Competition and facilitation within and between a snail and a mayfly larva and the effect on the grazing process

We studied the competitive effects within and between two taxonomically distant freshwater herbivores, a snail and a mayfly, common in Swedish lakes, Lymnaea peregra and Cloeon dipterum, respectively, and their effect on grazing in a laboratory experiment. The experimental set-up consisted of 2-l aquaria, each containing a periphyton covered tile. Intra- and interspecific effects were tested by increasing the density of one species at a time in four different treatments, (1) snails (intraspecific treatment), (2) mayflies (intraspecific treatment), (3) mixed-snails (interspecific treatments, snails kept constant) and (4) mixed-mayflies (interspecific treatments, mayflies kept constant). Intraspecific competition affected both snails and mayflies negatively, i.e. increasing mortality with increasing con-specific density. Furthermore, there was a decrease in snail growth with increasing snail density. In the mixed-species treatments both species changed their microhabitat use indicating interspecific competition. Despite this, we also found a positive effect of mayfly density on snail growth, most likely due to indirect commensalism. No density-dependent effect of grazing on periphyton was found, probably due to interference competition between grazers. However, there was a significant difference in periphyton biomass, due to species composition of grazers. Irrespective of their densities, if they co-existed, the two grazer species decreased the periphyton biomass significantly compared with both single-species treatments. We considered this as a joint action of facilitation and interaction. Our results suggest that competition can be an important structuring factor in macroinvertebrate communities and that species composition can be significant for ecosystem processes within lentic environments.     

Intra- and interspecific competition among coexisting lotic snails

The competitive interactions of two lotic snails, Elimia cahawbensis and Elimia carinifera , were examined in a second-order spring-fed stream. We first demonstrated food limitation in laboratory microcosms where snails grew faster when exposed to enhanced periphyton levels. We then tested the magnitude and relative strengths of intra- and interspecific competition in similar stream and laboratory mesocosm experiments. Treatments were maintained in Plexiglas enclosures over a 7-week period with 0, 1×, 2× and 4× ambient biomass of each species alone, as well as mixed species treatments at 2× and 4× ambient. Snail responses to treatments were almost identical in field and laboratory experiments. Growth rates of both species were reduced by increased density of snails indicating strong intra- and interspecific competition among E. cahawbensis and E. carinifera . An analysis of the strengths of intra- and interspecific competition indicated minimal differences for either species, implying a lack of competitive dominance. Although periphyton biomass was generally highest without snails, there was little difference in periphyton biomass and snail production over the four-fold density range, regardless of species composition. These results suggest that E. cahawbensis and E. carinifera are functionally redundant with density-dependent responses in growth rate resulting in similar grazing pressure across a density gradient. This clearly demonstrates that species impact is not necessarily reflected by measures of abundance or biomass, and that secondary production should be considered.     

Competitive Outcome of Daphnia-Simocephalus Experimental Microcosms: Salinity versus Priority Effects

Competition is a major driving force in freshwaters, especially given the cyclic nature and dynamics of pelagic food webs. Competition is especially important in the initial species assortment during colonization and re-colonization events, which depends strongly on the environmental context. Subtle changes, such as saline intrusion, may disrupt competitive relationships and, thus, influence community composition. Bearing this in mind, our objective was to assess whether low salinity levels (using NaCl as a proxy) alter the competitive outcome (measured as the rate of population biomass increase) of Daphnia-Simocephalus experimental microcosms, taking into account interactions with priority effects (sequential species arrival order). With this approach, we aimed to experimentally demonstrate a putative mechanism of differential species sorting in brackish environments or in freshwaters facing secondary salinization. Experiments considered three salinity levels, regarding NaCl added (0.00, 0.75 and 1.50 g L⁻¹), crossed with three competition scenarios (no priority, priority of Daphnia over Simocephalus, and vice-versa). At lower NaCl concentrations (0.00 and 0.75 g L⁻¹), Daphnia was a significantly superior competitor, irrespective of the species inoculation order, suggesting negligible priority effects. However, the strong decrease in Daphnia population growth at 1.50 g L⁻¹ alleviated the competitive pressure on Simocephalus, causing an inversion of the competitive outcome in favour of Simocephalus. The intensity of this inversion depended on the competition scenario. This salinity-mediated disruption of the competitive outcome demonstrates that subtle environmental changes produce indirect effects in key ecological mechanisms, thus altering community composition, which may lead to serious implications in terms of ecosystem functioning (e.g. lake regime shifts due to reduced grazing) and biodiversity.     

Differential herbivory tolerance of dominant and subordinate plant species along gradients of nutrient availability and competition

We tested whether differences in the herbivory tolerance of plant species is related to their abundance in grassland communities and how herbivory and nutrient availability affect competitive balances among plant species through changes in their tolerance. The experimental approach involved a simulated grazing treatment (clipping) of two competitive grass species (Arrhenatherum elatius and Holcus lanatus) and two subordinate forb species (Prunella vulgaris and Lotus corniculatus) along a gradient of nutrient availability and under conditions of competition. Total standing, aboveground, root, and regrowth biomass were evaluated at the end of the experiment as an estimate of the capacity to compensate for twice removing aboveground biomass at different nutrient levels (NPK). Although clipping had a more pronounced negative effect on dominant plant species (Arrhenatherum and Holcus) than on subordinate species, the negative effects on dominant species were offset by the application of fertilizer. The combined effect of fertilizer and competition had more negative effects on the performance of Lotus and Prunella than on the dominant species. In terms of competition, the regrowth ability of Arrhenatherum and Holcus increased with the application of fertilizer, while the opposite pattern was observed for Lotus and Prunella. The addition of fertilizer has a positive effect on both grass species in terms of growth in clipped pots and competition, while subordinate species did not respond to the addition of fertilizer to the clipped pots and were negatively affected by competition with both grass species. The results suggest (1) that species replacement towards subordinate species as a function of herbivory is partially dependent on the herbivory tolerance of that species, (2) competitive relations between competitive grass species and subordinate forb species change under different environmental conditions, and (3) although grazing disturbance significantly influences competitive relations in favor of less competitive species, increasing nutrient levels counteract the negative effect of grazing on dominant competitive plant species.     

Relative importance of competition and plant–soil feedback,their synergy,context dependency and implications for coexistence

Plants interact simultaneously with each other and with soil biota, yet the relative importance of competition vs. plant–soil feedback (PSF) on plant performance is poorly understood. Using a meta‐analysis of 38 published studies and 150 plant species, we show that effects of interspecific competition (either growing plants with a competitor or singly, or comparing inter‐ vs. intraspecific competition) and PSF (comparing home vs. away soil, live vs. sterile soil, or control vs. fungicide‐treated soil) depended on treatments but were predominantly negative, broadly comparable in magnitude, and additive or synergistic. Stronger competitors experienced more negative PSF than weaker competitors when controlling for density (inter‐ to intraspecific competition), suggesting that PSF could prevent competitive dominance and promote coexistence. When competition was measured against plants growing singly, the strength of competition overwhelmed PSF, indicating that the relative importance of PSF may depend not only on neighbour identity but also density. We evaluate how competition and PSFs might interact across resource gradients; PSF will likely strengthen competitive interactions in high resource environments and enhance facilitative interactions in low‐resource environments. Finally, we provide a framework for filling key knowledge gaps and advancing our understanding of how these biotic interactions influence community structure.     

Investigations of density interactions among breeding birds in ponderosa pine forests: correlative and experimental evidence

Summary We assessed the importance of interspecific competition among insectivorous birds breeding in northcentral Arizona's ponderosa pine forests. We examined density interactions among species using two analytic approaches; correlative and experimental. The correlative approach examined patterns of change in breeding densities over four years at the community level and within two foraging guilds; picker-gleaners and aerial feeders. The relationships between morphological and behavioral similarity with pairwise density interactions were also assessed. Our experimental approach involved placement of nest boxes on two treatment plots to increase breeding densities of secondary cavity nesting birds that were in foraging guilds with open nesting insectivores.We found little evidence of interspecific competition. Patterns of density fluctuations indicated large positive covariances among species at both the community level and within guilds. Pairwise density interactions were independent of morphological or behavioral similarity. Nest boxes significantly increased breeding densities of the secondary cavity nesters. However, these increases did not induce reprocal density changes in the open nesting species. Interspecific competition for food during the breeding season appears to be unimportant in ponderosa pine bird communities.     

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

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

Grazing-induced effects on soil properties modify plant competitive interactions in semi-natural mountain grasslands

Plant-soil feedbacks are widely recognized as playing a significant role in structuring plant communities through their effects on plant-plant interactions. However, the question of whether plant-soil feedbacks can be indirectly driven by other ecological agents, such as large herbivores, which are known to strongly modify plant community structure and soil properties, remains poorly explored. We tested in a glasshouse experiment how changes in soil properties resulting from long-term sheep grazing affect competitive interactions (intra- and inter-specific) of two graminoid species: Nardus stricta, which is typically abundant under high sheep grazing pressure in British mountain grasslands; and Eriophorum vaginatum, whose abundance is typically diminished under grazing. Both species were grown in monocultures and mixtures at different densities in soils taken from adjacent grazed and ungrazed mountain grassland in the Yorkshire Dales, northern England. Nardus stricta performed better (shoot and root biomass) when grown in grazing-conditioned soil, independent of whether or not it grew under inter-specific competition. Eriophorum vaginatum also grew better when planted in soil from the grazed site, but this occurred only when it did not experience inter-specific competition with N. stricta. This indicates that plant-soil feedback for E. vaginatum is dependent on the presence of an inter-specific competitor. A yield density model showed that indirect effects of grazing increased the intensity of intra-specific competition in both species in comparison with ungrazed-conditioned soil. However, indirect effects of grazing on the intensity of inter-specific competition were species-specific favouring N. stricta. We explain these asymmetric grazing-induced effects on competition on the basis of traits of the superior competitor and grazing effects on soil nutrients. Finally, we discuss the relevance of our findings for plant community dynamics in grazed, semi-natural grasslands.     

螺类牧食对伊乐藻与苦草种间关系的影响

动物牧食可以调节植物种间的相对竞争能力,从而改变物种在群落中的竞争地位.外来植物伊乐藻生长迅速、具有较强的光竞争能力,土著种苦草根系发达、具有较强的地下资源竞争能力.选择这两种各具竞争特色的沉水植物为模式生物,通过室外受控实验研究了螺类牧食对两种沉水植物种间关系的影响.结果表明:不管有无螺类牧食,伊乐藻的相对生长率为苦草的2～5倍,伊乐藻具有明显的竞争优势.苦草低密度种植时,螺类牧食与种间竞争对其生长没有显著影响;高密度种植时,螺类牧食活动促进了苦草的生长,种间竞争则使苦草的生长率明显降低.无论伊乐藻种植密度如何,螺类牧食均使其生长率明显降低,混栽高密度的苦草也能抑制伊乐藻生长.探讨了螺类牧食对沉水植物的种间竞争关系的作用机理.     

Investigating the community consequences of competition among clonal plants

Although clonal plants comprise most of the biomass of several widespread ecosystems, including many grasslands, wetlands, and tundra, our understanding of the effects of clonal attributes on community patterns and processes is weak. Here we present the conceptual basis for experiments focused on manipulating clonal attributes in a community context to determine how clonal characteristics affect interactions among plants at both the individual and community levels. All treatments are replicated at low and high density in a community density series to compare plant responses in environments of different competitive intensity. We examine clonal integration, the sharing of resources among ramets, by severing ramets from one another and comparing their response to ramets with intact connections. Ramet aggregation, the spacing of ramets relative to each other, is investigated by comparing species that differ in their natural aggregation (either clumped growth forms, with ramets tightly packed together, or runner growth forms, with ramets loosely spread) and by planting individual ramets of all species evenly spaced throughout a mesocosm. We illustrate how to test predictions to examine the influence of these two clonal traits on competitive interactions at the individual and community levels. To evaluate the effect of clonal integration on competition, we test two predictions: at the individual level, species with greater clonal integration will be better individual-level competitors, and at the community level, competition will cause a greater change in community composition when ramets are integrated (connected) than when they are not. For aggregation we test at the individual level: clumped growth forms are better competitors than runner growth forms because of their ability to resist invasion, and at the community level: competition will have a greater effect on community structure when ramets are evenly planted. An additional prediction connects the individual- and community-level effects of competition: resistance ability better predicts the effects of competition on relative abundance in a community than does invasion ability. We discuss additional experimental design considerations as revealed by our ongoing studies. Examining how clonal attributes affect both the individual- and community-level effects of competition requires new methods and metrics such as those presented here, and is vital to understanding the role of clonality in community structure of many ecosystems.     

Species richness and evenness respond in a different manner to propagule density in developing prairie microcosm communities

Diversity has two basic components: richness, or number of species in a given area, and evenness, or how relative abundance or biomass is distributed among species. Previously, we found that richness and evenness can be negatively related across plant communities and that evenness can account for more variation in Shannon’s diversity index (H′) than richness, which suggests that relationships among diversity components can be complex. Non-positive relationships between evenness and richness could arise due to the effects of migration rate or local species interactions, and relationships could vary depending on how these two processes structure local communities. Here we test whether diversity components are equally or differentially affected over time by changes in seed density (and associated effects on established plant density and competition) in greenhouse communities during the very early stages of community establishment. In our greenhouse experiment, we seeded prairie microcosms filled with bare field soil at three densities with draws from a mix of 22 grass and forb species to test if increased competition intensity or seedling density would affect the relationships among diversity components during early community establishment. Increased seed density treatments caused diversity components to respond in a different manner and to have different relationships with time. Richness increased linearly with seed density early in the experiment when seedling emergence was high, but was unrelated to density later in the experiment. Evenness decreased log-linearly with seed densities on all sampling dates due to a greater dominance by Rudbeckia hirta with higher densities. Early in the experiment, diversity indices weakly reflected differences in richness, but later, after the competitive effects of Rudbeckia hirta became more intense, diversity indices more strongly reflected differences in evenness. This suggests that species evenness and diversity indices do not always positively covary with richness. Based on these results, we suggest that evenness and richness can be influenced by different processes, with richness being more influenced by the number of emerging seedlings and evenness more by species interactions like competition. These results suggest that both diversity components should be measured in plant diversity studies whenever it is possible.     

Regional effects on competition–productivity relationship: a set of field experiments in two distant regions

We studied the effect of productivity on competition intensity and the relationship between competition intensity and community species richness, using a removal experiment with the perennial plant Solidago virgaurea . The experiment was conducted in 16 different communities from two geographically distant areas (western Estonia and northern Norway). The results were compared with the results of previous experiments with Anthoxanthum odoratum from the same areas. Removal of neighbors had a positive effect on the biomass of both Solidago and Anthoxanthum , and this response was stronger in communities with higher productivity. Thus, the corrected index of relative competition intensity, CRCI, increased with increasing community productivity. Species richness was negatively correlated with CRCI in Estonia but not in Norway and not in the case of the pooled material. The results suggest that competitive exclusion operates at least in these communities which species pool is large.
Our results indicate that the relationship between competition intensity and productivity is non-linear. In our data, competition prevails in communities where living plant biomass exceeds 200 g m⁻², whereas in less productive communities, competition remains undetected and direct plant–plant relationships might at times be even mutualistic. Moreover, we found that the relationship between competition intensity and productivity is strongly dependent on regional differences and is intimately connected to a concordant variation in the intensity of grazing. The least productive communities both in Estonia and in Norway are characterized by intensive grazing, which reduces importance of competition. Hence, the contrasting results corroborates the predictions of the hypothesis of exploitation ecosystems, predicting that trophic dynamics account for the relationship between competition intensity and primary productivity.     

Separating the effects of number of individuals sampled and competition on species diversity: an experimental and analytic approach

1 Species richness typically increases with the number of individuals sampled, although many ecological processes that influence species richness are also well known to depend on density of individuals. We separated the effects of density on species richness that are due to sampling, from those due to density-dependent ecological processes such as competition or predation, by manipulating the density of an entire community.
2 A seed bank from a community of desert annual plants that occur on semi-stabilized sand dunes in Israel was collected from the field and sown in an experimental garden at a range of densities from 1/16 to eight times the natural density. The species pool observed in the lowest density plots was used as the null community, which was repeatedly sampled to calculate the species richness (and other diversity indices) in higher density plots that would be expected from sampling considerations alone. The significance of deviations of observed diversity from this expected diversity was then evaluated.
3 Both observed and expected number of species increased substantially with the experimental increase in density. However, observed species richness, the Shannon–Wiener diversity index and Simpson's diversity index were often significantly lower than that expected based on sampling considerations. The magnitude of the deviation from expected increased significantly with increasing density for richness and the Shannon–Wiener index. This provides some of the first direct experimental evidence from diverse natural assemblages that increasing competition among all the individuals in a community can lead to competitive exclusion.     

Linking sheep density and grazing frequency to persistence of herb species in an alpine environment

Large herbivores are important drivers in ecosystems worldwide. Changes in herbivore densities are predicted to especially affect herbs that are strongly preferred by herbivores. The persistence of herbs could be challenged by enhanced grazing, but also grazing cessation may affect persistence, especially for prostrate herbs, which might be out-competed. To test how different herbivore densities (high, low, and no sheep) affect grazing frequency and plant responses (plant height, flowering frequency, and plant density) at the herb community and species level, we conducted a fully replicated, landscape-scale experiment in an alpine environment. We found that none of the herb species changed their densities after 5 years with experimental changes in grazing pressure. Sheep density affected grazing and flowering frequency at the herb community level. Eight herb species were more grazed at high sheep density as compared to enclosures with no sheep. Herb height decreased at high sheep density as compared to no sheep for the two species with the highest grazing frequency. Increased height and flowering frequency were found for small herbs at high sheep density. Our experiment clearly shows that herbs do not constitute a homogeneous functional group and that, in particular, tall and small herbs are affected in contrasting ways but all species (n = 15) tolerated changes in grazing regimes as densities were maintained at both enhanced grazing and grazing cessation.     

植物功能群去除对高寒草甸群落结构、多样性及生产力的影响

群落中物种的丧失在干扰下普遍存在,但对生态系统过程和功能的影响仍存在较大不确定性。选取青藏高原东缘典型高寒草甸为对象,开展优势植物功能群的梯度去除试验,以模拟长期过牧干扰下物种的损失。经过连续两个生长季的功能群去除,我们对群落的物种组成、结构、多样性和生物量等特征进行了分析,探讨了上述指标的响应过程和机制。研究结果表明：（1）功能群的去除降低了群落高度,增加了物种均匀度,并显著影响了禾草、杂草优势比以及功能群多样性和优势度;（2）同时,去除操作显著减小了凋落物量与禾草生物量,并显著影响了群落地上生物量;（3）进一步分析还发现,禾草、莎草和杂草功能群之间存在显著的竞争关系,群落生产力主要取决于禾草功能群并随物种均匀度的增大而显著减小。上述结果表明,禾草在高寒草甸群落中占据竞争优势地位,植物功能群的损失主要通过改变种间竞争关系、引起有机物质丢失影响群落过程和功能。     

A shift from exploitation to interference competition with increasing density affects population and community dynamics

Intraspecific competition influences population and community dynamics and occurs via two mechanisms. Exploitative competition is an indirect effect that occurs through use of a shared resource and depends on resource availability. Interference competition occurs by obstructing access to a resource and may not depend on resource availability. Our study tested whether the strength of interference competition changes with protozoa population density. We grew experimental microcosms of protozoa and bacteria under different combinations of protozoan density and basal resource availability. We then solved a dynamic predator–prey model for parameters of the functional response using population growth rates measured in our experiment. As population density increased, competition shifted from exploitation to interference, and competition was less dependent on resource levels. Surprisingly, the effect of resources was weakest when competition was the most intense. We found that at low population densities, competition was largely exploitative and resource availability had a large effect on population growth rates, but the effect of resources was much weaker at high densities. This shift in competitive mechanism could have implications for interspecific competition, trophic interactions, community diversity, and natural selection. We also tested whether this shift in the mechanism of competition with protozoa density affected the structure of the bacterial prey community. We found that both resources and protozoa density affected the structure of the bacterial prey community, suggesting that competitive mechanism may also affect trophic interactions.     

A novel approach to assess livestock management effects on biodiversity of drylands

In drylands livestock grazing is the main production activity, but overgrazing due to mismanagement is a major cause of biodiversity loss. Continuous grazing around water sources generates a radial gradient of grazing intensity called the piosphere. The ecological sustainability of this system is questionable and alternative management needs to be evaluated. We apply simple indicators of species response to grazing gradients, and we propose a novel methodological approach to compare community response to grazing gradients (double reciprocal analysis). We assessed degradation gradients of biodiversity under different management strategies in semiarid rangelands of the Monte desert (Argentina) by analyzing changes in vegetation, ants and small mammal richness and diversity, and variation due to seasonality. At the species level, we determined the trend in abundance of each species along the gradient, and the potential cross-taxa surrogacy. At the community level, the new methodological consists of assessing the magnitude of biodiversity degradation along different piospheres by comparing the slopes of linear functions obtained by the double reciprocal analysis. We found that most species showed a decreasing trend along the gradient under continuous grazing; while under rotational grazing fewer species showed a decreasing trend, and a neutral trend (no change in the abundance along the gradient of grazing intensity) was the most common. We found that vegetation cannot be used as a surrogacy taxon of animal response. Moreover, weak cross-taxa surrogacy was found only for animal assemblages during the wet season. The double reciprocal analysis allowed for comparison of multi-taxa response under different seasons and management types. By its application, we found that constrains in precipitation interacted with disturbance by increasing the negative effect of grazing on vegetation, but not on animal assemblages. Continuous grazing causes biodiversity loss in all situations. Rotational grazing prevents the occurrence of vegetation degradation and maintains higher levels of animal diversity, acting as an opportunity for biodiversity conservation under current scenarios of land use extensification. Our approach highlights the importance of considering multi-taxa and intrinsic variability in the analysis, and should be of value to managers concerned with biodiversity conservation.     

Interspecific competition in plants: how well do current methods answer fundamental questions?

Accurately quantifying and interpreting the processes and outcomes of competition among plants is essential for evaluating theories of plant community organization and evolution. We argue that many current experimental approaches to quantifying competitive interactions introduce size bias, which may significantly impact the quantitative and qualitative conclusions drawn from studies. Size bias generally arises when estimates of competitive ability are erroneously influenced by the initial size of competing individuals. We employ a series of quantitative thought experiments to demonstrate the potential for size bias in analysis of four traditional experimental designs (pairwise, replacement series, additive series, and response surfaces) either when only final measurements are available or when both initial and final measurements are collected. We distinguish three questions relevant to describing competitive interactions: Which species dominates? Which species gains? and How do species affect each other? The choice of experimental design and measurements greatly influences the scope of inference permitted. Conditions under which the latter two questions can give biased information are tabulated. We outline a new approach to characterizing competition that avoids size bias and that improves the concordance between research question and experimental design. The implications of the choice of size metrics used to quantify both the initial state and the responses of elements in interspecific mixtures are discussed. The relevance of size bias in competition studies with organisms other than plants is also discussed.     

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.     

放牧干扰梯度下川西亚高山植物群落的组合机理

为了阐明放牧干扰对川西亚高山区域植物群落的组合过程以及群落结构的影响, 研究了放牧干扰梯度下的功能群均匀度和群落谱系结构的变化趋势。结果显示: 在干扰较轻的阔叶林与针叶林样地, 部分样方的功能群均匀度显著高于无效模型, 随着干扰梯度的增强, 功能群均匀度呈线性下降, 样方平均值从0.930降至0.840, 其高于无效模型的次数也逐渐降低, 干扰程度较大的草甸中出现部分样方的功能群均匀度显著低于无效模型。随着干扰程度的增强, 群落的谱系结构指数也呈逐渐上升趋势, 净关联指数平均值由-0.634逐渐增加至2.360, 邻近类群指数由-0.158上升至2.179。草甸与低矮灌丛受干扰较为严重, 其大部分样方的谱系结构指数显著高于随机群落, 表明干扰群落的谱系结构呈聚集分布。功能群均匀度与谱系结构的变化趋势一致, 表明生境筛滤效应与种间竞争作用的平衡决定着群落的组合过程。干扰降低了竞争作用, 促进了少数耐干扰功能群的优势地位, 造成功能群均匀度下降, 同时通过生境筛滤作用, 使群落的谱系结构呈现出聚集分布; 而未干扰的群落中由于竞争作用的效应, 功能群均匀度较高, 谱系结构也更加分散。研究区域植物群落的功能群均匀度与物种丰富度呈负相关, 表明物种间特别是相似物种间的竞争限制了群落的物种多样性。研究结果说明, 生态位分化和物种间的相互竞争在物种共存与群落组合中具有重要作用。