Girdled Trees, Rain, and Fish

Summer 2018 began a radical new era for Fourth Pond. We were expecting possible rare plants, more amphibians, rare dragonflies? We never considered invading fish.

But last year, like this year, featured Great Torrents of Rain, mixing natural with human-caused complexity. We got more change than we bargained for.

To understand why we girdled trees around Fourth Pond, you have to consider some ecology.

Somme Woods benefits from five larger ephemeral ponds and half a dozen smaller "pools." They are important because they harbor some rare and endangered species - and even more so as recovering ecosystem remnants. A natural pond in a regularly burned tallgrass oak woodland is a rare thing. Over evolutionary time, the open oak woods and tall grasslands included ponds that dried up every summer, grew sedges, rushes, and other vegetation over their bottoms, and then burned during dry autumns. This natural community did not survive well over the last hundred years. How much of the original biota of such ponds has survived over the decades of no fires? Are there rare dragonflies, snails, or other biota that need such fire-maintained ponds and wetlands? 

Death By Cat-tail

One of the main culprits in the loss of this biota has been cattails. Solid masses of this rank vegetation fill all, leaving thick mulch that doesn't burn - an especially unnatural problem with the spring-only burns that once were standard with many stewards. (Pond bottoms that were burnable in late summer or fall will be full of water in spring.) Many conservationists judge dense solid stands of cattails to be both unnatural and one of the main causes of loss of wetland diversity (from plants to birds and beyond).

Death By Shade

Another culprit is too many trees. At Fourth Pond, before we started cutting invasive trees from around its edge, the dark was so deep that nothing at all grew. The bottom when the pond dried each summer was just mud. Some Somme ponds have big old oaks (often bur oaks) at their edges, and these oaks sometimes have big old limbs reaching out over the pond. That's because no other trees were growing in the pond or on the edge. 

But over the decades without fire, most ponds filled with green ash, cottonwood, and other trees and shrubs that would not have been so frequent when a sward of vegetation resisted their seedlings in spring and then burned off saplings in fall.  

Early stages of restoration

In the 1938 aerial photo, Fourth Pond is surrounded mostly by open savanna grassland. But by 2016 when we started restoring it, this pond was surrounded and heavily shaded by basswoods, maples, and a couple of dozen large cottonwoods. By 2018 only the cottonwoods were left. Stewards girdled the smaller trees and Forest Preserve staff girdled the larger.

All Somme's ponds all dry completely by August every year, But some hold water long enough for chorus frogs, peepers, and blue-spotted salamanders to mature. We sometimes see great numbers of little frogs hopping into the vegetated uplands, where they'll spend the summer. But last summer at Fourth Pond, as it dried down to about three inches deep and we approached, we were puzzled to see what for all the world looked like a pandemonium of fish racing back and forth. The amphibians that live at Somme are rare in part because they depend on ephemeral ponds which, because of their transient nature, don't have fish. Dr. Karen Glennemeier has been studying these ponds, especially their salamanders, and she suggested we seek counsel from Eve Barrs of the Shedd Aquarium.

When the water was down to two inches, we caught the fish with our hands and a butterfly net. Here's our note to Eve and her reply:

   

To: Eve Barrs at the Shedd Aquarium:

We’ve never seen fish when this pond (or any of the Somme ponds) dried out before. (Perhaps the herons got them first?) Perhaps they show up only in a year as phenomenally wet as this? 

Attached see photos of the four species of fish we found, as Fourth Pond dried down.

Could it really be that these fish swam a half-mile up a creek that's mostly dry and flows only briefly for a couple of days after a big rain, past many log-jam waterfalls?

If fish can reach this pond (or these ponds?) in some years, do you suppose the young chorus frogs, peepers, and blue-spotted would mostly get eaten in those years? 

  

Best,

The Somme Team

From Eve Barrs to the Somme Team: 

Thanks for the photos.  You’ve got a great community of native species - tolerant of warm, low-oxygenated waters like an ephemeral pond: (in order of photos) Green sunfish (Lepomis cyanellus), golden shiner (Notemigonus crysoleucas), mudminnow (Umbra limi), fathead minnow (Pimphales promelas).

These guys probably did come up that stream.  Other species may have as well, but didn’t survive as the waters warmed up.  Cold water holds more oxygen, so other fish species don’t last when those levels go down.  And yes, heron, raccoon, kingfishers, waterfowl etc will enjoy them, especially as the water shallows!  The distance isn’t surprising, and log jams are great habitat for all these species and more (jams are full of leaves which feed inverts which feed fish!).

I’m sure their presence alters recruitment of amphibians from year to year.  It would matter when the fish arrived vs breeding season for herps. Karen can speak more to that with all her research on this!  Peepers, chorus, toads typically are the early layers and their tadpoles don’t take too long to morph…  So they might have been OK if the fish arrived after the amphibians morphed out.    

It’s all a cool, complicated web. There are some ephemeral studies in and around the Shawnee Forest that are looking at how/why amphibians choose which pond to breed in.  In an area with many ephemerals, there are hordes of salamanders in some, almost none right next door. They look identical!  Neat research to be teased out of the data once “we” all start logging it. Go Karen!

Seems like a natural occurrence, but a good thing to take note of for this year and that pond!

Eve

How the pond looked when we caught the fish.
Two days later, there was no water at all.

We counted and found that, just before drying entirely, Fourth Pond still had five sunfish, three mudminnows, and one each of the fathead minnow and golden shiner. We wonder what was there before the herons and raccoons went to work on them. We saw no frog tadpoles as the pond dried up, but there was one nearly mature salamander larva. 

Commonest was the green sunfish. 

Fun fact: The green sunfish is said to have polarization sensitive vision not found in humans and other vertebrates. This "polarization difference imaging" is thought to enhance visibility of targets in scattered light.

Vertical bars help identify a mudminnow. 

A mudminnow may bury itself tail first in mud to avoid predators. A female can lay 2,500 eggs per season. 

Golden shiner.

Golden shiners make use of an alarm substance contained in special skin cells. If a predator bites one, the substance is released, and other shiners in the vicinity detect it and leave the area.

Fathead minnow. 

Fathead minnow females deposit eggs in nests built by males. The males then care for the eggs until they hatch. He aerates them, provides a protective chemical, removes diseased eggs from the clutch, and wards off egg predators such as crayfish.

For more detail on this kind-of-amazing photo, see below.

When this photo was taken, July 27, 2018, Fourth Pond was already three-quarters dried. The water in spring would have nearly filled this photo - surrounding the bases of the now-girdled cottonwoods. Essentially all the grass, sedge, and wildflower vegetation around the edge here now is the result of recent restoration. To get a sense of how dark it was before we started, look behind the pond to the right. No restoration had begun there yet. It is so dark that the ground is just bare dirt, eroding away. Due to the cottonwoods, much of the pond was still too dark for natural vegetation. Now that will change massively. Girdling was a good way to kill these invading cottonwoods because no herbicide is needed, a good thing on the edge of a fragile and perhaps barely-surviving pond ecosystem. The cottonwoods probably had invaded here shortly after the Forest Preserves bought the land - on the denuded pond margin from which livestock had now been removed.  

There was no indication that any fish had travelled up other streams to any of the other ponds, fortunately for our amphibians. Perhaps that's because Middle Brook goes up steeper slopes and higher waterfalls than North Brook, the one that leads for Fourth Pond. 

The adventure of the biodiversity recovery of Fourth Pond is off to a predictably unpredictable start. More to come!

Thanks
To Dr. Karen Glennemeier for her ongoing work with Somme's amphibians, to Eve Barrs and the Shedd Aquarium for fish expertise, and to Kathy Garness for edits.