Competitive Hierarchy Research Articles

Functional equivalence, competitive hierarchy and facilitation determine species coexistence in highly invaded grasslands.

Alien and native plant species often differ in functional traits. Trait differences could lead to niche differences that minimize competitive interactions and stabilize coexistence. However, trait differences could also translate into average fitness differences, leading to a competitive hierarchy that prevents coexistence. We tested whether trait differences between alien and native species translated into average fitness or stabilizing niche differences, and whether competition could explain observed coexistence within invaded grassland communities (New Zealand). Trait differences reflected marked competitive hierarchy, suggesting average fitness differences. Species coexistence was determined by a trade-off between species susceptibility to herbivory vs competitive hierarchy and facilitation. Importantly, although aliens and natives differed in their trait values, they did not differ in their competitive response, highlighting the importance of equalizing mechanisms in structuring invaded communities. Only a few alien species with a particular set of traits were able to jeopardize species coexistence when grazing was ceased. Our study explains why some alien species coexist with natives, whereas others have strongimpacts on native communities. It highlights that trait differences can underlie several coexistence processes and that the demonstration of trait differences between aliens and natives is only a first step to understanding the role of biotic interactions in structuring invaded communities.

Habitat degradation modifies the strength of interspecific competition in coral dwelling damselfishes

Habitat degradation is predicted to exacerbate competition for critical resources; however, the relationship between habitat quality and competition is poorly understood. In this study, we used a manipulative experiment to test the effects of habitat degradation on competition between two planktivorous, coral‐dwelling damselfishes, Chrysiptera parasema and Dascyllus melanurus. Experimental reefs were constructed with either healthy (100% live) or degraded (10% live) Acropora longicyathus coral, stocked with varying densities of these two fish species, and monitored for two months. On healthy habitat, the mortality of C. parasema was density dependent, and increased substantially in the presence of the dominant interspecific competitor D. melanurus. In contrast, on reefs where habitat was degraded, C. parasema mortality was highly variable, density independent, and was no longer influenced by the presence of dominant competitor D. melanurus. Behavioral observations revealed that agonistic interactions for both species increased with density on degraded habitat, but not on healthy habitat. In addition, on degraded reefs, both species displayed a reduced association with reef habitat and ventured further away from shelter with increasing densities of the dominant competitor D. melanurus. These results suggest that reduced habitat quality can have such a profound effect on reef fishes, that it eliminates density‐dependent mortality and competitive dominance hierarchies, thereby substantially altering the mechanisms that structure reef fish communities.

Utilization of mangrove crab-burrow micro-habitats by the goby Redigobius dewaali: Evidence for dominance hierarchy

When a range of life-history groups within a population exploit similar niches, hierarchies are often established whereby optimal resources are capitalized by dominant individuals. The present study investigates the feeding and movement of a size structured population of gobies residing in crab burrow micro-habitats, within this context. All life-history stages of the goby Redigobius dewaali were found to utilize burrow-pools formed by the crab Scylla serrata in the mangrove intertidal zone of an estuary at low tide. Gut content analysis was therefore conducted to test for feeding overlap and potential intraspecific competition among various sizes of fish residing in these habitats. In addition, an exclosure experiment was conducted to determine whether these fish leave the burrow-pools at high tide to forage over the intertidal flats. While ontogenetic dietary shifts across size-classes occurred, copepods and amphipods comprised the most important components of the diet for most size-classes, resulting in a large degree of dietary overlap. Of the adult R. dewaali sampled from pools, a single large mature male was sampled from each burrow, suggesting territoriality, a trait well observed in males of many goby species. The experimental component of the study highlighted the movement of smaller individuals, but not larger individuals, from the burrow-pools at high tide. As all life-history stages occur in the same microhabitats, and forage on much of the same prey, we suggest that the existence of a competitive hierarchy resulted in the need for the smaller individuals to find alternate feeding areas, consistent with dominance hierarchy theory.

Structural changes in arboreal ant assemblages (Hymenoptera: Formicidae) in an age sequence of cocoa plantations in the south-east of Bahia, Brazil

A study of succession of ant species in plantations of different ages and development may assist with our understanding of the dynamics of their assemblages. The aim of this study was to characterise the relationship between development of Brazilian cocoa plantations and the nature of their dominant ant assemblages. A chronosequence of cocoa plantations aged 1, 3, 4, 8, 15 and 33 years was sampled by several methodologies. Data were analysed in terms behavioural dominance and Berger-Parker's dominance index (here based on frequency data), and also by principal component analysis and analysis of co-occurrence. Apart from lower numbers of species being found in the 1-year-old plantation, there was no consistent trend in ant richness with plantation age. According to the criteria we adopted, only one species reached behavioural dominance in most age classes of plantation, although this increased to three in the 8-year-old one, before declining to zero in the oldest plantation. No species reached Berger-Parker's dominance in the youngest plantation, whereas all other age classes contained one to three dominants. Particular species showed non-age-related variations in their degree of Berger-Parker's dominance and this could in part be related to which species initially colonised the plantation. Principal component analysis axis 1 was partly related to plantation age, indicating an age-related change in assemblage composition. Ant species co-occurrence could only be effectively detected in cocoa plantations from 3 to 15 years of age. The arboreal ant assemblage is dynamic in nature, with the competitive hierarchy among species oscillating along the cocoa development chronosequence. The assemblage structure could be influenced by the initial founding ants, as well as by the invasive Monomorium floricola.

A mechanistic study of plant and microbial controls over R* for nitrogen in an annual grassland.

Differences in species' abilities to capture resources can drive competitive hierarchies, successional dynamics, community diversity, and invasions. To investigate mechanisms of resource competition within a nitrogen (N) limited California grassland community, we established a manipulative experiment using an R* framework. R* theory holds that better competitors within a N limited community should better depress available N in monoculture plots and obtain higher abundance in mixture plots. We asked whether (1) plant uptake or (2) plant species influences on microbial dynamics were the primary drivers of available soil N levels in this system where N structures plant communities. To disentangle the relative roles of plant uptake and microbially-mediated processes in resource competition, we quantified soil N dynamics as well as N pools in plant and microbial biomass in monoculture plots of 11 native or exotic annual grassland plants over one growing season. We found a negative correlation between plant N content and soil dissolved inorganic nitrogen (DIN, our measure of R*), suggesting that plant uptake drives R*. In contrast, we found no relationship between microbial biomass N or potential net N mineralization and DIN. We conclude that while plant-microbial interactions may have altered the overall quantity of N that plants take up, the relationship between species' abundance and available N in monoculture was largely driven by plant N uptake in this first year of growth.

Environmental nutrient supply alters prevalence and weakens competitive interactions among coinfecting viruses.

The rates and ratios of environmental nutrient supplies can determine plant community composition. However, the effect of nutrient supplies on within-host microbial interactions is poorly understood. Resource competition is a promising theory for understanding microbial interactions, because microparasites require nitrogen (N) and phosphorus (P) for synthesis of macromolecules such as nucleic acids and proteins. To better understand the effects of nutrient supplies to hosts on pathogen interactions, we singly inoculated and coinoculated Avena sativa with two virus species, barley yellow dwarf virus-PAV (BYDV-PAV) and cereal yellow dwarf virus-RPV (CYDV-RPV). Host plants were grown across a factorial combination of N and P supply rates that created a gradient of N:P supply ratios, one being replicated at low and high nutrient supply. Nutrient supply affected prevalence and the interaction strength among viruses. P addition lowered CYDV-RPV prevalence. The two viruses had a distinct competitive hierarchy: the coinoculation of BYDV-PAV lowered CYDV-RPV infection rate, but the reverse was not true. This antagonistic interaction occurred at low nutrient supply rates and disappeared at high N supply rate. Given the global scale of human alterations of N and P cycles, these results suggest that elevated nutrient supply will increase risks of virus coinfection with likely effects on virus epidemiology, virulence and evolution.

Functional trait differences and the outcome of community assembly: an experimental test with vernal pool annual plants

Functional trait‐based approaches have seen rapid development in community ecology and biogeography in recent years, as they promise to offer a better mechanistic and predictive understanding of community structure. However, several key challenges remain. First, while many studies have explored connections between functional traits and abiotic gradients, far fewer have directly tested the common assumption that functional trait differences influence interspecific interactions. Second, empirical studies often ignore intraspecific trait variation within communities, even though intraspecific variation has been known to have substantial impacts on community dynamics. Here we present an experiment designed to assess the role of functional trait differences in predicting the outcome of interspecific species interactions among a suite of California vernal pool annual plants. Eight species were grown in pairwise combinations in two levels of inundation in a greenhouse and functional traits were measured on all individuals. Nested models predicting focal plant performance were fit to the data. For seven of the eight species in the experiment, the best model included a functional trait difference term that was consistent with a competitive hierarchy, indicating that focal species tended to do better when they had larger leaf size, lower specific leaf area, and greater investment in lateral canopy spread than their neighbors. Models that included individually measured trait values generally performed better than models using species trait averages. We tested if the same trait measurements predicted tolerance of inundation (a feature of vernal pool habitats), and species depth distributions from extensive field surveys, though we did not find strong relationships. Our results suggest that functional traits can be used to make inferences about the outcome of interspecific interactions, and that greater predictive power can come from considering intraspecific variation in functional traits, particularly in low diversity communities.

Inclusive-fitness logic of cooperative breeding with benefits of natal philopatry

In cooperatively breeding species, individuals help to raise offspring that are not their own. We use two inclusive-fitness models to study the advantage of this kind of helpful behaviour in social groups with high reproductive skew. Our first model does not allow for competition among relatives to occur but our second model does. Specifically, our second model assumes a competitive hierarchy among nest-mates, with non-breeding helpers ranked higher than their newborn siblings. For each model, we obtain an expression for the change in inclusive fitness experienced by a helpful individual in a selfish population. The prediction suggested by each expression is confirmed with computer simulation. When model predictions are compared to one another, we find that helping emerges under a broader range of conditions in the second model. Although competition among kin occurs in our second model, we conclude that the life-history features associated with this competition also act to promote the evolutionary transition from solitary to cooperative breeding.

Applying trait-based models to achieve functional targets for theory-driven ecological restoration.

Manipulating community assemblages to achieve functional targets is a key component of restoring degraded ecosystems. The response-and-effect trait framework provides a conceptual foundation for translating restoration goals into functional trait targets, but a quantitative framework has been lacking for translating trait targets into assemblages of species that practitioners can actually manipulate. This study describes new trait-based models that can be used to generate ranges of species abundances to test theories about which traits, which trait values and which species assemblages are most effective for achieving functional outcomes. These models are generalisable, flexible tools that can be widely applied across many terrestrial ecosystems. Examples illustrate how the framework generates assemblages of indigenous species to (1) achieve desired community responses by applying the theories of environmental filtering, limiting similarity and competitive hierarchies, or (2) achieve desired effects on ecosystem functions by applying the theories of mass ratios and niche complementarity. Experimental applications of this framework will advance our understanding of how to set functional trait targets to achieve the desired restoration goals. A trait-based framework provides restoration ecology with a robust scaffold on which to apply fundamental ecological theory to maintain resilient and functioning ecosystems in a rapidly changing world.

Bashing ‘the Chinese’: contextualizing Zambia's Collum Coal Mine shooting

The 2010 shooting of 13 miners at Zambia's small, privately-owned ‘Chinese’ Collum Coal Mine (CCM) has been represented by Western and Zambian politicians and media as exemplifying the ‘neo-colonial’ and ‘amoral’ practices of ‘China’ and ‘the Chinese’ in Africa. CCM has been used to provide a sharp contrast to the supposed ways of the Western firms that own most of Zambia's mines. Embedded in racial hierarchy and notions of strategic competition between the West and China, the discourse of the CCM shootings further shapes conceptions of global China and Chinese overseas. While examining all the oppressive conditions that have given rise to protest at the mine, we contextualize the shooting and subsequent conflicts. In analyzing CCM's marginal and troubled development, we discuss aspects of the 2010 shooting incident known to miners and union leaders, but ignored by politicians and media. We look at the shooting's political fallout, focus also on the epilogue that was the 2012 CCM riot—in which one Chinese person was murdered and several others seriously injured—and trace the sometimes violent discontent manifested at other foreign-owned mines in Zambia since their privatization in the late 1990s. The empirical data for this detailed study derive from hundreds of documentary sources and interviews with union leaders, workers, officials and others in Zambia from 2011 to 2013.

Gall insects and selection on plant vigor: can susceptibility compromise success in competition?

Gall insects select vigorously growing plants and plant parts when initiating gall formation. Vigor is associated with rapid growth rate, and in turn, rapid growth confers competitiveness. Are there conditions under which the cost of vigor, in the form of increased susceptibility to attack, outweighs the benefit of competitive success? I present a simulation model to explore the interaction between susceptibility and competition on the selective advantage of increased growth rate. Assuming size-symmetric competition, the model shows that in general, vigor is favored (benefit > cost) at low to intermediate gall loads. At very high plant densities, however, plants with high gall loads may lose standing in the competitive size hierarchy from which they cannot recover. The details of this result, however, change somewhat when competition is size-asymmetric, that is, when a larger focal plant suppresses smaller neighbors, but smaller neighbors cannot exert a reciprocal effect on the focal. At low densities, the pattern of selection on growth rate is qualitatively similar to the size-symmetric case. At higher plant densities, however, fast-growing genotypes can suppress slow ones so much during the preattack phase that even at the highest gall loads they maintain their standing in the competitive hierarchy. Thus, heavy gall insect attack on vigorous plants can impose selection against high intrinsic growth rates under strong symmetric competition, but not strong asymmetric competition. While life history traits can evolve as a correlated response to selection on defensive traits that reduce susceptibility, this model reveals that susceptibility can evolve as a correlated response to selection on basic life history traits.

Mechanisms of species‐sorting: effect of habitat occupancy on aphids' host plant selection

Interspecific interactions such as competition are important factors affecting insect herbivore fitness. Host choice in herbivorous insects including aphids has largely been studied with respect to host plant condition, while the role of competition is also very important. Tansy (Tanacetum vulgare L.) hosts three specialised aphids, Macrosiphoniella tanacetaria (Kaltenbach), Metopeurum fuscoviride Stroyan and Uroleucon tanaceti (L.) (Hemiptera: Aphididae). In this study, a set of greenhouse and field experiments were carried out to investigate whether aphid host plant choice was affected by the presence of the other aphid species. When winged individuals were given the choice between unoccupied plants and plants occupied by the same or another aphid species, choices generally reflected the outcome of competitive interactions among aphids. Colonisation of plants by winged individuals was influenced not only by the presence of other aphids on the host plant but also by previous infestation. The host choice of winged individuals basically reflected competitive hierarchies, i.e. aphids in most cases selected plants where future competition was less likely. By contrast, unwinged aphids did not show any host plant preference. For M. tanacetaria previously infested plants promoted the production of winged offspring. Our results show that competitive interactions could affect host selection behaviour by aphids. In a metacommunity context, such preferences in colonising different habitats leads to species‐sorting through habitat heterogeneity.

Role of National Law in International Investment Law System: A Competitor, a Supplementary or a Cooperator?

As a typical area of transnational law, international investment law is piercing the veil of sovereignty and blurring, if not dissolving, the boundaries between municipal law system (NL) and international investment law system (IIL). The interaction between NL and IIL is further complicated when considerable discretion of arbitral tribunal over reviewing governmental actions encounters inconclusive but multifaceted relevance of law of host state in arbitral proceedings. The legal status, the impact and the potential function of national law at both specific arbitral proceeding level and institutional level in international investment law system are under-theorized, which partly present itself as and partly generate the seemingly divergent arbitral practices of tribunals. Therefore, it is of great practical and theoretical significance to figure out the (potential) role of national law in international investment law system from a dynamic institutional perspective. This paper first appraises the role of NL in specific investment treaty arbitration from three perspectives, namely, NL as fact, NL as evidence and NL as governing law. The inquiry then goes beyond the sphere of individual arbitration to the systematic dynamics of IIL: NL as a template for negotiation and renegotiations of investment agreements, NL as state practices leading to the development of customary law and NL as recognition and enforcement of IIL.The paper further explores the institutional structure of governance system over transnational investment activities and argues that IIL and Investor-State Dispute Resolution (ISDS) mechanisms are smart governance mechanisms for purely or mainly economic disputes concerning investment. However, IIL is not of institutional advantages over NL when investment related disputes are of great social influence and political impacts. The normative delimitation of regulatory boundary of NL vis-a-vis IIL over transnational investment related activities lies in the equilibrium between competing parameters, namely marketable interest concerns and socialized (and political, to the most extent thereof) interest concerns. The more marketable the targeted investment related matter is, the more involved IIL should be. The paper further argues that an interdependent, hybrid and poli-contextual governance network is emerging in transnational investment regulation. Based on comparative advantages of both NL and IIL system, the governance network is and shall be distinct from both totally free competitive approach and total hierarchy approach, and of the institutional capacity to exercise the comparative advantages of both systems without functionally distorting the other while capable of coping cope with the constantly changeable and multifaceted regulatory context in transnational investment sphere.

Both like it hot? Influence of temperature on two co-occurring intertidal barnacles in central Chile

Jehlius cirratus and Notochthamalus scabrosus, two chthamalid barnacles common to the upper rocky intertidal coast of Chile, often overlap extensively in their vertical distribution, with Jehlius extending slightly higher on the shore. Previous studies examining competition between Jehlius and Notochthamalus have come to differing conclusions regarding the nature and direction of their relationship within a hierarchy for space, but together suggest that temperature may play an important part in meditating their competitive interactions. Here, we address the role of temperature during emersion on Jehlius and Notochthamalus via species-specific responses in abundance, growth and reproduction to varying thermal conditions during tidal emersion. Results from a shading experiment conducted at two high intertidal elevations indicated that both species responded similarly to reduced temperatures in terms of abundance and space occupation. Growth rates differed between species, however, suggesting that Notochthamalus may be more limited by higher temperature than Jehlius, but growth rates of both species were similar under the warmest (high zone, unshaded) and coolest (mid zone, shaded) treatments. Although we found a greater proportion of Notochthamalus individuals brooding eggs in unshaded control areas (where sample size allowed analysis) and greater numbers of Jehlius individuals brooding under shades, no differences in reproductive output (egg mass) were found among brooding individuals between shading treatments and tidal elevation for either species. Overall, we found no evidence of temperature mediated tradeoffs in performance between adult Jehlius and Notochthamalus. Instead, our results indicate a lack of a strong competitive hierarchy between Jehlius and Notochthamalus and suggest that temperatures experienced during emersion may reinforce vertical patterns of adult abundance established during settlement.

Hierarchy of root functional trait values and plasticity drive early‐stage competition for water and phosphorus among grasses

Summary The link between species' functional traits and competitive abilities has been described as a major factor structuring plant communities. However, two diverging hypotheses have been proposed to explain this process: competition‐trait similarity and competition‐trait hierarchy. We performed a greenhouse experiment to determine whether grasses' root foraging strategies, from acquisitive or conservative functional groups, are linked to plant competitive ability and to test which hypothesis better explains interactions during the early stage of grass establishment under contrasting growth conditions. Two grass species of each functional group were grown with and without a neighbour under two levels of water and phosphorus supplies. Three functional traits related to plant competitive ability were measured on all plants grown without neighbours: specific root length (SRL), root phosphorus use efficiency and root length density. Above‐ground biomass was measured on plants grown with and without neighbours to evaluate the intensity of plant interaction. We demonstrated that for the three traits the intensity of interaction is driven mainly by hierarchical trait distance, that is, trait distance between target and neighbour, and not by trait similarity. Growth conditions strongly affected the significance of the relation between hierarchical distances and competition intensity. For the SRL hierarchical distance, this effect may be due to the most competitive species (with high SRL) being strongly impacted by water shortage, which modified the competitive hierarchy. Trait plasticity in response to stresses also appeared an important factor influencing the competitive ability of species, that is, species with the most plastic SRL in response to P stress were also the most competitive under P stress. A strong hierarchy exists among grasses' competitive abilities in non‐limiting growth conditions that is linked to their root functional traits and investment in the root system. Consequently, our results support the trait hierarchy hypothesis in its ability to describe competitive interaction among grasses during early stages of establishment. Our study provides evidence that root functional hierarchical trait distance and plasticity explain how grasses interact with their neighbours. This distance enables species to be ranked according to their competitive ability; however, this ranking may be influenced by the growth conditions and traits considered.

Categorization and systematization of institutionally- procedural and politically-behavioral attributes and features of semi-presidentialism: theoretical and methodological cut

The article is devoted to theoretical and methodological categorization and systematization of institutionally-procedural and politically-behavioral attributes and features of semi-presidentialism. The author identified at least two groups of causes and factors, i.e. exogenous and endogenous, that define and categorize semi-presidentialism: the actual content of a constitution, the combination of traditions and circumstances (exogenous causes and factors), the composition of parliamentary majority and the position of a president in relation to parliamentary majority (endogenous causes and factors). It was motivated that, within the same constitutional delineation, a specific semi-presidential country can be classified in practice, first of all in the cut of varying institutional rules and formal/actual powers of presidents, governmental cabinets/ prime ministers and parliaments. The researcher found out that taking into account the place and role of political institutions in inter-party competition and party hierarchy has a significant influence on this process. That is why semi-presidentialism was generalized as a system of government, which is comprehensively updated and taxonomied formally and actually. At the same time, it was recorded that semi-presidentialism uses specific hierarchical and transactional relations in the triangle “the head of state–governmental cabinet–parliament”. Their combination affects the formal and actual positioning and taxonomy of different types of heterogeneous semi-presidentialism. It was generalized that semi-presidentialism is definitively and permanently characterized with: a restrained and moderate model of separation of powers and appropriate system of checks and balances; popular election of a president for a fixed term; the collective responsibility of a prime minister and a cabinet to a legislature; the actual “securitization” of a president from interference in his or her activities by other institutions and branches of state power; actual (or at least formal), but multi-step and different deconcentration or dualization of the executive between a president (the head of state) and a prime minister (the head of governmental cabinet); double or dual nature of the origin and implementation of the executive, but not a double or dual nature of the responsibility of the executive.

Against the odds of the ant competition hierarchy: submissive Myrmica rugulosa block access of the dominant Lasius fuliginosus to its aphids

Many ant assemblages are organised in competitive hierarchies. Myrmica rugulosa, a submissive species at the bottom of the ant competition hierarchy, temporarily took over groups of the myrmecophilic aphid Stomaphis quercus, which is strictly associated with the territorial ant species Lasius fuliginosus. Such previously unknown intervention happened repeatedly on the nest tree of L. fuliginosus. Masses of M. rugulosa intruded into bark crevices harbouring the aphids, and blocked with their own bodies access of L. fuliginosus to the aphids. Lasius fuliginosus showed no aggression towards the intruders, but walked around and on M. rugulosa clusters, palpating foreign ants with their antennae. Their only countermeasure was to briefly drag or carry individual M. rugulosa workers that pressed themselves down and froze for the moment, and then resumed their activities. We interpret the behaviour of M. rugulosa, in the context of its already known interspecific relations, as a specific competitive means of existence with higher-ranked species. The behaviour—by strict definition neither interference nor exploitation competition—effectively combines submissiveness, appeasement and tenacity. We discuss similarities between such behaviour and related behaviour reported earlier in a few other Myrmica species and Manica rubida while confronting physically stronger ants.

Plant species size and density‐dependent effects on growth and survival

AbstractQuestionExperimental evidence suggests that competition among plant species is generally hierarchical and that relatively large species are at a competitive advantage when competition is predominantly above‐ground. However, regional species pools are dominated numerically by relatively small plant species, and small species generally have higher local densities of resident plants within natural communities. One explanation is that larger plant species suffer disproportionately more under effects of intra‐specific competition (i.e. greater density dependence). We tested this prediction using ten herbaceous plant species in a competition experiment.LocationKingston, Ontario, Canada, glasshouse.MethodsUsing a glasshouse experiment, we tested whether relatively large species suffer disproportionately more in monoculture relative to mixtures of all ten herbaceous plant species. We measured the effects of competition on biomass production and survival by monitoring both in monocultures and mixtures of our species.ResultsLarger plant species suffered more under intra‐specific relative to diffuse inter‐specific competition in terms of survival; however, the slope of this relationship was not significantly greater than one, indicating that larger species did not suffer disproportionate density‐dependent suppression.ConclusionsOur results support a role for size in plant competition, but also indicate that this role is reduced because relatively larger species suffer greater density‐dependent mortality when competing with other, equally large plants. As such, size‐based competitive hierarchies may not function as clearly in natural systems because the increased negative density dependence for larger species contributes to balancing out competition across size hierarchies.

Community assembly by limiting similarity vs. competitive hierarchies: testing the consequences of dispersion of individual traits

Summary The degree of dispersion of trait values among species in a community has frequently been used to infer processes of community assembly. However, multiple assembly processes can lead to the same pattern of trait dispersion or the same process can lead to different patterns of dispersion. In particular, competitive processes can lead both to trait overdispersion (if the trait controls niche differentiation and only substantial differences allow coexistence) or to trait underdispersion (if the trait controls position in a competitive hierarchy and only similar values enhance coexistence). Because different traits are likely to contribute to stabilizing (niche differentiation) and equalizing (competitive hierarchies) mechanisms of coexistence, we compare the role of dispersion of a number of individual traits for species diversity by conducting in silico experiments using field‐parameterized spatially explicit models of communities of clonally growing plants. We manipulate both dispersion and means of different traits and examine consequent changes of species diversity in the whole community. We hypothesize that growth traits, which are directly linked to resource acquisition, are likely related to position in competitive hierarchies and thus diversity and trait dispersion will be negatively associated. In contrast, we hypothesize that architectural traits, which control spatial deployment of new plants and are thus less directly linked to resource acquisition, are more likely to be linked to niche differentiation and thus diversity and trait dispersion will be positively associated. Individual traits differed considerably in effects of trait dispersion on community diversity. Specifically, increasing dispersion in growth traits often decreased diversity, presumably reflecting widening competitive differences and exclusion of weaker competitors, which is consistent with the action of equalizing mechanisms of species coexistence. In contrast, increasing dispersion in architectural traits either increased diversity or had no effect, which possibly indicates niche‐based mechanisms of diversity maintenance. Changes of community‐wide trait means can have large effects on diversity for a given degree of dispersion, although not usually changing the sign of the relationship. Synthesis. These results suggest inference of community assembly processes from patterns of trait dispersion without understanding how particular traits function in community assembly may often be misleading. Effects of dispersion of traits that are likely associated with position in a competitive hierarchy are very different from those of traits associated with niche differences.

Correlates of tree species sorting along a temperature gradient in New Zealand rain forests: seedling functional traits, growth and shade tolerance

Summary It is widely believed that species sorting on environmental gradients reflects trade‐offs between competitive ability and physiological tolerance of stresses such as frost and desiccation. One specific expression of this general idea is the hypothesis that tree species sorting on temperature gradients in temperate regions involves a trade‐off between growth rate and frost resistance, because adaptations to frost reduce light interception and carbon gain potential. We measured seedling growth of 17 New Zealand rain forest angiosperm trees in a glasshouse, as well as biomass partitioning, gas exchange and hydraulic traits. We then related these variables to the mean July (winter) minimum temperatures most frequently experienced throughout the range of each species. Species associated with mild winters on average had wider vessels and more conductive stems and were leafier (i.e. developed more foliage area per unit sapwood area) than species from frostier sites. Species' positions on the temperature gradient were not significantly correlated with relative growth rates of seedlings; they were, however, negatively correlated with two measures of species light requirements in the field: the light compensation point for growth, and low‐light mortality rates obtained from the literature. Synthesis. Although seedling growth rates of warm‐ and cool‐temperate New Zealand angiosperm trees were similar on average, the former are more shade tolerant. Competitive hierarchies associated with tree species sorting on temperature gradients thus probably involve a shift in the relationship between shade tolerance and growth rate, rather than a simple trade‐off of growth with cold tolerance. This shift is associated with variation in light interception potential per unit of seedling biomass, possibly reflecting a trade‐off between stem conductivity and resistance to freeze‐thaw embolism.