|American bullfrog (common in CA but not native)|
|Stagnant Pool on Santa Clara River Oxnard. Duane Nash|
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|
|Dried beaver pond. Santa Ynez river CA. Duane Nash|
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.
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|
|Heron/Egret tracks on drying beaver pond. Santa Ynez river|
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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