Exotic plant invasion in the highly degraded upland landscape of Hong Kong, China

Plant communities in the continental tropics have suffered less from exotic plant invasions than their oceanic island counterparts. Most studies have focused on near-pristine communities. By contrast, we examine the resistance of semi-natural continental plant communities in Hong Kong, which have been suffering from chronic and massive human impacts. We compiled a list of all naturalized non-native species recorded in Hong Kong and then sampled the plant communities for exotic species along roadsides, a stream through semi-natural vegetation, and in semi-natural vegetation away from both roads and streams on Tai Mo Shan, Hong Kong’s highest peak (957 m). Similar surveys were repeated in other areas of Hong Kong. More than 162 naturalized exotic plant species have been recorded in Hong Kong. On Tai Mo Shan, 29 exotic species were recorded in roadside vegetation, with the diversity but not percentage cover declining significantly with altitude. Fifteen exotic species were found along the stream, including two not found along the roadside. Only six exotic species were found away from roads and streams, all in unshaded areas disturbed by feral cattle. In all surveys, no exotics were found in closed woody vegetation or in open areas without feral cattle, except for one species. The shade-tolerant tropical Asian tree Syzygium jambos was found invading along some streamsides without anthropogenic disturbance. Despite centuries of massive human impacts, exotic plant invasions in Hong Kong are still largely confined to habitats that suffer from chronic human disturbance. Feral cattle promote invasion where people are absent, but this problem still seems potentially reversible. Only Syzygium jambos is of possible current conservation concern.     

Identifying the most noxious invasive plants in China: role of geographical origin, life form and means of introduction

In a heterogenous world, invasive plants often differ in noxiousness and nature of impacts, and there is a difference between strong and weak invaders. That knowing which invaders are most noxious and their nature of impacts is of great value to invasive ecology and management. Here we show that in China the most notorious invasive plants are with a perennial life cycle, clonal growth ability and from the American continent. They often form monocultural populations in land and water, replacing almost all other plants, and are mainly distributed in Central and South China. Following are some accidentally introduced annuals. Annuals from the American continent are often common, dominant or in monocultural populations in ruderal and agricultural habitats whereas those from Eurasia are highly noxious only in agricultural habitats. Annuals are not restricted in distribution and could spread fast to all of the country. There are also two perennial grasses from Europe and Mediterranean area and four intentionally introduced annuals from South America that were identified in this study. Factors that may account for these patterns include relatedness between invasives and natives, vegetative propagation, and plant strategies. The invasive plants identified in this study have caused significant negative impacts to native biodiversity, environment, economics and agriculture, with different groups being different in their extent and nature of impacts.     

Soil type, microsite, and herbivory influence growth and survival of Schinus molle (Peruvian pepper tree) invading semi-arid African savanna

Naturalization of Schinus molle (Anacardiaceae) has been observed in semi arid savanna of the Northern Cape Province of South Africa. However, with high dispersal ability, the species is expected to achieve greater densities and invade more widely. The study involved a field manipulation experiment over 14 months using a factorial block design to examine transplanted seedlings in different savanna environments. The experiments examine the effects of soil type (sandy and clay), microsite, and herbivores on seedling performance (establishment, growth and survival). Seedlings were grown in a greenhouse and individually transplanted into four treatment groups: in open grassland, under tree canopies, and with and without cages to exclude large herbivores (cattle and game). The same experiment was repeated in two different soil types: coarse sand and fine-textured clay soil. Results suggest that protection provided by canopies of large indigenous Acacia trees facilitates S. molle invasion into semi-arid savanna. In the field, S. molle seedlings performed considerably better beneath canopies of indigenous Acacia trees than in open areas regardless of soil type. Whether exposed or protected from large herbivores, no seedlings planted in open grassland survived the first winter. Although, seedlings grew better and had higher survival rates beneath tree canopies than in the open sites, exposure to large herbivores significantly decreased heights and canopy areas of seedlings compared with those protected from large herbivores. The effect was greater on clay soil than on sandy soil. The results suggest that low temperature (frost), and possibly inter-specific competition with grasses, may limit S. molle seedling establishment, survival and growth away from tree canopies in semi arid savannas. Low soil nutrient status and browsing may also delay growth and development of this species. The invasive potential of S. molle is thus greatest on fertile soils where sub-canopy microsites are present and browsing mammals are absent.     

Distribution of alien vs. native plant species in roadside communities along an altitudinal gradient in Tenerife and Gran Canaria (Canary Islands)

Roadside plant communities were studied along two roads following an altitudinal gradient in Gran Canaria and Tenerife (Canary Islands). Our aim was to investigate variation in plant species richness, particularly of the alien flora, along a gradient from coastal shrubland to summit vegetation (1950 m a.s.l. in Gran Canaria, 2300 m in Tenerife) in relation to variation in habitat factors (altitude, habitat structure, roadside disturbance, distance to urban nuclei). We compared different species groups that were classified in terms of their biogeographical status, origin and life form. Altitude was the most important factor determining species richness and composition along both roadside transects. Alien plants showed a unimodal distribution pattern along the altitudinal gradient, with less species and lower abundance at low and high altitudes, and highest abundance at intermediate altitude. Alien plant species were also relatively more frequent near urban centres. The number of native and alien species was significantly positively correlated along the altitudinal gradient. Both alien and native, non-endemic species showed differences in their distribution along the altitudinal gradient according to their biogeographical affinities and climatic tolerances. Despite considerable differences in species pools these patterns were consistent among the two islands. Environmental (abiotic) stress is proposed as a primary, altitude-related factor acting as a filter against most alien plants at coastal and high-mountain altitudes. A higher frequency or intensity of disturbance at intermediate altitudes may be a further causal factor promoting alien plants in this zone. Future management efforts to control alien plants along roads should, therefore, concentrate on intermediate altitudinal zones of the higher Canary Islands.     

Effects of anthropogenic disturbances on the butterfly assemblage in an urban green area: the changes from 1990 to 2005 in Kanazawa Castle Park, Japan

Kanazawa Castle Park (KCP) is an isolated urban green area in Kanazawa, Japan. A mature forest had developed there since 1949, but it was partially deforested from 1996 to 1999. Moreover, most of the grasslands in KCP were removed from 2000 to 2002. In this study, we report the damage to the butterfly assemblage by these anthropogenic disturbances and subsequent recovery after the disturbances, using previous literature on the butterfly assemblage in KCP from 1990 to 1994 and our monitoring data from 1999 to 2005. In addition, to understand urbanization effects, we also compared the butterfly assemblage of KCP with that of Tawara, a satoyama area 6 km away from KCP. Before the urbanization around KCP, KCP might have had a butterfly assemblage similar to Tawara. The previous articles indicate that 33 species lived in KCP before the partial deforestation. The number of generalist species (multivoltine and polyphagous species) was similar in KCP and Tawara (22 vs. 19), but that of specialist species (uni-bivoltine and oligophagous species) was lower in KCP than in Tawara (5 vs. 15). Six species, composed of a bamboo grass feeder, mantle feeders, herb/grass feeders, and a multi-feeder, disappeared from KCP after the deforestation and have not yet recolonized. The grassland removal also affected herb/grass feeders negatively, but they have been recovering quickly following the partial grassland recovery. Some species that disappeared after the deforestation were sporadically found from 2000 to 2005 probably due to immigration from the surrounding rural areas (the nearest one is 2 km away). For recolonization of the lost species, recovery of the interior and edge of the forest, which are the habitats for these species, is necessary.     

Invasions by alien plant species of the agro-pastoral ecotone in northern China: Species-specific and environmental determinants

The establishment, reproduction, dispersal, and distribution of alien plants are affected by various factors during the transition from being newly introduced in a habitat to being invasive. In the agro-pastoral ecotone of northern China, comprising farmlands and natural grasslands, the biological characteristics of alien plant species were the key intrinsic factors (propagation characteristics and competitive ability), followed by such extrinsic factors as human interference and environmental heterogeneity. Among biological characteristics, the life form may be an important and useful indicator of the invasive ability of a species, and the risk of invasion is greater from alien species that are poisonous, inedible, and have traits that facilitate wide dispersal. Farmlands may serve as initial shelters for alien species, from which they spread into neighbouring habitats, whereas natural grassland may act as a barrier to plant invasions. Management practices detrimental to grasslands, including overgrazing, reclamation, and road construction, often facilitate the invasions; therefore, counter measures such as reseeding and a ban on grazing need special attention. Environmental factors including precipitation, nutrients, prevailing winds, fires, and topography may be other factors that promote or block the process of invasion. In studying ways of preventing or controlling such invasions, alien plants with short life cycle, prolific seed production, and strong competitiveness, deserve particular attention and so do human activities that may damage the environment and fragile habitats.     

Linking biotic homogenization to habitat type, invasiveness and growth form of naturalized alien plants in North America

Aim Biotic homogenization is a growing phenomenon and has recently attracted much attention. Here, we analyse a large dataset of native and alien plants in North America to examine whether biotic homogenization is related to several ecological and biological attributes. Location North America (north of Mexico). Methods We assembled species lists of native and alien vascular plants for each of the 64 state‐ and province‐level geographical units in North America. Each alien species was characterized with respect to habitat (wetland versus upland), invasiveness (invasive versus non‐invasive), life cycle (annual/biennial versus perennial) and habit (herbaceous versus woody). We calculated a Jaccard similarity index separately for native, for alien, and for native and alien species. We used the average of Jaccard dissimilarity index (1 ? Jaccard index) of all paired localities as a measure of the mean beta diversity of alien species for each set of localities examined in an analysis. We used a homogenization index to quantify the effect of homogenization or differentiation. Results We found that (1) wetland, invasive, annual/biennial and herbaceous alien plants markedly homogenized the state‐level floras whereas non‐invasive and woody alien plants tended to differentiate the floras; (2) beta diversity was significantly lower for wetland, invasive, annual/biennial and herbaceous alien plants than their counterparts (i.e. upland, non‐invasive, perennial and woody alien plants, respectively); and (3) upland and perennial alien plants each played an equal role in homogenizing and differentiating the state‐level floras. Main conclusions Our study shows that biotic homogenization is clearly related to habitat type (e.g. wetland versus uplands), species invasiveness and life‐history traits such as life cycle (e.g. annual/biennial and herbaceous versus woody species) at the spatial scale examined. These observations help to understand the process of biotic homogenization resulting from alien vascular plants in North America.     

Concordance between life history traits, invasion history, and allozyme diversity of the Everglades invader Melaleuca quinquenervia

During the century following its initial introduction in 1886, the Australian tree Melaleuca quinquenervia (Myrtaceae) dispersed from a few introduction points to occupy over 200,000 ha, primarily in historic Everglades wetlands of southern Florida. Cellulose acetate gel electrophoresis (CAGE) was used to investigate the allozyme diversity and population genetic structure of 208 individuals in a dozen populations resulting from this invasion. The analyses showed that these populations have a high (82%) rate of polymorphic loci and an average of 2 alleles/locus. There was substantial heterozygosity (mean H_e = 0.356), which concords well with recent studies reporting a greater number of introduction events and sources than generally recognized. The introduction history and distributional patterns within Florida have led to geographic structuring (G_ST = 0.419) in which the Gulf Coast metapopulation has a greater effective number of alleles and greater heterozygosity than the Atlantic Coast metapopulation. The gene diversity in M. quinquenervia was comparable to other tropical woody species. Its strong population divergence was reminiscent of pioneer species and consistent with its status as a plant invader in Florida.     

Minimum residence time, biogeographical origin, and life cycle as determinants of the geographical extent of naturalized plants in continental Chile

The geographical extent of exotic plant species is a major component of invasiveness, which has been explained by intrinsic attributes of the plants, such as growth rate, reproductive type, life form, and biogeographical origin. We assessed quantitatively life cycle and biogeographical origin as determinants of the geographical distribution of naturalized plants in continental Chile, using minimum residence time (MRT) as an estimator of introduction date. We assembled a database with information on 428 plants (principally herbs) in continental Chile, corresponding to 61% of the exotic naturalized flora. For each species we recorded: (1) minimum residence time (introduction date or first recorded date in the country); (2) biogeographical origin (American, Eurasian, others); (3) life cycle (annual, perennial, others); (4) number of Chilean regions occupied by the plant. We found that 82 species (19%) have been recorded in only one region of Chile, while only three species have been found in all 13 regions of the country. About 89% of the species (381) have been found only in central Chile (Regions IV to VIII), while the remaining 11% (47) are found only in the northern (Regions I to III) or southern parts of the country (Regions IX to XII). We detected significant differences in regional spread of naturalized plants according to minimum residence time: those species with shorter MRT had more limited spread ranges than those with longer MRT. Biogeographical origin and life cycle did not explain geographical extent in Chile. This study shows that historical factors are more important than biological ones in determining the geographical extent of naturalized plants in continental Chile. Thus, caution should be exercised when assigning value to biological attributes that may confer invasiveness to naturalized plants.     

Phenology and life cycle of the annual, <Emphasis Type="Italic">Chamaesyce maculata</Emphasis> (L.) Small (Euphorbiaceae), with multiple overlapping generations in Japan

The phenology of germination, vegetative growth and sexual reproduction in the annual Chamaesyce maculata (L.) Small (Euphorbiaceae) were investigated in a natural population in western Japan. Seedlings emerged from mid-June to early October, with three peaks: mid-June, late July and late August. Plants that emerged in June commenced sexual reproduction from late July, and thereafter both vegetative growth and sexual reproduction occurred together until early November, the plants showing no switching from vegetative growth to sexual reproduction. Seedlings that emerged in June and July suffered high mortality, but most seedlings that emerged from August onward survived until the reproductive stage. The minimum size for reproduction was largest for plants that emerged early in the season, and it decreased with a delay in seedling emergence. The late emergence of seedlings that resulted in low reproductive output may be to some extent compensated for by the increased probability of survival in the seedling stage. A transplant experiment clarified that C. maculata can repeat a maximum of three overlapping generations within a year. Multiple generations per year were attained by non-dormant seeds produced in the first and second generations and clearly resulted in an increased reproductive output per year. The life cycle with multiple overlapping generations may have been acquired in habitats where unpredictable disturbance results in temporally unsuitable conditions for germination, vegetative growth and sexual reproduction of annual plants, but where suitable conditions frequently continue over a period longer than the single generation time of annual plants.     

Nest-site selection and reproductive success of a critically endangered parrot,the Great Green Macaw (Ara ambiguus), in an anthropogenic landscape

Nest-site selection is the principal way in which secondary cavity-nesting species mitigate the negative effects of factors such as predation, parasitism and exposure on reproductive success. Large-bodied secondary cavity-nesting birds rely on large cavities in mature trees that are often absent or reduced in anthropogenically disturbed forests. Hence, the availability of high-quality nest-sites may be limited for these species, potentially reducing reproductive success. The aim of this study was to investigate nest-site selection and the effect of nest-site features on reproductive success in the critically endangered Great Green Macaw Ara ambiguus in northern Costa Rica. We show that Great Green Macaws select nest-sites according to the characteristics of the cavity and the tree in which they are located. Moreover, reproductive success is a function of certain cavity features. However, the availability of high-quality cavities, those with features associated with higher reproductive success, is potentially limited. By studying nest-site selection and productivity together, we have identified that the availability of high-quality cavities could potentially limit the future recovery of the critically endangered Great Green Macaw.     

Invasion history, habitat preferences and phenology of the invasive ladybird Harmonia axyridis in Belgium

Harmonia axyridis Pallas (Coleoptera: Coccinellidae) is native to Asia, and was widely introduced as a biocontrol agent of aphids and coccids in Europe and North America. In Europe, H. axyridis is considered to be an invasive alien species because of its potential to disrupt native ladybird communities. Since 1999, the Belgian Ladybird Working Group mapped all Belgian Coccinellidae and recorded data on substratum plants and habitat. The first feral H. axyridis population in Belgium was recorded in 2001, but the expansion rate is decreasing because it now colonised the whole country. Recorded occupancy in Belgium showed an average rate of increase of 189% between 2002 and 2006. In Belgium, H. axyridis occurred in a wide range of habitats, including those of high conservation value. However, habitat and land cover analysis showed that H. axyridis is more frequently found in urbanised landscapes than in semi-natural landscapes. A niche overlap analysis based on plant use data showed that the potential to affect native species is higher for generalist, deciduous and coniferous tree ladybird species than for heathland and wetland specialist species. Phenology data showed that H. axyridis is able to reproduce later in the year than native species. Based on recorded distribution, ecology and phenology, we discuss the potential of H. axyridis to disrupt native ladybird assemblages in Belgium.     

Use of life table statistics and degree-day values to predict the invasion success of Gonatocerus ashmeadi (Hymenoptera: Mymaridae), an egg parasitoid of Homalodisca coagulata (Hemiptera: Cicadellidae), in California

Life table statistics and degree-day requirements for Gonatocerus ashmeadi Girault, a parasitoid of the glassy-winged sharpshooter Homalodisca coagulata (Say), were used to estimate the number of expected parasitoid generations in California (USA). Between two to 51 and one to 37 generations per year were estimated across different climatic regions in California, using life table and degree-day models, respectively. Temperature-based values for net reproductive rate, R_o, were estimated in California using a laboratory-derived equation and ranged from zero to approximately 48 and analyses indicate that a minimum of eight generations are required each year to sustain a population increase of G. ashmeadi. Long-term weather data from 381 weather stations across California were used with an Inverse-Distance Weighting algorithm to map temperature-based estimations for the entire state of California. This Geographic Information Systems model was used to determine number of G. ashmeadi generations based on day-degree accumulation, T_c, and R_o. GIS mapping indicated that Californian counties in the north, central west coast, central west and Sierra Nevada regions may be climatic conditions unfavorable for supporting the permanent establishment of invading populations of G. ashmeadi should H. coagulata successfully establish year-round populations in these areas. Southern counties in California that experience warmer year round temperatures and support year round populations of H. coagulata, appear to be conducive to the establishment of permanent populations of G. ashmeadi. The mechanisms facilitating G. ashmeadi invasion and the implications of these temperature-based estimates for biological control of H. coagulata are discussed.