I have mad garden envy of my friends who can garden. My friend Jonathan puts a lot of work into his, from hauling in pick-ups full of eye-watering not-quite-ready compost to determinedly watering the blank spot where his pipe vine used to be before his vacation until it re-sprouts. A former housemate, Emma, harvests greens and strawberries and radishes from a plot in a community garden (and blogs about it!).
By contrast, my yard is a haven for whatever can survive the brutal Arizona heat and the adorable trampling of my housemate’s dogs. I have yet to figure out how bribe myself to work out there. The tentative promise of plump cherry tomatoes I ate from our backyard a kid just doesn’t do it. That’s what grocery stores are for, right?
My point is that different houses offer very different growing conditions. A watermelon seed falling in Jonathan’s little Eden may live to grow and produce fruit. I can spit all the seeds I want into my backyard, secure in the knowledge that they won’t stand a chance.
Seeds can arrive not only through loving planting and tender care, but also via birds pooping, the wind blowing, or children playing. Ecologists call this process seed dispersal, as the seeds disperse themselves throughout your neighborhood (or the radius of the hardware store where you bought the packet).
Here is another ecologist-y phrase to describe the collection of well-watered manicured gardens and the dandelion-spotted lots you see as you bike around a city: spatial variation. Means that in some yards roses grow better than dandelions (in Tucson it’s probably only if some human is maintaining their water supply), but in other yards the dandelions win.
Spatial variation can promote biodiversity. You can see more species along a street with a range of landscaping preferences or conditions than you might in a neighborhood where everyone has picture-perfect Bermuda grass coating every inch of space. It is a common phenomenon in nature, too, one I observed in the Rocky Mountains on a recent vacation to Banff. Different trees and plants dominate the dry, windswept ridges than do the pockets in alpine meadows that remain swampy from melted snow. We can see with our own eyes that species have their own specific responses to environmental conditions.
But if that seems too obvious, environmental conditions are only part of what determines which species can coexist. The other major part is competition. As gardeners who start their delicate basil plants indoors know, if you sprinkle several seeds into each tiny pot, you’re almost sure to get at least one sprouting (that’s called bet hedging, and plants do it, too). But very likely, you will have a number of seedlings poking out their first shoots and leaves in every pot, as Emma’s April 21st photos show. As they grow larger, not all of them can survive. There is just not enough room, never mind nutrients, light, moisture, or other things plants need.
This process ecologists (and gardeners) call thinning. If the gardener does not pinch off the smaller plants early, they will die out eventually, but only after a long and grueling battle with its siblings, sapping the energy of the survivors, reducing its eventual output. This is competition, a familiar word from sports and family life, but much closer in meaning ecologically to the way we use it to talk about economics. Remember Adam Smith and his “invisible hand?”
Just as a University neighborhood (or gloomy sky like Seattle’s) may provide fertile grounds for coffee shops, yet a plethora of espresso joints ensures stiff competition and low profits, so do plants that find the choicest habitat have to battle contenders for space, for water, for light, for nitrogen, for pollinators, and the list goes on and on. (Check out Emma’s March 1 photo of a field of daffodils in Washington for an image of competition!) Environment and competition are obviously related and can determine local coexistence (and larger regional biodiversity). But how? Should a “good” environment be more diverse or less diverse than “poor” habitat?
In introductory biology classes, we received this answer a maddening amount of the time: it depends. But that just leads to more questions, the most obvious being: it depends on what?!
That’s where biology starts to get fun, but you’ll have to excuse me for a bit. The monsoon has started here in Tucson (raining in small patches, providing more spatial variation!), and I have a garden to go imagine. Stay tuned.