Flexible Life History Research Articles

Estimating the impacts of fishing on dependent predators: a case study in the California Current

Juvenile rockfish (Sebastes spp.) are important prey to seabirds in the California Current System, particularly during the breeding season. Both seabird breeding success and the abundance of pelagic juvenile rockfish show high interannual variability. This covariation is largely a response to variable ocean conditions; however, fishing on adult rockfish may have had consequences for seabird productivity (e.g., the number of chicks fledged per breeding pair) by reducing the availability of juvenile rockfish to provisioning seabird parents. We tested the hypothesis that fishing has decreased juvenile rockfish availability and thereby limited seabird productivity over the past 30 years. We quantified relationships between observed juvenile rockfish relative abundance and seabird productivity, used fisheries stock assessment approaches to estimate the relative abundance of juvenile rockfish in the absence of fishing, and compared the differences in seabird productivity that would have resulted without rockfish fisheries. We examined the abundance of juvenile rockfish and the corresponding productivity of three seabird species breeding on Southeast Farallon Island (near San Francisco, California, USA) from the early 1980s to the present. Results show that while the relative abundance of juvenile rockfish has declined to approximately 50% of the estimated unfished biomass, seabirds achieved 75-95% of the estimated un-impacted levels of productivity, depending upon the species of bird and various model assumptions. These results primarily reflect seabirds with "conservative" life histories (one egg laid per year) and may be different for species with more flexible life history strategies (greater reproductive effort). Our results are consistent with the premise that the impacts of local rockfish fisheries on seabird productivity are less than impacts that have occurred to the prey resources themselves due to ocean climate and the ability of seabirds to buffer against changes in prey availability through prey-switching and other behavioral mechanisms.

The short-term responses of small mammals to wildfire in semiarid mallee shrubland, Australia

Context. Wildfire is a major driver of the structure and function of mallee eucalypt- and spinifex-dominated landscapes. Understanding how fire influences the distribution of biota in these fire-prone environments is essential for effective ecological and conservation-based management. Aims. We aimed to (1) determine the effects of an extensive wildfire (118 000 ha) on a small mammal community in the mallee shrublands of semiarid Australia and (2) assess the hypothesis that the fire-response patterns of small mammals can be predicted by their life-history characteristics. Methods. Small-mammal surveys were undertaken concurrently at 26 sites: once before the fire and on four occasions following the fire (including 14 sites that remained unburnt). We documented changes in small-mammal occurrence before and after the fire, and compared burnt and unburnt sites. In addition, key components of vegetation structure were assessed at each site. Key results. Wildfire had a strong influence on vegetation structure and on the occurrence of small mammals. The mallee ningaui, Ningaui yvonneae, a dasyurid marsupial, showed a marked decline in the immediate post-fire environment, corresponding with a reduction in hummock-grass cover in recently burnt vegetation. Species richness of native small mammals was positively associated with unburnt vegetation, although some species showed no clear response to wildfire. Conclusions. Our results are consistent with the contention that mammal responses to fire are associated with their known life-history traits. The species most strongly affected by wildfire, N. yvonneae, has the most specific habitat requirements and restricted life history of the small mammals in the study area. The only species positively associated with recently burnt vegetation, the introduced house mouse, Mus domesticus, has a flexible life history and non-specialised resource requirements. Implications. Maintaining sources for recolonisation after large-scale wildfires will be vital to the conservation of native small mammals in mallee ecosystems.

Advances in herring biology: from simple to complex, coping with plasticity and adaptability

AbstractGeffen, A. J. 2009. Advances in herring biology: from simple to complex, coping with plasticity and adaptability. – ICES Journal of Marine Science, 66: 1688–1695. At least two centuries of investigations on herring have been absorbed by scientific journals, and applied and basic research has produced groundbreaking concepts in fisheries, population biology, and marine ecology. By the 1970s, a firm understanding of herring biology formed the basis for more sophisticated research. At that point, herring populations had been delineated, and their migration patterns described. The reproduction and early stage biology were characterized in ways that could be applied to fisheries management. However, over the subsequent four decades, new approaches and technology overturned many of the earlier findings. Behavioural studies revealed a repertoire of patterns that extended the concept of schooling, and genetic analyses showed high levels of stock mixing. Application of otolith analysis to larval, juvenile, and adult fish revealed the scope of plasticity in growth and life-history strategies. Developments in physiological research have revealed that herring are not “primitive”, as once believed, but highly adaptable in their nutrition and metabolism. These advances fundamentally changed our view of herring, and the resulting challenge is to synthesize current knowledge to help explain the significance of adaptability and plasticity in its flexible life history.

Life history buffering in Antarctic mammals and birds against changing patterns of climate and environmental variation

AbstractThe consequences of warming for Antarctic long‐lived organisms depend on their ability to survive changing patterns of climate and environmental variation. Among birds and mammals of different Antarctic regions, including emperor penguins, snow petrels, southern fulmars, Antarctic fur seals and Weddell seals, we found strong support for selection of life history traits that reduce interannual variation in fitness. These species maximize fitness by keeping a low interannual variance in the survival of adults and in their propensity to breed annually, which are the vital rates that influence most the variability in population growth rate (λ). All these species have been able to buffer these rates against the effects of recent climate‐driven habitat changes except for Antarctic fur seals, in the Southwest Atlantic. In this region of the Southern Ocean, the rapid increase in ecosystem fluctuation, associated with increasing climate variability observed since 1990, has limited and rendered less predictable the main fur seal food supply, Antarctic krill. This has increased the fitness costs of breeding for females, causing significant short‐term changes in population structure through mortality and low breeding output. Changes occur now with a frequency higher than the mean female fur seal generation time, and therefore are likely to limit their adaptive response. Fur seals are more likely to rely on phenotypic plasticity to cope with short‐term changes in order to maximize individual fitness. With more frequent extreme climatic events driving more frequent ecosystem fluctuation, the repercussions for life histories in many Antarctic birds and mammals are likely to increase, particularly at regional scales. In species with less flexible life histories that are more constrained by fluctuation in their critical habitats, like sea‐ice, this may cause demographic changes, population compensation and changes in distribution, as already observed in penguin species living in the Antarctic Peninsula and adjacent islands.

Colorado Potato Beetle Resistance to Insecticides

The Colorado potato beetle, Leptinotarsa decemlineata (Say), is widely regarded as the most important insect defoliator of potatoes. Its current range covers about 16 million km2 in North America, Europe, and Asia and continues to expand. This insect has a complicated and diverse life history, which is well-suited to agricultural environments, and makes it a complex and challenging pest to control. Dispersal, closely connected with diapause, feeding, and reproduction, allow the Colorado potato beetle to employ “bet-hedging” reproductive strategies, distributing its offspring in both space (within and between fields) and time (within and between years). The Colorado potato beetle played a large role in creating the modern pesticide industry, with hundreds of chemicals tested against it. High selection pressure, together with natural propensity to adapt to toxic substances, eventually resulted in a large number of insecticide-resistant Colorado potato beetle populations. Since the middle of the last century, the beetle has developed resistance to 52 different compounds belonging to all major insecticide classes. Resistance levels vary greatly among different populations and between beetle life stages, but in some cases can be very high (up to 2,000-fold). Known mechanisms of Colorado potato beetle resistance to insecticides include enhanced metabolism involving esterases, carboxylesterases and monooxygenases, and target site insensitivity, as well as reduced insecticide penetration and increased excretion. There is also some evidence of behavioral resistance. Resistance mechanisms are sometimes highly diverse even within a relatively narrow geographical area. Resistance is usually inherited as an incompletely dominant or incompletely recessive trait, with one or several genes involved in its determination. Because of pleiotropic effects of resistant alleles, insecticide-resistant beetles often have reduced relative fitness in the absence of insecticides. Rotating different classes of insecticides and reducing insecticidal pressure on pest populations by provision of temporal and spatial refuges from exposure to toxins have been proposed to delay evolution of resistance. However, insecticide resistance in this insect will likely remain a major challenge to the pest control practitioners. Still limited understanding of beetle biology, its flexible life history, and grower reluctance to adopt some of the resistance management techniques create impediments to successful resistance management. Overcoming these obstacles is not an easy task, but it will be crucial for sustainable potato production.

Reproductive Flexibility: Genetic Variation, Genetic Costs and Long-Term Evolution in a Collembola

In a variable yet predictable world, organisms may use environmental cues to make adaptive adjustments to their phenotype. Such phenotypic flexibility is expected commonly to evolve in life history traits, which are closely tied to Darwinian fitness. Yet adaptive life history flexibility remains poorly documented. Here we introduce the collembolan Folsomia candida, a soil-dweller, parthenogenetic (all-female) microarthropod, as a model organism to study the phenotypic expression, genetic variation, fitness consequences and long-term evolution of life history flexibility. We demonstrate that collembola have a remarkable adaptive ability for adjusting their reproductive phenotype: when transferred from harsh to good conditions (in terms of food ration and crowding), a mother can fine-tune the number and the size of her eggs from one clutch to the next. The comparative analysis of eleven clonal populations of worldwide origins reveals (i) genetic variation in mean egg size under both good and bad conditions; (ii) no genetic variation in egg size flexibility, consistent with convergent evolution to a common physiological limit; (iii) genetic variation of both mean reproductive investment and reproductive investment flexibility, associated with a reversal of the genetic correlation between egg size and clutch size between environmental conditions ; (iv) a negative genetic correlation between reproductive investment flexibility and adult lifespan. Phylogenetic reconstruction shows that two life history strategies, called HIFLEX and LOFLEX, evolved early in evolutionary history. HIFLEX includes six of our 11 clones, and is characterized by large mean egg size and reproductive investment, high reproductive investment flexibility, and low adult survival. LOFLEX (the other five clones) has small mean egg size and low reproductive investment, low reproductive investment flexibility, and high adult survival. The divergence of HIFLEX and LOFLEX could represent different adaptations to environments differing in mean quality and variability, or indicate that a genetic polymorphism of reproductive investment reaction norms has evolved under a physiological tradeoff between reproductive investment flexibility and adult lifespan.

The effect of diet quality on physiological and life-history traits in the harvestman Pachylus paessleri

The balance between energy acquisition and expenditure is critical to the survival and reproductive success of animals. Here we investigate the long-term effects of diet quality on physiological and life-history flexibility in the harvestman, Pachylus paessleri. We used cow meal as a protein-rich diet and potatoes as a carbohydrate-rich diet in order to reproduce two extreme conditions regarding food quality in harvestmen natural habitat. As proxy variables of the energy expenditure process, we quantified standard metabolic rate (maintenance), changes in body mass (somatic condition), and fecundity (reproduction). We found that animals consuming the protein-rich diet were able to increase both their body condition and fecundity. However, the increment in these two life-history traits was correlated with higher maintenance costs. In contrast, the carbohydrate-rich diet did not provide enough specific nutrients for reproductive events, although it may have allowed animals to survive for a long time. Thus, according to the quality of the diet available in the environment, harvestman females can adopt different life-history strategies correlated with phenotypic adjustments at anatomical and physiological levels. In the Mediterranean region, spatial and temporal changes in food quality are typical, so greater phenotypic flexibility is expected to cope with this kind of environmental variation.

Life history flexibility allows Sargassum polyceratium to persist in different environments subjected to stochastic disturbance events

Stochastic, stage-based matrix models were used to investigate the life history strategy of the seaweed Sargassum polyceratium in shallow intertidal and deep-water (18 m) populations. Matrix models were parameterized with 3 years of yearly transitions among four plant stages quantified from three bays on Curacao (Netherlands Antilles). There were years without a storm, with a moderate (winter) storm and with a strong storm (Hurricane Lenny). The stochastic population growth rate varied among populations (λs: 0.54–1.03) but was not related to depth. The most important stages for population growth were reproductive adults (shallow) and non-reproductive adults (deep). With the occurrence of storms, vegetative growth (mainly deep) and fertility (mainly shallow) became the most important processes. Recruitment (shallow) and regeneration from holdfasts (deep) only contributed to population persistence after the hurricane. It is concluded that S. polyceratium has a flexible, depth-dependent, life history strategy that is adjusted to disturbance events.

Flexible Life History Strategies of Ninespine Sticklebacks, Genus Pungitius

We studied the life histories of the ninespine sticklebacks, Pungitius pungitius and Pungitius tymensis, collected from Japanese freshwater and brackish (sea) water habitats by examining the strontium (Sr) and calcium (Ca) concentrations in their otoliths. The Sr:Ca ratios in the otoliths changed with the salinity of the habitat regardless of identification as freshwater or brackish water type based on morphological characteristics. The ninespine sticklebacks living in a freshwater environment showed consistently low Sr:Ca ratios throughout the otolith. These samples were identified as a standard freshwater type. In contrast, all freshwater-type fishes collected from the intertidal zone showed higher otolith Sr:Ca ratios than those in the standard freshwater type, and the ratios fluctuated with the growth phase. All brackish water-type fishes collected in the intertidal zone showed the highest otolith Sr:Ca ratio throughout the otolith. In the present study, besides the two representative life history types of P. pungitius, i.e., freshwater and brackish water life history types, other sticklebacks had an anadromous life history type. These findings clearly indicate that the ninespine stickleback has a flexible migration strategy with a high degree of behavioral plasticity and an ability to utilize the full range of salinity in its life history.

A continuum of life history tactics in a brown trout (Salmo trutta) population

Life history tactics of the brown trout (Salmo trutta) population of the Oir River (Normandy, France) were studied using passive integrated transponder (PIT) tagging data of five consecutive cohorts (5900 individuals) monitored between 1995 and 2002. Results demonstrate that (i) life history traits vary among cohorts, chiefly caused by environmental variability, (ii) juvenile growth, particularly second-year growth, plays an important role in the determination of the growing environment and trout exhibit variable migratory behaviour (from remaining in the natal brook to migrating in the sea) related to their juvenile growth rate, and (iii) the description of life history tactics (including juvenile growth, fine-scale migratory behaviour, and reproduction) can be clarified. Tactics are expressed along a continuum in time (age to reproduce) and space (distance of migration). Flexible life history tactics varying with juvenile growth is consistent with previous studies, but the use of empiric data on growth and migration from PIT tagging allows refining the description of life history tactics, taking into account their continuous distribution in time and space.

Effects of climate on relative predation by scyphomedusae and ctenophores on copepods in Chesapeake Bay during 1987-2000

The mesohaline region of Chesapeake Bay had high densities of the scyphomedusan Chrysaora quinquecirrha and low densities of the ctenophore Mnemiopsis leidyi in 1987-1990 and 1995. By contrast, 1996-2000 had much lower medusa and higher ctenophore densities. Predation on copepods (Acartia tonsa) was intense in 1996-2000, and copepod densities were low when ctenophores were abundant. At equivalent sizes, the feeding potentials of ctenophores were greater than those of medusae, with clearance rates about 1.2 times greater by volume and about 3 times greater by carbon biomass. M. leidyi ctenophores more negatively influence copepod populations than C. quinquecirrha medusae because they are more effective predators of copepods and have broader spatial and temporal occurrence, wider salinity and temperature ranges, greater densities, and a more flexible life history. Because they consume ctenophores, C. quinquecirrha medusae positively affect copepod abundance. C. quinquecirrha and M. leidyi could be considered keystone predators because of their far-reaching effects on the plankton food web. The balance between medusae and ctenophores in Chesapeake Bay was greatly affected by climatic factors. Medusa abundances were high when dry years prevailed prior to and during 1987-1990 and 1995. By contrast, medusa abundances were low when dry and wet years alternated before and during 1996-2000. Significant variables that favored medusae in 1987-1990 and 1995 were high salinity, warm temperature, and high solar irradiance. The North Atlantic Oscillation Index was significantly inversely correlated with medusa numbers from 1960-1995. Climate clearly affects gelatinous predator abundances, with consequences that cascade throughout the plankton food web.

Flexible life history responses to flower and rosette bud removal in three perennial herbs

In a garden experiment we investigated the response to continuous removal of either flower buds or rosette buds in three perennial grassland species (Hypochaeris radicata, Succisa pratensis and Centaurea jacea), which differ in longevity and flowering type. We distinguished two possible responses: compensation for lost buds by making more buds of the same type, and switching towards development of other life history functions. Both responses were demonstrated in our experiment, but bud removal had significantly different effects in each of the three species. The degree of compensation and the expression of trade‐offs between life history functions differed markedly between species and seem related to longevity and developmental constraints. With respect to switching, our results suggest costs of reproduction and a trade‐off between life history functions, at least for Hypochaeris and Succisa. For these species weight of new rosettes increased when resource allocation to flowering was inhibited. In Hypochaeris, we see that both compensation for lost flower buds and switching from lost rosette buds increased production of flower buds, underscoring the pivotal role of sexual reproduction in this short‐lived species. The most prominent response seen in Centaurea is compensation for lost rosette buds, indicating that this long‐lived species with monocarpic rosettes relies on rosette formation. Although Succisa does respond to bud removal, time is an important constraint in this species with long‐lived rosettes and preformed flowering stalks. Trade‐offs in Succisa seem to operate at a larger time scale, requiring long‐lasting experiments to reveal them. We conclude that the response of these species to inflicted damage is likely to be linked to their longevity and developmental constraints.

The biology and life history of arctic populations of the littoral mite Ameronothrus lineatus (Acari, Oribatida)

The present study attempts to elucidate possible microevolutionary adaptations of life-history traits of high-latitude populations of the holarctic, littoral oribatid mite Ameronothrus lineatus by comparing arctic and temperate populations. Additionally, the paper provides an overview of the limited research on general ecology and population biology of arctic populations. In the Arctic the larviparous A. lineatus has a 5-year life cycle (larva-to-larva), and adults survive a further 2-3 years. High survival to maturity is consistent with a low lifetime reproductive output of ca. 20 larvae. The life history can be regarded as an extreme version of the typical oribatid life history. However, several life-history features suggest specific adaptations of arctic populations. In particular, the pre-moult resting stage is synchronized with the warmest part of the arctic summer, which shortens this vulnerable part of development. High reproductive investment by females at relatively low temperatures may represent a physiological adaptation to the cool arctic summer. Finally, prolonged cold exposure positively affects reproduction and survival the following summer, suggesting adaptation of the species to the highly seasonal arctic environment. On the other hand, the ability of all life-cycle stages to overwinter, and a flexible life history with the species being able to take advantage of favourable climatic conditions to accelerate development and larviposition, seem to be ancestral features. Thus, the success of A. lineatus in arctic habitats is probably attributable to a combination of derived and ancestral life-history traits. Studies of conspecific temperate populations are required to elucidate further local adaptations of arctic populations.

The biology of Bothrometopus elongatus (Coleoptera, Curculionidae) in a mid-altitude fellfield on sub-Antarctic Marion Island

Bothrometopus elongatus is one of four Ectemnorhinus-group species restricted to the epilithic biotope on the Prince Edward Islands. Here we examine the biology of this species over a full year at Kerguelen Rise, a mid-altitude fellfield site on Marion Island. B. elongatus adults eclose from April onwards, reaching maximum densities (ca. 17 individuals m−2) in September. Females mature approximately three eggs at a time and these commence hatching in July. Larval eclosion reaches a peak in November, during which time larval densities are also highest (ca. 153 individuals m−2). The larvae develop through six instars, which is within the range found for other Ectemnorhinus-group species. The high densities of B. elongatus in fellfield habitats, and its single, virtually discrete annual generation, make this species unusual among insects indigenous to the sub-Antarctic. The latter generally have low densities compared to other micro-arthropods, prolonged life-cycles, and flexible life-histories. We suggest that the diversity of life-histories found amongst the indigenous insects at Marion Island presents considerable potential for testing environmental effects on insect life-histories. An overview of sub-Antarctic insect life-history data suggests that the indigenous species, with generally prolonged life-cycles, are at a disadvantage relative to introduced species that have more rapid cycles and often complete several generations per year. This is reason for concern given rapid climate change at these islands.

Genetic Population Structure and Origin of Life History Types in Chinook Salmon in British Columbia, Canada

We used protein electrophoresis to examine genetic population structure and origin of life history types of chinook salmon Oncorhynchus tshawytscha in British Columbia, Canada. Among 31 allozyme loci resolved in 91 samples from 63 populations of chinook salmon in rivers and hatcheries throughout British Columbia, population heterozygosities averaged 0.084 (range 0.048–0.108) and were typical of values for populations in other regions. A hierarchical gene diversity analysis indicated that 91.3% of the total allele-frequency diversity was attributable to within-population variability; the remaining 8.7% was attributable to geographic variability among populations, which was partitioned into among-river (3.3%), among-area (3.5%), and among-region (1.9%) components. Two major groups of populations appeared in the principal components analysis and in cluster analysis of genetic distances. A coastal group included populations in four subgroups: Central coast, Georgia Strait, lower Fraser River, and west Vancouver Island. An inland group included six subgroups: Nass River, Skeena River, north Thompson River, upper and mid-Fraser River, south Thompson River, and lower Thompson River. The geographic extents of the inland and coastal groups largely coincided with the geographic distributions of stream- and ocean-type juvenile forms and may reflect postglacial colonization by two ancestral lineages that survived in Pleistocene refugia. The presence of genetically undifferentiated stream-type fish in coastal streams populated by ocean-type fish may reflect either postglacial life history differentiation from ancestral ocean-type fish or life history flexibility of ocean-type fish.

Climatic signals in the life histories of insects: the distribution and abundance of heather psyllids (Strophingia spp.) in the UK

1. The population density and age structure of two species of heather psyllid Strophingia ericae and Strophingia cinereae, feeding on Calluna vulgaris and Erica cinerea, respectively, were sampled using standardized methods at locations throughout Britain. Locations were chosen to represent the full latitudinal and altitudinal range of the host plants. 2. The paper explains how spatial variation in thermal environment, insect life‐history characteristics and physiology, and plant distribution, interact to provide the mechanisms that determine the range and abundance of Strophingia spp. 3. Strophingia ericae and S. cinereae, despite the similarity in the spatial distribution patterns of their host plants within Britain, display strongly contrasting geographical ranges and corresponding life‐history strategies. Strophingia ericae is found on its host plant throughout Britain but S. cinereae is restricted to low elevation sites south of the Mersey–Humber line and occupies only part of the latitudinal and altitudinal range of its host plant. There is no evidence to suggest that S. ericae has reached its potential altitudinal or latitudinal limit in the UK, even though its host plant appears to reach its altitudinal limit. 4. There was little difference in the ability of the two Strophingia spp. to survive short‐term exposure to temperatures as low as – 15 °C and low winter temperatures probably do not limit distribution in S. cinereae. 5. Population density of S. ericae was not related to altitude but showed a weak correlation with latitude. The spread of larval instars present at a site, measured as an index of instar homogeneity, was significantly correlated with a range of temperature related variables, of which May mean temperature and length of growing season above 3 °C (calculated using the Lennon and Turner climatic model) were the most significant. Factor analysis did not improve the level of correlation significantly above those obtained for single climatic variables. The data confirmed that S. ericae has a 1 year life cycle at the lowest elevations and a 2 year life cycle at the higher elevations. However, there was no evidence, as previously suggested, for an abrupt change from a one to a 2 year life cycle in S. ericae with increasing altitudes or latitudes. 6. By contrast with S. ericae, S. cinereae had an obligatory 1 year life cycle, its population decreased with altitude and the index of instar homogeneity showed little correlation with single temperature variables. Moreover, it occupied only part of the range of its host plant and its spatial distribution in the UK could be predicted with 96% accuracy using selected variables in discriminant analysis. 7. The life histories of the congeneric heather psyllids reflect adaptations that allow them to exploit host plants with different distributions in climatic and thereby geographical space. Strophingia ericae has the flexible life history that enables it to exploit C. vulgaris throughout its European boreal temperate range. Strophingia cinereae has a less flexible life history and is adapted for living on an oceanic temperate host. While the geographic ranges of the two Strophingia spp. overlap within the UK, the psyllids appear to respond differently to variation in their thermal environment.

Flight and Fecundity of Colorado Potato Beetles (Coleoptera: Chrysomelidae) Fed on Different Diets

Overwintered and summer generation adult female Colorado potato beetles, Leptinotarsa decemlineata (Say), were reared on 8 different diets (7 plant species and no food) for 30 d, during which time individual fecundity and flight mill performance were measured. Diet did not affect survival of overwintered females, but strongly affected survival of the newly eclosed summer generation. Fecundity was related positively for both generations to larval success on the different plant species. Tethered flight was greatest amongst starved overwintered females and generally greater in the overwintered generation than in the summer generation. Across treatments, total flight duration was related positively to fecundity for summer generation females; in contrast, it was related negatively to fecundity for the overwintered generation. However, there was no relationship between flight and fecundity within any of the treatments in either generation. Flight was greatest on days 3–6 for overwintered females, peaking later after eclosion for the summer generation, and continuing throughout the 30 d in at least some individuals. Flight was greater for summer generation females with greater emergence weight, and for overwintered females which emerged earlier. Among females nearer to death, or smaller females of the summer generation, reproduction took precedence over flight. The temporal patterns of fecundity and flight in female L. decemlineata constitute a flexible life history, which seems to represent an adaptation against disappearance of host plants when this risk is highest.

Performance Thresholds and Life‐History Flexibility in Salmonids

Performance Thresholds and Life‐History Flexibility in Salmonids

Fitness consequences of alternative life histories in water striders, Aquarius remigis (Heteroptera: Gerridae).

Using field and laboratory observations and experiments over 3 years, I investigated whether reproductive trade-offs shape individual life histories in two natural populations of the water strider, Aquarius remigis, in which univoltine and bivoltine life cycles coexist. Both later eclosion dates and food shortages, even after adult eclosion, induced diapause in females, thus deferring reproduction to the following spring. Adult body size was positively affected by food availability during juvenile development. Higher food levels also increased the reproductive output of females, but not their longevity or oviposition period. When compared to spring breeders (univoltine life cycle), direct (summer) breeders (bivoltine life cycle) experienced reduced lifetime egg numbers and longevity, as well as reduced survivorship of their second-summer-generation offspring; these reproductive costs offset, at least in part, the advantage in non-decreasing populations of having two generations per year. Fecundity was correlated with body size, and among summer-generation females direct breeders were larger than non-breeders. The time remaining before the onset of winter and/or the time since adult eclosion augmented cumulative energy uptake, and consequently the lipid reserves and winter survival probability of non-breeding (diapausing) summer adults approaching hibernation. Overwintered spring reproductives died at faster rates than non-reproductive summer individuals despite greater food availability in spring, indicating a mortality cost of reproduction. Body length correlated with absolute and not with proportional lipid content but showed no consistent relationship with survivorship in the field. These results are in agreement with current theory on the evolution of insect voltinism patterns, and further indicate high degrees of life history flexibility (phenotypic plasticity) in the study populations in response to variable environmental factors (notably photoperiod and food availability). This may be related to their location in a geographic transition zone from uni- to bivoltine life cycles.

The Effect of Inflorescence Bud Removal on Tuber Production in Helianthus Tuberosus L. (Asteraceae)

I investigated allocation to sexual and asexual reproduction in the Jerusalem artichoke, Helianthus tuberosus L. (Asteraceae), a clonal pseudoannual that is an herbaceous ramet for one season, but propagates as a perennial through tubers that quickly dissociate from the parent plant. H. tuberosus ramets grown from tubers allocated 24% of their end—of—season biomass to tubers and 9% to sexual organs, with substantial variation attributable to four genetic lines cultured from eastern Iowa. Experimentally induced failure of sexual reproduction increased allocation to asexual reproduction under common garden conditions. Plants with inflorescence buds removed produced more (82 vs. 69) and larger (4.4 g vs. 3.8 g) tubers than those with unlimited sexual reproduction. Total biomass allocation to reproduction in plants receiving the bud—removal treatment was equivalent to combined allocation to sexual and asexual reproduction in plants that produced flower heads and achenes. This suggests a complete diversion of resources from sexual to asexual reproduction when sexual reproduction is thwarted. Aboveground plant size did not change with treatment. These results suggest a flexible life history that can respond to sexual failure resulting from low pollinator availability or damage to inflorescences from herbivory or weather, with enhanced facultative production of asexual tubers capable of sexual reproduction the following year. Hereditary variation in allocation leaves open the possibility of local adaptation.