Ecological and socioeconomic impacts of invasive alien species in island ecosystems

Minimizing the impact of invasive alien species (IAS) on islands and elsewhere requires researchers to provide cogent information on the environmental and socioeconomic consequences of IAS to the public and policy makers. Unfortunately, this information has not been readily available owing to a paucity of scientific research and the failure of the scientific community to make their findings readily available to decision makers. This review explores the vulnerability of islands to biological invasion, reports on environmental and socioeconomic impacts of IAS on islands and provides guidance and information on technical resources that can help minimize the effects of IAS in island ecosystems. This assessment is intended to provide a holistic perspective on island-IAS dynamics, enable biologists and social scientists to identify information gaps that warrant further research and serve as a primer for policy makers seeking to minimize the impact of IAS on island systems. Case studies have been selected to reflect the most scientifically-reliable information on the impacts of IAS on islands. Sufficient evidence has emerged to conclude that IAS are the most significant drivers of population declines and species extinctions in island ecosystems worldwide. Clearly, IAS can also have significant socioeconomic impacts directly (for example human health) and indirectly through their effects on ecosystem goods and services.These impacts are manifest at all ecological levels and affect the poorest, as well as richest, island nations. The measures needed to prevent and minimize the impacts of IAS on island ecosystems are generally known. However, many island nations and territories lack the scientific and technical information, infrastructure and human and financial resources necessary to adequately address the problems caused by IAS. Because every nation is an exporter and importer of goods and services, every nation is also a facilitator and victim of the invasion of alien species.Wealthy nations therefore need to help raise the capacity of island nations and territories to minimize the spread and impact of IAS

Spatio-temporal Analysis of An Invasive Alien Species, Vachellia nilotica, on Rodrigues Island, Mauritius, Using Geographic Information Systems and Remote Sensing Techniques

Invasive alien species (IAS) constitute a large and growing environmental and socio-economic problem. Tropical islands, one of the richest habitats in the world, are especially vulnerable to invasions because of their island-specific flora and fauna. The aim of this study is thus to assess the viability of monitoring IAS distributions on small tropical islands using Geographic Information Systems (GIS) and remote sensing techniques, focusing on the invasive plant species Vachellia nilotica on the island of Rodrigues as a case study. Freely available satellite images are used to conduct the analysis with resulting classified maps having accuracy levels in the high 70s. The results reveal a significant increase in Vachellia coverage from 2013 to 2023 especially along the coasts while the simulation for 2033 indicates an inward migration from coasts to the central plateau which could have severe repercussions on the native vegetation and human activities. Given the high invasive potential of Vachellia, the present findings can support conservation actions and decision making and even support community participation in managing this IAS. In the broader context, the study demonstrates the potential of GIS and remote sensing as cost-effective tools for monitoring certain invasive plant species

A test of methods for estimating population size of the invasive land snail Achatina fulica in dense vegetation

1. Physical inaccessibility often complicates censuses of poorly mobile organisms. We therefore assessed the effectiveness of using a sample of quadrat counts to generate a population estimate corrected for inaccessible areas. The result is directly applicable to management of the introduced snail Achatina fulica on Ile aux Aigrettes, a small island off Mauritius, but also has implications for counting this and similar species elsewhere. Accurate counting of A. fulica is important given that this species is such a widespread and serious pest. 2. Counts were made in 17 quadrats taken from a grid covering the island. These were used to produce one population estimate by interpolating for the rest of the grid using GIS software (method 1). A second estimate assumed equal density of snails in accessible and inaccessible parts of the 17 quadrats, again with the population estimate interpolated (method 2). 3. Four further quadrats were cleared of vegetation and, by comparison of counts before and after clearance, the relationship between initial count and true snail number was estimated. This resulted in two further population estimates, with the relationship used to adjust counts in the 17 experimental quadrats before interpolation (methods 3 and 4). 4. All four estimates were tested using 35 additional quadrats of two types. Type 1 quadrats were physically cleared of vegetation; type 2 quadrats were fully accessible without clearance. Predicted counts in these quadrats were assessed for accuracy by comparison with actual counts. 5. The method 1 estimate was clearly inadequate; method 4 gave a consistent overestimate; method 2 gave the smallest error in both quadrat types. In type 1 quadrats, method 2 and 3 estimates were not significantly different and method 2 had a slight tendency to underestimate. Overall, for studies of A. fulica , method 2 is recommended. However, it should be noted that the study took place towards the end of the wet season. In the dry season, damp refuges under inaccessible vegetation may be more important and methods 3 and 4 may then give a better estimate. 6. The population of A. fulica with shell length > 10 mm on Ile aux Aigrettes near the end of the wet season in 2000 was between 37 300 and 45 100, with 39 700 being the best estimate. 7. The results underline the importance of considering inaccessible areas when accurate counts of species are needed, and a method is suggested by which a simple census technique can be adjusted. In the case of A. fulica , more accurate estimates of population size and distribution are invaluable in the management, monitoring and eradication of this invasive species