Invasions of alien gammarid species and retreat of natives in the Vistula Lagoon (Baltic Sea, Poland)

During the last decades of the twentieth century, the alien gammarid species Gammarus tigrinus, Dikerogammarus haemobaphes, Pontogammarus robustoides and Obesogammarus crassus invaded the lower Vistula River and its deltaic, partly brackish regions. In brackish waters of the Vistula Lagoon the native Atlantic-boreal species Gammarus zaddachi and Gammarus duebeni have been replaced or at least outnumbered by the aliens. As compared to our earlier studies, through the years 1998–2004 we could observe nearly total decline of the native gammarid populations along the coasts of the Lagoon, and overdomination of the North-American G. tigrinus in most places. Possible reasons for the observed phenomena are e.g. increasing pollution and eutrophication of the Lagoon accompanied by competition between the native and the alien species.     

Invasive alien plant species in China: regional distribution patterns

Plant invasions have been attracting increasing attention from ecologists because of their worldwide environmental impacts and huge economic costs. Research on the characteristics of the recipient regions is essential for understanding the process of plant invasion. However, few previous studies on invasibility of habitats include social factors, although human activities are critical in the process of plant invasion. China is a vast country with high plant species diversity and a long history of introduction of exotic plant species and is particularly vulnerable to invasive plant species. Alien plant species are widespread in the country. Therefore, the study of invasive plants in China is urgent in practice and theoretically important for developing invasion ecology. For the present study, 126 species were selected to represent the major invasive plant species in China. We then collected data on their species richness in 31 provincial administrative units of China and performed Spearman rank correlations between species richness and possible natural and socio‐economic factors. We found that socio‐economic factors, such as human density and GDP, correlated positively with the species richness of invasive plants in China. In conjunction with the natural and socio‐economic correlations in the study of regional distribution pattern of the major invasive plants, we discussed the factors influencing the regional distribution pattern of the major invasive plants in China. We suggest that native plant species richness was mainly determined by the natural conditions of the regions, while invasive species richness was influenced by natural conditions and human disturbance together.     

Impact of the invasive alien grass Melinis minutiflora at the savanna-forest ecotone in the Brazilian Cerrado

Exotic grasses are a serious threat to biodiversity in the cerrado savannas of central Brazil. Of particular concern is the possible role they may have in impeding tree regeneration at gallery (riverine) forest edges and increasing fire intensity, thereby driving gallery forest retreat. Here we quantify the effect of roads and distance from gallery forests on the abundance of the African grass Melinis minutiflora Beauv. and test for an effect of this species on woody plant regeneration and leaf area index. Melinis was present at approximately 70% of the sites near gallery forest edges, with its frequency declining sharply at greater distances from the edge. Melinis frequency was 2.8 times greater where roads were present nearby. Leaf area index (LAI) of the ground layer was 38% higher where Melinis was present than where it was absent. LAI was strongly correlated to fine fuel mass (r² = 0.80), indicating higher fuel loads where Melinis was present. The abundance of tree and shrub species in the ground layer was negatively related to LAI and to the presence of Melinis. The greater fuel accumulation and reduced tree regeneration caused by Melinis may cause a net reduction in forest area by increasing fire intensity at the gallery forest edge and slowing the rate of forest expansion.     

Segregation,nestedness and homogenisation in plant communities dominated by native and alien species

ABSTRACT

Background: Highly modified landscapes offer the opportunity to assess how environmental factors influence the integration of alien plant species into native vegetation communities and determine the vulnerability of different communities to invasion.

Aims: To examine the importance of biotic and abiotic drivers in determining whether alien plant species segregate spatially from native plant communities or become integrated and lead to biotic homogenisation.

Methods: Ordination and classification of a floristic survey of over 1200 systematically located 6 m × 6 m plots were used to examine how plant community segregation, nestedness and homogenisation varied in relation to climate, environmental and human-related factors across Banks Peninsula, New Zealand.

Results: The analyses of community structure indicated that native and alien plant communities were spatially and ecologically segregated due to different responses primarily to an anthropogenic impact gradient and secondly to environmental factors along an elevation gradient. Human-land use appeared most strongly linked to the distribution of alien species and was associated with increased vegetation homogenisation. However, despite spatial segregation of alien and native plant communities, biotic homogenisation not only occurred in highly managed grasslands but also in relatively less managed shrublands and forest.

Conclusions: The role played by anthropogenic factors in shaping alien and native plant species community structure should not be ignored and, even along a marked environmental gradient, if the recipient sites have a long history of human-related disturbance, biotic homogenisation is often strong.     

The emergy of metabolism in different ecosystems under the same environmental conditions in the agro-pastoral ecotone of northern China

The sustainability of ecosystem productivity and rules governing ecosystem development are important topics of scientific research. The emergy approach is an effective method for investigating these topics, especially when used to evaluate systems that have developed under the same environmental conditions, such as climate and soil. In this paper, emergy differences between terrestrial ecosystems were studied in Guyuan County, a region representative of the agro-pastoral ecotone in Hebei Province, China. A combination of field tests and a questionnaire survey were carried out between June and August 2015. The ecosystems studied included natural grassland, artificial grassland, field crops and commercial crops. These four ecosystems were further subdivided into a total of ten ecosystems. Natural grassland was divided into free-grazing and mowed ecosystems; artificial grassland consisted of oat, Chinese leymus and corn silage; field crops included naked oats, flax and wheat; and commercial crops consisted of cabbage and potatoes. The results showed that the rain input of 4.78 × 10¹⁴ seJ/ha/yr constituted the highest renewable natural resource emergy and that the purchased emergy inputs of the ten ecosystems ranged from 3.53 to 147.67 × 10¹⁴ seJ/ha/yr. Natural resource emergy input was the basic power to maintain the ecosystem, and purchased emergy input was the direct cause of the development of the ecosystems. Groundwater was the most important non-renewable purchased energy for the production of economic crops. The emergy investment ratios (EIR) for potatoes (27.81) and cabbage (19.03) were higher than those of the other ecosystems, but mowed and artificial Chinese leymus grassland had the higher emergy self-sufficiency rates (ESR). Natural grassland, artificial Chinese leymus grassland and traditional grain crops had a low environmental load and high sustainability, whereas potatoes and cabbage had a high environmental load and low sustainability. Overall, rain-fed artificial grassland has a high development potential from the perspective of environment and productivity.     

Determinants of native and alien species richness in the urban flora of Rome

Until now, analytical studies on European urban floras have mostly concentrated on the central and north‐western parts of the continent. In this paper, factors determining species richness of urban flora were studied for the city of Rome, Italy, based on a comprehensive floristic survey carried out between 1985 and 1994, and updated in 2005. All species were recorded in grid cells of 1.6 km² and classified into native and alien (the latter divided into archaeophytes and neophytes). The grids were classified with respect to the prevailing habitat type, area available to vegetation, level of disturbance and geographical position within the city. Data were analysed using minimal adequate models. Total species number was determined by habitat and its interaction with position on the north‐west gradient; other variables explained much less variance. Holding other variables constant, the average species number per grid cell was highest in archaeological sites and parks, followed by woodlands and rivers, and grasslands and recent developments. Residential areas and the historical centre were poorest in species number. Towards the north of the city, species richness in corresponding habitats increases because of higher landscape heterogeneity and closer association with diaspore pools in the surroundings. Native species make up on average 84% of the total species numbers, and trends opposite to those for the total number of species were found for the proportional representation of aliens. The occurrence of alien and native species in the flora of Rome is driven by similar factors, but factors that increase representation of aliens decrease that of natives and vice versa. The representation of neophytes and native species in grid cells was easier to explain (74% of variation accounted for) than that of archaeophytes (27%); this result reflects that in terms of ecology and response to factors, archaeophytes take an intermediate position between native plants and neophytes. Proportional representation of neophytes decreased with increasing area available to vegetation, reflecting that semi‐natural vegetation is better developed where less fragmented.     

Invasive alien plant control: The priority to save one of the most rapidly declining island-endemic plant species worldwide

Up to 6,800 plant species endemic to oceanic islands are highly threatened with extinction. Although habitat destruction and fragmentation have greatly contributed to this, it is generally recognised that invasive alien species currently pose the single most important threat to island plants. Most studies exploring the role of novel interspecific interactions in driving declines of island plants, focus on threats mediated by animals, be it direct (e.g. browsing, seed predation, mutualism disruption) or indirect (e.g. extinction of seed dispersal or pollination mutualists). Relatively few studies have investigated the specific role of plant-plant interactions, particularly in-situ. We studied a threatened island endemic plant in rapid decline to evaluate the short (1–2 years) and medium-term (about 1–2 decades) influence of invasive alien plants (IAPs) on individuals and a variety of proxies of plant fitness. We compared mortality of traceable individuals that were recorded 12–20 years previously between habitats that are invaded with IAPs and habitats where IAPs are absent, or have been removed decades ago. We also carried out an in-situ manipulative experiment using 14 randomly chosen plants from around which IAPs were removed, paired with controls, at two sites. Canopy cover change before and after IAPs’ removal was quantified along with above ground biomass of IAPs removed for use as potential explanatory variables of change in proxies of plant fitness. Ten branches were randomly selected per plant and branch dynamics, leaves’ sizes and reproductive structure production were monitored quarterly for two years. Over the medium term, plant mortality was recorded only in presence of IAPs (X² = 4.80, df = 1, p < 0.05). Over the short term, at the plant level, IAPs’ removal triggered overall weak to moderate improvements in the number of surviving and new branches as well as change in number of branches at one of the sites. At the leaf and branch levels, we found weak evidence for positive effects of IAPs removal on surviving leaves, flower buds produced and difference in leaf surface area per branch in one site. We therefore provide some experimental evidence of negative effects of alien plants on overall fitness of the threatened species in-situ presumably through competitive interactions. We posit that these effects were found to be weak to moderate due to the short experimental period over which they could develop (1–2 years). Overall, IAPs stand out as the most severe threat from among all documented threats to the species, for being the only one capable of causing mortality of adult plants. Results hence highlight island plants’ vulnerability to IAPs, and how their timely control would improve the survival and fitness of threatened plants, even at the scale of single individuals. Such a strategy could be more often employed. Our study stresses on prioritising IAPs’ control for rescuing long-lived threatened plants that grow in habitats invaded by alien plants (itself a very common situation on oceanic islands) before addressing other subtler, slower-acting threats, like disrupted pollination or seed dispersal mutualisms, florivory or seed predation.     

Temperature-dependent performance of competitive native and alien invasive plant species

To assess the likely impacts of environmental change, the responses of two well-known invasive plant species, native Pueraria lobata and alien Humulus japonicus, to differences in growth temperature were studied in South Korea. Habitat preferences, physiological responses such as photosynthetic rates and chlorophyll contents, growth rates, and nutrient contents were quantified for each species. A competition experiment was conducted to evaluate the temperature preferences of the two species. All results indicated that the alien species H. japonicus can take advantage of elevated temperatures (35 °C) to enhance its competitive advantage against the native species P. lobata. While H. japonicus took advantage of elevated temperatures and preferred high-temperature areas, P. lobata showed reduced performance and dominance in high-temperature areas. Therefore, in future, due to global warming and urbanization, there are possibilities that H. japonicus takes advantage of elevated temperature against P. lobata that could lead to increased H. japonicus coverage over time. Therefore, consistent monitoring of both species especially where P. lobata is dominated are required because both species are found in every continents in the world. Controlling P. lobata requires thorough inspection of H. japonicus presence of the habitat in advance to prevent post P. lobata management invasion of H. japonicus.     

Invasion dynamics and potential spread of the invasive alien plant species Ageratina adenophora (Asteraceae) in China

Ageratina adenophora (Sprengel) R. King & H. Robinson (=Eupatorium adenophorum Sprengel) is one of the worst invasive alien species in China. Since A. adenophora was first noticed in Yunnan Province of China in the 1940s, its rapid spread has caused an ecological problem in south‐western China. Understanding its historical invasion pattern and its potential for further spread is needed to plan the management of the species. We reconstructed the historical process of its invasion and analysed its ecological preferences in the invaded region. After a lag phase of 20 years (1940–60), A. adenophora spread rapidly throughout the south and middle subtropical zones in Yunnan, Guizhou, Sichuan, and Guangxi, China, with an average expansion rate of 20 km per year. It spread relatively slowly in north subtropical areas, with an average expansion rate of 6.8 km per year. It has not established in warm temperate areas within the invaded regions. Although range expansion in Yunnan stopped after 1990, the expansion of its range into neighbouring provinces indicates that A. adenophora has not reached the full potential of its distribution and its range is still rapidly expanding within China. We applied ecological niche modelling (GARP — Genetic Algorithm for Rule‐set Prediction) to predict potential invasion areas in mainland China on the basis of occurrence points within colonized areas where A. adenophora has reached equilibrium. The predictions, confirmed by the range of values of four key environmental parameters, generally match the parameters of the geography and ecology in the invaded region. Southern and south‐central China have climatic conditions suggestive of a high potential for invasion by A. adenophora. Climatic conditions in northern and western China appear unsuitable for A. adenophora. Urgent measures should be taken to prevent this species from further spreading into the vast areas of potential habitat in southern and south‐central China.     

The role of species traits and taxonomic patterns in alien bird impacts

Aim To test whether the distribution of alien bird impacts varies across bird families and regions of origin, and to investigate whether species traits associated with successful introductions can predict which species will have negative impacts in the new area of introduction. Location Europe and the Mediterranean Basin. Methods Combining historical information and published literature about negative economic, biological and human health impacts, we compared the distribution of impacts among bird families and native origins of bird species for three major types of impact (economic, biodiversity and human health). We examined the relationships between ecological, biological and reproductive characteristics of species and the severity of the impacts. Results The majority of alien species with reported impacts originated from the Afrotropical, Indo‐Malayan and Palaearctic biogeographical regions. The distribution of alien bird species in Europe with reported impacts shows a taxonomic bias and largely mirrors patterns of establishment. While most species had primarily either economic or biodiversity impacts, several species in the Anatidae, Corvidae, Passeridae, Phasianidae and Sturnidae families were associated with moderate to serious negative impacts on both economic resources and native biodiversity. After controlling for taxonomic effects, species with the greatest overall impacts were habitat generalists and multi‐brooded, while species with smaller bodies and the tendency to form large feeding or roosting flocks were linked with greater impacts on native biodiversity. Main conclusions This study presents the first synthesis of published impact data for alien birds and provides a broad‐scale perspective on factors that contribute to their impacts. The results show that accounting for both species traits and taxonomy improves our ability to predict the impacts of alien bird species. Because several species are currently in the early stages of establishment in Europe, there may be an opportunity to limit negative impacts with efforts that promote proactive strategies against species and families possessing the above characteristics.     

Alien plant species composition and associations with anthropogenic disturbance in North American forests

A probability-based sampling scheme was used to survey plant species composition in forests of 16 states in seven geopolitical regions of the United States (California, Colorado, Minnesota, and parts of the Pacific Northwest, Southeast, Mid-Atlantic, and Northeast) in 1994. The proportion of alien species relative to the total species number and to canopy cover in the ground stratum (0–0.6 m height) was evaluated in 279 plots. Visually evident anthropogenic disturbances (e.g., artificial regeneration, logging, prescribed burning, and grazing by livestock), if any, were recorded on each plot. In each of the seven regions we quantified (1) the percentage of the number of species and total cover comprised of alien species, (2) the difference in these percentages for disturbed and undisturbed plots, and (3) the origin or native range for the alien species.The percentage of alien species ranged from approximately 4.5% (Colorado) to approximately 13.2% (California). The percentage of alien species cover ranged from approximately 1.5% in Colorado to 25% in California. In five regions, species introduced from temperate Eurasia comprised the largest proportion of alien species and cover. In the Southeast, species introduced from far eastern and subtropical Asia dominated the alien flora. In the Mid-Atlantic, the majority of alien species was Eurasian and the majority of alien species cover consisted of far eastern and subtropical Asian species.The proportion of plots in which at least one alien species was recorded was significantly higher in disturbed than undisturbed plots in the Southeast and marginally significantly higher ($p=0.053$) in the Northeast. These results are consistent with other published studies that indicate that anthropogenic disturbance affects the structure and composition of both the ground stratum and upper canopy of forest habitats. In other regions, however, no significant differences were found.     

Influence of settlement time, human population, park shape and age, visitation and roads on the number of alien plant species in protected areas in the USA

Abstract. I examined a data set of 77 protected areas in the USA (including national and state parks) to determine which of the following variables most strongly influence alien plant species richness: park area, climate (temperature and precipitation), native species richness, visitation rate, local human population size, total road length, park shape and duration of European settlement. Many of these predictor variables are intercorrelated, so I used multiple regression to help separate their effects. In support of previous studies, native species richness was the best single predictor of alien species richness, probably because it was a good estimator of both park area and habitat diversity available for establishment of alien species. Other significant predictors of alien species richness were years of occupation of the area by European settlers and the human population size of adjacent counties. Climate, visitation rate, road length and park shape did not influence alien species richness. The proportion of alien species (alien richness/native richness) is inversely related to park area, in agreement with a previous study. By identifying which variables are most important in determining alien species richness, such findings suggest ways to reduce alien species establishment.     

Disentangling the relative effects of environmental versus human factors on the abundance of native and alien plant species in Mediterranean sandy shores

Aim Mediterranean coastal sand dunes are characterized by both very stressful environmental conditions and intense human pressure. This work aims to separate the relative contributions of environmental and human factors in determining the presence/abundance of native and alien plant species in such an extreme environment at a regional scale. Location 250 km of the Italian Tyrrhenian coast (Region Lazio). Methods We analysed alien and native plant richness and fitted generalized additive models in a multimodel‐inference framework with comprehensive randomizations to evaluate the relative contribution of environmental and human correlates in explaining the observed patterns. Results Native and alien richness are positively correlated, but different variables influence their spatial patterns. For natives, human population density is the most important factor and is negatively related to richness. Numbers of natives are unexpectedly lower in areas with a high proportion of natural land cover (probably attributable to local farming practices) and, to a lesser degree, affected by the movement of the coastline. On the other hand, alien species richness is strongly related to climatic factors, and more aliens are found in sectors with high rainfall. Secondarily, alien introductions appear to be related to recent urban sprawl and associated gardening. Main conclusions Well‐adapted native species in a fragile equilibrium with their natural environment are extremely sensitive to human‐driven modifications. On the contrary, for more generalist alien species, the availability of limited resources plays a predominant role.     

Soil carbon and nitrogen stores and storage potential as affected by land-use in an agro-pastoral ecotone of northern China

Equilibrium carbon stock is the result of a balance between inputs and outflows to the pool. Changes in land-use are likely to alter such balance, resulting in different carbon stores under different land-use types in addition to the impacts of global climate change. In an agro-pastoral ecotone of Inner Mongolia, northern China, we investigated productivity and belowground carbon and nitrogen stores under six different types of land-uses, namely free grazing (FG), grazing exclusion (GE), mowing (MW), corn plantation (CP), fallow (FL), and alfalfa pasture (AP), and their impacts on litter and fine roots in semiarid grassland ecosystems. We found that there were great variations in aboveground net primary production (ANPP) across the six land-use types, with CP having markedly high ANPP; the FG had significantly reduced soil organic carbon (SOC) and nitrogen stores (SON) to 100 cm depth compared with all other types of land uses, while very little litter accumulation was found on sites of the FG and CP. The top 20 cm of soils accounted for about 80% of the root carbon and nitrogen, with very little roots being found below 50 cm. About 60% of SOC and SON were stored in the top 30 cm layer. Land-use change altered the inputs of organic matters, thus affecting SOC and SON stores accordingly; the MW and GE sites had 59 and 56% more SOC and 61% more SON than the FG. Our estimation suggested that restoring severely degraded and overgrazed grasslands could potentially increase SOC and SON stores by more than 55%; conversion from the native grasses to alfalfa could potentially double the aboveground biomass production, and further increase SOC and SON stores by more than 20%. Our study demonstrated significant carbon and nitrogen storage potential of the agro-pastoral ecotone of northern China through land-use changes and improved management in the context of mitigating global climate change.     

Environmental gradients shift the direction of the relationship between native and alien plant species richness

Aim

To assess how environmental, biotic and anthropogenic factors shape native–alien plant species richness relationships across a heterogeneous landscape.

Location

Banks Peninsula, New Zealand.

Methods

We integrated a comprehensive floristic survey of over 1200 systematically located 6 × 6 m plots, with corresponding climate, environmental and anthropogenic data. General linear models examined variation in native and alien plant species richness across the entire landscape, between native‐ and alien‐dominated plots, and within separate elevational bands.

Results

Across all plots, there was a significant negative correlation between native and alien species richness, but this relationship differed within subsets of the data: the correlation was positive in alien‐dominated plots but negative in native‐dominated plots. Within separate elevational bands, native and alien species richness were positively correlated at lower elevations, but negatively correlated at higher elevations. Alien species richness tended to be high across the elevation gradient but peaked in warmer, mid‐ to low‐elevation sites, while native species richness increased linearly with elevation. The negative relationship between native and alien species richness in native‐dominated communities reflected a land‐use gradient with low native and high alien richness in more heavily modified native‐dominated vegetation. In contrast, native and alien richness were positively correlated in very heavily modified alien‐dominated plots, most likely due to covariation along a gradient of management intensity.

Main conclusions

Both positive and negative native–alien richness relationships can occur across the same landscape, depending on the plant community and the underlying human and environmental gradients examined. Human habitat modification, which is often confounded with environmental variation, can result in high alien and low native species richness in areas still dominated by native species. In the most heavily human modified areas, dominated by alien species, both native and alien species may be responding to similar underlying gradients.

     

Island biogeography theory predicts plant species richness of remnant grassland patches in the agro-pastoral ecotone of northern China

Habitat loss and fragmentation are critical threats that cause biodiversity loss. However, the driving factors of species diversity in fragmented habitats are complex. This study was conducted in the agro-pastoral ecotone of northern China, a region that has suffered from severe habitat fragmentation due to agricultural activities. Based on the plant species richness, patch characteristics (patch area, fractal dimension, and isolation), landscape characteristics (grassland area ratio within buffer zones with different radii (50–500 m) around each patch), and environmental characteristics (mean annual precipitation, mean annual temperature, altitude, extremely high temperature, extremely low temperature, slope, and aspect) of 82 remnant grassland patches, we first explored the appropriate buffer zone with the most powerful impact on species richness of remnant grassland patches (SRP). Furthermore, we analyzed the relationships between a series of predictors and SRP. Finally, we confirmed the critical driving factors of SRP through a random forest analysis. The results showed that the landscape scale with the most powerful impact on SRP was 150 m. The patch area, grassland area ratio of a buffer zone with a 150 m radius, mean annual precipitation, and slope all had significantly positive effects on SRP. Among these factors, patch area and patch isolation were the top two driving factors of SRP. Our study suggests that large and highly connected patches should be protected for the construction of natural reserves. Meanwhile, the habitat around patches within a suitable range should also not be ignored. We highlight that the island biogeography theory could be used to predict plant species richness of remnant grassland patches in the agro-pastoral ecotone of northern China.     

Distribution of alien plant species in relation to human disturbance on the Georgia Sea Islands

Abstract. This study investigates the effects of human disturbance and environmental factors on the distribution of alien plant species on the Georgia Sea Islands (GSI), USA. We sampled the absolute cover of native and alien plant species on two tourist islands (St. Simons Island and Jekyll Island) and on two protected National Wildlife Refuge Islands (Blackbeard Island and Wassaw Island). On each island, vegetation composition and environmental variables (soil properties and salt spray) were measured in two habitats that differed substantially in their degree of environmental stress, the more exposed primary dune and the more sheltered and inland maritime forest. Sites were further stratified within each habitat into areas that had different levels of human disturbance. Many alien species were present on all islands and the absolute cover of alien species was not significantly different among islands even though they varied substantially in their degree of accessibility and overall land use. Alien plant cover was appreciably greater in severely disturbed sites than in less disturbed sites on all islands and within both habitats. However, the difference between disturbance categories was much less pronounced in the primary dunes where human disturbance agents do not mitigate the harsh environmental conditions of this habitat (salt spray and saline soils). Alien plant abundance on the GSI is evidently more dependent upon the availability of disturbed ground than the degree of accessibility or overall island development. It appears that human disturbance increases alien cover in general, but in environments where the stress levels are not mitigated, human disturbance does little to foster alien invasions.