Beyond the ecological: biological invasions alter natural selection on a native plant species

Biological invasions can have strong ecological effects on native communities by altering ecosystem functions, species interactions, and community composition. Even though these ecological effects frequently impact the population dynamics and fitness of native species, the evolutionary consequences of biological invasions have received relatively little attention. Here, I show that invasions impose novel selective pressures on a native plant species. By experimentally manipulating community composition, I found that the exotic plant Medicago polymorpha and the exotic herbivore Hypera brunneipennis alter the strength and, in some instances, the direction of natural selection on the competitive ability and anti-herbivore defenses of the native plant Lotus wrangelianus. Furthermore, the community composition of exotics influenced which traits were favored. For example, high densities of the exotic herbivore Hypera selected for increased resistance to herbivores in the native Lotus; however, when Medicago also was present, selection on this defense was eliminated. In contrast, selection on tolerance, another plant defense trait, was highest when both Hypera and Medicago were present at high densities. Thus, multiple exotic species may interact to influence the evolutionary trajectories of native plant populations, and patterns of selection may change as additional exotic species invade the community.     

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.     

Human impacts, plant invasion, and imperiled plant species in California.

Invasive species are one of the fastest growing conservation problems. These species homogenize the world's flora and fauna, threaten rare and endemic species, and impose large economic costs. Here, we examine the distribution of 834 of the more than 1000 exotic plant taxa that have become established in California, USA. Total species richness increases with net primary productivity; however, the exotic flora is richest in low-lying coastal sites that harbor large numbers of imperiled species, while native diversity is highest in areas with high mean elevation. Weedy and invasive exotics are more tightly linked to the distribution of imperiled species than the overall pool of exotic species. Structural equation modeling suggests that while human activities, such as urbanization and agriculture, facilitate the initial invasion by exotic plants, exotics spread ahead of the front of human development into areas with high numbers of threatened native plants. The range sizes of exotic taxa are an order of magnitude smaller than for comparable native taxa. The current small range size of exotic species implies that California has a significant "invasion debt" that will be paid as exotic plants expand their range and spread throughout the state.     

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.     

Biotic resistance via granivory: establishment by invasive, naturalized, and native asters reflects generalist preference

Escape from specialist natural enemies is frequently invoked to explain exotic plant invasions, but little attention has been paid to how generalist consumers in the recipient range may influence invasion. We examined how seed preferences of the widespread generalist granivore Peromyscus maniculatus related to recruitment of the strongly invasive exotic Centaurea stoebe and several weakly invasive exotics and natives by conducting laboratory feeding trials and seed addition experiments in the field. Laboratory feeding trials showed that P. maniculatus avoided consuming seeds of C. stoebe relative to the 12 other species tested, even when seeds of alternative species were 53-94% smaller than those of C. stoebe. Seed addition experiments conducted in and out of rodent exclosures revealed that weakly invasive exotics experienced relatively greater release from seed predation than C. stoebe, although this was not the case for natives. Seed mass explained 81% of the variation in recruitment associated with rodent exclusion for natives and weak invaders, with larger-seeded species benefiting most from protection from granivores. However, recruitment of C. stoebe was unaffected by rodent exclusion, even though the regression model predicted seeds of correspondingly large mass should experience substantial predation. These combined laboratory and field results suggest that generalist granivores can be an important biological filter in plant communities and that species-specific seed attributes that determine seed predation may help to explain variation in native plant recruitment and the success of exotic species invasions.     

Abiotic constraints eclipse biotic resistance in determining invasibility along experimental vernal pool gradients.

Effective management of invasive species requires that we understand the mechanisms determining community invasibility. Successful invaders must tolerate abiotic conditions and overcome resistance from native species in invaded habitats. Biotic resistance to invasions may reflect the diversity, abundance, or identity of species in a community. Few studies, however, have examined the relative importance of abiotic and biotic factors determining community invasibility. In a greenhouse experiment, we simulated the abiotic and biotic gradients typically found in vernal pools to better understand their impacts on invasibility. Specifically, we invaded plant communities differing in richness, identity, and abundance of native plants (the "plant neighborhood") and depth of inundation to measure their effects on growth, reproduction, and survival of five exotic plant species. Inundation reduced growth, reproduction, and survival of the five exotic species more than did plant neighborhood. Inundation reduced survival of three species and growth and reproduction of all five species. Neighboring plants reduced growth and reproduction of three species but generally did not affect survival. Brassica rapa, Centaurea solstitialis, and Vicia villosa all suffered high mortality due to inundation but were generally unaffected by neighboring plants. In contrast, Hordeum marinum and Lolium multiflorum, whose survival was unaffected by inundation, were more impacted by neighboring plants. However, the four measures describing plant neighborhood differed in their effects. Neighbor abundance impacted growth and reproduction more than did neighbor richness or identity, with growth and reproduction generally decreasing with increasing density and mass of neighbors. Collectively, these results suggest that abiotic constraints play the dominant role in determining invasibility along vernal pool and similar gradients. By reducing survival, abiotic constraints allow only species with the appropriate morphological and physiological traits to invade. In contrast, biotic resistance reduces invasibility only in more benign environments and is best predicted by the abundance, rather than diversity, of neighbors. These results suggest that stressful environments are not likely to be invaded by most exotic species. However, species, such as H. marinum, that are able to invade these habitats require careful management, especially since these environments often harbor rare species and communities.     

薇甘菊入侵对深圳湾红树林生态系统碳储量的影响

外来植物入侵对生态系统碳过程的影响已经成为入侵生态学研究的热点问题。采用比较研究的方法分析了薇甘菊（MikaniamicranthaH．B．K．）入侵对深圳湾红树林生态系统碳储量的影响,分别设置红树林（Kandelia candel）群落、红树林．薇甘菊群落、薇甘菊群落以及光滩等样地,采集植被、凋落物和土壤剖面样品,分析生态系统各组分的碳储量变化。结果表明,薇甘菊入侵对红树林生态系统碳储量有着显著的影响,碳储量从未被入侵下215．73tC／hm^2减少到轻、高度入侵下的197．56tC／hm2和166．70tC／hm2,分别减少8．42％和22．72％。其中植被和土壤碳储量显著减少,凋落物碳储量显著增加。薇甘菊入侵一方面导致红树林枯萎,减少植被生物量,增加凋落物量,另一方面促进了土壤微生物活动,使土壤有机碳分解释放,降低土壤碳储量。薇甘菊入侵后红树林碳储量的降低将大大削弱生态系统的碳汇功能。     

澳门松山市政公园物种多样性特征

根据实际情况.将澳门松山市政公园植物群落分为阴香群落、假萍婆群落、假萍婆+石栗群落,调查并分析了各群落的物种多样性特征.结果表明,乔木层、灌木层和草本层的物种多样性水平表现为灌木层较高.草本层与乔木层较低.但总体而言.这些群落的植物种类都较少,物种多样性水平尤其乔木层物种多样性水平低.因此,应加强抚育,可通过生态恢复改善环境条件,为生物多样性的发展创造条件;通过林分改造促进群落物种多样性水平的提高,同时也能丰富森林景观.     

Indicators of ecosystem function identify alternate states in the sagebrush steppe

Models of ecosystem change that incorporate nonlinear dynamics and thresholds, such as state-and-transition models (STMs), are increasingly popular tools for land management decision-making. However, few models are based on systematic collection and documentation of ecological data, and of these, most rely solely on structural indicators (species composition) to identify states and transitions. As STMs are adopted as an assessment framework throughout the United States, finding effective and efficient ways to create data-driven models that integrate ecosystem function and structure is vital. This study aims to (1) evaluate the utility of functional indicators (indicators of rangeland health, IRH) as proxies for more difficult ecosystem function measurements and (2) create a data-driven STM for the sagebrush steppe of Colorado, USA, that incorporates both ecosystem structure and function. We sampled soils, plant communities, and IRH at 41 plots with similar clayey soils but different site histories to identify potential states and infer the effects of management practices and disturbances on transitions. We found that many IRH were correlated with quantitative measures of functional indicators, suggesting that the IRH can be used to approximate ecosystem function. In addition to a reference state that functions as expected for this soil type, we identified four biotically and functionally distinct potential states, consistent with the theoretical concept of alternate states. Three potential states were related to management practices (chemical and mechanical shrub treatments and seeding history) while one was related only to ecosystem processes (erosion). IRH and potential states were also related to environmental variation (slope, soil texture), suggesting that there are environmental factors within areas with similar soils that affect ecosystem dynamics and should be noted within STMs. Our approach generated an objective, data-driven model of ecosystem dynamics for rangeland management. Our findings suggest that the IRH approximate ecosystem processes and can distinguish between alternate states and communities and identify transitions when building data-driven STMs. Functional indicators are a simple, efficient way to create data-driven models that are consistent with alternate state theory. Managers can use them to improve current model-building methods and thus apply state-and-transition models more broadly for land management decision-making.     

Disturbance, Diversity, and Invasion: Implications for Conservation

Disturbance is an important component of many ecosystems, and variations in disturbance regime can affect ecosystem and community structure and functioning. The "intermediate disturbance hypothesis" suggests that species diversity should be highest at moderate levels of disturbance. However, disturbance is also known to increase the invasibility of communities. Disturbance therefore poses an important problem for conservation management, Here, we review the effects of disturbances such as fire grazing, soil disturbance and nutrient addition on plant species diversity and invasion with particular emphasis on grassland vegetation. Individual components of the disturbance regime can have marked effects on species diversity, but it is often modifications of the existing regime that have the largest influence. Similarly, disturbance can enhance invasion of natural communities, but frequently it is the interaction between different disturbances that has the largest effect. The natural disturbance regime is now unlikely to persist within conservation areas since fragmentation and human intervention have usually modified physical and biotic conditions. Active management decisions must now be made on what disturbance regime is required and this requires decisions on what species are to be encouraged or discouraged.     

High richness and dense seeding enhance grassland restoration establishment but have little effect on drought response

Restorations commonly utilize seed addition to formerly arable lands where the development of native plant communities is severely dispersal limited. However, variation in seed addition practices may profoundly affect restoration outcomes. Theory and observations predict that species-rich seed mixtures and seeding at high densities should enhance native plant community establishment, minimize exotic species cover, and may promote resistance and resilience to, and recovery from, environmental perturbations. We studied the post-seeding establishment of native plant communities in large grassland restoration plots, which were sown at two densities crossed with two levels of species richness on formerly arable land in Nebraska, USA, and their responses to drought. To evaluate drought resistance, recovery, and resilience of restored plant communities, we erected rainfall manipulation structures and tracked the response of seeded species cover and total plant biomass during experimental drought relative to controls and in the post-drought growing season. High seed richness and high-density seeding treatments resulted in greater richness and cover of native, seeded species per 0.5 m2 compared to low-richness and low-density treatments. Cover differences in response to seed mixture richness were driven by native forbs. Richness and cover of exotic species were lowest in high-richness and high-density treatments. We found little evidence of differential drought resistance, recovery, and resilience among seeding treatments. Increases in exotic species across years were restricted to drought subplots, and were not affected by seeding treatments. Grassland restoration was generally enhanced and exotic cover reduced both by the use of high-richness seed mixtures and high-density seeding. Given the lack of restoration treatment effects on the resistance, recovery, or resilience of seeded species exposed to drought, and the increases in exotic species following drought, other forms of active management may be needed to produce restored plant communities that are robust to climate change.     

Impact of Grazing Intensity during Drought in an Arizona Grassland

Abstract:  The ecological benefits of changing cattle grazing practices in the western United States remain controversial, due in part to a lack of experimentation. In 1997 we initiated an experimental study of two rangeland alternatives, cattle removal and high-impact grazing, and compared grassland community responses with those with more conventional, moderate grazing practices. The study was conducted in a high-elevation, semiarid grassland near Flagstaff, Arizona (U.S.A.). We conducted annual plant surveys of modified Whittaker plots for 8 years and examined plant composition shifts among treatments and years. High-impact grazing had strong directional effects that led to a decline in perennial forb cover and an increase in annual plants, particularly the exotic cheatgrass ( Bromus tectorum L.). A twofold increase in plant cover by exotic species followed a severe drought in the sixth year of the study, and this increase was greatest in the high-impact grazing plots, where native cover declined by one-half. Cattle removal resulted in little increase in native plant cover and reduced plant species richness relative to the moderate grazing control. Our results suggest that some intermediate level of cattle grazing may maintain greater levels of native plant diversity than the alternatives of cattle removal or high-density, short-duration grazing practices. Furthermore, episodic drought interacts with cattle grazing, leading to infrequent, but biologically important shifts in plant communities. Our results demonstrate the importance of climatic variation in determining ecological effects of grazing practices, and we recommend improving conservation efforts in arid rangelands by developing management plans that anticipate this variation.     

Seed and establishment limitation contribute to long-term native forb declines in California grasslands

The effects of exotic species invasions on biodiversity vary with spatial scale, and documentation of local-scale changes in biodiversity following invasion is generally lacking. Coupling long-term observations of local community dynamics with experiments to determine the role played by exotic species in recruitment limitation of native species would inform both our understanding of exotic impacts on natives at local scales and regional-scale management efforts to promote native persistence. We used field experimentation to quantify propagule and establishment limitation in a suite of native annual forbs in a California reserve, and compared these findings to species abundance trends within the same sites over the past 48 years. Observations at 11 paired sites (inside and outside the reserve) indicated that exotic annual plants have continued to increase in abundance over the past 48 years. This trend suggests the system has not reached equilibrium > 250 years after exotic species began to spread, and 70 years after livestock grazing ceased within the reserve. Long-term monitoring observations also indicated that six native annual forb species went extinct from more local populations than were colonized. To determine the potential role of exotic species in these native plant declines, we added seed of these species into plots adjacent to monitoring sites where plant litter and live grass competition were removed. Experimental results suggest both propagule and establishment limitation have contributed to local declines observed for these native forbs. Recruitment was highest at sites that had current or historical occurrences of the seeded species, and in plots where litter was removed. Grazing history (i.e., location within or outside the reserve) interacted with exotic competition removal, such that removal of live grass competition increased recruitment in more recently grazed sites. Abundance of forbs was positively related to recruitment, while abundance of exotic forbs was negatively related. Thus, exotic competition is likely only one factor contributing to local declines of native species in invaded ecosystems, with a combination of propagule limitation, site quality, and land use history also playing important and interactive roles in native plant recruitment.     

Native and exotic plant species exhibit similar population dynamics during succession

A growing body of literature has led to the debate in invasion biology whether exotic species perform within communities differently than native taxa due to inherent advantages. To address this issue, the population dynamics of native and exotic plant species were assessed from a 48-year record of permanent plot data from the Hutcheson Memorial Forest Center (New Jersey, USA) to determine rate of increase, lag time, maximum frequency, and the year of peak frequency. Overall, native and exotic species exhibited very similar population dynamics. Rates of increase and length of lag times were similar between native and exotic taxa but were strongly influenced by plant life form. Short-lived species were characterized by rapid population growth rates and short lag times. Growth rates decreased and lag times increased with species longevity. Overall, correlations between population metrics were the same in native and exotic taxa, suggesting similar trade-offs in life history patterns. The one difference observed was that, in native species, peak frequency was negatively associated with the year of peak frequency (i.e., early-successional species tended to become more abundant), while there was no relationship in exotic species. These analyses show that exotic species behave in essentially the same way as native taxa within dynamic communities. This suggests that abundant native and exotic plant species are exploiting the same range of ecological strategies resulting in similar roles within communities.     

三江源区不同建植年代人工草地群落演替与土壤养分变化

研究了了三源区不同建植期人工修复草地在不同演替阶段毒杂草[主要是甘肃马先蒿(Pedicularis kansuensis)]的入侵规律、数量特征,植物群落物种组成、生物苗和草地质最以及土壤养分、微生物活性的变化规律.结果表明,不同建植期人工修复草地植物群落的种类组成、植物功能群组成和群落数量特征存在显著差异.随着演替时间的推移,人工草地群落盖度、高度、物种数、生物最和多样性指数均表现出"V"字型变化规律,杂类草--甘肃马先蒿的数量特征变化尤为明显,在4 a的人工草地群落中开始局部入侵,在5～6 a的人工草地群落中大面积入侵,其入侵速度、入侵面积达到高峰期.土壤的含水量、容重、土壤中有机质、氮素和磷素在演替过程(7 a、9 a草地)中逐渐降低,到一定时期又逐步增加;随着演替的进行,不同建植期人工草地的土壤微牛物生物量碳和酶活性均呈"V"字型,变化.对于退化生态系统的恢复首先是植被恢复,其次是土壤肥力的恢复.土壤有机质等养分的积累、微生物活性的改善不仅能使土壤-植物复合系统的功能得以恢复,同时也能促进物种多样性的形成,有利于人工草地群落稳定性的提高.在试验区尽管植被恢复演替进行得比较缓慢,但从土壤发展的角度看,仍属进展演替.所以,在退化高寒草甸的恢复过程中,若降低和有效控制外界的干扰(如围栏封育),可为退化草地恢复提供繁殖体与土壤环境,实现人工草地逐步向恢复(正向)演替进行.图3表6参34     

Revealing how species loss affects ecosystem function: the trait-based Price Equation partition

Species loss can alter ecosystem function. Recent work proposes a general theoretical framework, the "Price Equation partition," for understanding how species loss affects ecosystem functions that comprise the summed contributions of individual species (e.g., primary production). The Price Equation partition shows how the difference in function between a pre-species-loss site and a post-loss site can be partitioned into effects of random loss of species richness (species-richness effect; SRE), nonrandom loss of high- or low-functioning species (species-composition effect; SCE), and post-loss changes in the functional contributions of the remaining species (context-dependence effect; CDE). However, the Price Equation partition is silent on the underlying determinants of species' functional contributions. Here we extend the Price Equation partition by using multiple regression to describe how species' functional contributions depend on species' traits. This allows us to reexpress the SCE and CDE in terms of nonrandom loss of species with particular traits (trait-based SCE), and post-loss changes in species' traits and in the relationship between species' traits and species' functional contributions (trait-based CDE). We apply this new trait-based Price Equation partition to studies of species loss from grassland plant communities and protist microcosm food webs. In both studies, post-loss changes in the relationship between species' traits and their functional contributions alter ecosystem function more than nonrandom loss of species with particular traits. The protist microcosm data also illustrate how the trait-based Price Equation partition can be applied when species' functional contributions depend in part on the traits of other species. To do this, we define "synecological" traits that quantify how unique species are (e.g., in diet) compared to other species. Context dependence in the protist microcosm experiment arises in part because species loss alters the diet uniqueness of the remaining species.     

紫茎泽兰入侵过程中生物群落的交互作用

生物入侵在全球范围内影响了生物群落的结构与功能,打破了群落内物种共存的生态格局,继而反馈性影响全球环境。该文就外来杂草紫茎泽兰入侵对生物群落之间交互作用的影响进行了分析。1）紫茎泽兰通过竞争排斥降低了土著植物群落的多样性,造成依赖于土著植物的节肢动物群落减少或丧失适宜的栖息环境。2）打破了土著植物与节肢动物之间相互依存的状态,并通过单优群落优势和强烈化感作用制约天敌昆虫的自然控制作用。3）通过改变地表生境和枯落物种类影响土壤动物群落。4）引起土壤微生物群落组成和功能的变化,改变土壤中可利用资源的形式和数量,影响并重塑了生物种间互作模式,并动态反馈于地面植物群落新格局的形成。分析指出：1）入侵过程中群落之间的交互作用通过多层次生态过程对群落结构与功能的生态改变发挥影响。2）入侵对生物群落的改变所产生的生态驱动反馈性作用于群落互作模式的重塑、群落和生态系统新格局的重建。同时,指出了生物入侵对群落影响的复杂性以及后续研究的方向。     

Susceptibility of a Northern Hardwood Forest to Exotic Earthworm Invasion

Abstract:  Numerous exotic earthworm species are colonizing northern hardwood forests of North America, where no native earthworms exist. Upon invasion, earthworms have been shown to alter the surface soil environment and plant populations and communities. We sought to identify land-use factors in the Ottawa National Forest (ONF), Michigan (U.S.A.), that contribute to earthworm invasion in forest dominated by sugar maple ( Acer saccharum Marsh.) so that the susceptibility to additional colonization could be evaluated. We sampled earthworm communities in Sylvania Wilderness Area, a unique old-growth hardwood forest, and nonwilderness sites influenced by recreational fishing, recent timber harvesting, or roads. All the nonwilderness sites contained one to five species of exotic earthworms. In contrast, only 50% of wilderness sites contained exotic earthworms, all of a single species. Nonwilderness sites also had thinner litter and duff layers, higher soil C and N content, and higher nitrogen mineralization potentials than Sylvania sites. Two central differences between Sylvania and nonwilderness sites were that all nonwilderness sites were in close contact with roads and had a history of timber harvest, whereas these factors were not present in Sylvania Wilderness Area. Using average rates of colonization, we constructed two geographic information system models to estimate the percentage of sugar maple on the ONF falling within a theoretical 100-year invasion distance of roads and of second-growth sugar maple as relative indices of susceptibility to invasion. Both models indicated high susceptibility to invasion, with 91.7% and 98.9% of sugar maple habitat falling within a theoretical 100-year invasion distance of roads or historical harvests, respectively.     

Abundance declines of a native forb have nonlinear impacts on grassland invasion resistance

The effects of declining plant biodiversity on ecosystem processes are well studied, with most investigations examining the role of species richness declines rather than declines of species abundance. Using grassland mesocosms, we examined how the abundance of a native, resident species, Hemizonia congesta (hayfield tarweed), affected exotic Centaurea solstitialis (yellow starthistle) invasion. We found that progressive H. congesta abundance declines had threshold effects on invasion resistance, with initial declines resulting in minor increases in invasion and subsequent declines leading to accelerating increases in invader performance. Reduced invasion resistance was explained by increased resource availability as H. congesta declined. We also found evidence that resident abundance might indirectly affect invasion by mediating invader impact on resident competitors; C. solstitialis disproportionately reduced H. congesta biomass in low-abundance rather than high-abundance populations. H. congesta's direct and indirect effects on invasion resistance illustrate that an individual species' declining abundance can have accelerating, deleterious effects on ecosystem functions of conservation value.     

Long term Changes in the Relative Abundances of Birds in Kings Park, Perth, Western Australia

Abstract. D. L. Serventy censused birds in Kings Park, Perth, between 1928 and 1937 and again between 1952 and 1955. Abundance was estimated by the frequency with which species were detected on censuses. The counts were repeated in 1986 by H. F. Recher. A total of 44 species were recorded during censuses. Since the first counts, were made, 14 species decreased in abundance by more than 15%, and 9 species went locally extinct. Eleven species increased in abundance by more than 20% (5 by more than 50%), and the abundance of 19 remained unchanged (frequency of occurrence changed by less than 15%).
Although nearly 300 ha of the park's 400 ha area remains as native vegetation, this vegetation has been degraded by logging, changed fire regimes, construction of trails, and invasion by exotic weeds. Changes in the ground vegetation have had a particularly significant impact. Nine of the 16 species that declined in abundance are ground foragers. Of the species that increased in abundance, a number benefit from the greater availability of nectar, fruits, and water in the suburban gardens that have developed around the park The Australian Raven (Corvus coronoides) has become particularly abundant and predation by ravens on nests was contribute to the decline of some species Changes to tbe park vegetation, suburban development of the park's immediate surroundings, and the extensive clearing of vegetation throughout southwestem Australia appear to have had a more signifcant effect on the park's avifauna than the sire of the park or its isolation in an urban environment. Poor management has seriously reduced the capacity of Kings Park to support native wildliye. Changes in park management designed to mbabilitate native vegetation, control weeds, and control ravens are necessary to prevent the further loss of species from the park's avifauna.