How do you reverse a Forest Invasion?

If an invasive species is a problem, isn't the answer simply to remove it? A new review by Wardle and Peltzer shows it's a bit more complicated than that.

Biological Invasions is putting together a special issue on Forest Invasions, and an Open Access paper Impacts of invasive biota in forest ecosystems in an aboveground–belowground context by Wardle and Peltzer has caught my eye.

…[A]lthough we have a growing understanding of the determinants of the effects of invasive plants, for invasive consumers we have yet to move from a series of iconic case studies to the development of general principles.

Wardle & Peltzer 2017

That word consumers is hugely important when you think about what a consumer is. One of the things you learn early on is that plants are autotrophs, they make their own food. Heterotrophs don’t. That means that when you think about consumers in a forest, you’re talking about pretty much everything in the forest that isn’t a plant, as Wardle and Peltzer’s diagram below shows.

Aboveground and belowground biota are linked in forest ecosystem
Aboveground and belowground biota are linked in forest ecosystems both by direct pathways (i.e., through soil organisms that interact directly with plant roots) and by indirect pathways (i.e., through decomposer organisms that mineralize nutrients required for plant nutrition and growth) (Wardle et al. 2004); these linkages collectively drive ecosystem functioning. These linkages are disrupted by both aboveground and belowground invasive organisms, representing all major trophic groupings, and through a wide variety of mechanisms. Wardle & Peltzer 2017.

It’s these various partnerships that can make simple predictions or rules impossible when examining invasive species. For example, an invasive plant is not only directly competing for nutrients, light and pollinators, it’s also a partner in the ecosystem. It can promote the growth of supporting species, like mycorrhiza or hinder the recycling of nutrients by dropping less useful leaf litter. As an example, Wardle and Peltzer cite recent work in AoB PLANTS of a garlic mustard that poisons local microbes, which breaks down relationships used by native plants.

One of the odder interactions to dramatically change a forest Wardle & Peltzer highlight is an invasive herbivore in South America. Invasive herbivores can be a major change for obvious reasons, for example being more voracious or having no natural predators to check its population. This isn’t the biggest problem with Castor canadensis, the North American beaver. It does what beavers do, which is fell trees to make dams, and this not only has a direct effect on the tree felled, it also changes the hydrology of the local ecosystem for the other trees.

Wardle and Peltzer also note that belowground consumers can be invasive too, with earthworms and other invertebrates moving into new soils thanks to human activity. Of course, predators can also be invasive. They don’t have to directly eat the plants to tip the balance for the herbivores, and by extension, the plants getting eaten. Or you can even have a predator eating another predator to relieve pressure on other herbivores.

Looking at a forest as a network of interactions has some obvious implications when it comes to restoration. It isn’t enough to simply remove the invasive species, you have to rebuild or replace the broken relationships. This was the topic of another AoB PLANTS paper that Wardle and Peltzer cite, Belowground legacies of Pinus contorta invasion and removal result in multiple mechanisms of invasional meltdown by Dickie et al.

An Invasional Meltdown sounds dramatic and it can be. The meltdown part refers not to the invasion but what happens after. Invasions can happen when a species is released into an area, but they can also happen when a native species is removed – creating an opportunity for something else. Dickie et al‘s paper shows that this kind of interaction can leave a long legacy.

The example they use is Pinus contorta. What this does is change how nutrients are cycled through the soil. It means that when Pinus is removed things don’t go back to how they were. The change in nutrients means the soil is now prepared for invasive grasses and herbs. Wardle and Peltzer include this as case b, the secondarily invaded ecosystem in another of their diagrams.

Three possible trajectories of change in forest ecosystems that may occur following removal or loss of an invasive species.
Three possible trajectories of change in forest ecosystems that may occur following removal or loss of an invasive species. These trajectories are: (a) return to the original native community. This may require additional interventions such as reintroduction of lost native species or mutualists of native species, or modification of habitat conditions to make them more suitable for native species establishment. (b) Persistence of the legacy of the removed invader through secondary invasion by other invasive species. (c) Movement of the ecosystem past a tipping point that prevents the ecosystem reverting back to its pre-invasion state and that differs fundamentally both from the originally uninvaded and invaded ecosystems. Note that although invasive plants are depicted here, exactly the same set of principles also applies to invasive aboveground and belowground consumers whenever they transform ecosystems. See Wardle & Peltzer 2017. for further discussion

My feeling after reading the paper is that my head is spinning slightly. In some ways it underlines how complex and chaotic ecosystems are. The sheer number of relationships connecting actors in an ecosystem makes detailed predictions very difficult. However, I don’t get the impression that Wardle and Peltzer are trying to reduce everything to a single equation to describe a forest. Rather, by focusing on the consumers above-ground and below-ground you can identify some of the web of interactions that can strengthen or weaken the stability of an ecosystem. The paper is a really helpful introduction into appreciating the complexity of what an invasion means. I can see this being a useful starting point in discussions about invasion species and ecosystem restoration in the future.

References

Wardle, D. A., & Peltzer, D. A. (2017). Impacts of invasive biota in forest ecosystems in an aboveground–belowground context. Biological Invasions. https://doi.org/10.1007/s10530-017-1372-x

Brouwer, N. L., Hale, A. N., & Kalisz, S. (2015). Mutualism-disrupting allelopathic invader drives carbon stress and vital rate decline in a forest perennial herb. AoB PLANTS, 7(0), plv014–plv014. https://doi.org/10.1093/aobpla/plv014

Dickie, I. A., St John, M. G., Yeates, G. W., Morse, C. W., Bonner, K. I., Orwin, K., & Peltzer, D. A. (2014). Belowground legacies of Pinus contorta invasion and removal result in multiple mechanisms of invasional meltdown. AoB PLANTS, 6(0), plu056–plu056. https://doi.org/10.1093/aobpla/plu056