A community‐level test of the leaf‐height‐seed ecology strategy scheme in relation to grazing conditions

Question: Is the assumption of trait independence implied in Westoby's (1998) leaf‐height‐seed (LHS) ecology strategy scheme upheld in a Mediterranean grazing system dominated by annuals? Is the LHS approach applicable at the community level? Location: Northern Israel. Methods: LHS traits (specific leaf area [SLA], plant height and seed mass), and additional leaf traits (leaf dry matter content [LDMC], leaf area, and leaf content of nitrogen [LNC], carbon [LCC], and phosphorus [LPC]), were analyzed at the species and community levels. Treatments included manipulations of grazing intensity (moderate and heavy) and protection from grazing. We focused on species comprising 80% of biomass over all treatments, assuming that these species drive trait relationships and ecosystem processes. Results: At the species level, SLA and seed mass were negatively correlated, and plant height was positively correlated to LCC. SLA, seed mass, and LPC increased with protection from grazing. At the community level, redundancy analysis revealed one principal gradient of variation: SLA, correlated to grazing, versus seed mass and plant height, associated with protection from grazing. We divided community functional parameters into two groups according to grazing response: (1) plant height, seed mass, LDMC, and LCC, associated with protection from grazing, and (2) SLA, associated with grazing. Conclusions: The assumption of independence between LHS traits was not upheld at the species level in this Mediterranean grazing system. At the community level, the LHS approach captured most of the variation associated with protection from grazing, reflecting changes in dominance within the plant community.     

New aspects of grassland recovery in old‐fields revealed by trait‐based analyses of perennial‐crop‐mediated succession

Classical old‐field succession studies focused on vegetation changes after the abandonment of annual croplands or on succession after the elimination of cultivated crops. Perennial‐crop‐mediated succession, where fields are initially covered by perennial crops, reveals alternative aspects of old‐field succession theories. We tested the validity of classical theories of old‐field succession for perennial‐crop‐mediated succession. We formulated the following hypotheses: (1) functional diversity increases with increasing field age; (2) resource acquisition versus conservation trade‐off shifts toward conservation at community level during the succession; (3) the importance of spatial and temporal seed dispersal decreases during the succession; and (4) competitiveness and stress‐tolerance increases and ruderality decreases at community level during the succession. We studied functional diversity, trait distributions and plant strategies in differently aged old‐fields using chronosequence method. We found increasing functional richness and functional divergence, but also unchanged or decreasing functional evenness. We detected a shift from resource acquisition to resource conservation strategy of communities during the succession. The role of spatial and temporal seed dispersal was found to be important not only at the initial but also at latter successional stages. We found an increasing stress‐tolerance and a decreasing ruderality during succession, while the competitiveness remained unchanged at the community level. Despite the markedly different starting conditions, we found that classical and perennial‐crop‐mediated old‐field successions have some similarities regarding the changes of functional diversity, resource acquisition versus conservation trade‐off, and seed dispersal strategies. However, we revealed also the subsequent differences. The competitive character of communities remained stable during the succession; hence, the initial stages of perennial‐crop‐mediated succession can be similar to the middle stages of classical old‐field succession. Moreover, the occupied functional niche space and differentiation were larger in the older stages, but resources were not effectively utilized within this space, suggesting that the stabilization of the vegetation requires more time.     

Species‐time‐area and phylogenetic‐time‐area relationships in tropical tree communities

The species‐area relationship (SAR) has proven to be one of the few strong generalities in ecology. The temporal analog of the SAR, the species‐time relationship (STR), has received considerably less attention. Recent work primarily from the temperate zone has aimed to merge the SAR and the STR into a synthetic and unified species‐time‐area relationship (STAR) as originally envisioned by Preston (1960). Here we test this framework using two tropical tree communities and extend it by deriving a phylogenetic‐time‐area relationship (PTAR). The work finds some support for Preston's prediction that diversity‐time relationships, both species and phylogenetic, are sensitive to the spatial scale of the sampling. Contrary to the Preston's predictions we find a decoupling of diversity‐area and diversity‐time relationships in both forests as the time period used to quantify the diversity‐area relationship changes. In particular, diversity‐area and diversity‐time relationships are positively correlated using the initial census to quantify the diversity‐area relationship, but weakly or even negatively correlated when using the most recent census. Thus, diversity‐area relationships could forecast the temporal accumulation of biodiversity of the forests, but they failed to “back‐cast” the temporal accumulation of biodiversity suggesting a decoupling of space and time.     

Asymptotically robust variance estimation for person‐time incidence rates

Person‐time incidence rates are frequently used in medical research. However, standard estimation theory for this measure of event occurrence is based on the assumption of independent and identically distributed (iid) exponential event times, which implies that the hazard function remains constant over time. Under this assumption and assuming independent censoring, observed person‐time incidence rate is the maximum‐likelihood estimator of the constant hazard, and asymptotic variance of the log rate can be estimated consistently by the inverse of the number of events. However, in many practical applications, the assumption of constant hazard is not very plausible. In the present paper, an average rate parameter is defined as the ratio of expected event count to the expected total time at risk. This rate parameter is equal to the hazard function under constant hazard. For inference about the average rate parameter, an asymptotically robust variance estimator of the log rate is proposed. Given some very general conditions, the robust variance estimator is consistent under arbitrary iid event times, and is also consistent or asymptotically conservative when event times are independent but nonidentically distributed. In contrast, the standard maximum‐likelihood estimator may become anticonservative under nonconstant hazard, producing confidence intervals with less‐than‐nominal asymptotic coverage. These results are derived analytically and illustrated with simulations. The two estimators are also compared in five datasets from oncology studies.     

A trait‐based trade‐off between growth and mortality: evidence from 15 tropical tree species using size‐specific relative growth rates

A life‐history trade‐off between low mortality in the dark and rapid growth in the light is one of the most widely accepted mechanisms underlying plant ecological strategies in tropical forests. Differences in plant functional traits are thought to underlie these distinct ecological strategies; however, very few studies have shown relationships between functional traits and demographic rates within a functional group. We present 8 years of growth and mortality data from saplings of 15 species of Dipterocarpaceae planted into logged‐over forest in Malaysian Borneo, and the relationships between these demographic rates and four key functional traits: wood density, specific leaf area (SLA), seed mass, and leaf C:N ratio. Species‐specific differences in growth rates were separated from seedling size effects by fitting nonlinear mixed‐effects models, to repeated measurements taken on individuals at multiple time points. Mortality data were analyzed using binary logistic regressions in a mixed‐effects models framework. Growth increased and mortality decreased with increasing light availability. Species differed in both their growth and mortality rates, yet there was little evidence for a statistical interaction between species and light for either response. There was a positive relationship between growth rate and the predicted probability of mortality regardless of light environment, suggesting that this relationship may be driven by a general trade‐off between traits that maximize growth and traits that minimize mortality, rather than through differential species responses to light. Our results indicate that wood density is an important trait that indicates both the ability of species to grow and resistance to mortality, but no other trait was correlated with either growth or mortality. Therefore, the growth mortality trade‐off among species of dipterocarp appears to be general in being independent of species crossovers in performance in different light environments.     

Trade‐offs between life‐history traits at range‐edge and central locations

The allocation of resources to different life‐history traits should represent the best compromise in fitness investment for organisms in their local environment. When resources are limiting, the investment in a specific trait must carry a cost that is expressed in trade‐offs with other traits. In this study, the relative investment in the fitness‐related traits, growth, reproduction and defence were compared at central and range‐edge locations, using the seaweed Ascophyllum nodosum as a model system. Individual growth rates were similar at both sites, whereas edge populations showed a higher relative investment in reproduction (demonstrated by a higher reproductive allocation and extended reproductive periods) when compared to central populations that invested more in defence. These results show the capability of A. nodosum to differentially allocate resources for different traits under different habitat conditions, suggesting that reproduction and defence have different fitness values under the specific living conditions experienced at edge and central locations. However, ongoing climate change may threaten edge populations by increasing the selective pressure on specific traits, forcing these populations to lower the investment in other traits that are also potentially important for population fitness.     

Species‐specific levels of ant attendance mediate performance costs in a facultative myrmecophilous butterfly

Trophobiont butterfly larvae offer caloric rewards to ants through specialised glands and, in return, gain ant‐derived protection from natural enemies. Thus, from the larva's perspective, the major cost of myrmecophily comprises the reward production. Larvae of the butterfly Parrhasius polibetes (Stoll) (Lycaenidae) are facultatively tended by several ant species, which might differ in the intensity of tending behaviour. The performance costs (development time, survival, pupal mass and adult dry mass) of P. polibetes are examined when tended by two ant species differing in size and foraging strategies (Camponotus melanoticus Emery and Camponotus crassus Mayr), along with the corresponding intensity of tending behaviour towards late instars. Untended larvae serve as controls. Larvae tended by C. melanoticus take longer to pupate compared with both C. crassus and control larvae. By contrast, pupae whose larvae are tended by C. crassus are lighter than control larvae but do not differ from those tended by C. melanoticus. No differences are found in the adult stage, indicating compensation in all cases. Both at short‐ and long‐term scales, C. melanoticus tends larvae of P. polibetes more intensely than C. crassus. The increase in tending activity of C. melanoticus presumably delays the development time of larvae tended by this ant species. The results of the present study show that tending intensity varies depending on the ant species, and that P. polibetes has compensatory mechanisms to minimise myrmecophily costs, regardless of tending intensity. To the authors' knowledge, this is the first experimental evidence that intensity of ant‐tending behaviour is species‐specific and affects performance in a trophobiont insect.     

Species‐specific responses to combined water stress and increasing temperatures in two bee‐pollinated congeners (Echium,Boraginaceae)

1. Water stress and increasing temperatures are two main constraints faced by plants in the context of climate change. These constraints affect plant physiology and morphology, including phenology, floral traits, and nectar rewards, thus altering plant–pollinator interactions.
2. We compared the abiotic stress responses of two bee‐pollinated Boraginaceae species, Echium plantagineum, an annual, and Echium vulgare, a biennial. Plants were grown for 5 weeks during their flowering period under two watering regimes (well‐watered and water‐stressed) and three temperature regimes (21, 24, 27°C).
3. We measured physiological traits linked to photosynthesis (chlorophyll content, stomatal conductance, and water use efficiency), and vegetative (leaf number and growth rate) and floral (e.g., flower number, phenology, floral morphology, and nectar production) traits.
4. The physiological and morphological traits of both species were affected by the water and temperature stresses, although the effects were greater for the annual species. Both stresses negatively affected floral traits, accelerating flower phenology, decreasing flower size, and, for the annual species, decreasing nectar rewards. In both species, the number of flowers was reduced by 22%–45% under water stress, limiting the total amount of floral rewards.
5. Under water stress and increasing temperatures, which mimic the effects of climate change, floral traits and resources of bee‐pollinated species are affected and can lead to disruptions of pollination and reproductive success.

     

Leaf palatability and decomposability increase during a 200‐year‐old post‐cultural woody succession in New Zealand

Question: How do aggregate trait values and functional diversity of leaf traits linked to palatability and decomposability change during a woody post‐cultural succession spanning 200 years? Location: Coastal Marlborough, South Island, New Zealand. Methods: The biomass of all woody species was determined in 32 20‐m × 20‐m plots ranging from 10 to 200 years in time since last disturbance. Species abundances were combined with data on leaf nutrient, secondary metabolite and structural carbohydrate content to calculate biomass‐weighted trait means (i.e. aggregate trait values) and functional diversity index values for each plot. Aggregate trait values and functional diversity were regressed on successional age and total live above‐ground carbon content to examine functional shifts with succession and one consequence of succession – increasing above‐ground carbon. Results: Almost all significant regressions between aggregate trait values and both successional age and above‐ground carbon indicated a shift toward increased leaf palatability and decomposability during succession. The relationships were all non‐linear, with aggregate trait value shifts occurring relatively early in the successional sequence. There was weak evidence for an increase in functional richness with succession, but this was a secondary effect relative to the shifts in aggregate trait values. Conclusions: These results are in direct contrast with studies of the early stages of herbaceous post‐cultural successions from grasslands to shrublands, which have found a shift towards communities of decreasing palatability and decomposability, suggesting that functional shifts in woody succession may be fundamentally different.     

Sex‐specific graphs: Relating group‐specific topology to demographic and landscape data

Sex‐specific genetic structure is a commonly observed pattern among vertebrate species. Facing differential selective pressures, individuals may adopt sex‐specific life history traits that ultimately shape genetic variation among populations. Although differential dispersal dynamics are commonly detected in the literature, few studies have used genetic structure to investigate sex‐specific functional connectivity. The recent use of graph theoretic approaches in landscape genetics has demonstrated network capacities to describe complex system behaviours where network topology represents genetic interaction among subunits. Here, we partition the overall genetic structure into sex‐specific graphs, revealing different male and female dispersal dynamics of a fisher (Pekania [Martes] pennanti) metapopulation in southern Ontario. Our analyses based on network topologies supported the hypothesis of male‐biased dispersal. Furthermore, we demonstrated that the effect of the landscape, identified at the population level, could be partitioned among sex‐specific strata. We found that female connectivity was negatively correlated with snow depth, whereas connectivity among males was not. Our findings underscore the potential of conducting sex‐specific analysis by identifying landscape elements or configuration that differentially promotes or impedes functional connectivity between sexes, revealing processes that may otherwise remain cryptic. We propose that the sex‐specific graph approach would be applicable to other vagile species where differential sex‐specific processes are expected to occur.     

Impact of inter‐ and intra‐specific competition among larvae on larval,adult, and life‐table traits of Aedes aegypti and Aedes albopictus females

1. Few studies have taken a comprehensive approach of measuring the impact of inter‐ and intra‐specific larval competition on adult mosquito traits. In this study, the impact of competition among Aedes aegypti (L.) and A. albopictus (Skuse) was quantified over the entire life of a cohort. 2. Competitive treatments affected hatch‐to‐adult survivorship and the development time to adulthood of females for both species but affected the median wing length of females only for A. albopictus. Competitive treatments had no significant effect on the median adult female longevity nor were there any effects on other individual traits related to blood feeding and reproductive success. 3. Analysis of life table traits revealed no effect of competitive treatment on the net reproductive rate (R₀) but there were significant effects on the cohort generation time (T_c) and the cohort rate of increase (r) for both species. 4. Inter‐ and intra‐specific competition among Aedes larvae may produce individual and population‐level effects that are manifest in adults; however, benign conditions may enable resulting adults to compensate for some impacts of competition, particularly those affecting blood‐feeding success, fecundity, and the net reproductive rate, R₀. The effect of competition, therefore, affects primarily larva‐to‐adult survivorship and the larval development time, which in turn impacts the cohort generation time, T_c, and ultimately the cohort rate of increase, r. 5. The lack of effects of the larval rearing environment on adult longevity suggests that effects on vectorial capacity owing to longevity may be limited if adults have easy access to sugar and bloodmeals.     

Isoform‐specific cleavage of 14‐3‐3 proteins in apoptotic JURL‐MK1 cells

The proteins of 14‐3‐3 family are substantially involved in the regulation of many biological processes including the apoptosis. We studied the changes in the expression of five 14‐3‐3 isoforms (β, γ, ε, τ, and ζ) during the apoptosis of JURL‐MK1 and K562 cells. The expression level of all these proteins markedly decreased in relation with the apoptosis progression and all isoforms underwent truncation, which probably corresponds to the removal of several C‐terminal amino acids. The observed 14‐3‐3 modifications were partially blocked by caspase‐3 inhibition. In addition to caspases, a non‐caspase protease is likely to contribute to 14‐3‐3's cleavage in an isoform‐specific manner. While 14‐3‐3 γ seems to be cleaved mainly by caspase‐3, the alternative mechanism is essentially involved in the case of 14‐3‐3 τ, and a combined effect was observed for the isoforms ε, β, and ζ. We suggest that the processing of 14‐3‐3 proteins could form an integral part of the programmed cell death or at least of some apoptotic pathways. J. Cell. Biochem. 106: 673–681, 2009. © 2009 Wiley‐Liss, Inc.     

Latitudinal variation in the competition‐colonisation trade‐off reveals rate‐mediated mechanisms of coexistence

Theories of species coexistence often describe a trade‐off between colonising and competitive abilities. In sessile marine invertebrates, this trade‐off can manifest as trends in species distributions relative to the size of isolated patches of substrate. Based on their abilities to find available substrate and competitively exclude neighbours, good colonisers tend to dominate smaller patches, whereas better competitors tend to monopolise larger patches. In theory, species with equivalent colonising and competitive abilities should display similar distributions across patch sizes. We used patch size to observe this manifestation of the competition‐colonisation trade‐off over 20° of latitude. The trade‐off was more readily observed at lower latitudes and was proportional to the ‘ecological age’ of communities (i.e. the degree of resource acquisition and likelihood of species interactions). Results suggest that ecological age may mediate the prominence of stochastic or deterministic coexistence mechanisms and will depend on the rate of ecological processes.     

Prey‐specific foraging tactics in a web‐building spider

相似文献