Insular threat associations within taxa worldwide

Abstract

The global loss of biodiversity can be attributed to numerous threats. While pioneer studies have investigated their relative importance, the majority of those studies are restricted to specific geographic regions and/or taxonomic groups and only consider a small subset of threats, generally in isolation despite their frequent interaction. Here, we investigated 11 major threats responsible for species decline on islands worldwide. We applied an innovative method of network analyses to disentangle the associations of multiple threats on vertebrates, invertebrates, and plants in 15 insular regions. Biological invasions, wildlife exploitation, and cultivation, either alone or in association, were found to be the three most important drivers of species extinction and decline on islands. Specifically, wildlife exploitation and cultivation are largely associated with the decline of threatened plants and terrestrial vertebrates, whereas biological invasions mostly threaten invertebrates and freshwater fish. Furthermore, biodiversity in the Indian Ocean and near the Asian coasts is mostly affected by wildlife exploitation and cultivation compared to biological invasions in the Pacific and Atlantic insular regions. We highlighted specific associations of threats at different scales, showing that the analysis of each threat in isolation might be inadequate for developing effective conservation policies and managements.

Figure 1

Graphical representation of species–threats interactions for (a) extinct species (n = 249) and (b) currently threatened species (n = 4,127) linked to the 11 threats. Colorful nodes reflect threats and gray nodes represent (a) extinct species or (b) species that are currently vulnerable (light gray), endangered (gray), and critically endangered (dark gray). Threat descriptions are given in Supplementary Table S4. The node size is proportional to their degree (i.e., number of interactions), and the percentage of the strongest interactions is indicated in the figure (see Supplementary Table S1 for further information). Top ten threats either acting alone or in association among (c) extinct (with 11 single threats and 23 threat associations) and (d) threatened species (with 11 single threats and 437 threat associations). Colorful bars reflect single threats, and white bars represent threat associations. Figures were created using Gephi 0.9.1 (https://gephi.org), R 3.3.1 (https://r-project.org), and Inkscape 0.91 (https://inkscape.org).

Figure 2

Graphical representation of networks connecting the proportion of threats (colored bars) to (a) extinct and (b) threatened species for eight taxa: birds, mammals, amphibians, reptiles, freshwater fish, plants, arthropods, and gastropods (top bars in gray). The width of links between taxa and threats is proportional to the sum of species–threat connections (see Supplementary Table S1 for further information). Figures were created using R 3.3.1 (https://r-project.org) and Inkscape 0.91 (https://inkscape.org). Icons made by Freepik from www.flaticon.com under a Flaticon Basic License.

Figure 3

Graphical representation of networks illustrating species–threats interactions according to regions for (a) extinct and (b) threatened species. Colorful nodes reflect threats, and gray nodes represent insular regions. The node size is proportional to their degree (i.e., number of interactions), and the width of their respective links is proportional to the sum of species–threats connections. For the sake of clarity, threat nodes were placed outside the world map. Figures were created using Gephi 0.9.1 (https://gephi.org), QGIS 2.18.2 (https://qgis.org), and Inkscape 0.91 (https://inkscape.org).