Invasive Plant Problems and Requirements for Weed Risk Assessment in the Galapagos Islands

Galapagos is a young oceanic archipelago with a native vascular flora of 560 species, few native trees, and an absence of many families present on the neighbouring mainland. It has only recently become subject to human influence. Plants introduced by man have only become problematic in the last 150 years, with most weed problems being less than 50 years old. The rise in plant introductions parallels the human population increase, with a recent introduction rate of 10 plant species per year, 100,000 times the natural colonization rate. Invasive plants include weeds and cultivated species, which cause detrimental effects on agriculture as well as natural habitats. Introduced species are the principal threat to the Galapagos ecosystem, and several species of alien plant are seriously damaging native habitats and threatening endemic species. Examples of some of the worst invaders and of the process and effects of invasions are presented. A weed risk assessment system for Galapagos must take account of the particular requirements of a fragile oceanic island ecosystem and flora. It must include consideration of potential invaders as well as plants already introduced. Factors that need to be included in an objective risk assessment system for Galapagos are discussed in light of the New Zealand Department of Conservation’s model. THE GALAPAGOS AND THEIR FLORA Galapagos is an isolated oceanic archipelago of volcanic islands lying 1000 km west of Ecuador, straddling the equator. They have never been connected to the mainland and range in age from 1 to 3.3 million years (Simkin 1984). Although it has been suggested that there were earlier visits to the islands by indigenous people from mainland South America, the official date of discovery of the archipelago is recognized as 1535 (Slevin 1959; Hickman 1985). In that year, Tomás de Berlanga’s ship was carried there by currents, while becalmed on his way from Panama to Peru (Slevin 1959; Hickman 1985). The islands were uninhabited at that time, and no evidence of earlier human use has been found. The first visitors after discovery were mainly buccaneers, passing sailors, whalers and sealers (Hickman 1985; Gordillo 1990). The first settler established on Floreana about 1807 and other groups there, after short intervals of unoccupancy, in 1832 and 1902; it has been permanently inhabited since 1929. San Cristóbal was settled permanently in 1869, Isabela in 1893 and Santa Cruz in the 1920s (Slevin 1959; Schofield 1989). The pirates and whalers deliberately or accidentally introduced a number of alien species, including goats, rats and, probably, insects and plants. Even before permanent settlement, Floreana contained large areas dominated by introduced plants such as Citrus spp. (Slevin 1959; Hamann 1984). The rapidly increasing settled population, growing at 8% per year in the 1990s through immigration and indigenous birthrate, has been accompanied by an enormous number of new introductions of alien plants and animals (Mauchamp 1997). Although agricultural development began at the time of settlement, the process has been uneven, leading to different rates of introduction of alien species. Floreana has the longest history of the presence of a large introduced flora, while agriculture on Santa Cruz was minimal until about 1960 (Moll 1990). The archipelago, although straddling the equator, has a semi-arid, subtropical climate, due to the prevailing Humboldt current from the Antarctic and prevailing winds which also come from the south for most of the year. These factors lead to a vegetation which forms certain well-defined zones according to altitude and aspect (Wiggins and Porter 1971). Progressing from lowlands to highlands, these zones are most commonly defined as: Littoral, comprising mangroves, dune vegetation and other coastal communities; “Arid” (actually semi-arid), comprising scrub and light woodland dominated by cacti; Transition, comprising more or less closed woodland; and Humid, which is broken into a number of sub-zones that vary between islands, but which include Scalesia Zone forest, Miconia zone dense scrub, and Fern-sedge zone highlands. The more humid zones extend lower on the southern, wetter sides of islands, and most islands, which are low, have no Humid Zone. Biodiversity is therefore highest on the larger, more climatically diverse islands. Plant endemicity is highest in the Arid Zone, which is represented on all except the smallest islets. The Galapagos islands support a native vascular flora of about 560 species, of which about 32% are endemic (Lawesson et al. 1987). This includes some of uncertain origin, principally pantropical weeds, which may have arrived naturally or may have been introduced by the earliest human visitors to the islands. Galapagos is a typical oceanic archipelago in that the native flora, like the fauna, is impoverished in the sense that a given area counts many fewer species than would a similar area in the mainland source areas (Loope and Mueller-Dombois 1989; Moll 1990). As a consequence, interspecific competition has also been weaker, and these two factors combine to render the vegetation highly susceptible to invasion by more competitive species (see Simberloff 1995 for review of this point). The relative impoverishment applies unevenly; for example, there are no tree species in some of the highland zones, although the climate and soils are suitable for tree growth. These zones are therefore especially susceptible to invasion by tree and shrub species that overtop and shade out the low-growing native vegetation. PLANT INTRODUCTIONS TO GALAPAGOS