Testing the intermediate disturbance hypothesis: when will there be two peaks of diversity?

Succession after disturbances generates a mosaic of patches in different successional stages. The intermediate disturbance hypothesis predicts that intermediate disturbances lead to the highest diversity of these stages on a regional scale resulting in a hump‐shaped diversity–disturbance curve. We tested this prediction using field data of forest succession and hypothetical succession scenarios in combination with analytical and simulation models. According to our study the main factors shaping the diversity–disturbance curve and the position of the diversity maximum were the transition times between the successional stages, the transition type, neighbourhood effects and the choice of diversity measure. Although many scenarios confirmed the intermediate disturbance hypothesis we found that deviations in the form of two diversity maximums were possible. Such bimodal diversity–disturbance curves occurred when early and late successional stages were separated by one or more long‐lived (compared to the early stages) intermediate successional stages. Although the field data which met these conditions among all those tested were rare (one of six), the consequences of detecting two peaks are fundamental. The impact of disturbances on biodiversity can be complex and deviate from a hump‐shaped curve.     

A mechanistic molecular test of the plant-sanction hypothesis in legume–rhizobia mutualism

The origin and persistence of mutualism is difficult to explain because of the widespread occurrence of exploitative, ‘cheating’ partners. As a policing strategy stabilising intraspecific cooperation, host sanctions against non-N₂ fixing, cheating symbionts have been proposed to stabilise mutualism in legume-rhizobium symbiosis. Mechanism of penalisations would include decreased nodular rhizobial viability and/or early nodule senescence. We tested these potential mechanisms of penalisations in split-root experiments using two soybean varieties and two rhizobial strains, a cooperative, normal N₂-fixing strain and an isogenic non-fixing derivative. We found no differences in the number of viable rhizobia recovered from nodules and no differential expression of a nodular senescence molecular marker. Thus, our results do not support the hypothesis of plant sanctions acting against cheating rhizobia in our experimental conditions.     

Why migrate? A test of the evolutionary precursor hypothesis

The question of why birds migrate is still poorly understood despite decades of debate. Previous studies have suggested that use of edge habitats and a frugivorous diet are precursors to the evolution of migration in Neotropical birds. However, these studies did not explore other ecological correlates of migration and did not control for phylogeny at the species level. We tested the evolutionary precursor hypothesis by examining the extent to which habitat and diet are associated with migratory behavior, using a species-level comparative analysis of the Tyranni. We used both migratory distance and sedentary versus migratory behavior as response variables. We also examined the influences of foraging group size, membership in mixed-species flocks, elevational range, and body mass on migratory behavior. Raw species analyses corroborated some results from studies that put forth the evolutionary precursor hypothesis, but phylogenetically independent contrast analyses highlighted an important interaction between habitat and diet and their roles as precursors to migration. Foraging group size was consistently associated with migratory behavior in both raw species and independent contrast analyses. Our results lead to a resource variability hypothesis that refines the evolutionary precursor hypothesis and reconciles the results of several studies examining precursors to migration in birds.     

Do microplastics impair male dominance interactions in fish? A test of the vector hypothesis

Microplastics (MPs) are widespread in aquatic environments and have become a critical environmental issue in recent years due to their adverse impacts on the physiology, reproduction, and survival of aquatic animals. Exposure to MPs also has the potential to induce sub‐lethal behavioral changes that can affect individual fitness, but these effects are understudied. Many plastic additives introduced during the manufacture of MPs are known endocrine‐disrupting chemicals (EDCs) that mimic the action of natural hormones, alter sexual and competitive behavior, and impair reproductive success in fish. In addition, EDCs and other aquatic contaminants may adhere to MPs in the environment, the latter of which may serve as transport vectors for these compounds (i.e., the vector hypothesis). In this study, we staged territorial contests between control males, and males exposed to virgin MP particles or to MPs previously immersed in one of two environmentally relevant concentrations of 17‐alpha ethinyl estradiol (EE2; 5 ng/L and 25 ng/L) to evaluate the independent and synergistic effects of exposure to MPs and a common environmental estrogen on male–male aggression and competitive territory acquisition in a freshwater fish, Pimephales promelas. Short‐term (30 days) dietary exposure to MPs did not impair the ability of males to successfully compete for and obtain a breeding territory. Overall levels of aggression in control and exposed males were also similar across trial series. These results help to fill a critical knowledge gap regarding the direct and indirect (vector‐borne) effects of MPs on the reproductive behavior of aquatic vertebrates in freshwater systems.     

Does understory vegetation reflect the history of fluvial disturbance in a riparian forest?

We have examined the relationship between the history of fluvial disturbance and understory vegetation in a riparian forest. The study site was divided into three sites, by use of aerial photographs and topographical maps, with different histories of fluvial disturbance: (1) Fagus-type on land that has not been flooded for the last 39 years, at least; (2) Populus-type on land that has not been flooded since debris flow occurred 34 years ago; and (3) Salix-type on land that has been flooded periodically from an abandoned channel since debris flow occurred 34 years ago. Species richness in the Salix-type was significantly higher than in the other types. Detrended correspondence analysis revealed obvious floristic differences among the three canopy types. Canonical correspondence analysis showed that herbaceous species were mainly found on lower plots with high moss cover, implying that moss layers may capture seeds transported by the stream. Tall herbs occurred in less shaded plots and on higher plots, suggesting that their rapid growth prevented the occurrence of other species. Fagus-type was dominated by species with ingested fruits which depended on animals for their dispersal. Populus and Salix-types were dominated by species with wind dispersal or no dispersal mechanism, which depended on physical phenomena for dispersal. Attributes of current understory vegetation were connected with historical events, suggesting that riparian vegetation reflects the history of fluvial disturbance.     

A test of vegetation–environment relationship in serpentine soils of Tuscany, Italy

The present study evaluates the relative importance of environmental factors in affecting the species composition and abundance of the plant communities on ultramafic soils in Tuscany, Italy. We used rigorous sampling techniques to test hypotheses generated from exploratory studies performed previously. Vegetation–environmental relationships were analyzed using 50 plots, each 1 m², randomly located throughout a 22-ha area in the Upper Tiber Valley. We confirm that the exchangeable fraction of nickel in the soil is almost never high enough to affect the vegetation. However, physical factors (e.g. substrate setting and elevation) are important in controlling the distribution of plant species. Tree cover (almost exclusively due to the introduced plantation pines) also had a significant affect on the vegetation composition and on soil features such as the C/N ratio. Other important factors significantly related to the gradients in vegetation composition (e.g. rockiness and total soil nitrogen) are interpreted as factors related to the vegetation composition through a positive feedback mechanism.     

Range-body mass interactions of a northern ungulate – a test of hypothesis

Summer diet, summer temperature, length of the growth season and animal density appeared to best explain annual and regional differences in calf and yearling body mass in moose from southeastern Norway. In general animals inhabiting steep, alpine landscapes had less body mass than animals using flat, low-altitude habitats. Autumn body mass of calves and yearlings decreased with increasing snow depth during the preceding winter and spring. However, calf body mass was more influenced by the summer range and less by the winter range than was body mass of yearlings. There was no indication that the effect of snow depth on autumn body mass was greater in moose living on poor than on good summer ranges. Body mass decreased with increasing competition for summer forage, while the winter range mainly had an density-independent effect. Habitat quality, expressed as regression lines between calf and yearling body mass and animal density (hunting yield), differed between regions. On ranges of medium and high altitude where birch (Betula spp.) rowan (Sorbus aucuparia) and bilberry (Vaccinium myrtillus) dominated moose summer diet, body mass decreased at a rapid rate with increasing animal density. Body mass decreased at a slower rate at low-altitude ranges and at high-altitude ranges where willow (Salix spp.) and forbs dominated the diet. Body mass of lactating cows decreased with increasing animal density, but animal density did not affect body mass of non-lactating cows. There was no indication that the decrease in autumn body mass with increasing moose density over the last 25 years has caused a decrease in animal condition (ability to survive the winter). The results are discussed in relation to the effect of summer and winter range on population regulation in moose. It is concluded that a density-dependent effect is apparent on the summer range even at low and intermediate population densities. On the winter range, on the other hand, density-dependence is likely to occur only at high levels of population density. Received: 4 February 1997 / Accepted: 1 February 1999     

Do metal-rich plants deter herbivores? A field test of the defence hypothesis

Some plant species growing on metalliferous soils are able to accumulate heavy metals in their shoots up to very high concentrations, but the selective advantage of this behaviour is still unknown. The most popular hypothesis, that metals protect plants against herbivores, has been tested several times in laboratory conditions, with contradictory results. We carried out the first large-scale test of the defence hypothesis in eight natural populations of the model Zn hyperaccumulator Thlaspi caerulescens J. and C. Presl (Brassicaceae). In two climatic regions (temperate, Belgium–Luxembourg, and Mediterranean, southern France), we worked in metalliferous and in normal, uncontaminated environments, with plants spanning a wide range of Zn concentrations. We also examined the importance of glucosinolates (main secondary metabolites of Brassicaceae) as antiherbivore defences. When exposed to natural herbivore populations, T. caerulescens suffered lower herbivory pressures in metal-enriched soils than in normal soils, both in Belgium–Luxembourg and in southern France. The trapping of gastropods shows an overall lower population density in metalliferous compared to normal environments, which suggests that herbivory pressure from gastropods is lower on metalliferous soils. In addition, foliar concentration of glucosinolates was constitutively lower in all populations from metal-enriched soils, suggesting that these have evolved towards lower investment in organic defences in response to lower herbivory pressure. The Zn concentration of plants had a protective role only for Belgian metallicolous plants when transplanted in normal soils of Luxembourg. These results do not support the hypothesis that Zn plays a key role in the protection of T. caerulescens against enemies. In contrast, glucosinolates appear to be directly involved in the defence of this hyperaccumulator against herbivores.     

Can the intermediate disturbance hypothesis and information on species traits predict anuran responses to fire?

Fire is a common form of recurrent disturbance in many ecosystems, but ecological theory has a poor record of predicting animal responses to fire, at both species and assemblage levels. As a consequence, there is limited information to guide fire regime management for biodiversity conservation. We investigated a key research gap in the fire ecology literature; that is, the response of an anuran assemblage to variation in the fire return interval. We tested two hypotheses using a spatially‐explicit fire database collected over a 40 year period: 1) species richness would peak at intermediate levels of disturbance. 2) Species with traits which enabled them to escape fire – burrowing or canopy dwelling – would be better able to survive fires, resulting in higher levels of occurrence in frequently burned sites. We found no evidence for either a reduction in species richness at locations with short fire return intervals, or a peak in species richness at intermediate levels of disturbance. Although we found some support for individual species responses to fire return intervals, these were inconsistent with the interpretation of burrowing or climbing being functional traits for fire‐avoidance. Instead burrowing and climbing species may be more likely to be disadvantaged by frequent fire than surface dwelling frogs. More generally, our results show that many species in our study system have persisted despite a range of fire frequencies, and therefore that active management of fire regimes for anuran persistence may be unnecessary. The responses of anurans to fire in this location are unlikely to be predictable using simple life‐history traits. Future work should focus on understanding the mechanistic underpinnings of fire responses, by integrating information on animal behavior and species’ ecological requirements.     

An experimental test of the independent action hypothesis in virus–insect pathosystems

The ‘independent action hypothesis’ (IAH) states that each pathogen individual has a non-zero probability of causing host death and that pathogen individuals act independently. IAH has not been rigorously tested. In this paper, we (i) develop a probabilistic framework for testing IAH and (ii) demonstrate that, in two out of the six virus–insect pathosystems tested, IAH is supported by the data. We first show that IAH inextricably links host survivorship to the number of infecting pathogen individuals, and develop a model to predict the frequency of single- and dual-genotype infections when a host is challenged with a mixture of two genotypes. Model predictions were tested using genetically marked, near-identical baculovirus genotypes, and insect larvae from three host species differing in susceptibility. Observations in early-instar larvae of two susceptible host species support IAH, but observations in late-instar larvae of susceptible host species and larvae of a less susceptible host species were not in agreement with IAH. Hence the model is experimentally supported only in pathosystems in which the host is highly susceptible. We provide, to our knowledge, the first qualitative experimental evidence that, in such pathosystems, the action of a single virion is sufficient to cause disease.     

Do northern bobwhite quail modulate intestinal nutrient absorption in response to dietary change? A test of an adaptational hypothesis

We acclimated northern bobwhite quail (Colinus virginianus) to either chow (high carbohydrate/low protein) or crickets (low carbohydrate/high protein) and tested predictions of hypotheses based on the premise of the economical design of animals. The adaptive modulation hypothesis predicts that d-glucose uptake would be higher and l-proline uptake lower in bobwhites acclimated to chow. The spare capacity hypothesis predicts that the capacity to absorb d-glucose actively will exceed the estimated nutrient load from daily food intake. There was no significant dietary effect on intestinal d-glucose (P = 0.8) and l-proline (P = 0.7) uptake rates measured in vitro using the everted sleeve technique. In chow eaters maximal mediated d-glucose uptake summed along the entire length of intestine (53 cm) was far too low (7.2 mmol/d) to explain observed rates of glucose absorption in vivo (>35 mmol/d). Hence, both predictions were falsified. In vitro uptake may not be an appropriate measure of the intestine's absorptive capacity because it does not measure possibly important pathways of passive absorption. There is increasing evidence that substantial passive glucose absorption occurs in some birds. If passive absorption predominates the adaptive modulation hypothesis might not apply.     

Stress as adaptation? A test of the adaptive boost hypothesis among Batswana men

Many studies have found elevated cortisol linked to negative events (“stress”) and subsequent negative outcomes, such as reduced immunity and stunted growth, leading to the conclusion that high cortisol is “bad.” However, a growing number of studies have found more advantaged groups showing relatively elevated cortisol. For example, higher morning cortisol followed by a steeper diurnal decline among Caucasians compared to ethnic minorities has been interpreted as a context-specific “adaptive boost” to meet daily demands. We tested the adaptive boost hypothesis using data on socioeconomic status, depressive affect and salivary cortisol among adult men (n = 32) in Botswana. Three findings emerged: (i) depressive affect was associated with lower morning cortisol (r = ? 0.43, p = 0.014); (ii) depressive affect was associated with a diurnal increase in cortisol when comparing morning and evening samples (r = 0.49, p = 0.004); and (iii) depressive affect was associated with lower income (r = ? 0.55, p = 0.001). Findings are consistent with the adaptive boost hypothesis and add to a growing body of evidence that elevated cortisol is not universally bad. Hypothalamic–pituitary adrenal axis (HPAA) activity, such as cortisol, may be adaptive depending on a person's contextual circumstances. Based on our findings and those of previous studies, we develop a “person-in-context” model of the threat appraisal process. Integrated with life history theory, our model facilitates testable hypotheses about intra- and inter-individual variability in HPAA and adaptive consequences.     

Do exaggerated sexual swellings function in female mating competition in primates? A comparative test of the reliable indicator hypothesis

The reliable indicator hypothesis proposes that exaggeratedsexual swellings in female primates serve as honest signalsof female quality that function in female—female competitionover mates. We examined a version of this hypothesis usinginterspecific data to test whether exaggerated sexual swellingsare associated with female mating competition, as measured usingthe adult sex ratio, female canine size, and expected femalemating synchrony. The ratio of females to males and relativecanine size declined over evolutionary transitions in swellingstate, thus providing no support for the reliable indicatorhypothesis. Expected female mating synchrony increased over evolutionary transitions in swelling state, but this patterndid not approach significance, and the patterns were oppositeto predictions when controlling for the number of males inthe group. In addition to these comparative tests, we reviewedevidence concerning individual attributes of females relativeto characteristics of their swellings. Contrary to the reliableindicator hypothesis, the least fertile females, or those leastlikely to raise surviving offspring, often have larger swellings.We consider the statistical power of our tests, discuss thetheoretical and empirical bases for our comparative predictions,and consider other lines of evidence needed to test the reliableindicator hypothesis. We also discuss an alternative hypothesis, the graded signal hypothesis, which combines the benefits ofbiasing and confusing paternity through a novel mechanism andis testable in the field and the laboratory.     

How reliable is the monitoring of permanent vegetation plots? A test with multiple observers

Questions: A multiple plot design was developed for permanent vegetation plots. How reliable are the different methods used in this design and which changes can we measure? Location: Alpine meadows (2430 m a.s.l.) in the Swiss Alps. Methods: Four inventories were obtained from 40 m² plots: four subplots (0.4 m²) with a list of species, two 10m transects with the point method (50 points on each), one subplot (4m²) with a list of species and visual cover estimates as a percentage and the complete plot (40 m²) with a list of species and visual estimates in classes. This design was tested by five to seven experienced botanists in three plots. Results: Whatever the sampling size, only 45‐63% of the species were seen by all the observers. However, the majority of the overlooked species had cover < 0.1%. Pairs of observers overlooked 10‐20% less species than single observers. The point method was the best method for cover estimate, but it took much longer than visual cover estimates, and 100 points allowed for the monitoring of only a very limited number of species. The visual estimate as a percentage was more precise than classes. Working in pairs did not improve the estimates, but one botanist repeating the survey is more reliable than a succession of different observers. Conclusion: Lists of species are insufficient for monitoring. It is necessary to add cover estimates to allow for subsequent interpretations in spite of the overlooked species. The choice of the method depends on the available resources: the point method is time consuming but gives precise data for a limited number of species, while visual estimates are quick but allow for recording only large changes in cover. Constant pairs of observers improve the reliability of the records.     

A circumventing role for the non-native intermediate in the folding of β-lactoglobulin

Folding experiments have suggested that some proteins have kinetic intermediates with a non-native structure. A simple G ?o model does not explain such non-native intermediates. Therefore, the folding energy landscape of proteins with non-native intermediates should have characteristic properties. To identify such properties, we investigated the folding of bovine β-lactoglobulin (βLG). This protein has an intermediate with a non-native α-helical structure, although its native form is predominantly composed of β-structure. In this study, we prepared mutants whose α-helical and β-sheet propensities are modified and observed their folding using a stopped-flow circular dichroism apparatus. One interesting finding was that E44L, whose β-sheet propensity was increased, showed a folding intermediate with an amount of β-structure similar to that of the wild type, though its folding took longer. Thus, the intermediate seems to be a trapped intermediate. The high α-helical propensity of the wild-type sequence likely causes the folding pathway to circumvent such time-consuming intermediates. We propose that the role of the non-native intermediate is to control the pathway at the beginning of the folding reaction.     

Abnormal motor patterns in the framework of the equilibrium-point hypothesis: a cause for dystonic movements?

Until now, the equilibrium-point hypothesis (λ model) of motor control has assumed nonintersecting force-length characteristics of the tonic stretch reflex for individual muscles. Limited data from animal experiments suggest, however, that such intersections may occur. We have assumed the possibility of intersection of the characteristics of the tonic stretch reflex and performed a computer simulation of movement trajectories and electromyographic patterns. The simulation has demonstrated, in particular, that a transient change in the slope of the characteristic of an agonist muscle may lead to temporary movement reversals, hesitations, oscillations, and multiple electromyographic bursts that are typical of movements of patients with dystonia. The movement patterns of three patients with idiopathic dystonia during attempts at fast single-joint movements (in the elbow, wrist, and ankle) were recorded and compared with the results of the computer simulation. This approach considers that motor disorders in dystonia result from faulty control patterns that may not correlate with any morphological or neurophysiological changes. It provides a basis for the high variability of dystonic movements. The uniqueness of abnormal motor patterns in dystonia, that precludes statistical analysis across patients, may result from subtle differences in the patterns of intersecting characteristics of the tonic stretch reflex. The applicability of our analysis to disordered multijoint movement patterns is discussed. Received: 26 July 1993/Accepted in revised form: 22 December 1993     

Does disturbance enhance genotypic diversity in clonal organisms? A field test in the marine angiosperm Zostera marina

Physical disturbance has often been invoked to control genotypic diversity in sessile clonal organisms, yet experimental evidence is lacking. I studied the effects of physical disturbance on genet dynamics and genotypic diversity in a clonal marine angiosperm, Zostera marina (eelgrass). In replicated plots of 1 m2, the vegetation canopy was removed in gaps of zero (control), 25%, 50% and 75% of the area (n = 6 replicates). Before removal and during two consecutive years, the genotypic composition was determined using genetic markers (DNA microsatellites) in a 5 x 5 pixel grid per plot. An aggregate index of genet dynamics summarizing recruitment, increase, loss and decrease of clones was maximal at intermediate disturbance levels (quadratic polynomial P = 0.02). Physical disturbance also increased the occurrence of new genotypes, possibly reflecting recruitment (linear model, P < 0.05). Contrary to expectations, there was no competitive advantage of more heterozygous genotypes over less heterozygous ones. In the absence of disturbance, in particular, clones with lower individual heterozygosity were more likely to increase in area over a 1-year time period than more heterozygous ones, while there was no such correlation in plots with disturbance (logistic model, P(disturbance x heterozygosity) = 0.036). Undisturbed plots revealed background recruitment independent of canopy gaps, suggesting that Z. marina exhibits a strategy of continual recruitment. Effects of experimental disturbance (linear or quadratic) on clonal diversity were not detectable. Instead, initial (pretreatment) clonal diversity accounted for between 68% and 91% of the variance in diversity, indicating remarkable resilience of genotypic diversity in the face of physical disturbance.