ecology of phasmids - KLUEDO - Universität Kaiserslautern

Community structure & host range 10
1973; Janzen & Schoener 1968; Smiley 1978; Willig et al. 1986, 1993; Willig & Camilo 1991; Braker
1991).
Studies assessing diversity and host specificity of herbivores are often descriptive and frequently based
on collection records (e.g. Wood & Olmstead 1984; Janzen 1988; Hodkinson & Casson 1991).
Conclusions from such approaches on specialization of herbivores are problematic because they
generally are based on the counts of occurrence of insect herbivores on different plant species.
Therefore, they depend on the extent of records (Barone 1998). With increasing sample size, there is an
inherent increase in plant species observed for a particular herbivore species. But more importantly,
collection records do not allow to differentiate if a plant species really is part of an insects diet or not
(Basset 1997; Novotny & Basset 2000). Therefore estimates of resource use from field observations
represent the multidimensional realized niche of herbivores, and plant records may diverge into
dimensions like food, mating, oviposition, or just transience (Begon et al. 1996).
One possibility to separate a herbivore’s food niche from other niche dimensions, and thereby giving a
more accurate estimate of specialization levels, is combining field records with feeding trials (e.g.,
Basset 1996, 1999; Basset et al. 1992, 1996; Barone 1998; Novotny et al. 2002a, 2002b). Feeding trials
result in an estimate of the absolute food niche of a herbivore (Krebs 1989). This is the potential host
range of a herbivore when released from biotic or abiotic factors which limit the use of palatable plants
under natural conditions. Information about the absolute food niche together with information of field
collection records (representing the multidimensional realized niche) allows approaching the realized
food niche. Differences between absolute and realized food niche then give a first insight into the
significance of resources, competition and predation in shaping plant-herbivore systems.
When discussing the factors that may have led to the observed restriction in using food resources,
evolutionary and ecological processes have to be considered. A first insight into host range evolution
can be gained by applying conventional definitions of host plant range on the results of feeding trials
(representing the absolute food niche). According to Bernays & Chapman (1994) the categories of host
range usually are recognized as: (1) monophagous, i.e. feeding on plants within a single genus; (2)
oligophagous, i.e. feeding on plants within a single family; and (3) polyphagous, i.e. feeding on plants
from different families. This concept refers to taxonomic relationships of host-plants and is based on the
classical theory that the capability of insects to handle allelochemicals of other plant taxa declines with
their increasing phylogenetic distance to the original host plant taxa (Ehrlich & Raven 1964).
On the other hand, restrictions in insect herbivore host range may be associated with chemical plant
properties that are not exclusively governed by phylogeny. For example, according to current plant
defense theories, mature leaves of shade-tolerant, slow growing plant species (persistent) are better
defended than mature leaves of shade-intolerant, fast growing plant species of gaps and forest edges
(pioneers) (Coley 1983; Coley et al. 1985) and hence are less digestible. Consequently, a generalist
herbivore should preferentially feed on leaves of pioneer plant species (Berenbaum et al. 1984).