Invasive Species Eradication: Prioritizing Efforts and Understanding Costs

Reference: Baker, C.M., Plein, M., Shaikh, R., and Bode, M. (2020). Simultaneous invasive alien predator eradication delivers the best outcomes for protected island species. Biological Invasions. 22: 1085-1095.

Eradication Benefits, Feasibility, and Costs

The introduction of invasive species (non-native species that subsequently establish, spread, and have major – negative – effects on local native biota) stands out as an especially detrimental impact of humans on the natural world. 

Some invaders, such as zebra mussels, gypsy moths, and emerald ash borers are not able to be fully eradicated due to their mobility, reproductive ability, large populations, and small body size. Once these species have successfully established, eradication is not an option. Others, however, such as feral cats and rats, are more feasible targets, and have been successfully removed from some areas resulting in the conservation success of many native prey species.  

Eradication is not an easy or straightforward process, however. Removing entire populations from large areas is challenging and costly. This is especially true when multiple invasive species interact. Eradication of one invader sometimes results in increases in another, putting even more pressure on native species. Feral cats, for instance, are often present in areas where rats have also invaded. In these situations, cats serve as apex (top) predators while the rats serve as mesopredators (second-tier predators), often consuming the same native prey. If all the cats, but not rats, were eradicated, the rats could reach high densities and have a much stronger impact on native prey than they had before. 

The possibility of interactions between coexisting invasive predators means that multiple eradications are often necessary. Historically, however, there has been much ambiguity about the order in which to perform multiple eradications – should you start with the apex predator, the mesopredator, or try and eradicate them both as once?  

Figure 1: Though domestic cats bring immense joy to their owners, wild (feral) cat populations can have major impacts on native bird and small mammal species. Source: Bramans,https://commons.wikimedia.org/w/index.php?sort=relevance&search=feral+cat+hunt&title=Special:Search&profile=advanced&fulltext=1&advancedSearch-current=%7B%7D&ns0=1&ns6=1&ns12=1&ns14=1&ns100=1&ns106=1#/media/File:Felis_silvestris_catus_(domestic_cat).jpg.
Modeling Can Help 

Christopher Baker – a postdoctoral fellow at the Queensland University of Technology – and colleagues decided to mathematically model how abundances of invasive predators and native prey species change through time under different eradication scenarios. 

Eradication scenarios are complex, and using a modeling method in which real-world interactions between species are simplified into mathematical terms allowed the researchers to get an idea of how major attributes of the species and budget could affect the outcome of conservation decisions. 

Their objective was to find a cost-effective way to remove multiple invasive species and protect vulnerable native prey. Because order of eradication could be important in the sequential methods, machine learning was employed to predict which predator would be best to target first. Using their models, the group tested how each species would respond to different eradication scenarios and the relative cost of each. 

Trade-offs

Simultaneous eradication of both invasive predators was found most effective, leading to the best outcomes for native prey in 99% of the scenarios tested. Unfortunately, however, sequential predator eradication was found to cost less in 98% of the simulations. 

There exists a trade-off between cost of eradication and outcome of the native species across the scenarios. Under the sequential strategies, native species outcomes were variable depending on initial model conditions. In the eradication scenarios where predator species were targeted simultaneously, however, native species outcomes were predicted to be excellent largely regardless of starting conditions. 

Despite the costliest method also being the most effective, conservation budgets are typically far from optimal. Thankfully, the machine learning method determined the most important factor in deciding which predator to target first is related to the ratio of mesopredator attack on target prey to attack on other species. When the lower order predator prefers the species that conservationists are working to protect, the mesopredator should be targeted first if a simultaneous eradication method is cost prohibited. 

Figure 2: Cats are considered the number one threat to birds in the US and Canada by the American Bird Conservancy, killing roughly 2.4 billion birds per year just within the US!  The piping plover (pictured) is one species risking extinction. Image Source: Kaiti Titherington/USFWS, https://commons.wikimedia.org/wiki/File:Piping_Plovers_(23072794576).jpg
Prevention is the Cheapest Option 

Modelling studies allow us to make more informed decisions about how to manage or eradicate established invaders. However, the best method of preventing biodiversity loss due to invasive species is to prevent invasion. As humans, we have a lot of power over the world around us, and that means we need to think carefully about how our actions can affect other species. Failing to spay or neuter your pets, releasing pets into the wild, forgetting to clean and drain boats between trips to different lakes, moving firewood from one campsite to the next, and sneaking food items onto flights are examples of common decisions that can lead to devastating invasions. 

Reviewed by: Jose Valdez

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Riley Lovejoy

Riley Lovejoy

I am a PhD candidate at the University of Alabama, where I completed a Master’s degree in 2017. My current research focuses on biological invasions of ecological communities, using freshwater plankton as a study system. I believe science is for everyone, and love connecting others with topics they can become passionate about. Because of this, I founded an organization called Delta Tree Initiative that introduces middle and high school girls to STEM research and careers. If I’m not at a microscope, in a pond, or doing outreach, you can likely find me hiking, baking, or spending time with family and friends. Instagram: @love.joy.science

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