ecology of phasmids - KLUEDO - Universität Kaiserslautern

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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).
Community structure & host range 11 Although phasmids are common herbivores in many tropical systems, little is known of their biology (Bedford, 1978; van den Bussche et al., 1989; Willig et al., 1986, 1993). Generally phasmids are considered as herbivore generalists but specialization ranges from strict monophagy to wide polyphagy (Bedford 1978). With about 3000 species worldwide (Whiting et al. 2003) phasmids are comparably poor in diversity and many species are known to occur in low abundances in humid tropical forests (Novotny & Basset 2000; but see Willig et al. 1986). Accordingly, studies to date indicate that the Panamanian phasmid community is poor in species (Hebard, 1923, 1929, 1933; Robinson, 1968a, 1968b, 1969) while there is no information available on density, distribution or specialization patterns. Ecological information for neotropical phasmids is entirely restricted to the studies on the abundant endemic Puerto Rican species Lamponius portoricensis (Willig et al. 1986, 1993; Willig & Camilo 1991, Sandlin & Willig 1993). Distribution patterns and patch densities of this polyphagous species could partially be explained by host plant availability (Willig et al. 1993) and drastic reductions in population densities followed natural disturbance of high intensity (Hurricane Hugo; Willig & Camilo 1991). However, the causal factors of temporal fluctuations in L. portoricensis densities largely remain unknown. Here, I assessed diversity, distribution, and density of the phasmid community on BCI by monitoring line-transects in the forest understory and along forest edges, and I determined the absolute food niche of four phasmid species by conducting feeding-trials. The possible range of host plants of phasmid species was determined by presenting them with four groups of plants. (1) I tested phylogenetic restrictions in host range by offering plant species out of the same genus, the same family and other families than the original host-plant belonged to (i.e. where they were observed on). To test for constraints in habitat selection connected to food digestibility and availability, I presented phasmids (2) with persistent and pioneer plant species representing the forest understory and forest edges or gaps, and (3) with a selection of 10 of the most abundant tree species on BCI (>1000 ind.*50 ha -1 ; Condit et al. 1998). (4) The validity of the assessed host range was tested by offering a random selection of plant species. 2.2 Materials and methods For details on study site and the maintenance of lab populations please refer to Chapter 1. 2.2.1 Line-transects To describe diversity, distribution and population densities of phasmid species in forest understory versus forest edge habitats, I collected data along line-transects from May 2000 to July 2001. I established 3 line-transects of 80 m length each in the forest understory and at forest edges (for locations
Page 1 and 2: ECOLOGY OF PHASMIDS (PHASMATODEA) I
Page 5 and 6: ECOLOGY OF PHASMIDS (PHASMATODEA) I
Page 7 and 8: Acknowledgements I Acknowledgements
Page 9 and 10: Table of Contents III 3 Life cycle,
Page 11 and 12: List of Figures V List of Figures F
Page 13: List of Abbreviations VII List of A
Page 16 and 17: Introduction 2 Regardless whether s
Page 18 and 19: Introduction 4 Description of phasm
Page 20 and 21: Introduction 6 Solanum and Piper (T
Page 22 and 23: Introduction 8 1.3.4 Data analysis
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Page 46 and 47: Life history & potential population
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Adult female feeding preference & n
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Predation mediated mortality & migr
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Concluding remarks 91 6 Concluding
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Abstract 93 7 Abstract Herbivory is
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Literature cited 96 Basset, Y. 1997
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Literature cited 98 Brown, B. J., M
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Literature cited 100 Croat, T. B. 1
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Literature cited 102 Hagerman, A. E
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Literature cited 104 Kearsley, M. J
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Literature cited 106 McNeill, S., T
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Literature cited 108 Reich, P. B.,
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Literature cited 110 Strong, D. R.,
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Appendix 113 9 Appendix Please cont
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Appendix 1: Phasmid species previou
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Tabellarischer Lebenslauf Angaben z
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Berger J., Schaefer, M., 1997. Wild
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