**2. Biodiversity and endemism of oceanic islands**

There are around 374,000 plants species on earth [11], but their distribution is uneven, with the tropical environments presenting larger numbers than other environments. This is a result of ecoevolutionary drivers which include the climatic stability over the past million years associated with time, energy availability, and biotic interactions [12]. Consequently, some areas of the globe have been recognised as global biodiversity hotspots as they exhibit exceptionally high species richness and high endemism levels [13]. Mittermeier et al. [14] have defined 35 biodiversity hotspots, many of which are oceanic and continental island archipelagos.

The colonisation of the small oceanic islands depends on geographical and environmental drivers, being inversely related to the distance to other lands [15]. Another important factor is the dispersal ability of the organisms. The geographical range of a *taxon* depends on its ability to disperse its pollen and its seeds. In the case of an island, this dispersal can occur through anemocory (wind dispersal), endozoochory (in the gut of animals), epizoochory (attached to the exoskeleton, fur, feathers of scales of animals) or thalassochory (floating in the water) [16].

When a plant species is able to reach a new territory, it depends on its ability to adapt to the physical and chemical characteristics of the island, and to other biotic factors such as competition, herbivory, parasitism, and symbiosis [3]. The few taxa that survived and adapted to the new environment may therefore evolve into new species. Due to the time these adaptive processes take, island age is an important factor for the biodiversity of oceanic islands, as older islands have a higher probability of successful colonisation. They also had more time for selection processes to act on the first colonisers, so that natural selection takes place, thus constituting a favourable factor for speciation. Because of their evolutionary processes, oceanic islands are poor in the number of species for their size, but present a remarkable high ratio of endemism, and the ecosystems exhibit much higher biodiversity than terrestrial ecosystems for the same area [1, 6]. E.g., the East Melanesian Islands,

**Figure 2.**

*Endemic plant species from oceanic islands. Top left:* Hibiscus arnottianus*, from Hawaii; top right:*  Brachycereus nesioticus *from Galapagos; bottom left:* Bikkia tetrandra *from Mariana; bottom right:* Viola paradoxa *from Madeira.*

comprising the Solomon islands, Vanuatu and Papua New Guinea, include around 8,000 plant species of which about 3,000 are endemic, the Atlantic islands of Macaronesia are the third richest hotspot in the world in terms of its plant biodiversity (25,000 species); 5,330 species of native vascular plants are native to Polynesia-Micronesia, of which more than 3,070 are endemic, Japan has more than 5,600 plant species of which roughly a third are endemic [17]. Hawaii archipelago also has about 1180 native vascular plants, of which 1000 are angiosperms. Of these, about 900 are endemic (**Figure 2**) [18].

These endemic species, however, present restricted geographical range, specialised environmental niche, limited dispersal ability and reduced size population and distribution [19]. The islands with high large proportion of endemic plants are mainly the high volcanic islands, while most the low islands are species poor. The smaller the island is, the more isolated, and the less the topographic relief, the poorer the island. This is due to the reduced variety of habitats and the broad mix of the typically sea-dispersed strand species that dominate their floras [1].
