When Beaver Introduction Backfires

A reintroduced ecosystem engineer species may exacerbate ongoing biological invasion: selective foraging of the Eurasian beaver in floodplains (2020) Juhasz et al., Global Ecology and Conservation, https://doi.org/10.1016/j.gecco.2020.e01383

The Crux

The reintroduction of species to an area from which they have been wiped out can have benefits which extend beyond that one species. Often they can restore ecological functions that have since been lost, which can result in everything from an increase in biodiversity to restructuring of an entire landscape.

That last example might seem a bit far-fetched, but beavers (Castor fiber) are capable of just that. Their damming activities can change river flows and restore healthy floodplains, and as such beavers are the target of a large reintroduction campaign now occurring throughout much of Europe.

But what happens when a species like the beaver is reintroduced to an ecosystem that has seen significant changes since it has been gone, like the introduction of invasive species? Today’s authors wanted to find out whether or not the presence of the beaver benefited native plants, or whether it made things easier for the invasive species.

What They Did

The study took place in lowland Hungary, where native softwood trees have been replaced by woody tree species like the green ash (Fraxinus pennsylvanica) and ash-leafed maple (Acer negundo). The researchers set up 20 different sites along rivers, with each site housing two transects of 500 metres, one at the waters edge and one ten metres further back. Along these transects they sampled 50 different sites, checking for the proportion of native vs. invasive species, and what proportion of both types had been used by beavers.

Did You Know: Beavers as Engineers

I’ve somewhat understated the effects beavers can have on ecosystems. Their ability to change the landscape through broadening rivers and raising the water table can increase persistence of otters, birds, fish and even increase water quality for farmers further downstream. Ben Goldfarb’s book Eager is a fascinating take on how important beavers are to landscapes, but for a cliffnotes version, try his article at the link below.

Beavers are the Ultimate Ecosystem Engineers

What They Found

In the areas where the invasive tree species were found, the native species were present in much lower abundances. This was the case both at the water’s edge and further back. Beavers utilised both native and invasive species, and there seemed to be a tendency to use the natives more than the invasives at many sites. This was particularly the case for thinner branches in the understory.

A beaver dam like this one can reorganise the ecology and hydrology of a river for kilometres in any direction (Image Credit: Franklin Vera Pacheco, CC BY-SA 3.0)


This week’s complaint comes admittedly from a place of ignorance. The paper mentions the foraging habits of beavers, but I would have really liked to know more about the effects of beaver proximity on the different transects. Did the beaver population being closer or further away from the transect, or local population density affect the results? It could be that this is obvious to any beaverologists, but it left me very curious.

So What?

In areas where invasive species weren’t so ubiquitous, increased foraging by beavers might not be a problem. They would likely be able to reproduce quickly and sustain populations. However in areas where native plants face heavy competition for resources from invaders, the reintroduction of the beaver could threaten native species, hastening their decline.

It throws a shadow on a good cause. Species reintroductions are important, and often landmark achievements for conservationists. Yet the other changes that ecosystems have faced since the local extinction of a species may mean that the species in question may not be welcomed back initially. It’s a tradeoff we’ve explored before – is a negative effect on the local ecosystem more important than the persistence of a threatened species?

Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology who once believed that beavers were capable of jumping ten metres and latching onto your throat (stupid Canadian friends). You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.

Title Image Credit: Roy Buri, Pixabay licence, Image Cropped

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