Island ecosystems are particularly vulnerable to biological invasions. Since the early 1800s, humans have introduced to the Galápagos Islands animals and plants that threaten the native vegetation Goats are the most abundant and destructive feral animals. Endemic members of the Cactaceae and Asteraceae have been reduced drastically. A control program has completely eradicated goats on several islands, where vegetation has returned but sometimes the species composition differs from the original. Hunting continues on Santiago, where 80,000 goats exist, and fenced quadrats protect some native plants. Cattle damage vegetation, particularly the endemic shrub, Miconia robinsoniana, by trampling and grazing. Ranchers on several islands have been urged to fence in cattle. Wild pigs dig up and eat the roots of woody plants and some rare orchids. Donkeys trample and eat grasses and shrubs. Control of pigs has started; there is no control of donkeys. Between 20 and 50 introduced plants have escaped cultivation and invaded the native vegetation. Guava is the most widespread, covering vast areas on several islands. Its dispersal is aided by free-roaming cattle, which eat the fruits and excrete the seeds. Quinine trees have spread extensively on one island via windblown seeds. Both species are being cut down, and herbicides are applied to trunks to prevent sprouting. Other aliens that are problems on one or more islands include lantana, hemp, avocado, sweet lime, and forage grasses. None of these is being eliminated yet, because of lack of funds and manpower. To ensure the future of the Galápagos flora, control measures must continue and expand to encompass all dangerous introductions.
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