Number Of Exotic Species Research Articles

The role of exotic species in homogenizing the North American flora

Exotic species have begun to homogenize the global biota, yet few data are available to assess the extent of this process or factors that constrain its advance at global or continental scales. We evaluate homogenization of vascular plants across America north of Mexico by comparing similarity in the complete native and exotic floras between states and provinces of the USA and Canada. Compared with native species, exotic plants are distributed haphazardly among areas but spread more widely, producing differentiation of floras among neighbouring areas but homogenization at greater distance. The number of exotic species is more closely associated with the size of the human population than with ecological conditions, as in the case of native species, and their distributions are less influenced by climate than those of native species.

HAWAII IMPORTATION PERMITS AND SPECIES IN THE ENVIRONMENT

Hawaii's aquaculture industry is driven by the introduction and production of exotic species. At issue is the establishment of exotic species in the environment. The number of import permits issued can represent the potential for exotics to establish in the environment. More than half of the permits issued annually by the Plant Quarantine Branch are multiple entry-permits related to live marine food and ornamental organisms. Single entry-permits amount to about 150 permits per year for more ornamental than food organisms. Known listings of exotic species in the environment can indicate trends and potential species establishment. Since the 1960s, the numbers of exotic species established in the environment has matched those released by the State prior to 1960 for forage or game purposes. The majority of species released is probably owing to hobby-related activity and not that of the aquaculture industry. The ornamental sector of the industry is not sufficiently developed and production by Hawaii aquaculture farms is exported.

PLANT COMPOSITION AND EROSION POTENTIAL OF A GRAZED WETLAND IN THE SALMON RIVER SUBBASIN, IDAHO

Wetlands are dynamic habitats with many unique, important functions including filtering sediments and providing diverse habitats for fish and wildlife. Wetlands in the western United States are particularly important because they offer habitat for a number of protected runs of endangered fish species. Historically, livestock grazing has altered wetland and riparian area form and function by facilitating exotic species invasions, altering spatial heterogeneity of vegetation, and increasing erosion. In this study we examined vegetation structure and erosion potential in a wetland meadow exposed to unregulated grazing along Deer Creek in the Salmon River subbasin, Idaho. We characterized the vegetation composition and structure within the study area and attempted to assess potential erosion conditions using the Revised Universal Soil Loss Equation (RUSLE), an empirical approach developed by the U.S. Department of Agriculture–Agricultural Research Service (USDA-ARS). We found no significant spatial variability in species richness and noted a moderate number of exotic species in the total plant composition. Plant cover was higher near slightly entrenched banks, indicating that uncontrolled livestock were primarily occupying gently sloped streambanks and the interior of the meadow. Based on current vegetation composition and RUSLE results, uncontrolled grazing may be negatively impacting the study area. If uncontrolled grazing were excluded or carefully managed in the wetland meadows of the upper portion of the Deer Creek watershed, a reduction in excess sediments to Deer Creek may occur.

The importance of marginal habitats for the conservation of old trees in agricultural landscapes

Deterioration of unfarmed habitats resulted in a considerable decrease of biological diversity in agricultural landscapes. The most important elements of cultural landscapes determining a high level of biological diversity are woodlots and the trees themselves. This paper presents the results of a study carried out in 2000–2001 on the distribution and number of champion (heritage) trees on the area of 5480 ha of intensively managed agricultural land (Lower Silesia, south-western Poland). The fieldwork consisted in localizing and measuring all champion tree specimens on the area of 5480 ha of an agricultural fragment of Wrocław Plain (Lower Silesia Province, SW Poland). Champion trees were found in 116 (50.1%) of all 228 surveyed sites. The overall number of champion trees in the study area was 493, belonging to 34 species. The most numerous were native species (276 specimens). The four species of cultivated trees amounted to 40% of all champion trees. The largest percentage of habitats with at least one champion tree specimen was characteristic for manor parks (100%), the lowest—for village areas (27.8%) and mid-field clumps (39.2%). A high, statistically significant correlation was found between the size of a mid-field clump and the number of champion trees it contained. The highest number of champion trees was found in water-edge hedgerows. The current occurrence of champion trees in the agricultural landscape is the result of many centuries of rural areas management. Apart from a number of exotic species of trees, introduced mainly for aesthetic reasons to the inhabited areas (villages, parks, cemeteries), many champion specimens of native trees have survived in mid-field woodlots, planted and managed in the past due to their high economic value ( Quercus robur, Fraxinus excelsior, Ulmus laevis). The pattern of occurrence of native champion trees indicates that some of them may arrived here as a result of natural processes, such as dispersion (e.g. Crataegus monogyna and Rhamnus cathartica that occurred in mid-field clumps).

The endemic tree Corynocarpus laevigatus (karaka) as a weedy invader in forest remnants of southern North Island, New Zealand

Native species can display invasiveness even in their natural systems. Corynocarpus laevigatus (Corynocarpaceae), karaka, is a tree species apparently native to the north of the North Island, New Zealand, with a history of human cultivation further south, where it appears to be outcompeting other native tree species. Invasiveness was assessed by comparing vegetation in paired plots with and without karaka at 14 sites in forest remnants in the lower half of the North Island. Potential to spread was estimated via dispersal and recruitment patterns around parent trees. Karaka has several “weedy” characteristics. Most seed falls near the parent, but recruitment apparently improves outside the shady canopy, resulting in concentric invasion rings. Compared with nearby non‐karaka plots, karaka plots have fewer non‐karaka trees, lower abundances of seedlings of other woody species, and slightly higher numbers of exotic species. Karaka appears to be homogenising vegetation, through reduction in herb cover and increase in shrub cover and probably eventual replacement of canopy trees. With its range anthropically extended, management of karaka's invasiveness appears necessary in lower North Island, South Island, and the Kermadec and Chatham Islands, involving elimination or control, depending on local cultural values.

Habitat Differences in Marine Invasions of Central California

We carried out a two-part investigation that revealed habitat differences in marine invertebrate invasions. First, we compared invasion levels of hard vs soft substrata in Elkhorn Slough, an estuary in Central California, by comparing abundance and richness of native vs exotic species in quantitative samples from each habitat type. Our results revealed that the hard substrata were much more heavily invaded than the soft substrata. Nearly all the hard substrata in Elkhorn Slough, as in most estuaries along the Pacific coast of North America, are artificial (jetties, rip-rap, docks). Some exotic species may by chance be better adapted to this novel habitat type than are natives. Two major vectors responsible for marine introductions, oyster culturing and ship-hull fouling, are also more likely to transport species associated with hard vs soft substrata. Secondly, we compared estuarine and open coast invasion rates. We examined species richness in Elkhorn Slough and adjacent rocky intertidal habitats along the Central California coast. The absolute number of exotic species in the estuary was an order of magnitude higher than along the open coast (58 vs 8 species), as was the percentage of the invertebrate fauna that was exotic (11% vs 1%). Estuaries on this coast are geologically young, heavily altered by humans, and subject to numerous transport vectors bringing invasive propagules: all these factors may explain why they are strikingly more invaded than the open coast. The finding that the more species rich habitat – the open coast – is less invaded is in contrast to many terrestrial examples, where native and exotic species richness appear to be positively correlated at a broad geographic scale.

Plant Species Richness On An Electric Transmission Right-Of-Way Using Integrated Vegetation Management

The State Game Lands (SGL) 33 Research and Demonstration Area, Centre County, Pennsylvania, U.S., has been studied each year since 1953, making this 52-year old project extremely valuable as a source of information on the effects of mechanical and herbicidal maintenance on flora and fauna along an electric transmission right-of-way (ROW). A desired objective of integrated vegetation management on the SGL 33 Research and Demonstration Area is to create a diversity of plant species. In this paper, our objective was to document plant species richness among treatment units and in relation to wire and border zones on the SGL 33 Research and Demonstration Area. We noted the presence of plant species from late May through mid-August in both 2003 and 2004 and observed 125 vascular plant species in the 15 treatment units. The total number of species per unit ranged from a low of 35 species in a mowing unit to a high of 63 species in a basal low-volume spray unit. Of the total number of plant species found on the right-of-way, 95 (76%) and 110 (88%) occurred in wire and border zones, respectively. In wire zones, the average number of plant species ranged from 31 in mowing units to 41 in stem­foliage spray units. In border zones, the average number of plant species varied from a low of 34 in mowing units to a high of 41 in handcut units. The proportion of exotic species did not vary appreciably between wire and border zones (19% and 22% of total, respectively) on the ROW. However, the three units adjacent to unpaved state forest roads with high rates vehicular traffic not only had the highest number of total species in the unit (55 to 63 species) but also the highest number of exotic species (13 to 16 species). Hence, vehicular traffic and roadside management may be responsible in part for the invasion of seeds or other means of plant dispersal to these units. Because units relatively isolated from state forest roads tended to have fewer exotic species, these sections of the ROW can serve as refugia for native flora.

Does fallen timber on floodplains influence distributions of nutrients, plants and seeds?

While much is conjectured about the ecological significance of fallen timber, there has been little work on the influence of logs on plants and nutrient distributions on periodically inundated floodplains. We compared distributions of seeds, germinated plants and a suite of soil properties around logs on (a) the floodplain proper, (b) in ‘flood runners’ (floodplain drainage channels) and (c) at randomly selected log-free locations on the floodplain. Sampling was conducted on logs oriented parallel to the river, so that we could compare the sides of logs closer to and further from the river. Numbers of exotic species germinating around flood-runner logs was much reduced compared with the other two locations. Native seeds accumulated to a greater extent on the river-side of floodplain logs compared to flood-runners logs. Densities of plants were lowest in the immediate proximity of logs, increasing up to two-fold 2 m away on both sides of logs. Gravel fraction was highest around flood-runner logs, while the fraction around floodplain logs was higher than at log-free locations. Extractable Na was higher around flood-runner logs (2.6%) than at log-free points (1.2%), but levels for floodplain logs were intermediate (2.1%). Soil samples taken near flood-runner logs were slightly more acidic than samples taken elsewhere. Our results generally were not consistent with expectations that floodplain logs would act as favourable areas for plant germination and survivorship. While certain sediment characteristics did appear to be influenced by the presence of logs (e.g., gravel fraction, pH and Na), effects were relatively small and most other measured characters were not affected by location.

Towards a really unified theory for metacommunities

Traditionally, community ecologists assumed that spe-cies differ in some aspects of their traits or responsesto the environment (i.e. their niches), which allow themto coexist in the same habitat (Hutchinson 1957, 1959).Recently, Hubbell (2001) and others (e.g. Bell 2001,2003) have suggested that this view is inadequate toexplain the diversity often observed in speciose systems.For example, hundreds to thousands of tree species livein tropical forests, which only have a handful of limitingresources such as water, light, and a variety of macro- andmicronutrients. Such high diversity, with so few resources,they argue, cannot be explained by niche theory.As an alternative to the traditional niche theory,Hubbell (2001) developed a neutral theory of commu-nity structure (see also Caswell 1976; Hubbell 1979;Hubbell & Foster 1986; Bell 2000, 2001, 2003; Chave L Chave 2004). In the neutral theory, pat-terns of species diversity, relative abundance, andcomposition are primarily a function of the size of themetacommunity, the dispersal rate of organisms withinthe metacommunity, and the rates of generation (spe-ciation) of new species (Bell 2001; Hubbell 2001; Chave2004). Because species are assumed to be identicalecologically, Hubbell termed his a ‘neutral theory’, bydirect analogy to neutral genes in population genetics.Hubbell further termed his theory ‘unified’ in that it issimultaneously able to predict diversity and relativeabundance of organisms in a locality, as well as bioge-ographic patterns of species composition.In this essay, I overview the key assumptions (inputs)required and the insights (outputs) that can be gainedfrom each framework. While a complete discussion ofall of the assumptions and predictions of these modelsis beyond the scope of this essay (but see Chase

Citizens as Propagules for Exotic Plants: Measurement and Management Implications1

Abstract There is a significant positive association between exotic plant species richness of an area and the human population size of that area. This association is notable not only for its high correlation (r2 = 0.69) but also because it spans five orders of magnitude for human population size. The correlation is apparently caused by the introduction of increasing numbers of exotic species as human population size grows. As population grows, so does exotic propagule importation and habitat disturbance. The regression is clearly asymptotic, indicating that per capita rates of exotic species introductions decline as human population continues to increase. A main implication for exotic species control is that efforts to limit exotic species will be most effective and least costly in areas (at many scales) where human population size is still relatively small. Another implication is that because human population size will continue to increase in many areas, educating the general public about harmful exotic ...

Weak interaction rates for astrophysical applications

An important effort has been developed in recent decades to measure the masses and β-decay rates of very neutron-rich nuclei at radioactive ion beam (RIB) facilities. However, major astrophysical applications involve a huge number of exotic species. Most of them cannot be synthesized in terrestrial laboratories and only theoretical predictions can fill the gap. We concentrate on the self-consistent predictions of the β-decay rates needed for stellar r-process modeling and for performing the RIB experiments. The continuum QRPA approach based on the self-consistent ground-state description in the framework of the density functional theory is briefly described. The model for the large-scale calculations of total β-decay half-lives accounts for the Gamow-Teller and first-forbidden transitions. Due to the shell configuration effect, the first-forbidden decays have a strong impact on the total half-lives of the r-process relevant nuclei at N=126, Z=60–70. The performance of existing global models for the nuclides near the r-process paths at N=126 is critically analyzed and confronted with the recent RIB experiments in the region “east” of 208Pb.

Flora diversity of farm forestry plantations in southeast Queensland

Summary Plant species composition in plantation monoculture of the native Gympie Messmate (Eucalyptus cloeziana F. Muell.) was assessed and compared with native eucalypt forest and cleared grazing land in southeast Queensland, Australia. A total of 18 sites (11 in the plantations, four in native eucalypt forests and three on cleared grazing land) were surveyed. The four plantation age classes during the 18‐month survey period were 0.3–1.8 (very young), 2.1–3.8 (young), 15–16.5 (mid‐aged) and 38–40.5 (old) years of age. Significantly more native plant species were recorded in the plantations, regardless of their age, than on cleared grazing land. The number of native plant species in the old plantations was not significantly different from the native eucalypt forests. Native tree and shrub species richness increased significantly with plantation age. Two species (Ricinocarpos speciosus Muell. Arg. and Xanthostemon oppositifolius F. M. Bailey) listed as Vulnerable and one species (Alyxia magnifolia F. M. Bailey) listed as Rare were recorded in the old plantations. Two Rare species (A. magnifolia and Acianthus amplexicaulis (F. M. Bailey) Rolfe) were recorded in the native eucalypt forests. Exotic plant species, consisting mainly of herbs, grasses and shrubs in the plantations, were significantly more abundant in the very young and young plantations. However, the number of exotic species decreased significantly with increasing age of the plantations. The results suggest that even small‐scale plantation can increase landscape heterogeneity and help protect biodiversity.

Ecological filtering of exotic plants in an Australian sub‐alpine environment

We investigated some of the factors influencing exotic invasion of native sub‐alpine plant communities at a site in southeast Australia. Structure, floristic composition and invasibility of the plant communities and attributes of the invasive species were studied. To determine the plant characteristics correlated with invasiveness, we distinguished between roadside invaders, native community invaders and non‐invasive exotic species, and compared these groups across a range of traits including functional group, taxonomic affinity, life history, mating system and morphology. Poa grasslands and Eucalyptus‐Poa woodlands contained the largest number of exotic species, although all communities studied appeared resilient to invasion by most species. Most community invaders were broad‐leaved herbs while roadside invaders contained both herbs and a range of grass species. Over the entire study area the richness and cover of native and exotic herbaceous species were positively related, but exotic herbs were more negatively related to cover of specific functional groups (e.g. trees) than native herbs. Compared with the overall pool of exotic species, those capable of invading native plant communities were disproportionately polycarpic, Asteracean and cross‐pollinating. Our data support the hypothesis that strong ecological filtering of exotic species generates an exotic assemblage containing few dominant species and which functionally converges on the native assemblage. These findings contrast with those observed in the majority of invaded natural systems. We conclude that the invasion of closed sub‐alpine communities must be viewed in terms of the unique attributes of the invading species, the structure and composition of the invaded communities and the strong extrinsic physical and climatic factors typical of the sub‐alpine environment.

Non indigenous species settle down in the Taranto Seas

Biogeographia vol. XXV - 2004 (Printed December, 30 q' 2004) Marine Biogeography of the Mediterranean Sea: patterns and dynamics of biodiversity Non indigenous species settle down in the Taranto Seas FRANCESCO MASTROTOTARO*, ANTONELLA PETROCELLI**, ESTER CECERE**, ALFONSO MATARRESE* *Dz;p/zrtimento di Z00/ogirz, Universitfz di Bari, Via Orzzeomz, 4, 7 0125 Bari (Italy) **[5tiz‘uto Ameients Maritza Cosziero (MMC) — CNR. — Sezione di Tzzrrmto — T aldssogmfico “A. Cerrutf’, Via Roma, 3, 74100 Tzzrzmto (Italy) e—mail: rmzstrotomro @ez'ologz'zz. uniezz. it Key words: non indigenous species, Gulf of Taranto, Ionian Sea, Mediterranean Sea. SUMMARY The presence and the distribution of seven exotic species (4 seaweeds and 3 animals) in the seas of Taranto are discussed. The seaweeds are: Czm/eipzz mmnorzz var. (.yli;zdrr1cM, Codizmzfiagile subsp. tamentosoides (Chlorophyceae), U721/rIritl}gir1I11ztiflt/:1 (pltaeopliyceae), and W/0772213113/all/1 retrzcerz (Rhodophyceae), all well~known as invasive species‘ The animal species are: Brmzc/Jiomm/z /ummmm (Polychaeta), Mzrrorosnms xqzmmiger (Ascidiacea), and Mmcz/Zixrzz I372/aousizz (Bivalvia). INTRODUCTION In the last 30-40 years, the number of exotic species, accidentally introduced in the Mediterranean Sea, has considerably increased. This was due to both the increasing of the naval traffic (transport offouling species and planktonic larvae in the ballast waters) (Occhipinti Ambrogi, 2001) and the aquaculture activity which, recently, went in for the breeding of non indigenous species [e.g. Cmssostrezz gzgrzs (Thunberg, 1793); Tapes pbilzppinarum (Adams and Reeve, 1850); Pemzezisjapaniczrs Bate, 1888; etc.] (Rossi, 1992). In the sea basins facing the town of Taranto (i.e. the Max Piccolo, the Mar Grande and a part of the gulf) there is a wide intercontinental naval traffic due to both the presence of the most important Italian Navy base and to the merchant harbour. Moreover, numerous mussel farms are present. The above mentioned activities together with industrial emissions and sewages, favoured

Do Exotics Homogenize or Differentiate Communities? Roles of Sampling and Exotic Species Richness

It is commonly asserted that exotic species promote the homogenization of biological communities. However, theoretical evidence implies that exotic species may often have the opposite effect, of differentiating biological communities where different exotic species become established in different areas. Furthermore, few empirical studies have directly measured the homogenizing effects of exotics. In this study, I used the plant inventories of 20 localities in the United States to measure whether exotic plant increased the similarity of those localities. I calculated Jaccard's index of similarity (JI) for exotic species and then for native species to see if the exotic JI was consistently higher when comparing two localities. I found that JI for both exotic and native species decline exponentially with increasing distance and latitude separation between localities so that localities that share many native species also tend to share many exotic species. More importantly, in nearly half of the pairwise locality comparisons (87 of 190), the (JIexotic/JInative) ratio is less than one, indicating a slight tendency for exotic species to differentiate rather than homogenize the localities analyzed. Also, the pattern of differentiation versus homogenization is strongly related to exotic species richness. When both localities have relatively few exotics, there is a clear tendency for exotics to produce biotic differentiation (JIexotic/JInative < 1). Much of this pattern seems related to the right-skewed frequency distribution of the geographic ranges of exotic species. As with native species, most exotics occur in few localities so there is a high probability that localities with low numbers of exotic species will share very few, if any, of those exotics. As exotic richness increases, the homogenizing effects become increasingly pronounced.

Ecological filtering of exotic plants in an Australian sub-alpine environment

We investigated some of the factors influencing exotic invasion of native sub-alpine plant communities at a site in southeast Australia. Structure, floristic composition and invasibility of the plant communities and attributes of the invasive species were studied. To determine the plant characteristics correlated with invasiveness, we distinguished between roadside invaders, native community invaders and non-invasive exotic species, and compared these groups across a range of traits including functional group, taxonomic affinity, life history, mating system and morphology. Poa grasslands and Eucalyptus-Poa woodlands contained the largest number of exotic species, although all communities studied appeared resilient to invasion by most species. Most community invaders were broadleaved herbs while roadside invaders contained both herbs and a range of grass species. Over the entire study area the richness and cover of native and exotic herbaceous species were positively related, but exotic herbs were more negatively related to cover of specific functional groups (e.g. trees) than native herbs. Compared with the overall pool of exotic species, those capable of invading native plant communities were disproportionately polycarpic, Asteracean and cross-pollinating. Our data support the hypothesis that strong ecological filtering of exotic species generates an exotic assemblage containing few dominant species and which functionally converges on the native assemblage. These findings contrast with those observed in the majority of invaded natural systems. We conclude that the invasion of closed sub-alpine communities must be viewed in terms of the unique attributes of the invading species, the structure and composition of the invaded communities and the strong extrinsic physical and climatic factors typical of the sub-alpine environment.

Invasion of native vegetation by Coolatai Grass Hyparrhenia hirta: impacts on native vegetation and management implications

Coolatai Grass Hyparrhenia hirta is an exotic perennial grass and environmental weed that is spreading rapidly in parts of southeastern Australia. This study examined the effects of Coolatai Grass invasion on the composition of ground strata vegetation within Kwiambal National Park in northern New South Wales. Plots that were heavily infested with Coolatai Grass together with matching control plots were surveyed. Coolatai Grass infestation reduced the richness of native ground strata plants and their projected groundcover, but did not affect the number of exotic species in plots. The control plots varied markedly in floristic composition, but infested sites were more homogenous, with Coolatai Grass clearly dominating the ground strata vegetation. Coolatai Grass appears to increase in abundance following fire; it persists under heavy grazing and is resistant to many herbicides. Spot spraying small swards and isolated tussocks with glyphosate or flupropanate provides prospects of control on a small scale, but there are no suitable methods for largescale control. Changes to roadside management practices could help to reduce the spread of Coolatai Grass, but research into seed bank dynamics and appropriate control techniques is required, as well as increased awareness of the threat to native vegetation posed by the species.

Plant diversity, composition, and invasion of restored and natural prairie pothole wetlands: Implications for restoration

Hundreds of wetlands comprising thousands of hectares have been restored in the Midwestern United States. In nearly all cases, restoration consisted of simply restoring wetland hydrology. For this reason, the success of these restorations relies on natural colonization. We compared the structure and composition of the vegetation in two types of wetlands: 10 natural wetlands and 17 five-to-seven-year-old restored wetlands. The overall vegetative composition of restored wetlands was different from that of natural wetlands. Restored wetland flora was formed from a subset of species found in natural wetlands. The species restricted to natural wetlands tended to be native perennials and were evenly represented along the elevational gradient. The few species that were restricted to restored wetlands were largely mudflat annuals whose presence is more indicative of the presence of more unvegetated habitat in restored wetlands than of the presence of a distinctive restored wetlands flora. In addition, restored wetlands had lower vegetative cover and species richness than natural wetlands. Both wetland types had similar numbers of exotic species at the whole wetland (4.4 species per wetland) and quadrat scale (1.5 species m2), and dominance of exotics increased with elevation. The lower species richness, greater compositional variability, and lack of a distinctive flora support the hypothesis that dispersal limitation is the primary cause of the differences between the vegetation in restored and natural wetlands.

Species composition and invasion in NW Argentinian secondary forests: Effects of land use history, environment and landscape

Abstract. Successional patches are a large component of forest ecosystems throughout the world and their vegetation composition is conditioned by multiple factors such as land use history, disturbances, environmental conditions and landscape context. We investigated the relative contribution of historical, environmental, biotic and spatial factors in determining vegetation composition and invasion by exotic species in secondary forest patches of Sierra de San Javier, Tucumán, Argentina. We estimated canopy cover for shrub, vine and tree species distributed over 51 patches with known land use history. We also recorded environmental, historical and spatial variables and used multivariate techniques to explore the relationship between forest composition and explanatory variables. Land use, time since abandonment, altitude, slope and cover of different strata were related to the vegetation pattern in the study site, and they were all significantly structured over space. Exotic species appeared to differ from natives in their response to explanatory variables. Overall, exotic species were dominant on the edges of young patches originated from herbaceous crops, but the total number of exotic species was related to the distance to urban areas and small farms identified as potential sources of exotic propagules. Vegetation composition of secondary forests in NW Argentina was related to historical and environmental factors, but spatial variables strongly influenced vegetation composition as well as the variation in explanatory variables.

Continental rain forest fragments in Singapore resist invasion by exotic plants

Abstract Aim In general, the plant communities of oceanic islands suffer more from exotic plant invasions than their continental equivalents. At least part of this difference may be contributed by differences in non‐biological factors, such as the antiquity and intensity of human impacts and the absence of internal barriers to dispersal, rather than differences in inherent invasibility. We tested the resistance of species‐rich continental rain forests to plant invasion on a small, continental island that has been subject to prolonged and intensive human impact.Location Singapore is a 683‐km2 equatorial island &lt;1 km from the Asian mainland and with a population of 4 million people. It has a continental biota but has been subject to human impacts as intense as on any oceanic island.Methods We sampled twenty‐nine sites in seven vegetation types, ranging from urban wasteland to fragments of primary lowland rain forest. In each sample plot, all plant species were identified, exotic cover was estimated, and a range of environmental variables measured. Additional qualitative surveys for exotic invasion were made in other forest areas in Singapore. The data were analysed by Spearman's rank correlation coefficient.Results The number of exotic species recorded at a site was unrelated to the number of native species. Across all sites, percentage canopy opening had the highest correlation with the number of exotic species, while soil pH (which largely reflects the incorporation of calcareous construction wastes) had the highest correlation if the mangrove sites were excluded. There were no exotics in mangrove forest and only a tropical American, bird‐dispersed shrub, Clidemia hirta (L.) D. Don (Melastomataceae: Koster's Curse), in primary and tall secondary forest patches. The species‐poor early stages of woody plant succession on highly degraded soils were also very resistant to exotic plant invasion.Main conclusions Long‐isolated rain forest fragments in an exotic‐dominated continental island landscape resist invasion by exotic plants, suggesting that the problems on oceanic islands may reflect an inherently greater invasibility. This study also adds to the increasing evidence that the floras of tropical rain forest fragments in South‐east Asia are remarkably resilient on a time‐scale of decades to a century or more.