Boom‐bust dynamics in biological invasions: towards an improved application of the concept

Boom‐bust dynamics – the rise of a population to outbreak levels, followed by a dramatic decline – have been associated with biological invasions and offered as a reason not to manage troublesome invaders. However, boom‐bust dynamics rarely have been critically defined, analyzed, or interpreted. Here, we define boom‐bust dynamics and provide specific suggestions for improving the application of the boom‐bust concept. Boom‐bust dynamics can arise from many causes, some closely associated with invasions, but others occurring across a wide range of ecological settings, especially when environmental conditions are changing rapidly. As a result, it is difficult to infer cause or predict future trajectories merely by observing the dynamic. We use tests with simulated data to show that a common metric for detecting and describing boom‐bust dynamics, decline from an observed peak to a subsequent trough, tends to severely overestimate the frequency and severity of busts, and should be used cautiously if at all. We review and test other metrics that are better suited to describe boom‐bust dynamics. Understanding the frequency and importance of boom‐bust dynamics requires empirical studies of large, representative, long‐term data sets that use clear definitions of boom‐bust, appropriate analytical methods, and careful interpretations.     

Can life‐history traits predict the fate of introduced species? A case study on two cyprinid fish in southern France

1. The ecological and economic costs of introduced species can be high. Ecologists try to predict the probability of success and potential risk of the establishment of recently introduced species, given their biological characteristics.
2. In 1990 gudgeon, Gobio gobio , were released in a drainage canal of the Rhône delta of southern France. The Asian topmouth gudgeon, Pseudorasbora parva, was found for the first time in the same canal in 1993. Those introductions offered a unique opportunity to compare in situ the fate of two closely related fish in the same habitat.
3. Our major aims were to assess whether G. gobio was able to establish in what seemed an unlikely environment, to compare populations trends and life-history traits of both species and to assess whether we could explain or could have predicted our results, by considering their life-history strategies.
4. Data show that both species have established in the canal and have spread. Catches of P. parva have increased strongly and are now higher than those of G. gobio .
5. The two cyprinids have the same breeding season and comparable traits (such as short generation time, small body, high reproductive effort), so both could be classified as opportunists. The observed difference in their success (in terms of population growth and colonization rate) could be explained by the wider ecological and physiological tolerance of P. parva .
6. In conclusion, our field study seems to suggest that invasive vigour also results from the ability to tolerate environmental changes through phenotypic plasticity, rather than from particular life-history features pre-adapted to invasion. It thus remains difficult to define a good invader simply on the basis of its life-history features.     

Vertebrate invaders and their ecological impacts in Chile

The South American country of Chile has been subjected to relatively few invasions in historic times. In comparison to the ca 610 vertebrate species found in continental Chile, only 24 species (3.9%) have invaded the country. Except for two bird and two or three mammal species that expanded their range and crossed the Andean Ranges from Argentina, the remaining 20 species were all introduced, or aided, by humans. Of the 24 invader species that currently occur in Chile, 11 (46%) were first introduced in central Chile, two (8%) in the northern part of the country, nine (38%) in the southern part, and two (8%) in Juan Fernández Archipelago. For ten (42%) of those species there is no information about their effects on the native flora, vegetation, or fauna. For eight species (33%), the information regarding the effects of the invaders is anecdotal. For six species (25%), there is adequate quantitative documentation available on their effects. Of a total of seven convincingly repo rted cases, two involve positive effects (increase in the prey base for native predators) and five have negative effects, of which three are on native vegetation and the remaining two on native fauna.     

Knowledge-based discovery for designing CRISPR-CAS systems against invading mobilomes in thermophiles

Clustered regularly interspaced short palindromic repeats (CRISPRs) are direct features of the prokaryotic genomes involved in resistance to their bacterial viruses and phages. Herein, we have identified CRISPR loci together with CRISPR-associated sequences (CAS) genes to reveal their immunity against genome invaders in the thermophilic archaea and bacteria. Genomic survey of this study implied that genomic distribution of CRISPR-CAS systems was varied from strain to strain, which was determined by the degree of invading mobiloms. Direct repeats found to be equal in some extent in many thermopiles, but their spacers were differed in each strain. Phylogenetic analyses of CAS superfamily revealed that genes cmr, csh, csx11, HD domain, devR were belonged to the subtypes of cas gene family. The members in cas gene family of thermophiles were functionally diverged within closely related genomes and may contribute to develop several defense strategies. Nevertheless, genome dynamics, geological variation and host defense mechanism were contributed to share their molecular functions across the thermophiles. A thermophilic archaean, Thermococcus gammotolerans and thermophilic bacteria, Petrotoga mobilis and Thermotoga lettingae have shown superoperons-like appearance to cluster cas genes, which were typically evolved for their defense pathways. A cmr operon was identified with a specific promoter in a thermophilic archaean, Caldivirga maquilingensis. Overall, we concluded that knowledge-based genomic survey and phylogeny-based functional assignment have suggested for designing a reliable genetic regulatory circuit naturally from CRISPR-CAS systems, acquired defense pathways, to thermophiles in future synthetic biology.

Electronic supplementary material

The online version of this article (doi:10.1007/s11693-015-9176-8) contains supplementary material, which is available to authorized users.     

Interactions among invaders: community and ecosystem effects of multiple invasive species in an experimental aquatic system

With ecosystems increasingly supporting multiple invasive species, interactions among invaders could magnify or ameliorate the undesired consequences for native communities and ecosystems. We evaluated the individual and combined effects of rusty crayfish (Orconectes rusticus) and Chinese mystery snails [Bellamya (=Cipangopaludina) chinensis] on native snail communities (Physa, Helisoma and Lymnaea sp.) and ecosystem attributes (algal chlorophyll a and nutrient concentrations). Both invaders are widespread in the USA and commonly co-occur within northern temperate lakes, underscoring the importance of understanding their singular and joint effects. An outdoor mesocosm experiment revealed that while the two invaders had only weakly negative effects upon one another, both negatively affected the abundance and biomass of native snails, and their combined presence drove one native species to extinction and reduced a second by >95%. Owing to its larger size and thicker shell, adult Bellamya were protected from crayfish attack relative to native species (especially Physa and Lymnaea), suggesting the co-occurrence of these invaders in nature could have elevated consequences for native communities. The per capita impacts of Orconectes (a snail predator) on native snails were substantially greater than those of Bellamya (a snail competitor). Crayfish predation also had a cascading effect by reducing native snail biomass, leading to increased periphyton growth. Bellamya, in contrast, reduced periphyton biomass, likely causing a reduction in growth by native lymnaeid snails. Bellamya also increased water column N:P ratio, possibly because of a low P excretion rate relative to native snail species. Together, these findings highlight the importance of understanding interactions among invasive species, which can have significant community- and ecosystem-level effects.     

A Characterization of the Non-indigenous Flora of the Azores Archipelago

The 9 Azores islands are located in the North Atlantic Ocean, about 1500 km from Europe. The vascular plant flora, consisting of 1002 taxa, was characterized using a database. Biogeographic and historic criteria were used to categorize the taxa as indigenous (31%) and non-indigenous (69%). The proportion of non-indigenous vascular plant taxa is higher than in other island ecosystems. This might have resulted from the removal of native vegetation and the introduction of many cultivated and ornamental plants, and from the relatively large extension of the agricultural landscape. The introduced plants were largely subcosmopolitan therophytes and hemicryptophytes, mainly Dicotyledoneae and Monocotyledoneae. The families with highest absolute contributions were similar to those in other areas of the world (Poaceae, Asteraceae, Fabaceae, Brassicaceae, Scrophulariaceae), but not all contributed as expected by their species richness worldwide. Many were introduced as ornamental or crop plants, but there were also many accidental introductions of weeds (about 55% of non-indigenous taxa). Considering the 9 islands, the percentage of introductions was positively correlated with human population density and island surface below 300 m, and negatively correlated with island surface allocated to natural areas.     

Relative Distributions of Dreissena bugensis and Dreissena polymorpha in the Lower Don River System,Russia

A survey was conducted in the lower Don River system in Russia to confirm the presence of Dreissena bugensis, and to compare its distribution relative to that of Dreissena polymorpha. In 1999 and 2001–2002, dreissenid mussels were collected at 15 sites in the main river, in connecting reservoirs, and in a major tributary, the Manych River. Collections were made near stations where long‐term monitoring data on total mineral (sum of principal ions) and calcium content were available. Both dreissenid species were found at all sites, with D. bugensis comprising 4–75% of all dreissenids at individual sites. D. bugensis was relatively more abundant than D. polymorpha in the Manych River where total mineral and calcium content was significantly higher than in the Don River, suggesting the two species may have different calcium requirements. Examination of archived samples indicated that D. bugensis was present in the Don River system as early as the 1980s, presenting the unresolved enigma of why D. bugensis has not displaced D. polymorpha as the dominant species as typically found over shorter time periods in other water bodies. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Variation in Desiccation Tolerance in Freshwater Snails and Its Consequences for Invasion Ability

The freshwaters of Martinique (French West Indies) have recently been invaded by snails belonging to the Thiaridae family (Gastropoda; Prosobranchia). Eight distinct Thiarid lines have been successively introduced in Martinique, and are still in the process of sequentially replacing one another within local habitats, revealing a range of increasing invasive abilities. Our aim was to test whether the variation in invasive ability can be partly explained by a specific life-history trait, desiccation tolerance, which might be important in view of the typical instability of tropical freshwater habitats. We therefore tested desiccation tolerance in both juveniles and adults under laboratory conditions. Our data show that, although all Thiarid lines resist desiccation quite well, they exhibit extreme variation in the degree of tolerance. These differences are mostly mediated by individual size, but are definitely of genetic origin given that our individuals were of similar ages and were raised under standardized laboratory conditions. The overall invasive success of a line in Martinique, deduced from field surveys, does not correlate with its desiccation tolerance. However, desiccation tolerance does seem to be a limiting factor for the invasion of the small fraction of habitats that are most exposed to drought. More generally, our study exemplifies the possibility that the invasion differential among habitats, rather than a general invasion ability, be predicted according to a particular life-history trait, within a set of closely related invasive taxa.     

A global risk assessment for the success of bird introductions

1.  Concern over the impact of invasive species has led to the development of risk assessment methodologies to identify potential invaders and prevent future ecological and economic problems. However, developing a risk assessment tool is challenging because of the difficulties of accurately predicting the outcome of species introductions.
2.  In this study, we develop a global risk assessment for birds. We integrate two approaches, generalized linear mixed models (GLMM) and hierarchical tree models, to help identify those introductions with the highest risk of establishment success.
3.  Past work has shown that the number of individuals released is the main factor influencing establishment success in animals, a conclusion that was supported in our analyses. Establishment success was also higher for species with broader ecological niches and larger brains relative to body size. These features should increase the likelihood of finding an appropriate niche in the region of introduction.
4.  The GLMM and tree model predicted the probability of establishment success of birds in Europe and Australia with high accuracy (over 80% of introductions correctly classified). This highlights that establishment risk can be reasonably assessed with information on general habitat use, brain size and the size of the founder population. When compared with an alternative risk assessment tool based on a qualitative ranking, our quantitative approaches achieved higher accuracy with less information.
5.   Synthesis and applications. Quantitative risk assessments based on traits related to establishment success are difficult but feasible, providing a useful tool for guiding preventive polices aimed at mitigating the impact of invasive species.