Invasive Species and Hellmann's Famous Five - Part One: Passage and Plasticity

Last week I began discussing the effect climate change will have on the proliferation of invasive species. We discussed two basic concepts; that of increasing ranges and climate change aiding successful invasions. This week, I’d like to take the opportunity to explore these concepts in a little more depth. Specifically, I’d like to discuss Hellmann et al.’s (2008) “Five Potential Consequences” of climate change for invasive species, and how it makes them a threat not only to native species but also to the biodiversity of ecosystems.

Firstly though, I feel it pertinent to readdress what we would call an “invasive species”. In a constantly changing ecological world, it can be hard to determine what species are invasive and which are not. The climate change induced range shifts of seemingly innumerable species only add to the confusion. Do these exile species count as invasives? In this blog, and all future blogs, I will be using what I find to be a very suitable definition of what counts as an invasive species; as Hellmann et al.(2008) so eloquently put it, an invasive species can be simply identified as “taxa that have been introduced recently and exert substantial negative impact on native biota, economic values or human health”.

As we learnt from Manchester and Bullock (2000) last week, invasive species can be incredibly harmful for native species and local biodiversity. Not only do invasive species outcompete and make vulnerable native species, but can also dramaticlly change the environmental regime, making enviurons ever more unsuitable for their original residents. But what makes them so successful? As Hellmann et al. (2008) observe, invasive species differ tremendously from those native to ecosystems.  Generally, invasive species have far greater climatic tolerances (and hence large geographic ranges), and often have characteristics that allow for rapid adaption, such as short generational times and rapid maturation of individuals. Some are also geared up for distribution, such as flora that feature low seed mass that can consequently travel greater distances.

Richards et al.(2006) portray a nice and succinct summary of the types of invasive species with their three (although very general) classifications. You have;

- The “Jack of all Trades”, essentially an invader which maintains fitness in a variety of environments
- The “Master of Some”, an invader that is able to actively increase its fitness in a few favourable environs
- The “Jack and Master”, wherein an invader has both of these abilities

Hellmann et al.’s(2008) idea is that essentially climate change affects invasive species in five major ways that are relatively unique to the ecological marauders. These include altering the methods of transport and introduction of invasives, altering the climatic constraints placed upon them, altering their distribution, altering their impact and finally altering the effectiveness of management strategies. Today I will be addressing the first two (the final three will be discussed in my next blog).

Altered Mechanisms of Transport and Introduction

Globalisation and increased human connectivity has had a large part to play in aiding the spread of invasive species. From Killer Shrimp (Dikerogammarus villosus) travelling to Britain in the ballast tanks of trade ships (Madgwick and Aldridge 2011) to Invasive Aquatic Plants being brought over as part of the plant trade and consequently escaping into the wild (Willby 2007), human factors play a large part in many invasions. Climate change, Hellmann et al. (2008) argue, has the potential to change these patterns of human behaviour, and hence the way that invasive species move around the globe.

This image, from the National Oceanic and Atmospheric Administration of the US, shows how ballast tanks can become transporters of invasive species

Hellmann et al. (2008) split these changes into three neat categories. Firstly, they see climate change altering patterns of tourism and commerce, meaning that we may see links starting to form between different geographical regions, opening up new areas to invasive species. They also argue that pathways of international transport could change; most obviously, if arctic sea ice experiences significant losses, the opening up of a viable Northern Passage will occur and will dramatically shorten travel time for some ships and hence increase the survival rate of invasive propagules or animals. Finally, Hellmann et al. (2008) predict that an increase in extreme weather events, such as hurricanes, could enhance invasive dispersal, as these events often carry birds, insects and marine larvae great distances from their native environments. 

The threat also exists for assisted migration. If humans were to begin to move organisms to new habitats in an effort to conserve species, they could potentially become invasive. This also applies to recreational assisted-migration, such as the transporting of fish for sport-fishing to new areas that are becoming climatically appropriate for the animals (Hellmann et al. 2008).

These consequences could have dramatic impacts on the number of invasions that occur. By improving chances of survivability and increasing the areas open to invasions, more ecosystems could be put at risk. This opens up areas to the increased competition rates, environmental impacts and even reduced biodiversity that destructive invasives can cause (Manchester andBullock 2000).

Altered Climatic Constraints/Climate-aided Invasives

Hellmann et al. (2008) also discuss how climate change can aid invasives in establishing significant populations. The group bring to our attention three key methods of how climate change can benefit invasives.

Firstly, climate change can dramatically increase climate suitability for invasives, who may otherwise be unable to establish successful populations in an environment. Climate change may aid in increasing their fitness to a point whereby they become viable competitors and can begin to successfully colonise the regions in which they find themselves. To give an example, plants and animals kept by humans which then escape, which before climate change would not be able to find a suitable habitat, may begin to establish populations.

Secondly, climate change may increase the fitness of invasive species relative to native species, which have effectively been shifted out of their optimum.  This process would dramatically reduce competition and make it far easier to invasive species to become dominant in their new habitats. This is demonstrated nicely by Knop and Reusser (2012) in their study of how phenotypic plasticity (the ability to respond to a wide range of temperatures) aided the Portuguese slug (Arion lusitancius) in invading the habitats of the native “Dusky Arion”, or Arion fuscus.

Here is the handsome chap himself; the Portuguese Slug

Through testing both species in a range of temperatures (using an altitudinal scale that ranged from 700 – 2400masl) and in low and high food conditions, they found that the Portuguese slug’s ability to adapt its behaviour to environmental conditions made it far more successful than the Dusky Arion. They found that across the temperature range the Portuguese slug had significantly higher survival rates, especially in areas that bordered on the limits to the Dusky Arion’s range. They also found that even in extremely low food conditions, the Portuguese slug still managed to reproduce, while the Dusky Arion laid no eggs at all. Knop and Reusser (2012) consequently argue that it is this ability to endure in a broad range of conditions (making the Portuguese slug, by Richards et al.’s (2006) classifications, a “Jack of all Trades”) that makes A. lusitanius such a successful invader.

Finally, Hellmann et al. (2008) also remark how changing climates may allow species that currently exist as non-natives in ecosystems to become invasive. They argue that climate change could effectively move these species closer to their optimal conditions, effectively increasing both their competitive ability and rate of spread. This could dramatically reduce the lag time of invasions (that is, the time between establishment of a non-native and, if it becomes invasive, its aggressive colonisation of the ecosystem).

These three changes dramatically increase the vulnerability of ecosystems to invasion by non-native species. Through changing conditions, climate change effectively allows invasive species to “get a foot in the door”, and dramatically increases the potential of invasive species to spread within ecosystems. This, as stated earlier, could have dramatic knock-on effects for the biodiversity of ecosystems, and the ability of individual native-species to survive in an environment that is open to invasions.

Unfortunately, that’s all we have room for this time. In the next blog, I will be continuing the discussion, talking about altered distributions, altered impact and the altered effectiveness of management strategies.