ecology of phasmids - KLUEDO - Universität Kaiserslautern

Introduction 1
1 Introduction
1.1 General aspects
Tropical rainforests are renowned for their vast biodiversity and high productivity. For understanding of
the factors that structure these communities and maintain ecosystem stability, the relationships among
trophic levels are of particular interest. Increasing attention has been paid to plant-animal interactions,
and herbivory is discussed as a key agent in maintaining forest dynamics and ecosystem stability
(Lowman 1984; Brokaw 1985; Schowalter et al. 1985; Brown et al. 1987). In tropical forests,
herbivores consume up to 11 percent of annual leaf production (Coley & Barone 1996). Such strong
pressure is thought to select for the evolution of plant defenses and high plant biodiversity (Ehrlich &
Raven 1964; Janzen 1970; Connell 1971; Rhoades & Cates 1976; Coley et al. 1985). In return, high
variety and high regimes of plant defenses are thought to favor high diversity of herbivores in the
tropics (Janzen 1973; Coley & Aide 1991). Alternatively, natural enemies may account for selection
towards narrow niches of their prey and thereby promote high diversity of tropical faunas (Pianka 1966;
Price et al. 1980; Bernays & Graham 1988). Whether populations at different trophic levels are limited
by resources (bottom-up, sensu Murdoch 1966; White 1978; McNeill & Southwood 1978) or due to
consumption by higher trophic levels (top-down, sensu Hairston et al. 1960) is subject to ongoing
debate.
Given the high biodiversity and productivity of tropical rainforests, it is most likely that processes that
structure tropical communities may be fundamentally different than processes in temperate ecosystems.
As a consequence, ecological paradigms developed in temperate systems may not be useful for
understanding tropical systems (Dyer & Coley 2001). In fact, relationships among trophic levels seem to
be converse depending on latitude. Tropical plants compared to temperate species seem to be richer in
plant defenses both in variability and amounts (e.g., Coley & Kursar 1996). But the impact of plant
defenses appears to be greater in temperate regions (Coley & Barone 1996). The opposite pattern
emerged for predators. There is evidence that the impact of predators on herbivores is stronger in the
tropics than in the temperate zone (reviewed in Dyer & Coley 2001). However, there is consensus that
tropical herbivores are kept at bay by both bottom-up and top-down regulation mechanisms (Pace et al.
1999; Persson 1999; Polis 1999). In other words: herbivores are between the devil (natural enemies) and
the deep blue sea (poor food) (Lawton & McNeill 1979). In the debate about the relative roles of
resource limitation and predators in controlling tropical herbivore populations, evidence suggests that
natural enemies may be the major control factor (Dyer & Coley 2001).