The fifth horseman of the biodiversity apocalypse

The book of Revelations tells of the four horsemen of the apocalypse: famine, war, conquest, and death. They are thought to foretell the beginning of the end times. In no less stark terms, the UN has warned that in order to have viable future, we must address the 5 horsemen of the biodiversity crisis. Most of us are familiar with the first four of the horsemen: land use change, over exploitation of species (e.g. destructive hunting), climate change, and pollution. But less well know is the fifth horseman: invasive species. Invasive species, such as kudzu and English ivy, contribute to 60% of global extinctions and cost a bare minimum of US$423 billion a year.

Still life with Flowers and Fruit by Jan van Huysum (1715) - could instead be titled “Alien and Invasive Flowers and Fruits from the Empire”.

As with climate change and the other horsemen, it is critical that individuals help solve the problem. However, talk of invasives seems to stir up more push back than most eco-conscious rants I’m known to offer. I’ve been told various versions of the following: all species were once invasive, ecosystems are always changing anyway, my one nandina won’t make any difference, and don’t begrudge me my Buddleja. I think some of the push back against invasives arises from irritation at hearing about “yet another way we ought to change our lifestyles”. I’m afraid there is no escaping this further call to action, but in this case we can have significant impacts pretty easily. And we are doing the hardest bit right now - learning. For once we’ve understood the what and why of invasives management, we’ll be empowered and perhaps even feel compelled to take the hugely impactful step of NOT planting (or releasing) invasive species. And I think we can all manage that simple lifestyle change.

Further opposition to considering invasive species stems from miscommunication about what invasive species are. When we say “invasive plant”, for instance, we never mean a native plant. And further more, even most foreign plants are not invasives. Rather, invasive species are a small subset of alien species that have a negative impact on biodiversity. And alien species, as opposed to native ones, are those that are established in the wild but whose presence is attributable to humans. There are an estimated 37,000 alien species world wide, or which about 3,500 are recognized as invasives. For the plant-aholics out there, about 6% of established alien plants, or 1061 plants, are considered invasive, globally. A classic example of an invasive species is the Emerald Ash Borer - an Asian beetle that is destroying ash trees in the US and Europe. In the parks near my home, the Emerald Ash Borer is estimated to have killed 73% of ash trees resulting in dramatic habitat change.

While the definitions of natives, aliens, and invasive species are fairly straight forward, it can be difficult to use these definitions to know which category a given species is in. To simplify classification, the USDA defines natives as those species that were present in the US prior to European settlement. But this is problematic. Native Americans moved plants around and managed the plains with controlled burns to increase habitat for bison.

Biologists prefer a more nuance categorization. For instance, Tallamy and Darke define a native as “A plant or animal that has evolved in a given place over a period of time sufficient to develop complex and essential relationships with the physical environment and other organisms in a given ecological community”. While a satisfying description, it leads some to conclude that we needn’t worry about invasives. These naysayers argue that given enough time, nature will find a new balance and any “alien species” that survive will form part of the new web of life. That is possibly true, except for one worrying caveat that I’ll draw out in the next paragraph. But, there is also the reality that the transition from today’s web to tomorrow’s will involve significant loss of individuals, species, and ecosystem services. It will not be a peaceful or abundant transition. Recall that invasives are estimated to have contributed to 60% of recorded global extinctions and these extinctions are just the tip of biodiversity loss - underpinned by a mountain of population reductions in other species. Invasives have also been shown to impede water flow making it unfit for drinking, to increase flood risks, to reduce soil quality, and to change pollinator populations. And directly afftecting humans, invasives can cause outbreaks of tick borne diseases, SARS, tuberculosis, AIDS, and virulent malaria. Over the long term, invasive species may well settle into a mutually beneficial role, but the journey to that new equilibrium would likely be very painful for humans and the rest of the biosphere.

And the big caveat to invasives contributing to a new ecological equilibrium? The damage caused by invasives species is not occurring in a vacuum - the other four horsemen of the biological apocalypse are also galloping across the land. The five horsemen are often co-occurring and they tend to amplify the each other’s impact. For example, deforestation is documented to drive more land use change, more climate change, more over harvesting of wild species, more pollution, and more invasives. In addition, the impact of multiple stresses on ecosystems is often synergistic - having more impact than just the sum of the individual stressors. This is similar to when a human has more than one disease, co-morbidity can lead to complications and even new health issues. There is a very real possibility that the cocktail of the five ravaging horsemen will lead to ecological collapses. It is imperative, for the survival of our species and for moral reasons, that we address each of these threats to the planet’s biosphere.

There is a lot that we, as individuals, can do to reduce the impact of invasive species. Our options for action parallel the three recognized stages of species’ invasions: introduction, establishment, and spread. Preventing the introduction of a potentially invasive species is the easiest and least expensive way to avoid problems. New Zealand takes this very seriously and checks the boots of visitors for seeds. But most other nations do not, indeed while they have targets for managing invasives, there is generally a massive gap between national targets and implementation. As citizens, we can support legislation to strengthen these controls. In my state, Maryland, new legislation is in the pipeline to increase the number of invasive plants banned from sale from the current paltry list of just 6. As gardeners, we can do better and not plant any invasives, even if we can buy them. You may be surprised to learn that horticulture is responsible for 46% of invasive plant species. That is massive. If you are a gardener, you can make an important contribution to slowing the introduction and spread of invasive species. The most ecological option is to plant native species.

The second stage of an invasion is the establishment of a self-sustaining population. For plants, this depends on the availability of a suitable site - the right soil, the right climate, the right state of the land. Recently disturbed land or fragmented ecosystems host more invasive species than established and intact ecosystems. Dense and diverse ecosystems are found to be less likely to be invaded by non-natives. So as a gardener, we can aim for a dense and diverse mix of native plants. Less weeding and more flowers. Yeah. And as a consumer, we can reduce our resource consumption to reduce the invasive precursors of land use change, ecosystem fragmentation, and carbon emissions. Outside our own homes, early detection and a rapid response are key to preventing the establishment of invasives. So as citizen scientists, we can make important contributions by using apps like iNaturalist to identify plants around us - natives, aliens, and invasives. Scientists harvest the observations made in iNaturalist to monitor the spread of invasives and to map biodiversity. Enthusiasts can do even more and join national programs like the Invader Detectives Program in the US or, in the UK, report sightings of invasives to the Non-native Species Secretariat.

The third stage of managing invasive plant species is the most difficult - control. Like many gardeners, I spend a lot of time cutting down invasive vines, digging up invasive shrubs, and pulling invasive grasses. It would be so much easier if those plants had never been introduced, but here they are. As a gardener, I have always focused on the mechanical removal of invasives - i.e. digging, pulling, cutting. But when we dig and pull, we disturb the soil microbiome and usually bring up unwanted seeds in the process. There is a growing ‘no-dig’ garden movement that instead advises cutting invasives off at ground level. For tough plants this technique is likely to require repeated cuttings but will eventually weaken and kill many invasives.

Other horticulturalists argue for the application of tiny amounts of herbicidesjust after cutting invasive shrubs and trees to ground level - it’s called cut and paint. Cut and paint is very different from previous practices of spraying herbicide on all the green leaves on a plant as it drastically reduces the amount of herbicide used. Until recently, I was adamantly opposed to herbicides. But the dig-cut-cut again methods I’ve been trying are not working in the tiny wild woodland I manage. And I want native plants there to support native insects, birds, and mammals. So this spring, I am going over to the dark side. I am going to try cutting and painting the invasive shrubs when they first start to green up. Then I will watch the stumps over the growing season and cut and paint those that survived my first poisoning. My instinct is that the small amount of herbicide will be less damaging than the non-natives themselves. For not only are the invasives in my garden crowding out native plants and animals, they also sending seeds into the neighborhood. So, I am going to use that old bottle of Roundup left in my garage by a previous owner to try to kill off the Japanese honeysuckle, the burning bush, the barberries, and the mahonia in my back garden.

An important tip if you do decided to remove invasives - it is wise to immediately fill the gap with a dense matrix of plants, ideally natives. Otherwise the gap that is likely to be filled by other unwanted plants. There are many resources online to help you find native plants and nurseries. I have found that other native plant gardeners are invaluable sources of information and plants, check out your local Wild One’s chapter. or Homegrown National Parks.

All of this thinking about invasives begs the question: “Are humans an invasive species?” Based on our current behaviors, yes. But we don’t need to be so destructive. We can be forces for good in the fight against invasives and for life. My own wild and chaotic garden stands in some contrast to my neighbors but it shimmers with lightening bugs, bees, birds, moths, frogs, lizards, snakes, and countless other critters. It may well be my greatest legacy.

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Switching to Sundays. Gentle readers, my son has pointed out that I should be more regular with my newsletters. I have therefore decided to post my newsletters on the last Sunday of every month. Let me know what you think about that or other thoughts this article has prompted. Thanks for reading and happy Sunday.