A Pragmatic Approach to the Management of Plant Invasions in Galapagos

Trait divergence and the ecosystem impacts of invading species

In The Origin of Species, Darwin (1859) drew attention to observations by Alphonse de Candolle (1855) that floras gain by naturalization far more species belonging to new genera than species belonging to native genera. Darwin (1859, p. 86) goes on to give a specific example: “In the last edition of Dr. Asa Gray’s ‘Manual of the Flora of the United States’ ... out of the 162 naturalised genera, no less than 100 genera are not there indigenous.” Darwin used these data to support his theory of intense competition between congeners, described only a few pages earlier: “As the species of the same genus usually have, though by no means invariably, much similarity in habits and constitution, and always in structure, the struggle will generally be more severe between them” (1859, p. 60). Darwin’s intriguing observations have recently attracted renewed interest, as comprehensive lists of naturalized plants have become available for various regions of the world. Two studies (Mack 1996; Rejmanek 1996, 1998) have concluded that naturalized floras provide some support for Darwin’s hypothesis, but only one of these studies used statistical tests. Analyses of additional floras are needed to test the generality of Darwin’s naturalization hypothesis. Mack (1996) tabulated data from six regional floras within the United States and noted that naturalized species more often belong to alien genera than native genera, with the curious exception of one region (New York). In addition to the possibility of strong competition between native and introduced congeners, Mack (1996) proposed that specialist native herbivores, or pathogens, may be

Ecological impact of phytoinvasions in Ukraine

Due to the relevance of protected areas to the conservation of native biota, the magnitude of invasions and threats posed by alien plants are currently important issues for the preservation of these areas. The paper summarises data on invasive alien plant species presence in the most valuable protected areas in Poland, i.e. national parks (NPs). We investigated the distribution of invasive alien plant species and management attempts concerning those species. We analysed data obtained from 23 national parks originating from published and unpublished sources. Invasive plants were present in all protected areas analysed, from two to 42 species in a particular national park, and 68 in total. The most widely distributed species were: Impatiens parviflora (present in 19 NPs), I. glandulifera (17), Solidago gigantea (17), Reynoutria japonica (17), and Robinia pseudoacacia (16). The conducted analyses showed that the number of invasive species decreased with the higher altitude (asl) of the national park. The most often managed species were Impatiens glandulifera (being removed in seven NPs), I. parviflora (six), Padus serotina (four) and Quercus rubra (four). In the majority of NPs, control activities are limited to small areas and singular species, thus having an incidental character. Only in five objects (Białowieża NP, Biebrza NP, Kampinos NP, Tuchola NP, Wigry NP), management has been focused on several species. We conclude that a lack of comprehensive management of invasive plant species in the majority of national parks currently limits the effectiveness of IAS (invasive alien species) eradication. Exchange of expertise among protected areas, documenting best practice examples, synthesising lessons learnt in IAS management, as well as the development of minimum standards for invasive plants surveillance and management are pivotal.

Characterisation of invasive plant proliferation within remnant riparian green corridors in Lusaka District of Zambia using Sentinel-2 imagery

Invasive alien species are a major problem for managers of protected areas (PAs) worldwide. Until the 1980s biological invasions were widely considered to be largely confined to anthropogenically disturbed sites and the widespread disruption of ecosystems in PAs by invasive species was not globally perceived as a major threat. A working group of the SCOPE program on biological invasions in the 1980s showed that PAs are not spared from major disruptive effects of invasions. Early research focused on descriptive studies of the extent to which PAs were invaded. More recent research explored drivers of invasion, and in the last decade much work has focused on understanding the impacts of invasions. We review the current understanding of alien plant invasions in PAs, focusing on four themes: (1) the status and macroecological patterns of alien plant invasions; (2) the threats that invasive alien plants (IAPs) pose and the impacts detected to date; (3) the current focus of invasion science in PAs; and (4) research priorities for advancing science-based management and policy. Of a sample of 59 widespread IAP species from a representative sample of 135 PAs globally, trees make up the largest proportion (32%), followed by perennial herbs (17%) and shrubs (15%). About 1857 papers have been published on alien species in PAs; 45% have focused on alien plants. Some textbook examples of impacts by IAPs originate from PAs, illustrating the severe threat to the core function of PAs. Impacts have been quantified at the species and community levels through the displacement and alteration of habitats. In some cases, native species abundance, diversity and estimated species richness have been altered, but reversed following control. At an ecosystem level, invasive plants have radically altered fire regimes in several PAs, in some cases causing regime shifts and transforming woodlands or savannas to grasslands. Invasions have also had a major impact on nutrient cycles. Protected areas are performing an increasingly important part of the global response to stem the rate of environmental change. Despite this, integrated efforts involving science, management and policy that are sufficiently resourced to generate insights on the status and dynamics of IAPs in PAs are insufficient or even lacking. Such efforts are needed to pave the way for monitoring trends, revising legislation and policies, and improving management interventions to reduce the extent and magnitude of impacts of invasive plants in PAs. While policy instruments to support management of non-native species date back to the 1930s, there has been a substantial increase in legislative support and general awareness since the early 2000s. Still, opportunities to improve research for PAs need to be created. Towards this goal, the establishment of a global PA research network could provide a unique vehicle to explore questions across species or functional groups and systems, at a scale currently beyond existing abilities. Developing an integrated global database with standardized, quantitative information could form part of such a networks function.

Cardiospermum grandiflorum

Invasive plant species in Indian protected areas have received relatively little attention until recently. This may partly be due to a historical emphasis on wildlife protection, rather than on a broader science-based approach to conservation of biodiversity and ecosystem functioning. A literature review of invasive plant species in India showed that nearly 60 % of all studies have been done since 2000, and only about 20 % of all studies are from protected areas. Studies from protected areas have largely focused on a small subset of invasive alien plants, and almost half these studies are on a single species, Lantana camara, probably reflecting the species’ ubiquitous distribution. The spread of alien plants in India has been both ecologically and human mediated. Efforts to manage plant invasions have, in the past, been diluted by the ambivalence of managers attempting to find beneficial uses for these species. Despite growing knowledge about the harmful impacts of certain invasive plants on native species and ecosystems, their deliberate spread has continued, even till quite recently. And, despite the successful implementation of management initiatives in some protected areas, these efforts have not expanded to other areas. The lack of a national coordinated effort for invasive species monitoring, research, and management largely underlies this.

Pingtan Island is the largest island in Fujian Province and the fifth largest island in China. The invasion of a large number of alien plants has had a profound impact on the local ecological environment. Because the harm caused by alien invasive plants varies greatly between different ecosystems and even in different habitats, the risk assessment index system suitable for one region may not be suitable for other regions. Therefore, it is necessary to establish a risk assessment index system for invasive alien plants on Pingtan Island. Alien plant communities in different habitats were studied by means of quadrat investigation and professional literature review. Some invasive alien species were selected and compiled into a list of invasive alien plants on Pingtan Island, and their species composition, origin, flora, life forms, and habitats were statistically grouped. There were 104 species in 80 genera and 37 families of alien invasive plants. Asteraceae, Fabaceae, Amaranthaceae, and Poaceae were the main families, accounting for 26.7%, 6.7%, 6.7% and 5.8% of the total species, respectively. The geographical components of families and genera have obvious tropical properties, accounting for 51.3% and 66.6% of the total species, respectively. These originated mainly from South America and North America, accounting for 45.5% and 30.1% of the total frequency, respectively. Annual herbs, biennial herbs, and perennial herbs accounted for 84.6% of the total species. Based on a DPSIR conceptual model and an AHP method, an invasion risk assessment of 104 invasive alien plants was conducted. The ecological adaptability, habitat distribution and landscape impact of species were considered in the selection of indicators and the formulation of standards. A total of 23 high-risk invasive species were identified at level I, 37 medium-risk invasive species at level II, and 44 low-risk invasive species at level III. Lantana camara L. had the highest risk score (49), followed by Cenchrus echinatus L. (45), Spartina alterniflora Loisel. (45), and Panicum repens L. (43.5). Suggestions are put forward to prevent the invasion of alien plants on Pingtan Island and to provide a theoretical basis for promoting the healthy and stable development of the ecological environment on the island.

Invasive process of alien species is affected by not only the invaded habitats, but also the surrounding landscape matrix. Understanding the effects of landscape matrix on alien species is of great significance for controlling invasive alien species. We surveyed plant communities along the water-level fluctuating zone (WLFZ) of the Three Gorges Reservoir. Invasive status of alien plant species was evaluated. Totally 10 spatial scales of the surrounding landscape matrix in the scope of 2000 m (including WLFZ) were classified, and 14 landscape indices were applied to analyze the landscape matrix composition and configuration. Using the principal component analysis and correlation analysis, the effects of landscape matrix on the alien invasive plant species and associated scale effect were tested. Results showed that a total of 42 alien invasive plant species were found in the WLFZ, belonging to 17 families and 36 genera. Fuling was a dividing place to differentiate invasive species distribution. The number of the alien invasive species between Fuling and the Three Gorges Dam was found more than that between Fuling and Jiangjin. For the all scales (within 2000 m). The higher the landscape matrix fragmentation was, the more difficult the alien species invading. The higher landscape connectivity was, the easier the alien species invading. The effects of landscape matrix composition and configuration on the invasive plant diversity at large scales (1200-2000 m) was more significant than those at small scales (200-1000 m), in which landscape matrix composition and configuration at 1200-1400 m showed the strongest effect, demonstrating a significant spatial scale effect. Different invasive plant species showed the scale effects of landscape matrix composition and configuration. At all scales, Xanthium strumarium and Bidens frondosa showed weak correlations with landscape indices, but Bidens tripartita and Erigeron canadensis showed strong correlations. Landscape matrix was closely related to invasive plant species, and demonstrated a significant scale effect. The alien invasive plant species could be traced to the landscape matrix at large scales. Grassland and forest patches at the small scales could be used as the 'stepping stone' for the alien species transiting before they arrived at the WLFZ. In order to control alien plants in the WLFZ, land-use management and optimization should be strengthened at different scales of landscape matrix on the basis of enhancement of habitat management. A diversified comprehensive control for alien species should thus be taken into account.

Native plant communities are often invaded by multiple alien species. It is still unclear how increasing diversity of alien invasive species suppresses the growth of native species and thus contributes to invasion success. In the subtropical monsoon region of southeast China, we experimentally created a native plant community with 18 herbaceous species. One week later, we let it be invaded by either zero (controls without invasion), one, two, four or eight alien plant species, with either high or low species evenness. After a four‐month growth period we harvested the aboveground biomass of each species. We found that increasing invasive species richness significantly increased invasive plant biomass, the biomass of all invasive and native plant species within the community, and invasion success (the ratio of invasive plant biomass to the biomass of all native and invasive plants), but it did not significantly reduce native plant biomass. Experimentally manipulating invasive species evenness did not influence invasion success and did not show any differential suppression effects on native plants. One invasive species, Sesbania cannabina , became dominant in terms of plant biomass, irrespective of its proportion in the alien plant mixtures. Throughout this experiment, effects of invasive species richness on invasion success were mainly due to such selection effects among the invasive species. On the other hand, the unchanged biomass of native species under increasing invasive plant richness suggests the presence of at least partly complementary resource niches between invasive and native species.

Aims Invasive alien plants can severely impact ecosystem diversity and function. While the effects of individual species are often studied, interactions between multiple invasive species are less understood. This study examines how Asclepias syriaca and Solidago spp. (including Solidago gigantea and S. canadensis ) influence taxonomic and functional diversity in sandy old‐fields. The aims are to: (1) assess the individual and combined impacts of Asclepias and Solidago on resident plant diversity and (2) determine whether interactions between these species alter their effects on the invaded community. Location Gödöllő Hills, Central Hungary. Methods I sampled 80 plots (2 m × 2 m) on sandy old‐fields with varying levels of Asclepias and Solidago cover. Plant species cover was visually estimated, and trait data were obtained from databases. Species richness, community completeness, and community‐weighted means (CWM) for plant traits, along with Rao functional diversity, were analyzed. Generalized linear and mixed models were used to evaluate the effect of invasive cover on resident diversity and traits, both with and without considering the invasives' traits in community indices. Results While Asclepias had no significant effect, Solidago cover significantly reduced species richness and community completeness. When invasive species traits were excluded from the analysis, only the CWM and Rao's diversity of the bud bank index were significantly related to Solidago cover. Including the invasives' traits revealed that their trait values influenced community indices significantly. Asclepias and Solidago cover were negatively correlated, but did not alter each other's effects. Conclusions Solidago has a stronger negative impact on resident species richness than Asclepias , which appears neutral. Trait‐based mechanisms underlying reduced species richness under Solidago dominance require further investigation. The lack of interaction between these invaders suggests their individual impacts dominate over potential synergies or antagonisms.

Microplastic (MP) pollution and alien plant invasions are two important threats to terrestrial ecosystems. Microplastics alter the physical and chemical characteristics of soil, potentially affecting the performance of alien plants. However, previous studies have overlooked the impact of weather on invasive plants in areas polluted by MPs. With the global increase in extreme rainfall events, it is imperative to redefine the correlation between MPs and invasive plants. Here, we conducted an experiment in a climate chamber to examine the effects of MPs on the growth and development of both native and invasive alien plants under constant and fluctuating water regime (FWR). The FWR simulated extreme water pulses during the 2016–2020 growing seasons in Wuhan, China. Our results indicated that biomass accumulation and root development were influenced by water conditions and MP pollution in both invasive and native species. The extent of the effects varied between the two groups of plant species. FWR promoted plant growth and fine root development in invasive plants but reduced the maximum quantum efficiency of photosystem II (Fv/Fm) and nonphotochemical quenching (NPQ) indices of native plants. Moreover, FWR attenuated the negative effects of polybutylene succinate (PBS, degradable MPs) on biomass and root characteristics (length, surface area and tips). FWR compensated the negative impacts of MPs on the total and below‐ground biomass of the invasive species Paspalum dilatatum and Sphagneticola trilobata, but not on the native species. Consequently, invasive species showed better performance than native species in the fine root development of biomass growth and chlorophyll fluorescence under the combined effects of MPs and FWR. Synthesis and applications. Our findings suggest that MP pollution enhances the competitiveness of invasive alien species over the native species when exposed to pronounced dry–wet water cycle conditions, potentially affecting the composition and biodiversity of the ecosystems. Thus, controlling MP pollution should be a part of the management strategy to conserve biodiversity and ecosystems.

Insights into ecological drivers of alien plant invasions can be gained through comparative studies of growth and fecundity of invasive alien plants versus those of co‐occurring non‐invasive alien plants and native plants across environmental conditions in common garden settings. Habitats that harbour alien plant species in many ecosystems globally are presently experiencing light pollution resulting from artificial light at night (ALAN) and increased rates of nutrient enrichment of the soil. However, the potential interactive effects of ALAN and nutrient enrichment on invasiveness of alien plant species remain unknown. Here, we performed a common‐garden experiment to test the interactive effects of ALAN and soil nutrient enrichment on the growth of a random set of ten alien (five invasive and five non‐invasive naturalized) and seven co‐occurring native ornamental plant species that are commonly cultivated within urban and peri‐urban areas of Nairobi city in Kenya. We predicted that a simultaneous increase in photoperiod via ALAN and nutrient enrichment will favour growth of invasive alien plant species over that of non‐invasive alien and native plant species. We grew the 17 plant species under natural daylight (ALAN−) versus natural daylight followed by ALAN (ALAN+) and fully crossed with two levels of nutrient enrichment (low vs. high) and competition (competition vs. no‐competition against a native plant Ocimum gratissimum ) treatments. Under simultaneous high‐nutrient and no‐competition treatments, ALAN enhanced mean total biomass of invasive and non‐invasive naturalized alien species by 61.1% and 131.4%, respectively but decreased that of native plant species by 34%. In contrast, under simultaneous high‐nutrient and competition treatments, ALAN enhanced mean total biomass of invasive alien plant species by 68.6% and that of non‐invasive naturalized alien species by 51.9% and native species by 35.4%. High‐nutrient treatment enhanced flower formation more strongly in invasive and non‐invasive naturalized alien plants than in native plants. The invasive and non‐invasive naturalized alien species grew taller than native species across the light, nutrient, and competition treatments. Synthesis : The present findings suggest that light pollution and nutrient enrichment may jointly confer growth advantage to invasive alien plant species over that of co‐occurring native plant species and enhance invasiveness of alien plant species.

In her recent paper, Shawna J. Dark (2004) concluded that the distributions of ‘invasive alien plants’ and ‘non-invasive alien plants’ in California were significantly correlated. She found the highest numbers of both ‘invasive’ and ‘non-invasive’ species in regions with low elevation, high road density, and high native species richness. Dark actually uses two definitions of ‘invasive’ in the paper; we discuss the implications of this on her conclusions. We point out that the most important message of Dark's paper is that the total number of naturalized species can be, at least at some spatial scales, a reliable predictor of the numbers of exotic pest species. We also provide another example to support this conclusion. We follow the definitions of native, non-native, naturalized, casual, pest, and weed in Pysek et al. (2004) in this note unless otherwise specified. It is necessary to examine how Dark defined ‘invasive’ in her paper because the data she used include only a small number of plant species (78) classified as ‘invasive’ while the remaining 1097 naturalized species listed in the most recent statewide flora and online updates (total 1175; Hickman, 1993; http://ucjeps.berkeley.edu/interchange.html) were classified as ‘non-invasive alien plants’. Dark first defines ‘invasive alien plants’ in the paper's second sentence as ‘alien plants that produce offspring in very large numbers and at large distances from parent plants and thus have potential to spread over a considerable area (Richardson et al., 2000)’. Later, however, we learn that species actually classified as ‘invasive’ for data analysis in this study are only those rated as ‘A’ and ‘B’ on the California Exotic Pest Plant Council's (1999) list of ‘Exotic Pest Plants of Greatest Ecological Concern’ (Dark, 2004; text under Fig. 1; CalEPPC, 1999). CalEPPC (1999) in turn defines its A and B categories as covering species that displace native species and disrupt habitats. Dark notes that this list is a work in progress and not yet complete, particularly for grasses but, using only this list to define invasive species, she states that ‘in the state of California there are over 1000 documented alien plant species, and an estimated 6% of these are considered to be invasive (Hickman 1993)’. Hickman (1993), however, does not list such a percentage and in fact does not use the term ‘invasive’. It is worth noting that far more alien plant species have been documented in California but that most of them grow only in cultivation. Hickman covers only alien plants that are naturalized plus some casual alien species. Relationship between the number of ‘noxious’ non-native weedy species and the number of remaining non-native naturalized or casual plant species in the 50 US states (P < 0.001). Note that the regression function starts to be meaningful only for X-values greater than 200. As both variables are negatively correlated with the area of individual states (see Rejmánek (2003) for explanation), the positive correlation between Y and X is not driven by species-area effect. Data: Kartesz & Meacham (1999). The 78 species from the CalEPPC list used in the paper include many serious exotic pests in natural areas and rangelands. However, they represent only a subset of what are sometimes called ‘environmental weeds’. These 78 species are undoubtedly invasive (i.e. they are spreading in California), but this list also does not include many alien (and invasive) agricultural, garden and yard weeds and many others invasive (spreading alien) species in California. Recently, Rejmánek (2000) estimated that about 650 naturalized plant species were invasive (spreading) in California. Even this number may be an underestimate since the most recent manuals of Californian weeds that explicitly cover all species regarded as pests in agriculture, forestry, rangelands, natural areas, natural and artificial waterways, yards and other urban settings in the state (DiTomaso & Healy, 2003, 2005) cover 701 non-native species in the main text (J. DiTomaso, pers. comm.). Even if not all alien weeds are invasive (spreading), most of them are. On the other hand, some invasive (= spreading non-native) species do not cause any detectable environmental or economic damage and therefore are not called weeds or pests (Richardson et al., 2000). To sum up, it would be safe to say that among the 1000–1300 naturalized plant species in California (Hickman, 1993; Rejmánek & Randall, 1994; Hrusa et al., 2002), at least 60% are invasive as defined in the second sentence of Dark's paper, and less than 40% can be classified as non-invasive. Unfortunately, terminological confusions are common in and between research, management and public policy publications and discussions regarding invasive species. These terminological differences have the potential to lead to confused or incorrect data interpretation, poor communication between different groups of researchers, and inappropriate management and public policy decisions (Pysek et al., 2004). Despite the problems outlined above regarding the exclusion of many invasive species in Dark's analysis, her results are very interesting—and for reasons in addition to those advanced in her paper. Dark's study indicates that the total number of naturalized species can be, at least at some spatial scales, a reliable predictor of the numbers of exotic pest species. Therefore, total numbers of naturalized species, as we are getting them from recently published manuals and checklists, seem to be good indicators of present or future problems with exotic pests. Dark's results probably have more general validity. Here we present another example from North America. We used non-native species that are listed as federal and/or state-level (listed in at least one state) ‘noxious weeds’ as a surrogate for exotic plant pest species in general. We found that the number of species in this group is significantly positively correlated with the number of remaining non-native naturalized and casual alien species in the US states (Fig. 1). The category non-native ‘noxious weeds’ (Kartesz & Meacham, 1999) includes 304 exotic pests that are difficult to manage (e.g. Alternathera philoxeroides, five Carduus species, 12 Centaurea species, Cortaderia jubata, eight Cuscuta species, Cyperus rotundus, Cytisus scoparius, eight Euphorbia species, Heracleum mantegazzianum, Hydrilla verticilata, Lepidium latifolium, Lonicera japonica, Melaleuca quinquenervia, five Pennisetum species, Pueraria montana (P. lobata), four Salsola species, Salvinia molesta, Silybum marianum, eight Solanum species, two Striga species, Taeniatherum caput-medusae, seven Tamarix species, Triadica sebifera (Sapium sebiferum), Ulex europaeus, Viscum album). However, this list of species is somewhat biased toward agricultural weeds and it does not include some important exotic pests of natural areas (e.g. Ailanthus altissima, Arundo donax, Foeniculum vulgare, Lonicera maackii, Lygodium spp., Microstegium vimineum, Rubus armeniacus). On the other hand, the fact that a species is declared as ‘noxious’ in one or more states does not mean that that species is a serious pest in all states where it is present. Therefore, ‘noxious weeds’ in Kartesz & Meacham (1999) is still not an ideal category for this analysis, but we believe that it includes, at least, a representative sample of exotic pests. Using this data and the regression equation in Fig. 1, we can conclude that 15–30% of naturalized species can be classified as ‘pests’ in the US states. Obviously, this is substantially larger proportion than 10%, the number proposed by Williamson (1996). The main mechanism behind correlations described above is likely just a simple sampling effect. With increasing numbers of introduced species, increasing numbers of species become naturalized, and, with increasing number of naturalized species, there is an increasing probability that a large number will cause some environmental or economic damage. Most naturalized species are incorporated into native floras without any serious consequences. They are concentrated mainly in disturbed areas and very often are just symptoms of human-created disturbances rather than causes of environmental changes. In general, however, a high numbers of naturalized species represents a red flag: problems with at least 15% of those species.