Convergence patterns and multiple species interactions in a designed plant mixture of five species

It is known that convergence and divergence can occur in complex plant communities, but the relative importance of biotic and abiotic factors driving these processes is less clear. We addressed this issue in an experiment using a range of mixed stands of five species that are common in Swiss fens (Carex elata, C. flava, Lycopus europaeus, Lysimachia vulgaris and Mentha aquatica) and two levels of water and nutrients. One hundred and seventy-six experimental mixtures were maintained in large pots (75 l) for two consecutive growing seasons in an experimental garden. The stands varied systematically in the initial relative abundance of each of the five species and in overall initial stand abundance. The changes in biomass over 2 years were modelled as linear functions of treatments and the initial biomass of each species. The dynamics of the system were mainly driven by differences in the identity of species and by a negative feedback mechanism but also by different abiotic conditions. In all mixtures, C. elata became more dominant over time, which caused an overall convergence of community composition. In addition, the rate of change of each species’ biomass was negatively related to its own initial abundance. Thus, a negative feedback further contributed to the convergence of communities. Species responded differently to water level and nutrient supply, causing community dynamics to differ among treatments. However, the different abiotic conditions only slightly modified the overall convergence pattern. Competitive interactions between more than two species were weaker than the negative feedback but still significantly influenced the species’ final relative abundance. The negative feedback suggests that there is niche partitioning between the species, which permits their coexistence. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Positive interactions in plant communities and the individualistic-continuum concept

The individualistic nature of communities is held as a fundamental ecological tenet by many ecologists. The empirical rationale for the individualistic hypothesis is largely based on gradient analyses in which plant species are almost always found to be arranged independently of one another in “continua” along environmental gradients. However, continua are correlative patterns and do not identify the processes that determine them, and so they do not necessarily preclude the possibility of interdependent interactions within plant communities. For example, the common occurrence of positive interactions suggests that plant species may not always be distributed independently of each other. If the distributions and abundances of species are enhanced by the presence of other species, their organization is not merely a coincidence of similar adaptation to the abiotic environment. Interpretations of gradient analyses also appear to assume that interactions among species should be similar at all points along environmental axes, and that groups of species should be associated at all points on a gradient if interdependence is to be accepted. However, virtually all types of ecological interactions have been shown to vary with changes in the abiotic environment, and a number of field experiments indicate that positive effects become stronger as abiotic stress increases. Furthermore, interactions among plants have been shown to shift from competition to facilitation along environmental continua. Thus, significant interdependence may occur even when species do not fully overlap in distribution. Higher-order, indirect interactions between animals and plants, and among plants, also suggest that interdependence within communities occurs. Eliminating a species involved in an indirect interaction may not necessarily mean that its beneficiary will be eliminated from a community, but the prospect that the distribution and abundance of any species in a plant community may be positively affected by the effects that other species have on their competitors suggests that communities are organized by much more than “the fluctuating and fortuitous immigration of plants and an equally fluctuating and variable environment” as stated by Henry Gleason. The ubiquity of direct and indirect positive interactions within plant communities provides a strong argument that communities are more interdependent than current theories allow. Received: 17 February 1997 / Accepted: 23 May 1997     

Resource limitation,habitat segregation,and species interactions of british tree-hole mosquitoes in nature

Summary The insect fauna of water-filled tree holes in southern Britain consists primarily of the mosquitoes Aedes geniculatus, Anopheles plumbeus, Culex torrentium, and a benthic detritivorous fauna that includes primarily the scirtid beetle Prionocyphon serricornis and the chironomid midge Metriocnemus martinii. Culex torrentium has been documented only relatively recently in tree holes but all three species of mosquitoes partition the resource in space and time. When mosquito larvae were forced to coexist in natural tree holes at limiting densities and at higher than natural levels of interspecific encounter, there was no evidence that Aedes geniculatus or Anopheles plumbeus affected pupation success, pupal weight, or development time of the other or that either Aedes geniculatus or C. torrentium affected the survivorship, pupation success, pupal weight, and biomass yield of the other. When A. geniculatus at limiting densities were forced in natural tree holes to live without or to coexist with natural or twice natural densities of P. serricornis and M. martinii, the presence, absence, or superabundance of the benthic insects did not affect pupation success or pupal weight of A. geniculatus; development time of A. geniculatus was faster when a superabundance of the benthic fauna was present. Effects of the benthic fauna on A. geniculatus are slight and the only significant interaction is facilitative, not competitive. The pattern of habitat segregation among treehole mosquitoes in southern Britain is characteristic of their respective genera and we propose that this pattern is more likely (but not certain) to have arisen through a process of independent evolution than through competitively driven niche shifts among already coexisting species.     

The role of reproductive plant traits and biotic interactions in the dynamics of semi-arid plant communities

The dynamics of semi-arid plant communities are determined by the interplay between competition and facilitation among plants. The sign and strength of these biotic interactions depend on plant traits. However, the relationships between plant traits and biotic interactions, and the consequences for plant communities are still poorly understood. Our objective here was to investigate, with a modelling approach, the role of plant reproductive traits on biotic interactions, and the consequences for processes such as plant succession and invasion. The dynamics of two plant types were modelled with a spatially-explicit integrodifferential model: (1) a plant with seed dispersal (colonizer of bare soil) and (2) a plant with local vegetative propagation (local competitor). Both plant types were involved in facilitation due to a local positive feedback between vegetation biomass and soil water availability, which promoted establishment and growth. Plants in the system also competed for limited water. The efficiency in water acquisition (dependent on reproductive and growth plant traits) determined which plant type dominated the community at the steady state. Facilitative interactions between plant types also played an important role in the community dynamics, promoting establishment in the driest conditions and recovery from low biomass. Plants with vegetative propagation took advantage of the ability of seed dispersers to establish on bare soil from a low initial biomass. Seed dispersers were good invaders, maintained high biomass at intermediate and high rainfall and showed a high ability in taking profit from the positive feedback originated by plants with vegetative propagation under the driest conditions. However, seed dispersers lost competitiveness with an increasing investment in fecundity. All together, our results showed that reproductive plant traits can affect the balance between facilitative and competitive interactions. Understanding this effect of plant traits on biotic interactions provides insights in processes such as plant succession and shrub encroachment.     

Detection of alternative stable states in marine communities

During the last 20 years there have been several attempts to test the theory of alternative stable states using marine systems. The results have been mixed, and there have been conflicting interpretations of the outcomes. In an attempt to resolve some of the problems, the theory and evidence for alternative stable states are reviewed. There are several different views of what is meant by alternative stable states, and so the key elements of the theory of alternative stable states are set in the context of marine systems. Appropriate experimental designs for detecting alternative states are discussed, and recent experimental studies in coral reefs, rocky intertidal systems, soft-sediment beds, and subtidal systems are briefly reviewed. Suggestions for improving experimental designs are proposed and unresolved issues are highlighted.     

植物邻体间的正相互作用

植物间的正负相互作用是构建植被群落的重要因素,也是群落生态学研究的中心内容之一。近20a来,植物间正相互作用的研究得到快速发展。综述了正相互作用的定义,不同植物群落中的直接、间接正相互作用及其发生机制,正相互作用研究的实验和模型方法,正负相互作用随胁迫梯度的变化及正相互作用对群落构建的影响。探讨了正相互作用研究前景:(1)进一步理解正负相互作用的平衡及其对群落构建的影响;(2)加深对全球变暖背景下的正相互作用的认识;(3)需把正相互作用研究同进化联系起来;(4)充分发挥正相互作用在生态系统中的推动力作用,把正相互作用应用到生态恢复中,为恢复退化生态系统服务。     

Competitive coexistence in a dynamic landscape

This paper investigates the effect of a dynamic landscape on the persistence of many interacting species. We develop a multi-species community model with an evolving landscape in which the creation and destruction of habitat are dynamic and local in space. Species interactions are also local involving hierarchical competitive trade-offs. We show that dynamic landscapes can reverse the trend of increasing species richness with higher fragmentation observed in static landscapes. The increase in the species-area exponent from a homogeneous to a fragmented landscape does not occur when dynamics are turned on. Thus, temporal aspects of the processes that generate and destroy habitat appear dominant relative to spatial characteristics. We also demonstrate, however, that temporal and spatial aspects interact to influence the persistence time of individual species, and therefore, rank-abundance curves. Specifically, persistence in the model increases in habitats with faster local turnover because of the presence of dynamic corridors.     

The ecology of restoration: historical links, emerging issues and unexplored realms

Restoration ecology is a young academic field, but one with enough history to judge it against past and current expectations of the science's potential. The practice of ecological restoration has been identified as providing ideal experimental settings for tests of ecological theory; restoration was to be the 'acid test' of our ecological understanding. Over the past decade, restoration science has gained a strong academic foothold, addressing problems faced by restoration practitioners, bringing new focus to existing ecological theory and fostering a handful of novel ecological ideas. In particular, recent advances in plant community ecology have been strongly linked with issues in ecological restoration. Evolving models of succession, assembly and state-transition are at the heart of both community ecology and ecological restoration. Recent research on seed and recruitment limitation, soil processes, and diversity–function relationships also share strong links to restoration. Further opportunities may lie ahead in the ecology of plant ontogeny, and on the effects of contingency, such as year effects and priority effects. Ecology may inform current restoration practice, but there is considerable room for greater integration between academic scientists and restoration practitioners.     

Positive vs. negative interactions in Picris hieracioides L., a mid-successional species of Mediterranean secondary succession

A field experiment was designed to evaluate the importance offacilitative and competitive interactions in Picrishieracioides, a facultative biennial that colonises the early andthemid-stages of secondary succession in the Mediterranean region. Seedlings ofPicris hieracioides from populations of the early- (1year)and the mid-stages of field abandonment (15–40 years) were transplantedintwo adjacent old fields, abandoned for 4 (F4) and 20 years (F20) and thatdiffered markedly in floristic composition and vegetation structure. For twoyears, we experimentally manipulated competition (no-neighbours vs. naturalvegetation) and resource availability (addition of water and fertiliser vs.controls) in an attempt to evaluate their influence on survival, reproductivetiming, growth and reproductive output throughout the life cycle. Earlymortality was higher in non vegetated plots in both fields. Mortality ofseedlings was mainly due to herbivory by larvae of genusAgriotes. Flowering throughout the whole experiment wasalso facilitated by vegetation in the F4 field as a result of the positiveeffect of annual vegetation and remained unaffected in the F20 field because ofthe high competitive effect of established perennial vegetation. The additionofresources altered the effect of facilitation and competition on late seedlingsurvival. Survival was enhanced in the vegetated plots of the F4 field, becauseresource addition increased the shade provided by the canopy of vegetation,protected seedlings from temperature extremes and reduced water loss. Seedlingmortality also decreased in the F20 field but in a similar manner to vegetatedand non-vegetated subplots, and consequently the outcome of positive andnegative interactions remained neutral. The net effect of facilitation andcompetition resulted in interference later in the life cycle and appearedthrough final lower growth and reproduction in both fields. However, thegreatercompetition in 1994 than in 1995 in both fields, probably because the size ofthe rosettes, makes them less susceptible to competition, illustrates thedifficulty in predicting the outcome of competition solely of one season forfacultative biennial plants. The relative competition intensity calculatedusingonly survivors (RCI₁) was unaffected by habitat fertility in bothfields. In striking contrast, the relative competition intensity calculatedusing seedling mortality (RCI₂) was significantly higher in subplotswithout resource addition in both fields because of high seedling mortality invegetated subplots. Finally, there were no differences in the net effect offacilitation and interference processes among populations from early and midsuccessional stages showing that phenotypic plasticity buffers theenvironmentalselective pressures linked to successional processes.     

Plant traits,species pools and the prediction of relative abundance in plant communities: a maximum entropy approach

Questions: To what extent can Shipley et al.'s original maximum entropy model of trait‐based community assembly predict relative abundances of species over a large (3000 km²) landscape? How does variation in the species pool affect predictive ability of the model? How might the effects of missing traits be detected? How can non‐trait‐based processes be incorporated into the model? Location: Central England. Material and Methods: Using 10 traits measured on 506 plant species from 1308 1‐m² plots collected over 3000 km² in central England, we tested one aspect of Shipley et al.'s original maximum entropy model of “pure” trait‐based community assembly (S₁), and modified it to represent both a neutral (S₂) and a hybrid (S₃) scenario of community assembly at the local level. Predictive ability of the three corresponding models was determined with different species pool sizes (30, 60, 100 and 506 species). Statistical significance was tested using a distribution‐free permutation test. Results: Predictive ability was high and significantly different from random expectations in S₁. Predictive ability was low but significant in S₂. Highest predictive ability occurred when both neutral and trait‐based processes were included in the model (S₃). Increasing the pool size decreased predictive ability, but less so in S₃. Incorporating habitat affinity (to indicate missing traits) increased predictive ability. Conclusions: The measured functional traits were significantly related to species relative abundance. Our results both confirm the generality of the original model but also highlight the importance of (i) taking into account neutral processes during assembly of a plant community, and (ii) properly defining the species pool.     

Perception,pattern, chance and the structure of reef fish communities

The recent history of attempts to understand the ecology of fish on coral reefs is surveyed as an example of the way in which science progresses. Scientists are trapped, both by their sensory equipment and by their preconceptions, into viewing the world in certain ways. Paradigms fall only slowly. Scientists are trained to seek pattern in their data, yet in some cases the largely stochastic variation of a system around its mean condition is the more important key to understanding its ecology. The reef fish assemblage provides such a case. It is unlikely that our present understanding of the nature of reef fish communities will survive unchanged by future research. And it is also unlikely that reef fish ecologists are the only ecologists who have difficulty discovering truth! Editorial     

Community‐level consequences of species interactions in an annual plant community

Question: How does the intensity of species interactions affect species and functional group composition of an annual plant community? Location: Sede Boqer, Negev Desert, Israel. Methods: The potential for competitive interactions in two annual plant communities (desert and coastal) from semi‐stabilized sand dunes was manipulated by varying seed bank density and therefore the number of potentially interacting individuals. Communities were exposed to three different irrigation regimes, mimicking precipitation at the desert site, the coastal site, and an intermediate precipitation level. Plots were maintained for 3 years, and percentage cover of each species in the plots was recorded at the end of each growing season. We used redundancy analysis to test for effects of initial density, irrigation, and year on the species and functional group composition of the communities. Results: Initial density had significant effects on species composition, and these effects remained significant over 3 years, even as total community percentage cover became more similar among treatments over time. Density effects did not depend on resource availability (irrigation level). Functional group identity or individual plant size did not predict which species would be good competitors, and a species' competitive ability did not predict its abundance in the field. Conclusions: Species interactions strongly affect community composition, and those effects carry over into subsequent years such that competition does not lead to convergence in community structure over time. However, the particular changes in composition observed were not predictable by some of the traits that have been found important in individual‐level experiments. We speculate that the outcome of competition in diverse communities will depend on multiple traits, in contrast to the outcome of individual‐level pairwise experiments. We also speculate that the shift in composition with density could mean that local variation in density may contribute to maintenance of diversity in this system.     

Competition and coexistence in submerged aquatic plant communities: the effects of species interactions versus abiotic factors

SUMMARY. 1. Aquatic plant biomass and species composition were studied at three sites in Long Lake, Alberta, Canada, to examine the role of biotic and abiotic factors in determining species diversity.
2. Results of controlled in situ experiments to test for interspecific competition showed that biomass of plants in four different mixed communities did not increase in response to selected species removals ( P >0.1, n =16).
3. In contrast, biomass of the dominant taxa (Myriophyllum exalbescens, Ceratophyllum demersum and Chara sp.) were correlated ( P <0.05) with abiotic factors (i.e. distance from shore, water depth, sediment exchangeable phosphorus concentration and/or sediment organic content).
4. These results suggest that interspecific competition between naturally coexisting species of submerged aquatic plants is slight and that spatial heterogeneity or differential utilization of abiotic resources promotes species diversity in submerged plant communities.     

Butterfly,spider, and plant communities in different land-use types in Sardinia,Italy

Butterfly, spider, and plant species richness and diversity were investigated in five different land-use types in Sardinia. In 16 one-hectare plots we measured a set of 15 environmental variables to detect the most important factors determining patterns of variation in species richness, particularly endemicity. The studied land-use types encompassed homogeneous and heterogeneous shrublands, shrublands with tree-overstorey, Quercus forest and agricultural land. A total of 30 butterfly species, among which 10 endemics, and 50 spider (morpho)species, were recorded. Butterfly and spider community composition differed according to land-use type. The main environmental factors determining diversity patterns in butterflies were the presence of flowers and trees. Spiders reacted mainly to habitat heterogeneity and land-use type. Traditional land-use did not have adverse effects on the diversity of butterflies, spiders, or plants. The number of endemic butterfly species per treatment increased with total species richness and altitude. Butterfly and spider richness did not co-vary across the five land-use types. Butterflies were, however, positively associated with plant species richness and elevation, whereas spiders were not. Conclusively, butterflies did not appear to be good indicators for spider diversity and species richness at the studied sites.     

Abiotic and biotic resistance to grass invasion in serpentine annual plant communities

Biological invasions severely impact native plant communities, causing dramatic shifts in species composition and the restriction of native species to spatially isolated refuges. Competition from resident species and the interaction between resource limitation and competition have been overlooked as mechanisms of community resistance in refugia habitats. We examined the importance of these factors in determining the resistance of California serpentine plant communities to invasion by three common European grasses, Avena barbata, Bromus diandrus, and Hordeum murinum. We added seeds of each of these grasses to plots subjected to six levels of resource addition (N, P, Ca, H₂O, all resources together, and a no-addition control) and two levels of competition (with resident community present or removed). Resource limitation and competition had strong effects on the biomass and reproduction of the three invaders. The addition of all resources together combined with the removal of the resident community yielded individual plants that were fourfold to 20-fold larger and sixfold to 20-fold more fecund than plants from control plots. Competitor removal alone yielded invaders that were twofold to sevenfold larger and twofold to ninefold more fecund. N addition alone or in combination with other resources led to a twofold to ninefold increase in the biomass and fecundity of the invaders. No other resource alone significantly affected native or invader performance, suggesting that N was the key limiting resource during our experiment. We found a significant interaction between abiotic and biotic resistance for Bromus, which experienced increased competitive suppression in fertilized plots. The threefold increase in resident biomass with N addition was likely responsible for this result. Our results confirm that serpentine plant communities are severely N limited, which, in combination with competition from resident species, promotes the resistance of these systems to invasions. Our work suggests that better understanding the relative sensitivities of invaders and residents to the physical environment is critical to predicting how abiotic and biotic factors interact to determine community resistance.     

Biotic relations affecting species structure in zooplankton communities

Mathematical modelling and laboratory experiments were used to study how exploitative competition and predation influence the species structure in cladoceran community. For five species of Cladocera (Sida crystallina, Daphnia magna, Simocephalus vetulus, Daphnia longispina, and Diaphanosoma brachyurum), representing a gradient of body size, population characteristics were described as functions of food concentration. Abundance dynamics were simulated in mixed species cultures and invasion experiments under different levels of food supply corresponding to oligo-, meso-, and eutrophic conditions. Separate simulations were also run including and excluding (fish) predation. The competitive ability of each species was estimated as the values of the population equilibrium food concentration. Simulation results showed that for the no-predator scenario, increases in the level of food supply promoted species coexistence while under lower food concentrations only one species remained at the end of the simulation runs. When predation was allowed, the number of species that coexisted at the end of the simulations increased up to four species, indicating that predator pressure facilitated species coexistence because it shortened periods of food depletion. Simulation results were verified in laboratory experiments which suggested that population equilibrium food concentration can be used as an estimate of competitive ability. Finally, species structure and relative abundance in Lake Naroch (Belarus) during the summer of 2004 was found to be consistent with our results from computer simulation and laboratory experiments with regard to competition and predation impacts on zooplankton community. Handling editor: S. I. Dodson     

Effects of variation in individual growth on plant species coexistence

Abstract. Both size structure and variability (spatial heterogeneity, disturbance, stochasticity, variation in species attributes, etc.) are regarded as regulatory mechanisms of species coexistence. However, none of the models so far proposed consider both size structure and variability simultaneously. A size-structured variation model for plant-community dynamics is proposed, which is based on the diffusion model for growth dynamics of plant populations. This model has four functions: (1) mean growth rate of individuals of size x at time t, G(t, x) (species-specific mean traits, e.g. competitive ability); (2) variance in growth rate of individuals of size x at time t, D(t, x) (stochastic factors due to genetic variation, environmental heterogeneity, spatial variation of individuals, etc.); (3) mortality rate of individuals of size x at time t, M(t, x); and (4) recruitment rate at time t, R(t), as a boundary condition. The interference function for individuals of size x at time t, C(t, x), is introduced, which expresses the degree of interactions between individuals and hence averaged effects of local neighbourhood competition; the G(t, x), D(t, x), M(t, x) and R(t) functions are given in terms of C(t, x). These four functions describe the growth dynamics of individuals of each species in the plant community. Effects of the G(t, x), D(t, x), M(t, x) and R(t) functions on species coexistence in plant communities were evaluated by simulation and the relative importance of the D(t, x) function as well as size structure was shown for species coexistence especially in plant communities where competition among species is non-transitive or niche limitation does not work.