Herbivory and Plant Defenses in Tropical Forests

Abstract

In this review, we discuss the ecological and evolutionary consequences of plantherbivore interactions in tropical forests. We note first that herbivory rates are higher in tropical forests than in temperate ones and that, in contrast to leaves in temperate forests, most of the damage to tropical leaves occurs when they are young and expanding. Leaves in dry tropical forests also suffer higher rates of damage than in wet forests, and damage is greater in the understory than in the canopy. Insect herbivores, which typically have a narrow host range in the tropics, cause most of the damage to leaves and have selected for a wide variety of chemical, developmental, and phenological defenses in plants. Pathogens are less studied but cause considerable damage and, along with insect herbivores, may contribute to the maintenance of tree diversity. Folivorous mammals do less damage than insects or pathogens but have evolved to cope with the high levels of plant defenses. Leaves in tropical forests are defended by having low nutritional quality, greater toughness, and a wide variety of secondary metabolites, many of which are more common in tropical than temperate forests. Tannins, toughness, and low nutritional quality lengthen insect developmental times, making them more vulnerable to predators and parasitoids. The widespread occurrence of these defenses suggests that natural enemies are key participants in plant defenses and may have influenced the evolution of these traits. To escape damage, leaves may expand rapidly, be flushed synchronously, or be produced during the dry season when herbivores are rare. One strategy virtually limited to tropical forests is for plants to flush leaves but delay "greening" them until the leaves are mature. Many of these defensive traits are correlated within species, due to physiological constraints and tradeoffs. In general, shade-tolerant species invest more in defenses than do gap-requiring ones, and species with long-lived leaves are better defended than those with short-lived leaves. 305

Effects of leaf herbivory on the bromeliad Aechmea blanchetiana: a study of selective feeding by the scraper Acentroptera pulchella

“…While the median leaf nitrogen content for angiosperms usually ranges between approximately 1% and 5% (Mattson 1980;Lorenzo et al 2010), that of the same nutrient in A. blanchetiana leaves is considerably lower, varying from 0.4% to 0.6%. Some studies have shown that nitrogen concentration is significantly higher in younger leaves, rendering them more palatable than adult ones (Coley & Barone 1996;Nichols-Orians & Schultz 1990). Our results show the opposite in that the attacked distal portions of mature and old leaves of A. blanchetiana presented higher nitrogen content than the soft younger leaves.…”

“…Intraspecific preferences of herbivores are commonly related to within-plant heterogeneity of mechanical, morphological, chemical and nutritional properties (Coley 1983;Coley & Aide 1991). Also, biotic (e.g., previous damage by herbivores) and abiotic (sun and shade leaves) effects, in addition to leaf rolling and tissue development stage or age (e.g., younger versus older leaves), can affect foraging behaviour (Coley et al 1985.…”

“…Moreover, younger leaves are usually more attacked than older leaves, as they have a higher concentration of water and nutrients in a softer structure (Coley et al 1985). This could occasionally be the opposite when less attacked younger leaves present a higher concentration of chemical defences (e.g., tannins) than older leaves (Coley & Aide 1991;Lokvam & Kursar 2005). In our study, however, the total phenolic content does not seem to work as an anti--herbivore defence in young leaves, as for these compounds higher values were found on the herbivore-damaged mid--upper leaf portions.…”

Comparação da morfologia foliar de espécies arbóreas de três estádios sucessionais distintos de floresta ombrófila densa (Floresta Atlântica) no Sul do Brasil

“…Essa diferença de espessura é comumente considerada uma prevenção contra a transpiração (Larcher 2000), uma vez que todas as folhas estudadas encontram-se expostas à radiação solar plena. Além disso, a cutícula espessa pode funcionar como um repelente de água, auxiliando na secagem mais rápida da folha (Smith & McClean 1989) e como defesa contra a herbivoria, pois a cutícula incrementa a espessura da folha, contribuindo com a sua textura coriácea (Coley & Barone 1996).…”

Tannins in Baccharis dracunculifolia (Asteraceae): effects of seasonality, water availability and plant sex

“…The concentration of tannins in plant tissue may be determined by many factors, such as environmental conditions (i.e., water and nutrient availability; Herms & Mattson 1992;Shure et ai. 1998) and parameters related to the plant, mainly plant phenology (Feeny 1970;Espírito-Santo & Fernandes 1998), sex (Jing & Coley 1990;Herms & Mattson 1992), and age (Coley & Barone 1996).…”