Comparative patterns of plant invasions in the mediterranean biome

The objective of this work was to compare and contrast the patterns of alien plant invasions in the world’s five mediterranean-climate regions (MCRs). We expected landscape age and disturbance history to have bearing on levels of invasion. We assembled a database on naturalized alien plant taxa occurring in natural and semi-natural terrestrial habitats of all five regions (specifically Spain, Italy, Greece and Cyprus from the Mediterranean Basin, California, central Chile, the Cape Region of South Africa and Southwestern - SW Australia). We used multivariate (hierarchical clustering and NMDS ordination) trait and habitat analysis to compare characteristics of regions, taxa and habitats across the mediterranean biome. Our database included 1627 naturalized species with an overall low taxonomic similarity among the five MCRs. Herbaceous perennials were the most frequent taxa, with SW Australia exhibiting both the highest numbers of naturalized species and the highest taxonomic similarity (homogenization) among habitats, and the Mediterranean Basin the lowest. Low stress and highly disturbed habitats had the highest frequency of invasion and homogenization in all regions, and high natural stress habitats the lowest, while taxonomic similarity was higher among different habitats in each region than among regions. Our analysis is the first to describe patterns of species characteristics and habitat vulnerability for a single biome. We have shown that a broad niche (i.e. more than one habitat) is typical of naturalized plant species, regardless of their geographical area of origin, leading to potential for high homogenization within each region. Habitats of the Mediterranean Basin are apparently the most resistant to plant invasion, possibly because their landscapes are generally of relatively recent origin, but with a more gradual exposure to human intervention over a longer period

Number of naturalized plant species in the five mediterranean-climate regions according to their native origin.

<p>Numbers in parenthesis are percentages within a region.</p>*<p><i>Mediterranean Basin and one region in Europe or Asia or N. Africa.</i></p>**<p><i>Regions in Africa and</i>/or <i>Europe and/or Asia.</i></p

Non-metric multidimensional scaling ordination (NMDS), (stress = 0.18) showing taxonomic differences in naturalized species among habitats in each of the five mediterranean-climate regions (MCRs).

<p>CA: California, CL: Chile, MB: Mediterranean Basin; SA: South Africa; SWA: SW Australia. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0079174#pone-0079174-t005" target="_blank">Table 5</a> for explanation of habitat acronyms.</p

Number of naturalized species for each of the five mediterranean-climate regions (a), number of species occurring in more than one of the five regions with indication of their percentage in parenthesis (b), and pooled number of species (c).

<p>Areas in 10⁶ km² (Cowling et al. 1996): California = 0.32; Chile = 0.14; Mediterranean Basin = 0.93 (for the 4 countries studied); South Africa = 0.09 SW Australia = 0.31.</p><p>The geographical distribution of the five MCRs included in this study is depicted in citation <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0079174#pone.0079174-Cowling1" target="_blank">[13]</a>.</p

Pair-wise numbers of shared naturalized species in the five mediterranean-climate regions of the world.

<p>Numbers in bold correspond to the number of species existing only in the assigned region. Numbers appearing in parentheses are percentages of the total number of aliens of the country in the same row.</p

Dendrogram (Bray Curtis metric, UPGMA) showing similarity of mediterranean-climate regions habitats based on the basis of naturalized species between habitat types regardless of region.

<p>See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0079174#pone-0079174-t005" target="_blank">Table 5</a> for explanation of habitat acronyms.</p

Number of naturalized species occurring in each habitat category identified in the five mediterranean-climate regions, regardless of the region.

<p>Percentages (in brackets) are calculated on the basis of the total number of plants in the last row. Note that the same taxon might appear as different growth form in each region and this explains the difference in total plants per habitat compared to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0079174#pone-0079174-t005" target="_blank">Table 5</a>.</p

Number of naturalized species invading each habitat category identified in the five mediterranean-climate regions.

<p>Totals represent the total number of species in each country and in each habitat. Percentages (in brackets) are calculated on the basis of the total number of plants in the last row. Empty cells denote that there are no alien species records but habitat category occurs in the region; dash (–) denotes that the habitat category is absent from the region.</p><p>For Australia details are given in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0079174#pone.0079174.s001" target="_blank">Appendix S1</a> with habitat categories.</p

Naturalized alien species similarity across the five regions of the mediterraean biome.

<p>Naturalized alien species similarity across the five regions of the mediterraean biome.</p

Examples of naturalised alien shrub and tree species in the five mediterranean-climate regions.

<p><b>2a</b>) The southern USA succulent shrub <i>Opuntia ficus-barbarica</i> in abandoned grazed lands, Stylida, Greece, Mediterranean Basin; <b>2b</b>) The Mediterranean basin region shrub, <i>Spartium junceum</i> in the Cape Region, South Africa; <b>2c</b>) The central Eurasian tree <i>Tamarix ramossissima</i>, in coastal California; <b>2d</b>) Two Australian shrub species, <i>Hakea gibbosa</i> and <i>Acacia longifolia</i> in the Cape Region, South Africa; <b>2e</b>) The European spiny fabaceous shrub, <i>Ulex europaeus</i> in Chillán, Región del BioBio; and <b>2f</b>) The east Asian tree <i>Ailanthus altissima</i> in central Greece, Mediterranean Basin. Each of these species have become naturalised in multiple Mediterranean-climate regions. Photo credits Margarita Arianoutsou <b>2a</b>, <b>2f</b>; Lesley Henderson <b>2b</b>, <b>2d</b>; Philip Rundel <b>2c;</b> Jonathan Urrutia <b>2e</b>.</p