Invasive species: the critical yet complex group driving biodiversity loss

In Europe alone, the damage costs from alien species, aka invasive species, on human societies is at least USD 21.52 billion. This is in part because the rate of introduction of invasive species has increased globally at an unprecedented rate, with no signs of slowing down.

The implementation of national legislation and international agreements aiming to reduce invasive species threats to biodiversity, the economy and human well-being have also increased during the last decades.

However, avoiding economic and biodiversity impacts involves multiple challenges, such as defining what is an invasive species, predicting which species are candidates to become invasive and which ecosystems are candidates to become hosts, and incorporating the social perceptions and the economic impact of invasion prevention and management.

What is an invasive species?

Invasive species can lead to the reduction of native species diversity through different mechanisms:

1. predation: where invasive species consume natives
2. disease transmission: where pathogens, such as fungi or viruses that arrive to the invaded range with the invasive species, are lethal for native species as they are not adapted to the new pathogen
3. competition: where invasive species use the same resources as native species and are better at exploiting them, thus replacing natives in their habitat (this could be due to an inherent higher performance of alien species or because of increases in performance during the invasion process, e.g. by escaping from their natural enemies)
4. by altering the local environment: leading to changes in local conditions that affect resource levels (e.g. excreting substances that modify the habitat of natives) or destroying, creating and modifying ecosystems structures (e.g. drying a pound or the emergence of tall new trees).

In most cases, invasive species don't lead to immediate species extinctions at local scales. Instead, we often see that invasive species increase biodiversity at local to regional scales, and homogenise and reduce biodiversity at global scales. It's important, therefore, to consider the scales of measuring invasive species when addressing their impact.

Figure 1: Global temporal trends in first record rates (dots) for all species (a) and taxonomic groups (b–q) with the total number of established alien species during the respective time periods given in parentheses. Data after 2000 (grey dots) are incomplete because of the delay between sampling and publication, and therefore not included in the analysis. For visualisation, 50-year periods are distinguished by white/grey shading. From Seebens et al. 2017 Nat com

How do we define an invasive species?

Currently, we only really know if something is a going to be a devastating invader when we observe negative impacts – and by then, the ship has sailed in terms of effectively managing it. In addition, the term "native" is often defined in terms of the species origin countries, with country borders being political rather than ecological realities.

For example, if a plant is native to northern France and is suddenly observed in southern Belgium, is it really an invader? Or is this just part of its natural spatial population fluctuations? Are plants that spread to new regions after glacial retreat (both historic and modern) non-native?

Management of invaders

In general, preventing the entry of alien species is more cost effective than control and wiping it out. Ideally, we would manage every single alien species on the chance that it may become invasive. However, the monitoring costs alone of trying to detect every new potential invader are beyond the available resources of most countries (although, there are some research projects looking improve the diagnosis process, like AlienImpacts).

There is a gradient of management strategies, where the most efficient ones are also extremely strict and bureaucratically costly, as well as more challenging to apply in countries with borders less easy to control than islands.

Most national prevention regulations are based on "black-lists" of potentially invasive or dangerous species. However, these lists are not enough to keep up with the increasing rates of species introductions, probably because many species are introduced "accidentally."

In the 1990s, New Zealand adopted the Biosecurity Act, which has proven to be the most efficient regulation on invasion prevention. It also represents the most strict and comprehensive national regulation. It is based on a "white-list" of permitted species – implying that any unlisted species is preventively forbidden to enter the islands unless a comprehensive risk assessment confirms its safety for national ecosystems.

Where does COP15 fits in?

This all raises questions about COP15’s ultimate goal of “living in harmony with nature”. For managing invasive species, what does this harmony look like? Is it trying to maintain the current status quo and stop any sort of change in distribution of any species, regardless of cost? (We would argue no).

Or is it acknowledging that we now live in a fully connected world that is always changing, and invasive species are an inevitable part of this, so we shouldn’t try to alter this? (We would argue definitely not).

The answer is probably somewhere in the middle of this continuum.

The problem for COP15 – a problem that applies to not just invasive species but all issues affecting biodiversity – is that some countries and regions are more impacted than others and have different resources (i.e. money) available to deal with this. Therefore, different actors will likely lie at different points along this continuum, depending on their specific contexts. This makes reaching a unilateral agreement exceptionally difficult – but doing so is vital for the maintenance of biodiversity on our planet.