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Interspecific competition for limited resources can drive ecological specialization and trait expression. Organisms released from intense competition may exploit a broader range of resources, but if reunited with stronger competitors, survivorship may depend on foraging behaviors that reduce competition. We compared the host selection behavior of the parasitoid Cotesia glomerata from two North American populations that differ in their association with Cotesia rubecula, a superior competitor. Both parasitoids originate from Europe and attack the imported cabbageworm (a.k.a. small cabbage white) Pieris rapae, but C. glomerata was introduced into North America almost a century before C. rubecula. After re-association in North America, C. rubecula has displaced C. glomerata in several regions, but not in other regions. Host selection was measured in female C. glomerata from Maryland (MD) where it coexists with C. rubecula, and in conspecifics from Colorado (CO) where C. rubecula is absent. Unparasitized and C. rubecula-parasitized P. rapae hosts were used in choice tests to examine whether C. glomerata host selection behavior differed based on the population's association history with C. rubecula. We found that C. glomerata from MD had a higher likelihood of avoiding hosts parasitized by C. rubecula (and thus avoiding competition) than did wasps from CO. The ability of C. glomerata to avoid hosts parasitized by C. rubecula may facilitate coexistence in MD; whereas, the lack of discrimination in CO populations of C. glomerata naïve to C. rubecula could contribute to the displacement of C. glomerata were C. rubecula to enter the same habitat.

Many invasive species are able to escape from coevolved enemies and thus enjoy a competitive advantage over native species. However, during the invasion phase, non-native species must overcome many ecological and/or physiological hurdles before they become established and spread in their new habitats. This may explain why most introduced species either fail to establish or remain as rare interstitials in their new ranges. Studies focusing on invasive species have been based on plants or animals where establishment requires the possession of preadapted traits from their native ranges that enables them to establish and spread in their new habitats. The possession of preadapted traits that facilitate the exploitation of novel resources or to colonize novel habitats is known as 'ecological fitting'. Some species have evolved traits and life histories that reflect highly intimate associations with very specific types of habitats or niches. For these species, their phenological windows are narrow, and thus the ability to colonize non-native habitats requires that a number of conditions need to be met in accordance with their more specialized life histories. Some of the strongest examples of more complex ecological fitting involve invasive parasites that require different animal hosts to complete their life cycles. For instance, the giant liver fluke, Fascioloides magna, is a major parasite of several species of ungulates in North America. The species exhibits a life cycle whereby newly hatched larvae must find suitable intermediate hosts (freshwater snails) and mature larvae, definitive hosts (ungulates). Intermediate and definitive host ranges of F. magna in its native range are low in number, yet this parasite has been successfully introduced into Europe where it has become a parasite of native European snails and deer. We discuss how the ability of these parasites to overcome multiple ecophysiological barriers represents an excellent example of 'multiple-level ecological fitting'., (© 2015 John Wiley & Sons Ltd.)

Substantial research has addressed adaptation of nonnative biota to novel environments, yet surprisingly little work has integrated population genetic structure and the mechanisms underlying phenotypic differentiation in ecologically important traits. We report on studies of the common milkweed Asclepias syriaca, which was introduced from North America to Europe over the past 400 years and which lacks most of its specialized herbivores in the introduced range. Using 10 populations from each continent grown in a common environment, we identified several growth and defense traits that have diverged, despite low neutral genetic differentiation between continents. We next developed a Bayesian modeling approach to account for relationships between molecular and phenotypic differences, confirming that continental trait differentiation was greater than expected from neutral genetic differentiation. We found evidence that growth-related traits adaptively diverged within and between continents. Inducible defenses triggered by monarch butterfly herbivory were substantially reduced in European populations, and this reduction in inducibility was concordant with altered phytohormonal dynamics, reduced plant growth, and a trade-off with constitutive investment. Freedom from the community of native and specialized herbivores may have favored constitutive over induced defense. Our replicated analysis of plant growth and defense, including phenotypically plastic traits, suggests adaptive evolution following a continental introduction.

Due to global warming, species are expanding their range to higher latitudes. Some range expanding plants have become invasive in their new range. The Evolution of Increased Competitive Ability (EICA) hypothesis and the Shifting Defense Hypothesis (SDH) predict altered selection on plant defenses in the introduced range of invasive plants due to changes in herbivore pressures and communities. Here, we investigated chemical defenses (glucosinolates) of five native and seven invasive populations of the Eurasian invasive range expanding plant, Rorippa austriaca. Further, we studied feeding preferences of a generalist and a specialist herbivore among the populations. We detected eight glucosinolates in the leaves of R. austriaca. 8-Methylsulfinyloctyl glucosinolate was the most abundant glucosinolate in all plants. There were no overall differences between native and invasive plants in concentrations of glucosinolates. However, concentrations among populations within each range differed significantly. Feeding preference between the populations by a generalist herbivore was negatively correlated with glucosinolate concentrations. Feeding by a specialist did not differ between the populations and was not correlated with glucosinolates. Possibly, local differences in herbivore communities within each range may explain the differences in concentrations of glucosinolates among populations. Little support for the predictions of the EICA hypothesis or the SDH was found for the glucosinolate defenses of the studied native and invasive R. austriaca populations.

Co-evolutionary theory underpins our understanding of interactions in nature involving plant-herbivore and host-parasite interactions. However, many studies that are published in the empirical literature that have explored life history and development strategies between endoparasitoid wasps and their hosts are based on species that have no evolutionary history with one another. Here, we investigated novel associations involving two closely related solitary endoparasitoids that originate from Europe and North America and several of their natural and factitious hosts from both continents. The natural hosts of both species are also closely related, all being members of the same family. We compared development and survival of both parasitoids on the four host species and predicted that parasitoid performance is better on their own natural hosts. In contrast with this expectation, survival, adult size and development time of both parasitoids were similar on all (with one exception) hosts, irrespective as to their geographic origin. Our results show that phylogenetic affinity among the natural and factitious hosts plays an important role in their nutritional suitability for related parasitoids. Evolved traits in parasitoids, such as immune suppression and development, thus enable them to successfully develop in novel host species with which they have no evolutionary history. Our results show that host suitability for specialized organisms like endoparasitoids is closely linked with phylogenetic history and macro-evolution as well as local adaptation and micro-evolution. We argue that the importance of novel interactions and 'ecological fitting' based on phylogeny is a greatly underappreciated concept in many resource-consumer studies., (© 2012 The Authors. Journal of Evolutionary Biology © 2012 European Society For Evolutionary Biology.)