Optimal Defense Theory in an ant–plant mutualism

Here we take a closer look at a fascinating plant defense article by Calixto et al., which has recently been published in Journal of Ecology.

Author Kleber Del-Claro explains different methods for anti-herbivore plant defenses and shares the results of this research, which is the first to show that ant recruitment via extrafloral nectar follows predictions from Optimal Defense Theory!

Plants and herbivores have interacted for millions of years. Herbivores have evolved to feed effectively, while plants have countered with a wide variety of anti-herbivore defenses. Some of these defenses are continuously expressed in plants across ontogeny (constitutive defenses), whereas other defenses are expressed or increase in expression only after damage or upon risk of damage (inducible defenses). Plant defenses can be also classified as either direct or indirect, with indirect defense represented by associations between plants and their herbivores’ own predators and parasitoids. These associations are mediated by a diversity of plant resources, notably nectar produced in organs located outside of flowers
(extrafloral nectaries – EFNs, Fig. 1).

Figure 1. Extrafloral nectaries in Qualea multiflora (Vochysiaceae). Active extrafloral nectaries a-b) on leaves, and c) on the most basal flower. d) Extrafloral nectaries on the abaxial surface after necrosis (on dead tissues).

Optimal Defense Theory (ODT) is one of the leading theories that seeks to explain patterns of anti-herbivore defense. According to this theory, to minimize costs and maximize defenses against herbivores, plants will allocate their defenses to structures according to their value and probability of attack. In this context, it is predicted that constitutive defenses should be used to protect structures of high value (including buds, flowers and fruits) that are subject to a high probability of attack, whereas induced defenses should be used in structures of low value (such as fully expanded leaves) that are subject to a low probability of attack. However, only recently have studies experimentally examined indirect defenses (i.e., ants, Fig. 2) such as ant-mediated defense in the context of ODT.

Figure 2. Ectatomma tuberculatum ant on Qualea multiflora, with a droplet of nectar between its mandibles . Photo: Denise Lange.

A few studies have successfully tested ODT predictions as a way of measuring ecological investment in EFN-mediated ant-plant interactions. However, while these studies have considered patterns in the production of extrafloral nectar, they have not looked at ant attraction to the EFNs. Extrafloral nectar is the resource provided by plants to mediate the interaction between defensive ants and host plants, but it is essential to assess ant attendance to fully evaluate ODT’s predictions concerning indirect defenses. In our study, we compared extrafloral nectar production and ant attractiveness to EFNs located on reproductive (Fig. 1c) versus vegetative (Fig. 1a) plant structures on Qualea multiflora plants subjected to simulated herbivory.

Figure 3. Summary of nectar production (volume and calories) by extrafloral nectaries (EFNs) on leaves and inflorescences of Qualea multiflora, and of ant abundance at nectaries, under control conditions and after simulated herbivory.

Our results, summarised in Fig. 3, showed that ants attending host plants are also influenced by the plant structure providing the food resource, as well as by the presence and level of damage to these structures. EFNs on vegetative and reproductive plant structures present distinct patterns of secretion: the former exhibit low levels of extrafloral nectar production, whereas the latter present high levels of production. However, extrafloral nectar in both cases can be induced. They respond in different ways, consistent with the value of the plant structure as well as the damage level. Thus, our study directly contributes to the understanding of how indirect plant defenses are allocated.

Kleber Del-Claro Universidade de São Paulo & Universidade Federal de Uberlândia, Brazil

You can find videos featuring Kleber Del-Claro’s research on his website Ciência que nós fazemos: https://www.cienciaquenosfazemos.org/videos

The full article is available online: Optimal Defense Theory in an ant–plant mutualism: Extrafloral nectar as an induced defence is maximized in the most valuable plant structures

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