Macroecology of a host-parasite relationship

The larvae of freshwater mussels are obligate ectoparasites on fishes while adults are sedentary and benthic. Dispersal of mussels is dependent on the movement of fish hosts, a regional process, but growth and reproduction should be governed by local processes. Thus, mussel assemblage attributes should be predictable from the regional distribution and abundance of fishes. At a broad spatial scale in the Red River drainage, USA, mussel species richness and fish species richness were positively associated; maximum mussel richness was limited by fish richness, but was variable beneath that constraint. Measured environmental variables and the associated local fish assemblages each significantly accounted for the regional variation in mussel assemblages. Furthermore, mussel assemblages showed strong spatial autocorrelation. Variation partitioning revealed that pure fish effects accounted for 15.4% of the variation in mussel assemblages; pure spatial and environmental effects accounted for 16.1% and 7.8%, respectively. Shared variation among fish, space and environmental variables totaled 40%. Of this shared variation, 36.8% was associated with the fish matrix. Thus, the variation in mussel assemblages that was associated with the distribution and abundance of fishes was substantial (> 50%), indicating that fish community structure is an important determinant of mussel community structure. Although animals commonly disperse plants and, thus, influence the structure of plant communities, our results show a strong macroecological association between two disparate animal groups with one strongly affecting the assemblage structure of the other.     

Patterns of variation in life history among South American fishes in seasonal environments

Summary Ten traits related to life history theory were measured or estimated for 71 freshwater fish species from two locations in the Venezuelan llanos. Multivariate statistics and cluster analysis revealed three basic endpoint patterns bounding a two-dimensional continuum. A suite of attributes associated with parental care and aseasonal reproduction appeared to correspond to an equilibrium strategy. A second group of small fishes was distinguished by traits associated with rapid colonizing ability: early maturation, continuous reproduction, and small clutches. The third basic pattern was associated with synchronized reproduction during the early wet season, high fecundity, absence of parental care, and breeding migrations. A subset of mostly small fishes exhibiting little or no parental care, small clutches, and two to four month reproductive seasons was intermediate between the opportunistic (rapidly colonizing) and seasonal strategies. All ten life history variables showed significant effects of phylogeny. The cluster of species corresponding to the equilibrium group was dominated by siluriform fishes and perciforms of the Cichlidae. The opportunistic cluster was dominated by cyprinodontiform and characiform fishes, whereas the seasonal cluster contained primarily characiform and siluriform fishes. Seven of nine traits were significantly correlated with body length. The three reproductive patterns are interpreted as being adaptative with respect to relative intensity and predictability of temporal and spatial variation in abiotic environmental parameters, food availability, and predation pressure.     

Egg production and individual genetic diversity in lesser kestrels

Fecundity is an important component of individual fitness and has major consequences on population dynamics. Despite this, the influence of individual genetic variability on egg production traits is poorly known. Here, we use two microsatellite-based measures, homozygosity by loci and internal relatedness, to analyse the influence of female genotypic variation at 11 highly variable microsatellite loci on both clutch size and egg volume in a wild population of lesser kestrels (Falco naumanni). Genetic diversity was associated with clutch size, with more heterozygous females laying larger clutches, and this effect was statistically independent of other nongenetic variables such as female age and laying date, which were also associated with fecundity in this species. However, egg volume was not affected by female heterozygosity, confirming previous studies from pedigree-based breeding experiments which suggest that this trait is scarcely subjected to inbreeding depression. Finally, we explored whether the association between heterozygosity and clutch size was due to a genome-wide effect (general effect) or to single locus heterozygosity (local effect). Two loci showed a stronger influence but the correlation was not fully explained by these two loci alone, suggesting that a main general effect underlies the association observed. Overall, our results underscore the importance of individual genetic variation for egg production in wild bird populations, a fact that could have important implications for conservation research and provides insights into the study of clutch size evolution and genetic variability maintenance in natural populations.     

Correlation between Genetic Variability and Body Size in Vertebrates

An analysis of allozymic variation carried out in the main groups of vertebrate animals revealed a tendency towards the increased level of genetic polymorphism in the species of small animals compared to the large ones. This tendency was clearly followed in caudate amphibians, fishes, and mammals. The data are discussed in terms of the integration of monogenic and polygenic systems in the populations. It is hypothesized that this relationship between heterozygosity and body size confirms more general regularity consisting in highly statistically significant correlation between polygenic heterozygosity, maturation rate and life span. It is suggested that high rate of development in small animals resulting in early sexual maturation, can serve as a mechanism determining correlation between heterozygosity and body size at the species level. As a result, compared to large animals, small animals display higher rates of generation change, resulting in accelerated growth of population size and faster accumulation of genetic variability.     

Correlation between variability and body size in vertebrates

An analysis of allozymic variation carried out in the main groups of vertebrate animals revealed a tendency towards the increased level of genetic polymorphism in the species of small animals compared to the large ones. This tendency was clearly followed in caudate amphibians, fishes, and mammals. The data are discussed in terms of the integration of monogenic and polygenic systems in the populations. It is hypothesized that this relationship between heterozygosity and body size confirms more general regularity consisting in highly statistically significant correlation between polygenic heterozygosity, maturation rate and life span. It is suggested that high rate of development in small animals resulting in early sexual maturation, can serve as a mechanism determining correlation between heterozygosity and body size at the species level. As a result, compared to large animals, small animals display higher rates of generation change, resulting in accelerated growth of population size and faster accumulation of genetic variability.     

LIFE-HISTORY VARIATION AND COMPARATIVE PHYLOGEOGRAPHY OF DARTERS (PISCES: PERCIDAE) FROM THE NORTH AMERICAN CENTRAL HIGHLANDS

Phylogeography seeks to evaluate the relationship between genetic variation and geographic distribution of a species to examine the influence of historical events on divergence. Congruent phylogeographic patterns in codistributed species indicate historical association of the taxa being compared, and the uniform action of biogeographic events in shaping genetic variation. We sought to evaluate the congruence of patterns of genetic variation of five closely related fish species across a well-defined biogeographic boundary. We gathered allozymic and mitochondrial DNA (mtDNA) sequence data for five species of darters (Percidae: Etheostomatini) from populations distributed among biogeographic regions in the Ozark and Ouachita highlands of the south-central United States. Comparisons among species revealed noncongruence in the magnitude of genetic divergence in both allozymes and mtDNA sequences. We hypothesized that noncongruence resulted, in part, from differences in life histories of the species in our comparison. To address this hypothesis, we evaluated the association between gene flow (measured by N_em) and variation in body size and fecundity variables because they have been shown to influence gene flow in fishes. Correlation analysis revealed an association between gene flow and fecundity (r = 0.88), but not with body size (r = 0.36) or reproductive investment per individual (r = –0.23). The result was similar when independent contrasts of the original variables were used in correlation analyses. Phylogeographic analysis of mtDNA sequence data indicated the importance of history, evident in gene trees of Percina nasuta and Percina phoxocephala. Divergence rates between these two taxa may differ because of historically persistent differences in population sizes, reflected in present-day abundance and fecundity differences. Conversely, Percina caprodes showed little evidence of divergence in mtDNA sequences and yielded the highest mean N_em values from allozyme data. Comparisons among closely related, codistributed taxa may help discriminate among the possibilities for noncongruence in biogeographic studies by allowing evaluation of the influence of life history on patterns of gene flow.     

Reproduction in the North Atlantic oceanic ichthyofauna and the relationship between fecundity and species' sizes

Synopsis The pelagic (589 spp.) and demersal (505 spp.) oceanic ichthyofaunas of the North Atlantic Basin have very different compositions at ordinal and family level. Yet the pattern of relationships between species' maximum size and maximum fecundity from data available (10% of the pelagic, 19% of the demersal species) was similar. A positive relationship between fecundity and weight was confirmed among most teleosts, but was not followed by the elasmobranchs represented. Species' reproductive styles are reviewed in ordinal groupings within a framework of the overall body size/fecundity distribution. Species size (maximum weight) spectra were synthesized for both pelagic and demersal assemblages to assess the allocation of potential reproductive effort throughout the North Atlantic oceanic ichthyofauna. The only available examples of species size spectra and biomass spectra from the pelagic and demersal ichthyofauna in this ocean basin imply geographic and bathymetric variation in overall reproductive effort among fishes whose fecundity is size dependent. Further implications concerning reproductive effort are discussed in the light of food availability.     

Comparative analysis of phylogenetic and fishing effects in life history patterns of teleost fishes

The effects of fishing on life history traits and life history strategies of teleost fishes are analysed by a new comparative method that splits traits into an allometric part (size effect), an autoregressive phylogenetic component, and an environmental component (fishing effect). Both intra- and inter-specific variation of age and size at maturity, fecundity, adult size and egg size are analysed by comparing 84 populations of 49 species submitted to various fishing pressures. Two axes of life history diversification are found among teleosts. One is the well-known slow-fast continuum separating short-lived and early maturing species (like Clupeiformes) from longer-lived species that mature late relative to their size and spawn larger eggs (like salmonids or Scorpaeniformes). An additional strategy involves the schedule of resource allocation to growth and reproduction. Indeterminate growth allows higher teleosts (e.g. Gadiformes) to reach a large size while maturing early and laying small eggs. Increasing fishing pressure decreases age at maturity and egg size, and increases fecundity at maturity, the slope of the fecundity-length relationship and relative size at maturity. These compensations for higher adult mortality differ among life history strategies. Indeterminate growth is associated with a greater flexibility in resource allocation to growth and reproduction that facilitates greater resilience to fishing mortality.     

Die paradoxe Fruchtbarkeit der Strahlenflosser

The paradox fecundity of Actinopterygii More than fifty percent of about 70,000 vertebrate species are bony fish or Actinopterygii. These are characterized by two prominent and unique features: very tiny early life stages, independent of species or size, and a seemingly paradox fecundity, depending on size and age of the spawning fish. Usually even small fish produce quantities of eggs outnumbering those of other vertebrates and many invertebrates by two to more than five orders of magnitude. This suggests an unusual reproduction strategy of bony fish compared to other animal species. Due to their abundance, growth rate and consumption habit, little fish convert large amounts of short‐lived plankton biomass to long‐lived fish biomass. This generates a huge depot of food which is exploited by all carnivores living in or at aquatic environments. Not least or even first of all bony fishes themselves exist on this biomass store when grown to a larger size and prefer larger food. This behaviour likely implies an energy gain in a habitat where most biomass is produced by tiny and short lived plankton organisms and stored as fish biomass.     

Inter‐seasonal persistence and size‐structuring of two minnow species within headwater streams in the Eastern Cape,South Africa

This study examined temporal variation in population dynamics and size structuring of two cyprinid minnows, Pseudobarbus afer and Barbus anoplus, in relation to their proximate physical habitats. Population estimates were determined using three‐pass depletion sampling during both summer and winter. The habitats were characterised by seasonal variation in all physico‐chemical conditions and spatial variation in substrata compositions. Whereas significant differences in population size were noted between seasons for B. anoplus, no differences were found between seasons for density and capture probability for either species. An increase in boulders was associated with increase in population size and density for P. afer; for B. anoplus, increased percentages of bedrock and bank vegetation were associated with an increase in population size and probability of capture, respectively. According to Canonical Correspondence Analysis, size structuring in P. afer was explained predominantly by seasonality, with smaller length classes associated with the seasonal variable of summer, while larger length classes were associated with pH that was higher in winter. By comparison, for B. anoplus, the habitat variables – bank vegetation and bedrock – accounted for much of the explained variance for size structuring. Recruitment appeared to be the major driver of size structuring for the two species; refugia, especially boulders and bank vegetation, also appeared to be important. Overall, the two species were adapted to the headwater streams that were generally variable in environmental conditions. Potential invasions by non‐native invasive fishes that occur within the mainstream habitats threaten these two species. Efforts should continue to protect these minnows from such invasions by constructing barriers to upstream migration of non‐native fishes into these headwater habitats.     

Female fecundity traits in wild populations of African annual fish: the role of the aridity gradient

The evolution of life history is shaped by life expectancy. Life‐history traits coevolve, and optimal states for particular traits are constrained by trade‐offs with other life‐history traits. Life histories contrast among species, but may also diverge intraspecifically, at the level of populations. We studied the evolution of female reproductive allocation strategy, using natural populations of two sympatric species of African annual fishes, Nothobranchius furzeri and Nothobranchius orthonotus. These species inhabit pools in the Mozambican savanna that are formed in the rainy season and persist for only 2–10 months. Using 207 female N. furzeri from 11 populations and 243 female N. orthonotus from 14 populations, we tested the effects of genetic background (intraspecific lineage) and life expectancy (position on the aridity gradient determining maximum duration of their temporary habitat) on female fecundity traits. First, we found that variation in female body mass was small within populations, but varied considerably among populations. Second, we found that fecundity was largely defined by female body mass and that females spawned most of their eggs in the morning. Third, we found that the trade‐off between egg size and egg number varied among lineages of N. furzeri and this outcome has been confirmed by data from two separate years. Overall, we demonstrate that local conditions were important determinants for Nothobranchius growth and fecundity and that eggs size in arid region was less limited by female fecundity than in humid region.     

Life-history correlates of the evolution of live bearing in fishes

Selection for live bearing is thought to occur when the benefits of increasing offspring survival exceed the costs of reduced fecundity, mobility and the increased metabolic demands of carrying offspring throughout development. We present evidence that live bearing has evolved from egg laying 12 times in teleost (bony) fishes, bringing the total number of transitions to 21 to 22 times in all fishes, including elasmobranchs (sharks and rays). Live bearers produce larger offspring than egg layers in all of 13 independent comparisons for which data were available. However, contrary to our expectation there has not been a consistent reduction in fecundity; live bearers have fewer offspring in seven out of the 11 available comparisons. It was predicted that live bearers would have a larger body size, as this facilitates accommodation of developing offspring. This prediction was upheld in 14 out of 20 comparisons. However, this trend was driven by elasmobranchs, with large live bearers in seven out of eight comparisons. Thus, while the evolution of live bearing in elasmobranchs is correlated with predicted increases in offspring size and adult size, teleost live bearers do not have such a consistent suite of life-history correlates. This suggests that constraints or selection pressures on associated life histories may differ in live-bearing elasmobranchs and teleost fishes.     

Fitness costs associated with low genetic variation are reduced in a harsher environment in amphibian island populations

A basic premise of conservation geneticists is that low levels of genetic variation are associated with fitness costs in terms of reduced survival and fecundity. These fitness costs may frequently vary with environmental factors and should increase under more stressful conditions. However, there is no consensus on how fitness costs associated with low genetic variation change under natural conditions in relation to the stressfulness of the environment. On the Swedish west coast, natterjack toad Bufo calamita populations show a strong population genetic structure and large variation in the amount of within-population genetic variation. We experimentally examined the survival of natterjack larvae from six populations with different genetic variation in three thermal environments corresponding to (a) the mean temperature of natural ponds (stable, laboratory), (b) a high temperature environment occurring in desiccating ponds (stable, laboratory) and (c) an outdoor treatment mimicking the natural, variable thermal conditions (fluctuating, semi-natural). We found that larvae in the outdoor treatment had poorer survival than larvae in the stable environments suggesting that the outdoor treatment was more stressful. Overall, populations with higher genetic variation had higher larval survival. However, a significant interaction between treatments and genetic variation indicated that fitness costs associated with low genetic variation were less severe in the outdoor treatment. Thus, we found no support for the hypothesis that fitness costs associated with low genetic variation increase under more stressful conditions. Our results suggest that natural thermal stress may mask fitness losses associated with low genetic variation in these populations.     

Fecundity and size in the housefly: investigations of some environmental sources and genetic correlates of variation

Environmental sources and genetic correlates of variation in fecundity and size were studied in field and laboratory housefly populations in central Iowa, U.S.A. Electrophoresis of enzymatic proteins was used to monitor genotypes and heterozygosity. In the field, mean fecundity declined as adult densities increased. Fecundity varied greatly within breeding seasons. Fly body size varied spatially and temporally during summer and winter. Fecundity and size were positively correlated. Size was independent of genotypes at the six loci studied. No relationship was observed between heterozygosity and the magnitude or variability of body size.     

Climate-driven changes in abundance and distribution of larvae of oceanic fishes in the southern California region

We examined climatic effects on the geographic distribution and abundance of 34 dominant oceanic fishes in the southern California region using larval fish data collected from the 50‐year long California Cooperative Oceanic Fisheries Investigations (CalCOFI) surveys. The oceanic species responses to environmental changes in their geographic distributions were not very pronounced, perhaps because they lived in the deep layer where temperature change was relatively small or because the environmental variation of the CalCOFI region is not strong enough (with an average temperature gradient of the upper 100 m around 91 km °C^?1). Among the 34 taxa, 16 showed a significant distributional shift (median latitude or boundaries) in relation to environmental variables, and eight species significantly shifted their geographic distribution from the 1951–1976 cold period to the 1977–1998 warm period. Interestingly, the vertically migrating taxa more often showed a significant response to environmental variables than the nonmigrating mesopelagic taxa, reflecting the more significant increase in heat content of the upper ocean (<200 m), compared with the deeper zone (300–500 m) where the mesopelagic fishes typically remain. Climate change has significant effects on the abundances of oceanic fishes. Twenty‐four taxa exhibited a significant change in abundance in relation to environmental variables, and 25 taxa, including both warm and cold‐water taxa, showed a significant increase in abundance from the cold to warm period. Analysis of physical data indicated that the surface‐layer (20–200 m) warmed significantly and the isotherms approached shoreward from the cold to the warm period. We further show that the spatial distribution of coastal–neritic fish retreated shoreward and oceanic fish extended shoreward from the cold to warm period. Our results suggest intensified stratification of the southern California region during the warm period may create a suitable habitat for the oceanic species. Moreover, such an unfavorable condition (e.g. changes in food habitat) for coastal–neritic species might result in competitive release for the oceanic fishes to flourish.     

Population genetic structure and effective population size of ayu (Plecoglossus altivelis), an amphidromous fish

The temporal and spatial population genetic structure of ayu Plecoglossus altivelis (Salmoniformes: Plecoglossidae), an amphidromous fish, was examined using analysis of variation at six microsatellite DNA loci. Intracohort genetic diversities, as measured by the number of alleles and heterozygosity, were similar among six cohorts (2001–2006) within a population (Nezugaseki River), with the mean number of alleles per cohort ranging from 11·0 to 12·5 and the expected heterozygosity ranging from 0·74 to 0·77. Intrapopulational genetic diversities were also similar across the three studied populations along the 50 km coast, with the mean number of alleles and the expected heterozygosity ranging from 11·33 to 11·67 and from 0·75 to 0·76, respectively. The authors observed only one significant difference in pair-wise population differentiation ( F _ST-value) between the cohorts within a population and among three populations. Estimates of the effective population size ( N _e) based on maximum-likelihood method yielded small values (ranging from 94·8 to 135·5), whereas census population size ranged from c. 4800 to 24 000. As a result, the ratio of annual effective population sizes to census population size ( N _e/ N ) ranged from 0·004 to 0·023. These estimates of N _e/ N agree more closely with estimates for marine fishes than that of the larger estimates for freshwater fishes. The present study suggests that ayu which is highly fecund and shows low survival during the early life stages is also characterized by having low value of N _e/ N , similar to marine species with a pelagic life cycle.     

Life-history Patterns of 25 species from European Freshwater Fish Communities

Fish life-history patterns were evaluated in relation to the trilateral continuum model by analyzing data from 25 species inhabiting European freshwaters. Multivariate tests identified a trend between later-maturing fishes with higher fecundity, larger size, and few spawning bouts per year and the opposite suite of traits with small fishes. A second trend contrasted fishes having parental care, smaller eggs, and longer breeding seasons against fishes with the opposite suite of traits. As a result, two extreme life-history patterns could be identified among European freshwater fish species: opportunistic and periodic. Nevertheless, intermediate patterns were also present. A true equilibrium life-history pattern was not represented among 25 fish species from European freshwaters. The high concordance of basic life-history patterns among distantly related taxa is probably caused by some universal trade-offs among life-history variables. As a consequence, only a limited life-history patterns may be recognizable among fish species, independently of the origin of fish communities.     

Life history variation among female surfperches (Perciformes: Embiotocidae)

Synopsis Life history variation within the family Embiotocidae is extensive and involves differences in age of first reproduction, fecundity schedules, growth rates, longevity and size of young. Based on maximum reported body lengths, there are three distinct size groups among the family's 23 species. Small species do not exceed 215 mm TL, medium-size species attain 275 to 335 mm TL, and the large species attain 380 to 470 mm TL. The longevity oh surfperches varies from two to ten years, growth is indeterminate, and females of the medium-and large-size groups may delay first reproduction beyond age one. With one exception, all species show increasing length-specific fecundities. The life history characteristics of females differ among the three size groups. Relative to smaller species, the largest species have moderately high fecundity, delayed maturity and long life. Medium-size species have low fecundity, may delay maturity for 1 to 3 years and have intermediate life spans. Small species have generally higher, but variable, fecundity, do not delay maturity, and are short lived. Among the small North American species, the trend in fecundity varies inversely with environmental predictability. Fecundity is highest in the species which occupies highly seasonal freshwater environments. Coastal species produce moderately large broods and species which occupy stable deep water environments produce the smallest broods.     

Sizing up community genetics: it's a matter of scale

The burgeoning field of community genetics posits that genetic variation within species affects the structure and dynamics of associated communities and ecosystems. While many experiments support this hypothesis, we argue that the most commonly employed experimental design suffers from a fundamental flaw that might result in overestimating the importance of genetic variation. Specifically, most studies collect genotypes from a wide area but perform experiments in small common gardens. Since environmental and genetic variation typically increase with spatial scale, this mismatch in scale is predicted to artificially inflate estimates of the ecological importance of genetic effects. Furthermore, most existing studies have used study systems with particular ecological characteristics, which might further inflate the inferred importance of genetic variation. To critically examine this potential problem, we reanalyze previous studies in community genetics and show how current methods lead to biased conclusions. More specifically, while a growing body of literature shows that intraspecific genetic variation can have an effect, it does not accurately estimate its effect size. As a remedy to this bias, we propose an experimental design that can accurately quantify the importance of genetic and environmental variation in affecting communities and ecosystems.     

Statistical Studies on Protein Polymorphism in Natural Populations II. Gene Differentiation between Populations

With the aim of testing the validity of the mutation-drift hypothesis, we examined the pattern of genetic differentiation between populations by using data from Drosophila, fishes, reptiles, and mammals. The observed relationship between genetic identity and correlation of heterozygosities of different populations or species was generally in good agreement with the theoretical expectations from the mutation-drift theory, when the variation in mutation rate among loci was taken into account. In some species of Drosophila, however, the correlation was unduly high. The relationship between the mean and variance of genetic distance was also in good agreement with the theoretical prediction in almost all organisms. We noted that both the distribution of heterozygosity within species and the pattern of genetic differentiation between species can be explained by the same set of genetic parameters in each group of organisms. Alternative hypotheses for explaining these observations are discussed.