The paradigm that secondary plant metabolites serve as chemical defence against predators is more or less beyond debate among ecologists and has also been utilized to develop the hypothesis of novel weapons for invasive plants. The spearhead of these putative novel weapons is catechin, a secondary metabolite that we also consume with tea and chocolate. It is the building block of condensed tannins, and also contributes to the dark colour of both mentioned beverages. In the meantime, some papers reporting those toxic properties of catechin that started its career as novel weapon have been retracted.

The paper by Lind and Parker represents a study that offers evidence that patterns of secondary metabolites do not account for the successful establishment of a plant species in the investigated community, neither for native nor for introduced ones. All published correlations of this trait with this phenomenon warrant a careful re-examination if they are not spurious. Moreover, in a broader context, this careful study also challenges the paradigm of secondary metabolites as chemical plant defence itself. The most abundant plants in the investigated community of indigenous and invasive plants were those that caused the least deterrent effects on a generalist herbivore that commonly occurs at this site.

Why do plants accumulate secondary metabolites in their tissues? What is the alternative benefit if it is not chemical defence? One of the papers cited by Lind and Parker (ref. 23) reports that parasitized butterfly larvae utilize pyrrolizidine alkaloids from their food plants that help them to survive those endoparasites, a form of self-medication. Indeed, Forbey et al. (Integr. Comp. Biol. 49:314) suggest that herbivores have developed sensory and behavioural means to exploit secondary metabolites to enhance reproduction, moderate thermoregulation and increase alertness, besides mitigating costs of parasites.

Accordingly, if plant secondary metabolites help the consumer to survive stress situations, why can they not contribute to the survival of of the producing organism too? Herbivory afflicts damage to the attacked tissue. The release of secondary metabolites from their compartments might facilitate a systemic response to survive or even overcompensate the attack. Such a view does not challenge the hypothesis that secondary metabolite contribute to the survival of a plant in an ecosystem, but the underlying mechanism (dx.doi.org/10.2203/dose-response.09-028.Hadacek). Gottfried Fraenkel's contribution to science was to change our view that plant secondary metabolites are waste products. Perhaps it is time to adopt a different view on secondary metabolites than that of efficient chemical weapons against predators? The nice and timely study of Lind and Parker illustrates the present dilemma in terms of developing a universial hypothesis for the accumulation of secondary metabolites in plants by attempting to explain the phenomenon with a bias of an indebatable paradigm.