Limited effects of above‐ and belowground insects on community structure and function in a species‐rich grassland

Question: Do above‐ and belowground insects differentially impact plant community structure and function in a diverse native grassland? Location: Rough fescue prairie in Alberta, Canada. Methods: Above‐ and belowground insects were suppressed with insecticides for 5 years using a randomised block design. During this experiment, a severe drought began in 2001 and ended in 2003. Aboveground plant growth was measured as cover and biomass from 2001 to 2005. Root demography was measured in 2002 using a minirhizotron. Mixed models and repeated measures ANOVA were used to determine treatment effects on a number of response variables. MRBP were used to test for treatment effects on community composition. Results: Five years of insect suppression had few significant effects on plant growth, species richness or community composition, and were limited primarily to the severe drought in 2002. During the drought, insect attack increased root mortality, reduced plant cover, and altered community composition. Following the drought, plant responses were unaffected by insecticide application for the remainder of the experiment. Conclusions: Five years of insect suppression had only minor effects on community structure and function in this diverse native grassland. There was no indication that these effects increased over time. The results are counter to studies conducted in productive old‐field communities that revealed large effects of insects on community structure. We suggest that the unique features of this system, such as high species evenness, abundance of generalist herbivores, and a lack of competition for light among plants, limited the potential for insects to greatly impact community‐level processes.     

Time and space in the community structure of a species-rich limestone grassland

Abstract. The community structure of a species-rich grassland was investigated at a small spatial scale (0.001 m²), to determine whether evidence suggesting assembly rules could be found in temporal or spatial variation in either species richness or guild proportions. The community was alvar limestone grassland on the island of Öland, Sweden. Three sites were sampled: two lightly grazed, the other recently ungrazed. Plots with and without fertilizer were compared. Evidence was sought for restriction on the ability of species to co-occur within a limited spatial area. Restriction due to a limited number of niches available,‘niche limitation’, could be manifest as lower variance in quadrat richness than expected under a null model (i.e. RV_r, the ratio of observed: expected variance in richness, would be < 1.0). In several cases, RV_r values were significantly < 1.0, even using a patch model to allow for possible spatial variation in the environment. Low RV_r values were found only at the smallest square quadrat size, 10 cm². On Fertilized plots in the years immediately after application of fertilizer, low RV_r could not be demonstrated. Explanations of low RV_r other than niche limitation are considered, such as environmental heterogeneity (present and/or historical) and limitations to the co-occurrence of individual plant modules. Assembly rules based on guild membership were sought by looking at the variance across quadrats in the proportions of species from morphological guilds. An assembly rule would be seen as relatively constant proportion, estimated via RV_gp, the ratio of observed: expected variances in guild proportions. Significant guild proportionality was found in some cases. There was no evidence of guild proportionality in the years after the application of fertilizer. The significant effects in RV_r were more numerous than expected on a random basis, though not observed in every site in every year. Similar trends were seen in RV_gp. At the space/time scales examined, the species in a plant community may be constrained by assembly rules only intermittently, e.g. when resources are more limiting (Wiens 1977). Under this concept, when competition is relaxed, such as following fertilizer application, there is a temporary microhabitat ‘waterhole’ in which more species can coexist, and the assembly rules break down, at least temporarily whilst the species composition adjusts. There was some indication of a return to more deterministic community structure four years after fertilization commenced. Variants of van der Maarel's Carousel model were tested. A Niche-limited Carousel Model (i.e. a model in which there is some limitation in the number of species that can occupy a microsite) would imply restricted variation in richness through time for a single quadrat (temporal RV_r). Overall differences between years in species richness were demonstrated, and their effect removed; after this adjustment there was support for the Niche-limited Carousel Model. The extent of this limitation varied between sites. There were also consistent differences between quadrats in species richness. There was little evidence for constancy of guild proportions through time. The site that showed the strongest community structure in time and space, least year-to-year variation in mean species richness, and least response to fertilizer perturbation, is that on the shallowest soil. Possibly the thin soil results in greater resource limitation, supporting suggestions that assembly rules are stronger when resources are more limited.     

Experimental effects of habitat fragmentation on old-field canopy insects: community, guild and species responses

We examined the effects of habitat fragmentation on the species distributions, guild membership, and community structure of old-field insects using a fine-scale experimental approach. A continuous 1-ha goldenrod field was fragmented into four treatments that varied in both patch size and degree of isolation. Each treatment was replicated four times and arranged in a Latin square design. Canopy insects in fragmented patches were sampled with sweep nets during early and late summer 1995. The species richness of insects was significantly lower in fragmented than in unfragmented treatments during July, but was similar among treatments in September. Overall community abundance showed no treatment effect during either month. We also found significant row and column effects, suggesting there was spatial heterogeneity in species richness and abundance apart from treatment effects. Differences in species richness during July were primarily due to the loss of rare species in highly fragmented plots. Overall abundance was less responsive to community change because deletions of rare species in fragmented areas were not detected in abundance analyses. Four feeding guilds showed different responses to fragmentation: the species richness of sucking herbivores and the abundance of parasitoids were significantly reduced by fragmentation but predators and chewing herbivores were largely unaffected. Analyses of a subset of individual species within guilds suggest that the greater effects of fragmentation on sucking herbivores and parasitoids may be due to the degree of habitat specificity of guild members. The effects of small-scale habitat fragmentation were therefore detectable at the level of community, guild, and individual species. Changes in species richness, guild structure and species distributions were likely due to differential effects of habitat alteration on individual movements and patch selection rather than dispersal or demographic change. Nonetheless, the selective loss of rare species, differential guild effects and changes in species occupancy that we found in this small-scale experiment are also factors that are likely to operate in fragmented habitats over broader spatial scales. Received: 11 May 1998 / Accepted: 27 September 1998     

施肥和放牧对青藏高原高寒草甸物种丰富度的影响

在草地生态系统中, 施肥通常会导致生物多样性下降, 但是关于引起生物多样性下降的机制还存在着很大的争议。该研究基于一个4年的施肥实验, 试图通过个体大小的不整齐性和单位植物氮含量, 定性地揭示青藏高原东部高寒草甸施肥后多样性下降的原因。研究显示: 在封育地, 施肥致使个体大小不整齐性增加了15%, 并不同程度地增加了物种的高度。同时, 施肥使物种间单位植物氮含量存在显著差异的数目降低了65%。施肥后光竞争加剧, 导致大个体植物排斥小个体植物, 进而引起了物种丰富度下降29.6%。与封育地不同, 放牧地施肥并没有改变个体大小不整齐性和物种的高度, 而是使物种间单位植物氮含量存在显著差异的数目增加了11.4%。施肥并没有改变放牧地的光竞争强度, 而是增加了物种间对土壤营养元素氮的竞争强度, 进而引起了物种丰富度下降17.3%。该研究还发现, 放牧施肥地的物种丰富度下降速度和等级显著低于封育施肥地的物种丰富度下降速度和等级, 这表明放牧减缓了施肥对物种丰富度的影响力。     

Effects of species interactions on community organization along a trampling gradient

Abstract. Effects of interspecific interactions on the organization of two trampled communities, Eragrostio ferrugineae - Plantaginetum asiaticae (EP) and Eleusino indicae - Digitarietum violascentis (ED), were examined by 4-yr field experiments. We compared changes in the relative abundance of the main component species of the communities in monoculture and mixed culture along a trampling gradient. At no trampling in mixed culture, Ambrosia artemisiifolia var. elatior and Erigeron annuus (roadside herbs, RH) predominated and excluded the trampled community species. Severe trampling markedly reduced the predominance of these roadside herbs, promoted Eragrostis ferruginea (a perennial grass of EP), but suppressed Eleusine indica (an annual grass of ED). These results suggest that the differentiation of trampled and roadside herb communities under heavy trampling are caused by asymmetric competition between their main species. Several species pairs showed a constant rank order of relative cover at high trampling levels. Pennisetum alopecuroides (a perennial grass of both RH and EP) and Eragrostis coexisted, indicating symmetric competition. Eragrostis and Plantago asiatica (a perennial herb of EP) or Poa annua (a winter-annual grass of both EP and ED) coexisted through separation in phenology. Eragrostis promoted the survival of Plantago by moderating unpredictable drought (commensalistic positive interaction). This suggests that community composition is maintained by several interspecific interactions.     

Effects of variable precipitation on the structure and diversity of a California salt marsh community

Abstract. We measured the effects of annual variation in climate and experimentally augmented rainfall on patterns of distribution and above-ground productivity in annual plant communities at Carpinteria Salt Marsh in central California. In the driest year, Hutchinsia procumbens was codominant throughout much of the upper marsh; however, Hutchinsia was very rare or not present in the wetter years. Conversely, Juncus bufonius was common in the wettest year and absent in the driest year. Elevational distributions of other annual species also differed among years with different total precipitation. In 1989–1990, an exceptionally dry season, supplemental water decreased soil salinity, increased above-ground productivity of annuals, and caused significant changes in spatial patterns and relative density. In the lowest zone, Hutchinsia occurred only in watered plots and supplemental water increased the density of Spergularia marina. At intermediate elevations Lasthenia glabrata occurred only in watered plots and supplemental water increased the density of Spergularia, Hutchinsia, and Parapholis incurva. At upper elevations, Juncus occurred only in watered plots, and Lasthenia was the only species that increased significantly in density with watering. Unlike natural shifts in species abundance, no species declined significantly in cover in any zone in the watered treatment. Although climatic variation has complex affects on annual plant communities, our experiments isolated important affects of total annual rainfall on the structure of annual plant communities that were similar to those that occurred with natural variation in rainfall. We conclude that variation in total annual precipitation promotes dynamic community composition and spatial distributions among years, and thus increases overall species diversity in the salt marsh.     

耕作和施肥对不同深度黑土中细菌群落结构的影响

【目的】为探讨耕作和施加有机肥、化肥对黑土表层(0-30cm)、中层(100-130cm)及深层(200-230cm)细菌群落结构的影响,【方法】应用DGGE技术对相应土层中细菌群落结构进行了解析。【结果】结果表明,与对照相比,耕作和施加有机肥、化肥对表层黑土细菌群落结构影响较大,二者差异度为4%;而对中层和深层细菌群落结构影响较小,二者差异度为2%。对于细菌群落结构的垂向变化,中层和深层中细菌群落结构的相似性远远高于同表层的相似性。【结论】可见,耕作和施加有机肥、化肥仅对黑土表层土壤(0-30cm)具有一定的影响,而对100cm以下土壤细菌群落影响较小,细菌群落随土壤深度不同的垂向变化要远高于土壤管理措施造成的影响。     

Temporal dynamics of an ephemeral plant community: species turnover in seasonal rock pools on Ivorian inselbergs

Inselbergs are monolithic, old and stable landscape elements with a worldwide distribution. On inselbergs, relatively old (i.e. hundreds of years old) rock pools form a unique habitat that offers locally wet conditions in an otherwise edaphically and microclimatically dry environment. They occur as seasonally water-filled depressions with a thin layer of soil, and are mainly inhabited by ephemerals and cryptogams. Even during the rainy season, they often become dry if precipitation is lacking for a few days. Between 1990 and 1999, about 200 rock pools in the Ivorian savanna zone were examined on a regular basis. Absolute and relative turnover rates were calculated: values ranged from 0.00 to 3.00 and 0 to 78%, respectively. Mean turnover was 0.19 (17%). The turnover in the SRPs increased significantly with soil depth and area. No correlation was found between turnover and species number of the pools, which increased significantly with area. The highest turnover was shown by the annual Cyanotis lanata. The turnover of therophytes was higher than those of other life-forms. Turnover and frequency of the species were only weakly correlated. In an environment of frequent stochasticity, climatic disturbances are the major driving force determining community dynamics (e.g. local extinctions and immigrations) of these habitat islands.     

The influence of vines on an oligohaline marsh community: results of a removal and fertilization study

The effects of competitive suppression by vines on the non-vine plant community have received little attention in temperate habitats. This study investigated the impact vines have on their herbaceous hosts in a wetland community at two soil fertility levels. Plots in an oligohaline marsh were treated in a 2 × 2 factorial design with vine removal and fertilization over two growing seasons. There was no significant interaction between removal and fertilization treatments on any of the measured variables. Vine removal initially caused an increase in light penetration through the canopy, but by the end of the study, plots with vines removed had less light due to a 25% increase in biomass by the plants released from competition with vines. For plots with vines removed, species richness was higher during a brief period in the spring of the second year, but by the end of the study, richness in removal plots decreased relative to controls. Fertilization caused a 40% increase in biomass overall, although only two species, Sagittaria lancifolia L. and Polygonum punctatum Ell., showed dramatic increases. Despite fertilization causing a 40% decrease in light penetration to the ground, no change in species richness was observed. Overall, these results show that vine cover in this wetland suppresses non-vine species and reduces community biomass. Removal of vines increased biomass of non-vine dominants but resulted in only an ephemeral change in species richness. Fertilization did not increase the effects of vines on the non-vine community. Received: 14 November 1996 / Accepted: 10 June 1997     

Small-scale spatial dynamics of plant species in a grassland community over six years

Abstract. In a species-rich mountain grassland in the Krkonosse Mts., Czechoslovakia, data from four permanent plots of 50 cm x 50 cm were recorded annually from 1985 to 1990 to study the spatial dynamics of the species. Plots were divided into 15 x 15 subplots and the number of vegetative units of all plants within each subplot was determined. There was not much net change at the plot level, but the subplots were very dynamic. Two aspects of the spatial dynamics of the species were followed: (1) persistence, i.e. the tendency of the species to remain in the same subplot, and (2) long-distance spreading, i.e. movement to subplots beyond the immediate neighbourhood. Species differed widely in their persistence and longdistance spreading and were classified into mobility types: long-range guerrilla, short-range guerrilla, phalanx and 'sitting’. The mobility types were, to a certain extent, correlated with the growth form of plants, but some species of one growth form showed different types of small-scale dynamics and some species with different growth forms had the same spatial dynamics.     

An experimental study of the impact of seasonal sheep grazing on formerly fertilized grassland

Abstract. The effects of grazing upon the composition of a grassland were examined in a replicated grazing experiment with sheep over eight years. Grazing was controlled to produce two levels of grazing in each of winter, spring and summer, combined in a 2×2×2 factorial experimental design. Particular species were either increased or decreased by winter grazing while, in general, dicotyledonous species were favoured by grazing in both winter and spring. A model fitting the main effects of grazing in three seasons showed the importance of grazing in winter and spring to achieve sward diversification.     

高原鼠兔对退化草地植物群落结构及恢复演替的影响

2009 年4 月至2011 年9 月采用围栏封育和去除法,对去除高原鼠兔的处理组和鼠兔扰动(对照组)区域植物群落结构、物种组成和地上生物量进行了比较分析,旨在阐明高原鼠兔对草地植物群落结构和群落演替的影响。结果表明:去除高原鼠兔处理组和对照区域植物群落平均高度和种盖度逐年上升,均在2011 年达到最大值,平均高度分别为6. 5 cm 和4. 2 cm,二者差异极显著(F =127. 80,df = 1,10,P < 0.01),种盖度分别为126. 5%和117. 9% ,差异不显著;植物群落组成中,处理区域禾草和莎草地上生物量比对照区域分别增加了1 965.1% 和33. 2% ,二者差异极显著(F = 41. 29,df = 1,10,P < 0.001),豆科植物和杂类草分别下降了89. 9% 和30.7% ;处理区域植物群落生态优势度发生了显著变化,由以杂类草为主改变为以禾草为主,而对照组均以杂类草为主,变化不明显。相似性指数2009 - 2011 年在对照区域年间变化范围为0.7168 - 0. 7550,在去除高原鼠兔处理区域年间变化范围为0.6464 - 0. 6732;对照区域和处理区间变化范围为0. 5354 - 0. 8956。根据试验结果我们认为,高原鼠兔的扰动可以有效降低植物群落的平均高度和植物种盖度,但植物种类组成主要以杂类草为主,年间变化不明显,表明鼠兔的扰动延缓了植物群落的恢复演替。     

Effects of N-free fertilization on ectomycorrhiza community structure in Norway spruce stands in Southern Sweden

The application of N-free fertilizer (i.e. lime combined with nutrients such as P, Ca, K and Mg) has been suggested as one way of compensating for nitrogen-caused eutrophication and losses of base cations due to atmospheric pollution. To study the effects of such a treatment on mycorrhizal fungi, fine-root samples were collected from the LFH-layer in four Norway spruce stands in southern Sweden. One stand was part of a larger experiment (Skogaby) and had four replicates. It was fertilized twice in 1988–89 (P:K:Ca:Mg:S 48:43:218:46:75 kg ha^-1), and sampling was carried out once yearly during 1991–93. The other three stands were fertilized once in 1988–89 (P:K:Ca:Mg:S|25:62:33:12:54 kg ha^-1) and sampled in 1992.Ectomycorrhizal fine-roots were classified into morphotypes on the basis of the structure and colours of their external hyphae and fungal mantle. The fungal biomass was estimated in 1992 using ergosterol analysis. In Skogaby, N-free fertilizer had no apparent effects on fungal biomass or on the total number of ECM types. Similar results were obtained for the other three stands. Previously reported 50% reductions in sporocarp production on the fertilized plots at Skogaby can probably be explained by a decrease in carbon allocation to the roots and by a decline in the abundance of a single morphotype which accounted for 3% of the total number of root tips, but ca. 30% of the sporocarp biomass in the control plots in the present study. It is concluded that moderate levels of N-free fertilization are not likely to drastically affect the community structure of the dominating ectomycorrhizal fungi. This result should be interpreted with some caution, however, since it remains to be determined whether the fertilizer treatments affect the function of the nutrient-absorbing soil mycelium of the mycorrhizal fungi.     

Tree competition and species coexistence in a cool-temperate old-growth forest in southwestern Japan

Abstract. The growth dynamics and mode of competition between adult trees ≥ 4 cm in DBH (stem diameter at breast height 1.3 m) of eight abundant species occupying ca. 90 % of the total basal area were investigated in a 4-ha study plot (200 m × 200 m) of a cool-temperate, old-growth forest on Mount Daisen, southwestern Japan. In the study plot, 30 tree species with individuals ≥ 4.0 cm DBH co-occurred. A bimodal DBH distribution showing upper and lower-canopy layers was found for the most dominant and largest species, Fagus crenata (ca. 78 % of the total basal area), whilst other tree species showed unimodal DBH distributions corresponding mostly to the lower-canopy layer. We developed a model for individual growth incorporating both intra and interspecific competition and the degree of competitive asymmetry. Onesided interspecific competition was detected only from Fagus crenata (upper-canopy species) to Acer japonicum and Acanthopanax sciadophylloides (lower-canopy species) on the scale of the 4-ha study plot. Only Acanthopanax sciadophylloides showed symmetric intraspecific competition. However, a positive (non-competitive) interspecific relationship between adult trees prevailed over a competitive relationship; for example, individual DBH growth rate of Fagus crenata (especially lower-canopy trees) was correlated with the abundance of Acer mono. The positive relationship represented a group of species with similar habitat preference [soil type (mature or immature) caused by landslide disturbance and the presence/absence of Sasa dwarf bamboos in the understorey], where tree densities were not so high as to bring about competition. Competitive interactions between adult trees ≥ 4 cm in DBH occurred only locally between a few specific species and were suggested to be almost irrelevant to the variation in species coexistence on the 4-ha scale of cool-temperate forest. Rather, the coexistence of 30 tree species (species diversity) on this large scale was suggested to be governed by the regeneration pattern of each component species (habitat preference, seedling establishment, sapling competition) with respect to landslide disturbance.