Aquatic plants

connecting aquatic and terrestrial worlds

Prompted by a discussion with another naturalist, I’ve been thinking about plants that grow in fresh or brackish waters and their unsung importance to animals. So this essay is about aquatic plants (collectively called macrophytes) such as pond lilies (Nuphar), milfoil (Myriophyllum), burreed (Sparganium) , buckbean (Menyanthes), pondweed (Potomogeton), water crowfoot (Ranunculus), ditch grass (Ruppia), arrowhead (Sagittaria), and some sedges (Carex) that play many ecological roles relative to animals. Therefore they also have numerous ramifying effects on many aspects of local ecosystems. Here are some examples.

Northern Milfoil. Photo by Bob Armstrong

These aquatic plants are eaten by animals. For example, Canada geese nibble the shoots of Lyngbye sedge out on the wetlands; later in the season they grub up the root, leaving characteristic divots. Sedge stands closer to the forest edge are grazed by bears, deer, and sometimes moose. Moose forage on buckbean and other aquatics (see this video by Bob Armstrong), which are reportedly very digestible forage plants and a good source of minerals. Geese, swans, and ducks graze on the leaves of milfoil, ditchgrass, burreed, ditch grass and other species too. Geese and ducks eat the seeds of sedge, milfoil, burreed, ditchgrass , and other species, in some cases passing undigested seeds through the digestive tract and thus dispersing the seeds. All of those animals are, at some point in their lives, potential prey for various predators.

Swans feeding on milfoil. Photo by Bob Armstrong

Beavers (and humans and muskrats) dig up and eat the nutritious tubers of arrowhead; beavers also eat the yellow pond lilies, buckbean, and soft leaves of several species. Beavers are habitat engineers, creating pond habitat for nesting birds and juvenile salmon. They are prey for carnivores such as wolves; a recent report concludes that wolves are able to plan ahead to set up ambushes for beavers, as well as just running them down on land.

Some of these macrophytes (e.g., water crowfoot, buckbean, arrowhead, pond lilies) produce flowers that are pollinated by insects. The visiting insects may obtain nectar or pollen as food, and they are prey for several kinds of birds.

Damselflies have evolved an unusual use for these plants: female damselflies insert their eggs in the leaves and stems of various aquatic plants, sometimes submerging themselves for several minutes. The emerging larvae are predators on other insects and are themselves (as both larvae and adults) prey for other insects, fish, birds, and frogs.

Macrophytes provide protective cover for small fish, such as sticklebacks and salmon fry, which in turn are prey for larger fish, birds (such as kingfishers and mergansers), otter, and mink. Similarly, toad tadpoles and some aquatic insects hang out in the watery ‘forests’ of pondweed or milfoil, temporarily hiding from predatory insects, fish, or birds.

In addition to providing food, cover, and egg-laying sites, the standing ‘forests’ of aquatic plants provide a handy substrate for dense coatings of algae. Photosynthesis of the algae produces oxygen that improves the breathability of the water. The algae are eaten by toad tadpoles and by herbivorous invertebrates such as snails, which in turn are prey for fish and birds.

These ecological connections are relevant to local ponds (such as Twin Lakes) that are sometimes managed to reduce the density of milfoil and other macrophytes. The species of milfoil in those ponds has been identified as a native species (northern milfoil, Myriophyllum sibiricum). A study of this species (and the invasive Eurasian milfoil, M. spicatum) in eastern North America showed that the native species generally supported more snails and other invertebrates than the invasive species. Those rich communities of invertebrates provide food for fish and waterfowl. Some of the waterfowl also graze directly on milfoil. Thus it becomes important to understand the ecological effects of reducing milfoil density in the lakes. How is the foraging of fish and birds changed? Also, perhaps reducing the density of the native milfoil facilitates invasion by the Eurasian species (widespread in North America and it might be in our area too), which supports poorer invertebrate communities. Furthermore, the invader can hybridize with the native species, changing its palatability or digestibility along with the associated composition of the algal community, with resultant effects on the animals that use milfoil. Hmmm, a potential research project awaiting attention…


A winter walk in Gustavus

feeder-watching, fresh snow, an owl sighting, and bone musings

The day began in quiet leisure—in a comfortable chair with a cup of hot tea, looking out a big window on a snowy field and waiting for birds to arrive at a seed-feeder. Soon, a crowd of juncos was having breakfast, milling about on a ground-level spread of seeds. The cat at my feet liked the juncos too, especially the ones just a few inches from the window.

An occasional chickadee flitted through, snatching a sunflower seed on the way. The sharp-shinned hawk that had tried for a feeding junco the previous day did not show up on this morning, so all was peaceful under the little arbor that kept snow off the feeder, except for minor altercations among the juncos themselves. Flared tail feathers and open beaks led to some jostling about. One junco with a gimpy leg held its own with the others.

The sun peeked through the trees, and it was time to go for a walk. Just outside the door, we found a perfect miniature snow-angel, where a junco had touched down and spread its wings for a quick flit to the feeder.

Walking through the woods on the way to the beach, the only ‘wildlife’ we saw was a spider dangling on a long silk thread and a ‘looper’-type of caterpillar, seemingly frozen solid but able to squirm when warmed in a hand.

A flight of pine siskins, over a hundred of them, swarmed by, overhead. Crossbills called in some of the spruces and left wings of spruce seeds on the snow. There are subtle distinctions among the red crossbills, based on calls and bill size. Usually, we have the hemlock (small bill) and spruce (medium bill) types, but recently the douglas-fir type (also a medium bill size but a different call) has been reported here, north of its usual range in the Pacific Northwest.

The grasses on the upper meadows made beautiful golden, frost-covered arches that caught the sunlight. A snipe was foraging in a nearby ditch. It was mostly concealed by the steep bank but occasionally it flew ahead to find a new spot in which to search. Although they typically nest in marshy places, in fall and winter we see them on forested streams. There were clusters of mallards at the edge of the river, foraging and sleeping. Small gangs of geese flew noisily overhead.

At the edge of the estuary, an otter had emerged and left a typical trail of footprints interrupted by a long, smooth slide. A raven had hopped up from the water’s edge and gone airborne, leaving the marks of jumping feet and just one wing. Perhaps it veered off to the side as it took off. A very small shorebird had left its prints in the mud. The snow on the high intertidal area was smooth and unmarked; no wolves or moose had passed by there recently.

Heading back toward the forest, the scattered, pioneering spruces gradually got denser and taller. From behind one small stand, a large bird went winging, almost overhead, into a bigger stand of taller spruces. Aha! A short-eared owl, a hoped-for sighting. These owls often frequent the meadows in winter, looking for voles and other small, vulnerable critters. They are fun to watch, with their distinctive style of flight.

Some crossbills landed on small spruces; they occasionally chose a vertical twig and perched right at the end of it. This led to the question of how they manage to perch there without getting stabbed by the sharp needles. Can they just somehow fit their toes among the erect needles? Or do they perhaps select twigs that have a large central, unopened bud to wrap their toes around?

Near one spruce grove, we stopped to look at something (now forgotten) and I heard an odd sound coming from the trees. It was hard to describe—I heard it as a soft, tonal ‘pop’, but my companion described it as a moan or wheeze. There were several of these ‘pops’, and my naturalist friend said they were made by a red squirrel. Really? Yup—they do it after each sharp, little bark, perhaps when inhaling. I couldn’t hear the barks, just the ‘pops’, but my friend saw the squirrel in the tree and was confident that it was the perpetrator. A new thing to listen for!

Back to the house to dig through a treasure box of animal bones and wonder about them. Both bats and birds fly, but birds have a big keel on the sternum (breastbone) and bats don’t. Why is the sternum of mice and squirrels so extremely narrow but that of deer is proportionately more substantial? The lower jaw of fox and skunk has a tiny, seemingly functionless last molar that does not occlude with teeth on the upper jaw.

The first cervical vertebra of mammals holds up the skull; it’s called the atlas, a name from Greek mythology. Atlas was on the losing side of a battle and was condemned to hold up the skies on his shoulders (although he was usually depicted as holding up the world). The atlases of bears and cats have wide, rounded lateral wing-like expansions, but those of moose and deer have narrower, straighter flanges. Many questions; I need a convenient functional morphologist for answers!


more friends than foes

There are tens of thousands of fungal species, which are classified in a kingdom all their own, neither plant nor animal. We become acquainted with some of them in unlikable ways, when they infect us or show up as mildew on our roses. Although we bemoan those fungal invasions, other fungi have been useful to humans in many ways.

Just consider, for a moment, what our lives might be like in the absence of fungi. There wouldn’t be yeast-leavened bread, so no hot-cross buns, chocolate eclairs, or ordinary pb&j sandwiches. No beer or wine, so although South Franklin Street might become more attractive, New Year’s Eve parties might be staid and quiet—and the highways would be much safer. Without penicillin and other antibiotics derived from fungi, we (collectively) would a lot sicker and maybe, in some cases, dead. Some folks would miss the hallucinogenic fungi and many would rue the lack of delectable chanterelles and boletes. Those that create fabric items would miss the many hues of dyes derived from fungi.

Of more fundamental importance, however, are the ecological roles of fungi. Many fungi form connections with the roots of trees, shrubs, and herbaceous plants, providing nutrients from the soil to the plant in trade for carbohydrates from the plant’s photosynthesis. These mycorrhizal fungi contribute significantly to the health and growth of the partnered plants. Our forest would be a poorer place without them!

Thus the fungi give, but they also take away: They are major decomposers of plant and animal matter. If dead trees and leaves didn’t decompose, ultimately the forest floor would be buried and no understory could grow. So, no winter food for deer, no high-bush cranberries or blueberries. Leaves and stems of dead grasses and sedges would make a thick, impenetrable mat over the meadows. Perhaps the only places new trees or grasses could grow would be on recently exposed bare soil (think of receding glaciers, landslides, post-glacial uplift), and then it would be only certain kinds of plants that could cope with conditions there. Furthermore, we’d be surrounded by carcasses of dead animals and small trailside mountains of dog poop that would be only partially diminished by bacterial action.

The activities of fungi sometime appear in unexpected places. For example, recent research has shown that fungi play a role in the formation of hair ice—those wonderful curls of extremely slender filaments of ice (only about one hundredth of a millimeter thick!) that emerge from sodden branches when the temperature is just below freezing, and the air is humid and still. This story has a beginning over a hundred years ago, with Albert Wegener, the astute fellow who recognized the fact that the continents move around. In addition, he noted that hair ice appeared on damp branches of deciduous trees and shrubs that were also laden with fungi; he then surmised that the fungi had something to do with the formation of that hair ice. After a long delay, recent research has confirmed that surmise. Although those old, wet branches harbor several kinds of fungi, one in particular is consistently associated with hair ice. That fungus (Exidiopsis effusa) is a decomposer of wood; it somehow also shapes the ice hairs, preventing the tiny crystals from coalescing into bigger ones. Organic matter in the hair ice, such as decomposed lignins from the wood, might be involved, but that remains to be determined.

Scientists have also discovered a new genus and species of fungus that has medicinal value, known and used in Chinese folk medicine for hundreds of years. This fungus grows on a certain kind of bamboo, as do other species in a related genus. These fungi contain compounds called hypocrellins, which are effective against viruses, bacteria, and fungi, when they are activated by light. And now we know that this new species does too. It just took science a while to catch up with traditional knowledge…

Invertebrate breathing

Invertebrates are a wildly diverse lot—far more diverse than the relatively small group known as vertebrates. Accordingly, the inverts have a huge diversity of life styles and body plans. Consider (for now) just the inverts that breathe air (and temporarily leave aside those that get oxygen from water).

Some invertebrates have lungs, although they are quite different from vertebrate lungs. Many snails have a well-vascularized lung derived from the mantle (that’s the tissue that makes the body cavity and surrounds the internal organs; it also secretes the shell). The mantle forms a thin-walled pouch that opens to the outside. Air enters the pouch and the oxygen diffuses into the blood. But, unlike the vertebrates, the blood is not contained in blood vessels (such as capillaries) but rather fills spaces in the body cavity. In these snails (and most other molluscs), oxygen is carried in solution (not in blood cells) by molecules of copper-containing hemocyanin (in contrast to vertebrates’ iron-containing hemoglobin); the blood is bluish when oxygenated. This arrangement is common among terrestrial snails and some freshwater snails that come to the surface to get air, and it also occurs in a few intertidal or estuarine marine species.

Land crabs have a sort of lung, made from a modified gill chamber. Functional gills are rudimentary or absent. The coconut crab is an example: this huge crab is related to hermit crabs but it is terrestrial, using lungs to breathe. Most terrestrial arthropods (crabs, spiders, insects etc.) have some form of hemocyanin in the blood. However, the larvae of certain midges, sometimes known as blood worms, are exceptional in that they capture oxygen with a type of hemoglobin (so they are red-colored).

Most spiders have structures called ‘book-lungs’, but these are quite unlike the lungs of vertebrates or snails or land crabs. They open to the outside underneath the front part of the abdomen and consist of a set of sacs, each containing parallel rows of thin blood-filled sheets separated by air passages (?like the leaves of a book?). Gas exchange occurs there, but in addition, spiders also have ‘tracheae’, which open to the outside via a spiracle; they are reinforced air-filled tubes that branch through the body.

Air-breathing insects generally depend on a tracheal system to deliver oxygen to the cells. Spiracles in the external skeleton lead to branching ‘trees’ of ever-smaller tracheae—the smallest ones reach individual cells (as in spiders too). Oxygen comes in through the tracheae and carbon dioxide goes out. Although the tracheal system is slightly supplemented by blood that moves around in the body cavity (not in blood vessels), the main function of insect blood is not respiratory; it serves mainly to move food from the digestive tract and waste products to the excretory organs, as well as hormones to particular tissues.

Some aquatic insects breathe air too. For example, rat-tailed maggots are the aquatic larvae of certain hoverflies (a.k.a. flowerflies) that pollinate flowers. A submerged larva has a tube at the end of its abdomen that reaches up to the surface of the water, where air can enter the tube. Some aquatic mosquito larvae do this too. Naturalists sometimes liken this habit to snorkeling.

Whirligig beetle. Photo by Bob Armstrong

Other aquatic insects take air with them when they submerge. Predaceous diving beetle adults have spiracles on the abdomen under the tips of the wing covers. An adult can rest at the surface with its head down and raise the wing covers to expose the spiracles to the air—and thus breathe air while resting there. When it dives, it can store air under the wing covers. Water boatmen have undersides covered with dense, unwettable hairs that trap a sheet of water up against the spiracles of the abdomen. These reservoirs of trapped air can exchange gases with the water and thus replenish the initial supply of oxygen.

Extremely small terrestrial invertebrates don’t have special respiratory systems at all. They are so small that there is a lot of surface area relative to the body volume, and oxygen and carbon dioxide just diffuse through the exterior covering. Similarly, earthworms, with their long thin shape and lots of surface area per unit of volume, just accomplish gas exchange through their moist skin.