Canada goose gains from eager beavers

Associate Professor Glynnis Hood, along with students and colleagues at the University of Alberta’s Augustana Campus, has conducted a study which shows that the activity of beavers helps Canada geese to get ahead in the northern nation’s snowy spring. The researchers believe that earlier accessibility to open water in beaver ponds as well as the elevated position provided by a nest on the side of a beaver’s lodge – out of the reach of predators – give the geese a better chance of successfully raising their young.

The North American beaver, Castor canadensis, is famous for using its strong teeth to fell trees with which it constructs dams and dens, known as lodges. In ponds where beavers are active, the snow and ice can give way to open water more than ten and a half days earlier than ponds where there are no beavers, especially in the area surrounding the lodge’s main entrance and food cache.

A four month project conducted by Professor Hood’s former BSc student Chantal Bromley from January to April 2008 revealed that the Canada goose, Branta canadensis, uses the beavers’ effects on the habitat to its own advantage. In an interview with ScienceOmega.com, Professor Hood went into the research findings and implications, as well as explaining how the study came about in the first place.

"Originally, we decided to investigate how beavers facilitate nesting for waterfowl in general, but realised the primary species to benefit was the Canada goose," she said. "The finding was likely because Canada geese migrate north and nest earlier than most other species of waterfowl, although we did see mallards and teal using the open water near the lodges during the study period."

In ecological terms, the team sought to answer two main questions. The first was whether water is available sooner in ponds with inhabited beaver lodges compared to those lodge and pond have been abandoned, and the second was whether geese choose active beaver lodges over other areas in a pond (including the abandoned lodges on the same pond and abandoned lodges in ponds without beavers).

"The main idea was that if the geese nest earlier, their young hatch earlier, leaving them a longer period of time to mature and increase their fitness for the fall migration," stated Professor Hood. "Indeed we found that geese did seek out and use active beaver ponds – particularly the area immediately around the lodge – over other sites that we surveyed, including ponds with abandoned beaver lodges." 

The study surveyed 32 active and 39 inactive beaver ponds in Miquelon Lake Provincial Park, situated in east-central Alberta. I asked Professor Hood what aspects of the beavers’ activity make it easier for the geese to nest, and nest earlier in the season.

"Firstly, early access to open water allows the geese a safer place to land and rest during the spring migration," she replied. "The water first opens up right next to the main, underwater, entrance to the lodge near the food cache. In the end the lodge acts like a ‘castle’ surrounded by a ‘moat’, despite the rest of the pond being frozen. This scenario allows geese greater protection from terrestrial predators such as coyotes and foxes."

Reported in the journal Mammalian Biology, the study also found that the snowpack was as much as six centimetres shallower on ponds where beavers were active. As the water opens up, waterfowl including Canada geese gain access to aquatic invertebrates for protein, aquatic vegetation, and drinking water, which can otherwise be a limiting factor during the winter months. 

"We had a theory that nesting on top of an active lodge would aid egg incubation due to increased heat from the lodge’s air vent, but we didn't note any difference in average temperatures, nor did we see geese nesting over the vent hole; they usually nested to the side," she continued. "The lodge does provide a raised platform surrounded by water, which geese prefer and, therefore, decreases predation risk.

"On more than one occasion we observed two pairs of geese fighting over a lodge on the ice, while a third pair of geese walked up the back side of the lodge and claimed it as their nesting territory. They really are prime real-estate."

The relationship between the busy beavers and their neighbours seems harmonious to Professor Hood, who related that she had never seen any indication that either species was bothered by the presence of the other, either during the study or in the years following it. Due to the nature of the study, the researchers were not able to measure gosling success relative to nesting times and so on, and thus the question of how the Canada geese would fare without the hard work of the beavers goes unanswered as yet.

"We can speculate that they might have a higher fitness level because they have had a longer time to mature and grow than geese that nest later, but that would be another study," Professor Hood commented. "One thing that would aid the geese is that if their first clutch of goslings wasn't successful, the adults would have more time to lay a second clutch."

Without beavers, the coveted space provided by their lodges would not exist and there would be fewer ideal nesting structures as a consequence. The risk of predation might also increase.

"In a previous study I conducted with Dr Suzanne Bayley, we determined that the very presence of beavers and their activities – digging, damming, and so on – creates areas of more extensive open water, even during drought," Professor Hood carried on. "In a study that I am just writing up at the moment, my colleague Dr David Larson and I have determined that ponds with beavers in them are significantly deeper and more complex than those that have been abandoned by beavers. The active management of water levels and provision of nesting structures would be the two key advantages for nesting in ponds with beavers present."

Professor Hood and her colleagues have carried out other research into the way that beaver activity impacts on other species, and further studies are in the pipeline; she is also hoping to examine small mammal use of beaver-modified landscapes in future. The work, current and ongoing, highlights the role of beavers as a ‘keystone’ species – these rodents have a disproportionate effect on the environment and on other species relative to their own needs.

MSc student Nils Anderson is currently completing his thesis, under Professor Hood’s guidance, on how the modification of aquatic habitats by beavers influences amphibians. It will detail interesting findings about the way that digging of channels by beavers aids in the dispersal of metamorphosed wood frogs. There is also some indication that breeding adults returning to the pond might use these channels in a preferential nature.

"Dr Larson and I currently have a paper in review that presents results relating to aquatic macro-invertebrates," Professor Hood told me. "Again the channels dug by beavers in their ponds seem to be hotspots for various taxa of aquatic macro-invertebrates, the predators in particular. We also have a paper in preparation that will reveal the dramatic physical alterations beavers make to these ponds and how those alterations influence landscape connectivity."