Induced resistance and biological invasions
In contrast to the expectations of the BRH, the large pine weevil consumed more on the native P. pinaster than on the exotic P. radiata when both species were offered as cut material in in vitro bioassays. As cut stem twigs are unable to respond to the damage caused by the weevils, results of the in vitro bioassays likely reflect the variation in preference because of constitutive resistance between the two species, which appeared to be higher in the exotic than in the indigenous pine species. However, under field conditions, damage by the pine weevil was clearly greater on the exotic than on the native species, suggesting lower effective resistance of the former. Both species were planted simultaneously, and the pine weevil attack started early after planting and lasted several months until weevil damage was assessed. During this time, weevils were continuously feeding on the pine seedlings, which should have had enough time to respond by activating induced resistance mechanisms. As an evidence of this, the resin canal density in the xylem of the P. pinaster seedlings in the attacked stand was nearly twice as high as that found in the nearby unattacked stand; the differences were attributed to the defensive response induced by the weevil damage (Moreira et al. 2008b).
The results of our induction experiment revealed large differences between the species in the induction of resin in the stem after wounding. Although the damage inflicted by the weevil during the induction experiment did not differ between pine species, the increment in the resin content in the indigenous species just 72 h after insect damage was more than 2-fold than in the nonindigenous species. Moreover, differences between species in this main resistant trait were only significant after exposure to the insect. Conversely, the response to the application of MeJa did not differ between the species, suggesting similar MeJa-inducible physiological capabilities (i.e. inducibility potential) between the two species. Based on the differences between the response to the pine weevil and to the MeJa application, we infer, as predicted by the BRH, that the native pine species is better adapted to this local insect, being able to elicit a stronger induced resistance mechanism that likely will deter further damage. These results suggest that because of the greater induced resistance in the native species, the initial preference of the insect for the native pine could switch with time towards greater damage on the exotic, as observed in the field. The lack of preference between the two pine species shown by the weevil in the induction experiment does not conflict with this idea, as the period of time that the weevils were feeding on the seedlings (just 72 h) was probably insufficient for the induced defences to become effective against the weevil.
Induced resistance is a key strategy for defending Pinus sp. against bark beetles and phloem feeders (Lieuter et al. 2004; Franceschi et al. 2005) and varies greatly among species of the Pinaceae (Hudgins, Christiansen & Franceschi 2004). Plants are known to be able to recognize specific cues of their enemies and elicit the appropriate defensive mechanisms (Heil 2009; Hilker & Meiners 2010). Thus, plant species can respond specifically to different enemies (Lieutier 2002), with larger responses of the trees to the enemies that normally attack them (Raffa 1991). Results presented here are in concordance with this cue specificity of the induced responses, which were more intensely elicited in the pine species with a common evolutionary history with the enemy.
Empirical evidences of the ability of plants to recognize different herbivorous species and elicit differential responses against them are, however, equivocal (Heil 2009). For example, the specificity of the tree response to the bark beetle–fungus association may be more related to protein–protein interactions with the fungus species than to plant recognition of the insect species (Karban & Kuc 1999). Hylobius abietis could also vector different native pathogenic fungus species to which P. radiata may be not adapted, but information on the transmission of diseases by weevils is very limited (Lieuter et al. 2004).
Although other factors (e.g. the environmental and ontogenetic differences between the field trials and the bioassays with greenhouse potted plants) may have differentially affected the results, our findings indicate that the intensity and/or rapidity of the induced responses to a native insect can differ between native and non-native host species. If induced defences are more specific mechanisms than constitutive resistance (Franceschi et al. 2005), the differences in inducibility between native and exotic species may be relevant for understanding the role of local enemies in invasion ecology. The lower inducibility of the exotic pine species supports the predictions of the BRH (Elton 1958), as it could imply greater damage in the field, which could contribute towards preventing the exotic pine from becoming invasive in the area. As pointed out by Orians & Ward (2010), however, little attention has been paid to the implications of induced resistance in invasion ecology. To date, only two studies have examined induced defences in exotic plants, comparing patterns of inducible defences in native and introduced ranges (Cipollini et al. 2005; Eigenbrode et al. 2008). These studies explore how induced defences have evolved after the relaxation of herbivore pressure in the exotic range, using either chemical elicitors or mechanical wounding to assess inducibility. However, responses to specific herbivore species can potentially differ from those to artificial induction treatments (as observed in the present study), and these differences could be highly relevant in the context of invasion ecology. To our knowledge, results presented here are the first reporting how failure to elicit effective induced responses against local herbivores might be a factor in impeding an introduced species from becoming invasive.
Tolerance in the context of biological invasions
Differences in tolerance to herbivore damage can also be important in interpreting the invasion ecology hypotheses (Ashton & Lerdau 2008; Chun, van Kleunen & Dawson 2010). Besides the release from specialist enemies, the ERH also assumes a lower impact of generalist enemies on the introduced species, because native consumers will be better adapted to consume native, rather than exotic hosts (Keane & Crawley 2002). According to the BRH, however, the exotic hosts may be either more heavily damaged by the resident herbivores than native plants or just poorly adapted to tolerate the damage caused by novel enemies with which they lack an evolutionary history (Parker & Hay 2005). This idea is well accepted among foresters, who commonly consider that trees can be more susceptible to pests and pathogens when they are planted off-site, i.e. outside the soil, climate system and biotic interactions to which they are adapted (Lombardero, Vázquez-Majuto & Ayres 2008).
In support of the BRH, we found that the native pine was more tolerant of the coevolved herbivore than the exotic pine, suggesting no fitness advantage of the exotic pine species in its introduced area. For the same level of insect damage, survival of P. pinaster families was higher than that of P. radiata, indicating a lower impact of the herbivore on the former. Fitness evaluation is especially complicated in forest trees with long lifespans and reproduction. However, early survival is a clear component of pine fitness, and thus the relationship between survival and damage can be utilized to assess tolerance to the attack. Early growth in light-demanding species such as the pines studied is another component of plant fitness, which evaluates the opportunity of a tree to arrive to the dominant stratus, and thus reproduce and contribute to the next generation. Slow-growing pines are expected to be suppressed by dominant trees and die. Considering early growth, the native P. pinaster appeared to be, again, more tolerant than the exotic P. radiata. Although the pine weevil reduced the expected regular size of P. pinaster during the immediate following years after the attack, the positive relationship between initial weevil damage and relative medium-term growth reveals that this pine species was able to likely compensate after just 5 years. In contrast, in P. radiata, neither immediate consecutive growth nor medium-term relative growth was related to initial damage, suggesting no compensatory growth capability in the exotic pine. The differences in the growth trends in the attacked and unattacked stands were in concordance with this finding, although the results from this comparison should be interpreted with care as the two sites may not have been equally suitable for the two pine species (see Appendix S1 in Supporting Information).
The detection of intraspecific genetic variation in tolerance of P. pinaster to the insect supports the possibility of evolution of tolerance traits as an adaptation to this (and other) local insects. The cost–benefits balance, together with the temporal and spatial heterogeneity in the selective pressure imposed by these (and other) insects, would explain why these apparent adaptive traits are not fixed within the native population and genetic variation for tolerance is maintained.
The lower tolerance of the exotic pine species suggest poor adaptation to its new range, which is in consonance with the predictions of the BRH. In a recent study, Lombardero, Vázquez-Majuto & Ayres (2008), studying the impact of the local bark beetle Tomicus piniperda on the same pine species in the same region, also found the native P. pinaster to be more tolerant than the exotic P. radiata. Although the insect damage was more than twice as great in the native P. pinaster as in the exotic P. radiata, the productivity of the exotic was more adversely affected, suggesting higher tolerance of the native species. The results were consistent across a wide range of tree ages and beetle incidence levels. Additionally, productivity of the non-native P. radiata was further reduced by the pathogen Sphaeropsis sapinea, a fungus that seems to be associated with T. piniperda and was only present in the exotic pine.
Few studies have considered the importance of tolerance in invasion ecology (Chun, van Kleunen & Dawson 2010). After the formulation of the evolution of increased competitive ability (EICA) hypothesis (Blossey & Notzold 1995), however, it has been postulated that the invasiveness of exotic species might be related to increased tolerance of the introduced populations as a result of a relaxation in herbivore pressure in the new range (Stastny, Schaffner & Elle 2005; Zou, Rogers & Siemann 2008). However, P. radiata was introduced in Spain very few generations ago, so few opportunities to evolve tolerance are expected in this species. Besides, the lack of genetic variation in tolerance found in the present study would further constrain the possibility to evolve towards higher tolerance. As stated by Lombardero, Vázquez-Majuto & Ayres (2008), the most obvious implications of EICA in forestry would be for tree breeding programmes, because natural selection is constrained in trees by their long generation times.
In summary, we found that P. pinaster and P. radiata coexisting in NW Spain differed markedly in their responses to a native insect herbivore. Although the insect intrinsically preferred the native pine species with which it shared an evolutionary history, the native P. pinaster responded more strongly to the attack and was more tolerant of it than the exotic P. radiata. Thus, in this area, the exotic P. radiata shows no advantages compared with the native pine against this enemy. These findings are in agreement with the fact that P. radiata is not invasive in this area (Carrillo-Gavilán & Vilá 2010). Other authors have already suggested that the failure of invasions by alien conifers in Central Europe might be, among other reasons, because of the presence of pathogens and phytophages in the introduced range because of the presence of native conifer congeners (Adamowski 2004; Carrillo-Gavilán & Vilá 2010). Indeed, exotic conifers in Europe are known to be colonized by as much herbivore insect species as native conifers (Bertheau et al. 2009), and the majority of native insects species that colonize alien trees are generalist herbivores (Bertheau et al. 2010). Additionally, there is some evidence that herbivory by generalist vertebrates might limit the invasiveness of P. radiata in some areas of the Southern Hemisphere (Becerra & Bustamante 2008). However, that P. radiata might show signs of invasiveness in Europe in the absence of herbivory and that this pine has become invasive in other parts of the world because of escape from herbivory remain untested. Based on the results presented here, the weak ability to elicit induced defences and the low tolerance of the exotic species against native herbivores appeared as key concepts for understanding how native enemies may contribute to limit the invasiveness potential of exotic plants, as stated by the BRH. Further studies are needed to confirm this by testing whether P. radiata may show signs of invasiveness in the absence of herbivory in Europe.