The immediate effects of tropical cyclones are treefalls and canopy defoliation. If cyclones are frequent and intense, this will result in shorter and denser forests (see my earlier post). Furthermore, more severe cyclone impacts, as are forecast as a result of anthropogenic climate change, would produce more openings in the forest canopy. Such conditions should favour fast-growing, disturbance-adapted species and, therefore, early successional species will likely benefit from the forecast intensification of cyclones. If there are winners, will there also be species that are likely to lose out?
Our recent study suggest that species that are rare in a community may be the losers, as forests on islands that were heavily impacted by cyclones had fewer rare species than those on less impacted islands. Such rare species are likely to be trees that require shady conditions for successful establishment and growth, making them highly susceptible to cyclones. Many of the endemic species in the Pacific would be expected to be slow-growing, shade tolerant species, suggesting that increased cyclones impacts due to climate change may negatively affect many endemic species in Oceania.
This threat of increased cyclones to rare endemic species is exemplified by anecdotal evidence about the critically endangered conifer Acmopyle sahniana, which is endemic to Fiji’s largest island (Viti Levu). One of the six subpopulations of this rare conifer, known as drautabua, occurred in the Mount Koroyanitu Range but had disappeared by the 1990s. This extirpation is believed to be the result of several cyclones that severely affected this mountain range in the 1970s and 1980s. While the impacts of the cyclones on this subpopulation were not documented, this case highlights the potential impacts severe cyclones could have on small populations of susceptible species.
Are therefore many of the endemic species in Oceania doomed to be driven to extinction by increasingly more intense cyclones? Fortunately, many endemic species, such as Acmopyle sahniana, have more than one subpopulation and the impacts of cyclones are spatially variable, meaning that in most cases part of a species’ population should survive. This, however, does not diminish the impacts future cyclones will have in reducing population numbers and extirpating subpopulations. In some cases, this will likely lead to the eventual extinction of some endemic species.
Related article: Ibanez T, Keppel G, Baider C, Birkinshaw C, Florens FV, Laidlaw M, Menkes C, Parthasarathy N, Rajkumar M, Ratovoson F and Rasingam L. 2020 (early view). Tropical cyclones and island area shape species abundance distributions of local tree communities. Oikos, https://onlinelibrary.wiley.com/doi/full/10.1111/oik.07501.