Species interactions and random dispersal rather than habitat filtering drive community assembly during early plant succession

Theory on plant succession predicts a temporal increase in the complexity of spatial community structure and of competitive interactions: initially random occurrences of early colonising species shift towards spatially and competitively structured plant associations in later successional stages. Here we use long‐term data on early plant succession in a German post mining area to disentangle the importance of random colonisation, habitat filtering, and competition on the temporal and spatial development of plant community structure. We used species co‐occurrence analysis and a recently developed method for assessing competitive strength and hierarchies (transitive versus intransitive competitive orders) in multispecies communities. We found that species turnover decreased through time within interaction neighbourhoods, but increased through time outside interaction neighbourhoods. Successional change did not lead to modular community structure. After accounting for species richness effects, the strength of competitive interactions and the proportion of transitive competitive hierarchies increased through time. Although effects of habitat filtering were weak, random colonization and subsequent competitive interactions had strong effects on community structure. Because competitive strength and transitivity were poorly correlated with soil characteristics, there was little evidence for context dependent competitive strength associated with intransitive competitive hierarchies.     

Competitive interactions change the pattern of species co‐occurrences under neutral dispersal

Non‐random patterns of species segregation and aggregation within ecological communities are often interpreted as evidence for interspecific interactions. However, it is unclear whether theoretical models can predict such patterns and how environmental factors may modify the effects of species interactions on species co‐occurrence. Here we extend a spatially explicit neutral model by including competitive effects on birth and death probabilities to assess whether competition alone is able to produce non‐random patterns of species co‐occurrence. We show that transitive and intransitive competitive hierarchies alone (in the absence of environmental heterogeneity) are indeed able to generate non‐random patterns with commonly used metrics and null models. Moreover, even weak levels of intransitive competition can increase local species richness. However, there is no simple rule or consistent directional change towards aggregation or segregation caused by competitive interactions. Instead, the spatial pattern depends on both the type of species interaction and the strength of dispersal. We conclude that co‐occurrence analysis alone may not able to identify the underlying processes that generate the patterns.     

Trait plasticity alters the range of possible coexistence conditions in a competition–colonisation trade‐off

Most of the classical theory on species coexistence has been based on species‐level competitive trade‐offs. However, it is becoming apparent that plant species display high levels of trait plasticity. The implications of this plasticity are almost completely unknown for most coexistence theory. Here, we model a competition–colonisation trade‐off and incorporate trait plasticity to evaluate its effects on coexistence. Our simulations show that the classic competition–colonisation trade‐off is highly sensitive to environmental circumstances, and coexistence only occurs in narrow ranges of conditions. The inclusion of plasticity, which allows shifts in competitive hierarchies across the landscape, leads to coexistence across a much broader range of competitive and environmental conditions including disturbance levels, the magnitude of competitive differences between species, and landscape spatial patterning. Plasticity also increases the number of species that persist in simulations of multispecies assemblages. Plasticity may generally increase the robustness of coexistence mechanisms and be an important component of scaling coexistence theory to higher diversity communities.     

不同氮素水平下入侵种豚草与本地种黄花蒿、蒙古蒿的竞争关系

已有研究表明,土壤氮素增加可提高外来植物的入侵性,降低本地植物的竞争力.为揭示全球氮沉降对入侵种与本地种之间竞争关系的影响,我们于2010年5-8月在中国科学院北京森林生态系统定位研究站温室内,采用取代系列实验方法(standard replacement experiment),研究了3个氮素水平下入侵种豚草(Ambrosia artemisiifolia)与本地种黄花蒿(Artemisia annua)、蒙古蒿(Artemisia mongolica)的生长特征及种内、种间竞争关系的变化.实验采用双因素-随机区组设计,设置了低氮、中氮和高氮3个氮素水平,每一氮素水平分别设置豚草和黄花蒿、豚草和蒙古蒿组成的竞争实验,生长90 d后测量株高和生物量.结果表明:单栽情况下,随氮素水平的增加3个物种的株高均增加,而生物量均无显著变化;混栽情况下,3个物种株高和生物量随氮素水平的增加变化各异,豚草呈极显著增加趋势,而黄花蒿无明显变化,蒙古蒿则先增加后减少.豚草的快速生长使其在竞争中处于优势地位,对本地种黄花蒿和蒙古蒿产生明显的竞争效应.但不同氮素水平下,豚草对本地种的竞争力不同:低氮素水平下,豚草＜两个本地种;中氮素水平下,黄花蒿＜豚草＜蒙古蒿;高氮素水平下,豚草＞两个本地种.氮素添加显著提高了豚草的种间竞争力,改变了豚草与本地种之间的竞争关系,使竞争有利于入侵种.据此推测,在全球变化的背景下,氮沉降的增加将会促进外来种豚草的入侵,增加本地群落的可入侵性.     

Plant competitive ability and the transitivity of competitive hierarchies change with plant age

Plant competitive effect and response ability are known to change with plant age, however it remains unclear how competitive hierarchies among plant species change as plants age and transition between life stages. We examined the competitive interactions among seven species in all pairwise combinations in a greenhouse experiment. Competitive effect and response were measured as the relative yield (RY) for each target-neighbor species combination for both seedling and adult plants. Competitive hierarchies were constructed based on competitive effect and response scores, and we examined the degree of transitivity in the seedling and adult competitive hierarchies. The competitive effect hierarchy did not vary substantially with plant age, while the competitive response hierarchy was highly variable between juvenile and adult plants. Competitive effect and response ability were positively correlated at both plant stages. The seedling relative yield matrix was predominantly transitive, while there were far fewer transitive competitive relationships among the adult plants. The breakdown of the clear competitive hierarchy among seedlings as plants aged may explain why competition does not appear to be an active mechanism structuring some late-successional plant communities. In early-successional communities, interactions among seedlings with a clear competitive hierarchy may establish competitive ability—abundance relationships that persist as a legacy effect even though the breakdown of the competitive hierarchy among adult plants removes competition as an active mechanism structuring some late-successional plant communities.     

Secondary contact during adaptive radiation: a community matrix for Lake Malawi cichlids

We used a community of Lake Malawi rock‐dwelling cichlids to study secondary contact during adaptive radiation. Using abundance data from survey plots we constructed a matrix of pair‐wise interaction coefficients for males of 21 native and eight transplanted species. After controlling for the effects of habitat variation, correlations among residual male abundances suggest that coevolved species compete less than those brought into artificial secondary contact 30 years ago and that species with the same body colour compete more than those with different body colours. The latter result provides evidence that a trait related to reproductive isolation affects competitive interactions and the distribution of individuals throughout an entire community. Our results further suggest lake level fluctuations that divide and reconnect communities act to increase local (alpha), as well as total (gamma) diversity, in this adaptive radiation. The communities are not, however, unsaturated in the simplest sense; new species can enter a community, but they disproportionately reduce the abundance of original community members.     

Rules of the seed size game: contests between large‐seeded and small‐seeded species

The coexistence of multiple seed size strategies within plant communities have been considered puzzling, based on a theoretical expectation of the existence of an optimal seed size under each set of specific environmental conditions. A model aimed at explaining the coexistence of different seed sizes has been suggested, where a seed size – seed number tradeoff is connected to a tradeoff between competition and colonization, leading to a competitive advantage in larger‐seeded species and a colonization advantage in smaller‐seeded species. Recently an alternative model has been suggested, based on a tradeoff between stress tolerance and fecundity, associated with the variation from large to small seeds. Here, we examine the role of seed size for recruitment in two‐species contests subjected to various treatments. In a garden experiment seeds of 14 plant species were combined pair‐wise into seven pairs, each with one larger‐seeded species and one smaller‐seeded species. Each species‐pair was sown with sparse and dense seed densities and subjected to different treatments of shading and litter. Recruitment was recorded during two years. Our results showed a general advantage of larger‐seeded species over smaller‐seeded species. This seed size advantage increased in treatments with litter, whereas there were minor effects of shade, and no effect of seed density was found. We thus found little support for a density dependent seed size game as assumed in models of a competition‐colonization tradeoff, whereas our results fit well with a model based on a tradeoff between stress tolerance and fecundity. Our experiment provides novel empirical data to theoretical models on co‐existence between multiple seed size strategies.     

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

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Simulated herbivory counteracts the effects of grass competition on the invasive fireweed (Senecio madagascariensis)

Competition, herbivory and their interaction play a significant role in determining the competitive ability and survival of individual plant species. Understanding these processes and interactions can improve the efficacy of biocontrol programs against invasive weeds. Senecio madagascariensis (fireweed) is an invasive weed of South African origin that reduces pastoral productivity and poisons livestock in several countries, notably Australia. Although competitive pastures can suppress the weed’s growth in Australia, its competitive nature is poorly understood in relation to its invasion success. This greenhouse study assessed the growth and reproductive yield of fireweed growing in competition with six native and introduced grasses present in both South Africa and Australia. Since fireweed is a target for biocontrol in Australia, we examined whether its response to grass competition changed with herbivory (simulated by 40% leaf removal). The effect of grass competition and herbivory on the weed’s biomass and floral productivity was examined during a 12‐week pot trial in South Africa. Floral numbers were unaffected by both grass competition and herbivory. Biomass was used to calculate Relative Interaction Indices (RII) to quantify the weed’s competitive or facilitative response. This index compares a specific measurable trait, such as biomass, of fireweed growing alone, to fireweed growing with grass to determine the level of competitive suppression or facilitation resulting from the interaction. Despite the lack of species‐specific effects of grass competition, the presence of grass suppressed fireweed’s foliar, root and whole plant biomass the most when herbivory was absent. With herbivory, fireweed did not suffer from any measurable competitive suppression. This lack of competitive suppression may be due to an induced allelopathic response, given the levels of pyrrolizidine alkaloids common in many Senecio species. Since this result may weaken the case for biocontrol, the weed’s competitive responses should be verified in relation to actual insect herbivory.     

Competition-colonization trade-offs, competitive uncertainty, and the evolutionary assembly of species

We utilize a standard competition-colonization metapopulation model in order to study the evolutionary assembly of species. Based on earlier work showing how models assuming strict competitive hierarchies will likely lead to runaway evolution and self-extinction for all species, we adopt a continuous competition function that allows for levels of uncertainty in the outcome of competition. We then, by extending the standard patch-dynamic metapopulation model in order to include evolutionary dynamics, allow for the coevolution of species into stable communities composed of species with distinct limiting similarities. Runaway evolution towards stochastic extinction then becomes a limiting case controlled by the level of competitive uncertainty. We demonstrate how intermediate competitive uncertainty maximizes the equilibrium species richness as well as maximizes the adaptive radiation and self-assembly of species under adaptive dynamics with mutations of non-negligible size. By reconciling competition-colonization tradeoff theory with co-evolutionary dynamics, our results reveal the importance of intermediate levels of competitive uncertainty for the evolutionary assembly of species.     

Components of ‘competitive ability’ in the LHS model: Implication on coexistence for twelve co-occurring Mediterranean grasses

The leaf–height–seed (LHS) model has been proposed as a simple trait-based functional classification. We investigated whether the two components of competitive ability, i.e. competitive response (CR) and competitive effect (CE), are captured by the LHS model and whether these two components are independent for 12 coexisting Mediterranean grasses. Two greenhouse experiments were conducted to estimate competitive effect and response of 12 coexisting grass species from Mediterranean habitats in Jordan. We applied a phytometer design and calculated CR and CE using the relative interaction index (RII). Mature plant height, seed mass and leaf dry matter content (LDMC, used as the leaf trait) were measured for each species. Correlations and trade-offs between the three traits and the components of competitive ability, CR and CE, were analyzed with principal components analysis (PCA). The LHS model was a good predictor of competitive ability but CR and CE were independent and related to different traits. CR was positively correlated with seed mass and CE with plant height. LDMC was neither correlated to CR nor to CE. Based on these findings, we suggest that there are three primary strategies allowing coexistence in Mediterranean communities, which are related to competition: (1) large CE, i.e. large negative impact on other species associated with large stature, (2) large CR, i.e. resistance to competition associated with large seeds, and (3) competition avoidance associated with small seeds.     

Fear in the dark? Community‐level effects of non‐lethal predators change with light regime

The total effect of predators on prey is a combination of direct consumption, and non‐consumptive effects (NCEs), such as predator‐induced changes to prey morphology, behaviour and life history. Past research into NCEs has tended to focus on pair‐wise interactions between predators and prey, while in natural ecosystems, species exist in complex communities with several trophic levels made up of multiple autotrophic and heterotropic species. To address how predator NCEs alter the photosynthetic and heterotrophic components of communities, we exposed microbial microcosms to one of three predator treatments: live predators (full predator effect), freeze‐killed predators (NCEs only) or no predators (control), and incubated them under either 12 h:12 h light:dark conditions or continual darkness. Under 12 h:12 h light:dark conditions, NCEs‐only communities never differed from predator‐free communities, but differed from live predator communities. Under conditions of continual darkness, the structure of NCEs‐only communities differed from predator‐free controls, but not from live predator communities, suggesting NCEs can be strong enough to structure communities. Predation threat may cause certain prey to induce defences, such as reductions in movement, which make them less competitive in a community setting. This reduction in competitive ability could lead to these species being driven to extinction through interspecific competition, resulting in similar communities to those in which live predators are present. Heterotrophic species whose rates of resource acquisition depend on movement rates may be affected to a greater extent than autotrophs by predator‐induced reductions in movement, accounting for our observed differences in predator NCEs in ‘dark’ and ‘light’ communities. Our results suggest that the community‐level consequences of fear are greater in the dark. Synthesis Predators affect prey through consumptive and non‐consumptive effects (NCEs) such as alterations to prey behaviour, morphology, and life history. However, predators and prey do not exist in isolated pairs, but in complex communities where they interact with many other species. Using a long term study (>10 predator generations), we show that predator NCEs alone can alter community structure under conditions of darkness, but not in a 12h:12h light:dark cycle. Our results demonstrate for the first time that although the community‐level consequences of predator NCEs may be dramatic, they depend upon the abiotic conditions of the ecosystem.     

Effects of multiple competitors for pollination on bumblebee foraging patterns and Mimulus ringens reproductive success

When co‐occurring plant species overlap in flowering phenology they may compete for the service of shared pollinators. Competition for pollination may lower plant reproductive success by reducing the number of pollinator probes or by decreasing the quality of pollen transport to or from a focal species. Pair‐wise interactions between plants sharing pollinators have been well documented. However, relatively few studies have examined interactions for pollination among three or more plant species, and little is known about how the outcomes and mechanisms of competition for pollination may vary with competitor species composition. To better understand how the dynamics of competition for pollination may be influenced by changes in the number of competitors, we manipulated the presence of two competitors, Lythrum salicaria and Lobelia siphilitica, and quantified reproductive success for a third species, Mimulus ringens. Patterns of pollinator preference and interspecific transitions in mixed‐species arrays were significantly influenced by the species composition of competitor plants present. Both pair‐wise and three‐species competition treatments led to a similar ～ 40% reduction in Mimulus ringens seed set. However, the patterns of pollinator foraging we observed suggest that the relative importance of different mechanisms of competition for pollination may vary with the identity and number of competitors present. This variation in mechanisms of competition for pollination may be especially important in diverse plant communities where many species interact through shared pollinators.     

On the indices of plant–plant competition and their pitfalls

The index of relative competition intensity (RCI) has serious built‐in biases, due to its asymptotic behavior when competition intensity is high and its tendency to obtain very low values when plants with neighbors intact perform better than neighbor removal plants. These biases have been partially corrected in the index of relative neighbor effect (RNE), but the existence of fixed upper and lower bounds (?1≤RNE≤+1) still creates problems and biases in communities where the average intensity of competition or facilitation is high and plant performance pronouncedly varies in space. The third commonly used index, the logarithm of response ratio (lnRR), is mathematically and statistically sound, but when computed from pair‐wise comparisons between neighbor removal and control plants, this index reflects the geometric mean of the treatment effect. Moreover, linear patterns in lnRR reflect exponential patterns in the intensity of competition. As the interest of ecologists usually focuses on arithmetic means, we propose a corrected index of relative competition intensity, CRCI=arc sin (RNE). This index is fairly linear within the observed ranges of competition and facilitation, and for the range of competition intensities where RNE behaves reasonably, the two indices obtain almost identical values. We compared the performance of the four indices, using both imagined and real data, the latter from systems where the responses of plants to neighbor removal ranged from weak to moderate, so that RNE and CRCI were expected to behave similarly. The indices were computed both from pooled data for each community and as averages of pair‐wise comparisons. lnRR and CRCI were found to behave in a consistent and bias‐free manner, yielding similar results regardless of method of computation. This was, by and large, the case with RNE, too, but as the values of indices grew, the values from pair‐wise comparisons became increasingly smaller than values computed from pooled data. RCI yielded grossly aberrant results in computations based on pair‐wise comparisons. Therefore, the further use of RCI is unadvisable and studies where RCI has been derived from pair‐wise comparisons should be excluded from meta‐analyses.     

Interactions between root and shoot competition vary among species

Understanding how the competition varies with productivity is essential for differentiating among alternative models of plant community organization. Prior attempts to explain shifts in root and shoot competition along gradients have generally assumed an additive interaction between the two competitive forms, using an experimental design which does not fully separate both above‐ and belowground processes. At the most basic level, few field studies have separated root and shoot competition, and we have limited knowledge about both the relative importance of these processes, and how they interact to affect plant growth in the field. Presented here are findings from a field study in which root and shoot competition were experimentally separated by using root exclusion tubes and neighbor tiebacks in an early successional community. Individuals of four species (Abutilon theophrasti, Amaranthus retroflexus, Rumex crispus, and Plantago lanceolata) were grown at two levels of fertilization with full competition, aboveground competition only, belowground competition only, or neither above‐ nor belowground competition. Competition was measured as competitive response, which is the natural log of the relative biomass of a target plant grown with competition compared to growth without competition. In contrast to predictions from current models of productivity‐competition relationships, but in agreement with other experimental studies, there was no change in the strengths or root, shoot, or total competition with a modest increase in productivity. Despite no effect of fertilization on the strength of competition, the form of interaction between root and shoot competition varied both as a function of species identity and fertilization. For both of the rosette forming species, the combined effects of root and shoot competition were less than predicted assuming no interaction (a “negative interaction”), with one species switching from a negative to an additive interaction with fertilization. The fact that fertilization caused a shift in the root‐shoot interaction, but not in the total strength of root and shoot competition, suggests that the root‐shoot interaction is itself a highly labile variable. If root‐shoot interactions are common in natural systems, then simply measuring the strength of one form of competition in no way provides any information about the overall importance of that competitive form to plant growth.     

Resource competition and fire‐regulated nutrient demand in carnivorous plants of wet pine savannas

Question: What are the mechanisms by which fire reduces competition for both a short‐lived and a long‐lived species in old‐growth ground‐cover plant communities of wet pine savannas (originally Pinus palustris, replaced by P. elliottii)? Location: Outer coastal plain of southeastern Mississippi, USA. Methods: I reviewed previous competition experiments and proposed a new hypothesis to explain the relationship between fire, competition, and species co‐existence in wet longleaf pine savannas. The first study is about growth and seedling emergence responses of a short‐lived carnivorous plant, Drosera capillaris, to reduction in below‐ground competition and above‐ plus below‐ground competition. The second study deals with growth and survival responses of a long‐lived perennial carnivorous plant, Sarracenia alata, to neighbour removal and prey‐exclusion to determine if a reduction in nutrient supply increased the intensity of competition in this nutrient‐poor system. Results: Fire increased seedling emergence of the short‐lived species by reducing above‐ground competition through the destruction of above‐ground parts of plants and the combustion of associated litter. Prey exclusion did not increase competitive effects of neighbours on the long‐lived species. However, because the experiment was conducted in a year without fire, shade reduced nutrient demand, which may have obviated competition for soil nutrients between Sarracenia alata and its neighbours. Conclusion: Repeated fires likely interact with interspecific differences in nutrient uptake to simultaneously reduce both above‐ground competition and competition for nutrients in old‐growth ground cover communities in pine savannas. Restoration practitioners should consider the possibility that the composition of the plant community is just as important as fire in ensuring that frequent fires maintain species diversity.     

Intransitive competition and its effects on community functional diversity

Can competitive interactions be inferred from the analysis of community functional diversity patterns? Originally, at the scale of a community, competitive interactions were supposed to generate trait overdispersion patterns due to limiting similarity process. More recently, by highlighting the importance of competitive hierarchies, it has been shown that when only one resource limits species coexistence, competition can also lead to patterns of trait clustering. However, these two expectations (overdispersion and clustering) ignore potential multi‐species indirect competitive interactions, and especially intransitive competition. Indeed, little is yet known about intransitive competition and its influence on community's functional diversity. Here I propose a brief appraisal of empirical evidence for intransitive competition in nature, and an overview of the current understanding of this mechanism and its properties. I then demonstrate with a theoretical model that intransitive competitive interactions can actually generate random‐like functional diversity patterns. The variety of diversity patterns (overdispersion, clustering, randomness) that can emerge from diverse types of competitive interactions makes it difficult to identify the presence of competition in nature, potentially leading to an underestimation of its importance as a structuring force. New methodologies able to capture both simple and complex competition mechanisms are thus urgently needed.     

Networks and dominance hierarchies: does interspecific aggression explain flower partitioning among stingless bees?

1. The distribution of consumers among resources (trophic interaction network) may be shaped by asymmetric competition. Dominance hierarchy models predict that asymmetric interference competition leads to a domination of high quality resources by hierarchically superior species. 2. In order to determine the competitive dominance hierarchy and its effect on flower partitioning in a local stingless bee community in Borneo, interspecific aggressions were tested among eight species in arena experiments. 3. All species tested were strongly mutually aggressive in the arena, and the observed interactions were often lethal for one or both opponents. Aggression significantly increased with body size differences between fighting pairs and was asymmetric: larger aggressors were superior over smaller species. Additional aggression tests involved dummies with surface extracts, and results suggest that species‐ and colony‐specific surface profiles are important in triggering the aggressive behaviour. 4. Sixteen stingless bee species were observed foraging on 41 species of flowering plants. The resulting bee–flower interaction network showed a high degree of generalisation (network‐level specialisation H₂’ = 0.11), corresponding to a random, opportunistic distribution of bee species among available flower species. 5. Aggressions on flowers were rare and only occurred at a low level. The dominance hierarchy obtained in the arena experiments did not correlate significantly with plant quality, estimated as the number of flowers per plant or as total bee visitation rate. 6. Our findings suggest that asymmetries in interference competition do not necessarily translate into actual resource partitioning in the context of complex interacting communities.     

Scale, Experimental Design and the Detection of Ineterspecific Competition within Plant Communities

Abstract We report preliminary results of a series of experiments designed to explore the importance of interspecific competition within arable weed communities at different scales. Competition hierarchies were apparent from a pot experiment with different levels of nutrients and water. Two field experiments looked at Bromus sterilis, Galium aparine and Papaver rhoeas in winter wheat in the field, in a range of combinations and management treatments, and a fourth field experiment included a wider variety of species. There was little effect of fertilizer on population behaviour in the the field. Bromus increased around ten fold per year on minimum-tilled plots, regardless of other treatments. Galium increased on organically-fertilized and minimum-tilled plots, but only in the absence of Bromus. Papaver densities remained low, but again were depressed in the presence of high densities of Bromus . Taken together, the experiments demonstrate the existence of competition between weed species. However, as the design of the experiment increased to include greater levels of environmental variation, so competition became more difficult to detect, and less useful for interpreting the results than knowledge of the biology of the individual species. At the scale of interest to the farmer, the level of competition is not a good predictor for weed population dynamics.     

Estimating competition coefficients: strong competition among three species of frugivorous flies

Despite the abundance of studies on competitive interactions, relatively few experiments have been used to fit explicit competition models and estimate competition coefficients. Such estimates are valuable for making contact between theoretical and empirical studies, which tend to measure competition in different units. To quantify the strength of competitive interactions among the larvae of three species of frugivorous flies, I manipulated the densities of each species to investigate all three pairwise interactions. The densities of each species were changed independently (i.e., using a response surface experimental design), which allowed maximum likelihood estimation of the competition coefficients for each species, based on the Hassell and Comins competition model. The effects of competitor density on larval survival, time to emergence, and the weight of emerging adults were also analyzed to investigate the responses of individual species to density. The estimates of the competition coefficients suggest that the larvae of these flies experience strong asymmetric competition for resources, and raise questions as to how these species coexist. For each pair, one of the species was largely unaffected by interspecific competition, but decreased the performance of the other. Received: 8 February 1999 / Accepted: 5 April 1999