Demography of the invasive geophyte Oxalis pes-caprae across a Mediterranean island

BACKGROUND AND AIMS: Success during the early stages of the life-history of alien plants is essential for invasion to occur. The reproductive components of plant invaders have mostly been studied in species reproducing sexually but little is known about invaders that depend exclusively on vegetative reproduction. In this paper, the importance of the different recruitment stages on population growth is quantified and, thus, the invasion potential of the South African annual geophyte Oxalis pes-caprae invading Mediterranean ecosystems is assessed. METHODS: Tests and experiments were conducted across Menorca (Balearic Islands) to analyse the spatial variability of Oxalis pes-caprae reproductive components (i.e. bulb production, bulb bank, bulb predation, bulb mortality, bulb dormancy, bulb germination, plant establishment and survival). KEY RESULTS: Oxalis pes-caprae has a transient bulb bank that remains dormant in the soil during summer. High levels of bulb predation after dispersal, followed by bulb mortality during summer or a failure to germinate in autumn were the most critical factors limiting plant establishment. Bulb germination was high. However, plant establishment and bulb production is constrained by intraspecific competition, but is not affected by soil disturbance. No symptoms of spatial discordance could be found between recruitment stages because the spatial variability of the life cycle was extremely low at all the scales examined (i.e. among populations, habitats and microsites). It was estimated that, on average, 4 % of bulbs can become plants the following year and the field rate of population increase (lambda) to be 0.08. CONCLUSIONS: The results suggest that invasion is constrained by post-dispersal bulb predation, loss of viability of the propagule bank due to summer drought and high intraspecific competition. However, a high spatial concordance between recruitment stages and probably a high propagule pressure due to human and livestock bulb dispersal determine the success of this invader across Menorca Island.     

Foraging in Non-Native Environments: Comparison of Nile Tilapia and Three Co-Occurring Native Centrarchids in Invaded Coastal Mississippi Watersheds

We examined the diet of the alien Nile tilapia and bluegill, redear sunfish, and largemouth bass over a two-year period in coastal Mississippi. Nile tilapia diet was visually separated from the three natives based on group-average linkage cluster analysis. Sequential two-way nested analysis of similarities indicted there was no season effect (Global R = 0.026, P = 24.3%), but there was a moderate size class effect (Global R = 0.457, P = 0.1%) and a strong species effect (Global R = 0.876, P = 0.1%). Pairwise tests indicated species fed on different components of and locations within the environment, with bluegill, redear sunfish and largemouth bass (all R ≤ 0.683, P = 0.1%) having the most similar dietary components and Nile tilapia (all R ≥ 0.953, P = 0.1%) having the most distinct. Multivariate dispersion indicated that largemouth bass (1.425) and bluegill (1.394) had the most diverse diets compared to redear sunfish (0.906) and Nile tilapia (0.918). Similarities of percentages indicated that diets were separated based on prey: bluegill and redear sunfish consumed chironomids and insects; largemouth bass consumed fish and insects; and Nile tilapia fed most often on sediment resources such as nematodes, rotifers, bryozoans and hydrozoans. Nile tilapia had the highest frequency of mud, sand and detritus in their stomachs, suggesting they fed directly on bottom sediments. These data and the fact that Nile tilapia has a 1.3–7.6 times longer intestine on average than its body length, support our contention that this alien species feeds at the base of the food web and is well adapted to survive and proliferate in non-native environments.     

The new threat to Italian inland waters from the alien crayfish “gang”: the Australian Cherax destructor Clark, 1936

Biological invasions inflict damage to the ecology, economy, and human health, and pose serious threats to the native communities. Among the many invasive taxa, crayfish have attracted much attention by scientists and policy makers. Recently, an established population of an alien species of crayfish, new for Italy, was found in the Natural Preserve of “Laghi di Ninfa” (central Italy). Based on morphological and genetic evidence (molecular barcoding of COI and 16S rDNA), we classified it at subspecies level as the Parastacidae Cherax destructor destructor, native to Australia. Its introduction possibly occurred at the end of the 1980s but the species seems to be still confined in the preserve. The low temperature of the adjacent waters may be a barrier against its natural spreading but not against its intentional translocation into natural waters by man. Because of the invasive history of C. destructor, eradication of this population is urgent and still economically profitable. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: Pierluigi Viaroli     

Does global warming induce segregation among alien and native beetle species in a mountain-top?

The last few centuries have seen an increase in the mean air temperature of the planet, a phenomenon that is called “global warming”. One of the most sensitive habitats to the effects of global warming is the high-elevation mountain environments, because these habitats are characterized by low temperature. Cushion plants are one of the best-adapted growth forms in this habitat, generating more suitable sites for other plants and insects. In the present study, we experimentally evaluated the effects of global warming by open-top chambers on the abundance and interaction of two ladybirds at 3,600 m, growing over cushions of the Azorella monantha species in the Andes of central Chile. Additionally, we measured variation in temperature, water content, and food availability by the presence of open-top chambers as possible mechanisms of spatial segregation between ladybirds. Without open-top chambers, the abundance of native and alien beetles was similar; but with open-top chambers, the native beetle species is spatially segregated by alien species, decreasing in abundance. The open-top chambers increase temperature and food availability, but not water content. We suggest that under the global warming scenario, the native insects will decrease in abundance or become extinct by the presence of alien insects, at least in the high-elevation mountain environments.     

Patterns of alien plant distribution at multiple spatial scales in a large national park: implications for ecology, management and monitoring

Aim Spatial scale is critical for understanding and managing biological invasions. In providing direction to managing alien plant invasions, much emphasis is placed on collecting spatially explicit data. However, insufficient thought is often given to how the data are to be used, frequently resulting in the incompatibility of the data for different uses. This paper explores the role of spatial scale in interpreting, managing and monitoring alien plant invasions in a large protected area. Location Kruger National Park, South Africa. Methods Using 27,000 spatially‐explicit records of invasive alien plants for the Kruger National Park (> 20,000 km²) we assessed alien plant species richness per cell at nine different scales of resolution. Results When assessing the patterns of alien plants at the various scales of resolution, almost identical results are obtained when working at scales of quarter‐degree grids and quaternary watersheds (the fourth level category in South Africa's river basin classification system). Likewise, insights gained from working at resolutions of 0.1–0.5 km and 1–5 km are similar. At a scale of 0.1 × 0.1 km cells, only 0.4% of the Kruger National Park is invaded, whereas > 90% of the park is invaded when mapped at the quarter‐degree cell resolution. Main conclusions Selecting the appropriate scale of resolution is crucial when evaluating the distribution and abundance of alien plant invasions, understanding ecological processes, and operationalizing management applications and monitoring strategies. Quarter‐degree grids and quaternary watersheds are most useful at a regional or national scale. Grid cells of 1 to 25 km² are generally useful for establishing priorities for and planning management interventions. Fine‐scale data are useful for informing management in areas which are small in extent; they also provide the detail appropriate for assessing patterns and rates of invasion.     

Population genetics of an invasive riparian species,<Emphasis Type="Italic"> Impatiens glandulifera</Emphasis>

We assess the population genetic structure of the invasive riparian weed Impatiens glandulifera, and where possible, determine whether natural or anthropogenic dispersal best explains the observed patterns. Results are compared with a similar contemporary analysis for Heracleum mantegazzianum undertaken in the same catchments, and we suggest that some of the observed differences in genetic structure could be because of life history differences between these species. Our results confirm the importance of at least occasional dispersal events mediated by human activity in the colonisation and subsequent spread of invasive plants in river catchments. However, processes related to river structure, dispersal range and genetic drift also appear to be structuring these populations over short temporal scales. The implication is that local populations can be established as small founders, and therefore eradication programs need to be thorough and undertaken at the catchment scale. Effective management needs to consider the natural spread of riparian species along rivers, but also prevent long-distance dispersal from sources outside the catchment. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Habitat engineering by the invasive zebra mussel <Emphasis Type="Italic">Dreissena polymorpha</Emphasis> (Pallas) in a boreal coastal lagoon: impact on biodiversity

Habitat engineering role of the invasive zebra mussel Dreissena polymorpha (Pallas) was studied in the Curonian lagoon, a shallow water body in the SE Baltic. Impacts of live zebra mussel clumps and its shell deposits on benthic biodiversity were differentiated and referred to unmodified (bare) sediments. Zebra mussel bed was distinguished from other habitat types by higher benthic invertebrate biomass, abundance, and species richness. The impact of live mussels on biodiversity was more pronounced than the effect of shell deposits. The structure of macrofaunal community in the habitats with >10³ g/m² of shell deposits devoid of live mussels was similar to that found within the zebra mussel bed. There was a continuous shift in species composition and abundance along the gradient ‘bare sediments—shell deposits—zebra mussel bed’. The engineering impact of zebra mussel on the benthic community became apparent both in individual patches and landscape-level analyses.     

Early warning signals of demographic regime shifts in invading populations

The Allee effect can cause alternative stable states in population abundance of invasive species. Sudden eruption of invading populations from low to high abundance may be viewed as a regime shift from one alternative state to another. Previous research proposed several types of early warning signals to predict regime shifts in ecological systems such as polluted lakes and semiarid grasslands. This paper explores theoretically the potential of such indicators in predicting demographic regime shifts of invading populations. I analyzed a stochastic differential equation model for the population dynamics of an invasive species subject to Allee effects and propagule pressure. Diffusion approximation to the stochastic model suggests that persistent propagule pressure makes demographic regime shifts inevitable, but Allee effects can lengthen the mean time until regime shifts virtually indefinitely. To compare the potential of indicators, I examined standard deviation, skewness, and estimated return rates of longitudinal population abundance. I found that standard deviation showed a distinct increase as regime shifts became more likely, but skewness and return rates showed no clear trends. This result suggests that standard deviation might be a useful warning signal for forecasting an impending demographic regime shift of invading populations during the period when their abundance is still low.     

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.