In my last post, I shown that not all green is good. That some plant species are a threat and can even make other plant species disappear. The bad guys plants are what is called invasive alien plants, non-native plants or exotics plants.
Most people would know the problem that invasive alien animals can cause. For example, only one species of mosquito, the tiger mosquito, is responsible increasing cases of dengue, chikungunya and zika in the last decades.
But the impact of invasive alien plants is less known as fewer species can disrupt human life as quick as non-native animals. The famine weed, Parthenium hysterophorus, it is a small herb that is disrupting lives in Africa – it decreases crop yields, make meat unfit for consumption, causes health problems like asthma, among other problems.
But many invasive alien species are a slower trouble-maker. Well, slower just on our on perception of time. Large trees can survive for decades and centuries, so competition with invasive alien plants needs to be measure on larger time scales.
In the island of Mauritius, in the Western Indian Ocean, this process the very few best preserved and protected native forests are today dominated by alien species! In average, nearly 80% of the stems of these best preserved forest are of non-native species.
The main problem is that the levels of invasion is set to get worst. The invasion is progressing, slowly but surely. When comparing todays number of species, number of stems and basal area of forest on forest areas where surveys were done in 1930s and in 1980s, we see a clear tendency of the invasion getting worst making the native forest, little by little, be submerged under this tsunami of alien species.
Some endemic species have disappeared from these forests because of the invasion. The number of trees in one of these forests have more than halved in near 70 years. Also the biomass of native trees in this forest is less than half of it was in 1930s.
This high invasion is also seen in other forest on tropical oceanic islands around the world. The invasion is putting the planet on the way to lose many unique species, many unique interactions, many unique ecosystems. All these without even considering the effects that global warming (that is a fact and not a hoax) is and will cause.
Florens et al in press. Long term declines of native trees in a oceanic island’s tropical forests invaded by alien plants. Applied Vegetation Science. DOI: 10.1111/avsc.12273
Florens et al 2016. Invasive alien plants progress to dominate protected and best-preserved wet forests of an oceanic island. Journal for Nature Conservation 34: 93–100