Invaders Must Die? The Prodigy Vs The Ecology

It's Friday, so I'm forgoing today's piece in favour of having a rave.

Over and out

The Invader Inspector

Ok not really... but I need to check something. Are you feeling pumped? Are you? Because if not then you should be. Today we are turning the tables, ripping up the rulebook and other analogies for looking alternative viewpoints.

Because today, we're looking at positive impacts that invasive species can have!

Did you see that coming? You should have because I said it was going to happen in the quiz. (Which, incidentally, Endemol have expressed an interest in. They think it could be the new Deal or No Deal). 

So far we've only really looked at negative impacts of invasive species (remember those poor red squirrels? REMEMBER THEM?!?!). and I'm all in favour of trying to cover multiple angles. I mean this blog isn't for propaganda...

No place for propaganda here (and yes I succumbed to this terrible trend. And yes, that is a dog #ashamed)

Yet...

Before addressing some studies, I decided to do a crude bit of data collection myself. I searched Google Scholar for: 'invasive species negative impact' and 'invasive species positive impact'. Results are in the table below.

	All time	Since 2013
Invasive species negative impact	427,000	17,500
Invasive species positive impact	385,000	17,300

To summarise, in all time search results 'negative' comes up with approximately 10.9% more results than 'positive'. BUT since the start of 2013, the difference is only 1.10%. Hardly the most extensive data analysis ever undertaken, but it seems to indicate an increasing awareness that invaders can also have a positive impact.

So, what are those positive impacts?

Well, McLaughlin et al (2013) examined the ecosystem function and service impact of what Vila et al (2010) identified as the 10 'worst' European invasive species. The study found that negative impacts of invasive species are often assumed rather than evidenced quantitatively (!!!!) and shows that some invasive species are covered far more often in literature than others.

The ten 'worst' invasive species in Europe and their total no. of articles retrieved from ISI web of knowledge.

The paper also examined provisioning, regulatory and cultural ecosystem services. Out of 11 services examined, the 10 invasive species were shown to impact all but one of them (climate and air quality), mostly in a variety of ways. Only Balanus improvisus (the Bay barnacle) and Branta canadensis (The Canada Goose) were shown to have no positive impact, so Justin Bieber isn't alone in his category as a Canadian pest.

All impacted services had mixed results apart from new products/ industries from biodiversity (only positive) and regulation of floods and fires (only negative). Full results of McLaughlin et al's study are summarised in the table below.

Summary of impacts of 10 worst invasive species according to literature. Key: Black = -ve impact only. Grey = +ve and -ve impacts. Diagonal lines = +ve only. (IMHO being able to read this >> being outside my margins)

Clawing out the specific case of Procambarus clarkii (the freshwater crayfish) reveals a multitude of complex positive and negative impacts. The crayfish can be farmed or caught by fisherman, which can generate income. However, farming costs could outweigh benefits as they can cause physical damage to the habitat.

Their impact on trophic pathways is also mixed. Many large predators consume P. clarkii, reducing the ecosystem's number of trophic levels, leaving it more vulnerable to an abrupt change of state. However, at the same time, by providing a food source to higher predators, a positive effect is realised with regard to several birds of high conservation value.

Procambarus clarkii : its presence puts us in a bit of a pinch

As my extensive data collection suggested, the study of invasive species' positive effects is indeed receiving increasing attention. However, an issue precluding this is that invasion ecologists often present significant subjectivity and emotive reasoning against invasive species in literature (see Schlaepfer et al., 2013 first paragraph in particular).

Given the fact that invasive species can have positive impacts, I view this lack of objectivity as unprofessional. Invasive species can act as ecosystem engineers, boost vital services like pollination and restore ecosystem functions that were previously provided by another species, subsequently lost to habitat change.

An example of this is ecological restoration of the degraded Cape Cod salt marshes in New England, U.S.A. by an invader (Bertness and Coverdale, 2013). Overfishing (human activity again...) depleted salt marsh predators and allowed Sesarma reticulatum (a herbivorous crab) to denude hundreds of hectares of low marsh. However, an invasion of carnivorous Carnicus maenas (green crabs) into the marshes partially reversed decades of cordgrass die off, either by consuming the herbivorous crab or evicting it from its burrows, thus reducing herbivory and promoting cord grass recovery.

(A) Regression analysis of Carnicus abundance and cordgrass recovery demonstrating a positive correlation and that Carnicus denisty explained 37% of inter-site variation in cordgrass regrowth. (B) Carnicus presence significantly reduced Sesema burrow usage, rendering it more vulnerable to predation and desiccation.

Conversely, invasive plants can also have positive effects for an ecosystem's animals. For example, Malo et al. (2013) report that the invasive shrub Rhododendron ponticum, can have a positive effect on Apodemus sylvaticus (wood mouse) abundance in Southern England, primarily by providing a 'shield' from aerial predators.

A cute little wood mouse, where's a Rhododendron at when you need one?

Wood mice play an important role in many terrestrial ecosystems as they disperse seeds and act as a staple prey for numerous predators, whose breeding success is affected by changes in rodent density (King, 1985). However, rodents can have negative effects on agriculture (Stenseth et al., 2003) and pose a human threat by spreading disease. Hence, once again, positive effects are present but there are still negatives too.

To conclude, it seems that sometimes we, including professional ecologists, can be too quick to write off invasive species as only having negative impacts. In reality the ecological picture is, as always, much more complex and a lot of the time we lack sufficient data to know precisely what is going on.

For this to even become remotely possible, I think it is imperative that subjectivity is removed from research on the matter and not let the more obvious negative impacts overshadow the positives. Overall, we would be very wrong to say 'invaders must die', which just goes to show, when it comes to ecology, you shouldn't take advice from musicians.

Over and out

The Invader Inspector