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.

Home on the Range... Shifts - Bonus Chapter: Unforeseen Consequences

I know I have been going on about range shifts for quite some time now, but I ask that you just humour me for just one more week, because I think I’ve found something you’ll find interesting.

During my weekly dive into the literature, I stumbled across a relatively new paper by Cobben et al. (2012) that revealed to me a side of range shifts which I’d never really thought about before. What Cobben et al. discuss is the genetic impacts of changing range shifts and how these can massively disadvantage a species. What I'd like to talk about today then is quite theoretical, but nonetheless is hugely interesting.

The genetic composition of a species is largely dependent upon interactions within the population and the species’ adaptations to their environmental conditions. This means that the genetic make-up of a species is rarely uniform; in fact, it can see large variations across its range, specifically so between the central and marginal regions. Those living in the central region are at the optimum environmental conditions for their survival, and it is here that you generally find members of the species that are more specialised – that is, they have specialist gene alleles that provide them with greater fitness in their environment. Move to the outskirts, however, and you find more challenging environments, with more diverse and generalist species members who are designed genetically to deal with it; here there is a strong trend towards animals with more generalist alleles that allow them to take on a greater variability of environments, although with reduced fitness across the board.

I’m just going to take a second to explain some basic genetics to assure we are all on the same page. An allele can be defined as a variant form of a gene; that is, many different alleles do essentially the same job, but in different ways. An easy example is eye colour in humans – this is controlled by a single gene, which can be made of up many different types of alleles which determine whether you have blue eyes (like me) or brown eyes (like my brother). It is the same for other species; a single allele can determine whether you are a specialist, adapted specifically for certain environmental conditions, or a generalist, a jack of all trades but master of none. Different combinations of alleles create different genotypes (the part of the DNA sequence that is responsible for a certain characteristic), and so, we can have generalist genotypes, or specialist genotypes.

Here you can see different allles making up the genes for a flower; note how different alleles carry with them different characteristics.

 

What Cobben et al. argue is that range shifts can have massive impacts on this genetic make-up, and, following the rather depressing theme of this blog, this is usually for the worst. As the potential range increases with temperature, you have what are called “founder events” – these are merely colonisations at the expanding margin. These are conducted mostly by generalists, who show better fitness than specialists in less than optimum habitats and hence have greater success, meaning new populations generally tend to have a significant majority of members with generalist genotypes. These “generalist-polarised” founder events can therefore affect the local evolutionary process and cause dramatically reduced allelic variation at the margins of populations, leading to a genetic bias towards generalists as the generalist allele becomes more dominant/prominent.

As ranges continue to shift polewards, this polarisation tends to increase, and the locations of generalists, originally at the range margins shift closer towards the centre of the distribution. Specialists are also observed to move towards the lower margins of the distribution. This tends to happen in times of rapid warming (such as that we are experiencing at the moment) and can be augmented by habitat fragmentation preventing successful “migration” of the species to track their optimum conditions. This leads to general maladaptation of the species to their environs; generalists at the centre of the distribution are significantly less fit than other species adapted to said niche, and the specialists, who find themselves pushed towards the margins as range shifts overtake them are removed from their niche and put at a distinct disadvantage.

Cobben et al. model this process using METAPHOR, a simulation model designed to test changes to metapopulation demography. Specifically they modelled the Middle Spotted Woodpecker (known to his friends as Dendrocopos medius), as within our wood-pecking friend a single gene determines adaptation to local conditions, making it the ideal test subject. Throughout the model, various climate change scenarios (matching predicted change) and weather variability were applied.

This the little guy himself, doing some modelling of his own for us

They found that the original distribution of genotypes within the distribution was changed dramatically by climate change induced range shifts. Generalists were seen to increase in relative number and size of area – an increase that occurred at the cost of specialist populations. After a relative period of temperature increase, the positions of both specialists and generalists seemed to be at odds to the position of their respective optimum environments; generalists occurred at the temperature optimum, where specialists had a selective advantage, and specialists were seen at the range margin, where the generalists tended to have a greater fitness. Eventually, after a long period, the specialist allele was observed to go extinct. Put simply, the range shifts resulted in the centre of the distribution moving to areas where generalists were originally on the margins, whilst specialists, originally at the centre of the distribution, found themselves now at the edge. Due to reduced genetic variation within the generalist populations, there was little room for specialists to retain dominance, and hence generalists relatively grew in number.

Here you can see the results of Cobben et al.'s (2012) modelling in graph form. As you can see, across the years, the specialsits (light grey bars) tend to migrate (or more accurately, are forced) towards the lower bound of the distribution, whilst generalists (black bars) move to occupy both the centre and upper bounds of the distribution. By year 600, the specialist genotypes are all but gone.

 

This transformation in the genetic make-up of species has numerous knock-on effects, most noticeably on the size and persistence of populations. The occurrence of genotypes in areas where they had less than optimum fitness will generally result in a decreased metapopulation size, as species are far less successful in surviving to maturity and producing offspring. The persistence of a species is also threatened; not only are they disadvantaged, putting them at risk of invasive species or extreme events dramatically reducing population sizes, but also the decreased genetic diversity reduces their ability to adapt to future changes. This genetic polarisation hence makes species extremely vulnerable.

It is shocking just how deep the threats from range shifts run for the species of our planet. Not only are external factors reducing their ability to survive, but threats are also coming from within the species itself. Contemporary range shifts truly are one of the biggest challenges to the flora and fauna of our planet that they have seen in thousands, if not millions, of years, and their futures at the moment hang in the utmost uncertainty. Humans really have to turn things around soon, and truly devote themselves to helping our fellow residents, before it is too late.

If today's blog hasn't upset you enough, then you may want to read this article that explores the reasons behind the mystery of 85,000 Saiga antelope dying in just one day (later revealed to be 211,000, or 70% of the entire species). It is an interesting, yet very morbid read - find it here