How beavers shape insect communities

Beavers in Europe

Beavers (Castor sp.) are large, semi-aquatic rodents found across the Northern hemisphere. Two species exist, the Eurasian beaver (Castor fiber) and the North American beaver (Castor canadensis). By the early 20th century, the beaver population in Europe had plummeted to around 1200 individuals due to hunting. However, conservation efforts have resulted in a remarkable recovery, and today, beavers are once again thriving in freshwater and woodland habitats across Europe. Most of Europe is home to the Eurasian beaver, except for Finland, where the North American beaver was introduced instead of the Eurasian beaver in 1937. 

Ecosystem engineer

Beavers are renowned as ecosystem engineers – species that alter their surroundings in ways that affect many other organisms. Their impact is often more noticeable than the animals themselves. While beavers can be elusive, their presence is revealed by gnawed tree stumps, felled logs, and intricate dams made of wood, stones, and mud.

Beavers modify their habitats in many ways. As they settle into an area, trees are felled, waterways are redirected, and both standing and fallen deadwood accumulates, in various stages of decomposition. These changes influence the entire hydrology of the area, creating beaver wetlands that change resource availability and habitat structures for countless species. This transformation introduces new ecological niches while others deteriorate, altering the attractiveness of the area for different species.

Natural succession

Natural succession is the process in which a natural ecosystem evolves and changes over time. Beaver wetlands experience this both during and after beaver occupancy. Flooding occurs while beavers inhabit an area often throughout their presence. Once beavers leave and their dams deteriorate, water levels drop exposing bare soil. Soon, sedges and grass will colonize this land, followed by shrubs and saplings once it dries out even more, and eventually by trees, returning the once flooded beaver wetland to forest. 

This natural succession creates dynamic environments where, depending on the successional stage, certain niches appear while others disappear. 

Beavers and biodiversity

Numerous studies have highlighted the profound effect beavers have on other species. Beaver wetlands support higher species richness of waterbirds compared to non-beaver wetlands. Bats, too, benefit from beaver activity, likely due to increased abundance of insects, their staple food. 

Beavers and insects: a complex relationship

Since beavers alter landscapes, it’s natural to ask: do beavers increase insect diversity? Research suggests that the answer depends on the insect group and habitat in question. Most studies agree that insect communities differ between beaver wetlands and free-flowing streams. Some find that beaver wetlands do indeed enhance certain insect populations, while other studies find no such effect, and yet others detect a decline in specific insect groups.

Aquatic insects

Most studies have focused on how beavers affect the aquatic invertebrate community. Many studies report that flowing water habitats support greater aquatic insect richness than beaver wetlands. However, at a landscape level, beaver-created ponds increase the overall insect diversity by promoting large habitat heterogeneity. 

Some aquatic insect species will benefit from the creation of beaver wetlands, for example, water beetles, where a Finnish study showed that both abundance and diversity of water beetles were higher in beaver ponds compared to ponds with no beavers. Beaver wetlands also favor species such as Asellus aquaticus, a generalist crustacean that benefits from beaver disturbance.

Terrestrial insects

The beaver’s influence extends beyond water, affecting terrestrial insect populations as well. The initial flooding of an area will likely have a detrimental effect on numerous species not adapted to submersion, but other species will thrive during the various successional stages.

A good example of beavers promoting terrestrial biodiversity is their impact on the St. Francis’ satyr butterfly in North America. The butterfly depends on sedges (Carex sp.), grass-like plants, as host plants for its larvae. In this case, beaver wetlands harbored more sedges compared to wetlands without beavers, and the increased number of sedges significantly correlated with the butterfly population size. Similarly, a Danish study found that overall moth species richness and diversity were higher in beaver wetlands, which were also more heterogeneous and greener than areas without beavers. Additionally, research from Finland and North America has shown that bark beetles (Scolytinae) are more abundant in beaver wetlands.

Overall, beavers play an important role in freshwater ecosystems by creating disturbance, increasing habitat heterogeneity and thereby altering insect communities. Their impact varies depending on species: while some insects thrive in the niches beavers create, others perish. By engineering their environments, beavers contribute to biodiversity in complex and often surprising ways.