visual capability, and macroevolutionary patterns reveal that taxa inhabiting dim environments have larger eyes than taxa from bright environments. This suggests that the light environment is a key driver of variation in eye size. Yet other factors not directly linked with visual tasks (i.e., non-sensory factors) may influence eye size. We sought to jointly investigate the roles of sensory (light) and non-sensory factors (food) in determining eye size and ask whether non-sensory factors could constrain visual capabilities. We tested environmental influences on eye size in four species of the freshwater crustacean Daphnia, crossing bright and dim light levels with high and low resource levels. We measured absolute eye size and eye size relative to body size in early and late adulthood. In general, Daphnia reared on low resources had smaller eyes, both absolutely and relatively. In contrast to the dominant macroevolutionary pattern, phenotypic plasticity in response to light was rarely significant. These patterns of phenotypic plasticity were true for overall diameter of the eye and the diameter of individual facets. We conclude that non-sensory environmental factors can influence sensory systems, and in particular, that resource availability may be an important constraint on visual capability.     

ADAPTIVE GENETIC VARIATION IN GROWTH AND DEVELOPMENT OF TADPOLES OF THE HYBRIDOGENETIC RANA ESCULENTA COMPLEX

The distribution and proportion of the sexual species Rana lessonae to the hemiclonal hybrid R. esculenta among natural habitats suggests that these anurans may differ in adaptive abilities. I used a half-sib design to partition phenotypic and quantitative genetic variation in tadpole responses at two food levels into causal variance components. Rana lessonae displays strong phenotypic variation across food levels. Growth rate is strictly determined by environmental factors and includes weak maternal effects. Larval period and body size at metamorphosis both contain moderate levels of additive genetic variance. The sire x food interactions and the lack of environmental correlations indicate that adaptive phenotypic plasticity is present in both of these traits. In contrast, R. esculenta displays less phenotypic variation across food levels, especially for larval period. Variation in body size at metamorphosis is underlain by genetic variation as shown by high levels of additive genetic variance, yet growth rate and larval period are not. Significant environmental correlations between larval period at high food level and growth, larval period, and body size at low food, indicate phenotypic plasticity is absent. A positive phenotypic correlation between body size at metamorphosis and larval period for R. lessonae at both food levels suggests a trade-off between growing large and metamorphosing quickly to escape predation or pond drying. The lack of a similar correlation for R. esculenta at the high food level suggests it may be less constrained. Different levels of adaptive genetic variation among larval traits suggest that the sexual species and the hybridogenetic hemiclone differ in their abilities to cope with temporally and spatially heterogeneous environments.     

Zooplankton community size structure and taxonomic composition affects size-selective grazing in natural communities

The body size of an individual zooplankton is well related to its grazing rate and to the range of particle sizes it can ingest, and since cladocerans and copepods feed differently, they follow different relationships. Based on these general patterns in individual organisms, we tested whether the size structure and taxonomic composition of more complex natural zooplankton communities are related to their in situ grazing rate and to the range of algal sizes they graze. We compared community grazing rates on individual algal taxa in two communities dominated by small cladocerans, three communities dominated by large cladocerans and three copepod-dominated communities. Small algae were usually grazed most intensively, but grazing rates were poorly related to algal size alone. The range in size of grazed algae increased with increasing mean zooplankton body size, but differed systematically with their taxonomic composition. Communities dominated by Ceriodaphnia or Holopedium grazed a narrower size range of algae [maximum greatest axial length dimension (GALD)=16–36 μm)] than communities with large biomasses of Bosmina or Daphnia (maximum GALD=28–78 μm). Copepod-dominated communities followed the same general relationship as cladocerans. Daphnia-dominated communities grazed the broadest range of algal sizes, and their total grazing rates were up to 2.4 times their grazing rates on small (<35 μm) “highly edible” algae, a difference of similar magnitude to those found in successful trophic cascade biomanipulations. Received: 31 March 1998 / Accepted: 19 October 1998     

Spatial autocorrelation and dispersal limitation in freshwater organisms

Dispersal can limit the ranges of species and the diversity of communities. Despite its importance, little is known about its role in freshwater habitats and its relation to habitat type (lentic vs. lotic), especially for organisms with cryptic dispersal methods such as plankton. Poor dispersers are expected to show more clumped distributions or greater spatial autocorrelation (SA) in community composition than good dispersers. We examined patterns of SA across freshwater taxa with different dispersal modes (active vs. passive) and their association with habitat type (lake vs. stream) using 18 spatially explicit community composition data sets. We found significant relationships between SA and body size among taxa in lake habitats, but not in streams. However, the increase in SA with body size in lakes was driven entirely by fishes—organisms ranging in size from diatoms to macro-invertebrates showed equivalent levels of SA. These results support the idea that large organisms are less effective dispersers in aquatic environments, resulting in greater SA in community structure over broad scales. Streams may be effectively more connected than lakes as patterns of SA and body size were weaker in lotic habitats. Our data suggest that the critical threshold where greater body size increases dispersal limitation seems to come at the juncture between invertebrates and vertebrates rather than that between unicellular and multicellular organisms as has been previously suggested. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Habitat transitions alter the adaptive landscape and shape phenotypic evolution in needlefishes (Belonidae)

Habitat occupancy can have a profound influence on macroevolutionary dynamics, and a switch in major habitat type may alter the evolutionary trajectory of a lineage. In this study, we investigate how evolutionary transitions between marine and freshwater habitats affect macroevolutionary adaptive landscapes, using needlefishes (Belonidae) as a model system. We examined the evolution of body shape and size in marine and freshwater needlefishes and tested for phenotypic change in response to transitions between habitats. Using micro‐computed tomographic (µCT) scanning and geometric morphometrics, we quantified body shape, size, and vertebral counts of 31 belonid species. We then examined the pattern and tempo of body shape and size evolution using phylogenetic comparative methods. Our results show that transitions from marine to freshwater habitats have altered the adaptive landscape for needlefishes and expanded morphospace relative to marine taxa. We provide further evidence that freshwater taxa attain reduced sizes either through dwarfism (as inferred from axial skeletal reduction) or through developmental truncation (as inferred from axial skeletal loss). We propose that transitions to freshwater habitats produce morphological novelty in response to novel prey resources and changes in locomotor demands. We find that repeated invasions of different habitats have prompted predictable changes in morphology.     

Composition,size, and biomass of zooplankton in large productive Florida lakes

Crustacean zooplankton data were compiled from long-term observational studies at seven large shallow Florida lakes, to determine whether there are general characteristics in regard to species composition, body size, and biomass. In particular, we examined whether patterns in body size and species richness fit empirical models developed by Stanley Dodson. The lakes included range in size from 125 to 1730 km² and encompass mesotrophic to hyper-eutrophic conditions. We found that zooplankton biomass was strongly dominated by one species of calanoid copepod—Arctodiaptomus dorsalis. Large daphnids were absent, and Cladocera assemblages were dominated by small taxa such as Ceriodaphnia, Chydorus, and Eubosmina. The total number of species of pelagic cladocerans (8–12) was consistent with Dodson’s predictions based on lake area. The average size of crustacean zooplankton in Florida lakes is small in comparison with temperate communities. A. dorsalis is the smallest calanoid copepod in North America, and the mean length of Cladocera (0.6 mm) is consistent with Dodson’s results that size decreases from temperate to tropical zones. Total biomass of crustacean zooplankton was very low, ratios of zooplankton to phytoplankton biomass (0.01–0.1) are among the lowest reported in the literature, and the zooplankton displayed short-lasting early spring peaks in biomass. Cladocera were almost entirely absent in spring and summer. Factors known to occur in Florida lakes, which appear to explain these characteristics of biomass, include intense fish predation and high summer water temperature.     

Identifying biotic drivers of population dynamics in a benthic–pelagic community

Benthic species and communities are linked to pelagic zooplankton through life‐stages encompassing both benthic and pelagic habitats and through a mutual dependency on primary producers as a food source. Many zooplankton taxa contribute to the sedimentary system as benthic eggs. Our main aim was to investigate the nature of the population level biotic interactions between and within these two seemingly independent communities, both dependent on the pelagic primary production, while simultaneously accounting for environmental drivers (salinity, temperature, and oxygen conditions). To this end, we applied multivariate autoregressive state‐space models to long (1966–2007) time series of annual abundance data, comparing models with and without interspecific interactions, and models with and without environmental variables included. We were not able to detect any direct coupling between sediment‐dwelling benthic taxa and pelagic copepods and cladocerans on the annual scale, but the most parsimonious model indicated that interactions within the benthic community are important. There were also positive residual correlations between the copepods and cladocerans potentially reflecting the availability of a shared resource or similar seasonal dependence, whereas both groups tended to correlate negatively with the zoobenthic taxa. The most notable single interaction within the benthic community was a tendency for a negative effect of Limecola balthica on the amphipods Monoporeia affinis and Pontoporeia femorata which can help explain the observed decrease in amphipods due to increased competitive interference.     

Growth pattern and survival in populations of Poecilia vivipara (Teleostei; Poeciliidae) inhabiting an environmental gradient: a common garden study

We performed a common garden experiment to assess the existence of genetic differences on growth and body size between two populations of Poecilia vivipara inhabiting extremes of an environmental gradient caused by water salinity in lagoons of Northern Rio de Janeiro State, Brazil: the Campelo lagoon (freshwater) and Açu lagoon (brackish/saltwater). The two populations show extreme differences in average phenotypes for body size, shape and life history (freshwater populations with smaller body size, lower fecundity and larger reproductive allotment). Pregnant females were brought to the lab and the offspring from both groups were kept in a common recirculating system with freshwater. Standard length and survival were measured weekly over a period of 200 days and growth models were fitted and selected with information criteria. The offspring originally from the brackish water lagoon presented larger asymptotic length, higher maximum growth rate but lower survival than the offspring originally from the freshwater lagoon. Potential confounding variables such as density differences due to mortality and maternal effects (offspring size) were included as covariates in comparisons of growth rates between groups. The results are consistent with phenotypic differences among populations having some genetic basis, and with the existence of a trade-off between growth and maintenance due to the high growth/low survival observed in the group that changed from salt to freshwater. Comparisons of captive and natural populations suggest that the influence of environmental factors, such as salinity, food availability, fish density and predation should also be considered relevant to explain phenotypic variation in this system.     

Food spectra of the farm juveniles of the Chum Salmon <Emphasis Type="Italic">Oncorhynchus keta</Emphasis> (Salmoniformes,Salmonidae) in the Ryazanovka River (Southern Primor’e)

Qualitative data are presented on the diet of juvenile chum salmon Oncorhynchus keta released from the Ryazanovka experimental fish hatchery farm situated in the south of Primor’e Territory. The stomach fullness, frequency of occurrence of food items, their average weight value, and selectivity are estimated. Food spectra of chum salmon juveniles during downstream migration comprised representatives of 51 taxa of bottom freshwater invertebrates and terrestrial insects. The bulk of the diet consisted of larvae of aquatic insects—chironomids, other Diptera, and mayflies. Their diversity increased with the growth of fry. Fry of chum salmon had a wide spectrum of feeding preference, favorite food items were larvae and pupae of chironomids.     

The evolution of caste polymorphism in social insects:0 Genetic release followed by diversifying evolution

Caste polymorphism, defined as the presence within a colony of two or more morphologically differentiated individuals of the same sex, is an important character of highly eusocial insects both in the Hymenoptera (ants, bees and wasps) and in the Isoptera (termites), the only two groups in the animal kingdom where highly eusocial species occur. Frequently, caste polymorphism extends beyond mere variations in size (although the extent of variations in size can be in the extreme) and is accompanied by allometric variations in certain body parts. How such polymorphism has evolved and why, in its extreme form, it is essentially restricted to the social insects are questions of obvious interest but without satisfactory answers at the present time. I present a hypothesis entitled ‘genetic release followed by diversifying evolution’, that provides potential answers to these questions. I argue that genetic release followed by diversifying evolution is made possible under a number of circumstances. One of them I propose is when some individuals in a species begin to rely on the indirect component of inclusive fitness while others continue to rely largely on the direct component, as workers and queens in social insects are expected to do. Thus when queens begin to rely on workers for most of the foraging, nest building and brood care, and workers begin to rely increasingly on queens to lay eggs—when queen traits and worker traits do not have to be expressed in the same individual—I postulate the relaxation of stabilizing selection and new spurts of directional selection on both queen-trait genes and worker-trait genes (in contrasting directions) leading to caste polymorphism.     

Correction factors used for estimating prey biomass in the diet of American minkMustela vison

Feeding experiment on nine feral minkMustela vison Schreber, 1777 was carried out to find differences in digestibility of particular prey types. The values of correction factors (CF), which show the ratio between consumed prey biomass and undigested remains, were calculated for seven food types. The experiment revealed that results obtained by commonly used percentage of occurrence methods differed from the real intake of food. Food types which were intensively digested were underestimated by percentage of occurrence methods and prey which had hardly digestive elements of their body were overestimated. The calculated values of CF are as follows: crayfish — 14.8, fish — 30.8, frog — 61.3, small passerine bird — 17.2, chicken — 41.3, rodent — 17.3, egg — 687.5. The variability in CF values did not depend on mink individuals but could be explained by the mean weight of prey items eaten by mink. The positive correlation between the prey size and its digestibility was recorded for crayfish, fish and chicken. The biomass of frog, passerine and rodent remains was not related to the body mass of eaten prey. The use of evaluated correction factors enables more accurate estimates of the food composition of wild living mustelids.     

Potential Effects of a Species’ Name

In systematics, the importance of a species’ name is obvious and very considerable — it may even affect the names of other taxa (genera and families). Here, I argue that in some specific circumstances the name of a species may also indirectly play some role in the public understanding of the theory of evolution and creationism — or at least, has the potential to play such a role.     

Competition between rotifers and cladocerans of different body sizes

Summary We conducted laboratory experiments to test the hypothesis that rotifers could coexist with small (<1.2 mm) but not large (>1.2 mm) cladocerans. Keratella cochlearis was excluded in <8 days by the large cladocerans Daphnia pulex and D. magna, probably through both interference and exploitative competition. On the other hand, K. cochlearis persisted for 8 weeks with two small cladocerans (Bosmina longirostris and Ceriodaphnia dubia) and excluded a third small cladoceran (D. ambigua). Similarly, Synchaeta oblonga coexisted with B. longirostris for >7 weeks, and K. testudo coexisted with D. ambigua for >4 weeks. Coexistence of small cladocerans and rotifers was always accompanied by suppression of one or both populations, probably primarily if not exclusively by exploitative competition for limiting food resources. These results indicate that the competitive dominance of cladocerans over rotifers decreases markedly with cladoceran body size and that factors other than body size may determine the competitive outcome between rotifers and small cladocerans. Our study provides a mechanistic explanation for a commonly observed pattern in natural zooplankton communities: planktonic rotifers often are abundant when only small cladocerans occur but typically are rare when large cladocerans are present.     

Relative contributions to seed production by floral visitors of slickspot peppergrass, Lepidium papilliferum (Brassicaceae)

Assessing the relative contributions to seed production made by different types of floral visitors is fundamental to understanding the evolution of floral morphology and the influence that particular pollinator taxa have on plant fitness and reproduction. This 3-year study examined the pollinator activity and the seed production in three populations of Lepidium papilliferum, a threatened mustard endemic to sagebrush-steppe habitat in southwest Idaho. Relative amounts of time visitor taxa spent foraging on flowers, visitation rates (number of flowers visited per unit time during a foraging bout), and pollination effectiveness (fruit set per single visit to a virgin flower) were recorded for each of 12 insect taxa that visited L. papilliferum flowers. Relative contributions to seed production were calculated as the product of relative interaction frequencies (the relative number of flowers visited by each taxon—the “quantity” component of pollination) and pollination effectiveness (fruit set per single visit to a virgin flower—the “quality” component of pollination). Despite the superiority of some insect taxa in terms of pollination effectiveness, estimates of relative pollinator contributions to seed production were influenced primarily by an insect taxon’s interaction frequency with flowers. Pollinator assemblages varied widely both spatially and temporally, which suggest that L. papilliferum is not under strong selective pressure to shift from its generalist pollination strategy toward greater specialization. For this threatened plant, reliance on a diverse assemblage of insect pollinators may serve as an important buffer against disruption in reproductive success caused by fluctuations in population sizes of individual pollinator taxa.