Optimal strategy of plant antiherbivore defense: Implications for apparency and resource‐availability theories

Abstract

Plants produce chemicals as methods against animal herbivory. Such chemical defenses are dassified into two major categories: (i) quantitative defenses with massive production of indigestible substances; and (ii) qualitative defenses with production of poisonous substances. A mathematical model was developed that identified factors that favored the evolution of quantitative defenses. Selecting an annual plant for simplicity, the allocation of photosynthetic production between growth substances and defense substances was considered. If the plant invests more in defense substances, it can protect itself more efficiently from herbivory but with a reduced growth rate. If it invests more in growth substances, the contrary holds. Using Pontoryagin's maximum principle, the following results were obtained: (i) the plant should conduct quantitative defenses when the growth rate (G), reflecting resource-availability, is low and the growth period (T) is long as well; (ii) if the plant invests in quantitative defenses, the optimal proportion of defense substances (x*) should be higher as G is smaller, but it is independent of T; and (iii) the value of x * is not monotone for the effectiveness of defense substance (A), but has a maximum at an intermediate value of A. Predictions of the modal partly supported both Feeny's apparency theory, claiming that apparent plants or their parts for herbivores should quantitatively defend themselves, and Coley's resource-availability theory, claiming that plants with rich resources should invest in growth rather than defense.

Optimal Distribution of Defences: Are Plants Smart Investors?

“…In accordance with these theories, our model also predicts that more effective defences should be produced in smaller amounts than less effective defences. Although this result seems obvi ous, the opposite (the more effective the defence, the more should be produced) might hold in other parts of the parameter space, especially at very low effective ness of the defence chemical and high herbivory pres sures relative to growth rate (Yamamura & Tsuji 1995).…”

“…Iwasa & Roughgarden (1984) showed that usually the alloca tion schedule that realizes the fastest growth over an indefinite period, is also the one that maximizes repro ductive success, even if the growth period is not very long. Similarly Yamamura & Tsuji (1995) showed that the optimal allocation to defense is constant and is not affected by the length of the growing season (their fig. 2C).…”

“…If plants grow slowly they should allocate more to defense (Table 1). Similarly, if the relative growth of a plant is high early in life and low late in life, it is optimal for a plant to allocate less to defense when young and more when old (discussed in Yamamura & Tsuji 1995). We would expect this result to hold when growth is not exponential.…”

“…Most theories of optimal defence against herbivores compare whole plants that belong to different plant species (Feeny 1976;Coley, Bryant & Chapin 1985;Simms 1992;Yamamura & Tsuji 1995). However, as Janzen (1979) noted, herbivores do not eat 'Latin binomials' but usually consume specific plant parts.…”

Does plant apparency matter? Thirty years of data provide limited support but reveal clear patterns of the effects of plant chemistry on herbivores

“…Consequently, this hypothesis of plant defense has been dismissed as lacking generality and being ineffective in application (Bernays, 1981;Duffey & Stout, 1996;Agrawal et al, 2006). Nevertheless, it is still prominent in the literature, and a modified plant-apparency hypothesis has not been proposed (Haukioja, 2003;Yamamura & Tsuji, 1995;Loehle, 1996;Silvertown & Dodd, 1996;Bustamante et al, 2006). Although it is clear that many exceptions, discrepancies, and faults can be found in this hypothesis, a more quantitative approach is needed to objectively refute or revise it.…”