Can Beaver Ponds Actually Reduce Invasive Species?

American bullfrog (common in CA but not native)

One of the chief indictments against beavers, especially in areas where they are not yet established, is that the modifications that they make will greatly augment the available habitat for non-native invasive species. Now depending on where you live this can be a whole assortment of wetland loving critters. Beaver ponds being lentic habitats, that is they are bodies of slow moving waters, invasive species that are attracted and benefit from this environment include a whole slew of things where I live in California. But the usual suspects include both signal and Louisiana crayfish, African clawed frogs, American bullfrogs, catfish species but most commonly bullhead, large and small mouthed bass, and common carp. There are others but these are the main culprits. They all love sluggish, stagnant water- and they like it warm. Beaver ponds can create certainly make for warmer waters in the shallower parts of their pond.

Stagnant Pool on Santa Clara River Oxnard. Duane Nash

Although it makes intuitive sense that beaver modifications would greatly augment the habitat for these destructive invasives, as is often posited, we should not be satisfied with mere assertions but actually investigate this question more assiduously.

Let us look and see if there are any characteristics that unify these California invasives. As mentioned earlier they like the water slow and they like it warm. It should also be noted that the species I listed above are all highly fecund. They make a load of babies. A common carp can produce about a million offspring in a season. So when conditions are right, food is plentiful and the water is warm- every species listed can reproduce and grow fast. The big down side to all these non-natives is that not only do they crowd out the natives and out compete for spawning sites, critical refugia- they are more often than not highly predaceous on them during one or more of their life stages. For instance bullfrogs can eat hatchling western pond turtles, carp can eat the eggs of all fish/amphibians but not the adults.

Bullfrogs Eat Everything

Yep these invasives in California are especially troublesome due to their often voracious and omnivorous dining habits. Some such as the African clawed frog- a problem in the Santa Clara river watershed -are just ridiculously gluttonous. These weird, fully aquatic, frogs were actually commonly used as proxies for pregnancy tests!!

African Clawed Frogs Are Piggies

But with regards to these nasty invasives- predatory as they may be- these animals are not top predators. Bullfrogs, bass, crayfish are better thought of as mesopredators "middle predators". They eat animals, but animals also commonly eat them. And at least in California these non-natives have a host of predators that can and do eat them such as raccoons, herons, mergansers, garter snakes, and ospreys among others. The trick is to get the predators to the prey....

Walt and the Beaver (link)

Please do yourself a favor and watch professor Walt talk about his experience with beaver on the upper Verde river of Arizona. The whole video is outstanding but what is most interesting and pertinent to our discussion here starts at about 3:30. Here he goes on to describe that although the river system he is discussing, the upper Verde river, has an invasive species problem his qualitative observations (he is a professor) have suggested that beaver ponds have actually lowered invasives while natives have rebounded.... How is this possible?

The key player in this scenario Walt posits is the river otter (Lontra canadensis). With the return of beaver ponds otter have returned to the river as well. These deep, slow pools give the otter the habitat it prefers. Otters, like any predator, have an optimal foraging strategy. Why waste your efforts on catching multiple small prey items when you can catch a few larger animals and get the calories you need without all that extra effort? And in these beaver ponds the larger, non-native fish such as bass and large bullfrogs are being preferentially preyed on by otter. The smaller, quicker, and more cryptic native fish and amphibians are being consumed as well but not to the same extent. Several interesting ecological processes are putatively occurring here.

1) Beaver, a keystone species, is living up to its name in providing habitat conducive to otter.

2) Otter, being top predators within the aquatic realm have reestablished and are starting to exert strong top down control. Top predators are now recognized as crucial elements in optimally functioning ecosystems.

3) These invasives, which prior to the return of beaver and otter to the system, were enjoying optimal success due to a phenomena referred to as "mesopredator release hypothesis" now have to contend with a growing and hungry population of river otters.

4) Being larger, commoner, and more conspicuous than the native aquatic organisms otters feed on the non-natives preferentially. Again, exerting top down control and following optimal foraging strategy otter start to make significant impacts on the ecology of the ecosystem.

5) With their chief competitors and predators lessened in population density due to otter predation native species can expand.

6) Because beaver ponds allow interactions between native and non-native species this allows non-native species to evolve defenses against them, which in the long run may be most pragmatic because it is highly unlikely that humans will exterminate completely non-native aquatics.

And in this manner beaver ponds may actually be minimizing the impacts of non-native species on the upper Verde river of Arizona.

Beaver Lodge and dam Upper Verde River. Nature Conservancy

Now obviously much of the story involving beaver/otter/invasive species on the Upper Verde river of Arizona is anecdotal, we need proper peer reviewed studies, but I would like to add my own observations on beaver ponds and invasive species on the Santa Ynez river in Santa Barbara county. As I detailed on  a recent post the Santa Ynez has dropped precipitously due to drought and the low amount of water in Cachuma reservoir. But I also suggested that these beaver have developed strategies for dealing with the low water. Never the less it was a bit of a disturbing site to see and photograph the wonderful beaver ponds dried up.

Dried beaver pond. Santa Ynez river CA. Duane Nash

But as distressing as this image is here this is where I will suggest such occurences may be actually doing some ecological benefit. Compared to many of our native fish/amphibians the invasive aquatic organisms are highly dependent on water. Bullfrogs are very aquatic compared to native amphibians which can survive drought and go underground and into dormancy. When these beaver ponds start to dry up, although they served a great home and sanctuary to non-native species when there is water, these very same beaver ponds may serve as death traps.

Bullfrog Tadpoles Stranded in Drying Beaver Pond. Santa Ynez River.

Now as the video above succinctly documents, these bullfrog tadpoles- whose parents probably enjoyed life in these lush, well vegetated ponds -are most likely doomed. Birds, raccoons, or simply lack of water will do them in.

Another common sight I witnessed in these drying beaver ponds were loads of chewed up Louisiana crayfish remains. Most likely raccoons did the damage, wading birds will simply swallow the crustacean whole.

Unlike the situation in the Verde river, we do not have river otters in southern California nor is there any suggestion that they were ever native here. But, in addition to raccoons, what we have a lot of is abundant and diverse avian predators of aquatic animals.

Santa Clara River estuary. Great Blue Heron and White Pelicans. Duane Nash. March 2014

The photo above is of the Santa Clara river estuary in Ventura county and you can see two high level avian aquatic predators. The great blue heron is a startlingly efficient and ruthless predator. It is a bird that really denotes the dinosaurian heritage of birds. As you can imagine great blue herons love beaver ponds and as I was exploring the Santa Ynez during this dry spell I kept on disturbing a flock of great egrets and one loud, croaking pissed off great blue heron. These guys were having a buffet on the stranded crayfish, fish, and tadpoles left in the ponds.

Heron/Egret tracks on drying beaver pond. Santa Ynez river

Great Blue herons are efficient predators but let us not pay short thrift to American white pelicans. These birds may actually be the most efficient and diligent of all freshwater predators, maybe even more so than river otters. Unlike our brown pelicans, white pelicans are cooperative predators of freshwater and estuarine environments. They do not plunge dive but they cooperatively shoal fish in shallow water.

So not only are white pelicans cooperative feeders they can also snag and swallow huge fish- including pretty huge carp, a non-native fish and probably what the white pelicans in the Santa Clara estuary were feeding on. I found this video Nygren Pelicans feeding frenzy that really highlights what a mass feeding spectacle a group of white pelicans can achieve, I highly recommend it.

White Pelican going after Asian Carp. Bill Rudden

White pelicans have already been posited as a control on invasive Asian carp in the Great Lakes region and so why not Californian waterways? Well, white pelicans will not frequent narrow, confined channels. They like big, wide shallow bodies of water. In California this is primarily lakes/reservoirs/estuaries. But it is not without reason to suspect that big beaver ponds could provide such foraging habitats for white pelicans. If you do a little internet sleuthing you can find pics of white pelicans in beaver ponds or even standing on beaver dams. Maybe white pelicans are another species that has been setback due to loss of beaver habitat on this continent?

Osprey with carp

Well hopefully you found this post interesting. The more I look at how complicated some of the issues I highlight are the more I see it is important to stay away from blanket statements. Can beaver ponds benefit invasives? Sure. But if you have abundant top predators the opposite may occur. Non-natives are expeditiously slaughtered and the native return. Beaver ponds, by actually creating the ideal habitat for non-natives, may actually be creating an environmental cul-de-sac that allows top predators to move in and decrease non-native populations- perhaps leaving the natives to repopulate.

Great Egret and Great Blue Heron. Fillmore fish hatchery. Fillmore CA. Duane Nash

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