Selection for commercial forestry determines global patterns of alien conifer invasions

Question Are the patterns of alien conifer (Pinaceae, Cupressaceae) invasions different between continents, and how is invasion success influenced by commercial forestry practices? Location Temperate and subtropical countries and regions (n = 60) from five continents spanning both hemispheres. Methods We used generalized linear mixed models to test how continent identity, region area and use in commercial forestry affect probabilities of Pinaceae and Cupressaceae species to escape following introduction and cumulative logit regression models to assess how these predictors affect the likelihood that a species becomes naturalized or invasive. Results Sixty Pinaceae of a global total of 232 and 26 Cupressaceae of a total of 142 species have escaped from cultivation across the study regions examined. Average numbers of both alien Pinaceae and Cupressaceae species per region were highest in Oceania, followed by Africa. Moreover, the probability of alien Cupressaceae and Pinaceae becoming naturalized or invasive was particularly high in these two continents. For both families, species used in commercial forestry have a significantly higher probability of escape than those which are only introduced for ornamental or other purposes. In the case of Pinaceae, forestry species also become naturalized or invasive more frequently than non‐forestry species, while no such effect was detectable for Cupressaceae. Conclusions We found that non‐native conifers are more likely to escape from cultivation, naturalize and turn into invasive weeds on the continents of the Southern Hemisphere. In addition to this biogeographic signal, introduction effort strongly determines the behaviour of introduced Pinaceae, and less so, Cupressaceae. A clear conflict exists between the economic benefits of conifer forestry and the risks to the environment from invasions. Future expansion of commercial forestry should address spatial planning to ecosystems vulnerable to invasion and adopt comprehensive risk assessment procedures.     

Lake construction has facilitated calanoid copepod invasions in New Zealand

Aim We tested the hypothesis that construction of lakes and ponds has facilitated both inter‐ and intracontinental invasions of calanoid copepod species. Location North Island, New Zealand. Methods We sampled both natural and constructed lakes, ponds and reservoirs for calanoid copepods in the North Island, New Zealand. Species records were supplemented by examining historically collected samples and literature review. Distributions of non‐indigenous calanoid copepod species were compared between constructed and natural waters. Species distributions of native species were compared with the basement terranes (microplates) of the North Island to determine if they possess ‘natural ranges’, and to assess whether construction of new water bodies had altered these distributions. Results Ten calanoid copepod species have been recorded. At least four, and possibly five, of these species are non‐indigenous and were restricted to constructed water bodies. Occurrences in constructed water bodies were not restricted to dammed valleys, but also included ponds constructed on farms, ornamental ponds, disused quarries and retired mines. Four Boeckella species had distributions in natural waters closely related to the North Island basement terranes, and therefore possess ‘natural ranges’ on the island. One species, Boeckella propinqua, was found in natural lakes over a small geographical range only, but has spread with construction of new water bodies to now be widely distributed over the island. Main conclusions Construction of lakes and ponds has facilitated the invasion of calanoid copepod species at both inter‐ and intracontinental scales. Our findings suggest that resident native calanoid copepod species may reduce the risk of invasion to natural water bodies, as similar‐sized species are commonly unable to co‐occur. Spread of the non‐indigenous representatives from constructed into natural waters is inevitable, with established populations providing local propagule supplies for regular introductions.     

Seed predation as a barrier to alien conifer invasions

Interactions between exotic plants and animals can play a major role in determining success or failure of plant introductions. Seed predation has been seen as important in explaining biotic resistance to plant invasion, but this hypothesis has rarely been tested. We studied seed predation on exotic forest plants on an island in Patagonia, Argentina where 43 pine species, including 60% of the world’s known invasive Pinaceae, were introduced ca. 80 years ago, but where exotics attain relatively high densities only near the original plantings. To test if seed predation limits exotic conifer establishment in this area, we compared seed predation in areas close to plantations (colonized by exotics) and far from them (not invaded). Seeds of exotics were preferred over seeds of native species, possibly because exotic seeds are bigger. Predation was more intense in areas far from plantations than in areas close to them, substantially reducing the chances of exotic seed establishment. Using automatic cameras, we found that both rodents and birds preyed on exotic seeds. This study suggests that native seed predators can be an important component of biological resistance to plant invasion.     

Pine invasions in the southern hemisphere: modelling interactions between organism,environment and disturbance

Current theories of plant invasion have been criticized for their limited heuristic and predictive value. We explore the heuristic and predictive potential of a model which explicitly simulates the mechanisms of plant invasion. The model, a spatially-explicit individual-based simulation, is applied to the invasion of pine trees (Pinus spp.; Pinaceae) in three vegetation types in the southern hemisphere. The model simulates factors which have been invoked as major determinants of invasive success: plant traits, environmental features and disturbance level. Results show that interactions between these determinants of invasive success are at least as important as the main effects. The complexity of invasions has promoted the belief that many factors must be invoked to explain invasions. This study shows that by incorporating interactions and mechanisms into our models we can potentially reduce the number of factors needed to predict plant invasions. The importance of interactions, however, means that predictions about invasions must be context-specific. The search for all-encompassing rules for invasions is therefore futile. The model presented here is of heuristic value since it improves our understanding of invasions, and of management value since it defines the data and models needed for predicting invasions.     

Disentangling the effects of land use, shrub cover and climate on the invasion speed of native and introduced pines in grasslands

Aims To determine how changes in land use, climate and shrub cover affect the invasion dynamics of native (Pinus sylvestris L.) and introduced (Pinus nigra Arn. subsp. nigra) pines in grasslands. To analyse how these factors interact and affect seedling recruitment, a bottleneck in the lifecycle of many trees. Such information is required to manage the dynamics of these species. Location Grands Causses, calcareous plateaus (Southern France). Methods We used both published and unpublished demographic and dispersal data to assess population growth and invasion speed of invading pines. A demographic and spatially explicit model, which included density dependence and stochasticity in dispersal, demography and environment, was run for different scenarios of sheep grazing pressure (nil, extensive or intensive), shrub cover (0, 10 or 20%) and drought frequency (past‐to‐present or future). For each scenario, population growth rate, invasion speed and elasticity of invasion speed to each demographic and dispersal parameter were computed. Results Grazing was the main factor for limiting invasion speed. Shrub cover reduced tree spread under nil or extensive grazing pressure, but increased it under intensive grazing pressure. Although dry years led to nil seedling establishment rates, an increase in their frequency had surprisingly few effects on pine invasion speed. This last result remained unchanged when very dry years, inducing seedling, but also sapling mortality were introduced. In most environmental conditions, population growth rate and invasion speed were higher for the introduced than for the native pine. Elasticity analysis highlighted the importance of demographic parameters on invasion speed, notably adult and sapling survival. Main conclusion Tree invasion speed may rely at least as much on human activities, like sheep grazing, tree cutting and non‐native trees introduction, as on changes in climate factors. Therefore, human activities need to be explicitly taken into account in the prediction and management of tree dynamics.     

Pine invasions in treeless environments: dispersal overruns microsite heterogeneity

Understanding biological invasions patterns and mechanisms is highly needed for forecasting and managing these processes and their negative impacts. At small scales, ecological processes driving plant invasions are expected to produce a spatially explicit pattern driven by propagule pressure and local ground heterogeneity. Our aim was to determine the interplay between the intensity of seed rain, using distance to a mature plantation as a proxy, and microsite heterogeneity in the spreading of Pinus contorta in the treeless Patagonian steppe. Three one‐hectare plots were located under different degrees of P. contorta invasion (Coyhaique Alto, 45° 30′S and 71° 42′W). We fitted three types of inhomogeneous Poisson models to each pine plot in an attempt for describing the observed pattern as accurately as possible: the “dispersal” models, “local ground heterogeneity” models, and “combined” models, using both types of covariates. To include the temporal axis in the invasion process, we analyzed both the pattern of young and old recruits and also of all recruits together. As hypothesized, the spatial patterns of recruited pines showed coarse scale heterogeneity. Early pine invasion spatial patterns in our Patagonian steppe site is not different from expectations of inhomogeneous Poisson processes taking into consideration a linear and negative dependency of pine recruit intensity on the distance to afforestations. Models including ground‐cover predictors were able to describe the point pattern process only in a couple of cases but never better than dispersal models. This finding concurs with the idea that early invasions depend more on seed pressure than on the biotic and abiotic relationships seed and seedlings establish at the microsite scale. Our results show that without a timely and active management, P. contorta will invade the Patagonian steppe independently of the local ground‐cover conditions.     

Differential invasion success among biotypes: case of Bemisia tabaci

Studies on success or failures of biological invasions by different insect biotypes are scarce and could provide interesting insights into the traits that determine greater or lower ability to invade. Life history traits of invasive whiteflies Bemisia tabaci of the B biotype (known as a worldwide invasive biotype) and of the indigenous biotype Ms (not known as an invader anywhere in the world), both from the island of La Réunion (Indian Ocean), were compared for this purpose. In our study we demonstrated that within a cultivated host plant (tomato), the B biotype differs from the Ms by a combination of several life-history traits. This combination gives the invasive biotype an advantage over the resident both in terms of rapid demographic growth (increased intrinsic rate of increase and associated traits such as short developmental times and high fecundity) and in terms of competition (large adult and offspring sizes), without any recorded trade off. However, in the field the resident biotype remains dominant on non-cultivated hosts (weeds) and in a particular climate (high humidity). This suggests that invasive biotypes are characterized by physiological, morphological and biological adaptations to a disturbed environment created by anthropic activities at different places in the world, while resident biotypes may persist in less altered habitats.     

Plant invasion across space and time: factors affecting nonindigenous species success during four stages of invasion

Invasive nonindigenous plant species (NIPS) threaten native diversity, alter ecosystem processes, and may interact with other components of global environmental change. Here, a general framework is outlined that attempts to connect patterns of plant invasion to processes underlying these patterns at four well-established spatio-temporal stages of the invasion process: transport, colonization, establishment, and landscape spread. At each stage we organize findings and ideas about the filters that limit NIPS success and the interaction of these filters with historical aspects of introduction events, NIPS traits, and ecosystem properties. While it remains difficult to draw conclusions about the risk of invasion across ecosystems, to delineate universal 'invader traits', or to predict large-scale extinctions following invasions, this review highlights the growing body of research that suggests that the success of invasive NIPS is controlled by a series of key processes or filters. These filters are common to all invasion events, and will interact throughout the stages of plant invasion, although the relative importance of a filter may be stage, species or location specific. It is suggested that both research and management programs may benefit from employing multiscale and stage approaches to studying and controlling invasion. We further use the framework to briefly examine potential interactions between climate change and filters that limit NIPS invasion.     

The fundamental and realized niche of the Monterey Pine aphid, Essigella californica (Essig) (Hemiptera: Aphididae): implications for managing softwood plantations in Australia

Essigella californica is a pine aphid native to western North America. In Australia, E. californica is considered an invasive pest that has the potential to cause severe economic loss to the Australian forestry industry. Two CLIMEX models were developed to predict the Australian and global distribution of E. californica under current climate conditions based upon the aphid's known North American distribution. The first model (model I) was fitted using the reasonably contiguous set of location records in North America that constituted the known range of E. californica, and excluded consideration of a single (reliable) location record of the aphid in southern Florida. The second model (model II) was fitted using all known records in North America. Model I indicated that the aphid would be climatically restricted to the temperate, Mediterranean and subtropical climatic regions of Australia. In northern Australia it would be limited by hot, wet conditions, while in more central areas of Australia it is limited by hot, dry conditions. Model II is more consistent with the current Australian distribution of E. californica. The contrast in geographical range and climatic conditions encompassed between the two models appears to represent the difference between the realized niche (model I) and fundamental niche (model II) of E. californica. The difference may represent the strength of biotic factors such as host limitation, competition and parasitism in limiting geographical spread in the native range. This paper provides a risk map for E. californica colonization in Australia and globally. E. californica is likely to remain a feature of the Australian pine plantations, and any feasibility studies into establishing coniferous plantations in lower rainfall areas should consider the likely impact of E. californica.     

Phylogenetic isolation increases plant success despite increasing susceptibility to generalist herbivores

Aim Theory suggests that introduced species that are phylogenetically distant from their recipient communities should be more successful than closely related introduced species because they can exploit open niches and escape enemies in their new range, i.e. Darwin’s Naturalization Hypothesis. Alternatively, it has also been hypothesized that closely related invaders might be more successful than novel invaders because they are pre‐adapted to conditions in their new range; a paradox coined Darwin’s Naturalization Conundrum. To date, these hypotheses have been tested primarily at the regional scale, not within local plant communities where introduced species colonize, compete and encounter herbivores. Location Global. Methods and Results We used community phylogenetics to analyse data from 49 published experiments to examine the importance of phylogenetic relatedness and generalist herbivory on native and exotic plant success at the community level. Plants that were categorized as ‘invasive’ were indeed less related to the recipient community than ‘non‐pest’ exotic plants. Distantly related exotic plants were also more abundant than closely related species. Phylogenetic relatedness predicted herbivore impact, but in a way that was opposite to predictions, as herbivores had stronger, not lesser, impacts on distantly related plants. Importantly, these same patterns generally held for native plants, as distantly related native plants were more abundant and more susceptible to herbivores than closely related species, ultimately resulting in herbivores suppressing community‐level phylogenetic diversity. Main conclusions Distantly related plants were more locally successful despite experiencing stronger control by generalist herbivores, a finding that was robust across native and exotic species. To our knowledge, this is the first evidence that phylogenetic matching influences the local success of both native and exotic species and that herbivores can influence community phylodiversity. Phylogenetic relatedness explained a relatively small portion of the variance in the data even after taking herbivory into account, however, suggesting that phylogenetic matching works in combination with other factors to influence community assembly.     

Over-invasion in a freshwater ecosystem: newly introduced virile crayfish (Orconectes virilis) outcompete established invasive signal crayfish (Pacifastacus leniusculus)

Biological invasions are a key threat to freshwater biodiversity, and identifying determinants of invasion success is a global conservation priority. The establishment of introduced species is predicted to be hindered by pre-existing, functionally similar invasive species. Over a five-year period we, however, find that in the River Lee (UK), recently introduced non-native virile crayfish (Orconectes virilis) increased in range and abundance, despite the presence of established alien signal crayfish (Pacifastacus leniusculus). In regions of sympatry, virile crayfish had a detrimental effect on signal crayfish abundance but not vice versa. Competition experiments revealed that virile crayfish were more aggressive than signal crayfish and outcompeted them for shelter. Together, these results provide early evidence for the potential over-invasion of signal crayfish by competitively dominant virile crayfish. Based on our results and the limited distribution of virile crayfish in Europe, we recommend that efforts to contain them within the Lee catchment be implemented immediately.