The green porcelain crab, Petrolisthes armatus, has recently invaded oyster reefs of the South Atlantic Bight at mean densities of up to several thousand individuals m-². Despite the crab’s tremendous densities and wide-spread occurrence, its population dynamics, the reasons for its success, and its ecological impacts have remained unknown. We used field monitoring in two estuaries of coastal Georgia to assess spatial and temporal patterns of distribution, demographics, reproduction, and effects on native crabs. We used field and mesocosm experiments with constructed oyster reef communities of varying native species richness and adult porcelain crab additions to assess why the invader is successful and how it impacts native species and communities. We found P. armatus distributed throughout the estuaries, primarily in the lower regions and low intertidal. Sex ratios were 1:1 throughout the year. During warmer months mean densities ranged from 1,000-11,000 crabs m-², 20-90% of mature females were gravid, and numerous recruits were present. Despite decreases in density of 64->99% in the winter, populations rebounded in the spring. Maximum mean densities were 37 times the highest densities ever recorded and population fecundity exceeded that of the native range by an order of magnitude, but correlations did not show significant negative effects of P. armatus on native crabs. Field experiments suggested that invasion was successful due to tremendous recruitment overwhelming biotic resistance by native species richness or predation. The crab only needed structure to invade, but the presence of adult conspecifics significantly enhanced recruitment (i.e., intraspecific “invasional meltdown”). We documented several impacts on native biota, including the (1) suppression of oyster growth, benthic algal biomass, native crab recruitment, and native goby densities and the (2) enhancement of bivalve recruitment, macroalgal cover, and survivorship of oyster drills. We did not, though, see an effect on native taxonomic richness. The large direct and indirect effects of P. armatus on growth, survivorship, and recruitment of virtually all of the most common native species on oyster reefs in the short-term (4-12 weeks) and at relatively low experimental densities (750-1500 crabs m-²) imply considerable long-term consequences for a major hard-substrate habitat of the South Atlantic Bight.

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Non-native species richness has been observed to peak at mid-temperate latitudes, shaping a pattern of richness and abundance that is distinct from native species patterns that peak in the tropics. Stronger species interactions, and therefore biotic resistance, may lower invasion success in the tropics and help explain the discrepancy between native and non-native richness and abundance. To test the hypothesis that strong predation and competition in the tropics could limit invasion success, we conducted a distributed experiment on sessile marine invertebrate communities in four regions spanning 47-degrees latitude of the eastern Pacific Ocean. We manipulated predator access and resource availability at 12 sites and sampled experimental communities in early and late stages of assembly. Overall, our results suggest that biogeographic location, assembly timescale, and predation interactively shape invasion success across latitude. Strong predation reduced richness of non-native species in the tropics at both assembly timescales but increased non-native species richness in the subtropics during early assembly. Predation also increased non-native abundance in the tropics by late-stage assembly and shaped non-native composition at both assembly stages. Effects of predation at higher latitudes were weak or undetectable, and increasing resource availability never had a positive impact on non-native richness or abundance at any latitude. Further, non-native richness was greater at early relative to late assembly stages at mid to low latitudes and was consistently low in our high latitude region at both timescales. In a complementary experiment, short-term predator exposure reduced non-native abundance in Panama, further confirming the influence of predation in this tropical region. Our results highlight important biogeographic differences in invasion dynamics and disentangle local mechanisms that can shape regional patterns. / Biology

Predicting the impacts of non-native species remains one of the greatest challenges to invasion ecologists. Because of their insularity, freshwater systems are particularly vulnerable to invasions, especially from non-native predators. The research in this thesis explores the role of abiotic and biotic factors in mediating the distribution and impact of Bythotrephes longimanus, a predatory cladoceran that has been introduced to freshwater systems in North America. Although the general impacts of this invasion have been documented, little is known about the factors that modulate them. Using a combination of field surveys and experiments, I tested several hypotheses concerning the influence of interactions with native species, as well as the role of heterogeneity in the light environment, in mediating the impact of Bythotrephes. Results demonstrated that biotic resistance by native macroinvertebrate predators does not play a limiting role in the establishment success of Bythotrephes. However, the within-lake distribution of Bythotrephes was influenced by these macroinvertebrates, suggesting that the native predator context matters when trying to understand the impacts of non-native predators. This was demonstrated with a mesocosm experiment in which the impact of Bythotrephes was constrained by the native Chaoborus larvae. In terms of the abiotic environment, in situ feeding experiments demonstrated that refuges from impact could exist for some prey taxa, as the outcome of predation by Bythotrephes was dependent on light availability and some prey taxa were more successful at evading predation under low light conditions. Finally, results show that adaptive behaviour by prey is also an important determinant of impact, as migrating Daphnia can escape predation effects by Bythotrephes. The combination of light-limited predation and a shallow distribution by Bythotrephes selects for prey that occupy relatively deeper positions during the day. As a whole, this research highlights the importance of complex interactions in mediating the impact of Bythotrephes and may help to explain some of the variation that has been documented among invaded lakes. A better understanding of these complex interactions can improve our ability to anticipate impacts as Bythotrephes continues to spread, as well as provide insight on some of the long-term effects following invasion. / Thesis (Ph.D, Biology) -- Queen's University, 2013-06-17 09:26:35.221

Invasions of non-native species cause problems in ecosystems worldwide, and despite the extensive effort that has been put into research about invasions, we still lack a good understanding for why some, but not other, communities resist these invasions. In this doctoral thesis I test hypotheses on biotic resistance using a large dataset of more than 1000 both failed and successful introductions of freshwater fish into Swedish lakes. We have found that the classic species richness hypothesis is a poor descriptor of introduction success because it fails to acknowledge that resident species contribute to the resistance in different ways. We developed a new measure of biotic resistance, the weighted species richness, which takes into account that the resident species contributes to the resistance with different strength and sign. Further, we correlated performance traits of species in their role as an invader and as a resident species to predict how the biotic resistance of these communities would develop over time. We found a positive correlation between performance traits: Some species have high introduction success, they make a large contribution to the resistance, and they cause extinctions when introduced but do not go extinct themselves when other species establishes, whereas other species are weak performers in these respects. Thus, the biotic resistance of these communities should grow stronger as non-native species accumulates. These results give us clues about what type of communities that should be most sensitive to further invasions, i.e., communities harboring species weak performers.  My results show that the biotic resistance of communities is an important factor in determining invasibility of a community. They also show that methods for quantifying resistance must take into account how interactions are structured in nature. What determine the biotic resistance of a community is the type of interactions that the resident species have with the invader and not the species richness of the community.

La croissance d'une plante est un processus dynamique qui répond aux différentes caractéristiques de l'environnement. La baisse de production de biomasse induite par les différents stress, les perturbations ou la compétition détermine la tolérance des plantes à ces contraintes. Dans le cas d'une plante invasive, caractériser cette tolérance est crucial pour déterminer les habitats sensibles et rechercher des moyens de prévention ou de lutte. Les taxa du genre Fallopia sont ici étudiés dans le cadre de l'invasion du département de la Loire. Leurs réponses au stress nutritif, salin, et à une perturbation sont associées aux conditions environnementales favorisant ou limitant l'invasion. Les résultats obtenus apportent des éléments de réflexion sur les mécanismes à l'origine de l'invasion par les renouées et sur l'efficience de différentes méthodes de lutte et de prévention de l'invasion / Plant growth is a dynamic process that responds to environmental characteristics. The decrease of the plant biomass production induced by various stresses, disturbance, or competition, determines the tolerance to these constraints. In the case of invasive plants, assessing this tolerance is crucial to determine invasibility and to find prevention or control methods. The taxa of the genus Fallopia are here considered in the context of the invasion of the Loire department. Their responses to nutrient stress, salt stress, and disturbance are associated with environmental conditions favouring or limiting the invasion. The results give some evidences about mechanisms implied in the success of Fallopia spp and about the effectiveness of different prevention or control methods

The introduction and establishment of exotic species can profoundly alter ecosystems. Two exotic species drastically changing the landscape of deserts in western North America are Bromus tectorum L. and Bromus rubens L. Through the buildup of biomass and slow decomposition rates in deserts these two exotic annual grasses can alter fire regimes that change the plant and animal community dynamics in the ecosystems. To better understand the ecological mechanisms that could restrict or alter the patterns of invasive plant establishment we established a replicated full factorial experiment in the Great Basin and Mojave Desert. The combinations of factors being manipulated are burned or intact plant communities, and presence or exclusion of small mammals. Generally invasive species establishment is thought to be a result of competitive superiority or lack of natural enemies, but if that is the case then why do not all invasive species establish and become highly abundant in their new ecosystems? To understand why some invasive species establish and others do not we monitored three dominant exotic species from the Great Basin and the Mojave Desert, B. tectorum, Halogeton glomeratus (M. Bieb.) C.A. Mey., and B. rubens. We observed that the presence of small mammals create a biotic resistance to B. tectorum, H. glomeratus, and B. rubens. This pattern was observed in both intact and burned plant communities; however, it was most prevalent in the burned plant communities. The strength of the biotic resistance on these invasive species varied between species and the years sampled. In deserts both plant and small mammal communities are tightly tied to precipitation. We wanted to understand how invasive species establishment is affected by small mammal presence after a fire disturbance, and manipulating total precipitation. Total precipitation was manipulated through three different treatments: 1) drought or 30% reduction of ambient precipitation; 2) ambient precipitation; 3) water addition or an increase of 30% ambient precipitation. We focused on B. rubens establishment in the Mojave Desert as our model organism by monitoring it beneath rain manipulation shelters nested in burned/intact and small mammal presence/absence full factorial plots. What we observed was that again small mammals created a biotic resistance on the density of B. rubens regardless of the burn or precipitation treatments. This biotic resistance also translated into decreasing B. rubens biomass and seed density. Under the drought and ambient precipitation treatments we found that small mammals kept the density and biomass equal but under increased precipitation the efficacy of biotic resistance on B. rubens density and biomass was lessened by the availability of the added water.

Non-indigenous species have become a global issue of increasing importance in recent years, with many causing significant environmental and economic damage. Identifying locations vulnerable to invasion allows for focus of management efforts towards prevention of invasions at those locations. In order to determine whether constructed water bodies, such as reservoirs, ornamental lakes or retired mines, are more easily invaded environments than natural water bodies, owing to decreased biotic resistance, the distributions of native and non-indigenous freshwater calanoid copepod species in the North Island were examined. Calanoid copepods in ports and other coastal environments were also examined, in order to determine whether ports are more frequently invaded owing to increased propagule supplies from visiting ships and other sources. The distributions of the native freshwater calanoid copepod species Boeckella hamata, B. propinqua, B. delicata and B. tanea are confined in the North Island of New Zealand to specific technostratigraphic terranes when natural waters only are examined, and as such each species can be considered to have a native range. The recently colonised calanoid copepod species Boeckella minuta (6 locations), Skistodiaptomus pallidus (3 locations) and Sinodiaptomus valkanovi (2 locations) are to date confined to constructed water bodies. Boeckella symmetrica (2 locations) may be confined to constructed water bodies, but the status of one location is unclear. Boeckella triarticulata, a species common in the South Island, is known only from a single farm dam in the North Island. The native species Boeckella hamata, B. propinqua and B. delicata were found to occur in constructed waters, but only B. propinqua was found in constructed water bodies outside their natural ranges (9 locations). Calamoecia lucasi is found in lakes throughout most of the North Island, and is not confined to any one terrane. My results indicate that constructed water bodies are more easily invaded by non-indigenous species than natural water bodies, represents a potential pathway for future invaders to establish, and provides locations for species to spread. In order to determine whether recently established freshwater calanoid copepod species have the potential to spread from their present habitats into other water bodies, the prosomal lengths of non-indigenous calanoid copepod species were measured and compared with those for native species. The results suggest that dietary overlap should prevent the non-indigenous species present to date from spreading into any water bodies with established Boeckella populations, although Sinodiaptomus valkanovi and Boeckella triarticulata could potentially spread to lakes containing only Calamoecia lucasi. Data on the co-occurrences of native freshwater calanoid copepod species support the theory of dietary exclusion, as Boeckella species have not been found to coexist. In order to test whether New Zealand marine environments have been invaded by non-indigenous calanoid copepods, and whether ports have been more regularly invaded than non-port areas, calanoid copepods were sampled from various coastal locations around the North Island. With the possible exception of Sulcanus conflictus, no non-indigenous species were found, indicating that non-indigenous marine calanoid copepod species are not establishing in New Zealand despite a history of invasion elsewhere.

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