development and approbation of methodology for monitoring invasive plant species: the case of Latvia.
Categorisation of invasive alien species based on their impact is an important way to improve the management of biological invasions. The impact of 35 alien plant species in Latvia was evaluated based on information in the literature and certain studies of their environmental and socio-economic impacts. As a result, 15 priority monitorable species, or the Black List, have been evaluated, for which seven of the nine criteria set were met. The other 20 invasive plant species make up the list of monitorable invasive plants, or the Grey List. The list and methodology developed during the study were approbated in the vegetation season of 2016 in 16 randomly selected monitoring quadrates, which were further stratified according to geobotanical regions of Latvia, which is proportionally 1,57% from all 1017 quadrates of Latvia. In total, 34 alien species were identified during approbation of methodology as showing signs of invasiveness: 10 from Black List, 10 from Gray List and 14 other invasive species, not included in the monitorable species list of developed monitoring methodology. In general, half of the species found during approbation of the methodology are trees and shrubs that were deliberately imported into Latvia in the past to be used in landscape gardening. A large proportion (41%) of the invasive species encountered come from North America. The most commonly encountered invasive species are Elodea canadensis Michx., Impatiens parviflora DC. and Solidago canadensis L. The data obtained through the field approbation show that the areas rich in invasive species are covered by forests and transit corridors, but the areas where no invasive species have been detected are transition mires and raised bogs as well as intensively managed farmland. Following the developed methodology, it was concluded that 50% of the quadrates to be monitored should be randomly selected while maintaining the principle of geobotanical regions, while the other 50% should be selected in previously known invasive plant populations.