Can we infer island introduction and naturalization rates from inventory data? Evidence from introduced plants in Galapagos

Studies of human-mediated rates of introduction of organisms to islands are few, results fall into two models (linear and exponential), and relationships with human population and activities are obscure. Owing to their late settlement and good scientific record, data from Galapagos may be exceptionally informative. The rate of introduction of vascular plant species to Galapagos has been suggested to be exponential, paralleling the rise in human population. However, detailed reconstruction of growth in numbers of introduced plant species, based on historical and recent records, reveals changes in study criteria over the last two centuries, which obscure the true introduction rate. At first, cultivated species were deliberately excluded from most studies. From the 1960s, naturalized cultivated species were included more consistently, but non-naturalized species were still excluded. From the mid-1980s, the latter were deliberately included. Accidental introductions increased linearly from 1807 (the first records) to the present. Escapes from cultivation show increases in rate around 1906 and in the period 1970-1990, the latter coinciding with the first studies directed at areas affected by human activities. Non-naturalized cultivated species rose abruptly from the late 1980s, as they became deliberately studied. There seems to be no direct link with human population size. Data represent rate of discovery rather than true introduction rate, and the changing overall rate reflects changing botanical interests and research effort. Data from other islands also suggest that linear increases in naturalized plants are the norm. Galapagos data do not permit confident statements about the introduction rate of cultivated species, but suggest that this may depend more on human activities than human population size.