Invasive success and the evolution of enhanced weaponry

Abstract

A key hypothesis that has been proposed to explain plants’ invasive success suggests that some invasive species produce allelochemicals that are novel against naïve neighbours at the introduced range and therefore provide an advantage there (novel weapons hypothesis – NWH). However, a seldom‐studied hypothesis suggests that invasive populations could not only possess novel weapons, but might also evolve their enhanced production. Moreover, so far no study has examined both the novelty and evolution of allelopathic effects. Here, we examined these two hypotheses in a set of experiments with the highly invasive plant Impatiens glandulifera. In the first experiment, we examined the evolution of allelopathic ability by comparing the inhibitory effects of leaf extracts from native versus invasive I. glandulifera on the germination success of its dominant neighbour Urtica dioica. In the following experiments, we examined the NWH by comparing the germination success of U. dioica seeds collected at the native versus invasive range of I. glandulifera, in response to either leaf extracts or soil trained with invasive I. glandulifera. The results of the first experiment indicate that invasive I. glandulifera exert a stronger inhibitory effect on the germination of U. dioica compared to their native counterparts, providing support for the hypothesis that allelopathic ability can evolve at the invasive range. However, the results of the two following experiments reveal no difference in the response of U. dioica from the native versus invasive range of I. glandulifera, to the allelopathic effects of either the leaf extracts or the trained soil. These results therefore do not provide support for the NWH, and suggest that increased allelopathy in invasive I. glandulifera might have been selected for by other processes. The results of this study call for biogeographical experiments that will examine not only the novelty but also the evolution of allelopathic effects in invasive plants.

Evolution of plant defences along an invasion chronosequence: defence is lost due to enemy release – but not forever

“…Finally, we measured herbivore load in the different populations within the invasive range. We predicted that (i) invasive populations will exhibit decreased herbivore resistance compared to native populations (expressed as increased damage by a herbivore with no common evolutionary history), but these differences will diminish in older populations, due to an increase in herbivore load, and a subsequent evolution of increased resistance; (ii) invasive populations will exhibit enhanced allelopathic ability compared to native populations (expressed as greater inhibition of germination of the co‐occurring species), as has been shown in a previous study (Gruntman, Zieger & Tielbörger ), but these differences will also diminish in older populations; (iii) herbivore resistance and allelopathic ability will be negatively correlated across I. glandulifera individuals due to an allocation trade‐off that could drive their evolutionary shifts (Bossdorf ; Uesugi & Kessler ); (iv) contrary to their predicted reduced damage by a herbivore with no common evolutionary history (prediction i), older populations will incur greater damage by local herbivores that will become adapted to the invasive stands with time and colonize them.…”

“…It is possible that the lack of strong differences found in allelopathic effect in this study might be attributed to our use of senescent rather than young green leaves, the latter of which have been commonly used in previous allelopathy studies of I. glandulifera (Vrchotová, Sera & Krejcova ; Ruckli et al . ; Gruntman, Zieger & Tielbörger ). Although senescent leaf leachates can provide a more realistic estimate of the way allelochemicals are released into the environment from leaf litter (Inderjit & Callaway ), Ruckli et al .…”

“…Recently, evolutionary shifts have also been shown to encompass increases in allelopathic ability (Zheng et al . ; Gruntman, Zieger & Tielbörger ), suggesting that release from native enemies can select for increased competitive ability not only via increased investment in size but also by means of increased allelochemical production (Inderjit et al . ; Bossdorf ).…”

“…Moreover, recent studies suggest that invasive populations of this species have evolved enhanced allelopathic ability compared to populations in the native range (Gruntman et al . ; Gruntman, Zieger & Tielbörger ).…”

“…These ecological processes have in turn been proposed to select for increased competitive ability via evolutionary shifts in resource allocation patterns, particularly the shift from herbivore defence to increased size (evolution of increased competitive ability (EICA) hypothesis: Blossey & N€ otzold 1995;Joshi & Vrieling 2005). Recently, evolutionary shifts have also been shown to encompass increases in allelopathic ability (Zheng et al 2015;Gruntman, Zieger & Tielb€ orger 2016), suggesting that release from native enemies can select for increased competitive ability not only via increased investment in size but also by means of increased allelochemical production Bossdorf 2013). Such enhanced allelopathic ability can be selected for either due to an allocation trade-off with decreased herbivore resistance (Uesugi & Kessler 2013) or due to the adaptive advantage it provides against na€ ıve neighbours (Zheng et al 2015).…”