The role of insect leaf herbivory on the mangroves Avicennia marina and Rhizophora stylosa

Burrows, Damien Wayne (2003) The role of insect leaf herbivory on the mangroves Avicennia marina and Rhizophora stylosa. PhD thesis, James Cook University.

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Abstract

This thesis examines insect leaf herbivory and the insect herbivore community on two common mangrove species – Avicennia marina and Rhizophora stylosa, at two sites near Townsville, northern Queensland. Despite its widely recognised importance in other forested ecosystems, the role of insect herbivory in mangrove ecosystems is often downplayed and remains relatively unexplored. It is generally considered that mangroves lack a diverse and specialised insect herbivore fauna, and are populated with insects from adjoining habitats. It is also commonly believed that mangrove leaves are less palatable and nutritious than leaves of other tree species, and that herbivory levels are less than those found in other forest ecosystems. Sixty-one insect species were confirmed feeding upon A. marina and R. stylosa at the study sites, more than doubling the number of published host records for Australian mangroves. Herbivore diversity on the two mangrove species was similar, but only four of the 61 herbivore species were in common between them and the composition of the two faunas was substantially different. The two faunas show a high degree of host-specificity and adaptation to their mangrove hosts and there were substantial similarities between the faunas found in this study and those of other Avicennia and Rhizophora species elsewhere in the world. The diversity of folivores on these two mangrove species was similar to that of other nearby non-mangrove trees and to that reported for other trees elsewhere in the tropics. Nearly all mangrove herbivory studies have measured damage on discrete leaf samples collected at one point in time. This approach fails to detect entirely consumed leaves, or partly damaged leaves that have been abscised. These leaves can be accounted for by an alternative long-term method that makes repeated herbivory estimates on tagged leaves. Application of the long-term method in this study found herbivory to be 3-6 times higher than estimated by discrete measurements. R. stylosa had only 2-3% loss of leaf area in discrete samples but 7-13% loss in the long-term study. A. marina had 6-7% loss of leaf area in discrete samples but 28-36% loss in the long-term study. For both species, most herbivory occurred whilst the leaves were young. Once past the juvenile phase, R. stylosa leaves were rarely attacked. In contrast, mature A. marina leaves suffered significant insect damage and leaf loss. Herbivory reduced average leaf longevity of all leaves by 4-5% for R. stylosa and 12-13% for A. marina. In mangroves, loss of entire leaves is reported to be rare, but in R. stylosa and A. marina in this study, 4-5% and 19-29% of leaves, respectively, were either entirely consumed, or aborted due to insect damage. For both species, loss of leaf material through premature abscission of damaged leaves was as great as that actually consumed by insects, indicating a role for herbivory in promoting leaf fall. Destruction of developing leaf buds by insects resulted in greater leaf losses than that suffered by emerged leaves. This was especially important for R. stylosa, which can only produce leaves from the apical bud. Damage to R. stylosa apical buds was common, frequently resulting in the loss of leaves before they emerged or, where damage resulted in the destruction of the apical bud, cessation of leaf production and death of that shoot. In some cases, new apical buds regenerated from suppressed lateral buds immediately below the destroyed apical bud. Death of existing shoots and regeneration from suppressed laterals are potentially major sources of architectural change to tree growth form. Damage to apical buds also resulted in the loss of developing inflorescences and lateral branches. The chemical and physical nature of leaves changed significantly as they aged. Young leaves had a higher nutrient and chlorophyll concentration, but lower leaf thickness and leaf mass per unit area. Thus they were more nutritious and less tough for herbivores. These young leaves were especially prone to premature leaf fall because of insect damage. Leaves re-translocated over half of their nutrients prior to senescence but consumption or premature abscission of leaves before this process is complete may increase nutrient loss from the trees to microbial and detrital food chains on the forest floor. Thus herbivore-mediated leaf fall may also impact upon nutrient dynamics in mangrove forests. This thesis demonstrates that: mangroves support a diverse and distinctive insect herbivore community; leaf herbivory is much higher than previously reported; there are additional mechanisms (eg, apical bud damage) by which herbivores affect leaf loss and other aspects of tree performance; and insects significantly affect leaf longevity, the timing of leaf fall and the quality of leaf litter. These results indicate that the role of herbivorous insects in mangrove ecosystems is more important than previously thought and that its evaluation needs to go well beyond simple static measures of leaf area missing from mature leaves.

Item ID: 1174
Item Type: Thesis (PhD)
Keywords: Insect leaf herbivory, Insect herbivore community, Avicennia marina, Rhizophora stylosa, Diversity, Host-specificity, Tagged leaves, Average leaf longevity, Premature abscission of damaged leaves, Destruction of developing leaf buds, Nutrient and chlorophyll concentration, Nutrient dynamics in mangrove forests, Quality of leaf litter, Folivorous insects, Australian mangroves, Townsville, Queensland
Date Deposited: 08 Nov 2006
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 0%
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