Because of the pandemic, I’ve spent much more time in our gardens this year. There is so much to see and do on our little half-acre lot. Over the decades, trees have filled in and gardens expanded and, through neglect, much has “gone wild.”
New England gardeners know all too well what happens when you take your watchful eye off bittersweet, wild grape and multiflora rose. Add in hundreds of tree seedlings and you get the picture.
And the animals! Orioles, robins, wrens, warblers, cat birds, woodchucks, rabbits, chipmunks, voles, mice, several kinds of bees, butterflies, ants, spiders, garter snakes and, of course, the local bear and wayward moose all provide endless entertainment and wonder. I am reminded of an essay I read as an undergraduate, written by G. Evelyn Hutchinson in 1959, called “Homage to Santa Rosalia, or Why Are There So Many Kinds of Animals?”
An eminent ecologist and student of invertebrates in lakes, among many other things, Hutchinson concluded “because there are so many types of plants!”
As a botanist, I loved his answer. He was not being ironic or just clever, but posing what came to be a fundamental theory in ecology. The amount of energy that flows through an ecosystem is a primary control on how many animals and how many types of animals there can be.
Energy flow — from the sun to plants to herbivores and then to carnivores — directly influences the complexity of local food webs. This is called the energy-richness hypothesis. Low-energy habitat like Arctic tundra will have fewer species than in a tropical rainforest. So as our garden ran riot, it was filling up with new types of plants, thus supporting a wider array of animals, becoming more like a jungle.
Here’s how the food web theory goes. Everything that is not a plant (except photosynthetic bacteria) must consume a plant, in some form. In turn, other animals must eat plant eaters to survive. The number of steps up a food chain largely determines the complexity and diversity of the food web. Organisms need a certain amount of food to thrive and reproduce, based upon their size, metabolic requirements and abundance, and every step up the chain loses 80-90% of the energy available to the level before.
So even the most productive ecosystem can support only a small mass of top carnivores. But what determines plant diversity?
Plant growth is primarily limited by the availability of water, light and nutrients. Local populations change due to reproduction, colonization and extinction. As gardeners, we constantly alter plant diversity with seeds, plant exchanges and visits to the garden center. Birds and wind add additional (perhaps unwanted) diversity as seeds of colonizing species land on our soil. Diseases, competing plants (aka weeds) and insects can be factors that can limit some species and favor others, though we can often manage these.
We gardeners can do a lot to create the ecosystem we want. But we can only work with the solar input we’re given. The path of the sun and the capricious clouds are beyond our control. But even there we can manipulate canopy structure, taking advantage of the myriad ways plants reach for the sun.
In the natural world, plants don’t have a gardener to determine their fate. Rooted in place, stuck with the light given them, plants must adapt by how they use the carbon they capture in photosynthesis.
Some create woody carbon-rich structures that lift their leaves high above the rest — trees! Some parasitize trees by climbing existing trunks and shading out their owners — dreaded bittersweet vine! And some just sit and take it, doing fine in shade, minimizing energy loss and waiting for sunflecks or breaks in the canopy.
But no plant can do all of these tricks at once — they are mutually exclusive and so there is no one sure solution. These trade-offs are a primary basis for the many types of plants. Deep roots, shallow roots, annual growth with persistence as seeds, biennials that store energy in roots for a year before bursting into flower, tall trees that must bring enough water to great heights and protect precious leaves against predators and disease, all before dropping those leaves to conserve water in the winter — there are so many different ways to make a living.
The energy-richness hypothesis doesn’t explain diversity in all ecosystems, of course. But it can be useful in your garden. And every garden choice is an ecology experiment.
Hot, dry south side of the house? Highly productive if watered and fertilized, a good place for fruiting vines. Shady maple glade? A wonderful place for bloodroot to establish and spread as ants carry the seeds everywhere. And if you plant a hemlock hedge and manage to keep woolly adelgid at bay, you’ll see that the deep canopy of this iconic conifer removes any chance of an understory.
Happy gardening, and ecological study!
Lawrence J. Winship is emeritus professor of botany at Hampshire College.
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