Conceptual Frameworks and Methods for Advancing Invasion Ecology

Invasion ecology has much advanced since its early beginnings. Nevertheless, explanation, prediction, and management of biological invasions remain difficult. We argue that progress in invasion research can be accelerated by, first, pointing out difficulties this field is currently facing and, second, looking for measures to overcome them. We see basic and applied research in invasion ecology confronted with difficulties arising from (A) societal issues, e.g., disparate perceptions of invasive species; (B) the peculiarity of the invasion process, e.g., its complexity and context dependency; and (C) the scientific methodology, e.g., imprecise hypotheses. To overcome these difficulties, we propose three key measures: (1) a checklist for definitions to encourage explicit definitions; (2) implementation of a hierarchy of hypotheses (HoH), where general hypotheses branch into specific and precisely testable hypotheses; and (3) platforms for improved communication. These measures may significantly increase conceptual clarity and enhance communication, thus advancing invasion ecology

Glacial History of the North Atlantic Marine Snail, Littorina saxatilis, Inferred from Distribution of Mitochondrial DNA Lineages

The North Atlantic intertidal gastropod, Littorina saxatilis (Olivi, 1792), exhibits extreme morphological variation between and within geographic regions and has become a model for studies of local adaptation; yet a comprehensive analysis of the species' phylogeography is lacking. Here, we examine phylogeographic patterns of the species' populations in the North Atlantic and one remote Mediterranean population using sequence variation in a fragment of the mitochondrial cytochrome b gene (607 bp). We found that, as opposed to many other rocky intertidal species, L. saxatilis has likely had a long and continuous history in the Northwest Atlantic, including survival during the last glacial maximum (LGM), possibly in two refugia. In the Northeast Atlantic, several areas likely harboured refugial populations that recolonized different parts of this region after glacial retreat, resulting in strong population structure. However, the outlying monomorphic Venetian population is likely a recent anthropogenic introduction from northern Europe and not a remnant of an earlier wider distribution in the Mediterranean Sea. Overall, our detailed phylogeography of L. saxatilis adds an important piece to the understanding of Pleistocene history in North Atlantic marine biota as well as being the first study to describe the species' evolutionary history in its natural range. The latter contribution is noteworthy because the snail has recently become an important model species for understanding evolutionary processes of speciation; thus our work provides integral information for such endeavours

Consistent Pattern of Local Adaptation during an Experimental Heat Wave in a Pipefish-Trematode Host-Parasite System

Extreme climate events such as heat waves are expected to increase in frequency under global change. As one indirect effect, they can alter magnitude and direction of species interactions, for example those between hosts and parasites. We simulated a summer heat wave to investigate how a changing environment affects the interaction between the broad-nosed pipefish (Syngnathus typhle) as a host and its digenean trematode parasite (Cryptocotyle lingua). In a fully reciprocal laboratory infection experiment, pipefish from three different coastal locations were exposed to sympatric and allopatric trematode cercariae. In order to examine whether an extreme climatic event disrupts patterns of locally adapted host-parasite combinations we measured the parasite's transmission success as well as the host's adaptive and innate immune defence under control and heat wave conditions. Independent of temperature, sympatric cercariae were always more successful than allopatric ones, indicating that parasites are locally adapted to their hosts. Hosts suffered from heat stress as suggested by fewer cells of the adaptive immune system (lymphocytes) compared to the same groups that were kept at 18°C. However, the proportion of the innate immune cells (monocytes) was higher in the 18°C water. Contrary to our expectations, no interaction between host immune defence, parasite infectivity and temperature stress were found, nor did the pattern of local adaptation change due to increased water temperature. Thus, in this host-parasite interaction, the sympatric parasite keeps ahead of the coevolutionary dynamics across sites, even under increasing temperatures as expected under marine global warming

The Whereabouts of an Ancient Wanderer: Global Phylogeography of the Solitary Ascidian Styela plicata

Genetic tools have greatly aided in tracing the sources and colonization history of introduced species. However, recurrent introductions and repeated shuffling of populations may have blurred some of the genetic signals left by ancient introductions. Styela plicata is a solitary ascidian distributed worldwide. Although its origin remains unclear, this species is believed to have spread worldwide by travelling on ship's hulls. The goals of this study were to infer the genetic structure and global phylogeography of S. plicata and to look for present-day and historical genetic patterns. Two genetic markers were used: a fragment of the mitochondrial gene Cytochrome Oxidase subunit I (COI) and a fragment of the nuclear gene Adenine Nucleotide Transporter/ADP-ATP Translocase (ANT). A total of 368 individuals for COI and 315 for ANT were sequenced from 17 locations worldwide. The levels of gene diversity were moderate for COI to high for ANT. The Mediterranean populations showed the least diversity and allelic richness for both markers, while the Indian, Atlantic and Pacific Oceans had the highest gene and nucleotide diversities. Network and phylogenetic analyses with COI and ANT revealed two groups of alleles separated by 15 and 4 mutational steps, respectively. The existence of different lineages suggested an ancient population split. However, the geographic distributions of these groups did not show any consistent pattern, indicating different phylogeographic histories for each gene. Genetic divergence was significant for many population-pairs irrespective of the geographic distance among them. Stochastic introduction events are reflected in the uneven distribution of COI and ANT allele frequencies and groups among many populations. Our results confirmed that S. plicata has been present in all studied oceans for a long time, and that recurrent colonization events and occasional shuffling among populations have determined the actual genetic structure of this species

Glacial Refugia in Pathogens: European Genetic Structure of Anther Smut Pathogens on Silene latifolia and Silene dioica

Climate warming is predicted to increase the frequency of invasions by pathogens and to cause the large-scale redistribution of native host species, with dramatic consequences on the health of domesticated and wild populations of plants and animals. The study of historic range shifts in response to climate change, such as during interglacial cycles, can help in the prediction of the routes and dynamics of infectious diseases during the impending ecosystem changes. Here we studied the population structure in Europe of two Microbotryum species causing anther smut disease on the plants Silene latifolia and Silene dioica. Clustering analyses revealed the existence of genetically distinct groups for the pathogen on S. latifolia, providing a clear-cut example of European phylogeography reflecting recolonization from southern refugia after glaciation. The pathogen genetic structure was congruent with the genetic structure of its host species S. latifolia, suggesting dependence of the migration pathway of the anther smut fungus on its host. The fungus, however, appeared to have persisted in more numerous and smaller refugia than its host and to have experienced fewer events of large-scale dispersal. The anther smut pathogen on S. dioica also showed a strong phylogeographic structure that might be related to more northern glacial refugia. Differences in host ecology probably played a role in these differences in the pathogen population structure. Very high selfing rates were inferred in both fungal species, explaining the low levels of admixture between the genetic clusters. The systems studied here indicate that migration patterns caused by climate change can be expected to include pathogen invasions that follow the redistribution of their host species at continental scales, but also that the recolonization by pathogens is not simply a mirror of their hosts, even for obligate biotrophs, and that the ecology of hosts and pathogen mating systems likely affects recolonization patterns

Host-switching among crabs: species introduction results in a new target host for native parasites

Invasive species can introduce parasites to, and/or acquire new parasites from, novel regions, thereby greatly influencing community interactions, including symbiotic relationships involving parasites. Host-switching of native and non-native parasites could enhance or dilute parasite transmission and spread among hosts. We investigated the effect of host invasion on trematode parasitism in 2 Newfoundland (Canada) bays: one invaded by European green crabs Carcinus maenas and the other not yet invaded. To determine the influence of C. maenas on host-parasite relationships, we assessed trematode prevalence in 3 native hosts: 2 Littorina spp. snails and Atlantic rock crabs Cancer irroratus (first- and second-intermediate hosts for microphallid trematodes, respectively). We found no difference in trematode prevalence between the bays among the 4 host species. However, cyst abundance was significantly higher in C. maenas versus C. irroratus in the bay where the crab distributions overlap, while it was lower in C. irroratus in the invaded versus uninvaded bay, suggesting a dilution of infection in the native host. Sequencing data of microphallid trematodes detected 4 genetically divergent lineages: a cosmopolitan lineage found in all host species; 2 lineages dominant in C. irroratus, suggesting a native origin for the trematodes that now use C. maenas as an additional host; and 1 lineage represented just in C. maenas in Europe. This is the first study to demonstrate the magnitude of trematode infection in crab hosts in Newfoundland, including the commercially valuable native C. irroratus. Our results demonstrate the influence that species introductions can have on parasite life cycles in native systems under recent host invasion.</jats:p

A baitbox for all seasons: temporal shifts in a vector’s propagule supply characteristics and implications for invasion ecology

Invasion dynamics are influenced by both vector operation and propagule pressure. Which propagules are entrained in a vector depends on how, where, and when a vector operates, but the timing and effects of vector operations on species delivery patterns is poorly resolved. Using the live marine baitworm trade, we tested vector selectivity across 3 boreal seasons (summer 2011, fall 2011, and spring 2012). We compared macroinvertebrate assemblages at the source (Maine, US east coast field) and in baitboxes upon delivery (Mid-Atlantic distributors, US east coast) and quantified live and dead biota to test for interactive effects of season and vector stage (i.e. source vs. destination) on per capita abundance, species richness, diversity, functional richness, and community composition. In all, we identified 46262 hitchhiking macro-organisms from 56 distinct taxa. Among live biota, taxonomic richness, functional group richness, and abundance differed by vector stage and season. Community composition showed seasonality for functional groups, but not for taxonomic groups. Vector stage affected dead community composition more than season, implying that vector operations (i.e. handling at source and during shipping) filter species transfers differentially. Dead communities were typically composed of the most abundant live organisms in the same baitboxes, emphasizing how important propagule pressure is to successful transport. Some combinations of 5 key functional traits (body size, feeding mode, growth form, modularity, and motility) were associated with increased survival during vector transfer. Successful species transfers are correlated with specific functional traits and propagule pressure, both of which are influenced by seasonal variation.</jats:p

Year-round reproduction in a seasonal sea: biological cycle of the introduced ascidian Styela plicata in the Western Mediterranean

10 páginas, 6 figuras.The widely introduced ascidian Styela plicata is very common in the Western Mediterranean, an area that can act as a source for secondary introductions due to its high shipping activity. In order to understand the potential of this species to colonize new habitats, its reproductive features were assessed in the Western Mediterranean by means of monthly monitoring of two populations (Vilanova i la Geltru´ 41 1205300N, 1 4401100E; Blanes 41 4002900N, 2 4705600E) from January 2009 to December 2010. The reproductive activity of this species was assessed through gonad histology and a gonad index. Population size-structure was measured monthly in order to study recruitment dynamics. No clear seasonal pattern was observed, and mature gametes and recruits were present all year long. Spawning was potentially continuous, although it seemed punctuated with pulses of gamete release, particularly in spring. A prolonged reproductive period is likely to confer a competitive advantage on S. plicata in temperate seas, where most species reproduce seasonally, and may promote recurrent introductions as larvae are available for settlement on transport vectors over much of the year.This research was supported by the Marie Curie International Reintegration Grant FP7-PEOPLE-2010-RG 277038 within the 7th European Community Framework Program, by the Spanish Government projects CTM2010-22218 and CTM2010- 17755, the Catalan Government grant 2009SGR-484 for Consolidated Research Groups, and by a University of Barcelona APIF fellowship to MCP.Peer reviewe