Experimental test of the Invasional Meltdown Hypothesis: an exotic herbivore facilitates an exotic plant,but the plant does not reciprocally facilitate the herbivore

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The influence of abiotic stress and phenotypic plasticity on the distribution of invasive Alternanthera philoxeroides along a riparian zone

Relatively few studies have compared invasibility and species invasiveness among microhabitats within communities, synchronously. We surveyed the abundance and performance of non-native Alternanthera philoxeroides (Mart.) Griseb. (alligator weed), its co-occurring native congener, Alternanthera sessilis (L.) DC. (sessile joyweed), and other species in a wetland community along a riparian zone in southeast China to test the hypotheses that: i) degree of invasion differs between different types of microhabitats within the community; and ii) microhabitat types that differ in invasibility also differ in soil resource availability or in sediment characteristics likely to affect resource availability; iii) phenotypic plasticity of A. philoxeroides may play a key role in its adaptation to diverse habitats as can be concluded from its extremely low genetic diversity in China. The study riparian zone comprises different types of microhabitats including wet abandoned field, swamp, marsh dunes and gravel dunes. Consistent with these hypotheses, cover of A. philoxeroides was high in abandoned fields (73 ± 2.9%) and swamps (94 ± 1.3%), which had high soil nutrients and water availability. On the contrary, cover of native A. sessilis was relatively high in marsh dunes and grave dunes, which had coarse gravel surfaces, low soil nutrients and low water availability. A. philoxeroides showed greater morphological plasticity in response to habitat variation. In abiotically harsh habitats, stems had limited growth, and were prostrate with weak adventitious roots at nodes, forming thin, scattered patches. In the two richer habitats, the highly branched plants spread over the water or soil surface, supporting dense stronger leaf-bearing stems which grew vertically. The growth pattern of A. sessilis among microhabitats did not exhibit significant variations. These results suggest that morphological plasticity and microhabitat types with high soil resources may facilitate invasions of A. philoxeroides.     

Direct effects of nitrogen addition on seed germination of eight semi‐arid grassland species

Seed germination plays an important role in mediating plant species composition of grassland communities under nitrogen (N) enrichment. Shifts of plant community structure with N‐enhanced deposition in terrestrial ecosystems have occurred globally. Despite numerous studies about the effects of enhanced N deposition on mature plant communities, few studies have focused on seed germination. Using a laboratory experiment, we report the effects of five N concentrations, including 0, 5, 10, 20, and 40 mM N (NH₄NO₃) on seed germination of eight semi‐arid grassland species. Results showed that low N concentrations (5‐ and 20‐mM N) promoted mean final germination proportion of all eight species by 4.4% and 6.4%, but high concentrations (40 mM N) had no effect. The mean germination rate was decreased 2.1% and 5.1% by higher N concentration (20‐ and 40‐mM N) levels, but germination start time showed the opposite trend, delayed by 0.7, 0.9, and 1.8 d for the 10, 20, and 40 mM N treatments. Final germination proportion, mean germination rate, and germination start time were significantly different among species in response to N concentration treatments. The final germination proportion of Allium tenuissimum and Chenopodium glaucum were suppressed by increased N concentration, whereas it increased for Potentilla bifurca, Plantago asiatica, and Setaria viridis. Our findings provide novel insights into N deposition‐induced species loss based on seed germination factors in semi‐arid grassland communities.     

Environmental Productivity and Biodiversity Effects on Invertebrate Community Invasibility

Productivity influences the availability of resources for colonizing species. Biodiversity may also influence invasibility of communities because of more complete use of resource types with increasing species richness. We hypothesized that communities with higher environmental productivity and lower species richness should be more invasible by a competitor than those where productivity is low or where richness is high. We experimentally examined the invasion resistance of herbivorous meiofauna of Jamaican rock pools by a competitor crustacean (Ostracoda: Potamocypris sp. (Brady)) by contrasting three levels of nutrient input and four levels of species richness. Although relative abundance (dominance) of the invasive was largely unaffected by resource availability, increasing resources did increase the success rate of establishment. Effects of species richness on dominance were more pronounced with a trend towards the lowest species richness treatment of 2 resident species being more invasible than those with 4, 6, or 7 species. These results can be attributed to a ‘sampling effect associated with the introduction of Alona davidii (Richard) into the higher biodiversity treatments. Alona dominated the communities where it established and precluded dominance by the introduced ostracod. Our experimental study supports the idea that niche availability and community interactions define community invasibility and does not support the application of a neutral community model for local food web management where predictions of exotic species impacts are needed.     

Flow intermittency decreases nestedness and specialisation of diatom communities in Mediterranean rivers

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The influence of abiotic stress and phenotypic plasticity on the distribution of invasive Alternanthera philoxeroides along a riparian zone

Relatively few studies have compared invasibility and species invasiveness among microhabitats within communities, synchronously. We surveyed the abundance and performance of non-native Alternanthera philoxeroides (Mart.) Griseb. (alligator weed), its co-occurring native congener, Alternanthera sessilis (L.) DC. (sessile joyweed), and other species in a wetland community along a riparian zone in southeast China to test the hypotheses that: i) degree of invasion differs between different types of microhabitats within the community; and ii) microhabitat types that differ in invasibility also differ in soil resource availability or in sediment characteristics likely to affect resource availability; iii) phenotypic plasticity of A. philoxeroides may play a key role in its adaptation to diverse habitats as can be concluded from its extremely low genetic diversity in China. The study riparian zone comprises different types of microhabitats including wet abandoned field, swamp, marsh dunes and gravel dunes. Consistent with these hypotheses, cover of A. philoxeroides was high in abandoned fields (73 ± 2.9%) and swamps (94 ± 1.3%), which had high soil nutrients and water availability. On the contrary, cover of native A. sessilis was relatively high in marsh dunes and grave dunes, which had coarse gravel surfaces, low soil nutrients and low water availability. A. philoxeroides showed greater morphological plasticity in response to habitat variation. In abiotically harsh habitats, stems had limited growth, and were prostrate with weak adventitious roots at nodes, forming thin, scattered patches. In the two richer habitats, the highly branched plants spread over the water or soil surface, supporting dense stronger leaf-bearing stems which grew vertically. The growth pattern of A. sessilis among microhabitats did not exhibit significant variations. These results suggest that morphological plasticity and microhabitat types with high soil resources may facilitate invasions of A. philoxeroides.     

Amphibian richness patterns in Atlantic Forest areas invaded by American bullfrogs

The relationship between invasion success and native biodiversity is central to biological invasion research. New theoretical and analytical approaches have revealed that spatial scale, land‐use factors and community assemblages are important predictors of the relationship between community diversity and invasibility and the negative effects of invasive species on community diversity. In this study we assess if the abundance of Lithobates catesbeianus, the American bullfrog, negatively affects the richness of native amphibian species in Atlantic Forest waterbodies in Brazil. Although this species has been invading Atlantic Forest areas since the 1930s, studies that estimate the invasion effects upon native species diversity are lacking. We developed a model to understand the impact of environmental, spatial and species composition gradients on the relationships between bullfrogs and native species richness. We found a weak positive relationship between bullfrog abundance and species richness in invaded areas. The path model revealed that this is an indirect relationship mediated by community composition gradients. Our results indicate that bullfrogs are more abundant in certain amphibian communities, which can be species‐rich. Local factors describing habitat heterogeneity were the main predictors of amphibian species richness and composition and bullfrog abundance. Our results reinforce the important role of habitats in determining both native species diversity and potential invasibility.     

植物群落的生物多样性及其可入侵性关系的实验研究

 生物入侵已经成为一个普遍性的环境问题,并为许多学者所关注。尽管一些理论研究和观察表明生物多样性丰富的群落不容易受到外来种的入侵,但后来有些实验研究并没能证实两者的负相关性,多样性 可入侵性假说仍然是入侵生态学领域争论比较多的一个焦点。人为构建不同物种多样性和物种功能群多样性（C3 禾本科植物、C4植物、非禾本科草本植物和豆科植物）梯度的小尺度群落,把其它影响可入侵性的外在因子和多样性效应隔离开来,研究入侵种喜旱莲子草(Alternanthera philoxeroides)在不同群落里的入侵过程来验证多样性 可入侵性及其相关假说。研究结果显示,物种功能群丰富的群落可入侵程度较低,功能群数目相同而物种多样性不同的群落可入侵性没有显著性差异,功能群特征不同的群落也表现出可入侵性的差异,生活史周期短的单一物种群落和有着生物固氮功能的豆科植物群落可入侵程度较高,与喜旱莲子草属于同一功能群且有着相似生态位的土著种莲子草(A. sessilis)对入侵的抵抗力最强。实验结果表明,物种多样性和群落可入侵性并没有很显著的负相关,而是与物种特性基础上的物种功能群多样性呈负相关,群落中留给入侵种生态位的机会很可能是决定群落可入侵性的一个关键因子。     

Intraspecific functional diversity of common species enhances community stability

Common species are fundamental to the structure and function of their communities and may enhance community stability through intraspecific functional diversity (iFD). We measured among‐habitat and within‐habitat iFD (i.e., among‐ and within‐plant community types) of two common small mammal species using stable isotopes and functional trait dendrograms, determined whether iFD was related to short‐term population stability and small mammal community stability, and tested whether spatially explicit trait filters helped explain observed patterns of iFD. Southern red‐backed voles (Myodes gapperi) had greater iFD than deer mice (Peromyscus maniculatus), both among habitats, and within the plant community in which they were most abundant (their “primary habitat”). Peromyscus maniculatus populations across habitats differed significantly between years and declined 78% in deciduous forests, their primary habitat, as did the overall deciduous forest small mammal community. Myodes gapperi populations were stable across habitats and within coniferous forest, their primary habitat, as was the coniferous forest small mammal community. Generalized linear models representing internal trait filters (e.g., competition), which increase within‐habitat type iFD, best explained variation in M. gapperi diet, while models representing internal filters and external filters (e.g., climate), which suppress within‐habitat iFD, best explained P. maniculatus diet. This supports the finding that M. gapperi had higher iFD than P. maniculatus and is consistent with the theory that internal trait filters are associated with higher iFD than external filters. Common species with high iFD can impart a stabilizing influence on their communities, information that can be important for conserving biodiversity under environmental change.     

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.     

Resource limitation,biodiversity, and competitive effects interact to determine the invasibility of rock pool microcosms

The success of species invasions depends on both the characteristics of the invaded habitat and the traits of the invasive species. At local scales biodiversity may act as a barrier to invasion; however, the mechanism by which biodiversity confers invasion resistance to a community has been the subject of considerable debate. The purpose of this study was to test the hypothesis that productivity and diversity affected the ability of a regionally available species to colonize communities from which it is absent. We hypothesized that the invasibility of rock pool invertebrate communities would increase with increasing nutrients and decrease with increasing diversity. We tested this possibility using naturally invaded outdoor aquatic microcosms. We demonstrated that the invasibility of an experimental multi-trophic aquatic community by a competitive native midge species (Ceratopogonidae: Dasyhelea sp.) was determined by an interaction between resource availability, diversity, and the densities of two competitive ostracods species. Nutrient enrichment increased invasion success; however, within nutrient-enriched microcosms, invasion success was highest in the low-diversity treatments. Our results suggest that resource availability may in fact be the principal mechanism determining invasibility at local scales in multi-trophic rock pool communities; however resource availability can be determined by both nutrient input as well as by the diversity of the biotic community.     

Replanting of first‐cycle oil palm results in a second wave of biodiversity loss

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Biodiversity of traits and species both show weak responses to hydromorphological alteration in lowland river macroinvertebrates

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Intraspecific plant–soil feedback and intraspecific overyielding in Arabidopsis thaliana

Understanding the mechanisms of community coexistence and ecosystem functioning may help to counteract the current biodiversity loss and its potentially harmful consequences. In recent years, plant–soil feedback that can, for example, be caused by below‐ground microorganisms has been suggested to play a role in maintaining plant coexistence and to be a potential driver of the positive relationship between plant diversity and ecosystem functioning. Most of the studies addressing these topics have focused on the species level. However, in addition to interspecific interactions, intraspecific interactions might be important for the structure of natural communities. Here, we examine intraspecific coexistence and intraspecific diversity effects using 10 natural accessions of the model species Arabidopsis thaliana (L.) Heynh. We assessed morphological intraspecific diversity by measuring several above‐ and below‐ground traits. We performed a plant–soil feedback experiment that was based on these trait differences between the accessions in order to determine whether A. thaliana experiences feedback at intraspecific level as a result of trait differences. We also experimentally tested the diversity–productivity relationship at intraspecific level. We found strong differences in above‐ and below‐ground traits between the A. thaliana accessions. Overall, plant–soil feedback occurred at intraspecific level. However, accessions differed in the direction and strength of this feedback: Some accessions grew better on their own soils, some on soils from other accessions. Furthermore, we found positive diversity effects within A. thaliana: Accession mixtures produced a higher total above‐ground biomass than accession monocultures. Differences between accessions in their feedback response could not be explained by morphological traits. Therefore, we suggest that they might have been caused by accession‐specific accumulated soil communities, by root exudates, or by accession‐specific resource use based on genetic differences that are not expressed in morphological traits. Synthesis. Our results provide some of the first evidence for intraspecific plant–soil feedback and intraspecific overyielding. These findings may have wider implications for the maintenance of variation within species and the importance of this variation for ecosystem functioning. Our results highlight the need for an increased focus on intraspecific processes in plant diversity research to fully understand the mechanisms of coexistence and ecosystem functioning.     

Unravelling intricate interactions among atmospheric bromeliads with highly overlapping niches in seasonal systems

• Biotic and abiotic interactions are important factors that explain community assembly. For example, epiphytic communities are shaped by tree traits that can act as environmental filters, but also by positive and/or negative interactions among coexisting epiphytes on a tree.
• Here, we studied interactions among three widespread atmospheric bromeliads with overlapping niches (Tillandsia recurvata, T. tricholepis and T. pohliana), using experimental data about facilitation through seed capture, interspecific interaction during seed germination and competition among adult individuals. We aim to understand how species interactions are reflected in the natural coexisting patterns of epiphytes in communities at high and low tree densities.
• Tillandsia pohliana showed higher facilitation by capturing almost all wind‐dispersing seeds, and had the largest reduction in seed germination in the presence of any adult individual, also presenting a relatively high rate of adult mortality in the presence of other individuals. Our results indicate higher colonisation ability for T. pohliana and distinct strategies of rapid exploitation of T. recurvata and T. tricholepis individuals.
• In natural communities, the coexistence among atmospheric bromeliads may be hampered by dispersal limitations in wind‐dispersed epiphytes at high tree density conditions, but a negative effect of T. recurvata on T. pohliana is still reflected in their reduced coexistence. However, competitive patterns observed in the experimental data may be overshadowed by a possible mass effect driving large communities under such conditions. Our results show the joint effect of positive interactions and high dispersal levels on the community patterns of atmospheric bromeliads.

     

Salamander diversity alters stream macroinvertebrate community structure

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Responses of the soil fungal communities to the co‐invasion of two invasive species with different cover classes

• Soil fungal communities play an important role in the successful invasion of non‐native species. It is common for two or more invasive plant species to co‐occur in invaded ecosystems.
• This study aimed to determine the effects of co‐invasion of two invasive species (Erigeron annuus and Solidago canadensis) with different cover classes on soil fungal communities using high‐throughput sequencing.
• Invasion of E. annuus and/or S. canadensis had positive effects on the sequence number, operational taxonomic unit (OTU) richness, Shannon diversity, abundance‐based cover estimator (ACE index) and Chao1 index of soil fungal communities, but negative effects on the Simpson index. Thus, invasion of E. annuus and/or S. canadensis could increase diversity and richness of soil fungal communities but decrease dominance of some members of these communities, in part to facilitate plant further invasion, because high soil microbial diversity could increase soil functions and plant nutrient acquisition. Some soil fungal species grow well, whereas others tend to extinction after non‐native plant invasion with increasing invasion degree and presumably time. The sequence number, OTU richness, Shannon diversity, ACE index and Chao1 index of soil fungal communities were higher under co‐invasion of E. annuus and S. canadensis than under independent invasion of either individual species.
• The co‐invasion of the two invasive species had a positive synergistic effect on diversity and abundance of soil fungal communities, partly to build a soil microenvironment to enhance competitiveness of the invaders. The changed diversity and community under co‐invasion could modify resource availability and niche differentiation within the soil fungal communities, mediated by differences in leaf litter quality and quantity, which can support different fungal/microbial species in the soil.

     

Community dominance patterns, not colonizer genetic diversity, drive colonization success in a test using grassland species

Successful colonization and/or invasion depend on characteristics of the invaded community and of the colonizer itself. Although many studies have documented a negative relationship between invasibility and biodiversity, the importance of community evenness is rarely examined and thus poorly understood. However, colonizer characteristics, including population genetic diversity, can also be important determinants of colonization success. We conducted a greenhouse experiment to assess the relative importance of community evenness and colonizer population genetic diversity using the weed Arabidopsis thaliana. We added seeds of A. thaliana (varying genetic diversity while keeping propagule pressure constant) to four types of constructed plant communities: those dominated by legumes, grasses or forbs, or with equal abundances of all three functional groups. We selected community members from a large pool of species to avoid the confounding effects of species identity. We also assessed the success of multiple seedbank colonizers to assess generality in the effects of our evenness treatments. Equal-abundance communities were no better at suppressing colonization than communities dominated by a single functional group. Forb-dominated communities suppressed A. thaliana colonization better than grass-dominated communities and suppressed seedbank colonizers better than legume-dominated communities. Equal-abundance communities were similar to forb-dominated ones in their eventual composition and in their invasibility, suggesting that forbs drove colonizer suppression in that treatment rather than high evenness itself. Most of our forbs grew quickly, yielding productive forb-dominated communities; this points to the importance of growth and colonization phenology in our system. A. thaliana genetic diversity did not affect colonization success, perhaps because strong interspecific competition substantially limited A. thaliana seedling emergence.     

Mass–abundance scaling in avian communities is maintained after tropical selective logging

1. Selective logging dominates forested landscapes across the tropics. Despite the structural damage incurred, selectively logged forests typically retain more biodiversity than other forest disturbances. Most logging impact studies consider conventional metrics, like species richness, but these can conceal subtle biodiversity impacts. The mass–abundance relationship is an integral feature of ecological communities, describing the negative relationship between body mass and population abundance, where, in a system without anthropogenic influence, larger species are less abundant due to higher energy requirements. Changes in this relationship can indicate community structure and function changes.
2. We investigated the impacts of selective logging on the mass–abundance scaling of avian communities by conducting a meta‐analysis to examine its pantropical trend. We divide our analysis between studies using mist netting, sampling the understory avian community, and point counts, sampling the entire community.
3. Across 19 mist‐netting studies, we found no consistent effects of selective logging on mass–abundance scaling relative to primary forests, except for the omnivore guild where there were fewer larger‐bodied species after logging. In eleven point‐count studies, we found a more negative relationship in the whole community after logging, likely driven by the frugivore guild, showing a similar pattern.
4. Limited effects of logging on mass–abundance scaling may suggest high species turnover in logged communities, with like‐for‐like replacement of lost species with similar‐sized species. The increased negative mass–abundance relationship found in some logged communities could result from resource depletion, density compensation, or increased hunting; potentially indicating downstream impacts on ecosystem functions.
5. Synthesis and applications. Our results suggest that size distributions of avian communities in logged forests are relatively robust to disturbance, potentially maintaining ecosystem processes in these forests, thus underscoring the high conservation value of logged tropical forests, indicating an urgent need to focus on their protection from further degradation and deforestation.

     

Population recovery,seasonal site fidelity,and daily activity of pirarucu (Arapaima spp.) in an Amazonian floodplain mosaic

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