Chapter 20 4.3 Random routes to a stable ecosystem

However, there are still tricks in it.As things progressed, Packard noticed that the order in which species were added mattered.He learned that other ecologists had found the same thing.A colleague of Leopold discovered that a closer approximation of the real North American prairie could be obtained by sowing the seeds of the North American prairie on overgrown land rather than on newly cleared land, as Leopold had done.Leopold had worried that aggressive weeds would smother wildflowers, but weedy land resembles the North American prairie more than cultivated land.On old weedy plots, there are some weeds that are latecomers, and some of them are members of the savannah.Their early arrival can hasten the transition to grassland systems.On bare, plowed ground, the rapid-sprouting weeds are extremely aggressive, and the beneficial "latecomers" join the group too late.This is like pouring the concrete foundation first when building a house, and then the steel bars arrive.Therefore, order is important.

Tennessee State University ecologist Stuart Pym compares sequences—such as classic slash-and-burn farming—to sequences that play out countless times in nature. "In an evolutionary sense, the players who play the game know what the sequence is." Evolution not only develops the function of the colony, but also fine-tunes the process of forming the colony until the colony can finally become a whole.Restoring ecosystem communities is going in the opposite direction. "When we're trying to restore a prairie or a wetland, we're going down a path that the community hasn't tried," Pym said.Our starting point is an old farm, and the starting point of nature may be an ice sheet thousands of years ago.Pym asked himself: Can we assemble a stable ecosystem by randomly adding species?You must know that the way humans restore the ecosystem has a strong randomness.

In their lab at Tennessee State University, ecologists Pym and Jim Drake have been combining elements of microecosystems in different random orders to reveal the importance of order.Their microcosm is a microcosm.They started with 15 to 40 different individual algae plants and microorganisms, and sequentially put these species in different combinations and sequences into a large flask. After 10 to 15 days, if all goes well, the aquatic hybrid forms a stable, self-reproducing mud ecology—a unique mix of interdependent species.In addition, Drake also established artificial ecology in the aquarium and running water respectively.After mixing them together, let it run naturally until it settles down. "You look at these communities, and ordinary people can see the difference," Pym commented. "Some are green, some are brown, some are white. The interesting thing is that there is no way to know in advance how a particular combination of species will develop. Like most complex systems, they have to be set up first to run. in order to discover its secrets.”

At first, it wasn't very clear that it would be easy to get a stable system.Pym had thought that randomly generated ecosystems might "wander endlessly, transitioning from one state to another and back again, never reaching a constant state." However, man-made ecosystems And didn't linger.Instead, Pym found, surprisingly, "all sorts of wonderful phenomena. For example, these random ecosystems have absolutely no trouble stabilizing. Their most common feature is that they all reach some kind of constant state, and Usually each system has its own unique constant state."

It's easy to get a stable ecosystem if you don't mind what the system you get looks like.It's surprising."We know from chaos theory that many deterministic systems are extremely sensitive to initial conditions -- a small difference can throw it into chaos," Pym said. And the stability of such ecosystems is opposed to chaos theory. Starting with complete randomness, you see these things coalesce into something more orderly than can be explained by common sense. That's anti-chaos." To complement their in-test tube research, Pym also set up computer simulations—creating simplified ecological models in a computer.He coded artificial "species" that required the existence of other specific species to survive, and set up a chain of the jungle: if the number of species B reaches a certain density, species A can be extinct. (Pym's stochastic ecological model is similar to Stuart Kaufman's stochastic genetic network system. See Chapter 20).Each species is loosely related to other species in a vast distributed network.After running thousands of random combinations of the same species list, Pym got how often the system stabilized.The so-called stability means that under small disturbances, such as the introduction or removal of individual species, the stability of the whole will not be disrupted.Pimm's results echo those of his bottled microcosm.

According to Pym, computer models show that "when there are 10 to 20 components in a mixture, there can be anywhere from a dozen to hundreds of peaks, or plateaus. If you replay the course of life, you get to Different peaks." In other words, after the same few species were released, the initial state of disorder would lead to a dozen or so endpoints.And changing the order of inputs in even one species is enough to change the system from one outcome to another.The system is sensitive to the initial conditions, but usually turns into an ordered state. Pym sees Packard's work on the Illinois prairie (or should I say savannah) as proof of his discovery: "Packard's first attempt to assemble that community failed, In a sense it's because he can't get the species he wants, and has a lot of trouble getting rid of the ones he doesn't want. Once the oddball but suitable species are brought in, it's pretty close to a constant state So it can easily get to that state and probably stay there.”

Pym and Drake discovered a principle that is an important lesson for anyone concerned with the environment and interested in creating complex systems. "You don't want to get a wetland by just flooding it with a lot of water and expecting everything to work out," Pym told me. "You're dealing with a system that's been around for tens of millions of years. Just make a list of rich and diverse species. It's not enough either. You also have to have a composition guide."