Long-term dynamics of biotic and abiotic resistance to exotic species invasion in restored vernal pool plant communities

Invasion of native ecosystems by exotic species can seriously threaten native biodiversity, alter ecosystem function, and inhibit conservation. Moreover, restoration of native plant communities is often impeded by competition from exotic species. Exotic species invasion may be limited by unfavorable abiotic conditions and by competition with native species, but the relative importance of biotic and abiotic factors remains controversial and may vary during the invasion process. We used a long-term experiment involving restored vernal pool plant communities to characterize the temporal dynamics of exotic species invasion, and to evaluate the relative support for biotic and abiotic factors affecting invasion resistance. Experimental pools (n=256) were divided among controls and several seeding treatments. In most treatments, native vernal pool species were initially more abundant than exotic species, and pools that initially received more native seeds exhibited lower frequencies of exotic species over time. However, even densely seeded pools were eventually dominated by exotic species, following extreme climatic events that reduced both native and exotic plant densities across the study site. By the sixth year of the experiment, most pools supported more exotics than native vernal pool species, regardless of seeding treatment or pool depth. Although deeper pools were less invaded by exotic species, two exotics (Hordeum marinum and Lolium multiflorum) were able to colonize deeper pools as soon as the cover of native species was reduced by climatic extremes. Based on an information-theoretic analysis, the best model of invasion resistance included a nonlinear effect of seeding treatment and both linear and nonlinear effects of pool depth. Pool depth received more support as a predictor of invasion resistance, but seeding intensity was also strongly supported in multivariate models of invasion, and was the best predictor of resistance to invasion by H. marinum and L. multilorum. We conclude that extreme climatic events can facilitate exotic species invasions by both reducing abiotic constraints and weakening biotic resistance to invasion.     

Facilitation by Pinus flexilis during succession: a hierarchy of mechanisms benefits other plant species

Studies of facilitation have primarily been limited to single mechanisms, species, or environments. We examined interacting mechanisms governing the facilitative effects of Pinus flexilis on two later successional understory species, Pseudotsuga menziesii and Ribes cereum, in different microhabitats and seasons at the ecotone between the Rocky Mountain forests and Great Plains grasslands in Montana, USA. In field surveys, 69% of Pseudotsuga and 91% of Ribes were located beneath P. flexilis even though P. flexilis subcrowns accounted for a small proportion of available habitat. For three years, we monitored the survival of Pseudotsuga and Ribes seedlings experimentally planted beneath P. flexilis and in the open at a windward and a leeward site. Survival of both species was highest beneath P. flexilis at a site topographically protected from strong unidirectional winds (38% for Pseudotsuga and 63% for Ribes), and lowest at a windward site and in the open where tree crowns did not provide shelter from winds (2% and 6%, respectively). These results suggest that wind amelioration contributed to the facilitative effect of P. flexilis. However, even at the leeward site, where wind speed was low, survival of Pseudotsuga and Ribes was higher beneath P. flexilis, suggesting the importance of shade. To explore the relative importance of different mechanisms, we designed an experiment with six treatments: "shade," "shade + wind," "shade + drift," "wind," "drift," and a "control." After two years, we found shade to be of overwhelming importance for the survival of Pseudotsuga and Ribes. Without shade, no other treatments were significant, but once shade was provided, wind amelioration and snow pack accumulation increased survival of Pseudotsuga, suggesting that these different facilitative mechanisms functioned in a nested hierarchical manner: some mechanisms were important only when others were already functioning. Many studies have demonstrated multiple interacting mechanisms in the way that plants interact, but to our knowledge hierarchical interactive processes have not been previously documented. If the effects of positive or competitive mechanisms are often hierarchical, then studies of isolated mechanisms may not accurately assess their importance in nature.     

Increased plant size in exotic populations: a common-garden test with 14 invasive species

The "evolution of increased competitive ability" (EICA) hypothesis predicts that exotic species will adapt to reduced herbivore pressure by losing costly defenses in favor of competitive ability. Previous studies often support the prediction that plants from exotic populations will be less well defended than plants from native populations. However, results are mixed with respect to the question of whether plants from exotic populations have become more competitive. In a common-garden experiment involving plants from two native and two exotic populations of 14 different invasive species, we tested whether exotic plants generally grow larger than conspecific native plants, and whether patterns of relative growth depend on the intensity of competition. We found a quite consistent pattern of larger exotic than native plants, but only in the absence of competition. These results suggest that invasive species may often evolve increased growth, and that increased growth may facilitate adaptation to noncompetitive environments.     

Habitat-specific impacts of multiple consumers on plant population dynamics

Multiple consumers often attack seeds, seedlings, and adult plants, but their population-level consequences remain uncertain. We examined how insect and small mammal consumers influenced the demography and abundance of the perennial shrub, bush lupine (Lupinus arboreus). In grassland and dune habitats we established replicate experimental lupine populations in 81-m2 plots that were either protected from, or exposed to, herbivorous voles and granivorous mice (via fencing) and/or root feeding insects (via insecticide treatment). Populations were initiated with transplanted seedlings in 1999 and 2000. We followed the demography of these cohorts, subsequent generations, and the seed bank for 5.5 years. Voles and insects killed many seedlings in dune (1999 only) and grassland (1999 and 2000) habitats. After 2000, insects and voles had minimal effects on seedling or adult survival. Seed predation by granivorous mice, however, greatly depressed seedling recruitment, resulting in lower adult lupine abundance in control plots vs. those protected from rodents. In grasslands, initial effects of voles and insects on seedling survival produced large differences among treatments in adult plant density and the cumulative number of seeds produced throughout the experiment. Differences among grassland populations in seed rain, however, had little influence on the magnitude of seedling recruitment into this habitat. Instead, recruitment out of a preexisting seed bank compensated for the lack of seed production in populations exposed to consumers. Shading by dense adults in plots protected from consumers limited seedling establishment within these populations. Although differences among populations in cumulative seed rain did not influence adult establishment, populations protected from consumers accumulated substantially larger seed banks than controls. These results illustrate how density dependence, habitat-specific seed dynamics, and particular demographic impacts of consumers interact to shape plant population responses to consumers.     

Clustering species using a model of population dynamics and aggregation theory

The high species diversity of some ecosystems like tropical rainforests goes in pair with the scarcity of data for most species. This hinders the development of models that require enough data for fitting. The solution commonly adopted by modellers consists in grouping species to form more sizeable data sets. Classical methods for grouping species such as hierarchical cluster analysis do not take account of the variability of the species characteristics used for clustering. In this study a clustering method based on aggregation theory is presented. It takes account of the variability of species characteristics by searching for the grouping that minimizes the quadratic error (square bias plus variance) of some model’s prediction. This method allows one to check whether the gain in variance brought by data pooling compensate for the bias that it introduces. This method was applied to a data set on 94 tree species in a tropical rainforest in French Guiana, using a Usher matrix model to predict species dynamics. An optimal trade-off between bias and variance was found when grouping species. Grouping species appeared to decrease the quadratic error, except when the number of groups was very small. This clustering method yielded species groups similar to those of the hierarchical cluster analysis using Ward’s method when variance was small, that is when the number of groups was small.     

Seasonal variations in plant species effects on soil N and P dynamics

It is well established that plant species influence ecosystem processes, but we have little ability to predict which vegetation changes will alter ecosystems, or how the effects of a given species might vary seasonally. We established monocultures of eight plant species in a California grassland in order to determine the plant traits that account for species impacts on nitrogen and phosphorus cycling. Plant species differed in their effects on net N mineralization and nitrification rates, and the patterns of species differences varied seasonally. Soil PO4- and microbial P were more strongly affected by slope position than by species. Although most studies focus on litter chemistry as the main determinant of plant species effects on nutrient cycling, this study showed that plant species affected biogeochemical cycling through many traits, including direct traits (litter chemistry and biomass, live-tissue chemistry and biomass) and indirect traits (plant modification of soil bioavailable C and soil microclimate). In fact, species significantly altered N and P cycling even without litter inputs. It became particularly critical to consider the effects of these multiple traits in order to account for seasonal changes in plant species effects on ecosystems. For example, species effects on potential rates of net N mineralization were most strongly influenced by soil bioavailable C in the fall and by litter chemistry in the winter and spring. Under field conditions, species effects on soil microclimate influenced rates of mineralization and nitrification, with species effects on soil temperature being critical in the fall and species effects on soil moisture being important in the dry spring. Overall, this study clearly demonstrated that in order to gain a mechanistic, predictive understanding of plant species effects on ecosystems, it is critical to look beyond plant litter chemistry and to incorporate the effects of multiple plant traits on ecosystems.     

Seasonal timing of first rain storms affects rare plant population dynamics

A major challenge in forecasting the ecological consequences of climate change is understanding the relative importance of changes to mean conditions vs. changes to discrete climatic events, such as storms, frosts, or droughts. Here we show that the first major storm of the growing season strongly influences the population dynamics of three rare and endangered annual plant species in a coastal California (USA) ecosystem. In a field experiment we used moisture barriers and water addition to manipulate the timing and temperature associated with first major rains of the season. The three focal species showed two- to fivefold variation in per capita population growth rates between the different storm treatments, comparable to variation found in a prior experiment imposing eightfold differences in season-long precipitation. Variation in germination was a major demographic driver of how two of three species responded to the first rains. For one of these species, the timing of the storm was the most critical determinant of its germination, while the other showed enhanced germination with colder storm temperatures. The role of temperature was further supported by laboratory trials showing enhanced germination in cooler treatments. Our work suggests that, because of species-specific cues for demographic transitions such as germination, changes to discrete climate events may be as, if not more, important than changes to season-long variables.     

Divergent composition but similar function of soil food webs of individual plants: plant species and community effects

Soils are extremely rich in biodiversity, and soil organisms play pivotal roles in supporting terrestrial life, but the role that individual plants and plant communities play in influencing the diversity and functioning of soil food webs remains highly debated. Plants, as primary producers and providers of resources to the soil food web, are of vital importance for the composition, structure, and functioning of soil communities. However, whether natural soil food webs that are completely open to immigration and emigration differ underneath individual plants remains unknown. In a biodiversity restoration experiment we first compared the soil nematode communities of 228 individual plants belonging to eight herbaceous species. We included grass, leguminous, and non-leguminous species. Each individual plant grew intermingled with other species, but all plant species had a different nematode community. Moreover, nematode communities were more similar when plant individuals were growing in the same as compared to different plant communities, and these effects were most apparent for the groups of bacterivorous, carnivorous, and omnivorous nematodes. Subsequently, we analyzed the composition, structure, and functioning of the complete soil food webs of 58 individual plants, belonging to two of the plant species, Lotus corniculatus (Fabaceae) and Plantago lanceolata (Plantaginaceae). We isolated and identified more than 150 taxa/groups of soil organisms. The soil community composition and structure of the entire food webs were influenced both by the species identity of the plant individual and the surrounding plant community. Unexpectedly, plant identity had the strongest effects on decomposing soil organisms, widely believed to be generalist feeders. In contrast, quantitative food web modeling showed that the composition of the plant community influenced nitrogen mineralization under individual plants, but that plant species identity did not affect nitrogen or carbon mineralization or food web stability. Hence, the composition and structure of entire soil food webs vary at the scale of individual plants and are strongly influenced by the species identity of the plant. However, the ecosystem functions these food webs provide are determined by the identity of the entire plant community.     

子午岭植被演替过程中土壤生物学特性的动态

土壤生物学特性在土壤有机质的形成和降解、营养循环等方面起重要作用。植被的恢复演替显著影响土壤生物学特性,尤其影响土壤酶活性。植被演替过程中土壤酶活性的研究结果表明,随着植被恢复年限的延长,土壤脲酶和转化酶的活性逐渐提高,17 a达到最大值,随后有所降低。土壤酶活性和土壤化学特性和微生物量的相关性分析表明,土壤转化酶和脲酶不仅互相之间具有显著的相关性,而且它们与土壤有机碳、全氮、微生物碳氮之间都具有显著的正相关性,说明土壤酶活性与土壤有机质紧密相关,与微生物的大小紧密相关,所以土壤酶活性可以表征土壤生物学肥力。     

Rapid ecosystem shifts in peatlands: linking plant physiology and succession

Stratigraphic records from peatlands suggest that the shift from a rich fen (calcareous fen) to an ombrotrophic bog can occur rapidly. This shift constitutes a switch from a species-rich ecosystem to a species-poor one with greater carbon storage. In this process, the invasion and expansion of acidifying bog species of Sphagnum (peat mosses) play a key role. To test under what conditions an acidifying bog species could invade a rich fen, we conducted three experiments, contrasting the bog species S. fucsum with the rich-fen species S. warnstorfii and S. teres. We first tested the effect of calcareous water by growing the three species at different constant height above the water table (HWT; 2, 7, and 14 cm) in a rich-fen pool and measured maximum photosynthetic rate and production and difference in length growth as an indicator of competition. In none of the species was the photosynthetic capacity negatively affected when placed at low HWT, but S. fuscum was a weaker competitor at low HWT. In our second experiment we transplanted the three species into microhabitats with different and naturally varying HWT in a rich fen. Here, S. fuscum nearly ceased to photosynthesize when transplanted to low HWT (brown moss carpet), while it performed similarly to the two rich-fen species at the intermediate level (S. warnstorfii hummock level). In contrast to S. fuscum, the rich-fen sphagna performed equally well in both habitats. The brown moss carpet was seasonally flooded, and in our third experiment we found that S. fuscum, but not S. teres, was severely damaged when submerged in rich-fen water. Our results suggest two thresholds in HWT affecting the ecosystem switch: one level that reduces the risk of submergence and a higher one that makes bog sphagna competitive against the rich-fen species.     

The pattern of species turnover resulting from stochastic population dynamics: The model and field data

The model of random population dynamics in a sampling site returns geometric distribution of longevities of continuous presence (=persistence) and Poisson distribution of the presence–absence transitions. This discrete-time stochastic process describes the presence–absence pattern observed in the beetles surveyed 6 years on Mount Carmel, Israel. Homogeneous pools of species mostly on the Families rank, exhibit the predicted by the model patterns. Conformity to an ergodic hypothesis is the criterion of ecological homogeneity. This criterion assumes the equivalence of short-term behavior of entire pool and long-term behavior of any species from this pool. The pool of all 801 species of Order Coleoptera does not match the model. Thus a taxon of an arbitrary rank may not be considered a priory as a unit of ecological study. Determined from field data parameters of the model are biased and magnitude of the bias depends on longevity of the survey. Parameter of distribution depends also on species tolerance, which is the level adaptation of given species to given environment in given time interval. Random process of species turnover may be considered as a game of species to gain their presence against deteriorative fluctuations of environmental conditions.     

Ungulate population dynamics and optimization models

Optimal harvesting strategies for an ungulate population are estimated using stochastic dynamic programming. Data on the Llano Basin white-tailed deer (Odocoileus virginianus) population were used to construct a 2-variable population dynamics model. The model provided the basis for estimating optimal harvesting strategies as a feedback function of the current values of the state variables (prefawning older deer and juveniles). Optimal harvest strategies were insensitive to assumptions about the probability distributions of the stochastic variable (rainfall). The response of the population components to harvesting and the returns obtained from applying optimal strategies were explored through simulation. Mean annual harvest is about 15% of the population. Simplified harvesting strategies based on age-ratios as well as a simplified version based on optimal strategies—but assuming persisting equilibrium juvenile deer density—were compared to optimal strategies through examining values of information. Simplified harvesting strategies lead to a lower harvest over a 50-year simulation period.     

Recovery of plant species richness during long-term fertilization of a species-rich grassland

Nutrient enrichment of habitats (eutrophication) is considered to be one of the main causes of plant diversity decline worldwide. Several experiments have shown a rapid loss of species in the first years after fertilization started. However, little is known about changes in species richness in the long term. Here, we use a 50-year-old field experiment with a range of fertilization treatments in grasslands that were mown twice each year in the center of The Netherlands. We show that species richness in all plots initially declined but started to recover after approximately 25 years of continued fertilization. This was also true for the heavily fertilized treatment (NPK). In NPK-fertilized plots, the decline was strongest and associated with a strong divergence of plant trait composition from the control, reflecting a shift to a plant community adapted to nutrient-rich conditions. During the subsequent period of increase in species richness, the trait composition remained stable. These results show that plant species richness can, at least partially, recover after an initial diversity decline caused by fertilization.     

外来入侵植物化感作用与土壤相互关系研究进展

文章以全球普遍关注的外来植物入侵为背景,综述了外来入侵植物化感物质与土壤化学性质(土壤pH,土壤有机碳和有机质,土壤中的化学元素等)、土壤生物群落(土壤微生物、土壤动物)的关系。并探讨了未来研究需要加强的几个方面,包括将化感作用与野外实际情况相结合研究,更真实的反映田间状况;进一步探讨微生物在化感物质生物变化过程中的作用及其机理;化感物质作为信号物质如何影响微生物代谢,从而影响地上植物;深入研究化感作用与土壤之间的相互关系,为入侵植物生物防治的安全性提供理论指导等方面。     

Defoliating insect immune defense interacts with induced plant defense during a population outbreak

During population outbreaks, top-down and bottom-up factors are unable to control defoliator numbers. To our knowledge, details of biotic interactions leading to increased population density have not been studied during real population outbreaks. We experimentally assessed the strength of plant defenses and of insect immunocompetence, assumed to contribute to active insect resistance against parasitoids and pathogens, in the geometrid Epirrita autumnata during a steep increase in population density. We demonstrated rapid (same-season) induced resistance in the foliage of its host, mountain birch. The response was systemic, spreading throughout the tree, and retarded larval growth rate by approximately 10%. On the other hand, no direct delayed carry-over effects were found in the next season in larval growth rate, mortality, or pupal mass. Larval damage to a tree during the previous year, however, significantly (by approximately 13%) accelerated the advance of the immune response (measured as melanization of an implant inserted into the pupal hemocoel). The encapsulation rate correlated positively with larval mortality in trees in which larvae had been introduced the previous year, but not in control trees. Both of these observations suggest that induced plant defense was associated with an increased insect immunocompetence during the population increase.     

盐沼植物群落研究进展：分布、演替及影响因子

盐沼是全球温带及亚热带地区的主要滨海湿地类型之一,在我国分布广泛。盐沼湿地生态系统敏感、脆弱且具有重要的生态系统服务功能。理解盐沼植物群落时空分布动态的一般规律与生态学机制,是开展盐沼生态系统研究的基础与关键。海陆交界的特殊环境特征是影响盐沼湿地植物群落的空间分布及演替过程的主要因素。在海洋潮汐作用下,盐沼湿地中的盐度、水淹强度、氧化还原电位等非生物因子往往呈梯度分布,这也导致了生物群落中种内、种间关系的变化。在非生物及生物因子的共同作用下,盐沼植物群落也往往沿高程梯度呈带状分布。环境变化是盐沼植物群落演替的驱动因素,在海岸线相对较为稳定的盐沼,植物群落的演替多属自发演替,而在靠近的大型河口的一些持续淤涨的盐沼,植物群落演替通常属于异发演替。沿海地区的水产业、流域上游及沿海地区的工程、污染及生物入侵等直接或间接的人类活动已对盐沼湿地植物群落的产生了深刻影响。经过数十年发展,国际上盐沼植物群落学研究的热点领域主要包括盐沼植物群落与其他生物群落的相互关系、植物群落在盐沼生态系统过程中的作用等。在全球变化背景下,盐沼植物群落对气候变化与海平面升高也日益成为盐沼植物群落学相关的热点。     

Native plant diversity resists invasion at both low and high resource levels

Human modification of the environment is causing both loss of species and changes in resource availability. While studies have examined how species loss at the local level can influence invasion resistance, interactions between species loss and other components of environmental change remain poorly studied. In particular, the manner in which native diversity interacts with resource availability to influence invasion resistance is not well understood. We created experimental plant assemblages that varied in native species (1-16 species) and/or functional richness (defined by rooting morphology and phenology; one to five functional groups). We crossed these diversity treatments with resource (water) addition to determine their interactive effects on invasion resistance to spotted knapweed (Centaurea maculosa), a potent exotic invader in the intermountain West of the United States. We also determined how native diversity and resource addition influenced plant-available soil nitrogen, soil moisture, and light. Assemblages with lower species and functional diversity were more heavily invaded than assemblages with greater species and functional diversity. In uninvaded assemblages, experimental addition of water increased soil moisture and plant-available nitrogen and decreased light availability. The availability of these resources generally declined with increasing native plant diversity. Although water addition increased susceptibility to invasion, it did not fundamentally change the negative relationship between diversity and invasibility. Thus, native diversity provided strong invasion resistance even under high resource availability. These results suggest that the effects of local diversity can remain robust despite enhanced resource levels that are predicted under scenarios of global change.     

人工封育区植物群落恢复演替系列种群生态位动态特征——以宁夏盐池为例

为了解干旱半干旱区草原植物生态位格局,运用Levins生态宽度指数及Pianka生态位重叠指数,对宁夏盐池人工封育草原3种不同措施下的植物生态位宽度及生态位重叠进行计量。结果表明:茵陈蒿(Artemisia capillaries Thunb)和新疆猪毛菜(Salsola sinkiangensis A.J.Li)的生物、生态学特性决定了它在该区的优势地位,其生态位宽度远远高于其他物种,并且出现下降的趋势;生态位重叠结果均表明,生态位宽度大的物种不一定和其他物种有大的重叠值,较高的生态位宽度和较高的生态位重叠之间并不存在直接的线性关系;较高的Pianka生态位重叠值都出现在生态位宽度较小的物种之间。这一现象从另外一个角度说明植被恢复过程中环境资源存在着高度的空间异质性。     

Context matters: matrix vegetation influences native and exotic species composition on habitat islands

The extensive research on plant communities of natural-habitat islands has primarily focused on the "islands." The island analogy, however, potentially limits understanding of processes influencing composition on habitat islands because the nature of their matrix is overlooked. We determine how plant community structure of the surrounding matrix influences vegetation on volcanic outcrops in the modified landscape of Banks Peninsula, New Zealand. Our primary purpose is to address whether the matrix is more important for recently established exotic species than it is for well-established native species and whether such invasion by exotics has led to homogenization of the outcrop flora. To test this, we examined our data at three spatial scales: that of the entire outcrop flora, between individual outcrops and their immediate surrounding matrix, and between individual outcrop faces and the individual relevés of the immediate surrounding matrix. We found that 81% of the native flora and 90% of the exotic flora also occur in the matrix. This high level of species shared between the outcrop and matrix persists at the scale of individual outcrop faces (68% of the total flora of individual faces is shared with the matrix). We predicted that floras from different outcrops would vary in their distinctiveness from their immediate matrix. We found Bray-Curtis distance coefficient values to range from 0.26 to 0.64; these were even higher at the outcrop-face scale. Variability in outcrop distinctiveness relates primarily to the outcrop face properties of area, vegetation height, and soil depth, and matrix properties of vegetation structure and vegetation heterogeneity. The effect of the vegetation structure of the matrix is more pronounced on the exotic than on the native outcrop flora. The component of composition and structure of the matrix that was independent of outcrop properties and local environment accounts for similar levels of explainable variation in total and native composition (29-31%), but considerably more (40%) in composition of exotic species. Our results support our prediction that, as the surrounding matrix becomes more modified, invasion by exotics makes outcrop vegetation less distinct from its matrix.     

沿黄河下游湖泊湿地植物群落演替及其多样性研究

沿黄河下游湖泊湿地是黄河健康生命系统的重要组成部分,其植物群落演替及其多样性研究对湿地及其生物多样性保护有重要意义。通过野外样地调查,分析了沿黄河下游湖泊湿地植物物种组成及其群落的演替模式,并计算了群落植物多样性特征指数,包括物种丰富度指数（S）、多样性Simpson指数（D）、Shannon-wiener指数（H′）、均匀度指数Pielou（E）。结果表明,研究区湖泊湿地共有77种植物,属于28科61属,除榆树Ulmus pumila Linn.为木本植物外,其余76种为草本植物,其中有1种轮藻门植物,1种蕨类植物;从空间来看植物群落演替过程为沉水植物菹草群落→纯水烛群落→水烛群落→水烛＋扁秆藨草群落→藨草群落→芦苇群落,青龙湖湿地水环境的变化,使湿地植物群落的类型也随之发生变化;群落多样性指数Shannon-wiener指数均低于1.12,Simpson指数为0.41~1,Pielou均匀度指数为0.8左右;芦苇群落的物种数最多,有32个物种,水烛群落以12种次之,菹草群落、水烛＋扁秆藨草群落和扁秆藨草群落中的物种数差别不大。沿黄河下游湿地植物群落多样性水平较低,湿地系统有向陆地系统过渡的趋势。