Can We Infer Island Introduction and Naturalization Rates from Inventory Data? Evidence from Introduced Plants in Galapagos

Abstract

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. ResumenLos estudios de la tasa de introduccio´n de los organismos a islas causada por los seres humanos son pocos; sus resultados caben dentro de dos modelos (linear y exponencial), y la relacio´n con la poblacio´n humana y sus actividades es oscura. Los datos de Gala´pagos pueden ser excepcionalmente informativos, debido a su colonizacio´n tardı´a y registro cientı´fico bueno. Se ha sugerido que la tasa de introduccio´n de las plantas vasculares a Gala´pagos es exponencial, en paralelo al crecimiento de su poblacio´n humana. Sin embargo, una reconstruccio´n detallada del incremento en nu´mero de especies de plantas introducidas, basada en los datos histo´ricos y recientes, revela cambios en los criterios de estudio durante los u´ltimos dos siglos, lo que oscurece la verdadera tasa de introduccio´n. Al principio, se excluı´an de la mayorı´a de los estudios las especies cultivadas. Desde los 1960, se incluyeron con ma´s regularidad las especies cultivadas naturalizadas, mientras las no-naturalizadas quedaban excluidas. Desde los 1980, estas u´ltimas fueron intencionadamente incluidas. Las especies de introduccio´n accidental demuestran un aumento linear en su nu´mero desde 1807 (los primeros registros) hasta la presen...

Conservation of oceanic island floras: Present and future global challenges

“…A few invasive plant species can successfully invade undisturbed vegetation patches, where they may become dominant; unfortunately, these species are difficult to identify and predict (Tye, 2006a;Kueffer et al, 2010).…”

“…Tye, 2006a) although palaeobotanical research may be able to clarify status. An illustrative example is the recent use of pollen records to confirm as native in the Galá pagos six plant species that were formerly believed or suspected to be alien species there (Van Leeuwen et al, 2008).…”

“…Alien species richness is indeed strongly correlated with indices of economic development across oceanic islands (Denslow et al, 2009;Kueffer et al, 2010), possibly in part because anthropogenic habitat alteration is a strong driver of plant invasions (cf. Kueffer et al, 2010) or because economic development favours the deliberate introduction of a wider range of alien species (Tye, 2006a).The past role of plant invasions versus habitat destruction in the decline of island plant diversity may thus be difficult to disentangle. However, it should be of high priority to understand how invasive plants currently affect endemic plants, especially where habitats are heavily disturbed and invaded, and population sizes of many native species are small, and studies are now beginning to reveal cases in which invasive plants are clearly the direct cause of declines in native plants on islands.…”

Residence time and human-mediated propagule pressure at work in the alien flora of Galapagos

“…Alien plants in Galapagos have been introduced alongside human settlement on the four inhabited islands of Floreana, Isabela, Santa Cruz and San Cristobal (Mauchamp 1997;Tye 2006). A few species were introduced by visitors to the archipelago prior to settlement, but the vast majority were introduced during the 20th century (Tye 2006).…”

When is an invasive not an invasive? Macrofossil evidence of doubtful native plant species in the Galápagos Islands

“…Unfortunately the Gala´pagos Islands, despite their relatively late human occupation (human presence did not begin until European discovery in AD 1535 [Froyd et al 2010]) and unusually high native species retention with ;95% of terrestrial biodiversity remaining intact , still face significant threat from invasive species (Magee et al 2001). Recent plant introductions are estimated to be over 13 000 times the historic natural species arrival/establishment rate (Tye 2006). Today, the pressures placed by humans upon the Gala´pagos are dramatically transforming ecosystems particularly in the humid highlands of the larger more developed islands, where approximately 88% of the habitat on the island of Santa Cruz and 94% on San Cristobal are now classed as transformed (Watson et al 2009).…”