Positive potential

It's time to take a more optimistic approach, and look at the positive effects that invasive species can potentially have on ecosystems. I’ll start by proposing counter-arguments for the 3 problems described in ‘Why the bad press’. 

1. Predation
Let’s take a look at the European Green Crab, which was a successful invader to America at locations where the native predator (Blue Crab) had been removed as a result of human activity. Due to the local extinction of Blue Crabs, the population of an herbivorous crab species lower down the food chain dramatically increased. This herbivore was able to strip salt marshes of hundreds of hectors of vegetation, namely cordgrass, which resulted in salt marsh die-off and impacted negatively on the ecosystem. The invasion of the Green Crab is thought to have significantly reduced the population of the herbivorous crab via predation. Optimistically there was found to be a positive correlation with Green crab abundance and cordgrass recovery. (Bertness and Coverdale, 2013)

2. Competition

When an invasive species expands its geographic range, it competes for resources with pre-existing species. If some native individuals are able to survive, then invasives may drive the evolution of these native species and select for traits which increase fitness. Ferrero-Serrano et al., (2009) found this to be true for native grass populations. The authors demonstrated that as a consequence of previous competition imposed by invaders, evolved species were more likely to be able to tolerate new invaders. Importantly the authors argue that this knowledge may help advance the understanding and practice of restoration ecology. 

3. Disease

Ok so this one was a little bit harder….even after trawling the internet I have failed to come up with a valid argument! 

To compensate I have found a case study to share with you that I thought was particularly interesting. Pejchar and Mooney (2009) reviewed whether different ecosystem services in South Africa were positively or negatively impacted on by the invasion of woody plant species.

(Pejchar and Mooney, 2009)

This review by Pejchar and Mooney (2009) highlights just how many ecosystem services invasive species can affect. Moreover, there is only one ecosystem service where invasive species had a strictly positive influence. Whilst reading the literature in preparation for this blog post I found that this was often the case. As a result I am personally still not swayed; to me it seems evident that invasive species have a net negative effect.

Let me know what you think!

Elena 

Survival Guide

From Camley Street Nature Park to the Arctic, it seems that invasive species have the potential to crop up just about everywhere! But what is it about these species that allows them to become so well established in the new locations across the globe? I've combined the traits of some of the top invaders to create the ultimate invasive species survival guide...

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Invasive Species Survival Guide

1. Climate Matching

Species that are introduced to a location where the environmental conditions are similar to that of their native range may be more likely to succeed (Bomford et al., 2005). This is as their physiological adaptations and behavioural responses are already suited to seasonal variations in temperature, precipitation.  

2. High competitive Ability

Species with a high competitive ability such as the Argentine ant (Holway, 1999) are more likely to acquire food, water and habitat resources that are necessary in order to survive.

3. High intrinsic rate of increase
The intrinsic rate of increase takes into consideration the number of surviving reproductive individuals, the proportion of these which are females, and the age at which females can reproduce. Thus the higher the intrinsic rate of increase, the more rapid population growth is. As a result, species that have a high intrinsic rate of increase are more likely to be successful invaders (Crawley et al., 1986).

4. High abundance in native range

Firn et al., (2011) studied the abundance of invasive plant species in their native and invasive range across eight countries, revealing that there was often a similar abundance in both locations. This suggests that if a species is very abundant in its native range it is likely to be abundant in its invasive range.

5. Availability of suitable habitat and vacant niche

Due to an increased availability of resources, a vacant niche means that an invasive species is more likely to become established in a new location. For example, ant species with different sized mandibles consume different sized food sources; allowing the species to be separated dependant on their food niche. Arndt (2006) showed that when the food niche was empty, species were successful invaders. However, when ants were introduced to a site where their niche was already occupied by an existing species, the invader did not establish.

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*Terms & Conditions:

To succeed as an invasive species not all traits are necessary (Daehler, 2003; Gallagher et al., 2010) however, a combination of multiple traits can be advantageous!  

Why the bad press?

More often than not invasive species are portrayed in a negative light; this is fuelled by both the research of ecological conservationists and by personal experiences of recreational gardeners. I have also put a large emphasis on the need to remove invasive species and prevent the introduction of future invasives throughout my blog. Before I progress any further, I feel that it is necessary to present the basis for this school of thought by outlining some of the key ways invasive species can have a negative impact on the environment.

Predation

Prior to the introduction of an invasive species, a given ecosystem is likely to be at an equilibrium state with an established food web. Invasions have the potential to introduce a new predator to the ecosystem, which will consequently alter the population dynamics. In particular this could have extreme consequences if the invasive species preys on a keystone species. Predation may also result in the loss of endemic species. When the Brown Tree Snake was introduced to Guam in the late 1940s it had a devastating effect on the island’s bird populations. The snake was associated with the extinction of at least seven avian species from the island, some of which were endemic, and was also linked to a decline of many other bird species (Pimm,1987).

Competition

Water, nutrients, minerals and light are just a few of the resources that are often essential for life. What’s more is that these are often limited, and despite differences in the relative requirement of resources between species, there is often interspecific competition to acquire them. It may not come as a surprise to you that invasive species are often very strong competitors. For example the Himalayan Balsam (mentioned here), has the ability to outcompete a variety of plant species that may also be present along the canal edge, excluding other organisms and decreasing species richness (Hulme and Bremner, 2006)

Perhaps a less obvious way that invasive species can drive biodiversity decline is via competition for reproductive mates, leading to hybridisation (Mooney and Cleland, 2001). During the 1950s the Ruddy duck was introduced to the UK from North America. This species can sexually reproduce with the native White-headed duck population. Unfortunately, the reproductive behaviour of White-headed duck females indicates a sexual preference for male Ruddy ducks, resulting in an increased incidence of hybridisation and a decline in the native endangered population (Green and Hughes, 1996). Consequently wide scale culling programmes have been implemented across Europe to reduce Ruddy duck populations (NNSS).

Disease

As invasive species move around the globe, they may also act as disease vectors, transporting potential harmful pathogens to new geographic locations. If animals, plants and humans are not equipped with the appropriate immune response, then transported diseases may have severely detrimental effects. There is increasing research into whether the presence of certain invasive species can have a negative effect on human health as result of associated pathogens (e.g. Allan et al., 2010). 

I hope that these examples help to illustrate just some of the reasons why the Millennium Ecosystem Assessment highlighted invasive species as one of the major drivers of biodiversity decline. Furthermore, not only do invasive species have an ecological impact, but they also have an economic impact. Surprisingly it is estimated that as much as $137 billion per year is spent controlling and compensating for the effects of invasive species populations across the USA (Pimentel et al., 2000). 

I will leave you with this video, explaining that as a consequence of the negative effect on biodiversity and human health, the EU is currently proposing to implement a ban on the future intorduction of invasive species...

Until next time!

Elena