Invasive Species - Climate Change's own Migrant Crisis?

Invasive species have always been a headache for ecosystems and ecologists alike. Put simply, these invaders are non-natives; they are species that have colonised areas either through accidental human transport or through range expansion. And, living up to their name as “invaders”, more often than not they bring with them a whole load of problems.

Invasive species can have massive impacts on ecosystems, making them more vulnerable or reducing biodiversity. One of the most obvious is issues that arise from increased competition with native species for resources, a fight that, due to their highly adaptive nature, invasive species usually win. Invasives may also predate on native species, adding additional stressors that can cause populations to shrink. They can also have a direct impact on the environments themselves, through changing fire regimes, soil properties, vegetation structure and the water table, which can make habitats highly unsuitable for native species. Invasive species can even have genetic impacts through hybridisation with native species, which can affect fitness of native species to their environment (Manchester and Bullock 2000).

Such effects undoubtedly lead to severe ecosystem and biodiversity problems. This video below, from Augnitia, provides a pretty good summary of the effects that invasions can have both on individual species and on a macro-scale.

But where does climate change come into all of this? Many, such as Hintzen (2015) (in their academic article for Michigan State University) and Harvard Magazine (2010), believe that a link exists between climate change and the proliferation and capability of invasive species. This argument appears twofold; on the one hand there are those such as Nijhuis (2013) who see climate change as aiding invasive species in outcompeting natives through the changing of conditions, and on the other, you have those such as the National Wildlife Federation of America who see climate change as aiding in the spread of invasives through the landscape.

The idea that climate change and its effects on environs aids invasive species is not unfounded.  Hintzen (2015) notes that, due to climate change, growing seasons are becoming extended and extreme storm events are becoming more frequent and are growing in intensity. She argues that these factors create stressors for both native and invasive species alike, however suggest that invasive species are generally much better equipped to deal with said stressors. This arguably stems from what makes invasive species so successful in the first place; invaders generally have the ability to adapt quickly to new environments and conditions. As Nilhuis (2013) explains, they are highly flexible, with generally short generation times, high dispersal ability and show rapid responses to changing environments. This means that invaders are far more able to adjust to changing timing of annual activities such as blooming and fruiting (hence showing far greater phenotypic plasticity), allowing them to capture a larger share of nutrients, water or pollinators whilst shading out the competition.

One such offender is none other than Purple Loosestrife (Lythrum salicaria)

One such invasive that has done just that is the Purple Loosestrife (its business name is Lythrum salicaria). This plant adapted rapidly to sync its flowering schedule with the lengthening growing season in the UK, allowing it to outcome natives who generally show a much slower response.

Range shifts, which we have been talking about in excess over the last few weeks, are also understandably beneficial to invasive species. By opening up more areas where these species can persist, climate change also makes a lot more areas susceptible for invasion. The National Wildlife Federation for example lists numerous cases of invasive species rapidly expanding their range in the wake of a changing climate (please forgive the American examples; this is supposed to be a Euro-centric blog).

The Deer Tick (Ixodes ricinus), for example, is expected to increase its range by up to 68% in North America due to milder winters. Similarly, the Red Imported Fire Ant (Solenopsis Invicta) could expand its range by 80 miles and total area by 21%. Cheatgrass (Bromus tectorum), is also set to expand as long as summer precipitation continues to decline. The estimated expansion is around 45%, and could make habitats more susceptible to wildfire. Furthermore, Pine Bark Beetles (of the sub-family Scolytinae) are predicted to undergo a population explosion due to the absence of severe winter cold, and threaten to migrate northwards, causing the untimely demise of many healthy trees as they go (Mersereau 2014) (National Wildlife Federation).

A more European example is that of the Arctic Fox (Vulpes Iagopus). These animals, which are highly adapted to severe arctic conditions, find their southern boundary determined not by climate but by competition, mainly from their cousin the Red Fox (Vulpes vulpes). For the Red Fox, their northern boundaries are determined instead by harsh conditions. In the event of climate change, therefore, it is estimated that the range of Red Fox will expand northwards and consequently that of the Arctic Fox will decline (Elmhagen et al. 2015).

The Arctic Fox's southern boundary is limited by competition - meaning that if its main competitor, the Red Fox can advance north, it will be forced to retreat in response.

The effects of climate change on invasive species then are twofold. The friendship that climate change provides to a large amount of invasives however is to the detriment of the majority. As Charles Davis (in Harvard Magazine 2010) aptly put it, “climate change will lead to an as-yet unknown shuffling of species, and it appears that invasive species will become more dominant”. Whilst some will thrive, most will fall, and with them, so will the resilience of ecosystems and the state of biodiversity be called into question. This is something I look forward to exploring in more detail in the coming weeks.