Individual Life-history Traits Research Articles

Maternal and paternal effects determine size, growth and performance in larvae of a tropical reef fish

Larval mortality in marine fishes is strongly linked to individual life history traits such as size and growth, but the processes that influence variability in these traits are poorly understood. We explore the relative importance of maternal and paternal influences and water temperature on the larval growth and performance characteristics of the tropical clownfish Amphiprion melanopus (Pomacentridae). Larvae were reared from an incomplete 4 male × 4 female diallel breeding cross at 2 temperatures (25 and 28°C). Paternity interacted with maternity and affected traits immediately prior to and after hatching. Size of larvae at metamorphosis was primarily affected by maternal and paternal influences, but not by rearing temperature. Paternity explained 52% of the variance in growth rates to metamorphosis, while the combination of paternity, maternity and temperature explained 30%. This strong paternal influence may be due to the extensive role males play in nest-tending coupled with the relatively long embryonic duration of the species. A negative relationship between larval growth rate and mortality within a tank emphasised the importance of these parental effects to larval survival and also provides support for the ‘bigger is better’ hypothesis.

The influence of nonylphenol on life-history of the earthworm Dendrobaena octaedra Savigny: linking effects from the individual- to the population-level

We conducted a study to look at the effects of nonylphenol (NP) on the life-history of the parthenogenetic earthworm, Dendrobaena octaedra . During a 196-day study, we observed that the growth rate of juveniles and the percentage of worms producing cocoons were the only traits significantly affected by NP, while the total number of cocoons produced was marginally affected. Despite some fairly large changes in the average values of individual life-history traits caused by NP, the effects were difficult to detect statistically due to large interindividual variability. A declining trend was observed for population growth rate ( λ ) with increasing NP concentration, but the decline was not statistically significant. The percent reduction in λ was less than the percent reduction in the most sensitive life-history trait (fecundity). An elasticity analysis showed that λ was more sensitive to changes in survival than to changes in reproductive traits. However, neither juvenile nor adult survival were affected by NP, and decomposition analysis showed that the minor changes in λ were mainly caused by effects of NP on time to first reproduction, time between reproduction events and fecundity. The present study suggests that extrapolation from laboratory studies to population effects in the field may be greatly enhanced by combining ecotoxicological and demographic methods.

The influence of nonylphenol on life-history of the earthworm Dendrobaena octaedra Savigny: linking effects from the individual- to the population-level

We conducted a study to look at the effects of nonylphenol (NP) on the life-history of the parthenogenetic earthworm, Dendrobaena octaedra. During a 196-day study, we observed that the growth rate of juveniles and the percentage of worms producing cocoons were the only traits significantly affected by NP, while the total number of cocoons produced was marginally affected. Despite some fairly large changes in the average values of individual life-history traits caused by NP, the effects were difficult to detect statistically due to large interindividual variability. A declining trend was observed for population growth rate ( λ) with increasing NP concentration, but the decline was not statistically significant. The percent reduction in λ was less than the percent reduction in the most sensitive life-history trait (fecundity). An elasticity analysis showed that λ was more sensitive to changes in survival than to changes in reproductive traits. However, neither juvenile nor adult survival were affected by NP, and decomposition analysis showed that the minor changes in λ were mainly caused by effects of NP on time to first reproduction, time between reproduction events and fecundity. The present study suggests that extrapolation from laboratory studies to population effects in the field may be greatly enhanced by combining ecotoxicological and demographic methods.

Interspecific competition in small rodents: from populations to individuals

The role of interspecific competition in shaping animal and plant communities has formed one of the major issues in ecology for decades. Small mammals, mainly rodents, have been among the model systems used for research on interspecific competition. Most studies within small mammal systems in the past have examined effects of competition on population attributes such as on population size, habitat use, or population dynamics. Population-level responses are the cumulative effects of individual responses, however, the influence of competition on individual life-history traits has rarely been studied. Research on life-histories may bridge gaps between population biology and effects of competition on individual behaviour. In this paper, we review recent research approaches to interspecific competition in rodents based on census data and species assemblages, that use regression analysis, time series analysis, removal and exclusion experiments, and showcase our own experimental research on the effects of interspecific competition on individual life-history traits in boreal voles.

Determining Demographic Effects of Cypermethrin in the Marine Copepod Acartia tonsa : Stage-Specific Short Tests Versus Life-Table Tests

Short-term lethal and sublethal responses of the calanoid copepod Acartia tonsa to cypermethrin were compared with life-table responses to assess whether or not it is necessary to use exposure periods longer than 5 days to estimate demographic responses to stress. More specifically, by limiting exposure periods to sensitive age classes (eggs, nauplii, copepodids, and adults) and including measurements on survival, egg production, and feeding rates, it was possible to derive a short test design of similar sensitivity and ecological relevance as full life-table tests. Short-term exposures to cypermethrin reduced copepodid's feeding rates at concentrations well below those affecting egg production rates and survival of eggs and adult stages. Lethal effects on naupliar stages occurred at lower concentrations than any other effect observed on eggs and adults. Life-table sensitivities of the intrinsic rate of increase ( r(m)) to cypermethrin were similar to those observed in short-term exposures. More specifically, exposure to cypermethrin impaired r(m) responses at concentrations (7.4 ng x L(-1)) that also affected feeding and naupliar responses. Our results show that by quantifying and separating combined toxic effects on ecologically relevant individual life-history traits, it is possible to develop toxicity test designs of similar ecological relevance yet that are less labor-intensive and costly than existing demographic tests.

Size-dependent life-history traits promote catastrophic collapses of top predators.

Catastrophic population collapses such as observed in many exploited fish populations have been argued to result from depensatory growth mechanisms (i.e., reduced reproductive success at low population densities, also known as Allee effect). Empirical support for depensation from population-level data is, however, hard to obtain and inconclusive. Using a size-structured, individual-based model we show that catastrophic population collapses may nonetheless be an intrinsic property of many communities, because of two general aspects of individual life history: size- and food-dependent individual growth and individual mortality decreasing with body size. Positive density dependence, characteristic for depensatory growth mechanisms and catastrophic behavior, results as a direct and robust consequence of the interplay between these individual life-history traits, which are commonly found in many species.

Variation in cadmium uptake, feeding rate, and life‐history effects in the gastropod Potamopyrgus antipodarum: Linking toxicant effects on individuals to the population level

A life-table response experiment was performed to investigate the effects of sediment-bound cadmium on individual life-history traits and feeding rates of four clones of Potamopyrgus antipodarum. Demographic effects were evaluated using a simple two-stage model to estimate population growth rate (lambda). Decomposition analysis was performed to investigate the contributions of each of the affected life-history traits to the effects observed on lambda, and elasticity analysis was applied to examine the relative sensitivity of lambda to changes in each of the life-history traits. Interclonal differences in tolerance to sediment-bound cadmium were statistically significant but were within an order of magnitude. There were no consistent patterns among clones in terms of which individual life-history trait was most or least sensitive to cadmium exposure. The relative performance of clones did not rank consistently across the cadmium gradient and was dependent on which trait was measured. Although lambda was most sensitive to changes in survival terms, the effects of cadmium on time to first reproduction and reproductive output were the major causes of reductions in lambda. Large percent reductions in some of the individual life-history traits were attenuated at the population level, but very small effects on population growth rate were statistically significant.

Effects of chronic fluoranthene exposure on sibling species of Capitella with different development modes

Toxic effects of the polycyclic aromatic hydrocarbon fluoranthene (FLU) on life-history traits and their demographic consequences were investigated in 3 non-interbreeding Capitella sib- ling species with different physiological tolerances and developmental modes: sensitive Capitella sp. S from oxygen-rich intertidal sediments of the North Sea (Germany); tolerant Capitella sp. M from sediments near shallow hydrothermal vents off Milos (Greece), a habitat low in organic matter with steep abiotic gradients and high sulfide concentrations; tolerant Capitella sp. I from New York (USA), known to dominate eutrophicated/polluted environments. Both Capitella spp. M and I can develop into hermaphrodites and have lecithotrophic larval development. In contrast, Capitella sp. S appears to be dioecious and has direct development with benthic juveniles. In life-table-response experi- ments (LTRE), juveniles from the 3 species were raised under different FLU concentrations (0 to 95 µg g -1 FLU), and data on age-specific survival, growth and life-history parameters were recorded at weekly intervals. Under control conditions, the 3 Capitella species differed markedly in a number of life-history traits and population growth rates (λ), with Capitella sp. S showing the lowest λ (1.05), and Capitella sp. M the highest (1.42). Chronic exposure to inreasing FLU concentrations also revealed species-specific differences in individual- and population-level toxic responses. Highest FLU con- centrations (95 µg g -1 ) markedly reduced juvenile survival and completely inhibited reproduction in Capitella sp. S, whereas individual life-history traits in Capitella spp. M and I were affected little, if at all. At the population level, the highest FLU exposures resulted in λ values of effectively zero in Capitella sp. S, whereas λ of Capitella spp. M and I remained >1. In conclusion, the combination of opportunistic life-history features, reproductive flexibility, and physiological adaptations enables Capitella spp. M and I to colonize habitats rapidly after local disturbance and to persist in stressed and unpredictable environments; whereas in Capitella sp. S, population extinction under toxicant stress mainly results from its physiological sensitivity.

Exploring Cost Constraints on Stem Elongation in Plants Using Phenotypic Manipulation

Negative associations between individual life-history traits of an organism are referred to as life-history trade-offs (Stearns 1992; Zera et al. 1998). The existence of costly trade-offs is thought to have favored the evolution of phenotypic plasticity as a mechanism through which organisms can account for environmental heterogeneity while modulating costs and benefits incurred by fixed allocation to competing functions (Bradshaw 1965; Stearns 1992; Sultan 1995; Dudley and Schmitt 1996; Pigliucci 1996). In plants, the ability to modify stem elongation in response to environmental cues appears to be a classic form of adaptive phenotypic plasticity (Sultan 1995; Gedroc et al. 1996). Plastic increases or decreases in stem elongation rate in plants can be triggered by light-quality and lightquantity cues (Smith 1982; Ballare et al. 1989, 1990; Rajapakse and Kelly 1995; Petit and Thompson 1997) and exposure to mechanical stimuli (e.g., wind) (Gartner 1991; Jaffe and Forbes 1993; Emery et al. 1994; Cipollini 1997b). Increased stem elongation in response to light cues can enable plants to escape shading by neighbors and capitalize on heterogenous resources, while reduced elongation in response to mechanical cues can enable plants to resist mechanical damage (Ballare et al. 1991, 1995; Jaffe and Forbes 1993). The best-studied plastic stem response in plants is the “shade avoidance” response, cued by altered red : far-red

LIFE HISTORY CONSEQUENCES OF FOOD QUALITY IN THE FRESHWATER COPEPODBOECKELLA TRIARTICULATA

Food quality often has profound effects on life history traits and individual fitness, altering rates of growth and development, changing the timing of reproduction, and shifting the trade-off between egg size and egg number. Few data are available on the effects of food quality on copepod life history traits. We measured several life history traits on a large number of individuals to document the effects of food quality on individual traits, on life history correlates, and on a composite measure of individual fitness in the freshwater copepod Boeckella triarticulata Thomson. Nauplii were raised individually on two diets: one consisted of the high quality alga Cryptomonas sp. (abbreviated as CR), and the second diet consisted of a combination of Cryptomonas sp. and the low quality cya- nobacterium Anabaena flos-aquae (CA). The mixed CA diet slowed growth and develop- ment so that individuals raised on this diet were older and smaller at metamorphosis and maturity. Despite these effects, there were no differences between diets in survival to maturity, and male copepods raised on the mixed diet lived significantly longer than females or than either sex raised on Cryptomonas alone. Females raised on the mixed diet produced more and larger clutches than those raised on CR, so that total egg production increased on this diet, although large intradiet variation obscured statistical differences between diets in these parameters. Intradiet variation was due to a large range in the number of clutches produced by individual females: some individuals produced 10-15 clutches, contradicting previous descriptions of this species as semelparous. Although diet affected age at first reproduction, it had no significant effect on individual fitness, estimated as X. Boeckella triarticulata achieved high fitness either by minimizing age at first reproduction (CR diet) or by increasing reproductive output (CA diet). Data collected from cast-off exuviae allow nondestructive measures of individual life history traits along with a composite measure of individual fitness. Combining these two analyses is an important step in unraveling life history correlates and in identifying the selective forces driving life history evolution in these crustaceans.

A framework for understanding Atlantic salmon (Salmo salar) life history

We took a hierarchical approach to understanding Atlantic salmon (Salmo salar) life history patterns by first comparing salmonids to other teleosts, next comparing Atlantic salmon to other salmonids, and finally, mapping correlations among individual life history traits within Atlantic salmon. The combination of anadromy, large eggs, nest construction and egg burial by females, and large size at maturity differentiates salmonids from most other teleosts. Within the family Salmonidae, there is considerable variation in all traits but Atlantic salmon are generally in the middle of the range. Within Atlantic salmon, we were able to map correlations among individual life history traits, but we found that we still lacked an understanding comprehensive and quantitative enough to allow us to predict how the entire life history should respond to environmental changes. Thus, we proposed several general courses of action: (i) use models to synthesize complex patterns and relationships, (ii) collect long time series of data in individual systems, and (iii) design experiments to assess phenotypic plasticity and how environmental influences differ from genetic effects and constraints.

A framework for understanding Atlantic salmon (<i>Salmo</i> <i>salar</i>) life history

We took a hierarchical approach to understanding Atlantic salmon (Salmo salar) life history patterns by first comparing salmonids to other teleosts, next comparing Atlantic salmon to other salmonids, and finally, mapping correlations among individual life history traits within Atlantic salmon. The combination of anadromy, large eggs, nest construction and egg burial by females, and large size at maturity differentiates salmonids from most other teleosts. Within the family Salmonidae, there is considerable variation in all traits but Atlantic salmon are generally in the middle of the range. Within Atlantic salmon, we were able to map correlations among individual life history traits, but we found that we still lacked an understanding comprehensive and quantitative enough to allow us to predict how the entire life history should respond to environmental changes. Thus, we proposed several general courses of action: (i) use models to synthesize complex patterns and relationships, (ii) collect long time series of data in individual systems, and (iii) design experiments to assess phenotypic plasticity and how environmental influences differ from genetic effects and constraints.

Life-History Variation in a Colonial Ascidian: Broad-Sence Heritabilities and Tradeoffs in Allocation to Asexual Growth and Male and Female Reproduction.

Intraspecific variation in life-history strategy provides a valuable opportunity for examining how natural selection acts on life-history variants to mold reproductive strategies. Evaluating the consequences of selection requires knowledge of the range of phenotypic variation in life histories, the extent to which variation is genetically based, and possible correlations among different traits that might constrain or promote the effect of selection on individual traits. We explored life-history variation in the colonial ascidian Botryllus schlosseri (a cyclical hermaphrodite) by growing clonal replicates of 18 genotypes in a common-garden experiment. Colonies of this species have previously been shown to vary in egg production and growth rate. We demonstrate that genotypes also vary in sperm production, which is manifested as variation in testis size. We then calculate broad-sense heritabilities for a suite of life-history traits and demonstrate correlations among traits that suggest a three-way tradeoff in resource allocation to asexual growth and sexual reproduction via male and female function. This correlation structure suggests that selection cannot act independently on individual life-history traits.