Niche expansion and the niche variation hypothesis: does the degree of individual variation increase in depauperate assemblages?

The niche expansion and niche variation hypotheses predict that release from interspecific competition will promote niche expansion in depauperate assemblages. Niche expansion can occur by different mechanisms, including an increase in within-individual, among-individual, or bimodal variation (sexual dimorphism). Here we explore whether populations with larger niche breadth have a higher degree of diet variation. We also test whether populations from depauperate lizard assemblages differ in dietary resource use with respect to variation within and/or among individuals and sexual dimorphism. We found support for the niche expansion and niche variation hypotheses. Populations in assemblages with low phylogenetic diversity had a higher degree of individual variation, suggesting a tendency for niche expansion. We also found evidence suggesting that the mechanism causing niche expansion is an increase in variation among individuals rather than an increase in within-individual variation or an increase in bimodal variation due to sexual dimorphism.     

How does the magnitude of genetic variation affect ecological and reproductive character displacement?

While previous studies on character displacement tended to focus on trait divergence and convergence as a result of long-term evolution, recent studies suggest that character displacement can be a special case of evolutionary rescue, where rapid evolution prevents species extinction by weakening interspecific competition. Here we analyzed a simple model to examine how the magnitude of genetic variation affects evolutionary rescue via ecological and reproductive character displacement that weakens interspecific competition in exploitation of shared resources (i.e., resource competition) and in the mating process caused by incomplete species recognition (i.e., reproductive interference), respectively. We found that slow trait divergence due to small genetic variance results in species extinction in reproductive character displacement but not in ecological character displacement. This is because one species becomes rare in slow character displacement, and this causes deterministic extinction due to minority disadvantage of reproductive interference. On the other hand, there is no deterministic extinction in the process of ecological character displacement. Furthermore, species extinction becomes less likely in the case of positive covariance between ecological and reproductive traits as divergence of the ecological trait (e.g., root depths) increases the divergence speed of the reproductive trait (e.g., flower colors) and vice versa. It will be interesting to compare intraspecific genetic (co)variance of ecological and reproductive traits in future studies for understanding how ecological and reproductive character displacement occur without extinction.     

How does the presence of a conspecific individual change the behavioral game that a predator plays with its prey?

Behavioral games predators play among themselves may have profound effects on behavioral games predators play with their prey. We studied the behavioral game between predators and prey within the framework of social foraging among predators. We tested how conspecific interactions among predators (little egret) change the predator–prey behavioral game and foraging success. To do so, we examined foraging behavior of egrets alone and in pairs (male and female) in a specially designed aviary consisting of three equally spaced pools with identical initial prey (comet goldfish) densities. Each pool was comprised of a risky microhabitat, rich with food, and a safe microhabitat with no food, forcing the fish to trade off food and safety. When faced with two versus one egret, we found that fish significantly reduced activity in the risky habitat. Egrets in pairs suffered reduced foraging success (negative intraspecific density dependence) and responded to fish behavior and to their conspecific by changing their visiting regime at the different pools—having shorter, more frequent visits. The time egret spent on each visit allowed them to match their long-term capture success rate across the environment to their capture success rate in the pool, which satisfies one aspect of optimality. Overall, egrets in pairs allocated more time for foraging and changed their foraging tactics to focus more on fish under cover and fish ‘peeping’ out from their shelter. These results suggest that both prey and predator show behavioral flexibility and can adjust to changing conditions as needed in this foraging game.     

How does feedback from phage infections influence the evolution of phase variation in Campylobacter?

Campylobacter jejuni (C. jejuni) causes gastroenteritis following the consumption of contaminated poultry meat, resulting in a large health and economic burden worldwide. Phage therapy is a promising technique for eradicating C. jejuni from poultry flocks and chicken carcasses. However, C. jejuni can resist infections by some phages through stochastic, phase-variable ON/OFF switching of the phage receptors mediated by simple sequence repeats (SSR). While selection strength and exposure time influence the evolution of SSR-mediated phase variation (PV), phages offer a more complex evolutionary environment as phage replication depends on having a permissive host organism. Here, we build and explore several continuous culture bacteria-phage computational models, each analysing different phase-variable scenarios calibrated to the experimental SSR rates of C. jejuni loci and replication parameters for the F336 phage. We simulate the evolution of PV rates via the adaptive dynamics framework for varying levels of selective pressures that act on the phage-resistant state. Our results indicate that growth reducing counter-selection on a single PV locus results in the stable maintenance of the phage, while compensatory selection between bacterial states affects the evolutionary stable mutation rates (i.e. very high and very low mutation rates are evolutionarily disadvantageous), whereas, in the absence of either selective pressure the evolution of PV rates results in mutation rates below the basal values. Contrastingly, a biologically-relevant model with two phase-variable loci resulted in phage extinction and locking of the bacteria into a phage-resistant state suggesting that another counter-selective pressure is required, instance, the use of a distinct phage whose receptor is an F336-phage-resistant state. We conclude that a delicate balance between counter-selection and phage-attack can result in both the evolution of phase-variable phage receptors and persistence of PV-receptor-specific phage.     

How does variation in total and relative abundance contribute to gradients of species diversity?

Patterns of biodiversity provide insights into the processes that shape biological communities around the world. Variation in species diversity along biogeographical or ecological gradients, such as latitude or precipitation, can be attributed to variation in different components of biodiversity: changes in the total abundance (i.e., more‐individual effects) and changes in the regional species abundance distribution (SAD). Rarefaction curves can provide a tool to partition these sources of variation on diversity, but first must be converted to a common unit of measurement. Here, we partition species diversity gradients into components of the SAD and abundance using the effective number of species (ENS) transformation of the individual‐based rarefaction curve. Because the ENS curve is unconstrained by sample size, it can act as a standardized unit of measurement when comparing effect sizes among different components of biodiversity change. We illustrate the utility of the approach using two data sets spanning latitudinal diversity gradients in trees and marine reef fish and find contrasting results. Whereas the diversity gradient of fish was mostly associated with variation in abundance (86%), the tree diversity gradient was mostly associated with variation in the SAD (59%). These results suggest that local fish diversity may be limited by energy through the more‐individuals effect, while species pool effects are the larger determinant of tree diversity. We suggest that the framework of the ENS‐curve has the potential to quantify the underlying factors influencing most aspects of diversity change.     

How does variation in fetal loss affect the distribution of waiting times to conception?

Abstract

Unmeasured variation has long been a concern in analyses of the waiting time to conception. Recent work by Heckman and Walker (1991) and Trussell and Rodriguez (1990) has underscored the fact that statistical considerations alone cannot discriminate among likely models describing the distribution. Here, we specify a single theoretically important source of heterogeneity, namely variability in intrauterine mortality, and assess its effects on the waiting times to a conception which results in a live birth. We find that the effects of variation in fetal loss are confined to the tail of the distribution. Unless variation in fetal loss is extremely large or a substantial proportion of observed waiting times are initiated at late ages, variation in fetal loss does not appear to explain much variation in conception waits. We conclude that heterogeneity in fetal loss does not explain the variation in fecundability that has been observed for first birth intervals. This conclusion supports the hypothesis that at early ages (below age 35) variation in the waiting time to a fertile conception may largely reflect the proportion of nonsusceptible couples in the population. The analyses suggest that for the purposes of testing theoretically motivated models, future efforts should be directed toward examining reproductive experience after age 35 and toward incorporating information on characteristics of the fertile period as it becomes known.     

Can differences in phosphorus uptake kinetics explain the distribution of cattail and sawgrass in the Florida Everglades?

Background  

Cattail (Typha domingensis) has been spreading in phosphorus (P) enriched areas of the oligotrophic Florida Everglades at the expense of sawgrass (Cladium mariscus spp. jamaicense). Abundant evidence in the literature explains how the opportunistic features of Typha might lead to a complete dominance in P-enriched areas. Less clear is how Typha can grow and acquire P at extremely low P levels, which prevail in the unimpacted areas of the Everglades.     

How to prove the existence of metabolons?

Sequential enzymes in biosynthetic pathways are organized in metabolons. It is challenging to provide experimental evidence for the existence of metabolons as biosynthetic pathways are composed of highly dynamic protein–protein interactions. Many different methods are being applied, each with strengths and weaknesses. We will present and evaluate several techniques that have been applied in providing evidence for the orchestration of the biosynthetic pathways of cyanogenic glucosides and glucosinolates in metabolons. These evolutionarily related pathways have ER-localized cytochromes P450 that are proposed to function as anchoring site for assembly of the enzymes into metabolons. Additionally, we have included commonly used techniques, even though they have not been used (yet) on these two pathways. In the review, special attention will be given to less-exploited fluorescence-based methods such as FCS and FLIM. Ultimately, understanding the orchestration of biosynthetic pathways may contribute to successful engineering in heterologous hosts.     

How does the end begin? Formation and maintenance of telomeres in ciliates and yeast

Current models of telomere formation and replication involve either telomerase, a novel ribonucleoprotein, or recombination between the ends of DNA molecules. However, present models will have to be modified to explain recent data on telomere formation in yeast. An understanding of the mechanisms of telomere maintenance in yeast may reveal how other organisms with heterogeneous telomeric repeats replicate their chromosomal termini.     

Does the spatial variation in selective pressures explain among-site differences in seed mass? A test with Buxus balearica

Positive effects of seed size on germination and survival can be offset by a greater probability of predation or a poorer dispersal of larger seeds. We hypothesized that spatial variation in local selective pressures acting on seed mass may lead to differences in both optimal and observed seed mass among discrete populations. We first examined the variation in seed mass across a total of 14 populations of Buxus balearica; in six of such populations, we further examined the temporal variation by measuring seed mass during three consecutive years. Second, we evaluated seed mass effects on different recruitment phases (seed rain to seedling establishment) in three populations. Lastly, for these three populations, we estimated the resulting overall phenotypic selection on seed mass during recruitment and compared the observed and the predicted optimal seed masses. Most variation (c. 70%) in seed mass occurred among populations, and although we found inter-annual variation in seed mass, the differences in seed mass among populations were consistent over time. Conflicting selective pressures on seed mass appeared during recruitment, and their direction and strength varied among populations, depending on the relative local importance of seed predation vs. germination and establishment. Observed seed mass matched predicted optimal seed mass in two of the three examined populations, suggesting local adaptative responses to the spatial mosaic of selective pressures.     

How does temporal variation in habitat connectivity influence metapopulation dynamics?

Metapopulation persistence depends on connectivity between habitat patches. While emphasis has been placed on the spatial dynamics of connectivity, much less has been placed on its short‐term temporal dynamics. In many terrestrial and aquatic ecosystems, however, transient (short‐term) changes in connectivity occur as habitat patches are connected and disconnected due, for example, to climatic or hydrological variability. We evaluated the implications of transient connectivity using a network‐based metapopulation model and a series of scenarios representing temporal changes in connectivity. The transient loss of connectivity can influence metapopulation persistence, and more strongly autocorrelated temporal dynamics affect metapopulation persistence more severely. Given that many ecosystems experience short‐term and temporary loss of habitat connectivity, it is important that these dynamics are adequately represented in metapopulation models; failing to do so may yield overly optimistic‐estimates of metapopulation persistence in fragmented landscapes.     

How does sampling strategy influence detection of morphological variation in orchids? A case-study of the epiphyte Oberonia hosseusii

Effects of sampling strategy on detection of morphological variation were studied in the epiphytic orchid Oberonia hosseusii in Thailand. We compared the different effects of local versus regional sampling on the one hand and the size of local samples on the other. A local data set was collected by sampling a natural population on Doi Inthanon, during which process a preservation experiment was made. A regional data set was established by measurement of 18 preserved specimens from 12 different localities. Based on the preservation experiment, data from the regional sampling were calibrated (whenever needed) to make the two data sets more directly comparable. Heat drying and spirit preservation significantly influenced measurements for two mutually exclusive sets of characters. Increasing the local sample size from 18 to 52 individuals had no statistically significant effect on the mean value of any character. In contrast, local versus regional sampling significantly influenced the mean value of every character, virtually irrespective of local sample size. In the regional data set only, a difference was found in overall variability in vegetative versus floral characters. Three cluster analyses with equal representation of individuals from the local and regional samples were performed using different clustering techniques. Only in the analysis using neighbour-joining did pure “local clusters” prove to be significantly larger than pure “regional clusters”, suggesting that this clustering technique is particularly sensitive to effects from sampling strategy.     

Can color vision variation explain sex differences in invertebrate foraging by capuchin monkeys?

Invertebrates are the main source of protein for many small-to-medium sized monkeys. Prey vary in size, mobility, degree of protective covering, and use of the forest, I.e. Canopy height, and whether they are exposed or embed themselves in substrates. Sex-differentiation in foraging patterns is well documented for some monkey species and recent studies find that color vision phenotype can also affect invertebrate foraging. Since vision phenotype is polymorphic and sex-linked in most New World monkeys - males have dichromatic vision and females have either dichromatic or trichromatic vision - this raises the possibility that sex differences are linked to visual ecology. We tested predicted sex differences for invertebrate foraging in white-faced capuchins Cebus capucinus and conducted 12 months of study on four free-ranging groups between January 2007 and September 2008. We found both sex and color vision effects. Sex: Males spent more time foraging for invertebrates on the ground. Females spent more time consuming embedded, colonial invertebrates, ate relatively more "soft" sedentary invertebrates, and devoted more of their activity budget to invertebrate foraging. Color Vision: Dichromatic monkeys had a higher capture efficiency of ex posed invertebrates and spent less time visually foraging. Trichromats ate relatively more "hard" sedentary invertebrates. We con clude that some variation in invertebrate foraging reflects differences between the sexes that may be due to disparities in size, strength, reproductive demands or niche preferences. However, other intraspecific variation in invertebrate foraging that might be mistakenly attributed to sex differences actually reflects differences in color vision.     

Does intraspecific variation in social systems explain reported differences in the social structure of the proboscis monkey (Nasalis larvatus)?

The social structure of proboscis monkeys (Nasalis larvatus) was investigated in the Brunei Bay area of Brunei Darussalam (Northern Borneo). Intraspecific variability in social systems had been suggested to occur in this species, with social structure varying with habitat differences. Prior reports for this area suggested that groups contained several males, females, and offspring and were unstable. Contrary to these reports, I found that the monkeys were organized into groups containing one male, several females, and offspring. These groups associated at sleeping sites at the river’s edge, thus forming a secondary level of organization. This is the same social system reported in Sarawak, Malaysia and Kalimantan, Indonesia in different habitat types. It appears that methodological differences may account for the discrepancies between reports. I have found no evidence to suggest that intraspecific variation in social systems occurs in this species.     

Do reproductive characteristics of the species explain differences in the investment in weapon size present in males?

Sexually selected traits, such as male weapons, are highly variable in shape and size across species. However, little is known about the mechanisms that may govern this variation. Because males with greater investment in weapon size tend to win more fights, but also pay higher costs related to weapon development and maintenance, larger weapons should be expected only in species in which victory in male–male fights generate reproductive benefits that outweigh investment costs. Here, we hypothesized that the reproductive characteristics that increase the chances of winners to access females or to fertilize eggs will favor the evolution of larger weapons. To evaluate this hypothesis, we conducted a meta-analysis using arthropods as model organisms. To measure investment in weapon size, we gathered both Pearson correlation coefficient and the standardized (but non centralized) slope values for the relationship between weapon size and body size. We found that none of the reproductive characteristics we investigated was related to male weapon size. Thus, it seems that greater certainty of accessing a female or fertilizing female eggs with a victory does not modulate the investment in male weapon size. Perhaps the cost–benefit ratio between weapon size investment and reproductive success is not the main factor driving the variation in weapon size.