Loss Of Native Species Research Articles

First record of Eurasian Water-milfoil, <i>Myriophyllum spicatum</i>, for the Saint John River, New Brunswick

Eurasian Water-milfoil (Myriophyllum spicatum L.) is regarded by conservation practitioners as one of the most challenging invasive aquatic plants to manage. Owing to its broad tolerance to environmental conditions, vegetative propagation, and rapid establishment and growth, M. spicatum introductions have the potential to drastically alter macrophyte species assemblages via a loss of native species and their respective ecosystem functions. Following the discovery of a single specimen of M. spicatum in the Saint John River, near Fredericton, New Brunswick (Canada) we further investigated the localized distribution of this nonindigenous species. Thirteen areas were identified as potential M. spicatum habitat and were surveyed by wading or snorkeling. Specimens of M. spicatum were collected and morphological identifications were verified through genetic analyses (ITS2; rbcLa). The results of our investigation confirm the presence of M. spicatum at six different locations within the Saint John River. Here we discuss the implications of this discovery in the context of the contiguous aquatic habitats along a large river system.

Limited evidence of compositional convergence of restored vegetation with reference states after 20 years of livestock exclusion

AbstractNative pastures and rangelands may become degraded by intensive pastoral use through clearing of woody components, loss of sensitive native species, invasion of opportunistic exotics and soil erosion, compaction and nutrient imbalance. Livestock exclusion is commonly a first management step to restore ecosystems degraded by overgrazing. However, few studies evaluate the outcomes of livestock exclusion on plant species composition and soil chemistry, while those that do are inconclusive due partly to highly variable responses and to inexplicit performance criteria. We adopted a conceptual model that characterised responses to fencing in terms of divergence from initial states and convergence with targeted states that represent restoration goals. We used the model to design an investigation of fencing effects on grassy woodlands within livestock production landscapes in subhumid southeastern Australia. First, we asked whether fencing initiated divergence from continually grazed vegetation and convergence with reference states with respect to plant species composition and soil properties. Secondly, we asked how responses depend on livestock exclusion time, degradation of the initial state, and soil conditions. Compositional trajectories based on a 17‐year chronosequence showed that livestock exclusion initiated some divergence from unfenced controls but convergence with reference states was limited, variable among sites and unrelated to time‐since‐livestock‐exclusion. Fencing initiated the development of novel communities comprising exotic taxa and a subset of native taxa from the reference species pool. Initial state offered limited explanation for the variability in compositional divergence from unfenced controls. Soil properties did not converge with reference states over time, and initial state did not explain the degree divergence of fenced from unfenced plots. The findings suggest site selection is crucial to successful restoration and reinforce the need to consider supplementary management actions, such as planting, scalping, direct seeding and weed control, in addition to livestock grazing exclusion, to restore woodlands towards desired states.

High abundance of an invasive species gives it an outsized ecological role

Abstract Many communities are shifting composition, with losses of native species and increases of non‐indigenous species (NIS). At its extreme, such alteration of ecological guilds can result in simplification with a single NIS performing an ecological role once carried out by a suite of natives. This alteration has occurred in many rivers of the south‐eastern U.S.A., where the invasive filter‐feeding freshwater clam Corbicula fluminea has proliferated following the nearly complete extirpation of native mussels. We investigated the factors controlling the distribution and abundance of Corbicula, as well as estimated the ecological service it provides via water filtration. With a nested design, we surveyed multiple transects within four to six sites within each of four rivers that spanned three large catchments in the Georgia piedmont, collecting data on Corbicula density and physical habitat characteristics associated with its presence. We found Corbicula present in over half of the 1,536 sampled 0.044 m2 sampled plots, 90 of the 93 transects that spanned the width of the river, and all 1–2 km sample sites, underscoring the clam's ubiquity in the study region. At the river scale, Corbicula densities ranged from 50–212 Corbicula m−2, although individual sites ranged from 7–483 Corbicula m−2. Corbicula was more abundant in areas with higher proportions of gravel, and less abundant with higher proportions of bedrock. A hierarchical model with river, site, and these two substrate variables explained 32% of the variation in Corbicula density. Using observed densities and published per capita feeding rates, we calculated system‐wide collective filtration rates provided by Corbicula. In the four rivers surveyed and based on estimated residence times for median flows for the summer of 2012, Corbicula is estimated to filter water as many as seven times during median flows and 18 times during minimum flows before water flows out of a 10‐km reach. Due to high abundances and per biomass filtration rates, Corbicula plays an important role in these rivers. Invasive species, biotic homogenisation, and the loss of functional redundancy may mean that many more rivers are similar to our studied rivers, with a single, often invasive, species dominating ecosystem function. Understanding the influence of biotic homogenisation on ecosystem function is of foremost importance to evaluate the resilience of natural systems.

Fragmentación del paisaje empleando análisis multitemporal de imágenes de satélite Landsat TM y ETM+ en el municipio de Montelíbano, Córdoba-Colombia

La deforestación es un proceso que afecta negativamente la actividad de los ecosistemas y como consecuencia genera una fragmentación del paisaje, siendo ésta una amenaza directa a la biodiversidad y sostenibilidad de recursos bióticos ocasionando cambios espaciales que influyen en la pérdida de especies nativas de flora y fauna, reducción en el tamaño local de las diferentes coberturas naturales, lo cual altera el funcionamiento ecológico de los ecosistemas regionales. Con el propósito de abordar la dinámica espacial de la explotación a cielo abierto de los yacimientos de níquel al sur del departamento de Córdoba-Colombia, el objetivo del artículo es determinar el estado de la fragmentación del paisaje a partir del cambio de coberturas entre los años 1986 y 2013 utilizando imágenes Landsat TM y ETM+, las cuales fueron sometidas a varios procesos como correcciones radiométricas y transformaciones tasseled cap, los cuales suministran imágenes adecuadas para la correcta clasificación de las coberturas. Los resultados muestran alteraciones en la configuración del territorio, donde se identificó una reducción en rastrojos bajos en un 49,76% (2692,53 hectáreas) y una tala de bosques secundarios del 12% de los cambios totales (370,3 hectáreas). La dinámica antrópica generó un paisaje fragmentado con diferentes niveles de perturbación.

Resurrecting the Lost Flames of American Chestnut

Loss of native foundation tree species to introduced pests profoundly alters the structure and function of many forest ecosystems. Recent advances to resurrect or prevent the loss of species by developing resistant hybrids hold promise, but uncertainty remains about the potential impacts of introducing a novel genotype on ecological processes, such as fire. A classic example of a non-native, pathogen-caused loss of a foundation species is American chestnut (Castanea dentata), a species now functionally extinct from the eastern US but undergoing experimental trials to resurrect the species with a putatively disease-resistant genotype. We compared the litter flammability among American chestnut, Chinese chestnut (C. mollisima), and a population of first intercross, third-generation backcrossed hybrid (BC3F2) using laboratory burning experiments. Litter flammability of American chestnut was consistently greater than Chinese chestnut, whereas the hybrid had an intermediate flammability or more closely resembled Chinese chestnut flammability by some measures. Greater flammability in American chestnut was associated with a longer leaf structure with greater curling when dry. American chestnut had flammability properties consistent with other pyrophytic species present in contemporary fire-prone ecosystems. The loss of American chestnut may have altered litter flammability of some eastern US forest ecosystems, a result more commonly associated with compositional changes in conjunction with fire exclusion and other disturbances. Resurrecting American chestnut with a hybrid genotype may mitigate this change in areas where less flammable species have replaced American chestnut. Resurrection of lost foundation species through introduction of resistant genotypes may represent a resounding ecological success story, but unanticipated changes to ecological processes, such as fire, should be considered.

The origin and future of an endangered crater lake endemic; phylogeography and ecology of Oreochromis hunteri and its invasive relatives

Cichlids of the genus Oreochromis (“Tilapias”) are intensively used in aquaculture around the world. In many cases, when “Tilapia” were introduced for economic reasons to catchments that were home to other, often endemic, Oreochromis species, the loss of native species followed. Oreochromis hunteri is an endemic species of Crater Lake Chala on the slopes of Mount Kilimanjaro and is part of a small species flock in the upper Pangani drainage system of Tanzania. We identified three native and three invasive Oreochromis species in the region. Reconstructing their phylogeography, we found that O. hunteri is closely related to, but distinct from the other members of the upper Pangani flock. However, we found a second, genetically and phenotypically distinct Oreochromis species in Lake Chala whose origin we cannot fully resolve. Our ecological and ecomorphological investigations revealed that the endemic O. hunteri is currently rare in the lake, outnumbered by each of three invasive cichlid species. It is mitochondrially, phenotypically and trophically distinct from all others. The occurrence of the formerly abundant O. hunteri in such small numbers, its narrow habitat restriction and its limited morphological variability suggest recent population decline and loss of niche breadth in this critically endangered endemic cichlid species.

Effect of fragmentation of kerangas forest on small mammal community structure in Brunei Darussalam

Widespread and rapid forest loss and disturbance have resulted in increased fragmentation of tropical forests. The impacts of forest disturbance and fragmentation on small mammals have been widely studied across the tropics and these studies have highlighted the detrimental effects. However, there is limited understanding on the impacts on small mammals in Borneo. This study investigated the impacts of fragmentation on small mammal community structure in lowland coastal heath forests known as kerangas forests, in Brunei Darussalam. Twelve study sites were compared in three forest types: fragmented (2.07-17.6 ha), disturbed (443.55-483.79 ha) and undisturbed (>500 ha) forests. In addition, the correlations between species richness, abundance and biomass of small mammals, and forest size were investigated. There was a clear change in species composition in the different forest types. Fragmented forests had the lowest species richness but the highest pooled abundance and biomass compared with disturbed and undisturbed forests. Species richness increased with forest size as predicted by the theory of island biogeography. In contrast, abundance and biomass was negatively correlated to forest size. Factors that contribute to the pronounced decline in species richness in fragmented forests include loss of rare and native forest species, reduced forest size in fragmented forests and distance effect. We suggest that a release from top-down control by predators and favourable conditions as a result from forest fragmentation are responsible for higher abundance and biomass of small mammals in fragmented forests.

Invasion of Hawaiian rainforests by an introduced amphibian predator and N2‐fixing tree increases soil N2O emissions

AbstractInvasions of introduced species have homogenized ecological communities worldwide, leading to losses of native species and the services they provide. Some of these invaders substantially alter nutrient cycling, which changes conditions for all other organisms, but less is known about the potential influence of these species on nitrogen (N) trace gas emissions that affect atmospheric processes. We used a natural experiment to explore whether the establishment of an introduced nitrogen (N) fixing tree (Falcataria moluccana) and recent invasion of an amphibian predator, the Caribbean tree frog (Eleutherodactylus coqui), into native Hawaiian rainforests have affected soil emissions of nitrous oxide (N2O) and nitric oxide (NO), two atmospherically important trace gases produced by soil microorganisms. Soil N2O and NO emissions and rates of soil N cycling were significantly higher in F. moluccana‐dominated stands compared to native Metrosideros polymorpha (Ohi'a) stands. Additionally, invasion of E. coqui frogs moderately increased soil N2O emissions, primarily in non‐native F. moluccana forests where soil N availability was already elevated. N2O emissions were positively and significantly related to net potential N mineralization, and total N2O+NO fluxes increased with soil nitrate (NO3−) concentration and rates of nitrification. Previous work in these Hawaiian rainforest sites has shown that F. moluccana substantially increases N availability by increasing ecosystem N supply compared to uninvaded stands, and E. coqui accelerates N availability and litter decomposition, although moderately, due to enhanced fluxes of nutrient‐rich waste products. Here, we show that acceleration of nutrient cycling by introduced species can also alter biosphere–atmosphere exchange of N‐oxides.

An exotic woody weed in a pastoral landscape provides habitat for many native species, but has no apparent threatened species conservation significance

SummaryExotic woody plants are often used by native organisms, but may also be targets of expensive control justified by nature conservation. We determine the use of a weed of national significance, Gorse (Ulex europaeus L.), by native mammals, birds, reptiles and vascular plants in pastoral areas in an Australian biodiversity hotspot. Large numbers of fauna species were observed using Gorse within our 43 × 1 ha sample sites in riparian, woodland and pasture vegetation. Gorse cover and/or height positively influenced: the detection of mammals as a whole in an interaction with visibility at 50–75 cm above ground, but not their species richness or individual species abundances; bird abundance, but not richness; and, reptile richness but not abundance. In terms of flora, Gorse cover and/or height positively affected: non‐native plant species richness and the height and fecundity, but not the richness, of native grasses and forbs—but Gorse cover negatively influenced the height of native herbs. The only species of conservation significance using Gorse were three mammals, only one of which, the Tasmanian Pademelon (Thylogale billardieri), was sufficiently common to analyse. Its abundance had no relationship with Gorse cover or height. Even in the wider context of complementary work, there is no strong threatened species conservation justification for retaining Gorse thickets in the Northern Midlands pastoral landscape. Equally, expending scarce conservation resources to remove Gorse, as is taking place, is unlikely to achieve any threatened species conservation outcome but may help reduce long‐term loss of native animal and plant species.

Tropical rainforest conversion and land use intensification reduce understorey plant phylogenetic diversity

Abstract Conversion of rainforest into agricultural land affects multiple facets of tropical plant diversity. While the effects of tropical land use change and intensification on species diversity are comparatively well studied, the effects on phylogenetic diversity (PD) and structure of plant communities are largely unknown. Furthermore, it is not clear how the loss of native species and addition of alien species collectively affect PD and structure. We investigated the PD and structure of understorey plants; a diverse and ecologically important, yet poorly studied group. We studied four prominent land use systems (tropical lowland rainforest, jungle rubber agroforest, rubber plantations, and oil palm plantations) in the lowlands of Sumatra (Indonesia), a region experiencing dramatic land use changes. Across the four systems, we investigated differences in four metrics of phylogenetic community structure (PD, mean pairwise distance, mean nearest taxon distance, and their abundance‐weighted variants). Our analyses were based on a comprehensive vegetation survey consisting of 32 plots, 1,197 species of vascular plants, and 146,599 plant individuals. Our results showed that forest conversion into agricultural systems leads to a pronounced loss of PD. Furthermore, the standardized effect size of mean pairwise distance indicated a gradual change from clustered to overdispersed phylogenetic community structure with increasing land use intensity from forest over jungle rubber to the monoculture plantations. In most land use systems, the presence or absence of alien plant species did not affect phylogenetic structure. Only in oil palm plantations, removing alien species from the data led to a more overdispersed structure. In conclusion, conserving the PD and structure requires efficient protection of the last remaining rainforests. Synthesis and applications. Forest conversion into agricultural areas negatively affects phylogenetic understorey plant diversity and leads to a shift from clustered to overdispersed phylogenetic community structure. These trends are partly driven by alien species particularly in oil palm plantations. Protecting the remaining rainforests and considering multispecies agroforestry systems in favour of intensive monoculture plantations are thus imperative to conserve phylogenetic plant diversity and community structure.

Biological invasions undermine the functional diversity of fish community in a large subtropical river

Biological invasions are considered to be one of the main threats to biodiversity. Invasions lead to a loss of native species, changes to species composition, and a shift in the functioning and stability of ecosystems. In this study, derived from nine consecutive years of monitoring data and based on morphological functional trait values measured at the individual-level, we quantified the functional differences between native and non-native fish species and further assessed how biological invasions impact on species richness and functional diversity in the large subtropical Pearl River in southern China. Specifically, we differentiated intraspecific functional variability by separating individuals of a species according to their different life stages. Our results provided strong evidence that native and non-native fish were significantly different in their functional attributes. Invasion caused no obvious change in species richness; however, the yearly increase in non-native populations was accompanied by a significant decrease in functional niches of native species and change in several aspects of functional diversity in the fish community. Decreasing functional richness, and increasing functional divergence and specialization, indicated that most native species had been replaced by non-native species with different specific functional traits, which may affect ecosystem stability. Notably, this study provided empirical evidence that functional diversity was more sensitive to biological invasions than species richness. Our results show that control of non-native aquatic species is both necessary and urgent in the Pearl River. An understanding of the processes described in this study can form the basis of conservation in fish community, which is critical to sustainable and successful fisheries.

Responses of species and phylogenetic diversity of fish communities in the Lancang River to hydropower development and exotic invasions

Multiple anthropogenic disturbances have been modifying the structure and diversity of local assemblages worldwide through both introductions of nonnative species and disappearance of native species. Most previous studies have concentrated on the effects of human disturbances on species diversity, but their effects on phylogenetic diversity have been relatively seldom documented. Here, we measured the temporal changes in species and phylogenetic alpha diversity in 12 mainstream regions of the Lancang River (LCR) in the context of hydropower development and widespread exotic invasions. We found that disturbances by dams and widespread introductions of exotic fishes have induced a marked loss of native species (average 36.8 species disappeared per region, accounting for 47.2% of the historical native species) in the downstream regions over time, whereas few native species were disappeared (average 4 species disappeared, accounting for 23.8% of the historical native species) in the upstream regions. In terms of phylogenetic diversity, however, the average taxonomic distinctness (Δ+, a taxonomic proxy for phylogenetic diversity) of the entire assemblages significantly increased (an increase of 4.4% compared to historical Δ+) in all regions, while native Δ+ insignificantly decreased in downstream regions, and the variation in taxonomic distinctness (Λ+, a proxy for variation in phylogenetic diversity) of the entire assemblages significantly increased (24.6%) in downstream regions, while native Λ+ significantly decreased (12.0%) in all regions. Such changes in phylogenetic diversity were triggered by invasions by distantly related exotic species and disappearances of congeneric endemic species. Across the LCR, the relationships between species richness, Δ+, and Λ+ and altitude all became obscure due to the combined effect of introductions and disappearances. In dammed regions, the disappearance rate and rate of change in the taxonomic distinctness of native assemblages showed negative correlations with the distances of cascade dams. Our study showed that intensive hydropower development and exotic invasions have significantly affected both the taxonomic and phylogenetic diversity of fish assemblages in the LCR, especially in its downstream region. However, the response of phylogenetic diversity to human disturbance was inconsistent with that of species diversity, implying that examining the response of phylogenetic diversity could provide additional information for assessing the impacts of human domination.

Invasive plant species thresholds in the forests of Robinson Crusoe Island, Chile

ABSTRACTBackground: Invasion by exotic plants worldwide can lead to the loss of native species, particularly on islands with a high proportion of endemic plants, such as Robinson Crusoe Island (RCI).Aims: We studied the two most invasive exotic plant species occurring in the forest of RCI: Aristotelia chilensis and Rubus ulmifolius. We aimed at establishing thresholds for environmental and microsite variables related to invasion.Methods: Environmental and forest understorey variables, including canopy gaps and invasive species cover were measured in non-invaded and invaded forest sites. We expected more invasion in plots located close to invasive shrublands, and in large gaps with high solar radiation.Results: We found no relationship between the distance to invasive shrublands and invasion probability. Solar radiation tended to be slightly related with a higher cover of R. ulmifolius, the most abundant invasive exotic plant in RCI forests. Overall, the cover of native ferns appeared to inhibit invasion.Conclusions: The identification of variable thresholds related to invasion can be useful for guiding management decisions. Our results suggest that management should consider monitoring forest canopy gap formation and promote the establishment of ferns to reduce the probability of invasive species establishing.

Exotic flies maintain pollination services as native pollinators decline with agricultural expansion

Abstract Globally, conversion of natural habitat to agricultural land is a primary driver of declines in critical ecosystem services, including pollination. However, exotic species are often well‐adapted to human‐modified environments and could compensate for ecosystem services that are lost when native species decline. We measured pollination services (pollen delivery to stigma) provided by wild insects to a mass flowering crop, pak choi Brassica rapa at 12 sites across a gradient of increasing agricultural land use (agricultural expansion) in New Zealand. We found that pollination services increased as the proportion of agricultural land in the surrounding landscape increased; pollination from exotic species exceeded the loss of pollination from native species. However, pollination service delivery became increasingly dominated by a few exotic fly species that were active throughout the day, compared to native species, which had more constrained activity patterns. Synthesis and applications. The best way to ensure continued sufficient crop pollination is to protect and restore diverse natural habitats on or around farms, as species‐rich pollinator communities are relatively resilient to further environmental change. However, we show that where human‐driven disturbance has caused loss of native pollinator species, exotic pollinators can maintain sufficient pollination. Therefore, in areas where native species loss cannot easily be reversed, decisions about pesticide use and habitat provision that foster populations of beneficial exotic species are likely to maintain pollination service delivery, at least in the short term. This highlights the need for land managers to identify the pollinator communities that are present on their farms, whether native or exotic, and make decisions to support these important communities accordingly.

Habitat fragmentation provides a competitive advantage to an invasive tree squirrel, Sciurus carolinensis

Changes in the composition of biological communities can be elicited by competitive exclusion, wherein a species is excluded from viable habitat by a superior competitor. Yet less is known about the role of environmental change in facilitating or mitigating exclusion in the context of invasive species. In these situations, decline in a native species can be due to the effects of habitat change, or due to direct effects from invasive species themselves. This is summarized by the “driver-passenger” concept of native species loss. We present a multi-year study of tree squirrels that tested the hypothesis that tree canopy fragmentation, often a result of human development, influenced the replacement of native western gray tree squirrels (Sciurus griseus) by non-native eastern gray tree squirrels (Sciurus carolinensis). We tested this hypothesis along a continuum of invasion across three study sites in central California. We found that within the developed areas of the University of California at Santa Cruz campus and city of Santa Cruz, S. carolinensis excluded S. griseus from viable habitat. The competitive advantage of S. carolinensis may be due to morphological and/or behavioral adaptation to terrestrial life in fragmented hardwood forests. We classify S. carolinensis as a “driver” of the decline of native S. griseus in areas with high tree canopy fragmentation. Future habitat fragmentation in western North America may result in similar invasion dynamics between these species. Our study warrants consideration of existing and predicted interactions between potentially invasive species that co-occur with native species where land use change is proposed.

The Ecological and Conservation Status of The Guadalquivir River Basin (s Spain) Through The Application Of A Fish-based Multimetric Index

An index of biotic integrity (IBI) based on fish health, population age-structure, abundance and community richness of native and alien fish, originally developed for the Júcar basin (E Spain) (IBI-J), was adapted and validated here for another Mediterranean basin– the Guadalquivir (IBI-G). Until now, no one fish-based IBI − sensitive, useful in a large geographical context, simple and easily applied −has been employed in species-poor Mediterranean basins. Various attempts to develop such an index have generally failed to achieve all these objectives. In this paper, the applicability of the index was enhanced through several modifications to the metrics used in the original IBI-J, and a new version of the index was also developed which included an additional metric recommended by the author of the IBI-J. In the IBI-G each of the five metrics that make up the IBI (percentage of individuals with anomalies, age structure of native fish population, relative native fish density, loss of native species and alien fish pressure) were validated and adjusted to the specificities of a typical Mediterranean fish community. Both, IBI-J and IBI-G, are robust and coherent assessment tools, highly sensitive to the main pressure gradients observed in the study area, as demonstrated by its correlation with many pressure indicators (biotic and habitat indexes, habitat variables and physico-chemical parameters). It points to the real possibility of extending the same methodology to similar basins for the purposes of monitoring, management and ecological assessment, especially in contexts relating to the Water Framework Directive (WFD). In addition, this new index does not need undisturbed reference sites, a completely unreal requisite in Mediterranean basins, where there is a general scarcity of pristine sites to be used as reference conditions. The results of this study highlight the usefulness of IBI-J/IBI-G as a powerful biomonitoring tool, especially in application of the WFD.

Removal of perennial herbaceous species affects response of Cold Desert shrublands to fire

AbstractQuestionsTwo of the primary global change factors that threaten shrublands worldwide are loss of native perennial herbaceous species due to inappropriate livestock grazing and loss of native shrubs due to altered fire regimes. We asked: (1) how do the separate and interacting effects of removal of perennial herbaceous species and burning influence relative abundance of plant functional groups over longer time frames; and (2) how do interactions between perennial herbaceous species removal and burning differ along environmental gradients? We discuss implications of our findings for ecosystem resilience to these disturbances.LocationShoshone Mountain Range, Nevada and East Tintic Range, Utah, USA.MethodsWe used a factorial experiment to test effects of perennial herbaceous species removal (0%, 50% and 100%) and burning (burned and not burned) on plant functional group cover along elevation gradients within watersheds characterized by Artemisia tridentata Nutt. vegetation types. The experiment was conducted in two locations (Nevada and Utah) with sites located at low (1960 and 1710 m), mid (2190 and 2085 m) and high (2380 and 2274 m) elevations and was repeated in 2 yr. Percentage cover of native and exotic species and canopy area, density and size of the shrub A. tridentata were evaluated 12 and 13 yr after study implementation.ResultsOver a decade later, removal treatments resulted in highly significant decreases (40–62%) in perennial native grass and forb cover across site elevations. Burning decreased overall shrub and A. tridentata cover, but effects on perennial native grass cover differed among elevations. Removal had strong positive effects on A. tridentata seedling recruitment and resulted in progressive increases in density and canopy area following burning. A. tridentata canopy areas on burned plots with 0%, 50% and 100% removal were 0.19, 0.40 and 0.84 m2, respectively. Annual invasive grass density also increased with degree of removal, and both density and cover decreased with elevation.ConclusionsOur results show that loss of perennial herbaceous species, which can result from inappropriate livestock grazing, and loss of shrubs, which often results from fire, interact to affect key functional groups. The implications are that ecosystem resilience to disturbance in Cold Desert shrublands decreases when competition from perennial native grasses and forbs for available resources no longer prevents dominance by A. tridentata and other shrubs and/or annual invasive grasses. Managing livestock grazing to maintain or increase perennial herbaceous species, especially deep‐rooted grasses, which contribute to resilience along elevation gradients, can help prevent threshold crossings to undesirable states and retain critical ecosystem services following disturbances such as wildfire.

Native exotic relationships in plant communities: the role of exotic dominance in framing community composition

AbstractIn tropical urban areas, prone to invasions by multiple exotic species, there is a need for studies to understand the precise nature of the relationship between native and exotic species. This observational study was conducted in a rapidly urbanizing Indian metropolis. It examined the native and exotic species separately as a function of richness and abundance of two focal exotic species, namely Mikania micrantha and Alternanthera philoxeroides, to find support for the hypothesis that an overall native‐exotic relationship does not provide a true picture of the community composition. The richness of exotic species did not turn out to be a good predictor for the native‐exotic richness relationship even after focal exotics were considered. However, when the richness components were analysed separately, Poisson log‐linear models identified M. micrantha as the community‐dominant, by virtue of its extensive cover. Neither soil resource availability nor the presence of other exotics had any influence on native species richness. The negative relationship of M. micrantha cover with other exotic species could be associated with a lower risk of native species loss in a community invaded by multiple exotic species. M. micrantha appeared to be a ‘passenger’ of habitat alteration, but was likely to become a ‘driver’ once it attains high covers due to its reported superior competitive abilities. Therefore, in tropical areas there is a need to prioritize management initiatives in order to identify the dominant invader species in a community and effectively manage the dominant‐homogenized plots.

Components and drivers of change in European freshwater fish faunas

AbstractAimHuman‐induced loss of native species and introduction of non‐native species have altered richness and composition of species assemblages world‐wide. During the past 15 years many studies have focused on changes in taxonomic similarity and identified numerous yet often contrasting reasons for these changes. This study aims to quantify taxonomic changes in freshwater fish assemblages between the mid‐19th century and today, while explicitly separating its different components and drivers.LocationGeographic Europe, 251 river basins > 2500 km².MethodsPairwise catchment comparisons of historic and contemporary fish species inventories, with and without migratory fish, using Jaccard similarity; quantification of relative species turnover using a newly developed Reshuffling Index; determination of the threshold (here: number of catchments occupied) that displays how widespread a species must be to cause homogenization.ResultsThe European freshwater fish fauna changed profoundly since the mid‐19th century. All river catchments exhibited an average net gain of 5.7 species, leading to an overall increase in faunal similarity across Europe of 3.1% (4.6% if migratory species are excluded). However, species turnover was much higher than indicated by the net gain. On average, 20% of the historic assemblages became reshuffled. The native catchment range size of an introduced species primarily determined its impact on taxonomic similarity change, irrespective whether it is translocated within or introduced from outside Europe and whether the species is of fisheries importance or not.Main conclusionsThe concurrent use of multiple indices allowed disentangling the main components and drivers of taxonomic change. It became evident that prevention of intended or unintended species introduction will not lower the rate of taxonomic homogenization per se. However, most species actually caused taxonomic differentiation despite their range gain. All introduced species still considerably contributed to taxonomic change with potential negative effects on ecosystem functions.

High N, dry: Experimental nitrogen deposition exacerbates native shrub loss and nonnative plant invasion during extreme drought.

Hotter, longer, and more frequent global change-type drought events may profoundly impact terrestrial ecosystems by triggering widespread vegetation mortality. However, severe drought is only one component of global change, and ecological effects of drought may be compounded by other drivers, such as anthropogenic nitrogen (N) deposition and nonnative plant invasion. Elevated N deposition, for example, may reduce drought tolerance through increased plant productivity, thereby contributing to drought-induced mortality. High N availability also often favors invasive, nonnative plant species, and the loss of woody vegetation due to drought may create a window of opportunity for these invaders. We investigated the effects of multiple levels of simulated N deposition on a Mediterranean-type shrubland plant community in southern California from 2011 to 2016, a period coinciding with an extreme, multiyear drought in the region. We hypothesized that N addition would increase native shrub productivity, but that this would increase susceptibility to drought and result in increased shrub loss over time. We also predicted that N addition would favor nonnatives, especially annual grasses, leading to higher biomass and cover of these species. Consistent with these hypotheses, we found that high N availability increased native shrub canopy loss and mortality, likely due to the higher productivity and leaf area and reduced water-use efficiency we observed in shrubs subject to N addition. As native shrub cover declined, we also observed a concomitant increase in cover and biomass of nonnative annuals, particularly under high levels of experimental N deposition. Together, these results suggest that the impacts of extended drought on shrubland ecosystems may be more severe under elevated N deposition, potentially contributing to the widespread loss of native woody species and vegetation-type conversion.