Elton’s hypothesis revisited: an experimental test using cogongrass

In the 1950s Charles Elton hypothesized that more diverse communities should be less susceptible to invasion by exotic species (biodiversity–invasibility hypothesis). The biodiversity–invasibility hypothesis postulates that species-rich communities are less vulnerable to invasion because vacant niches are less common and the intensity of interspecific competition is more severe. Field studies were conducted at two sites, a logged site and an unlogged site in Santa Rosa County, Florida, U.S.A, to test Elton’s hypothesis using cogongrass (Imperata cylindrica), a non-indigenous grass invading large areas of the Southeastern United States. The logged site was under 17-year-old loblolly pine prior to clear cutting. The unlogged site, a longleaf pine forest, was at the Blackwater River State Forest. Both the logged site and unlogged site showed no significant relationship between the rate of cogongrass spread and native plant species richness, functional richness, and cover of the invaded community. Increased species or functional richness may increase the use of resources; however, the extensive rhizome/root network possessed by cogongrass and its ability to thrive under shade may allow for its persistence in a diverse community. The results from both the logged and unlogged sites do not support the general hypothesis of Elton that invasion resistance and compositional stability increase with diversity. Biodiversity does not appear to be an important factor for cogongrass invasion in the southern United States. Extrinsic factors in this study prevent the ability to draw a defined causal relationship between native plant diversity and invasibility. Underlying reasons for why no relationship was observed may be simply due to the tremendous competitive ability of cogongrass or the narrow range of species richness, functional richness and cover observed in our study.     

Amphibian Beta Diversity in the Brazilian Atlantic Forest: Contrasting the Roles of Historical Events and Contemporary Conditions at Different Spatial Scales

Current patterns of biodiversity distribution result from a combination of historical and contemporary processes. Here, we compiled checklists of amphibian species to assess the roles of long-term climate stability (Quaternary oscillations), contemporary environmental gradients and geographical distance as determinants of change in amphibian taxonomic and phylogenetic composition in the Brazilian Atlantic Forest. We calculated beta diversity as both variation in species composition (CBD) and phylogenetic differentiation (PBD) among the assemblages. In both cases, overall beta diversity was partitioned into two basic components: species replacement and difference in species richness. Our results suggest that the CBD and PBD of amphibians are determined by spatial turnover. Geographical distance, current environmental gradients and long-term climatic conditions were complementary predictors of the variation in CBD and PBD of amphibian species. Furthermore, the turnover components between sites from different regions and between sites within the stable region were greater than between sites within the unstable region. On the other hand, the proportion of beta-diversity due to species richness difference for both CBD and PBD was higher between sites in the unstable region than between sites in the stable region. The high turnover components from CBD and PBD between sites in unstable vs stable regions suggest that these distinct regions have different biogeographic histories. Sites in the stable region shared distinct clades that might have led to greater diversity, whereas sites in the unstable region shared close relatives. Taken together, these results indicate that speciation, environmental filtering and limited dispersal are complementary drivers of beta-diversity of amphibian assemblages in the Brazilian Atlantic Forest.     

Giant invasive Heracleum persicum: Friend or foe of plant diversity?

The impact of invasion on diversity varies widely and remains elusive. Despite the considerable attempts to understand mechanisms of biological invasion, it is largely unknown whether some communities’ characteristics promote biological invasion, or whether some inherent characteristics of invaders enable them to invade other communities. Our aims were to assess the impact of one of the massive plant invaders of Scandinavia on vascular plant species diversity, disentangle attributes of invasible and noninvasible communities, and evaluate the relationship between invasibility and genetic diversity of a dominant invader. We studied 56 pairs of Heracleum persicum Desf. ex Fisch.‐invaded and noninvaded plots from 12 locations in northern Norway. There was lower native cover, evenness, taxonomic diversity, native biomass, and species richness in the invaded plots than in the noninvaded plots. The invaded plots had nearly two native species fewer than the noninvaded plots on average. Within the invaded plots, cover of H. persicum had a strong negative effect on the native cover, evenness, and native biomass, and a positive association with the height of the native plants. Plant communities containing only native species appeared more invasible than those that included exotic species, particularly H. persicum. Genetic diversity of H. persicum was positively correlated with invasibility but not with community diversity. The invasion of a plant community by H. persicum exerts consistent negative pressure on vascular plant diversity. The lack of positive correlation between impacts and genetic diversity of H. persicum indicates that even a small founder population may cause high impact. We highlight community stability or saturation as an important determinant of invasibility. While the invasion by H. persicum may decrease susceptibility of a plant community to further invasion, it severely reduces the abundance of native species and makes them more vulnerable to competitive exclusion.     

First Evidence of <Emphasis Type="Italic">Batrachochytrium dendrobatidis</Emphasis> in China: Discovery of Chytridiomycosis in Introduced American Bullfrogs and Native Amphibians in the Yunnan Province,China

Although the chytrid fungus Batrachochytrium dendrobatidis (Bd), the etiological agent of amphibian chytridiomycosis, has been implicated in mass mortality and population declines on several continents around the world, there have been no reports on the presence of Bd infections in amphibians in China. We employed quantitative PCR and histological techniques to investigate the presence of Bd in introduced North American bullfrogs (Rana catesbeiana) (referred to hereafter as bullfrog) and native amphibians in bullfrog-invaded areas of the Yunnan Province, China. A total of 259 samples at five wild sites were collected between June and September in 2007 and 2008, including bullfrogs and four native amphibian species (Rana pleuraden, Rana chaochiaoensis, Odorrana andersonii, and Bombina maxima). In addition, 37 samples of adult bullfrogs were obtained from a food market. Bd infections were discovered in bullfrogs and three native amphibian species from all of the surveyed sites. Of the 39 Bd-positive samples, 35 were from wild-caught bullfrog tadpoles, postmetamorphic bullfrogs, R. pleuraden, R. chaochiaoensis, and O. andersonii, and four were from adult bullfrogs from the market. Our results provide the first evidence of the presence of Bd in Chinese amphibians, suggesting that native amphibian diversity in China is at risk from Bd. There is an urgent need to monitor the distribution of Bd in amphibians in China and understand the susceptibility of native amphibian species to chytridiomycosis. Strict regulations on the transportation of bullfrogs and the breeding of bullfrogs in markets and farms should be drafted in order to stop the spread of Bd by bullfrogs.     

Species-Rich Scandinavian Grasslands are Inherently Open to Invasion

Invasion of native habitats by alien or generalist species is recognized worldwide as one of the major causes behind species decline and extinction. One mechanism determining community invasibility, i.e. the susceptibility of a community to invasion, which has been supported by recent experimental studies, is species richness and functional diversity acting as barriers to invasion. We used Scandinavian semi-natural grasslands, exceptionally species-rich at small spatial scales, to examine this mechanism, using three grassland generalists and one alien species as experimental invaders. Removal of two putative functional groups, legumes and dominant non-legume forbs, had no effect on invasibility except a marginally insignificant effect of non-legume forb removal. The amount of removed biomass and original plot species richness had no effect on invasibility. Actually, invasibility was high already in the unmanipulated community, leading us to further examine the relationship between invasion and propagule pressure, i.e. the inflow of seeds into the community. Results from an additional experiment suggested that these species-rich grasslands are effectively open to invasion and that diversity may be immigration driven. Thus, species richness is no barrier to invasion. The high species diversity is probably in itself a result of the community being highly invasible, and species have accumulated at small scales during centuries of grassland management.     

Invasive ants as back-seat drivers of native ant diversity decline in New Caledonia

Biological invasions are typically associated with disturbance, which often makes their impact on biodiversity unclear—biodiversity decline might be driven by disturbance, with the invader just being a ‘passenger’. Alternatively, an invader may act as a ‘back-seat driver’, being facilitated by disturbance that has already caused some biodiversity decline, but then causing further decline. Here we examine the interactive effects of anthropogenic fire and invasive ant species (Anoplolepis gracilipes or Wasmannia auropunctata) on native ant diversity in New Caledonia, a globally recognized biodiversity hotspot. We first examined native ant diversity at nine paired burnt and unburnt sites, with four pairs invaded by Anoplolepis, 5 years after an extensive fire. In the absence of invasion, native epigaeic ants were resilient to fire, but native ant richness and the abundance of Forest Opportunists were markedly lower in invaded burnt sites. Second, we examined native ant diversity along successional gradients from human-derived savanna to natural rainforest in the long-term absence of fire, where there was a disconnection between disturbance-mediated variation in microhabitat and the abundance of the disturbance specialist Wasmannia. All native ant diversity responses (total abundance, richness, species composition, functional group richness and the abundance of Forest Opportunists) declined independently of microhabitat variables but in direct association with high Wasmannia abundance. Our results indicate that invasive ants are acting as back-seat drivers of biodiversity decline in New Caledonia, with invasion facilitated by disturbance but then causing further biodiversity decline.     

Community phylogenetic diversity and abiotic site characteristics influence abundance of the invasive plant Rhamnus cathartica L.

Aims Theory predicts that the success of introduced species is related to the diversity of native species through trait-based processes. Abiotic site characteristics may also affect a site's susceptibility to invasion. We quantified resident plant species richness, phylogenetic diversity and several abiotic site characteristics for 24 oak forests in Minnesota, USA, to assess their impact on the abundance of a widespread, introduced terrestrial plant species, common buckthorn (Rhamnus cathartica L.). Specifically, we asked (1) whether resident species richness and phylogenetic diversity affected the abundance of R. cathartica and (2) what site characteristics explained the overall abundance of R. cathartica .Methods Our survey included 24 oak-dominated stands in Minnesota's deciduous forests. In each stand, we identified all species in 16 plots. We also measured a series of environmental site characteristics, including canopy openness (a proxy for light availability), percent bare soil, soil pH, percent sand, an index of propagule availability, duff layer thickness (a proxy for earthworm activity), an index of insolation and slope. For all species present in at least one site, we estimated a community phylogeny. We combined all site-level characteristics, including phylogenetic diversity of the resident plant species, in a multiple regression model to examine site level drivers of community invasibility.Important findings Results indicate that sites with higher overall plant phylogenetic diversity harbor less R. cathartica, even though native species richness was not significantly related to R. cathartica abundance. Regression analyses indicated that, in addition to resident species phylogenetic diversity, the most important predictors of R. cathartica abundance were canopy openness and the amount of bare soil, both positively related to the abundance of the invader. By combining the effects of abiotic site characteristics and resident species phylogenetic diversity in a model that predicted the abundance of R. cathartica, we were able to simultaneously account for a wide range of factors that might influence invasibility. Overall, our results suggest that management strategies aimed at reducing disturbances that lead to increased bare soil and light levels may be more successful if they also maximize phylogenetic diversity of the resident plant community.     

Exotic plant species in a C4-dominated grassland: invasibility, disturbance, and community structure

We used data from a 15-year experiment in a C₄-dominated grassland to address the effects of community structure (i.e., plant species richness, dominance) and disturbance on invasibility, as measured by abundance and richness of exotic species. Our specific objectives were to assess the temporal and spatial patterns of exotic plant species in a native grassland in Kansas (USA) and to determine the factors that control exotic species abundance and richness (i.e., invasibility). Exotic species (90% C₃ plants) comprised approximately 10% of the flora, and their turnover was relatively high (30%) over the 15-year period. We found that disturbances significantly affected the abundance and richness of exotic species. In particular, long-term annually burned watersheds had lower cover of exotic species than unburned watersheds, and fire reduced exotic species richness by 80–90%. Exotic and native species richness were positively correlated across sites subjected to different fire (r = 0.72) and grazing (r = 0.67) treatments, and the number of exotic species was lowest on sites with the highest productivity of C₄ grasses (i.e., high dominance). These results provide strong evidence for the role of community structure, as affected by disturbance, in determining invasibility of this grassland. Moreover, a significant positive relationship between exotic and native species richness was observed within a disturbance regime (annually burned sites, r = 0.51; unburned sites, r = 0.59). Thus, invasibility of this C₄-dominated grassland can also be directly related to community structure independent of disturbance. Received: 9 February 1999 / Accepted: 12 May 1999     

Exotic tree monocultures play a limited role in the conservation of Atlantic Forest epiphytes

The Brazilian Atlantic Forest suffered a severe geographic contraction along the last five centuries that reduced drastically most vascular epiphyte populations. Among the range of man-made matrixes, tree monocultures have the potential to contribute positively to the maintenance of the regional epiphyte diversity. Here, we test the similarity in abundance, richness, and species composition between vascular epiphytic communities established in managed monocultures of exotic and native species with natural communities occurring in neighboring native Araucaria Forest patches. In the São Francisco de Paula National Forest (Rio Grande do Sul state, Brazil), we recorded 62 epiphyte species from 300 phorophytes occurring in 12, one-hectare plots of Araucaria Forest and managed plantations of Pinus, Eucalyptus and Araucaria. Species richness, rarefied richness and abundance were significantly higher in Araucaria Forest in comparison to the exotic stands. Species composition was also substantially differentiated as Araucaria Forest patches harbored a greater number of zoochorous species than those of the exotic stands. Additionally, plantations of Araucaria angustifolia, a native species, sustained more individuals and more species than the exotic plantations. Neither tree height nor DBH explained epiphyte richness; however, both phorophyte diversity and stand age together accounted for 92% of the among-site variation in epiphytic species richness. We conclude that substrate heterogeneity in combination with time available for colonization contribute significantly to beta-diversity of epiphytes in Araucaria forests. However, demographic experimental studies are necessary in order to disentangle the role of substrate quality from metapopulation processes, such as dispersal limitation, at both temporal and spatial scales.     

What drives invasibility? A multi‐model inference test and spatial modelling of alien plant species richness patterns in northern Portugal

Understanding and predicting biological invasions can focus either on traits that favour species invasiveness or on features of the receiving communities, habitats or landscapes that promote their invasibility. Here, we address invasibility at the regional scale, testing whether some habitats and landscapes are more invasible than others by fitting models that relate alien plant species richness to various environmental predictors. We use a multi‐model information‐theoretic approach to assess invasibility by modelling spatial and ecological patterns of alien invasion in landscape mosaics, and by testing competing hypotheses of environmental factors that may control invasibility. Because invasibility may be mediated by particular characteristics of invasiveness, we classified alien species according to their C‐S‐R plant strategies. We illustrate this approach with a set of 86 alien species in northern Portugal. We first focus on predictors influencing species richness and expressing invasibility, and then evaluate whether distinct plant strategies respond to the same or different groups of environmental predictors. We confirmed climate as a primary determinant of alien invasions, and as a primary environmental gradient determining landscape invasibility. The effects of secondary gradients were detected only when the area was sub‐sampled according to predictions based on the primary gradient. Then, multiple predictor types influenced patterns of alien species richness, with some types (landscape composition, topography and fire regime) prevailing over others. Alien species richness responded most strongly to extreme land management regimes, suggesting that intermediate disturbance induces biotic resistance by favouring native species richness. Land‐use intensification facilitated alien invasion, whereas conservation areas hosted few invaders, highlighting the importance of ecosystem stability in preventing invasions. Plants with different strategies exhibited different responses to environmental gradients, particularly when the variations of the primary gradient were narrowed by sub‐sampling. Such differential responses of plant strategies suggest using distinct control and eradication approaches for different areas and alien plant groups.     

Dominance not richness determines invasibility of tallgrass prairie

Many recent studies suggest that more diverse communities are more resistant to invasion. Community characteristics that most strongly influence invasion are uncertain, however, due to covariation of diversity with competition and crowding. We examined separately the effects of species richness and dominance on invasion by an exotic legume, Melilotus officinalis , in intact, native Kansas grassland. We manipulated dominance of C₄ grasses by reducing their abundance (i.e. ramet densities) by ∼25 and 50%. In addition, richness was reduced by removing species that were mainly rare and uncommon as might be expected with environmental changes such as drought and fragmentation. In both years of the study (2001–2002), invasibility, measured as peak establishment of Melilotus , was not affected by a 3-fold reduction in species richness, nor was there an interaction between loss of species and reduced dominance on invasion. In contrast, reductions in abundance of the dominants significantly reduced invasibility of the grassland plots in both years. Because the abundance of dominants was highly correlated with measures of competition (i.e. ratio of dominant biomass to total biomass) and crowding (total stem densities), this pattern was opposite to that expected if competition were indeed limiting invasion. Rather, invasion appeared to be facilitated by the dominant species, most likely because reduced dominance increased environmental stress. Our results suggest that dominance is the key community characteristic determining invasibility, because highly competitive and space-filling species can either enhance or reduce susceptibility to invasion depending on whether dominants create a more competitive environment or alleviate stressful conditions.     

Population genetics of introduced bullfrogs, Rana (Lithobates) catesbeianus, in the Willamette Valley, Oregon, USA

The American bullfrog, Rana (Lithobates) catesbeianus, is endemic to eastern North America, but has been introduced to approximately 40 countries on four continents and is considered one of the hundred worst invasive alien species in the world. Here, we investigated the genetics of invasive bullfrogs in the Willamette Valley, Oregon, USA, where bullfrogs are widespread and abundant to determine: (1) the minimum number of bullfrog introductions; (2) the native source population(s); and (3) whether genetic variation is reduced compared to source populations. To answer these questions, we analyzed partial sequences of the mitochondrial cytochrome b gene for 251 bullfrogs from the Willamette Valley and the native range. We found that bullfrogs from the Mississippi River basin and Great Lakes region were introduced at least once to the Willamette Valley. Genetic variation measured as haplotype diversity (h) and nucleotide diversity (?? _n ) was not significantly different between Willamette Valley and source populations. Our results were in contrast to a recent genetic analysis of invasive bullfrog populations in Europe, which found that genetic variation in European bullfrog populations was much lower than in source populations. European bullfrogs also originated from different source populations than Willamette Valley bullfrogs. The difference in genetic composition between Willamette Valley and European bullfrogs is likely due to differences in their invasion histories and may have implications for the potential of bullfrogs in these different regions to adapt and expand.     

Relation between environmental variables and aquatic megafauna in a first order stream of the Atlantic Forest, southern Brazil

The study was done in a first order stream in the southern portion of the Brazilian Atlantic Rain Forest. Samples of the aquatic megafauna (amphibians, crustaceans and fishes) were taken with the aim of describing spatial (longitudinal) and temporal (seasonal) patterns in species composition and abundance. Thirty four structural and limnological variables at macro and mesoscales from three sampling reaches were analysed. The spatio-temporal analysis of species richness and diversity indicated a gradient in which values increased in an upstream–downstream direction, independently of the season of the year. The results showed a strong influence of structural environmental variables on community structure. Furthermore, they revealed a hierarchical relation between macroscale and mesoscale variables and their influence on community abundance and composition in the various spatio-temporal sampling units analysed. The spatial distribution of species richness and diversity in the Carvão creek was strongly influenced by the presence of waterfalls, being progressively richer and more diverse downstream. Waterfalls seem to function as selective filters more than as absolute barriers, presenting different efficiencies for different species.     

Biotic and abiotic constraints to a plant invasion in vegetation communities of Tierra del Fuego

The biotic resistance theory relates invader success to species richness, and predicts that, as species richness increases, invasibility decreases. The relationship between invader success and richness, however, seems to be positive at large scales of analysis, determined by abiotic constraints, and it is to be expected that it is negative at small scales, because of biotic interactions. Moreover, the negative relationship at small scales would be stronger within species of the same functional group, because of having similar resource exploitation mechanisms. We studied the relationship between the cover of a worldwide invader of grasslands, Hieracium pilosella L., and species richness, species diversity and the cover of different growth forms at two different levels of analysis in 128 sites during the initial invasion process in the Fuegian steppe, Southern Patagonia, Argentina. At regional level, the invader was positively correlated to total (r = 0.28, P = 0.003), exotic (r = 0.273, P = 0.004), and native species richness (r = 0.210, P = 0.026), and to species diversity (r = 0.193, P = 0.041). At community level, we found only a weak negative correlation between H. pilosella and total richness (r = ?0.426, P = 0.079) and diversity (r = ?0.658, P = 0.063). The relationship between the invader and other species of the same growth form was positive both at regional (r = 0.484, P < 0.001) and community (r = 0.593, P = 0.012) levels. Consequently, in the period of establishment and initial expansion of this exotic species, our results support the idea that invader success is related to abiotic factors at large scales of analysis. Also, we observed a possible sign of biotic constraint at community level, although this was not related to the abundance of species of the same growth form.     

Native tree regeneration in native tree plantations: understanding the contribution of Araucaria angustifolia to biodiversity conservation in the threatened Atlantic Forest in Argentina

Deforestation is a global process that has strongly affected the Atlantic Forest in South America, which has been recognised as a threatened biodiversity hotspot. An important proportion of deforested areas were converted to forest plantations. Araucaria angustifolia is a native tree to the Atlantic Forest, which has been largely exploited for wood production and is currently cultivated in commercial plantations. An important question is to what extent such native tree plantations can be managed to reduce biodiversity loss in a highly diverse and vulnerable forest region . We evaluated the effect of stand age, stand basal area, as a measure of stand density, and time since last logging on the density and richness of native tree regeneration in planted araucaria stands that were successively logged over 60 years, as well as the differences between successional groups in the response of plant density to stand variables. We also compared native tree species richness in planted araucaria stands to neighbouring native forest. Species richness was 71 in the planted stands (27 ha sampled) and 82 in native forest (18 ha sampled) which approximate the range of variation in species richness found in the native forests of the study area. The total abundance and species richness of native trees increased with stand age and time since last logging, but ecological groups differed in their response to such variables. Early secondary trees increased in abundance with stand age 3–8 times faster than climax or late secondary trees. Thus, the change in species composition is expected to continue for a long term. The difference in species richness between native forest and planted stands might be mainly explained by the difference in plant density. Therefore, species richness in plantations can contribute to local native tree diversity if practices that increase native tree density are implemented.     

豚草入侵对新疆伊犁河谷林下本地草本植物群落结构的影响

为探究外来入侵植物豚草（Ambrosia artemisiifolia）对本地植物群落结构的影响,结合区内林下草本植物调查和室内分析方法,分析了豚草不同入侵压力下（无入侵、轻度入侵、中度入侵、重度入侵）本地草本植物丰富度、盖度等特征,探讨了地形、气候、土壤、光照、人口密度等外部因素与豚草种群特征、本地植物群落结构之间的关系。结果表明：豚草种群盖度与其高度、密度、生物量显著正相关。与对照相比,轻度入侵下本土草本植物Shannon-Wiener多样性指数和Pielou均匀度指数显著增加,丰富度、盖度未出现显著差异。随着豚草入侵程度加剧,本地草本植物物种丰富度、盖度极显著降低,样方Shannon-Wiener多样性指数先升高再降低,本土草本植物Pielou均匀度指数差异不显著。冗余分析（RDA）表明,土壤全氮、与道路距离、土壤全磷、郁闭度对研究区植物群落结构影响最大,土壤全氮含量随豚草盖度和生物量增加而减少,距道路越近或光照越强,豚草盖度和生物量越高。增强伊犁河谷地区交通往来货物豚草检验检疫力度、增加林内植被郁闭度或是抑制区内豚草入侵的有效手段。     

Ecological drivers of plant diversity patterns in remnants coastal sand dune ecosystems along the northern Adriatic coastline

Coastal sand dunes represent one of the most fragile ecosystems in the Mediterranean basin. These habitats naturally suffer the action of several limiting factors such as sand burial, marine aerosol and low soil fertility; on the other hand, they often host species of high conservation value. Over the last decades, they have also experienced a high level of biological invasion. In this study, we sampled psammophilous vegetation in two sites in the northern Adriatic coast belonging to the Natura 2000 network to describe diversity patterns and to identify the main ecological drivers of species diversity. Plant species richness and their abundance were assessed in each plot. Differences in species composition for native and alien species were compared via PERMANOVA analysis. Species complementarity was explored by partitioning beta diversity in its spatial components (richness and replacement). A Generalized Linear Model was also computed to assess the main environmental factors that may promote invasiveness in these ecosystems. For the investigated area, our results highlight the strong differentiation in community composition both in alien and native species: in particular alien species showed on average a lower complementarity among habitats compared to native species. Specifically, communities seem to be more diversified when larger spatial scales were considered. Beta diversity in both groups appears to be more dominated by the richness component with respect to the replacement component. Furthermore, in these habitats, the occurrence of alien species was shown to be related to geomorphological predictors more than climatic variables.     

The effects of land-use change on arthropod richness and abundance on Santa Maria Island (Azores): unmanaged plantations favour endemic beetles

We study how endemic, native and introduced arthropod species richness, abundance, diversity and community composition vary between four different habitat types (native forest, exotic forest of Cryptomeria japonica, semi-natural pasture and intensive pasture) and how arthropod richness and abundance change with increasing distance from the native forest in adjacent habitat types in Santa Maria Island, the Azores. Arthropods were sampled in four 150 m long transects in each habitat type. Arthropods were identified to species level and classified as Azorean endemic, single-island endemic (SIE), native, or introduced. The native forest had the highest values for species richness of Azorean endemics, SIEs and natives; and also had highest values of Azorean endemic diversity (Fisher’s alpha). In contrast, the intensive pasture had the lowest values for endemic and native species richness and diversity, but the highest values of total arthropod abundance and introduced species richness and diversity. Arthropod community composition was significantly different between the four habitat types. In the semi-natural pasture, the number of SIE species decreased with increasing distance from the native forest, and in the exotic forest the abundance of both Azorean endemics and SIEs decreased with increasing distance from the native forest. There is a gradient of decreasing arthropod richness and abundance from the native forest to the intensive pasture. Although this study demonstrates the important role of the native forest in arthropod conservation in the Azores, it also shows that unmanaged exotic forests have provided alternative habitat suitable for some native species of forest specialist arthropods, particularly saproxylic beetles.     

群落可侵入性及其影响因素

 可侵入性用于评价群落易遭受生物入侵的程度,受外来种死亡率、区域气候、干扰水平、生态系统抵抗入侵的能力、本地种竞争和抗干扰能力等因素的影响。当前对群落或区域间可侵入性的比较常以外来种数量或丰度为据,然而这两者仅代表了群落内单一的动态过程,不足以作为衡量群落可侵入性的广泛标准。借助一个描述外来种数量的简单模型阐明,由于影响可侵入性因素的复杂性,各地之间的可侵入性几乎不可比较。并从入侵过程、入侵种特性及本地种、本地群落对入侵的抵抗性几方面对群落可侵入性进行了阐述分析,其中着重介绍入侵生态中几个重要的概念,如可侵     

警惕北美牛蛙入侵海南的风险

牛蛙(Lithobates catesbeianus)的入侵已成为本地两栖动物种群下降的主要因素之一,该物种在中国广泛分布,但在海南尚无分布记录.2011年2～4月,在海南省万泉河沙洲岛附近3个点捕捉到8只成体,其中1只个体已有卵.捕捉点的土壤基质为沙质或泥质,距水边距离0.5～1.5 m,整体坡度为6°～ 25°,植被盖度75％ ～ 90％,生境附近本地两栖动物种类较为丰富.调查结果显示,市场上有牛蛙销售,销量约120只/d;琼海市有饲养牛蛙的记录,且存在粗放式(放养于田间)养殖情况,不排除养殖逃逸的可能;捕捉点附近有佛教组织定期放生,也存在个人放生行为.综合以上结果,万泉河发现牛蛙可能是养殖逃逸或放生所致,是否形成可繁殖的种群尚需进一步调查和研究.