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Biol Invasions DOI 10.1007/s10530-015-0874-7 PERSPECTIVES AND PARADIGMS How do native species respond to invaders? Mechanistic and trait-based perspectives Katherine Berthon Received: 28 July 2014 / Accepted: 6 March 2015 Ó Springer International Publishing Switzerland 2015 Abstract Adaptive responses of native species are important in enabling their persistence in the face of unprecedented biotic exchange. In the present paper I discuss how native species respond to invasive species both from a mechanistic and trait-based perspective. An earlier review by Strauss et al. (Ecol Lett 9:357–374, 2006) discussed a conceptual model of native species evolution in which the likelihood of an evolutionary response to an invader is dependent upon the strength of the selective pressure imposed (degree of variation in fitness between genotypes) and the adaptive capacity of the native (extent of pre-adaptation or genetic diversity). I aim to update and build upon this framework in light of new information on the interaction of phenotypic plasticity and evolutionary processes in adaptive responses of native species. Phenotypic plasticity can be a precursor to or an inhibitor of evolutionary responses and, under conditions of strong selection, phenotypic plasticity may enable adaptation where natives have a low evolutionary capacity. Based on current evidence, it is likely that phenotypic plasticity is the first front in native species adaptation, after which genetic changes occur via a genetic accommodation mechanism. Lastly, I K. Berthon Department of Biological Sciences, Macquarie University, North Ryde, NSW 2109, Australia K. Berthon (&) 79 Devon Street, North Epping, NSW 2121, Australia e-mail: [email protected] review the literature on behavioural, morphological, physiological and life history trait changes of responding native species in light of this framework. Knowledge of the genetic and physiological underpinnings of adaptive responses in native species is limited and would aid in distinguishing the contributions of phenotypic plasticity and evolutionary change in future studies. Keywords Evolution Invasion Native species Phenotypic plasticity Response Introduction Human activities have led to unprecedented rates of biotic exchange between previously isolated ecological communities (Strauss et al. 2006; Mooney and Cleland 2001). This has generated novel species combinations, enabling interaction of species with no shared evolutionary history (Mooney and Cleland 2001). In many cases, these altered interactions also generate changes in environmental conditions with ramifications for ecosystem function (Wright et al. 2012). Adaptation of exotic species to novel environments and new species interactions is well documented, and much research effort has been placed in understanding exotic traits or conditions that facilitate or prevent invasion (reviewed in Mooney and Cleland 2001; Strayer et al. 2006). 123 K. Berthon The research interest in invasive species is fuelled by the (often negative) impacts of exotics on native species persistence (Rodriguez 2006). Invasive species impose novel selection pressures on their recipient community through both direct interactions with native species [predation (Freeman and Byers 2006; Carlsson et al. 2009; Fisk et al. 2007; Latta et al. 2007; Nunes et al. 2014), herbivory (e.g. Lau 2008), competition (Oduor 2013; Lau 2008; Lankau 2012) or parasitism (e.g. Patten and Campbell 1998)], and indirect impacts on ecosystem processes (e.g. Wright et al. 2012) or trophic cascades (e.g. Knapp et al. 2001) (Fig. 1). Further, invasive species can cause changes in the distribution and abundance of resources (Reusch and Williams 1998) which may provide novel opportunities for native species (Carroll et al. 1997; Rodriguez 2006). For example, an invasive species may itself become a valuable food resource for a native predator (King et al. 2006) or herbivore (Carroll et al. 1998). The selection pressure exerted by an invasive species is thus dependent on its trophic relationship to the native species (Strauss et al. 2006). Native species that adapt to mitigate the negative impacts of invasive species or exploit the novel opportunities they provide will have a fitness advantage over those which do not (Carlsson et al. 2009). Failure to adapt to the negative impacts of an invasive species is a major threat to native species persistence, globally (Rodriguez 2006). However, there is an increasing body of evidence showing that native species are able to respond adaptively to the presence of invaders either through plastic phenotypic responses (Ortega et al. 2013; Latta et al. 2007) or evolutionary processes (Oduor 2013; e.g. Goergen et al. 2011). A large number of these associations have been found when investigating insect pests on introduced crop species (e.g. Zimmerman 1960) and thus have economic as well as ecological implications. Several studies have reviewed evidence for species adaptations generally (Strauss et al. 2006), as well as within the context of individual ecological interactions (Carlsson et al. 2009; Strauss et al. 2006), or with a focus on particular taxa (Bezemer et al. 2014; Oduor 2013; Shine 2012). These studies highlight four main aspects of native species biology that are altered under selection from invaders: behaviour, morphology, physiology and life history traits. However, a full synthesis of how these traits change across trophic relationships has not yet been performed. Further, previous studies have focused on evolutionary processes as mechanisms for native species adaptation to invaders. While such reviews do discuss plasticity and its contribution to evolution (e.g. Strauss et al. 2006), plasticity, particularly maladaptive changes, has not Invader Impact (A) (B) Direct Novel Competitor Novel Predator Indirect Novel Prey Trophic Interaction Environment Modification (C) Life History Physiological Behavioural Morphological Nov. Competitor 15 7 4 2 Nov. Predator 7 7 16 9 Fig. 1 Selection imposed by invasive species. An invasive species imposes selection pressure on its recipient community either a directly through interaction with native species or b indirectly through alteration of trophic linkages between species or changes in environmental conditions. c Four aspects of a species’ biology are impacted: life history, physiology, behaviour and morphology. The number of cases for each trait 123 Nov. Prey 7 2 12 7 Trophic Env. Mod. 1 1 1 change under each context are given based on analysis of 77 cases compiled from the literature. No references were found for native species altering their behaviour or morphology in the face of a novel competitor. Environmental effects were only found for morphological traits. The terms ‘‘predator’’ and ‘‘prey’’ are used generically and include herbivore-plant interactions. Further details are given in Table 1 Mechanistic and trait-based perspectives been explicitly incorporated into models of native adaptation. Considering recent advances in our understanding of the contribution of phenotypic plasticity to evolution (Schlichting and Wund 2014; Ghalambor et al. 2007a; Trussell and Smith 2000; Nylin et al. 2014), a mechanistic understanding of how native species respond adaptively to invaders should incorporate phenotypically plastic responses. In the present review, I aim to generate a comprehensive mechanistic understanding of how native species respond to invasive species. I begin by building upon a conceptual framework by Strauss et al. (2006) which outlines the conditions under which native species adaptation occurs (Fig. 2) and relate this to different types of selection pressure imposed by invaders (Fig. 1). Using this framework I discuss the interaction between phenotypic plasticity and evolutionary processes in adaptive responses of natives to invaders. I then review the literature on behavioural, morphological, physiological and life history trait changes of responding native species in light of this framework. This review is not exhaustive, but aims to synthesise information to cover all factors affecting how native species respond to invasive species. Mechanisms for adaptation: phenotypic plasticity or evolution? Selective pressure and capacity for evolution Selecve Impact of Invader (trophic interacons, resource distribuon changes) Capacity for Evoluon (genec diversity, pre-adaptaon) Low High a Minimal Effect c Phenotypic b Stable d Plascity (Co-)Evoluon Phenotype High Fig. 2 How native species adapt to invaders. The likelihood that a native species will adaptively respond to an invasive species is dependent on the strength and direction of selection imposed by the invader (i.e. the degree to which genotypes are differentially affected), and the capacity of the native for evolution. Four, non-mutually exclusive scenarios are possible. a If the ecological impact of the invader is weak and the adaptive capacity of the native species is low, the invasive will exert little selective pressure on native species primarily through lack of interaction. b Even if the native species has a large capacity for evolution, i.e. a high genetic diversity, pre-adaptation to the invader may result in minimal change in the native phenotype. c Under strong selection pressure and low adaptation capacity, native species are only able to respond using phenotypic plasticity. d Native species with a high capacity for evolution can respond adaptively to high selection pressure. This interaction may be two-sided if the invasive species is similarly capable of adapting. If the native species is able to adapt effectively, and the invader selection remains constant, the impact of the invader will decrease over time (red arrow). However, if the selection pressure is too strong, or the induced phenotype is initially maladaptive, the native species population may decline too rapidly to enable adaptation (blue arrow). Figure modified from Strauss et al. (2006) Native species may respond to the novel selection pressures imposed by invasive species using already existing phenotypes in relevant traits (Strauss et al. 2006). For example, species that respond to broad cues for anti-predator response may be more likely to respond to a novel predator (Nunes et al. 2014). The presence of such pre-adaptations to invader interactions is more common when the two species share some evolutionary history [e.g. through the presence of a phylogenetically similar host plant in the recipient ecosystem (Bezemer et al. 2014; Tuda et al. 2014), or an evolutionary history of exposure to other toxic prey (Llewelyn et al. 2011)]. In these instances, the selective impact of an invader may be small and the invader may readily integrate into the ecosystem without much change in trait frequencies of the native (Strauss et al. 2006; Figs. 2b, 3a). Alternatively, such pre-adaptations may hinder establishment of an invader through competitive exclusion or predation (Levine et al. 2004). In many cases, invasive species represent novel selection pressures for which species are not preadapted (Cox 2004; Strauss et al. 2006). Where the native species possesses inducible or heritable variation in traits that are involved in interaction with invaders, selective pressures imposed by invaders may result in differential fitness of different genotypes and consequently evolution (Fig. 2d; Strauss et al. 2006). The strength of the selection pressure is dependent on trophic relationships (e.g. direct predation risk versus optional novel prey [Robbins et al. 2013)] and extent to which the invader modifies environmental conditions 123 K. Berthon New phenotypic opmum b Phenotypic Change + c a 0 Ancestral phenotype e d – Time Invasion Fig. 3 Adaptive and maladaptive phenotypic changes of species exposed to invaders. a No change in phenotype either because the invader has no impact or because the existing phenotype enables the species to respond adaptively. b Where the ancestral phenotype is below a new phenotypic optimum, the species may adaptively change its phenotype. If this new phenotype is near the optimum there is likely to be no evolutionary change. c Most phenotypically plastic responses are incomplete leading to selection towards the new phenotypic optimum and the most rapid rates of evolution. Alternatively, the species may be trapped into displaying maladaptive phenotypes in the presence of the invader. Species may recover to more adaptive phenotype (e), or ultimately go extinct (d) [e.g. changes in nutrient availability (Reusch and Williams 1998) vs. habitat construction (Wright et al. 2012)] (Fig. 1). The stronger selection is on certain genotypes, the greater the fitness disparity between them, and the more likely it is a native species will evolve (Strauss et al. 2006; Fig. 2). The result of interaction between native and invasive species is not always one sided, with selection favouring both species adapting to better exploit, avoid or outcompete the other (Lankau 2012). In some cases, this interaction may be facultative for the native (Dijkstra et al. 2013) or the invader (e.g. Quinos et al. 1998). Such dynamics are important in determining the success of species invasion (Mooney and Cleland 2001), and the long-term consequences for ecosystem function thereafter (Strayer et al. 2006).This coevolution can lower the selective impact of invaders over time and allow for their integration into the ecosystem to a point where they may be considered ‘native’ (Carthey and Banks 2012). However, the point at which an invader becomes an integral part of an ecosystem is unclear, and any facultative interactions invasives have with native species are hotly debated in light of their frequently negative impacts (Rodriguez 2006). Due to the magnitude of human induced environment changes, many native species are impacted by 123 multiple threats and are already genetically impoverished (Strauss et al. 2006). A low genetic diversity, usually coupled with a low population size, confers a low capacity for an evolutionary response, as genotypes are either equally affected by the selection pressure, or are otherwise dominated by stochastic effects (an ‘ecological response’ as in Strauss et al. 2006; Fig. 2c). For some species, phenotypically plastic responses may aid in buffering against strong negative selection pressure from invaders (Brown 2012; Ghalambor et al. 2007a), as it allows for rapid responses to invasion without underlying genetic variation (Brown 2012; Schlaepfer et al. 2010). If these new phenotypes are close to the new phenotypic optimum (i.e. the fitness of individuals is maintained as close as possible to if not higher than that before invasion, Fig. 3b), phenotypic plasticity can decouple the selected phenotype from genotype frequencies, and may actually prevent formation of evolutionary responses (Schlichting and Wund 2014; Ghalambor et al. 2007a). In reality, most phenotypic responses are incomplete and do not fully restore native fitness in the face of the new invader (Ghalambor et al. 2007b; Fig. 3c). If, within the presence of the invader, the new phenotype has a higher fitness value than the ancestral state (i.e. the phenotype is adaptive), selection may act to increase the efficacy with which a preferred phenotype is produced. Hence, the phenotypic response facilitates a genetic response [‘genetic accommodation’, (Schlichting and Wund 2014; Ghalambor et al. 2007b)]. This may mean a loss of plasticity such that successful phenotypes become fixed in the population (e.g. Scoville and Pfrender 2010). Similarly, increasing instability of environments and species exchanges may favour increased capacity for phenotypically plastic responses [e.g. evolving an increased ability to learn (Caller and Brown 2013)]. Alternatively, phenotypic responses that are reliably produced within the presence of an invader may decrease the fitness of individuals, causing them to become ‘trapped’ into expressing maladaptive traits (Schlaepfer et al. 2005; Fig. 3d). The most commonly studied examples of this are in host plant selection for native insects where invaders are readily colonised but have poor nutritional quality compared to native hosts (e.g. Carroll et al. 1998). This may be because native insects are unable to discriminate between chemical cues of the invader and the native host, a key aspect of host recognition (Fox and Lalonde 1993). Similarly, Mechanistic and trait-based perspectives the similar appearance of the toxic cane toad (Bufo bufo) to native Australian frogs has caused several native predators to be decimated due to misrecognition of prey (Phillips and Shine 2006). In this way, phenotypically plastic responses may lower a species’ capacity for adaptation to invaders by moving the phenotype further from the optimum (Ghalambor et al. 2007b). Ultimately, such ecological traps may lead to extinction (Schlaepfer et al. 2005). However, evidence shows that species may not always be doomed to extinction by such evolutionary traps. For example, soapberry bugs have evolved to exploit novel hosts in spite of initial decreases in fitness (Carroll et al. 1997). Similarly, selection has favoured changes in snake morphology to overcome detrimental impacts of cane toad toxins [discussed below (Phillips and Shine 2006)]. Studies into genetic mechanisms have shown that trait phenotypes evolve independently (Carroll et al. 2003) but may be influenced by other traits (Lau 2008). Therefore, maladaptive phenotypic responses in one trait (e.g. host selection) may act to magnify selection pressure on another related trait (e.g. feeding morphology), thereby potentially increasing the rapidity of evolutionary change and releasing them from the trap (e.g. adoption of a new host) (Fig. 3e). Interactions of plastic and evolutionary changes As phenotypic plasticity and evolutionary responses are not mutually exclusive, there has been some interest in investigating the contribution of plasticity to rapid adaptive evolution (Scoville and Pfrender 2010; Trussell and Smith 2000; Latta et al. 2007; Ghalambor et al. 2007b; Hendry et al. 2008). Several studies have showed the importance of initial plasticity (Carroll et al. 1997; Latta et al. 2007; Nylin et al. 2014; Scoville and Pfrender 2010), but only one study has successfully quantified its relative contributions to adaptive responses in a native species (Latta et al. 2007). This study found that while phenotypic plasticity was important in the early stages of adaptation, it only explained 11 % of the change in phenotype. Similarly, studies of soapberry bugs (Carroll et al. 1997) have showed naı̈ve ancestral populations capable of responding plastically in the same direction as genetically distinct experienced populations. Interestingly, these populations also show a predominance of non-additive genetic mechanisms (Carroll et al. 2003), indicating a strong role of gene expression (and hence plasticity) in adaptation. Further, wider investigations of adaptive evolution have shown a large role of phenotypic plasticity in producing rapid rates of evolution (Hendry et al. 2008). Taken together, this suggests a genetic accommodation mechanism for native species adaptation, where initial phenotypic plasticity determines the evolutionary trajectory of the species (Schlichting and Wund 2014; Ghalambor et al. 2007a). Even in cases of maladaptive plasticity, evolution is rapid (Carroll et al. 2003) and the contribution of plasticity is likely to vary depending on the system (Hendry et al. 2008). Therefore, understanding the time scale of plastic and genetic processes and being able to distinguish between them may allow us to better track the adaptation process in native species and predict species capable of persistence in the face of invaders. Changes to species traits Selective pressure from invasive species alters native species biology in four ways (Fig. 1). Natives may alter their behaviour, morphology, physiology or lifehistory traits in order to reduce contact with or better exploit the opportunities presented by an invader (Strauss et al. 2006). A total of 77 cases of native species adaptation were compiled from searches of online databases and review articles (Table 1). Each native/invasive species pair was treated as a separate case, such that a native and/or invasive species may be involved in several cases. Trends in species trait changes were categorised in relation to different trophic interactions (Fig. 1). Behavioural changes Change in the anti-predator or foraging behaviours of native species is the most commonly studied form of adaptation (Strauss et al. 2006; Fig. 1c). Studies have shown adaptive changes in predator avoidance tactics (Berger et al. 2007; Whitlow et al. 2003; Vanderwerf 2012), dietary preferences (Robbins et al. 2013; Zimmerman 1960; Carroll et al. 1998; King et al. 2006; Dijkstra et al. 2013), prey handling methods (Carlsson et al. 2009; Robbins et al. 2013; Beckmann and Shine 2011; Phillips and Shine 2006), and habitat choice (Vanderwerf 2012). A large amount of this 123 K. Berthon Table 1 List of species found to respond adaptively to invasive species Native Species Non-native Selection Plastic Achilea millefoliuma Achilea millefoliuma Holcus lanatus Thymus pulegioides Centaurea maculosa Thymus pulegioides Thymus pulegioides Cenchrus ciliaris Competition Competition X X X X Strauss et al. in Oduor (2013) Grøndahl and Ehlers (2008) Competition X X Lesica and Atthowe (2007) Competition X X Jensen and Ehlers (2010) Competition X X Jensen and Ehlers (2010) Competition X Agropyron spicatuma Agrostis capillarisb Campanula rotundifollaa Cercidium microphyllum Coregonus hoyib Genetic M B LH P X Eilts and Huxman (2013) Alosa pseudoharengus Bombus tectorum Centaurea maculosa Acroptilon repens Competition X X Competition Competition X X X Competition X X Mealor and Hild (2007) Cirsium arvense Competition X X Ferrero-Serrano et al. (2011) Competition Competition X X X X Cipollini and Hurley (2008) Dostál et al. (2012) Salvelinus fontinalisb Sporobolus airoidesa Alliaria petiolata Impatiens parviflora Medicago polymorpha Alliaria petiolata Bombus tectorum Salmo trutta Oncorhynchus mykiss Catostomus commersoni Acroptilon repens Sporobolus airoidesa Stipa occidentalisa,b Elymus multisetus Festuca idahoensisa, b Hesperostipa comataa Hesperostipa comatea Impatiens capensisa Impatiens nolitangerea Lotus wrangelianusa,b Pilea pumila Poa secundaa Salmo salar Salmo salar Onychomys leucogasterb Anadara trapezia Zostera marina Cirsium canescensb Gentianella campestrisb Lotus wrangelianusb 123 X References Crowder (1986) X X Competition X X Competition Competition Competition Competition X X X X X Competition X X X X Rowe and Leger (2011) Callaway et al. (2005) X Lau (2006, 2008) X Lankau (2012) Goergen et al. (2011) van Zwol et al. (2012) van Zwol et al. (2012) X X X Bourke et al. (1999) Competition X X Cirsium arvense Competition X X Centaurea maculosa Yersinia pestis Competition X X Ferrero-Serrano et al. (2011) and Bergum et al. (2012) Ferrero-Serrano et al. (2011) and Sebade et al. (2012) Callaway et al. (2005) Disease X X Thomas et al. (1988) Caulerpa taxifolia Musculista senhousia Rhinocyllus conicus Livestock Environmental modification Environmental modification Novel herbivore Novel herbivore Novel herbivore Hypera brunneipennis X X X Wright et al. (2012) X Ruesch and Williams (1998) X X Rose et al. (2005) X X X X Lennartsson et al. (1997) X Lau (2006, 2008) Mechanistic and trait-based perspectives Table 1 continued Native Species Thuja plicata b Cassida piperata Colias philodiceb Euphydryas edithab Jadera haematolomab Leptocoris tagalicusb Lycaeides Melissab Ostrinia nubilalisb Papilio zelicaonb Pieris napib Prodoxus quinquepunctellusb Rhagoletis pomonellab Polioptila californicab Alytes muletensisb Amblyrhynchus cristatus Amblyrhynchus cristatus Anolis sagreib Daphnia retrocurvab Daphnia magnab Daphnia melanica Limnodynastes convexiusculus Melanotaenia duboulayi Mya arenaria Mytlius edulisb Nesameletus ornatusb Nucella lapillusb Perameles nausta Perameles nausta Non-native Selection Odocoileus hemionus Alternathera spinosus Medicago sativa Plantago lanceolata Koelreuteria elegans Cardiospermum grandiflorum Medicago sativa Zea maus Ammi visnaga Novel herbivore Novel host Plastic Genetic M B LH X X Novel host Novel host P References X Vourc’h et al. (2001) X X X Dai et al. (2014) X X X X X Tabashnik (1983) Thomas et al. (1988), Singer et al. (1993) Carroll et al. (1997, 1998, 2003) Carroll et al. (2005) Novel host X X X Novel host X X X Novel host Novel host Novel host X X X X Alliaria petiolata Yucca aloifolia Novel host Novel host X X Malus pumila Novel host X Molothrus ater Novel parasite Natrix maura Canis lupus familiaris Felis catus Novel predator Novel predator X X X X Griffiths et al. (1998) Berger et al. (2007) Novel predator X X X Berger et al. (2007) Leiocephalus carinatus Alosa pseudoharengus Introduced fish Salmanoid fish Novel predator X X Losos et al. (2004) Novel predator X X Wells (1970) Novel predator Novel predator X Rhinella marina Novel predator X Bufo marinus Novel predator Carcinus maenus Hemigrapsus sanguineus Salmo trutta Novel predator Novel predator Carcinus maenus Canis lupus familiaris Felis catus Novel predator Novel predator Novel predator X X X X X X X X X X X X X X X Novel predator X X X X Cousyn et al. (2001) Thomas et al. (1988) and Singer et al. (1993) Greenlees et al. (2014) Caller and Brown (2013) X X Smith et al. (1995) Dai et al. (2014) X X Nice et al. (2002) Malausa et al. (2005) Shapiro as in Strauss et al. (2006) Courant (1994) Groman and Pellmyr (2000) Whitlow et al. (2003) Tabashnik (1983) X Mcintosh and Townsend (1994) X X Vermeij (1982) Carthey and Banks (2012) X X Carthey and Banks (2012) X X 123 K. Berthon Table 1 continued Native Species Non-native Selection Plastic Pseudacris regilla Procambarus clarkii Rana catesbeiana Lepomis macrochrius Procambarus clarkii Rana catesbeiana Introduced fish Introduced fish Bufo marinus Novel predator X Novel predator Novel predator Novel predator Pseudacris regillab Rana aurora aurorab Rana aurora aurorab Rana aurorab Triturus alpestrisb Triturus helveticusb Denrelaphis punculatusb Egretta garzetta Haliastur sphenurus Milvus migrans Nycticorax caledonicus Porphyrio porphyrio Pseudechis porphyriacusb Sceloporus undulates Vestiaria coccineab M B X X Pease and Wayne (2014) X X X Chivers et al. (2001) Pearl et al. (2003) X X Pearl et al. (2003) X Novel Novel Novel Novel predator predator predator prey Novel Novel Novel Novel prey prey prey prey X X X X Bufo marinus Bufo marinus Novel prey Novel prey X Solenopsis invicta Novel prey X Feral ungulates Trophic cascades Bufo Bufo Bufo Bufo marinus marinus marinus marinus Genetic X X X X LH X X X X X X P References Kiesecker (1997) Denoel et al. (2005) Denoel et al. (2005) Phillips and Shine (2004) X X X X Beckmann Beckmann Beckmann Beckmann et al. (2011) and Shine (2011) and Shine (2011) et al. (2011) X X X X Beckmann et al. (2011) Phillips and Shine (2004, 2006) X X X X X Robbins et al. (2013); Langkilde (2009) Smith et al. (1995) Native and non-native species pairs, are listed by their trophic interaction as in Strauss et al. (2006). Both phenotypically plastic and genetic responses are given. Responses are marked according to the aspect investigated in each case study and absence of a mark does not indicate the absence of a genetic or plastic response. This list is not exhaustive, but is likely representative of current case studies. This list was used to inform Fig. 1 M morphological trait changes, B behavioural trait changes, LH life history trait changes, P physiological trait changes a References cited in Oduor (2013) b References cited in Strauss et al. (2006) behavioural change appears to result from phenotypic plasticity, primarily the ability of species to learn (Caller and Brown 2013; Brown 2012; Beckmann and Shine 2011; Vanderwerf 2012). Behavioural flexibility enables a rapid response that is often advantageous when a species is presented with the novel challenges of an invader (Brown 2012; Strauss et al. 2006). For example, the endangered Hawaiian forest bird (Chasiempis ibidis) has altered its nest placement behaviour in response to nest predation by introduced black rats (Rattus rattus), which has a clear fitness advantage (Vanderwerf 2012). However, all behaviours have an innate (genetic) component which usually relates to the stimuli required to initiate a particular behavioural response (Brown 2012). Examples may include particular cues 123 for host plant recognition that form the basis of host plant preference (Bezemer et al. 2014; Tuda et al. 2014), or predator recognition (Nunes et al. 2014). These innate elements may restrict the ability of species to respond behaviourally (Brown 2012). For example, species may be unable to recognise chemical cues of a novel predator and hence not display appropriate anti-predator behaviours (Freeman and Byers 2006). The innate elements of a species’ behaviour may be altered under selection (Brown 2012) and there is evidence that this is the case for invasive species responses (Caller and Brown 2013; Bourdeau 2013). For example, Caller and Brown (2013) showed that crimson spotted rainbow fish (Melanotaenia duboulayi) from sites with a history of cane toad invasion had an increased capacity for Mechanistic and trait-based perspectives aversion learning compared to their naive counterparts. Therefore, the capacity for phenotypic plasticity, and hence the tendency to display behavioural responses to novel invaders, can itself evolve (Brown 2012). Most behavioural studies have focused on the ability of species to learn to recognise and respond to predatory cues (e.g. Carthey and Banks 2012) or avoid toxic prey items (e.g. Caller and Brown 2013). The limited empirical evidence for predators responding to exploit novel prey (reviewed in Carlsson et al. 2009) is proposed to be an artefact of the strength of selection pressure. That is, the cost for an organism of losing a foraging opportunity is not as great as losing its life (Beckmann and Shine 2011), leading to a decreased tendency of adaptive change in predators compared to prey. However, a number of insect herbivores have evolved to utilise novel hosts (e.g. Carroll et al. 1998; Tuda et al. 2014) and there is evidence to suggest that vertebrate predators change their foraging behaviour to reduce consumption of prey toxins (Beckmann and Shine 2011; Robbins et al. 2013). Therefore, it is possible that the gap in empirical evidence for predator behavioural changes actually reflects a paucity of data rather than the rarity of the phenomena. Morphology changes Morphological changes in native species have been shown to occur under the same scenarios (such as predator–prey interaction) but are not as commonly documented as behavioural changes (Strauss et al. 2006; Fig. 1c). Similar to behavioural responses, these may be a consequence of phenotypic plasticity (e.g. induced anti-predator defences in the native mollusc, Mya arenaria, Freeman and Byers 2006). However, such changes form a part of organismal development and are often irreversible (Nunes et al. 2014; Freeman and Byers 2006) in contrast to more flexible behavioural plasticity. In some instances, morphological change has resulted in an alteration of native species’ adult body patterns, with a strong genetic basis (Langkilde 2009). For example, consumption of the toxic cane toad Bufo marina has led to selection in Australian snakes for a smaller gape size and longer body length (Phillips and Shine 2006). This new body plan limits the amount of toxin ingested by limiting the size of prey consumed, while increasing the resistance to the toxin based on body size. Similarly, the lizard (Sceloporus undulates), has evolved longer hind limbs in response to fire ant invasion (Langkilde 2009) and soapberry bugs show remarkably rapid changes in beak length to exploit the different fruit sizes of invasive species compared to natives (Carroll 2008; Carroll et al. 1997, 1998). Interestingly, indirect changes in the environment as a result of invasion have also led to altered morphology. For example, the native bivalve (Anadara trapezia) has been shown to have increased shell breadth in response to the altered water conditions produced by the introduced seaweed Caulerpa taxifolia (Wright et al. 2012). Indirect impacts of invaders are hard to predict as well as difficult to study, and therefore present a challenging area of future research. Changes in life history traits Life history strategy changes appear to be important in response to competition from invasive species (Oduor 2013), or predator–prey interactions (e.g. Nunes et al. 2014; Fisk et al. 2007; Fig. 1c). For example, plants often increase their growth rate or reproduction in response to competition (Oduor 2013). Alternatively, altered timing of reproductive events is beneficial in reducing the interaction of natives with their novel predators (Nunes et al. 2014) or competitors (Oduor 2013). Interestingly, life history traits have been found to be more readily modified than morphological traits under altered predation pressure (Nunes et al. 2014). Developmental elements of life history (such as time to maturity and growth rates) are strongly linked to environmental factors and have high fitness value (Fisk et al. 2007; Nunes et al. 2014). Life-history traits, therefore, may be more readily modified because they are under stronger selection pressure and evolve more rapidly. On the other hand, a strong linkage with environmental cues may indicate an important role of phenotypic plasticity in shaping trait optima. Contrastingly, requirements for specific developmental plans may buffer against environmental influence and prevent adaptation (Ghalambor et al. 2007b). Life history traits are often linked to morphology changes, especially body size (Blueweiss et al. 1978). It should not be surprising, then, that species which have altered morphology often have altered life history traits as well. For example, zooplankton display both a smaller body size and the production 123 K. Berthon of fewer, smaller offspring in the presence of a novel predator (Fisk et al. 2007). Physiological changes Physiological changes in response to invasive species are intimately linked to changes in a species’ life history (Nunes et al. 2014), or morphology (Berger et al. 2007, Freeman and Byers 2006) and are hence rarely studied explicitly. However, physiological changes can exist without a visible change in phenotype such as metabolic pathways for increased resistance to herbivory (Lau 2006) or disease (Thomas et al. 1988), and tolerance to allelopathic compounds (Callaway et al. 2005) or toxic prey (Phillips and Shine 2006). In most cases, the chemical mechanisms underlying these processes are poorly understood (Strauss and Agrawal 1999). Future directions Understanding the conditions under which native species respond to invaders is important in understanding the long-term impacts of current invasions (Strayer et al. 2006; Carthey and Banks 2012), as well as determining the future susceptibility of recipient communities to invasion (Mooney and Cleland 2001). Adaptive responses of native species also have economic implications either as pests of commercial crops (e.g. Zimmerman 1960) or as biological control agents (Flower et al. 2014). Though it is now clear that native species can and do respond adaptively to invasive species, little work has been done on the genetic mechanisms underlying these changes (Carroll 2007). It is becoming increasingly apparent that adaptive responses are the result of complex interactions between phenotypic plasticity and evolutionary processes. In many cases evolutionary adaptation is inferred based on population-specific differences in traits without directly testing for underlying genetic changes (Strauss et al. 2006; Fisk et al. 2007). A lack of explicit genetic approaches has led to misinterpretation of phenotypically plastic responses as evolved patterns (Trussell and Smith 2000). Considering the proliferation of cheap and accessible genetic techniques, there is promise that quantitative measures of genetic change may be readily integrated into future studies of native species, as they have been for invasive species 123 (reviewed in Lee 2002). Contrastingly, much has been gained by way of reciprocal transplant experiments (e.g. Lau 2006) and hybridization techniques (e.g. Carroll et al. 2003). Integrating both new and old techniques would allow for investigation of the interaction between phenotypic plasticity and evolutionary responses, and enable direct testing of genetic accommodation hypotheses to progress our understanding of the mechanisms of native species adaptation. In most of the above cases, more than one aspect of a species’ biology is altered in response to invaders. In combination these form complex adaptive strategies. For example, the longer hind limbs of S. undulates individuals is hypothesised to enhance the effectiveness of escape behaviours (Langkilde 2009), while alteration in foraging behaviours (i.e. eating passing fire ants) is likely to reduce incidence of predation at the risk of toxicity (Robbins et al. 2013). An understanding of the physiology underlying these changes may aid in linking these traits in terms of resource or energy trade-offs. However, our knowledge of the mechanisms of physiological change is severely lacking and should be a topic of further research. Finally, multi-species introductions are common and complicate the selective pressure experienced by native species. Additional invaders may modify the interactions between natives and existing invaders (Lau 2008) or between native species (Lankau 2012). However, only one study has directly compared responses of a native species exposed to multiple invasive species (Lau 2008). 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