Use of introduction characteristics of Mediterranean alien plants to predict their invasion success
The possibility for developing a model to evaluate the invasiveness of species introduced to Mediterranean islands was explored.
A database of all non-native plant records whether naturalized, casual or planted was compiled from 100 published flora covering 79 islands in the Mediterranean Basin. Species considered to be native in any part of the region were omitted, leaving 862 species. Descriptive information on plant species was provided as follows:
- to characterize propagule pressure, a frequency of introduction index was created
- mode of introduction: crop species, forestry species, horticultural uses (small scale), grown in public places, grown for ornamental purposes in gardens, introduced accidentally with seeds or soil, other accidental introductions
- date of introduction: archeophyte, 1500-1800, 1800-1900, 1900-1950, 1950+
- region of origin: Europe, Middle East and North Africa, North and Central Asia, Far East, South and South East Asia, Central Africa, South Africa, Australasia, Mexico-California, North America, South America, Neotropics, Oceanic islands, uncertain
- pollination strategy: water-pollinated, wind-pollinated, insect-pollinated with radial symmetry of the corolla, insect-pollinated with zygomorphic symmetry or tubular corolla, self-pollinated, pollinated by birds or mammals
- growth form: geophyte, therophyte, chamaephyte/hemicryptophyte, phanerophyte, suffrutescent, hydrophyte, vine, succulent
- range size: number of regions in which the species is native
- clonality: non clonal, clonal, clonal with rapid vegetative spread.
A Canonical Discriminant Analysis (CDA) was used to find species traits based on four invasion categories: never naturalized (140 species), naturalized on 3 islands or fewer (105 species), naturalized on 4-9 islands (31 species), naturalized on 10 or more islands (18 species).
Data suggest that the likelihood of naturalization was almost three times lower for archeophytes compared to species introduced in the past century. There is therefore a basic decrease in mean invasion success with increasing residence time. This may be because the historical origins of the Mediterranean flora remain obscure, and many of the better naturalizers from ancient times may be considered native.
The nature of introductions over the centuries was analyzed. The archeophyte flora was dominated by highly domesticated species imported for essential uses, mainly as food crops. Recent arrivals were more likely to be accidental and from the New World. The mode of introduction is a major factor affecting the rate of spread, and private introductions to gardens appear to be less of a risk than those planted widely in the public domain. Moreover, the rate of invasion was found to be higher for non-clonal species having good dispersal abilities than for clonal species.
Naturalization success was higher for species originating from other Mediterranean climates and succulents were the most successful growth form. Low naturalization ability is correlated with the regions of origin covering temperate Eurasia, and with phanerophytes and forestry species, suggesting that trees generally have a low success rate.
This estimate relied on the number of islands on which the species was naturalized, which does not reflect necessarily the negative ecological and economic impacts caused and is not necessarily a true reflection of the potential of a species to become invasive at small spatial scale.
Sources
Lambdon PW, Hulme PE (2006) Predicting the invasion success of Mediterranean alien plants from their introduction characteristics. Ecography. 29, 853-865
This study was conducted as part of the EU Framework 6 project ALARM and the database assembled during the framework 5 project EPIDEMIE.