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