Invasive Plants and Anthropogenic Environmental Change: Will Europe Soon Be Losing Its Rag Over Ragweed?

One of my assignments in the Masters course I'm doing is to make a poster on the theme of future environmental change. Me being me, the topic I chose was how invasive plant species will respond to anthropogenic environmental change. So without further ado, here's what I found out...

First some basic background information...

Invasive plants are non-native plants that spread and rapidly dominate over native species. Over the last century, the number of invasive plants has increased rapidly in many regions as global trade and travel have become more frequent (Vicente et al., 2013).  In recent decades invasive plants have impacted nearly all ecosystems and ecologists have increasingly recognised invasive plants as a major threat to native biodiversity.

Invasive plants compete for space, nutrients, water and light with natives and affect soil, water and light conditions. This can lead to replacement of diverse communities with a single invasive species. Vila et al (2011) showed that in 1041 field studies, invasive plants reduced native plant growth by 22.1%, abundance by 43.5% and diversity by 50.7%.

So as we can see, invasive plants could potentially cause a lot of serious issues, but how will they respond to anthropogenic environmental change?

Well, making predictions around this topic is a daunting task. Plants react to many climatic aspects including temperature, precipitation, length of frost free periods and magnitude and variation of extreme climates. The figure below gives an idea of of the many different forces that can affect invasions and also suggests that a positive feedback could occur with increasing invasions.

Impact of global change on invasions, and feedbacks from invaders to global change.

Climate change is expected to impact 5 key elements in the invasion process (Hellman et al., 2007): pathways, environmental constraints, distributions, impacts and management effectiveness.

The consequences of climate change for the invasion process.

In summary here are some of the things we can expect:

• Human transport will increase propagule pressure of some invasive plants, possibly beyond thresholds that permit establishment. 
• Some currently marginal invasive plants will be able to successfully colonise new areas if conditions become more similar to their native range. 
• Cold-temperature constraints on invasive species will be reduced at their higher-latitude or upper-elevation range limits. Warm-temperature constraints on invasive species will increase at their lower-latitude or lower-elevation range limits. 
• Relative impact of some invasive plants will increase when the abundance of natives decreases. 
• Finally, the tolerance of some invasive species to some herbicides will increase (e.g., due to increases in CO2), making management more difficult

We can also expect some other impacts: 

• Increased temperatures and CO₂  may disproportionately favour the spread of invasive plants to new regions as they have traits that help them adapt to new climates. such as rapid dispersal and evolutionary change (Maron et al., 2004), tolerance of a wide range of climate conditions and lack of dependence on co-evolved pollinators (Dukes et al., 2009; Sheppard and Stanley, 2014). 

• Climate change is also likely to lead to more frequent disturbance in ecosystems, from events such as wildfire, drought and storms (Bellard et al., 2013; CCEW, 2008).  This can also benefit the establishment of invasive species as native species will become stressed (CCEW, 2008) the ecosystem’s resistance to invasion will lower. If keystone species are lost, invasion vulnerability will dramatically increase (Zavaleta and Hulvey, 2004).

• Additionally,, climate change will affect phenology (Wolkovich and Cleland, 2014). Many invasive species flower later or earlier than natives. Theories around fluctuating resources and opportune invasion periods may be critical. If natives don’t track shifting climate, climate change may create phonological vacant niches which promote invasion success. 

Phenological impacts on invasion. Invasive plants should invade when other species are inactive.

I always like to include examples when talking about topics like this, and to illustrate how climate change could affect invasive plants, I found a cool model of projected increases of ragweed across Europe.

Ragweed: Doesn't look like much of a threat...

Again, first some background information...

Ambrosia ortemisiolia (Common ragweed) is an annual plant which originates from North America (Storkey et al., 2014). It first reached Europe in the mid-19^th century. However, rapid spread did not begin until the 1940s due to growing transport networks and contaminated seeds. The plant is highly invasive and rapidly spreading in Europe from its established areas in France, Austria, Hungary and Croatia.

Its pollen is highly allergenic to ~5% of Europeans (Richter et al., 2013), with health problems including hay fever, atopic dermatitis and asthma. A single plant produces 50,000 seeds (Brands and Nitzsche, 2007) and 1 billion pollen grains (Fumanal et al., 2007) and warming has been shown to increase pollen loads. Once established, control is labour intensive and expensive so there’s an urgent need to evaluate spread potential

To evaluate the possible impacts that climate change will have upon the range of Ambrosia ortemisiolia, a process based model incorporating growth, population dynamics and competition was utilised by Storkey et al (2014). Process based models are a useful tool for this purpose as they have the ability to capture interactions between climate change, land use and plant competition at local scales. 

Results of the model show that the European distribution will increase under future climate change. There is a risk of the population spreading to the United Kingdom and Denmark as growing conditions become more favourable. No southerly spread is predicted as lower future rainfall will make Spain, Italy and Greece climatically unsuitable.  This is shown in the figures below; there are a lot more areas that are highly suitable to ragweed in the bottom set of maps, which are for the future, compared to the top maps representing current conditions.

A: European distribution of common ragweed. B: Potential range under current climate. Orange areas are well established, yellow areas suitable for casual populations and blue areas are highly unsuitable.

A: Potential ragweed range in Europe from 2010-2050. B: Potential ragweed range in Europe from 2050-2070. Colour codes are the same as above.

So why is any of this important? And is there anything we can do about it?

Well, the level of management, likely to involve physical or herbicidal removal, will vary by country and have a large bearing on the impact of establishment. The species is ruderal and its persistence requires regular disturbance in the habitat, hence land use factors will be key to the spread.

Furthermore, Richter  et al (2013) produce evidence that a thorough, well executed management plan involving surveys and eradication can drastically inhibit ragweed spread and reduce allergy costs. A cost benefit analysis reveals that total savings from managing ragweed from 2011-2050 range from €9-11 billion for current to extreme climate changes. Without management, mean allergy costs could be as high as €365 million annually for Austria and Southern Germany with an annual temperature increase of 0.04 °C from 2011-2050.  

Those are big numbers, and given that the model discussed before showed that ragweed will soon be much better suited to large swathes of Europe than it is currently, it's not just Austria and Germany that will have to consider the potential economic impacts ragweed could cause them. The UK could also be facing the negative impacts of ragweed establishment unless we have plans in place if it becomes a more common species here. 

  

So will Europe soon be losing its rag over ragweed? Potentially yes, but time will tell..

Overall. climate change is likely to increase invasive plant prevalence. However, as with most things climate change related, the issues is far from clear-cut. Responses will vary greatly between species and locations (Corlett and Wescott, 2013). Furthermore, how we manage invasive plants under climate change will be key to the impacts they have.

Over and out

The Invader Inspector