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Alien species, one of the biggest threats to natural ecosystems worldwide, are of particular concern for oceanic archipelagos such as Galápagos. To enable more effective management of alien species, we reviewed, collated and analysed all available records of alien species for Galápagos. We also assembled a comprehensive dataset on pathways to and among the Galápagos Islands, including tourist and resident numbers, tourist vessels, their itineraries and visitation sites, aircraft capacity and occupancy, air and sea cargo and biosecurity interceptions. So far, 1,579 alien terrestrial and marine species have been introduced to Galápagos by humans. Of these, 1,476 have become established. Almost half of these were intentional introductions, mostly of plants. Most unintentional introductions arrived on plants and plant associated material, followed by transport vehicles, and commodities (in particular fruit and vegetables). The number, frequency and geographic origin of pathways for the arrival and dispersal of alien species to and within Galápagos have increased over time, tracking closely the increase in human population (residents and tourists) on the islands. Intentional introductions of alien species should decline as biosecurity is strengthened but there is a danger that unintentional introductions will increase further as tourism on Galápagos expands. This unique world heritage site will only retain its biodiversity values if the pathways for invasion are managed effectively.

Background: Plant invasions are causing habitat degradation in Galapagos. Problems are concentrated on the four inhabited islands. Plants introduced to rural areas in the humid highlands and urban areas on the arid coast act as foci for invasion of the surrounding Galapagos National Park. Methodology/Principal Findings: Here we present results of the most comprehensive inventory to date of alien vascular plants in the inhabited areas of Galapagos. The survey was conducted between 2002 and 2007, in 6031 properties (97% of the total) on Floreana, Isabela, San Cristobal and Santa Cruz Islands. In total 754 alien vascular plant taxa were recorded, representing 468 genera in 123 families. Dicotyledons represented 554 taxa, monocotyledons 183, there were 7 gymnosperms and 10 pteridophytes. Almost half (363) of the taxa were herbaceous. The most represented families were Fabaceae (sensu lato), Asteraceae and Poaceae. The three most recorded species in the humid rural areas were Psidium guajava, Passiflora edulis and Bryophyllum pinnatum, and in the dry urban areas, Aloe vera, Portulaca oleracea and Carica papaya. In total, 264 (35%) taxa were recorded as naturalized. The most common use for taxa was ornamental (52%). Conclusions/Significance: This extensive survey has increased the known alien vascular flora of Galapagos by 257 species, giving a ratio of alien to native taxa of 1.571. It provides a crucial baseline for plant invasion management in the archipelago and contributes data for meta analyses of invasion processes worldwide. A repeat of the survey in the future would act as an effective early detection tool to help avoid further invasion of the Galapagos National Park.

Mapping is an important tool for the management of plant invasions. If landscapes are mapped in an appropriate way, results can help managers decide when and where to prioritize their efforts. We mapped vegetation with the aim of providing key information for managers on the extent, density and rates of spread of multiple invasive species across the landscape. Our case study focused on an area of Galapagos National Park that is faced with the challenge of managing multiple plant invasions. We used satellite imagery to produce a spatially-explicit database of plant species densities in the canopy, finding that 92% of the humid highlands had some degree of invasion and 41% of the canopy was comprised of invasive plants. We also calculated the rate of spread of eight invasive species using known introduction dates, finding that species with the most limited dispersal ability had the slowest spread rates while those able to disperse long distances had a range of spread rates. Our results on spread rate fall at the lower end of the range of published spread rates of invasive plants. This is probably because most studies are based on the entire geographic extent, whereas our estimates took plant density into account. A spatial database of plant species densities, such as the one developed in our case study, can be used by managers to decide where to apply management actions and thereby help curtail the spread of current plant invasions. For example, it can be used to identify sites containing several invasive plant species, to find the density of a particular species across the landscape or to locate where native species make up the majority of the canopy. Similar databases could be developed elsewhere to help inform the management of multiple plant invasions over the landscape.

The Galapagos Archipelago is renowned for its high endemism but little effort has been made to quantify the human disturbance that compromises the islands' ecological integrity. We provide a quantitative assessment of anthropogenic degradation, which we define as areas either transformed by direct human activity or heavily invaded by four of the most prevalent alien plant species (Psidium guajava, Rubus niveus, Cinchona pubescens and Syzygium jambos). We assessed how the amount of degraded area varied among the six major vegetation zones (bare ground, littoral, arid, transition, humid and very humid) across five inhabited or formerly inhabited islands. Overall, we found that 37,833 ha (5.5%) of the Archipelago have been completely degraded. The islands that have suffered the greatest human impact (13,000–14,000 ha each) are Santa Cruz (the most populous) and Isabela (the largest). When vegetation type is considered the humid and very humid vegetation zones have been most affected by humans (29 and 45%, respectively). On San Cristobal and Santa Cruz 100 and 76%, respectively, of the very humid zone and 94 and 88%, respectively, of the humid zone have been transformed. These results are underestimations as mapping of the anthropogenic change in some vegetation zones (e.g. on Floreana) is poor, and the analysis did not take into account the effects of introduced animals. Nevertheless, this research points to an urgent need to prioritize restoration efforts in humid and very humid vegetation zones and to improve spatial mapping across the Archipelago to obtain a better understanding of the impacts of humans.

Introduced species present the greatest threat to the unique terrestrial biodiversity of the Galapagos Islands. We assess the current status of plant invasion in Galapagos, predict the likelihood of future naturalizations and invasions from the existing introduced flora, and suggest measures to help limit future invasions. There has been a 1.46 fold increase in plant biodiversity in Galapagos due to alien plant naturalizations, reflecting a similar trend on islands elsewhere. There are 870 alien plant species recorded in the archipelago. Of evaluated species, 34% species have naturalized. Within this group are the invasive species (16% of evaluated) and the transformers (3.3% of evaluated). We show that, as expected, naturalized species have been present in the archipelago longer than non-naturalized species. We also find that a higher human-mediated propagule pressure is associated with a greater human population and with properties that have been settled longer. This, combined with the relatively recent introduction of most species, leads us to the conclusion that Galapagos is at an early stage of plant invasion. We predict that more species from the existing alien flora will find an opportunity to naturalize and invade as propagule pressure increases alongside rapid human population growth associated with immigration to serve the booming tourism industry. In order to reduce future invasion risk, we suggest reviewing inter-island quarantine measures and continuing community education efforts to reduce human-mediated propagule pressure.

Improving our knowledge of pre-anthropogenic landscapes is vital for understanding landscape-scale heterogeneity and for setting goals and objectives for ecological restoration. This is especially important in highly modified landscapes that contain few remnants of pre-impact ecosystems. This study aims to develop new methodology to improve understanding of historical vegetation, using the now-degraded inhabited highlands of the Galapagos Islands as a case study. Our multidisciplinary approach innovatively combines data from interviews with residents who were familiar with the vegetation before most degradation occurred with the more traditional sources of historical aerial photography and information from early explorer and scientist reports. We reconstruct historical vegetation across the landscape by mapping it in the year 1960 and discussing this map in the historical context of anthropogenic change. Our results confirm published vegetation types but also define some other types not previously described, and suggest much greater spatial, temporal and structural heterogeneity than commonly understood. This result can be used by Galapagos land managers to better match species assemblages with sites and plan restoration actions that will maximise resilience against the ongoing and future threats of climate change and species invasions. Our methodology can be applied in extensive areas of the world where the majority of anthropogenic disturbance to natural ecosystems has been within the past 60 years.

Plant invasions are causing habitat degradation in Galapagos. Problems are concentrated on the four inhabited islands. Plants introduced to rural areas in the humid highlands and urban areas on the arid coast act as foci for invasion of the surrounding Galapagos National Park. Here we present results of the most comprehensive inventory to date of alien vascular plants in the inhabited areas of Galapagos. The survey was conducted between 2002 and 2007, in 6031 properties (97% of the total) on Floreana, Isabela, San Cristobal and Santa Cruz Islands. In total 754 alien vascular plant taxa were recorded, representing 468 genera in 123 families. Dicotyledons represented 554 taxa, monocotyledons 183, there were 7 gymnosperms and 10 pteridophytes. Almost half (363) of the taxa were herbaceous. The most represented families were Fabaceae (sensu lato), Asteraceae and Poaceae. The three most recorded species in the humid rural areas were Psidium guajava, Passiflora edulis and Bryophyllum pinnatum, and in the dry urban areas, Aloe vera, Portulaca oleracea and Carica papaya. In total, 264 (35%) taxa were recorded as naturalized. The most common use for taxa was ornamental (52%). This extensive survey has increased the known alien vascular flora of Galapagos by 257 species, giving a ratio of alien to native taxa of 1.57:1. It provides a crucial baseline for plant invasion management in the archipelago and contributes data for meta analyses of invasion processes worldwide. A repeat of the survey in the future would act as an effective early detection tool to help avoid further invasion of the Galapagos National Park.

Plant invasions are causing habitat degradation in Galapagos. Problems are concentrated on the four inhabited islands. Plants introduced to rural areas in the humid highlands and urban areas on the arid coast act as foci for invasion of the surrounding Galapagos National Park. Here we present results of the most comprehensive inventory to date of alien vascular plants in the inhabited areas of Galapagos. The survey was conducted between 2002 and 2007, in 6031 properties (97% of the total) on Floreana, Isabela, San Cristobal and Santa Cruz Islands. In total 754 alien vascular plant taxa were recorded, representing 468 genera in 123 families. Dicotyledons represented 554 taxa, monocotyledons 183, there were 7 gymnosperms and 10 pteridophytes. Almost half (363) of the taxa were herbaceous. The most represented families were Fabaceae (sensu lato), Asteraceae and Poaceae. The three most recorded species in the humid rural areas were Psidium guajava, Passiflora edulis and Bryophyllum pinnatum, and in the dry urban areas, Aloe vera, Portulaca oleracea and Carica papaya. In total, 264 (35%) taxa were recorded as naturalized. The most common use for taxa was ornamental (52%). This extensive survey has increased the known alien vascular flora of Galapagos by 257 species, giving a ratio of alien to native taxa of 1.57:1. It provides a crucial baseline for plant invasion management in the archipelago and contributes data for meta analyses of invasion processes worldwide. A repeat of the survey in the future would act as an effective early detection tool to help avoid further invasion of the Galapagos National Park.