ecology of phasmids - KLUEDO - Universität Kaiserslautern
ecology of phasmids - KLUEDO - Universität Kaiserslautern
ecology of phasmids - KLUEDO - Universität Kaiserslautern
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
Concluding remarks 92<br />
plant use <strong>of</strong> M. diocles may even be more restricted as shown here, because differential interspecific<br />
effects among host plants on nymph performance were great.<br />
6.2 Bottom-up or top-down or both<br />
This study provided evidence that herbivores suffer significant impact from both bottom-up and top-<br />
down factors. In the debate about the relative roles <strong>of</strong> higher and lower trophic levels in regulating<br />
tropical herbivore populations several authors recently reasoned that natural enemies (Pace et al. 1999;<br />
Persson 1999; Polis 1999; Dyer & Coley 2001) and in particular their marked effect on early lifestages<br />
(Cornell & Hawkins 1995; Cornell et al. 1998) exceeded the effects from plants.<br />
My results give further support to this view. Predation seems be the stronger factor explaining low<br />
abundances <strong>of</strong> M. diocles in the BCI forest because <strong>of</strong> its direct effects and because <strong>of</strong> indirect effects<br />
mediated by host plants.<br />
In two weeks nymphs suffered 54 % mortality from predators in their natural environment and predation<br />
pressure seemed to be spread equally in the forest understory (cf. Chapter 5). On the other hand I<br />
provided evidence that nymphs, when confronted with an inadequate host may suffer up to 100 %<br />
mortality in a comparable time span (cf. Chapter 4). I also showed that nymphs may leave a host <strong>of</strong><br />
minor suitability (although the evidence was weak; cf. Chapter 5). Sedentary behaviour on such a host<br />
involves two risks. The nymph may die because <strong>of</strong> toxic or minor nutritive host quality (particularly if<br />
this host was Piper grande or Dieffenbacchia longispatha, cf. Chapter 4 or e.g., Lill & Marquis 2001))<br />
or it will suffer decreased growth thereby remaining longer in the more vulnerable immature stage<br />
(Damman 1987). Leaving the host involves the risks <strong>of</strong> starvation, <strong>of</strong> not finding any or a more suitable<br />
host (van Dam et al. 2001), and <strong>of</strong> predation (Price et al. 1980).<br />
At least in the understory <strong>of</strong> the BCI forest M. diocles nymphs may find an acceptable host in the<br />
abundant Philodendron inaequilaterum (Croat 1978). This species pro<strong>of</strong>ed to be <strong>of</strong> high suitability for<br />
nymphs as they suffered least mortality. Albeit, growth on Phil. Inaequilaterum was decreased as<br />
compared to some other species. Despite the fact that a plant species may be <strong>of</strong> minor suitability<br />
considering growth, <strong>phasmids</strong> were shown to modulate feeding in their natural setting rather to<br />
availability than to acceptability (Willig et al. 1993).<br />
Consequently, low suitability <strong>of</strong> a host plant may either kill a nymph(demonstrated in Chapter 5), it may<br />
translate into higher predation pressure via decelerated growth (slow-growth high mortality; Feeny<br />
1976; Clancy & Price 1987), or it may increase the exposure to predators if the nymph searched a new<br />
host. In other words: effects <strong>of</strong> host suitability feed back into predation and thereby add on to direct top-<br />
down effects.<br />
Closing, I would like to come back to Lawton & McNeill (1979): the Devils can only be that evil<br />
because the Sea is so Deep and Blue.