Invasive Species and Hellmann's Famous Five - Part Two: Immigration and Impacts

Earlier this week we began the conversation about Hellmann et al.’s (2008) five predictions into how climate change will affect the proliferation and impact of invasive species. To recap, we discussed how climate would affect the way in which invasive species are transported and introduced to ecosystems and how climatic constraints on them would change, helping make them more successful and “putting a foot in the door” of previously highly resistant environs.

The time has come to finish the discussion, and bring our musings to a close. Today, we will discuss how climate change affects the distribution of invasives, their impact, and how easy they are to manage.

Climate Change and Altered Distributions

In this blog, we have extensively explored the topic of range shifts. It should come as no surprise then that the concept not only applies to native species, but also those with more invasive tendencies. For many invasive species, cliamte change brings with it an invitation to a range of new ecosystems and biomes. Europe alone for example saw its number of invasive species increase by 76% in just the last 30 years (Bellard et al. 2013).

Hellmann et al.(2008) expand upon this idea, stating that it is not only temperature change that will facilitate the spread of invasive species, but also the changing salinity of coastal estuaries, increasing wildfire frequency and soil moisture changes that will play a large role. Of course, we have seen numerous examples in previous blogs regarding changing species distribution. A meta-study conducted by Bellard et al. (2013) takes this further and examines how climate will change the distributions of what the International Union for the Conservation of Nature list as the "100 worst invasive species".

Using forecast projections derived from the species distribution models of the invasives, coupled with land cover and climate change scenarios, Bellard et al.(2013) uncover future hotspots of invasion (areas projected to see an increase of invasive species of >60). They found a general pattern that areas in the Northern Hemisphere were to become most vulnerable to invasions by the top 100, whilst tropical areas and other areas of extreme conditions (such as tundra and warm deserts) would generally be protected by their environments.  Within the Northern Hemisphere, it was found that biomes such as warm and temperate mixed forests would become most suitable for invasives (seeing a likely rise of about 4.5% of top invasives), as well as temperate deciduous forests. These predictions see large swathes of the eastern United States, North-East Europe, southwest Australia and New Zealand at particular risk.

These maps, from Bellard et al. (2013), show the distribution of the top 100 invasives globally in both the present and in future scenarios. Map (a) shows the richness of invasive species in 2000, (b) the relative change in richness between 2000 and 2100 (red meaning increase and green decrease), and (c)  the projected invasive species richness in 2100. As you can see, it is mainly temperate latitudes in the Northern Hemisphere that are most at risk.

An example of one invasive species making the most of its increased freedom is the infamous Yellow-legged Hornet (or Vespa velutina). Native to Southeast Asia, this hungry critter found its way into south west France and has since spread over 190,000km², even making its way to Northern Spain (Barbet-Massin et al. 2013). This invasive causes widespread damage to the ecosystems that it invades, primarily due to its predation upon honeybees (Apis) and attacks on colonies, which the poor honeybees have no real defence against, having not evolved alongside the invader. This predation has large knock-on effects for ecosystems, having considerable impacts on pollination and hence the ability of local flora to reproduce (Barbet-Massin et al. 2013).

This video below details the ruthless nature of the hornets predation on honeybees, and why they are such a destructive invasive. 

In order to assess areas that would become vulnerable to invasions under climate change, Barbet-Massin et al. (2013) modelled what the distribution could potentially look like in 2100, using several climate change scenarios. Their study found that there is very high probability that V. velutina will establish widespread populations across Western Europe, where climate and a lack of competition will allow the hornet to run rampant. Specifically, the areas found to be most vulnerable were large parts of Germany, Denmark, Poland, stretching even to Romania and parts of Scandinavia such as Southern Sweden. 

The area of climatic suitability for the Vespa velutina is set to dramatically expand by 2100 (Barbet-MAssin et al. 2013)

It seems then that climate change will loosen the reins on what are devastating species. This will put many organisms, both flora and fauna, in ever more vulnerable positions, which, combined with the difficulties caused by climate change alone, will make survival for species an incredibly difficult prospect.

Changing Impacts of Invasive Species

One of the most dangerous aspects of climate change for invasive species is the huge uncertainty in how it will affect their impact on ecosystems. As Hellmann et al. (2008) deftly point out, currently invasives have widespread effects on ecosystem structures, communities of organisms and even resource production (such as we have seen with the Yellow-legged hornet, which hinders pollination).  Arguably, the significance of this impact depends on a number of factors, such as the size of range of endemic species occupied by the invasive species, its abundance and it’s per unit-biomass impact. Hellmann et al. (2008) argue that as climate change is set to alter these factors, in ways that are far from easy to predict, thus making the impact of invasive species far more uncertain.

Whilst few good predictions exist of the changing impacts of invasive species, there are a limited number of studies that highlight potential situations. Seager et al. (2007), for example, discuss how the reduced precipitation pattern in the southwest United States (again I depart from our European focus, please forgive me) may affect the per-unit biomass impact of Tamarisk (Tamarix spp.), a family consisting of various shrubs and trees. According to Seager etal. (2007), they are more water intensive than native riparian species, meaning that, when combined with decreased water supply, their competitive impact will increase dramatically.

Here you can see Tamarisk itself, enjoying it's riparian habitat.

Changing impacts can obviously be a considerable threat for ecosystems. Where invasives already exert considerable pressure on environs, exacerbation of impacts by climate change could make challenges for native taxa ever more insurmountable.

Impact on Management Efficiency

Although this is somewhat removed from the flora and fauna focus that I usually enforce in my blogs, I felt it was necessary to touch on Hellmannet al’s (2008) last point that climate change may affect the efficiency of management techniques. Whilst in a large way this is a problem for humans (and hence none of this blog’s concern) as invasive species can be incredibly expensive, this can also present an issue for native species wherein conservation efforts are concerned. 

In some areas for example invasive species may be controlled by an artificially introduced predator or herbivore – however if climate change allows them to subvert these interspecies interactions, they may re-emerge as a highly problematic species. This also applies to other methods of management, such as the use of pesticides or other control techniques. Climate change can potentially make invasive species highly unpredictable and ergo incredibly difficult to manage. For the native species being protected, then, this is very bad news indeed.

Conclusions

Hellmann et al.(2008) paint quite the dystopian picture for the  invasive species/climate change relationship. Across the board, climate change can be seen to bring great benefits to them, by expanding their range (and their travel capabilities through new human movement), giving them a leg up against natives and making them even harder to get rid of. Invasive species come to the fight against natives with climate change as an ally, and it makes them far superior and far more dangerous.

Native species then are contending not only with one enemy, but two, and climate change is affecting them both directly and indirectly through proliferation of invasives. The picture at the moment seems so bleak, and I haven’t even reached the end of my list of topics to talk about. Is there a light at the end of the tunnel? I’ll guess we’ll just have to wait and see.