Invasive species impacting the functioning and services of island protected areas through losses of endemic species.
Lead Author: Carl Beierkuhnlein (UBT)
Contributors: Carl Beierkuhnlein (UBT), Severin Irl (UBT), Samuel Hofmann (UBT), Andreas Schweiger (UBT), Yrneh Ulloa (UBT)
Islands are of outstanding importance for speciation and for the development of species diversity. They host a high number and a high proportion of endemic species (see examples from ECOPOTENTIAL PAs in figures). However, these species are highly threatened by global extinction because they are not adapted to the pressures by non-native herbivory and invasive species that were introduced by humans to these isolated ecosystems. In consequence, biodiversity loss and subsequent loss of ecosystem functioning and services is likely to occur if no counteractive measures are implemented in order to avoid threats and restrictions for humans on islands that are directly dependent on the services provided by nature.
Figure 1: One of the last remaining individuals of the only remnant population of Lotus eremiticus, a species that is highly threatened by global extinction because it is intensely browsed by invasive herbivores (rabbits, goats).
Furthermore, the loss of endemic species would also mean a loss of aesthetic values (see Figure 1) and potential economic profit from ornamental plants. Especially the Canary Islands host a high number of taxa that have become enormously important in gardening worldwide (e.g. Argyranthemum spec., Aeonium spec. Echium spec.).
The conservation value, however, and the targets of protected areas are difficult to define, because the patterns of total species richness, richness of endemic species and of the proportion of endemic species (meaning a possible total loss of ecosystem functioning) differ. Areas that are of high value for these protection targets are not the same (see Fig. 2). Priorities must be defined.
Figure 2: Spatial interpolation maps of (a) species richness, (b) number of single-island endemics (nSIE), (c) number of archipelago endemics (nAE), (d) percentage of single-island endemics (pSIE) and (e) percentage of archipelago endemics (pAE) (Irl et al. 2015).
Particularly, island ecosystems are under threat by introduced species such as herbivores (rabbits, goats) and invasive plant species (Fig. 3). Island plants did not evolve (or loose) protective traits against herbivory such as thorns or poisonous compounds (Irl et al. 2014). Often they are also less competitive than non-native plant species, increasing the invasibility of island systems.
On the one hand, introduced herbivores pose a threat to endemics by preferentially browsing them, thereby increasing the extinction risk of already range-restricted species (because non-native plant species often possess mechanical and chemical defense mechanisms deterring introduced herbivores, while endemic species often have lost these traits as a result of speciation process in the absence of mammalian herbivores). On the other, invasive plant species that outcompete native island species especially at low elevations. Often non-native plant species are highly competitive owing to their wide ecological amplitude, fast reproduction, high anthropogenic disturbance tolerance, efficient resource use and effective defense mechanisms against herbivores.
Figure 3: Browsing of rabbits on the leaves and juvenile branches of the single-island endemic subalpine species Genista benehoavensis that was close to extinction in the summit region of La Palma Biosphere Reserve. The loss of sensitive species has lead to species poor subalpine scrubs that are expected to have low resilience towards climatic extremes.
Figure 4: Fenced population of the shrub species Chamaecytisus proliferus (Fabaceae) in pine forest ecosystems of La Palma. This endemic species is completely browsed by rabbits outside of the fences where the field layer consists almost only of pine needle litter and open soil.
References
Irl, S.D.H., Harter D.E.V., Steinbauer, M.J., Gallego Puyol, D., Fernández-Palacios, J.M., Jentsch, A., Beierkuhnlein, C. (2015) Climate vs. topography – spatial patterns of plant species diversity and endemism on a high elevation island. Journal of Ecology, 103, 1621-1633 (2015). doi:10.1111/1365-2745.12463
Irl, S; Steinbauer, M; Messinger, J; Blume-Werry, G; Palomares-Martínez, A; Beierkuhnlein, C; Jentsch, A (2014b) Burned and devoured - Introduced herbivores, fire and the endemic flora of the high elevation ecosystem on La Palma, Canary Islands, Arctic Antarctic and Alpine Research, 46, 859-869