Life-history variation in a hybrid species complex ofDaphnia

Summary Life-history variation was examined among members of theDaphnia longispina group, which consists ofD. galeata, D. hyalina, D. cucullata, and hybrids. Factorial experiments were conducted at two temperatures (14° and 20° C) and two food concentrations (0.2 and 1.0 mg Cl⁻¹). Differences in life-history features (size at maturity, age at first reproduction, size of first clutch, offspring size in first clutch) under the different environmental conditions were assessed among eightDaphnia clones, which represented members of this species complex. Significant differences between parentals and hybrids for most life-history features were observed under various treatments; generally, hybrid clones showed intermediate life-history traits when compared with parentals. When comparisons were made among clones within a given species (i.e.D. galeata, D. galeata xcucullata, D. cucullata), clonal differences were also noted for certain life-history traits. The data are discussed with reference to the formation and maintenance of hybrid species complexes in nature. This paper is dedicated to the memory of Hans Georg Wolf, who died suddenly in May 1990 at the age of 39 years.     

Life-history variation in relation to time constraints in a damselfly

Although variation within populations in plasticity to time constraints is expected with regard to hatching date, empirical studies are largely lacking. We studied life-history responses to time constraints manipulated by photoperiod and associated with hatching date in larvae of the damselfly Lestes viridis for two populations with a different hydroperiod. In a common garden experiment, early- and late-hatched larvae from both populations were reared at two photoperiods mimicking the start and the end of the egg-hatching season. In a reciprocal transplant experiment, early- and late-hatched larvae from both populations were reared in both ponds. In all these experiments, larvae were reared from egg hatching until adult emergence. Within both populations, larvae reared at the photoperiod indicating a late time point in the growing season, reduced development time to compensate for their perceived shorter development period. Growth rate, however, did not respond to photoperiod, resulting in a lower mass at emergence. As expected, both in the laboratory and in the field, larvae from eggs that hatched later in the season generally had a shorter development time and a faster growth rate, resulting in a higher mass at emergence compared to early-hatched larvae. This may explain the intriguing seasonal increase in mass at emergence in this species, and affect the predictions of optimality models. None of these life-history responses differed between the two populations, despite clear differences in time constraints linked to hydroperiod, suggesting the robustness of the observed patterns. Given the ubiquity of asynchronous hatching in nature, and the adaptive value of the observed differences between early- and late-hatched larvae, we expect the effects of hatching date on life-history plasticity to be widespread.     

Life-history variation within a three-spined stickleback population in the Camargue

Among individuals of female three-spined sticklebacks Gasterosteus aculeatus from a population in the Camargue, southern France, studied in 12 successive years, adult L _T ranged from 31–64 mm, clutch size ranged from 33–660 eggs, and mean egg diameter per clutch ranged from 1.15–1.67 mm. Because the population was strictly annual, inter-annual variation corresponded to variation among generations having experienced different environmental conditions. Body mass varied significantly among years, suggesting an effect of varying environmental conditions. Gonad mass and clutch size increased with body mass, but mean egg diameter was not correlated to body mass. Body mass-adjusted gonad mass, interpreted as reproductive effort per clutch, did not vary significantly among years, suggesting that this trait was not influenced by environmental conditions. Body mass-adjusted clutch size and egg size varied significantly among years. Inter-annual variation in body length at breeding, clutch size and egg size was of the same order of magnitude as inter-population variation reported by other authors for this species. During the breeding season, reproductive effort and clutch size tended to increase. Egg size tended to decrease during the breeding season but this seasonal pattern varied among years. Observed life-history variation is discussed both in terms of its evolutionary significance and methodological implications in the study of life-history variation among populations.     

Aspects of variation in a neotropical seed dispersal system

This paper explores the causes and consequences of seed and seedling mortality of the tree Virola nobilis (Myristicaceae) Central Panama in order to understand the advantage to local seed dispersal by birds and monkeys. Post-dispersal mortality due to insects (primarily Conotrachelus spp., Curculionidae) accounts for 30–35% of seed and seedling death during the first 12 weeks after seed fall. Because more seeds and seedlings are killed under and near fruiting trees than 15–45 m away, seed dispersal confers a 20–40 fold advantage on seeds carried 45 m from fruiting adults. In contrast, >60% of seed and seedling death during the first year is due to seed predation by mammals, with >90% due to mammals among the <2% seeds that survive until maternal endosperm is exhausted ±12 weeks after seed fall. Mortaliy due to mammals is independent of distance from parent trees, confering no advantage to seed dispersal. Insects account for variation in mortality attributable to distance effects, mammals to between site effects.Early weevil infestations put a premium on seed removal by large birds (Ramphastos swainsonii, R. sulfuratus, Penelope purpurascens), which carry >50% of the seeds that they eat >40 m, as compared with smaller birds (Baryphthengus martii, Tityra semifasciata, Trogon massena) and monkeys (Ateles geoffroyi), which leave most or all of the seeds that they eat under or near the tree crown.     

Life-history variation within a taxon: a comparative analysis of birds and mammals

Most studies on life-history evolution discuss the necessity of distinguishing between extrinsic and intrinsic sources of variability in life-history traits. I use log/log plots of yearly neonate production in daugters (b) versus adult mortality (Ma) for 75 bird species and 88 mammal species to compare graphically life-history "fields" arranged by these selective forces along a "slow-fast continuum". Under the assumptions of steady-state and linear relationship between adult mortality and reproductive effort, as well as between juvenile survival and relative neonate weight, it is possible to place additional axes in the two-dimentional plot, and to predict covariations among demographic and individual growth traits. The functional regression analysis shows, that the assumptions are completely fulfilled, at least for birds, but mammals show nonlinear relationship between adult mortality and reproductive effort. This can be explained by peculiarities of metabolism and parental care in small mammals with high reproductive output. Hence, for birds the axis of relative neonate weight approximately coincides in direction with the juvenile survivorship axis, but this is not a case for mammals. In both taxa, the relative neonate weight is an invariant in relation to fecundity and adult mortality (but not in relation to adult body weight). This important feature, together with other intrinsic (energetic and phylogenetic) constraints, explains well-documented close covariations among traits, even when the effect of body size is factored out. It is argued that life-history and body size variations in birds and mammals mainly depend on a pattern of temporal resource deficiency, although this impact cannot be separated from that of extrinsic juvenile mortality.     

Multiple sources of isotopic variation in a terrestrial arthropod community: challenges for disentangling food webs

Documenting trophic links in a food web has traditionally required complex exclusion experiments coupled with extraordinarily labor-intensive direct observations of predator foraging. Newer techniques such as stable isotope analysis (SIA) may facilitate relatively quick and accurate assessments of consumer feeding behavior. Ratios of N and C isotopes are thought to be useful for determining species' trophic position (e.g., 1 degrees consumer, 2 degrees consumer, or omnivore) and their original carbon source (e.g., C3 or C4 plants; terrestrial or marine nutrients). Thus far, however, applications of stable isotopes to terrestrial arthropod food webs have suggested that high taxon-specific variation may undermine the effectiveness of this method. We applied stable isotope analysis to a pear orchard food web, in which biological control of a dominant pest, pear psylla (Cacopsylla pyricola), involves primarily generalist arthropod predators with a high frequency of omnivory. We found multiple sources of isotopic variation in this food web, including differences among plant tissues; time, stage, and taxon-specific differences among herbivores (despite similar feeding modes); and high taxon-specific variation among predators (with no clear evidence of omnivory). Collectively, these multiple sources of isotopic variation blur our view of the structure of this food web. Idiosyncrasies in consumer trophic shifts make ad hoc application of SIA to even moderately complex food webs intractable. SIA may not be a generally applicable "quick and dirty" method for delineating terrestrial food web structure-not without calibration of specific consumer food trophic shifts.     

Seasonal variation in the sex allocation of a neotropical solitary bee

We carried out a field study on the life history and sex allocationof the ground-nesting solitary bee Diadasina distincta (Hymenoptera: Anthophoridae).This species is multivoltine, undergoing five generations a yearbetween February and September. The numerical sex ratio of thisspecies was female biased overall (approximately 38% males)and showed a strong and consistent seasonal pattern. The numericalsex ratio was extremely female biased (approximately 20% males)from February until May, and then slightly male biased (approximately60% males) from June until September. Females were 3.26 timesthe size of males, and so the overall investment ratio was female biasedthroughout the year. The overall female bias and seasonal variationin sex allocation is unlikely to be explained by models thatinvoke overlapping generations or competition between brothersfor mates (local mate competition). We suggest that a possibleexplanation for the female bias in the early part of the seasonis local resource enhancement (LRE): nesting near larger numbersof sisters reduces parasitism. LRE is likely to decrease in importancein the later part of the season, when the biased numerical and investmentratios may be explained by models in which male and female offspringgain different fitness returns from resources invested.     

Life-history variation in a seed beetle: adult egg-laying vs. larval competitive ability

Summary Populations of the seed beetle Callosobruchus maculatus differ genetically in several traits that mediate intraspecific competition. This study examined competitive interactions between larvae from two strains that differed in their propensity to oviposit on occupied hosts. In a strain (S) where females avoided laying >1 egg/seed, larvae were highly competitive; if two larvae entered a small host simultaneously, only one adult emerged. In a strain (I) whose females were sloppier in their egg-laying decisions, more than half of the seeds bearing two larvae yielded two adults. If seeds contained one larva from each strain, only one adult emerged per seed, and 70% of these adults belonged to the more competitive S strain. A larva's probability of emergence could be increased if it entered the seed before its competitor. A two-day headstart was needed merely for I larvae to compete equally with S larvae. Competition also affected development time and adult weight, but its effect was highly strain- and sex-specific. Adult life-history traits also differed substantially between strains. Compared to I beetles, S beetles exhibited decreased longevity, lower fecundity, a truncated period of oviposition, and larger egg and body size. Fecundity was linearly related to body size in the I strain, but was largely independent of size in the S strain. When faced with a shortage of hosts, S females (whose progeny are highly competitive) withheld eggs and died without depositing 40% of their lifetime supply. In contrast, the fecundity of I females was independent of seed availability. Comparisons with previous studies suggest that both highly uniform egg-laying and strong interference among larvae may be a consequence of chronic association with a relatively small host. Results from the larval competition experiments were used as parameter estimates for a simple, game-theoretic model that postulates interference vs. exploitative strategies. Fitness comparisons suggest that a strategy employing interference competition cannot be invaded by a more exploitative form of competition in a small host.     

Geographic variation in a widespread neotropical species,Mouriri myrtilloides (Melastomataceae)

Geographic variation inMouriri myrtilloides (Swartz) Poiret is analyzed on a morphological basis. Plants from Central America have medium puberulence, long calyx lobes, and short petioles and pedicels; those from Jamaica and Haiti are glabrous and have short calyx lobes, petioles, and pedicels; while those from Cuba have maximum puberulence, medium calyx lobes and pedicels, and long petioles. Glabrous or near-glabrous plants also appear rarely and more or less at random in Central and South America. Distribution in South America is limited to the northwest quarter of the continent. Most plants of this region are similar to those of Central America except that they have slightly shorter calyx lobes. However, the very short calyx lobes of plants from Jamaica and Haiti often occur in South America and rarely in Panama. Furthermore, most features of the Cuban plants appear in those of the Orinoco Basin, while individuals with one of these features, strong puberulence, are occasional elsewhere in South America. It is suggested that the complex originated in northwest South America and diversified there, subsequently radiating north and west. Considerations and problems related to such a dispersal are discussed. Taxonomically the complex is treated as consisting of four subspecies: ssp.parvifolia of Central America and much of the north-west quarter of South America, ssp.myrtilloides of Jamaica and southwest Haiti, ssp.acuta of Cuba, and ssp.orinocensis of the Orinoco Basin, the latter being a new taxon.     

Seed-size variation determines interspecific differential predation by mammals in a neotropical rain forest

It has been suggested that the anthropogenically driven loss of herbivorous mammals might lead to changes in the recruitment patterns of tropical rain forest plants, but few studies have examined the explicit mechanisms that might account for this effect. Here we propose a conceptual model linking differential mammalian defaunation and differential plant recruitment. We posit that in the absence of medium/large herbivores but with small rodent granivores still present (i.e. differential defaunation), predation pressure will be greater on small-seeded species than on large-seeded species. We tested such differential predation hypothesis (DPH) by means of a series of experiments directed to assess: 1) seed consumption by small rodents in laboratory cages; 2) seed-removal rates in small rodent enclosures in the field and 3) removal of seeds placed on the forest floor and exposed to either the full complement of mammals or only small rodents. Seeds used in the experiments were arranged in pairs consisting of species from the same taxonomic family but with a contrasting size (large, small). We found: 1) a significantly greater consumption of smaller seeds (a 2.3- to 20.5-fold difference) in cages and 2) a significantly greater removal of small-seeded species (a 3.7- to 65-fold difference) in field enclosures. Results of seed removal experiments in free-access plots and selective exclosures were more complex, with a general absence of significant differences among treatments but we found that predation was in general concentrated on small-seeded species and small rodents were the predominant visitors to the plots. This, together with the overall short distance of dispersal among large seeds suggest that in differentially defaunated forests large-seeded species are more likely to escape predation. We posit that such size related differential predation may lead to the floristic impoverishment observed in some defaunated forests.     

Leaf trait integration mediates species richness variation in a species-rich neotropical forest domain

Environmental gradients are known to drive changes in mean trait values, but changes in the trait integration strength across local communities are less well understood, particularly with regard to possible links with species richness variation. Here, we tested if climate, soil, and topography gradients drive species richness indirectly via constraints on trait integration in the Atlantic Forest of South America. We evaluated seven traits (from leaf, wood, seed, and plant size) of 1456 species occurring across 84 local communities. Generalized least square models and a path model were applied to test direct and indirect relationships. Correlations were higher between leaf traits (average r?=?0.28) and lower when other traits were included (average r?=?0.16). In line with this result, species richness was related to a multivariate index of interspecific trait integration (ITI) computed for leaf traits, but not to the ITI for all the seven traits. Abiotic gradients influenced species richness both directly and indirectly through the leaf trait integration. A total of 33% and 26% of the variation in species richness and ITI, respectively, were explained by the models, with climatic conditions showing higher contribution than topographic and edaphic factors. These results support a significant but reduced environmental selection role behind the trait-based community assembly and may suggest that other processes are involved in the constrain of trait integration at larger spatial scales. In addition, different directional trends in trait–trait relationships across local communities suggest that global trait relationships may not necessarily hold at local contexts.

     

Visual and nutritional food cues fine-tune timing of reproduction in a neotropical rainforest bird

Food may act as a proximate factor in the regulation of avian seasonal breeding. Food cues could provide particularly important seasonal information to birds living in variable tropical environments, but this has not yet been tested. Spotted antbirds (Hylophylax n. naevioides) inhabiting a humid forest in central Panama (9 degrees N) likely use changes in the tropical photoperiod to time reproduction on a long-term, seasonal basis. We predicted that these insectivorous birds also adjust reproduction to short-term cues such as food availability because the onset of the rainy season and the resulting increase in insect abundance varies considerably between years. To test this prediction, prior to their breeding season (when they had half-maximal gonads), we either exposed captive male spotted antbirds to an ad libitum standard diet only or added live crickets to this diet. Males that received live crickets significantly increased gonad sizes within 3 weeks over controls on the standard diet. Moreover, in six additional experiments cricket availability always increased song rate, usually within a few days. The stimulatory effect of live crickets on song activity may function independent of nutritional aspects: Freshly killed crickets, providing similar nutritional content as live crickets, did not stimulate the birds' song activity. However, song activity increased to intermediate levels when live crickets were shown under a clear plastic wrap, i.e., when birds could see but not eat crickets. We hypothesize that the opportunity to see and handle live insects stimulates song and reproductive activity in these birds. Our data indicate for the first time that a tropical rainforest bird can use food cues to evaluate the suitability of local environmental conditions for breeding. J. Exp. Zool. 286:494-504, 2000.     

Interbirth interval variation in three sympatric species of neotropical monkey

Relatively few papers have focused on interbirth intervals in primates, even though the spacing between births is one of the primary determinants of female reproductive success in long-lived mammals. We present life history data from a ten-year field study of Costa Rican capuchins (Cebus capucinus), howlers (Alouatta palliata), and spidei monkeys (Ateles geoffroyi). Analyses of intraspecific variability found no significant differences attributable to individual variation in age, parity, weight, or maternal rank. Loss of an infant significantly shortened the interbirth interval in all three species. There was no correlation between annual rainfall and birth rates, but there was a significant clustering of births in the dry season. Survival analyses demonstrated a significant difference between the median interbirth intervals of the three species. Howlers have the shortest intervals (19.9 months), capuchins exhibit longer intervals (26.36 months), and spider monkeys have the longest intervals (34.72 months;. This comparative pattern does not correspond to relative body weights of the three species, but does correspond to relative brain weights. Comparisons to other primates with similar life history characteristics demonstrate that interbirth intervals are best examined at the level of their three component phases: gestation, lactation, and cycling to re-conception. © 1995 Wiley-Liss, Inc.     

Condition and size of damselflies: a field study of food limitation

Summary Based on evidence from field manipulations, several authors have recently suggested that interference competition among larval odonates reduces individual growth rates and biomass by reducing foraging rates. This study was designed to test the effects of food shortage on condition (relative mass per unit head width) of larval Ischnura verticalis (Odonata: Coenagrionidae) under laboratory conditions and to use these results to estimate the degree of food shortage of larvae under naturally occurring field conditions. In the laboratory, there were marked differences in condition of larvae fed diets ranging from ad libitum feeding with worms to ad libitum feeding with Daphnia 1 day out of every 8. Condition of larvae collected from May through October from 17 different sites in southern Ontario indicated that, for most of the year, larvae had conditions similar to those fed ad libitum with Daphnia in the laboratory. There was no evidence that larval condition was related to population density. Condition of larvae in most sites during July was similar to that of larvae fed poor diets in the laboratory. It is unlikely that the low conditions were due to competition as there were no correlations with density across sites and population densities during July were at their lowest. Adult head widths showed a seasonal decline from mid June to the end of the flight season. There was no evidence that head widths were related to population density although there was some evidence that head widths of males were positively related to larval condition. My results do not support the hypothesis that competition is important in affecting foraging rates and subsequent development of larvae. Contrasts between my results and other studies may stem from difficulties with the interpretation of field experiments, that densities in my study may have been low due to fish predation, and/or that I. verticalis larvae are slow moving relative to other larvae and thus less likely to interact.     

Fluctuations in prey availability and food limitation for a terrestrial salamander

Summary A Virginia population of the forest-dwelling salamander Plethodon cinereus was repeatedly sampled over four years to determine volume and number of prey ingested under varying conditions of moisture and temperature. The proportion of the population on a daily negative energy budget was estimated by comparing actual foraging success with laboratory determined values of energetic requirements at various temperatures.Prey became limited in availability during rainless periods, apparently because salamanders were not able to forage in dry leaf litter. Foraging success increased with increasing rainfall. Food was a limiting resource for a majority of the population on most sampling days, as determined by energy budget analysis. While ambient moisture regulated food availability, ambient temperature set the metabolic requirements and assimilation efficiencies for the population. Consequently, food was more limiting on dry, warm days and less so on wet, cool days. These data support the hypothesis that intraspecific competition frequently occurs for a food resource that is periodically limited in availability.