Chapter 29 6.2 Which came first, stability or diversity?

If nature is based on perpetual change, then instability may be the reason for the richness and variety of biological types in nature.The idea that unstable forces of nature are at the root of diversity runs counter to an old environmentalist adage: that stability breeds diversity, and diversity breeds stability.But if it is true that natural systems do not tend towards delicate equilibrium, we should get used to dealing with instability. In the late 1960s, biologists finally got the help of computers and began to model dynamic ecosystems and food chain networks on silicon networks.One of the first questions they tried to answer was, where does the stability come from?If a relationship between predator and prey is created on a virtual network, what conditions will allow the two to stabilize as a long-term co-evolutionary duo, and what conditions will make these virtual creatures unsustainable?

One of the earliest papers on modeling stability was a collaboration between Gardner and Ashby in 1970.Ashbee was an engineer interested in the advantages of positive feedback loops and nonlinear control circuits.On a computer, the pair programmed hundreds of variations of simple network circuits, systematically changing the number of nodes and how connected they were.They found something surprising: If you increase the degree of correlation beyond a certain critical value, the ability of the system to recover from external disturbances suddenly decreases.In other words, complex systems are more likely to be unstable than simple systems.

The following year, theoretical biologist Robert May published similar conclusions.May ran ecological models on a computer, and some simulated ecological communities contained large numbers of interacting species, while others contained only a few.His conclusions contradict the stability/diversity consensus.He cautions against simply thinking that increasing the complexity of species mixing will bring stability.In contrast, May's simulated ecology argues that simplicity and complexity do not affect stability as much as patterns of interaction between species. "In the beginning, ecologists built simple mathematical models and simple laboratory microscopic systems, and they screwed up. Species disappeared rapidly," Stuart Pym told me. and a more complex system in the aquarium, they thought it would be better. They were wrong, and made it worse. The complexity just made things extremely difficult - because the parameters had to be just right. So, unless it really Simple (single-prey-single-resource population model), otherwise building a model at random will not work. Add diversity, strengthen interactions, or increase the length of the food chain, and they will soon reach the point of collapse. This is Gardner, The subject of early research on food webs by Ashby, May, and I. But keep adding species to the system, keep crashing them, and they end up mixing together, not collapsing, and suddenly gaining a natural order. They It took a lot of messy failures to get right. The only way we know of to get complex systems that last stably is to put them together over and over again. As far as I know, no one really understands why it works .”

In 1991, Stuart Pym and his colleagues John Lawton and John Cohen reviewed all field measurements of food webs in the wild, analyzed them mathematically, and concluded that "populations from catastrophe The rate of recovery in ... depends on the length of the food chain" and the number of prey and predators a species has.Insects eating leaves are part of a food chain.Turtles eat leaf-eating insects, forming two links in a food chain.The wolf may be on a link far from the leaf.Generally speaking, the longer the food chain is, the more unstable the interacting food chain will be due to the impact of environmental damage.

Another point drawn from May's simulations is best illustrated by a phenomenon observed a few years earlier by the Spanish ecologist Romon Maglev.Like May, Maglev noticed that a system with many members would be weakly connected to each other, while a system with few members would be strongly connected to each other.Maglev said: "Practical experience shows that those species that interact freely with other species tend to have a large social circle. On the contrary, species that interact closely with each other and have a strong degree of interaction often belong to a member with a large number of members. finite system.” This apparent trade-off within an ecosystem, with either a large number of loosely related members or a small number of closely related members, is very similar to the well-known trade-off in reproductive strategies of organisms: either produce a small number of offspring and keep them well protected , or produce countless offspring and let them find their own way of life.

Biology has shown that, in addition to regulating the number of joints at each node in the network, the system also tends to regulate the "connectivity" (the strength of the connection) between each pair of nodes in the network.Nature seems to preserve the invariance of connectivity.We should therefore expect to find similar conservation laws of connectivity in cultural, economic, and mechanical systems, although I'm not aware of any such studies.If there is such a law in all living systems, we can also expect that this connectivity is in flux, always in a state of constant adjustment.

"An ecosystem is a network of living things," Boggs said.Organisms are linked together with varying degrees of connectivity through food webs, smells, and sight.Every ecosystem is a dynamic network, always changing and in the process of reshaping itself. "Everywhere we look for invariance, we find change," Burtke wrote. When we embark on a pilgrimage to Yellowstone, or to the California mangroves, or to the Florida wetlands, we are always struck by the admirable and appropriate naturalness of the place.Bears seem to be in the deep valleys of the Rocky Mountains; redwood forests seem to be swaying on the coastal hills, and crocodiles seem to be on the plains.We have an urge to protect them from disturbance.But from a long-term perspective, they are all just passers-by, neither old residents of this place, nor will they live here forever."Nature itself," Bauken wrote, "is not static in form, structure, or composition, but changes all the time and all places."

Scholars have studied pollen fossils obtained from boreholes at the bottom of some lakes in Africa and found that African landforms have been in a state of flux over the past few million years.At one point in the past, the African landscape looked very different than it does now.What is now the vast Sahara Desert was a tropical forest in the geological past.And since then and now there have been many ecological types.We think wildness is eternal; in reality, nature is limited flux. Complexity injected into artificial media and silicon wafers will only flow further.Although we know that human institutions—those social and ecological systems that condense human efforts and dreams—must be in constant flux and repeated establishment, but when the changes begin, we are always surprised or resisted. (Ask a trendy postmodern American if he wants to change the 200-year-old U.S. Constitution. He'll suddenly become a medieval conservative.)

Change itself, not redwood forests or national parliaments, is eternal.The question then becomes: what controls change?How do we induce change?Can distributed life in loose groups such as governments, economies, and ecosystems be controlled in any deliberate way?Can we predict future states of change? Say you buy 100 acres of abandoned farmland in Michigan.You put a fence around it, keeping the cows and the people out.Then you walk away and monitor this wasteland for decades.In the first summer, weeds in the garden took over the land.Since then, new species outside the fence have been blown into the garden by the wind to take root every year.Some newcomers are slowly replaced by newer latecomers, the ecological combination organizes itself in this land, and the mix changes over the years.If a knowledgeable ecologist looked at this fenced wilderness, could he predict which wild species would occupy the land a hundred years from now?

"Yes, no doubt he can predict," said Stuart Pym, "but it will not be as interesting as one would think." Take any standard college ecology textbook and you'll find the last form of this Michigan land in the chapter on the concept of biological succession.The first year visiting weeds are annual flowering herbaceous plants, which are then replaced by tougher perennials such as sedgegrass and ragweed. Woody undergrowths shade and inhibit the growth of flowering plants, followed by pine trees. Shrub growth.However, the shade of the pine trees protected the young hardwoods such as beech and maple, which in turn firmly pushed the pines out of their turf.A hundred years later, the typical boreal forest almost completely covered the entire land.

The whole process is as if the land itself is a seed.The first year it grows a weed, and after some years it becomes a thick bush, and then it grows into a lush forest.The picture scroll of succession of this land unfolds gradually according to predictable stages, just as we can predict how frog eggs will turn into tadpoles. There are other odd sides to this development, if the new development starts with 100 acres of wet swamp area instead of a field, or is replaced by a dry, sandy dune in Michigan of the same size, the species that initially take over are different (sedges on swamps, raspberries on dunes), but the mix of species gradually converges towards the same end point, broadleaf forest.Three different seeds hatch into the same adult.This convergence phenomenon has led ecologists to conceive the idea of ​​the end of biological succession or the climax community.In a certain area, all ecological mixtures tend to shift until they reach a mature, ultimate, stable harmony. In the temperate north, what the land "wants" is hardwood forest.Given enough time, a dry lake or aeolian swamp can become a broad-leaved forest.If it is warmer, the mountain top will also have this desire.It's as if in a complex web of eating and being eaten food chains, the endless competition for survival stirs up the area's mixed species until the mixed state becomes the culmination of broadleaf forest (or in other climates, specific climax community), then, everything will calm down to a kind of peace acceptable to all, and the land will subside in the apex mixed state. At the climax of succession, the mutual needs of the diverse species are so beautifully tuned that the whole is difficult to destroy.In just three decades, North America's native chestnut trees—the mighty chestnut trees that make up the majority of North American forests—have been completely wiped out.However, the rest of the forest has not been greatly affected and the forest is still standing.The enduring stability produced by peculiar mixtures of species—ecosystems—shows a certain basin effect that resembles the harmony that belongs to organisms.Something whole and alive resides in each other.Maybe a maple grove is just a giant organism made up of smaller ones. On the other hand, Aldo Leopold wrote, "By ordinary physical measures, both mass and energy, the grouse is but a drop in the ocean in an acre of land ecosystem. But if you take away from the system Grouse, and the whole system shuts down."