The Invasion of Bayou Sauvage

It was an unusually cool, dry September day in New Orleans, feeling like spring with high temperatures in the mid 70s and low relative humidity. Perfect weather for our inaugural field study in Bayou Sauvage National Wildlife Refuge, the largest urban wildlife refuge in the United States.

I was surprised to see fresh green leaves sprouting on the sugarberry trees. It not only felt like spring, but it also looked like spring! I later learned that this was an adaptive strategy. To make and store food for the coming winter, the tree replaced leaves that had been stripped by the winds of Hurricane Ida four weeks earlier.

All living things have strategies to help them survive and to provide for the continuation of their species. From the larva of the Giant Swallowtail Butterfly, that looks like bird poop on a leaf thus discouraging any predators from assuming it will be a tasty snack, to the domesticated pig that, when released from captivity, will grow tusks and long hair and become more aggressive, living things must adapt. Adaptive strategies was just one of the many “threads” that we would become aware of as we studied the flora and fauna of south Louisiana during the coming semester.

Native vs. Non-native species is a thread that was evident throughout all of the field studies. The very name of this refuge, Bayou Sauvage, has its origin in what may have been the first non-native animal to the area, European settlers. When France governed the city of New Orleans, indigenous peoples lived along the banks of the bayou and were considered by the French to be wild. The French word for “wild” is “sauvage,” hence the name Bayou Sauvage or Wild Bayou.

In Bayou Sauvage refuge, at least three non-native species have become invasive, meaning they do harm to the natural balance of a habitat. Feral hogs (Sus scrofa), Chinese tallow (Triadica sebifera), and water spangle (Salvinia minima) are thriving in Bayou Sauvage.

Feral hogs in Bayou Sauvage and throughout all 64 parishes of Louisiana are an invasive species that might have been brought to this continent by that first invasive animal, European settlers. Although domestic pigs are diurnal, feral hogs tend to be nocturnal in this location due to human activity and during this time of year, when the heat of night is less intense than the day. (Contrary to the expression “sweating like a pig,” which actually is a simile referring to an iron smelting process, swine have no sweat glands and, therefore, no perspiration to regulate body temperature.) Although we saw no feral hogs, clues indicating their presence were evident: tracks left in damp mud, rooting marks where they foraged for food, scat, and wallows where they had rested. A sounder of hogs can do extensive damage to the wetlands by flattening grasses into trails and rooting out larger areas in the habitat. Because they are omnivores with voracious appetites, they cause major destruction to wetland plants and to some of the native animal species that live there. Feral hogs consume such large quantities of acorns and seedlings of the native Live Oak (Quercus virginiana) that they can greatly diminish the number of young oaks, providing more space for the invasive Chinese tallow tree.

The native sugarberry tree (Celtis laevigata), like the native Live Oak, competes with the Chinese tallow for space and soil nutrients. Both provide edible berries for migratory birds. The fruit of the sugarberry is rich in fats and provides long-term energy for birds to complete a strenuous migration. The Chinese tallow berries provide empty calories, giving the bird a sensation of being full but not the energy needed to complete its migration. This diverse habitat supports the diet of over 350 bird species throughout the year, with numbers rising during peak migratory seasons. As the Chinese tallow chokes out native species, food supplies for these birds will diminish.

The non-native Salvinia minima, also called water spangle, is over-taking native duckweed (Lemna minor) in bayous and canals. Duckweed reproduces asexually and has the ability to cover the water’s surface in a short period of time. It provides protein for fish and migratory waterfowl, who eat it and thus control its growth. Water spangle, low in nutrients, is not desired by wildlife and continues to gain territory in the duckweed’s habitat. A blue dragonfly rests on a carpet of Salvinia minima in the picture to the left. The tiny white flowers are those of duckweed, which is struggling to maintain its place in the bayou.

There has been another, non-living, invader to this refuge, which lies within the hurricane protection system of the city of New Orleans. In 2005, the storm surge from Hurricane Katrina broke the levees to the south, flooding the refuge with salt water. The saline water remained trapped in the area for weeks, until the water level in Lake Pontchartrain was low enough to pump it out. Prior to Katrina, this area had been a climax forest of mature Bald Cypress (Taxodium distichum) and other wetland trees in the swamp areas and Live Oaks along a coastal ridge. The saline intrusion from the flood waters killed all of the trees except a few remaining live oaks in the higher elevation.

I have lived in New Orleans for more than seven decades, yet this was my first visit to Bayou Sauvage. If I had started to visit the area as a teenager in the early 1960s, I would have had the privilege of seeing it as a climax forest…before Hurricane Ida, before the levees broke during Katrina, before it became a National Wildlife Refuge, before the levees were built, before the construction of Interstate 10, before Hurricane Betsy. I can’t go back in time, but I can impart this thought to my grandchildren’s generation: Observe, record, remember, and be kind to your planet.

Photos by Lori Passmore

(Photo in Becoming a Naturalist by Dr. Robert Thomas)