Distribution of Native and Invader Plant Species on the Island of Guam

Abstract

The structure of the vegetation of Guam and the position of invading species is investigated. Three species groups, endemic, native non-endemic, and invader, were found to have characteristic growth forms, ranges, and habitats on the island. Endemic species and native species of limited distribution are largely woody; invaders are generally herbaceous. The proportion of endemic species in each habitat was found to vary inversely with the proportion of invaders. Invasion was most successful in communities with large proportions of widely distributed native species. Invaders originating in the New World outnumber those from the Old World, but Old World invaders proved to be more successful in establishing themselves in highly organized, complex vegetation communities. The type of invading species and the organization of native species into communities appear to be important factors in the success of plant invasion. ONE CURRENTLY HELD IDEA in biogeography is that the place of evolution has some effect on an organism's competitive success. Certain areas may be considered evolutionary centers which, because of large size, high species diversity, and stable ecosystems, produce competitively dominant forms. Dispersal of species occurs from these centers to smaller, peripheral areas which are less likely to produce dominant species, and results in the displacement of the species which evolved in the peripheral areas. Islands are viewed as one type of peripheral area, and it has often been suggested that island forms are competitively inferior to mainland forms, and subject to rapid replacement by invading mainland species. Darlington (1965) maintains that the number of species present is related to area and climate. The more species present in an area, the more effective the evolution of the species. Dispersal of species occurs from these areas to smaller areas which are less likely to produce dominant forms. These dominant organisms have the ability to cross barriers and establish themselves in the peripheral areas, at the expense of organisms which have evolved here. Briggs (1966) draws similar conclusions regarding marine systems and additionally points out the possibility of greater and lesser dominant evolutionary centers. The susceptibility of islands to invasion has also been attributed to other characteristics of island biota. Carlquist (1965) suggests that islands are loosely packed with species and therefore have room for invaders. Thorne (1963) states that island forms are more generalized and therefore poor competitors when faced with the more specialized mainland forms. Although the actual species present may be subject to rapid turnover, it appears that an equilibrium species number and, in animals at least, an equilibrium trophic structure are maintained (MacArthur and Wilson 1967; Heatwole and Levins 1972). The present research investigates this idea of competitive superiority due to place of evolution by analyzing the competitive relationships between native and invading plants on the island of Guam. Most of the literature on the problem of nativeinvader plant interactions draws conclusions in contrast to those cited above. Persistence of native vegetation on islands in the face of invading mainland species has often been noted, and plant invaders appear to be transitory members of the community. Success of invading plant species has been largely attributed to man's disturbance of the native vegetation. Pioneer invading species appear to be most successful and most likely to become fully naturalized. Invasion is least successful in fully developed, highly diverse communities (Elton 1958; Harris 1963, 1965; Sauer 1967; Watts 1970, 1971). In this research two main factors are investigated, species range and growth forms. It is expected that the species most affected by invasion are the endemics, for these are the species which have evolved in the island habitat and should show the least competitive ability. More widespread species should be less affected. Smaller plants are both more likely to be successful invaders and less likely to go extinct than larger forms because of larger populations. Adjustment to invasion resulting in a mixed nativeinvader community is expected among these species. 1 This paper is a section of a Master's thesis presented to the University of Maryland. The author gratefully acknowledges the advice and assistance of Dr. G. J. Vermeij, Department of Zoology, University of Maryland, in the preparation of this paper. 2Present address: Department of Geography, University of Hawaii, Honolulu, Hawaii 96822 U.S.A. 158 BIOTROPICA 6(3): 158-164 1974 This content downloaded from 157.55.39.159 on Sun, 18 Sep 2016 06:42:37 UTC All use subject to http://about.jstor.org/terms