Canopy insect herbivores in the Azorean Laurisilva forests: key host plant species in a highly generalist insect community

Abstract

This article explores patterns of insect herbivore distribution in the canopy of the Laurisilva forests on seven islands in the Azores archipelago. To our knowledge, this is one of the first extensive study of this type in tree or shrub canopies of oceanic island ecosystems. One of the most frequently debated characteristics of such ecosystems is the likely prevalence of vague, ill-defined niches due to taxonomic disharmony, which may have implications for insect-plant interactions. For instance, an increase in ecological opportunities for generalist species is expected due to the lack of predator groups and reduced selection for chemical defence in host plants. The following two questions were addressed: 1) Are specialists species rare, and insect herbivore species randomly distributed among host plant species in the Azores? 2) Are the variances in insect herbivore species composition, frequency and richness explained by host plants or by regional island effects? We expect a proportional distribution of herbivore species between host plants, influenced by host frequency and distinct island effects; otherwise, deviation from expectation might suggest habitat preference for specific host tree crowns. Canopy beating tray samples were performed on seven islands, comprising 50 transects with 1 to 3 plant species each (10 replicates per species), giving 1320 samples from ten host species trees or shrubs in total. From a total of 129 insect herbivore species, a greater number of herbivore species was found on Juniperus brevifolia (s 0/65) and Erica azorica (s 0/53). However, the number of herbivore species per individual tree crown was higher for E. azorica than for any other host, on all islands, despite the fact that it was only the fourth more abundant plant. In addition, higher insect species richness and greater insect abundance were found on the trees of Santa Maria Island, the oldest in the archipelago. Insect species composition was strongly influenced by the presence of E. azorica , which was the only host plant with a characteristic fauna across the archipelago, whereas the fauna of other plant crowns was grouped by islands. The great insect occurrence on E. azorica reflects strong habitat fidelity, but only four species were clearly specialists. Our findings indicate a broadly generalist fauna. The simplicity of Azorean Laurisilva contributed to the understanding of insect-plant mechanisms in canopy forest habitats.

Spider biodiversity patterns and their conservation in the Azorean archipelago, with descriptions of new species

“…(1) Canopy-Laurel -canopy of native or endemic forest trees and shrubs (data available for all islands except Corvo and Graciosa) (see details on plants sampled in Ribeiro et al, 2005 Santos et al, 2005).…”

“…Another habitat surveyed was the canopy of both native and endemic trees (see Ribeiro et al, 2005; also BALA project) and of fruit orchards (see Santos et al, 2005;project INTERFRUTA). In the case of project INTERFRUTA data are only available for one island (Terceira).…”

“…A plastic bag was placed at the tip where arthropods, leaves and small branches were collected. For each selected plant, a branch was chosen at random, the beating tray placed beneath and the branch hit five times with a beating stick (for more details see Ribeiro et al, 2005;Santos et al, 2005).…”

Biotic integrity of the arthropod communities in the natural forests of Azores

“…This comes as no surprise given that arthropods, especially herbivores, are strongly influenced by the vegetation species composition (but see Ribeiro et al 2005). But this may also be because both, plant and arthropod communities, are probably influenced by the other disturbance factors (area, fragmentation, distance to border, paths) in much the same way, that is to say, both taxa respond to disturbance in similar manners.…”

Evidences that human disturbance simplify the ant fauna associated a Stachytarpheta glabra Cham. (Verbenaceae) compromising the benefits of ant-plant mutualism

“…We collected the arthropods with an entomological umbrella (Ribeiro et al, 2005). Each plant was hit 10 times with a 1m of length stick and the specimens that fell on the umbrella were collected.…”