In my last post on invasive species Human influence on invasive species, I showed that human had a huge influence on the geographic rearrangement of the earth’s biota.

Without human assistance species would have managed to spread, but much less often, much more slowly, and definitely not so far

I also told that some of those introductions became a threat to native species and ecosystems and for human well-being, because they became INVASIVE, with high monetary costs!

Today I want to explain how introduced species turns invasive

First, and fortunately, not all introduced species become invasive. For that, the non-native species must pass through at least three stages before they are able to inflict ecological or economic harm, and only a small amount has that ability.

Source: Lockwood JL, Hoopes MF, Marchetti MP (2007) Invasion Ecology. Blackwell Publishing, Oxford, UK, p. 9.

Invasion process starts with transportation of non-native species into a new geographic area, were are released into the wild (Transport). From these, some are able to establish a self-sustaining population without man direct interference (Establishment). The next step is taken by an established non-native population that is able to grow in abundance and expand its geographic range (Spread). Ecological and economic impacts are noticed only after the population got spread, earning the name "invasive".

Each of these stages represents biologically identifiable steps or ecological barriers that are overcome by invasive ones. Many factors contribute to this invasive process, namely biological attributes of the species, environmental factors, propagule pressure (introduction effort) or residence time.

Resume - Invasive species definition

An invasive species is one that arrives, whether deliberately or inadvertently with human assistance, in a habitat it had not previously occupied, then establishes a population, reproduces and spreads widely beyond the initial point of introduction.

Here in Portugal - some examples

Ceratonia siliqua - carob tree

The carob tree was introduced in Portugal by the Arabs in VIII century, and was first reported as naturalized in 1911 (source). This tree got naturalized in South of Portugal, and nowadays it gained environmentally and economically importance.

Adult carob tree

Carob tree mature pods

Acacia dealbata - silver wattle

The silver water tree is one of the worst invasive plants in our country. The first reference to its presence was registered in 1850, in a small farm close to Lisbon (source), being after disseminated by some enthusiasts and planted as a forest species. Nowadays, it occupies large areas on the mountains from northern and center of Portugal, but also in the riverside in all the territory.

Riverine vegetation was substituted by silver wattle tree invasion

Silver wattle inflorescence (really beautiful, hum? That was the problem)

These 4 photos were taken during my field work

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P.S. 1 - Did you find this matter of interest? Feel free to comment​ and upvote :)

Have a nice Friday and weekend!!