July

In our area, July is the peak month for vacations. From the Independence Day celebrations through the end of the month, campgrounds and cabins in the local forests and state parks are booked, public swimming sites are busy, and the aroma of barbeques hangs in the humid air of many evenings. Wild places offer a kaleidoscope of colors, as flowers advertise their nectar and pollen to bees, butterflies, hummingbirds, and other pollinators. Meanwhile, the birdsong that filled our world in May is largely gone, and we are left with the occasional burst of begging calls from nestlings hidden deep in a bush and companiable contact calls between mates or between parent and fledgling offspring. In the closing days of the month, a different category of sound starts up. It is the time for insect music makers. The first of the cicadas, together with the earliest of the crickets, grasshoppers, and katydids, are testing out their courtship songs.

Three of the essays this month focus on individual species: a strikingly beautiful butterfly that is on the wing for just a few weeks around midsummer (“The Baltimore Checkerspot”), the American toad with its voracious appetite (“A Natural Corridor for Toads”), and the American sycamore tree that inhabits the damp valley bottoms and has been planted in town (“Shedding Bark”). The final essay was inspired by a visit to Green Lakes State Park, with its clear, green waters created by a fascinating interplay of biology and chemistry (“The Making of a Green Lake”).

The Baltimore Checkerspot

The Baltimore checkerspot is a butterfly. It majors in orange, black, and white. The upper side of its wings is black with rows of white blobs, known as checkers, and there’s bright orange both around the margin and as spots on the inner, black region. When the butterfly is at rest with its wings up, its underside gleams with white and orange checkers. Continuing the theme, the butterfly’s body is a patchwork of white and orange checkers on black, its head and legs are orange, and its black antennae bear a flamboyant orange knob at the tip.

The Baltimore checkerspot does not get its name from the city of Baltimore but from its gaudy color scheme. This butterfly was formally identified in 1773 by Dru Drury, who lived in London, UK, and paid sailors to bring him insects from exotic lands, including North America. He likened the appearance of one of the dead butterflies that he received to the yellow and black coat of arms of George Calvert, the first Lord Baltimore, who was a renowned politician in the court of King James I of England. Not everything went well for George Calvert. He got into major difficulties in 1625 when he mishandled negotiations for James’s son Charles to marry one of the Hapsburg princesses and then was outed as a Roman Catholic. This double whammy ruined his political ambitions. Thereafter, he campaigned for the settlement of persecuted British Catholics in North America, starting with the Colony of Avalon in Newfoundland. That was a bit chilly, so he set up a second colony in Maryland, and his son Leonard Calvert became its first colonial governor. In 1729, one of the towns there was named Baltimore in honor of Cecil Calvert, another of George’s sons and the second Lord Baltimore. Perhaps it is fitting that Maryland has adopted the Baltimore checkerspot as its official insect.

Back to the “here” of Ithaca and the “now” of early July, several Baltimore checkerspot butterflies have been flitting about in a patch of damp meadowland at the southern edge of Cayuga Lake over the last few days. We were very lucky to see them because the Baltimore checkerspot flies for just three weeks, between late June and early July, and is not at all common.

The Baltimore checkerspot has just one generation per year. The adults emerge in late June, and the females lay batches of up to one thousand eggs on a suitable food plant for the caterpillars. Then the adults die. Most butterflies with a single generation per year overwinter as eggs or, alternatively, as pupae, but the Baltimore checkerspot isn’t an ordinary butterfly. It spends nearly 85 percent of its life—forty-four weeks, including the entire winter—as a caterpillar. If you are like me and care about these things, it spends just three weeks as an egg, two weeks as a pupa, and three weeks as an adult.

Let’s see how this works. Focus on one egg in a batch of, say, five hundred eggs, laid on the underside of a leaf of their preferred plant, the white turtlehead. Our egg hatches into a little caterpillar, along with its 499 siblings. It likes company, and the entire gang climbs to the top of the turtlehead. There they set about creating a communal tent of silk that encloses much or all of the plant. This is the so-called feeding web, within which the caterpillars munch steadily and grow and pass through two molts. As our caterpillar gets bigger, it may occasionally climb out of the tent and take a brief foray to feed on other plants, but it always comes back to the tent within a few hours. Then, sometime in August or September, all the caterpillars stop feeding and work together to create a thicker tent of silk, within which they do nothing. That’s the prehibernation web.

It starts to get cold, and our lazy, do-nothing caterpillars get fidgety. All five hundred caterpillars climb out of their web and go down into the dying grass and leaf litter beside their turtle-head plant. After a few days, groups of them leave the big group and wrap themselves up in a communal tent that they make of plant debris, held together with their silk. Then they do a lot more sitting around, much of the time under a layer of snow. When spring arrives, they wriggle out of their tent, eat some food, grow a bit more, and complete their development to adults.

At first sight, it makes no sense for all these juicy caterpillars to sit around doing nothing. You’d imagine they would take pride of place on the menu of any bird, mouse, frog, beetle, or spider. However, the caterpillars are safe because they are bright yellow with black stripes and decorated with lots of thick black spines, informing every hungry beast that they are poisonous. Their color signal is entirely honest. Their tissues are laced with toxic iridoid glycosides, which they derive from their turtlehead food plant.

Still, the caterpillars are not without enemies, especially one tiny species of wasp called Apanteles euphydryidis. The females of this wasp have a single ambition in life: to squirt an egg through her sharp ovipositor into a juicy Baltimore checkerspot caterpillar. The wasp egg hatches and the larva munches steadily on the tissues of the caterpillar, which stays alive for a while; eventually, the caterpillar dies, and the adult wasp emerges from the carcass. Protection against these wasps is provided by the communal webs. If the caterpillars stay inside their tent, the wasps can’t reach them. The wasps sit on the outside of the web, often in large numbers, and attack any caterpillar that sneaks out. Many wasp attacks are unsuccessful because the caterpillars can knock the wasps away with a jerk of the head, but some head jerks are not fast enough or strong enough.

The Baltimore checkerspot is one of many butterflies with declining populations because humans are destroying their preferred habitat—damp meadows—at an unprecedented rate. That said, I suspect this species has never been common because its preferred plant, the white turtlehead, is not abundant. Furthermore, the butterflies stay close to home. Once a butterfly has found a bunch of white turtleheads, it just stays put . . . and so do its children, and grandchildren, and so on. Some butterfly conservationists decided to help the Baltimore checkerspot by planting extra clumps of turtleheads, and it took the resident population five years to expand to use the new plants, just six hundred yards away.

Nevertheless, the prospects for the Baltimore checkerspot are far from gloomy. That’s because of a remarkable change in the behavior of some individual females, which have taken to laying their eggs on English plantain (Plantago lanceolata), a widespread invasive plant that favors dry grasslands rather than the damp meadows where turtleheads thrive. The caterpillars grow and develop on the plantain, which, like the turtleheads, contain the iridoid glycosides the insects need for protection against predators. In some locations with abundant plantains, the populations of Baltimore checkerspots are increasing. It appears that this butterfly is adapting to some aspects of habitat change caused by humans.

A Natural Corridor for Toads

Corridors in buildings tend to be unattractive but necessary routes to facilitate access between different rooms. Natural corridors serve a similar purpose as connectors between patches of natural habitat that would otherwise be isolated by human activities, but they are often interesting places as wildlife hotspots in their own right.

One of the most varied local natural corridors is Monkey Run, a stretch of tree-lined banks along a portion of Fall Creek, just upstream from the main Cornell University campus. One moment, you are sauntering beside the creek in a floodplain forest of maple, sycamore, willow, and quaking aspen. Then, suddenly, you come up through a steep hemlock grove and emerge at the top of a vertical cliff in a dry oak-hickory forest with a fantastic view of the creek a hundred feet below you. But this isn’t a standard Ithacan gorge of Devonian sedimentary rocks worn away by glaciers and rivers. Monkey Run’s cliffs are crumbly, turn to mud in rainy weather, and are continuously eroding away. In some places, it is ill-advised to go to the cliff edge because you may hear a tiny rumble and then find that there is nothing beneath your feet. These cliffs are glacial till from the last ice age and known locally as the Varna Cliffs. That sounds exotic to me, like something out of Star Wars, but the cliffs are named after the nearby settlement of Varna, which boasts two auto repair shops and a launderette.

We were walking along a wet patch of the trail beneath a dense maple canopy, focused mainly on swatting away insects that were hungry for our blood and sweat. As we took the next step, there in front of us was a full-grown American toad, which hopped languidly to the undergrowth on the edge of the path. It is very easy to declare that American toads are no beauties. The words used to describe all toads instruct us to see them as ugly. Toads are squat; their movements are ungainly, especially compared to leaping frogs; and their dry, wrinkly skin is littered with bumps that are usually referred to as warts. American toads tick all these ugly boxes with gusto. They are large beasts—our toad must have been a good three inches long—and their warts are numerous and irregular in size and shape.

The edge of Fall Creek in Monkey Run is the perfect place for American toads. As our National Audubon Society Field Guide to Reptiles and Amphibians (Behler and King 1979, 387) informs us, this species favors places where “there are abundant insects and moisture.” It has been estimated that a single toad consumes ten thousand insects in a single summer, although I struggle to imagine how that was calculated. The American toad is very common and widespread across the eastern part of the continent. What was special about our sighting was that one rarely sees these toads during the day. These animals tend to be nocturnal and spend the daytime under logs or big stones.

A little farther on, we saw three tiny toadlets walking along the path. I suspect they were newly minted American toads. The timing was right: the females deposit their egg strings, each containing thousands of eggs, into water in April, and the egg and tadpole stages generally last about two months or so. Our toadlets have a lot of growing to do, but they have time. American toads generally take two to three years to reach maturity. For better or worse, very few make it. Our toadlets have already survived the tadpole stage, which is much loved by hungry fish, birds, and water beetles—and toad, both large and small, is on the menu for every garter snake

This does not mean that the American toad is without defenses. Its oh-so-ugly skin is excellent camouflage . . . and the toad stage is poisonous. That is thanks to two bumps, one on either side, just behind the eyes. As we watched our toad, we saw the two bumps, both perfectly regular in shape and a uniform pale brown. They are the parotoid glands. A threatened American toad releases a toxic milky colored fluid from these glands. It is said that a dog that attacks an American toad will back off fast, foaming at the mouth and yelping, and will never make that mistake again. Garter snakes and other toad specialists have special detox enzymes that neutralize the toad’s defense.

The American toad isn’t the only beast with parotoid glands. These structures are found in a variety of toads, frogs, and salamanders. The poisons are generally alkaloids, and they do nasty things to the nervous system of unprotected animals.

Earlier, I explained that Monkey Run is a natural corridor. That’s important. It means that the creek is bounded by a relatively narrow strip of woodland. I am confident that one of the reasons why it is such a good spot for nature is that this corridor is sandwiched between large expanses of unimproved meadows that are only intermittently cut back. The meadows are a buffer between the forest strip and destructive human activities, and the wildlife in all the habitats is enriched by the proximity to the other habitats.

The trail took us from the damp woodland home of the toads into the bright sunshine of one of the meadows. At this time of year, the vegetation is home to red-winged blackbirds, savannah sparrows, and song sparrows—all singing at top volume—and to butterflies galore. We spotted seven species, including monarchs sizing up the milkweeds for laying their eggs, the fast-flying tiger swallowtails, and clouded sulfurs. A little farther on, there was a small group of meadowlarks, small birds that sport a brilliant yellow front and a vivid black V across the chest. Unfortunately, meadowlark populations are in decline. Their habit of nesting on the ground in fields and grasslands makes them vulnerable, but I like to think they are safe in the old field habitats around Monkey Run.

What we need are more places like this: diverse and rich habitats where animals and plants can lead their lives without disturbance.

Shedding Bark

The American sycamore tree (Platanus occidentalis) is a highlight of our walks from the door. A regimented line of mature and decidedly gnarled specimens graces the side of a nearby street, and it is the dominant tree along the valley bottom of the nearby Six Mile Creek, where it thrives in natural disorder.

In the last few weeks, though, something strange has been happening to the sycamore trees. As we walk under them, whether it’s along the creek footpath or down the neighboring road, the ground crunches. The crunching isn’t gravel; it’s lots of bark that has dropped off the trees in great chunks. The dark brown bark of the upper branches and, for some trees, the main trunk is peeling away, exposing an inner yellow or gray surface. Every now and again, these peelings drop to the ground with a thud. It almost looks like another dreaded disease, but it isn’t. The bark shedding is healthy but, for some reason, excessive this summer.

I decided to look into why sycamore trees engage in the strange exfoliation of their bark. At first sight, it seems decidedly unclever. Bark is always cited as the tree’s first line of defense. It’s the outer skin of dead cells, like the outermost layer of human skin, and it serves much the same purpose as our skin does for us. It’s a waterproof layer that keeps internal water in when it’s dry and external water out when it’s wet. It also guards against all but the most determined of pathogens, including bacteria and fungi, and against parasites, such as insects and worms. This protection is super important for the trunk and branches of a tree because its circulatory system is a narrow ring just under the bark.

All sorts of reasons have been proposed for why sycamore trees shed their bark. They seem to be cited as alternatives, but most of the explanations could be both-and rather than either-or. The first explanation is a bit like the reasons health spas give for subjecting their victims to facials. Exfoliation, meaning the removal of the outermost layer of dead cells, is meant to brighten you up. For a tree, the process removes all the algae and lichens that settle on the bark, along with the surface ecosystem of munching mites, insects, and worms. The argument that bark shedding is comparable to spring cleaning, brightening up, and “making a fresh start” sounds a bit like the advertising blurb for spa treatments to me, and it’s no more convincing.

A variant of this explanation is that the smooth trunk and branches of trees that have shed much of their bark are slippery. This exfoliated surface offers poor purchase for caterpillars and other insects that clamber up, down, and around, and it provides no cracks or crevices in which they can hide. Furthermore, caterpillars, other insects, and insect eggs—on the sheer, pale surface of an exfoliated sycamore tree—would be obvious to hungry birds, spiders, and squirrels. This idea seems plausible, and I wonder if this might be one reason why the sycamore tree is not badly affected by caterpillars of the spongy moth.

The second explanation couldn’t be more different. It holds that a tree can sometimes grow faster than its bark can extend. Let’s unpack that. Trees are not the same as insects, which are stuck inside an inextensible suit of armor. The only way an insect can grow is to crack open its skeleton and turn into a puddle of tissue while a new, bigger skeleton is made. By contrast, trees can add more bark as their trunk and branches expand. However, if the tree grows very fast, then this incremental patch filling may be insufficient, and it is better to remove the corset (meaning the bark) and build a new, bigger corset (that is one size bigger) after the growth spurt. We’ve had a very favorable spring and early summer with extremes of neither temperature nor rainfall. Perhaps our sycamore trees are simply taking the corsets off.

Now for the variant of this explanation. Without the bark, the ring of living tissue under the old bark is exposed to the light. That’s a great opportunity for some trunky or branchy photosynthesis to supplement the photosynthesis happening in the leaves. Perhaps our sycamore trees will need new corsets that are two sizes larger after this summer.

It is rather special that sycamore trees indulge in extensive bark shedding. Some trees, such as birches, do it in a small way and probably for similar reasons. Others, notably the shagbark hickory, hang onto their bark peelings as protection against fire.

I suspect that the sycamore is a supershedder because it grows fast and can get seriously big. I’ve read that the first European settlers in North America found enormously large and ancient sycamore trees in valley bottoms, so much so that some trees had naturally hollowed out on one side. Apparently, these settlers would often use, and even expand, the caverns in the sycamore trees for shelter. It was easier to do this than to make a cabin from scratch. There’s even a story about fifteen men on horseback who took shelter from a storm inside a sycamore tree hollow. Alas, these ancient and enormous sycamore trees were cut down in the frenzy of the forestry industry of the eighteenth and nineteenth centuries.

Perhaps the fast-growing and bark-shedding sycamore trees in our part of Ithaca will be left for centuries, until they hollow out and can provide shelter for . . . well, who knows what will be here in the 2320s, 2420s, or 2520s.

The Making of a Green Lake

Any excuse will do to visit Green Lakes State Park, situated some fifty miles northeast of Ithaca. The scenic wonder of these two lakes is celebrated in the name of the park. More accurately, the lake water is the glorious clear blue-green you would expect of the ocean surrounding a tropical island fringed with palm trees. Even in the summer heat of late July, the color seems strangely inappropriate for upstate New York. This local oddity is courtesy of the most astonishing geology, chemistry, and microbiology.

The best place to start is when the glaciers of the last ice age started to melt. At that time, a large glacial river ran roughly west to east near what is now the city of Syracuse. The river raced over a high cliff, probably about four hundred feet high. The force of the water was so intense that it eroded a deep, round plunge pool at its base. And I mean deep. It was some three hundred feet deep. If only we had a time machine to observe this magnificent and enormous waterfall, which was at least twice the height of Niagara Falls. At some point, the river changed direction, abandoning its plunge pool. I have to admit a small complication, that the river deserted not one but two plunge pools. There appears to be no easy explanation for the two pools. The plunge pool at the bottom of the cliff is round, as you’d expect, and is today called Round Lake. The second plunge pool is shaped like a comma and called Green Lake, giving the state park its name.

I’ll get back to the green bit in a moment. First, I need to explain how these isolated lakes don’t dry up. Sure, they get rain and snowmelt from above, but more than half of their water comes from underground—groundwater that seeps continuously from the bedrock at two levels, about thirty and fifty feet down. This water is very rich in minerals, a concentrated mix of dissolved dolomite, gypsum, and halites. It is a bit like seawater but rich in calcium ions, not sodium ions.

Now for the next step. There are two consequences of having groundwater that’s like calcium-rich seawater, and the second bit explains the green. The first bit is that the groundwater is dense—much denser than the fresh water coming in as rain and snowmelt from the top. We end up with two layers of water, one on top of the other, and they never mix. This is most unusual. The water column in most lakes gets mixed up whenever the temperature of the top layer reaches 39.2°F (4°C), the point at which water is at its greatest density and sinks, either increasing to 39.2°F in the spring or declining to 39.2°F in the fall. However, this so-called thermal mixing doesn’t work for Round and Green Lakes because the mineral-rich bottom water is denser than the top freshwater, even when the top is at 39.2°F. The result is that, down toward the bottom, there is essentially no oxygen—and no chance of worms, crabs, mussels, or fish. Remember this for later.

Now for the green bit. The reasons why the lakes are an amazing green-blue color like a tropical lake or sea are that the water column isn’t clogged up with lots of algae and detritus and, at least in the summer months, the water contains many tiny crystals of calcite, a form of calcium carbonate. These crystals reflect the light very efficiently, and the reflected light is blue-green. So how do these crystals come about?

Remember that the groundwater is calcium-rich. The calcium ions diffuse through much of the water column, which becomes supersaturated with calcium. This means that the slightest chemical tickle will precipitate the calcium. The tickle that counts in these lakes comes from a special kind of bacteria called cyano-bacteria. Don’t be distracted by the name. Although cyanobacteria means “blue-green bacteria,” the blue-green pigments of the bacteria don’t give the lakes their color. The bacteria are far too low in abundance for that.

Nevertheless, these bacteria are photosynthetically active. As any biochemist will tell you, a plant or cyanobacterial cell that photosynthesizes releases hydroxyl ions—which is just a fancy way of saying that they make their immediate surroundings a bit more alkaline (or less acidic). And as any inorganic chemist will tell you, alkaline conditions precipitate calcium carbonate, in this case into beautiful crystals of calcite. The calcite forms little concretions around each cyanobacterial cell, distributed throughout the water column. Then each little crystal causes more calcium carbonate to precipitate out from the supersaturated solution.

The main evidence for the calcite crystals is the blue-green color of the water. This chemistry is also visible to the naked eye as whitening of twigs and branches that fall into the lake. More than that, the cyanobacteria and other bacteria can get stuck together, making a reef. This is like a coral reef but called a thrombolite. (What an ugly word, like something out of a medical dictionary!) There’s a small shelf of this reef around the edge of the lakes, but the reef extends up to thirty feet into Green Lake and is some thirty feet deep in one place called Deadman’s Point.

Slowly but surely, the calcite crystals sink to the sediment at the bottom. Because there are no worms or other beasts disturbing the sediment surface or tunneling into the sediment (I asked you to remember this earlier), each summer’s production of calcite forms a coherent layer, sandwiched between the leaf debris that falls from the trees around the lake and accumulates at the bottom during the previous and subsequent fall. The layers of alternating black and white create a perfect record of the conditions in the lake over thousands of years. They are as informative as tree rings. These sediment layers are called varves, a useful word for Scrabble if you ever get two Vs.

I am sure that everyone who visits Green Lake agrees that it is a stunningly beautiful place. It is made even more special by taking a quick dip into the underlying biology and chemistry to understand how this local wonder works.