ecology of phasmids - KLUEDO - Universität Kaiserslautern

Introduction 2
Regardless whether strong impact from higher or lower trophic levels limits herbivore populations,
current theories assume that this selection pressure has led to narrow niches (Pianka 1966; MacArthur &
Wilson 1967). Since food is one of the most important dimensions of the niche (Krebs 1989), niche
breadth is generally expressed in terms of feeding specialization of herbivores (Bernays & Chapman
1994). This debate is of broad interest because estimates of global biodiversity are largely based on the
degree of specialization of tropical insects. Just recently global biodiversity estimates were corrected
from formerly stated 31 million (Erwin 1982) to about 4 to 10 million species (Ødegard 2000; Novotny
et al. 2002a) because increasing research effort indicates that specialization is less pronounced than
formerly supposed (Basset 1996, 1999; Basset et al. 1992, 1996; Barone 1998; Novotny et al. 2002a).
Generally, these studies state that most tropical insect herbivores are rare (Basset 1996, 1999; Basset et
al. 1992, 1996; Barone 1998; Novotny et al. 2002b), but they represent the plants view of herbivore
specialization, i.e. the herbivore communities of one or several plant species are sampled, and thus may
cover only a fraction of density and distribution patterns of insect herbivores.
There is an enormous lack of research which addresses the densities of insect communities, let alone the
densities and distribution of single insect herbivore species in tropical forests. The scarce information on
this topic derives from studies using trap techniques (Wolda 1978, 1980, 1983, 1992; Smythe 1982), or
describing the overall arthropod community (Janzen & Schoener 1968; Elton 1973). Beyond doubt,
these studies contributed important baseline data for the development of general ideas, but they either
incorporate species particularly prone to a certain trapping device, or all insect feeding guilds.
Surprisingly few studies have reported the most basic demographic parameters for tropical insect
herbivores, distribution and population density (Smiley 1978; Willig et al. 1986, 1993; Willig & Camilo
1991).
1.2 Current knowledge about tropical phasmid ecology
Although phasmids are common herbivores in many tropical ecosystems, little is known about the
biology of these hemimetabolous insects (Bedford 1978; Van den Bussche et al. 1989; Willig et al.
1986, 1993). The order Phasmatodea (walkingsticks and leaf insects) is one out of nine orders of
phytophagous insects and contains approximately 3000 described species that occur worldwide with a
concentration in tropical regions (Whiting et al. 2003). Generally, phasmids are considered as herbivore
generalists although empirical studies on diet breadth of natural populations are missing (Bedford 1978;
Willig et al. 1986, 1993; Sandlin & Willig 1993).
The significance of phasmids as herbivores has attracted some interest as they can reach plague numbers
and may cause severe damage to forests (temperate forests: Graham 1937; Australian Eucalypt forests:
Campbell 1960, 1961, 1974; Campbell & Hadlington 1967) or to agricultural plantations (Pacific
coconut plantations: Paine 1968; Swaine 1969). Such herbivores exerting strong pressure may be key