Climatic similarity and biological exchange in the worldwide airline transportation network

Recent increases in the rates of biological invasion and spread of infectious diseases have been linked to the continued expansion of the worldwide airline transportation network (WAN). Here, the global structure of the WAN is analysed in terms of climatic similarity to illuminate the risk of deliberate or accidental movements of climatically sensitive organisms around the world. From over 44,000 flight routes, we show, for each month of an average year, (i) those scheduled routes that link the most spatially distant but climatically similar airports, (ii) the climatically best-connected airports, and (iii) clusters of airports with similar climatic features. The way in which traffic volumes alter these findings is also examined. Climatic similarity across the WAN is skewed (most geographically close airports are climatically similar) but heavy-tailed (there are considerable numbers of geographically distant but climatically similar airports), with climate similarity highest in the June-August period, matching the annual peak in air traffic. Climatically matched, geographically distant airports form subnetworks within the WAN that change throughout the year. Further, the incorporation of passenger and freight traffic data highlight at greater risk of invasion those airports that are climatically well connected by numerous high capacity routes.     

The role of global trade and transport network topology in the human‐mediated dispersal of alien species

More people and goods are moving further and more frequently via many different trade and transport networks under current trends of globalisation. These networks can play a major role in the unintended introduction of exotic species to new locations. With the continuing rise in global trade, more research attention is being focused on the role of networks in the spread of invasive species. This represents an emerging field of research in invasion science and the substantial knowledge being generated within other disciplines can provide ecologists with new tools with which to study invasions. For the first time, we synthesise studies from several perspectives, approaches and disciplines to derive the fundamental characteristics of network topology determining the likelihood of spread of organisms via trade and transport networks. These characteristics can be used to identify critical points of vulnerability within these networks and enable the development of more effective strategies to prevent invasions.     

Integration of exotic seeds into an Azorean seed dispersal network

Seed dispersal plays a central role in plant ecology. Among animals, birds are particularly important seed dispersers, often incorporating exotic plants into their diets and facilitating their integration in the communities. Network theory offers a highly informative framework to study the structural and functional attributes of complex interactions networks. We used information from bird fecal samples to build a quantitative seed dispersal network for the last fragment of native laurel forest in the island of São Miguel—Azores with three specific objectives: (1) to assess the integration of exotic seeds into seed dispersal; (2) to evaluate the impact of exotic plants in network structure; (3) to test the potential of an exotic species to reduce the seed dispersal of a co-occurring native, via competition for seed dispersers. The seed dispersal network was based on the analysis of 1,121 droppings and described 74 unique interactions between 41 plant species and 7 bird species. Exotic seeds deeply infiltrated into the seed dispersal network forming the majority (59 %) of seeds in the droppings and including those of three globally invasive plants. Overall, birds depended equally on native and exotic fruits despite the lower abundance of the latter in the study area. In an experiment, birds did not show a preference for fruits of the exotic Leycesteria formosa over the native Vaccinium cylindraceum consuming them equally. However, the presence of the exotic plant negatively affected the number of native seeds dispersed, by diverting some of the consumers of the native fruits. Taken altogether the results reveal an alarming invasion level of seed dispersal systems in one of the last remnant native forests of the Azores.     

The role of migratory ducks in the long‐distance dispersal of native plants and the spread of exotic plants in Europe

Little is known about the role of migratory waterfowl in the long‐distance dispersal (LDD) of seeds. We studied the gut contents of 42 teals Anas crecca collected in the Camargue, southern France, and found intact seeds of 16 species. There was no relationship between the probability that a given seed species was found intact in the lower gut, and the seed hardness or size. The number of seeds found in the oesophagus and gizzard (a measure of ingestion rate) was the only significant predictor of the occurrence of intact seeds in the lower gut, so studies of waterfowl diet can be used as surrogates of dispersal potential. In a literature review, we identified 223 seed species recorded in 25 diet studies of teal, pintail Anas acuta, wigeon A. penelope or mallard A. platyrhynchos in Europe. We considered whether limited species distribution reduces the chances that a seed can be carried to suitable habitat following LDD. Overall, 72% of plant species recorded in duck diets in southern Europe (36 of 50) were also recorded in the north, whereas 97% of species recorded in duck diets in the north (137 of 141) were also recorded in the south. This suggests a great potential for LDD, since most dispersed plants species occur throughout the migratory range of ducks. Migratory ducks are important vectors for both terrestrial and aquatic plant species, even those lacking the fleshy fruits or hooks typically used to identify seeds dispersed by birds. Finally, we show ducks are important vectors of exotic plant species. We identified 14 alien to Europe and 44 native to Europe but introduced to some European countries whose seeds have been recorded in duck diet.     

Ecological traits influence the phylogenetic structure of bird species co‐occurrences worldwide

The extent to which species’ ecological and phylogenetic relatedness shape their co‐occurrence patterns at large spatial scales remains poorly understood. By quantifying phylogenetic assemblage structure within geographic ranges of >8000 bird species, we show that global co‐occurrence patterns are linked – after accounting for regional effects – to key ecological traits reflecting diet, mobility, body size and climatic preference. We found that co‐occurrences of carnivorous, migratory and cold‐climate species are phylogenetically clustered, whereas nectarivores, herbivores, frugivores and invertebrate eaters tend to be more phylogenetically overdispersed. Preference for open or forested habitats appeared to be independent from the level of phylogenetic clustering. Our results advocate for an extension of the tropical niche conservatism hypothesis to incorporate ecological and life‐history traits beyond the climatic niche. They further offer a novel species‐oriented perspective on how biogeographic and evolutionary legacies interact with ecological traits to shape global patterns of species coexistence in birds.     

Life history of peracarid species in South‐western Atlantic: Comparison of population traits between native and exotic species

Life‐history traits of exotic species are important to understand the process involved in their settlement and their potential impact on native biodiversity. In this context, the seasonal density, the population structure and the reproductive patterns of exotic and native peracarid species of two natural marine environments of Southwestern Atlantic were studied in order to determine the traits that favour the invasion success of exotic species. Five samples, consisted of algal patches (0.20 × 0.20 m quadrants), were collected seasonally from 2016 to 2017 in two intertidal environments (La Estafeta and Cerro Avanzado). Both environments presented high richness of cryptogenic and exotic species (Tanais dulongii, Monocorophium insidiosum, Ampithoe valida, Melita palmata and Jassa marmorata), and only two native species were recorded (Apohyale grandicornis and Exosphaeroma lanceolatum). The comparison of life‐history traits suggested that the distribution, dominance and the highest densities of some exotic species are closely related to their continuous reproductive and recruitment periods, their capability to adapt their life‐history strategies to different environmental conditions and to a more efficient distribution of resources during reproduction; however, in native species, only A. grandicornis registered similar life‐history traits than exotic populations, suggesting that their distribution could be limited by a latitudinal gradient. We expect that these results provide essential information to understand the invasion pattern of exotic species and their potential impact on native biodiversity in Southwestern Atlantic.     

Species traits and interaction rules shape a species‐rich seed‐dispersal interaction network

Species phenotypic traits affect the interaction patterns and the organization of seed‐dispersal interaction networks. Understanding the relationship between species characteristics and network structure help us understand the assembly of natural communities and how communities function. Here, we examine how species traits may affect the rules leading to patterns of interaction among plants and fruit‐eating vertebrates. We study a species‐rich seed‐dispersal system using a model selection approach to examine whether the rules underlying network structure are driven by constraints in fruit resource exploitation, by preferential consumption of fruits by the frugivores, or by a combination of both. We performed analyses for the whole system and for bird and mammal assemblages separately, and identified the animal and plant characteristics shaping interaction rules. The structure of the analyzed interaction network was better explained by constraints in resource exploitation in the case of birds and by preferential consumption of fruits with specific traits for mammals. These contrasting results when looking at bird–plant and mammal–plant interactions suggest that the same type of interaction is organized by different processes depending on the assemblage we focus on. Size‐related restrictions of the interacting species (both for mammals and birds) were the most important factors driving the interaction rules. Our results suggest that the structure of seed‐dispersal interaction networks can be explained using species traits and interaction rules related to simple ecological mechanisms.     

Influence of secondary dispersal by ants on invasive processes of exotic species with fleshy fruits

Biological Invasions - Invasive alien plant species (IAPS) are severely changing ecosystems on earth. Studying the interactions that allow IAPS to establish and spread in the new regions is...     

Naturalization of exotic plant species in north‐eastern North America: trends and detection capacity

Aim To reconstruct historical naturalization trends of exotic vascular plants in Quebec and Massachusetts, two regions that experienced fluctuations in their herbarium specimen collection efforts. We hypothesized that the observed trends are linked to the collection effort and that they differ according to the motive for plant introduction. Methods A checklist of naturalized exotic plant taxa for Quebec, including oldest proof of naturalization and motive for introduction, was built from herbaria, journals, dissertations, theses, bulletins, websites and unpublished records. For Massachusetts, we used the checklist of Sorrie (2005; Rhodora, 107 , 284–329). The collection effort was estimated from a pool of species with specimens from the five largest Quebec herbaria and from the Harvard University Herbarium. Results Naturalization trends of exotic plants in Quebec are similar to those observed in Massachusetts. A large part of the plants naturalized in the 17th, 18th or 19th centuries, with very little naturalization during the last 30–90 years. In general, there is a significant statistical link between herbarium specimen collection effort and the discovery of new naturalized plant taxa. Very few utilitarian plants naturalized in either region during the 20th century. In Quebec, a high number of ornamental plants naturalized during the last 100 years. Main conclusions The link between herbarium specimen collection effort and the discovery of new naturalized plants is real, although not as straightforward as it seems. Our analysis suggests that at least part of the decline in the number of new naturalized exotic plants observed in Quebec and Massachusetts during the last 30–90 years is a direct consequence of the low interest for traditional floristic studies. However, the possibility of a real decline cannot be ruled out. We nevertheless provide here one of the first pieces of evidence of the potential consequences of the decline of local plant collection for environmental management and especially for early detection systems of new invaders.     

Competitive interactions change the pattern of species co‐occurrences under neutral dispersal

Non‐random patterns of species segregation and aggregation within ecological communities are often interpreted as evidence for interspecific interactions. However, it is unclear whether theoretical models can predict such patterns and how environmental factors may modify the effects of species interactions on species co‐occurrence. Here we extend a spatially explicit neutral model by including competitive effects on birth and death probabilities to assess whether competition alone is able to produce non‐random patterns of species co‐occurrence. We show that transitive and intransitive competitive hierarchies alone (in the absence of environmental heterogeneity) are indeed able to generate non‐random patterns with commonly used metrics and null models. Moreover, even weak levels of intransitive competition can increase local species richness. However, there is no simple rule or consistent directional change towards aggregation or segregation caused by competitive interactions. Instead, the spatial pattern depends on both the type of species interaction and the strength of dispersal. We conclude that co‐occurrence analysis alone may not able to identify the underlying processes that generate the patterns.     

Dispersal and recruitment limitation in native versus exotic tree species: life‐history strategies and Janzen‐Connell effects

Life‐history traits of invasive exotic plants are typically considered to be exceptional vis‐à‐vis native species. In particular, hyper‐fecundity and long range dispersal are regarded as invasive traits, but direct comparisons with native species are needed to identify the life‐history stages behind invasiveness. Until recently, this task was particularly problematic in forests as tree fecundity and dispersal were difficult to characterize in closed stands. We used inverse modelling to parameterize fecundity, seed dispersal and seedling dispersion functions for two exotic and eight native tree species in closed‐canopy forests in Connecticut, USA. Interannual variation in seed production was dramatic for all species, with complete seed crop failures in at least one year for six native species. However, the average per capita seed production of the exotic Ailanthus altissima was extraordinary: > 40 times higher than the next highest species. Seed production of the shade tolerant exotic Acer platanoides was average, but much higher than the native shade tolerant species, and the density of its established seedlings (≥ 3 years) was higher than any other species. Overall, the data supported a model in which adults of native and exotic species must reach a minimum size before seed production occurred. Once reached, the relationship between tree diameter and seed production was fairly flat for seven species, including both exotics. Seed dispersal was highly localized and usually showed a steep decline with increasing distance from parent trees: only Ailanthus altissima and Fraxinus americana had mean dispersal distances > 10 m. Janzen‐Connell patterns were clearly evident for both native and exotic species, as the mode and mean dispersion distance of seedlings were further from potential parent trees than seeds. The comparable intensity of Janzen‐Connell effects between native and exotic species suggests that the enemy escape hypothesis alone cannot explain the invasiveness of these exotics. Our study confirms the general importance of colonization processes in invasions, yet demonstrates how invasiveness can occur via divergent colonization strategies. Dispersal limitation of Acer platanoides and recruitment limitation of Ailanthus altissima will likely constitute some limit on their invasiveness in closed‐canopy forests.     

Nitric oxide‐mediated dispersal in single‐ and multi‐species biofilms of clinically and industrially relevant microorganisms

Strategies to induce biofilm dispersal are of interest due to their potential to prevent biofilm formation and biofilm‐related infections. Nitric oxide (NO), an important messenger molecule in biological systems, was previously identified as a signal for dispersal in biofilms of the model organism Pseudomonas aeruginosa. In the present study, the use of NO as an anti‐biofilm agent more broadly was assessed. Various NO donors, at concentrations estimated to generate NO levels in the picomolar and low nanomolar range, were tested on single‐species biofilms of relevant microorganisms and on multi‐species biofilms from water distribution and treatment systems. Nitric oxide‐induced dispersal was observed in all biofilms assessed, and the average reduction of total biofilm surface was 63%. Moreover, biofilms exposed to low doses of NO were more susceptible to antimicrobial treatments than untreated biofilms. For example, the efficacy of conventional chlorine treatments at removing multi‐species biofilms from water systems was increased by 20‐fold in biofilms treated with NO compared with untreated biofilms. These data suggest that combined treatments with NO may allow for novel and improved strategies to control biofilms and have widespread applications in many environmental, industrial and clinical settings.     

Cones structure and seed traits of four species of large‐seeded pines: Adaptation to animal‐mediated dispersal

Seed dispersal selection pressures may cause morphological differences in cone structure and seed traits of large‐seeded pine trees. We investigated the cone, seed, and scale traits of four species of animal‐dispersed pine trees to explore the adaptations of morphological structures to different dispersers. The four focal pines analyzed in this study were Chinese white pine (Pinus armandi), Korean pine (P. koraiensis), Siberian dwarf pine (P. pumila), and Dabieshan white pine (P. dabeshanensis). There are significant differences in the traits of the cones and seeds of these four animal‐dispersed pines. The scales of Korean pine and Siberian dwarf pine are somewhat opened after cone maturity, the seeds are closely combined with scales, and the seed coat and scales are thick. The cones of Chinese white pine and Dabieshan white pine are open after ripening, the seeds fall easily from the cones, and the seed coat and seed scales are relatively thin. The results showed that the cone structure of Chinese white pine is similar to that of Dabieshan white pine, whereas Korean pine and Siberian dwarf pine are significantly different from the other two pines and vary significantly from each other. This suggests that species with similar seed dispersal strategies exhibit similar morphological adaptions. Accordingly, we predicted three possible seed dispersal paradigms for animal‐dispersed pines: the first, as represented by Chinese white pine and Dabieshan white pine, relies upon small forest rodents for seed dispersal; the second, represented by Korean pine, relies primarily on birds and squirrels to disperse the seeds; and the third, represented by Siberian dwarf pine, relies primarily on birds for seed dispersal. Our study highlights the significance of animal seed dispersal in shaping cone morphology, and our predictions provide a theoretical framework for research investigating the coevolution of large‐seeded pines and their seed dispersers.     

Genetic diversity,population structure and sex‐biased dispersal in three co‐evolving species

Genetic diversity and spatial structure of populations are important for antagonistic coevolution. We investigated genetic variation and population structure of three closely related European ant species: the social parasite Harpagoxenus sublaevis and its two host species Leptothorax acervorum and Leptothorax muscorum. We sampled populations in 12 countries and analysed eight microsatellite loci and an mtDNA sequence. We found high levels of genetic variation in all three species, only slightly less variation in the host L. muscorum. Using a newly introduced measure of differentiation (Jost’s D_est ), we detected strong population structuring in all species and less male‐biased dispersal than previously thought. We found no phylogeographic patterns that could give information on post‐glacial colonization routes – northern populations are as variable as more southern populations. We conclude that conditions for Thompson’s geographic mosaic of coevolution are ideal in this system: all three species show ample genetic variation and strong population structure.