Using Rose Galls for Field Exercises in Community Ecology & Island Biogeography

Abstract

Studying the attributes of biotic communities is integral to all courses in ecology and yet few communities are small enough, or available for effective examination, when classes begin in the fall. Communities discussed in textbooks are usually large and complex, leaving students with the view that detailed studies are difficult and all but impossible. However, it is common for ecologists to partition large and complex communities into smaller, more manageable units called Component communities are usually of sufficient size that all the players can be identified and studied. Component communities are of an ideal size for students to study within the confines of a laboratory period. One type of component community is the insects associated with a particular species of plant. Finding plants with an associated assemblage of insects is easy in most habitats; however, many of these insects are no longer present when students undertake their first field exercises in the fall. Gall inducers comprise one guild of plant-feeding insects still present in the fall and are ideal for studying community attributes. Galls represent one of the most complex insect-plant relationships in the natural world where, by some as yet poorly understood means, these specialized insects redirect the growth of attacked plant tissues and stimulate them into surrounding the larvae with highly nutritious cells and protective tissues. The resulting species-specific structures sustain the insects throughout the summer and shelter them from the elements and predators (Shorthouse & Rohfritsch 1992). Galls are commonly seen by students on field trips and make ideal subjects for explaining the complexities of insect-plant relationships and component communities. Large numbers of galls can be collected within the space of an hour and then returned to the lab or classroom for dissection or rearing. Each species of gall-inducing insect is responsible for a structurally distinct type of gall, making it easy to determine the number of insects present at a site without actually observing the insects themselves. We have found galls induced by cynipid wasps of the genus Diplolepis ideal for exercises in community ecology. Here we describe a simple field exercise that allows students to use cynipid galls for learning about community attributes and biogeography. Diplolepis wasps are restricted to inducing galls on wild roses. Galls are initiated in the spring when tiny adults (5 to 7 mm in length; see photographs in Shorthouse 1993) exit galls from the previous season and lay their eggs in leaf buds or at the tips of stems or sucker shoots. Galls start growing soon after the eggs hatch and immature larvae start feeding. Some galls have a single larva while those induced by other species have numerous larvae per gall. Diplolepis wasps have one generation per year and overwinter in the larval stage inside their galls. There are about 32 species of Diplolepis in North America with each species inducing a structurally distinct gall. Diplolepis larvae are attacked by numerous species of parasitoids (specialized parasitic wasps that complete growth on only a single host) throughout the summer season and the galls themselves are attacked and modified by inquiline cynipid wasps of the genus Periclistus. The assemblage of Diplolepis wasps, parasitoids and inquilines that emerge from a large collection of galls induced by the same species comprises a very distinct component community. Wild roses are common shrubs over much of North America. They are found throughout the northern U.S., Alaska and southern Canada, but some species and associated galls can be found as far south as Arizona or even Florida. They usually grow in patches of varying size and usually have reasonably sharp boundaries. Roses are ephemeral shrubs that frequently become established in disturbed habitats such as abandoned agricultural fields or recently burned land. Single rose plants grow into large patches through vegetative growth and as the years pass, they become colonized by various insects. If roses become shaded by trees and the patches disappear, their host-specific component communities disappear as well. Patches of rose can be considered islands and their colonization by Diplolepis is akin to the colonization of oceanic islands. The strong positive relationship between area and species diversity is one of the few rules in ecology. Demonstrations of island effects are legion, extending far beyond oceanic islands to include any discrete, colonizable habitat such as mountain tops, woodlots (Diamond & May 1981), and clusters of plants of the same species (Janzen 1968). This phenomenon is variously explained as a reflection of increasing habitat diversity as a function of area (Williamson 1981), or as a balance between rates of colonization and local extinction (MacArthur R.G. Lalonde is a Professor of Biology at Okanagan University College, Kelowna, B. C., V1V 1V7, Canada. J.D. Shorthouse is a Professor of Entomology in the Department of Biology, Laurentian University, Sudbury, Ontario, P3E 2C6, Canada.