SF Natural History Series

Michael McGowan spoke before the Natural History crowd on May 16, 2013 about his thinking and philosophy around invasive species in the San Francisco Bay.

Our current attitudes to managing invasive species come from Charles Sutherland Elton who brought this sub-discipline into being after the Second World War. He recommended quarantine, eradication, and controls to suppress invasive species. The chief goal of which was to prevent extinction of native species and a homogenization of the local ecosystem.

There are many examples of where invasive species have wrought havoc on local ecosystems. Many of these examples involve islands, where species diversity is already low. And there are places like Australia where there are many examples of introduced species gone wrong.

What can we say about the Bay, in this regard? There are any number of invasive species: striped bass, oyster drills, mitten crabs, clams, cord grass, various copepods, and a mud shrimp species that Michael only recently discovered. These have been introduced intentionally, and unintentionally and have all in some degree affected SF Bay, at the same time there have been changes wrought to the environment by man through other means: run-off from gold mining, the filling in of wetlands, the run-off from our growing towns and cities, ship traffic, damming and altering of rivers, climate change, etc.

One question then is how much harm have these invasive species brought to the bay? Of the species Michael went through, the Corbula clam may have had the greatest affect altering the processes of the North Bay, but there has been no species extinctions as a result of these species, and no homogenization of Bay diversity. In fact, one could say that the diversity of the Bay has increased, that there’s been some net gain.

That isn’t to say we shouldn’t try to preserve & protect native species, or introduce new species willy-nilly, or not worry about things like ballast water (the standard practice these days is for ships to exchange ballast water offshore where travelling species won’t be released into more protected waters). Michael stressed the importance of good data and monitoring over time to work towards understanding these complex systems on top of which we are living.

20 years ago, Nancy DeStefanis, was at Stow Lake and was lucky enough (although one imagines a keen eye probably helped) to find Great Blue Herons nesting at Stow Lake — the first Great Blue Herons recorded as being seen in the city in fact. Every since, then she has been following these big beautiful birds as they come to breed, nest, and rear their young. The pictures she presented at our lecture on April 18th, 2013, presented a shorthand version of their lives in and around the nest. (Forgive me if I get some of the details wrong — I was running the slideshow and not able to take notes.)

The males arrive first as a Christmas nest, looking to stake a claim on the nests left behind from previous years, waiting for the arrival of their mates. When the females arrive nest building starts, the males presenting sticks to be added to the ever growing nests. This leads into the courtship, and eventually mating. The eggs are small, only slightly bigger than chickens eggs, but the nestlings that hatch quickly begin to grow. The mother and father bring food to the nest, but don’t necessarily feed the chicks through regurgitation.

Great Blue Herons not only snack upon fish, they also hunt gophers and other rodents that populate our parks. Not much preys upon Great Blue Herons in turn although Great Horned Owls have been known to hunt them at times. There are 4 nests currently being used on the tree, but there is about 6 total. The most prized on is near the top of the tree, of enormous size as it has just been added to over the years. One nest fell off the tree at one point — which has since ended up at Audubon Canyon Ranch.

The young herons sitting in the nest are wonderfully silly to behold sporting black mohawks that get tossed about in the end. The birds quickly grow though and it is only some weeks before they are nearly as tall as their parents and are stretching their wings, and beginning to hop from branch to branch.

After the birds fledge, the birds go they own way. The juvenile young herons at this point are on their own. Parents don’t teach the young. Their survival rate is very low. 67% don’t make it past their first year.

You have a couple more weeks to be able to see the birds with the aide of Nancy and her crack crew of volunteers and great spotting scopes. At 1030am she walks her guest around (young kids are free, adults are $10) the lake and over to the Strawberry hill, looking at not only the herons, but with an eye for the other nests and species that inhabit the park. It’s well worth a trip — head over to the Stow Lake boathouse, and look for Great Blue Heron signs.