Invader Intolerance: A presumptuous viewpoint?

As my introductory post touched upon, there are many anthropogenic impacts effecting biodiversity (see below for a comprehensive list!). 

- Habitat destruction and fragmentation

- Agricultural intensification and changes in land use

- Changes in forest management practices

- Overfishing

- Atmospheric pollution

- Water pollution

- Climate change

- Human disturbance

- Invasive species

- Harvesting and collection of species (hunting)

- Ocean acidification

(Natural England, 2011) 

The focus for today is invasive species. Before I start chattering on about zebra mussels and the red squirrel, I believe it is crucial to understand just how scientists define and quantify biodiversity. The term 'biodiversity' originates from conservation biology and was coined by Walter Rosen in 1985. A formal definition was published by the Convention of Biological Diversity (1992) which stated:

'Biodiversity is the variability among living organisms including terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems'. 

Biodiversity measures have, however, also taken a battering over the years.  Wilson (1992) discussed the ambiguity surrounding measures of diversity and explored the possibility that biodiversity was in fact restricted by measuring species numbers alone. This has also been reiterated more recently by (McCann, 2007).  Nowadays there is still uncertainty when quantifying biodiversity (scientists can never make up their minds) and therefore this will be considered in much greater detail in a few weeks. 

So now back to the purpose of this post- 'INVASIVE SPECIES'. Although known worldwide as threatening and disruptive, non-native species and their impacts to biodiversity continue to be greatly debated throughout scientific literature. So why do invasive species have such a bad reputation? Invasive, known also as 'alien' and 'non- native' species can have significant effects on the existence of some species and their habitats. An example is the native red squirrel (Sciurus vulgaris) which has become threatened and outcompeted in the UK by the non-native 'grey' squirrel (Sciurus carolinensis). The first evidence of the red squirrel dates back to the end of the last ice age, 10,000 years ago. 

Save our native: The Red Squirrel

However, since the grey squirrel became introduced to the UK in the 19th Century it has replaced the red squirrel throughout most of England, Wales and parts of Scotland and Ireland (Bryce, 1997). The traditional explanation for this reduction in the red squirrel, is the overwhelming competition with the grey squirrel for food resources (Kenward and Holm, 1993). This was not the only reason for their gradual decline, another proposal is the introduction of devastation parapoxvirus (a fatal strand of squirrel "pox") that was brought from North America to the UK by the grey squirrel. The disease spread into the red squirrel population hence the rate of infection was extremely high. This induced a population crash, reducing the crowding pressures on non-native grey squirrels which subsequently increased in numbers (Gurnell et al. 2004). 

Squirrel distribution maps from 1945- 2010 showing the interaction between the red and grey squirrels (RSST, n/d). 

From the above maps, the red squirrel population can be seen to be rapidly declining especially in Northern England and Wales, with the grey squirrel taking over the South of England and the Eastern Coast of Wales. Wales and the the midlands, surprisingly, have habitats which are occupied by both species. From the maps above it is highlighted that urgent steps are needed to address the issue of invasive species in order to save native species. 

It is not just terrestrial ecosystems that can become jeopardised by non-native species, freshwater habitats are also extremely vulnerable. The ICAIS (International conference on aquatic invasive species) 2013 report stated that 'the introduction and spread of invasive species in freshwater and marine environments is a worldwide problem that is increasing in frequency'. A recent example is the invasion of the Asian Carp throughout the river systems of Illinois, USA. They were introduced, from China, into water treatment ponds to remove algae in 1970s. The carp escaped and migrated northwards through the Mississippi and Illinois rivers. Although the carp did not cause any extinctions of native species, there was a decline of certain commercial fish such as the big mouth buffalo (Ictiobus cyprinellus). 

Bigmouth buffalo threatened...

Invasive species (in some cases) are considered such a colossal threat because they have the upper hand when resources become scarce.  CSIRO, Australia's national science agency calls invasive species 'one of the greatest threats to biodiversity and to the ecological and economic well being of society and the planet'.

However... not every scientist shares the same assertive view on invasive species. In fact, only a week ago Professor Phil Roberts from the University of York published a paper which throws all opinions up in the air. This is why I love science, its contestable nature. Roberts (2013) discusses how the Anthropocene could raise biological diversity through the introduction of non-native species causing evolutionary hybridisation. He acknowledges that some invasive species damage ecosystems and can eradicate resident species (as I have already explored). However, his discussion remains focused on how people fail to acknowledge that invasive species can also be positive for biodiversity. On average, less than one native species dies out for each introduced species that becomes established. Roberts (2013) neglects cases of invasive species disruption and instead highlights that, despite the fact that we are losing irreplaceable populations, in some (some being the key word here) regions biodiversity is actually increasing.

I will leave you with Roberts (2013) departing words: 

'we have to rethink our "irrational" dislike of invading species'.

I, on the other hand, do not believe that the negative reputation of invasive species is 'irrational', due to the destruction they can and have previously caused. However, it is vital to understand both arguments in order to judge whether there is still hope for biodiversity, in particular at a regional level, despite the doom and gloom of inevitable global species decline.

Anthropocene 0-0 Biodiversity 

What do you think is in the lead? Suggestions welcome!