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 28<br />
acceptability. In contrast, B. ploiaria and O. martini that fed on plants from a number <strong>of</strong> different<br />
families (meaning they are polyphagous) most likely employ a number <strong>of</strong> different cues. The latter also<br />
applies for M. diocles whose diet included members <strong>of</strong> the Araceae and Piperaceae; two plant families<br />
that are not closely related (Judd et al. 2002). Such a feeding pattern <strong>of</strong> an insect that feeds on a small<br />
number <strong>of</strong> plants from different unrelated families is <strong>of</strong>ten described as disjunct oligophagous (Bernays<br />
& Chapman 1994).<br />
With its relation to plant taxonomy, the above definitions <strong>of</strong> host range involve conclusions regarding<br />
evolutionary processes. Monophagous species like Otocrania sp. are thought to be driven to<br />
specialization by physiological adaptations to host plant chemistry (Futuyma 1991). This may not apply<br />
for the disjunct oligophagous M. diocles. This term does not include any information on the connection<br />
<strong>of</strong> used host plants and Bernays et al. (1989) suggested that specializing on distantly related hosts<br />
indicates that factors such as competition or predation may have selected for restriction in host range.<br />
But what are the reasons that the realized food niches <strong>of</strong> <strong>phasmids</strong> (demonstrated by the overlap region<br />
in Figure 2-4) were smaller than their absolute food niches assessed in feeding trials? In other words:<br />
why did <strong>phasmids</strong> not use all available hosts that were confirmed edible in feeding trials? An<br />
explanation <strong>of</strong> these discrepancies between absolute and realized food niche may be found in biotic<br />
factors in the natural setting. For example, the observed distribution pattern <strong>of</strong> an herbivore among host<br />
plants can result from density dependent food selection, from food selection due to nutritional<br />
constraints, or from varying predation pressure on different hosts (Bernays & Chapman 1994). More<br />
specifically, Willig et al. (1993) found that <strong>phasmids</strong> modulated host selection to density <strong>of</strong> their host<br />
plants rather than to preference. The studied phasmid L. portoricensis occurred mainly on Piper<br />
treleaseanum, the least preferred plant species in feeding trials but the most abundant in the field.<br />
Sandoval (1994) proved that predation pressure can narrow down host use <strong>of</strong> phasmid nymphs to plant<br />
species where they suffered the least predation-related mortality. Such processes, for example, may<br />
explain why O. martini in line-transects never was found on Byttneria aculeata that was proved to be<br />
edible and was present in the habitat.<br />
Nevertheless, careful interpretation is inevitable because feeding trials involve some methodological<br />
imponderability that partially may have contributed to differences between realized and absolute food<br />
niche (see e.g. Gangwere 1961; Holecheck et al. 1982; Capinera 1985; Barone 1998). It is well known<br />
that feeding trials under lab conditions tend to produce more positive responses from insect herbivores<br />
than they would under natural conditions (e.g., Rowell-Rahier 1984). For instance, no-choice tests may<br />
overestimate the host range <strong>of</strong> an herbivore because there is no alternative to the <strong>of</strong>fered food. On the<br />
other hand the number <strong>of</strong> <strong>of</strong>fered plants is inevitably limited and may thus lead to an underestimation <strong>of</strong><br />
host range.