ecology of phasmids - KLUEDO - Universität Kaiserslautern

Concluding remarks 91
6 Concluding remarks
6.1 Is M. diocles a herbivore specialist?
This study could not provide clear evidence whether the restrictions in M. diocles host range reflect
specialization. The distant relationship of its host plant families, the Araceae and Piperaceae, disagrees
with the predictions of the concept of biochemical coevolution (Ehrlich & Raven 1964; Strong et al.
1984). Alternatively, restricted host range of M. diocles may reflect ‘clouded’ biochemical coevolution,
i.e. the taxonomic relationship among host plants is disconnected by other factors such as predation.
This concept of diffuse coevolution (Janzen 1979; 1980; Fox 1981) bases on multitrophic interactions
simultaneously exerting selective pressures on one another and being affected by changes in other
component members. For example, strong differential predation pressure on host plant species as
demonstrated by Sandoval (1994), may have led to the exclusion of missing links between Araceae and
Piperaceae in the host plant range of M. diocles. Such a scenario must remain purely speculative
because so far there is no incidence that these families are in a closer taxonomoic relation. In contrast,
the Araceae belong to the monocots and the Piperaceae remain on a somehow uncertain taxonomic
position among the paleoherbs between mono- and dicots (Judd et al. 2002). At least future field-
experiments assessing predation pressure on M. diocles on different host plants would give a principle
idea, whether diffuse coevolution may be involved.
However, Jermy (1976) suggested an alternative ‘null’ model of coevolution. Sequential evolution may
best apply to herbivores like M. diocles that feed on distantly related plant taxa. Jermy (1976) argued
that, because most insect herbivores have low population densities, they could hardly be important
selection factors for plants. Insect-host relationships need not be antagonistic, they may as well interact
in a mutually advantageous way by insects keeping their hosts at an ideal density. Consequently
secondary compounds may not reflect defense, as resistance is not necessary. According to sequential
evolution, biochemical plant diversity origins from abiotic selection factors and from plant-plant
interactions. Herbivores just adapt to plant changes but don’t influence them markedly. Generally, this
model applies when evidence for coevolution is not forthcoming, and parallel evolutionary lines of
plants and insects are few (Hartley & Jones 1995). As a consequence of sequential evolution, plants and
their interactions with the abiotic and biotic environment replace herbivores. Thus the initially stated
supposition that herbivores are central factors in maintaining forest dynamics and ecosystem stability of
tropical forests (Lowman 1984; Brokaw 1985; Collins et al. 1985; Schowalter 1985; Brown et al. 1987)
cannot hold if herbivore specialization is based on sequential coevolution.
Concluding, this study showed that M. diocles as a tropical herbivore is clearly restricted in its host
range, particularly facing the fact that this species seems to feed only on some plants from two families
out of more than 1200 plant species representing 120 families occurring on BCI (Croat 1978). Host