Invasive species denialism: a reply to Ricciardi and Ryan

This paper offers a defense against ad hominem aspersions cast in this journal by Ricciardi and Ryan (Biol Invasions 20(3):549–553, 2018) who allege that several articles I wrote represent “invasive species denialism” and “science denialism.” I summarize the arguments found in those articles. They are (1) science cannot define ecological “harm” and thus cannot measure its risk; (2) invasion biologists rely on tautologies, i.e., definitions of concepts like “biodiversity” and “ecosystem intactness,” that exclude exotic species; (3) no empirical evidence shows that introduced plants have been significant causes of extinction; (4) biologists cannot tell by observing a species whether it is native or a naturalized alien; and (5) debates over the meanings and measurements of key concepts in invasion biology have passed the point of diminishing returns. These arguments may be wrong but none is “similar to the denialism that has affected climate science and medical science” as Ricciardi and Ryan aver.     

Trade-offs between growth and defence in two phylogenetically close invasive species

The success of an invasive plant species could be explained by trade-off between growth and defence. The aim of this paper was to explore the responses of two non-native aquatic macrophytes Elodea canadensis and Elodea nuttallii to herbivores in their introduced range. We assessed the palatability of the two phylogenetically close aquatic plant species in field and their responses to gammarid consumption in spring, summer and autumn in a microcosm experiment. We measured the variation of functional traits for each season. The traits selected were those judged most closely related to the allocation of resources to growth or to resistance against herbivores. We clearly established that the strategies of the two species were different and that their consumption rate differed in summer. In summer, E. canadensis allocated more of its resources to structural defence (leaf toughness). The increase in leaf thickness reduced the palatability of E. canadensis, whereas E. nuttallii stimulated its growth. Moreover, a decrease in dry matter content in E. nuttallii was found during the growing season in field. In autumn, both plant species accumulated nitrogen and phosphorus in their tissues. We also demonstrated that neither species induced efficient chemical defences against the herbivores. The different strategies of these two Elodea species could be explained by their different resident times in the introduced area and by an adaptation of the naturalised E. canadensis to herbivores.     

Two-dimensional stochastic exponential growth models

Summary We consider a two niche model of two populations interacting through migration and undergoing exponential growth. Random effects enter through demographic stochasticity or randomization of the growth rates. Conditions for the moments to converge to zero or infinity are found. In addition, the asymptotic behavior of the paths of the diffusion process is characterized.     

Molar growth yield of Nitrobacter winogradskyi during exponential growth

A numerical analysis of experimental growth curves obtained for Nitrobacter by observing changes in cell numbers, substrate concentration and rate of heat evolution has allowed the calculation of the growth rate constants during the phase of balanced growth. The molar growth yield was smaller during that phase than during the phase preceding it. On the other hand, the rate of heat evolution was larger during exponential growth by a factor of about 1.5 than during the stages up to and including this phase. The two observations being in agreement since, if less efficient synthesis occurs during exponential growth, more free energy must be dissipated as heat.     

The contribution of passive surveillance to invasive species management

It has been recognised for some time that the community has an important role to play in invasive-species management. Reports from the community about new incursions can lead to significant cost savings when this early detection results in shorter management programs. Unfortunately there is little to guide invasive-species managers on cost-effective ways to elicit and incorporate information from the public in their pest-management programs. Not all community surveillance is equal: some information from the public about the presence of pests and diseases may arise from chance encounters, other data may be reported by stakeholders from a particular industry or by groups of volunteers organised on the basis of citizen science activities. While the resources, activities and effort required to encourage each type of community surveillance are known to differ, very little is known of the relationships that determine effectiveness, and thus the appropriate level of investment that would be required to encourage a particular level of reporting. In this research we focus on passive surveillance—the most fortuitous type of community surveillance—and review the current knowledge base on measuring its cost and effectiveness. We aim to stimulate the research required to improve our understanding of passive surveillance, and we provide guidance on the type of data that should be collected by agencies to enable this research. This information could then provide us with the ability to design optimal surveillance portfolios that integrate the surveillance opportunities provided by the public to best advantage.     

The stock of invasive insect species and its economic determinants

Invasions of nonindigenous organisms have long been linked to trade, but the contribution of individual trade pathways remains poorly understood, because species are not observed immediately upon arrival and the number of species arriving annually is unknown. Species interception records may count both new arrivals and species long introduced. Furthermore, the stock of invasive insect species already present is unknown. In this study, a state-space model is used to infer the stock of detected as well as undetected invasive insect species established in the United States. A system of equations is estimated jointly to distinguish the patterns of introduction, identification, and eradication. Introductions of invasive species are modeled as dependent on the volume of trade and arrival of people. Identifications depend on the public efforts at invasive species research, as well as on the established stock of invasive species that remain undetected. Eradications of both detected and undetected invasive species depend on containment and quarantine efforts, as well as on the stock of all established invasive species. These patterns are estimated by fitting the predicted number of invasive species detections to the observed record in the North American Non-Indigenous Arthropod Database. The results indicate that agricultural imports are the most important pathway of introduction, followed by immigration of people. Expenditures by the U.S. Department of Agriculture and the Agricultural Research Service are found to explain the species identification record well. Between three and 38 invasive insect species are estimated to be established in the United States undetected.     

Adaptive evolution in invasive species

Many emerging invasive species display evidence of rapid adaptation. Contemporary genetic studies demonstrate that adaptation to novel environments can occur within 20 generations or less, indicating that evolutionary processes can influence invasiveness. However, the source of genetic or epigenetic variation underlying these changes remains uncharacterised. Here, we review the potential for rapid adaptation from standing genetic variation and from new mutations, and examine four types of evolutionary change that might promote or constrain rapid adaptation during the invasion process. Understanding the source of variation that contributes to adaptive evolution in invasive plants is important for predicting future invasion scenarios, identifying candidate genes involved in invasiveness, and, more generally, for understanding how populations can evolve rapidly in response to novel and changing environments.     

Ecological genetics of invasive alien species

There is growing realisation that integrating genetics and ecology is critical in the context of biological invasions, since the two are explicitly linked. So far, the focus of ecological genetics of invasive alien species (IAS) has been on determining the sources and routes of invasions, and the genetic make-up of founding populations, which is critical for defining and testing ecological and evolutionary hypotheses. However an ecological genetics approach can be extended to investigate questions about invasion success and impacts on native, recipient species. Here, we discuss recent progress in the field, provide overviews of recent methodological advances, and highlight areas that we believe are of particular interest for future research. First, we discuss the main insights from studies that have inferred source populations and invasion routes using molecular genetic data, with particular focus on the role of genetic diversity, adaptation and admixture in invasion success. Second, we consider how genetic tools can lead to a better understanding of patterns of dispersal, which is critical to predicting the spread of invasive species, and how studying invasions can shed light on the evolution of dispersal. Finally, we explore the potential for combining molecular genetic data and ecological network modelling to investigate community interactions such as those between predator and prey, and host and parasite. We conclude that invasions are excellent model systems for understanding the role of natural selection in shaping phenotypes and that an ecological genetics approach offers great potential for addressing fundamental questions in invasion biology.     

A Panax-centric view of invasive species

Plant-centric sampling provides a novel approach to quantifying the potential impact of invasive species on native plant species. The aim of this study was to determine the level of exposure of individuals and populations of Panax quinquefolius to invasive plant species using this approach in thirty natural ginseng populations. A high level of invasion was found with 63–70% of ginseng populations containing at least one invasive species. Approximately one-third of all individuals were found in close proximity to invasive plants. The most prevalent invasive species were Rosa multiflora and Berberis thunbergii. The exposure to invasives of plants in different size classes varied among populations. Invasive species presence increased with greater ginseng population sizes and presence of harvest. The abundance of invasives plants within forest interiors near this valuable medicinal herb suggests that the economic and ecological costs of competitive interactions with native species could be high.     

L-DONAX,a growth model of the invasive weed species,Arundo donax L.

Arundo donax L. is a perennial reed and is an invasive weed of riparian systems in North America. A structural model (L-DONAX) of the species was constructed using L-system modelling in order to assist in understanding and demonstrating the complexities of the plant's development and structure. The model produces a realistic number of plant components from a single rhizome segment over the course of the first year of growth, using empirical relationships derived from outdoor experiments. Biomass production is also simulated, through the use of relationships found between aerial plant portion sizes and masses. L-DONAX demonstrates that control of A. donax clumps is likely to require more than annual biomass removal, due to the bulk of biomass being present underground, and the ability of remaining rhizome or stem segments to produce large clumps quickly. The model extrapolates to years of growth beyond the first, but is found to require some re-parameterisation to improve accuracy.     

Intra and interspecific competition among invasive and native species during early stages of plant growth

Plant competition is a primary ecological process limiting grassland restoration success. Appropriate restoration techniques require an understanding of the degree to which intra and interspecific competition control invasive and native plant growth. The objective of this study was to determine how the intensity of intra and interspecific competition changes during early stages of plant growth. Two invasive (Bromus tectorum and Taeniatherum caput-medusae) and two native (Pseudoroegneria spicata and Poa secunda) species were grown in a diallel competition experiment, either alone or in 1:1 binary combinations and exposed to two levels of N (no N or 400 mg N kg⁻¹ soil added) in a greenhouse. Total biomass for each species was quantified over four harvests and competitive effects were calculated. Our results show that the relative magnitude of intra and interspecific competition changes through time. Intraspecific competition was intense for native species at the initial harvests and therefore important in contributing to the outcome of final size of native species seedlings. Interestingly, bluebunch wheatgrass imposed interspecific competition on annual grasses at the first two harvests and appeared to be a better competitor than Sandberg’s bluegrass. We found that fast growing invasive species became more competitive compared to slow growing native species with increasing N and appear to establish a positive feedback mechanism between size and resource uptake. Opportunities to improve restoration success exist from determining the optimum combination of density, species proportion, and their spatial arrangement in various ecosystems and environments.     

Analysis of exponential growth data for yoghurt cultures

Obtaining accurate estimates of maximum specific growth rate, growth yield, and product yield is important for many fermentation processes. A systematic procedure is presented to select the exponential growth region and estimate the maximum specific growth rate using the covariate adjustment method with all the available measured variables (i.e. biomass, substrate, and product). The procedure is applied to data collected during growth of pure and mixed cultures of Lactobacillus bulgaricus and Streptococcus thermophilus on 3% dry milk under anaerobic conditions. The estimation procedure gives good estimates with relatively narrow confidence intervals even though biomass concentration is measured by an indirect method. The estimated values of maximum specific growth rate range from 0.2805 h(-1) for S. thermophilus (ATCC-19258) to 0.4672 h(-1) for S. thermophilus (Microlife). Growth and product yields are estimated using regression analysis and the data for the exponential growth region. The growth yields are compared to their theoretical maximum values.     

L-DONAX,a growth model of the invasive weed species,Arundo donax L.

Arundo donax L. is a perennial reed and is an invasive weed of riparian systems in North America. A structural model (L-DONAX) of the species was constructed using L-system modelling in order to assist in understanding and demonstrating the complexities of the plant's development and structure. The model produces a realistic number of plant components from a single rhizome segment over the course of the first year of growth, using empirical relationships derived from outdoor experiments. Biomass production is also simulated, through the use of relationships found between aerial plant portion sizes and masses. L-DONAX demonstrates that control of A. donax clumps is likely to require more than annual biomass removal, due to the bulk of biomass being present underground, and the ability of remaining rhizome or stem segments to produce large clumps quickly. The model extrapolates to years of growth beyond the first, but is found to require some re-parameterisation to improve accuracy.     

The relationship between invasive alien species and main climatic zones

Analysis of numbers of invasive species (insects, plants, plant diseases) in each of Chinese provinces and the world’s climatic zones respectively, demonstrated positive correlations between diversity of bioinvasion and air temperature and precipitation (the two main criteria for classifying world climate types). When the air temperature is within the range of 1–25°C, the degree of diversity of invasive alien species increases exponentially with air temperature. However, when the air temperature falls outside the range of 1–25°C, the diversity of harmful invasive alien species changes with air temperature in a parabolic pattern, showing a “mid-latitude bulge”. Namely, when air temperatures are too high or too low, a lower average invasion frequency of harmful alien species is observed. Invasion by harmful invasive alien species is more weakly related to precipitation. Different climatic zones showed dramatic differences in resistance to the invasion of invasive alien species due to their different characteristic climatic factors, mainly including air temperature and precipitation.     

Evolutionarily labile species interactions and spatial spread of invasive species

Both exotic and native species have been shown to evolve in response to invasions, yet the impacts of rapidly evolving interactions between novel species pairs have been largely ignored in studies of invasive species spread. Here, I use a mathematical model of an interacting invasive predator and its native prey to determine when and how evolutionary lability in one or both species might impact the dynamics of the invader's spatial advance. The model shows that evolutionarily labile invaders continually evolve better adapted phenotypes along the moving invasion front, offering an explanation for accelerating spread and spatial phenotype clines following invasion. I then analytically derive a formula to estimate the relative change in spread rate due to evolution. Using parameter estimates from the literature, this formula shows that moderate heritabilities and selection strengths are sufficient to account for changes in spread rates observed in historical and ongoing invasions. Evolutionarily labile native species can slow invader spread when genes flow from native populations with exposure to the invader into native populations ahead of the invasion front. This outcome is more likely in systems with highly diffuse native dispersal, net directional movement of natives toward the invasion front, or human inoculation of uninvaded native populations.     

Mechanisms of influence of invasive grass litter on germination and growth of coexisting species in California

In grasslands, litter has been recognized as an important factor promoting grass persistence and the suppression of forbs. The invasive European annual grass Bromus diandrus (ripgut brome) is widespread throughout California, where it produces a persistent and thick litter layer. The native grass, Stipa pulchra, is also common in some grassland settings and can also produce persistent litter, yet it is typically associated with more forbs. Very little is known about the mechanisms through which these two common grass species influence seedling establishment of both exotic invasive and native herbs. Here, we evaluated the effect of B. diandrus and S. pulchra litter on seedling establishment of two invasive (the grass B. diandrus and the forb Centaurea melitensis) and two native (the grass S. pulchra, and the forb Clarkia purpurea) herbaceous plants in a greenhouse setting. Our results showed that B. diandrus litter cover hindered seedling establishment of the four species tested, but that the degree and mechanism of inhibition was dependent on which species was tested, life form (e.g. monocot/dicot) and seed size. Seedling emergence of the two forb species was more vulnerable to litter cover than either grass species and both forbs had smaller seed size. After germination, only seedling biomass of B. diandrus itself was reduced by litter (both B. diandrus and S. pulchra). We found no significant effects of leachate of either grass species on seedling emergence of any species, while a high concentration of B. diandrus leachates inhibited root growth of all species including B. diandrus seedlings. Stipa pulchra litter leachates did not affect S. pulchra or C. melitensis seedlings although it did suppress B. diandrus and C. purpurea seedling growth. Our findings provide direct experimental evidence for the mechanism of effect of litter on these coexisting invasive and native species. Such evidence helps advance our understanding of role of B. diandrus and S. pulchra litter in California grassland.