On One Hand

December 29, 2009

How Life Transforms the Landscape: Evolution on a Global Scale

Filed under: science — ononehand @ 11:25 pm
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Most of us are familiar with the concepts that drive evolution and adaptability – that species with traits or features that allow them to thrive do thrive, populating the landscape, while species with traits that are not as well-adapted remain rare or go extinct. That’s why almost all the organisms we see in the wild are tough and well-adapted evolutionary “winners,” and while species come and go, life itself is nearly impossible to kill off.

But equally importantly, life has a tendency to bring other life with it; a successful new species creates a food source and shelter for other species to evolve or move in. Over billions of years, the net effect of this has been to expand the biosphere to cover the entire planet with increasing numbers of living things.

In the beginning of life on Earth, a few single-celled organisms stuck to the gentle shores of balmy, salty seas, while the rest of the planet was as barren and desolate as Mars or the Moon. The oceans filled with microorganisms first, aided by the mobility that sea currents provide but still few and far between. Early photosynthetic bacteria began to colonize into Stromatolites, forming grainy pillows of mud and sediment that dotted tropical coastal areas and are still visible today in some places, both in active growth and ancient fossils.

Left: Stromatolytes. Right: Red moss, a primitive, rootless plant that requires humid conditions.

When plants adapted ways of storing water or sucking it out of damp soil, significant numbers of species were able to move out of semi-submerged swamps and live on hills and plains. Early plants stayed within a few miles of the coast, places that are now known as marshes, cloud forests and coastal rainforest. But over time a few pioneers crept out onto continental interiors, and gradually evolved to larger sizes, for the first time making visual impact on the landscape on a global scale.

It was only recently in the timeline of life that an observer from space would see continents turn green with forests and grasslands. Deserts were quite barren until plants developed advanced strategies for storing water, allowing rocky, sandy places like the Mojave Desert to become littered with cactus and Joshua trees, dry grasses and woody shrubs with massive underground root systems. The establishment of consistent greenery, even if not as lush as a forest, allowed herbivores to move in, followed by predatory species, fungi, bacteria and other scavengers.

If you look at the kinds of plants that exist in Earth’s most difficult environments, you find that they are dominated by angiosperms – blooming plants – which emerged during the time of the dinosaurs and are recent introductions to Earth’s biosphere. It is hard to imagine ferns and cycads – Earth’s early plants with weak root systems and a strong preference for humid climates – growing comfortably in the desert or the arctic. Grass, a type of angiosperm and very recent development in the plant kingdom, was absent until about the time that dinosaurs went extinct, and now dominates semi-arid and arid areas. Before 65 million years ago, landscapes now covered in grass would be filled, instead, with caked dirt and only occasional greenery.

Grass changed the environment not only by introducing a food source for animals, but by taming it with extensive root systems. Grass has a dramatic, Earth-changing effect of controlling erosion, turning sand dunes into hills and muddy river banks into pastures. Dead grass decays and fertilizes sandy soil with water-absorbent organic material, which makes the land fertile. The systemic change of the landscape allows other drought-tolerant trees to begin growing, benefiting even some of the species that are less well-acclimated to the landscapes grass species take over.

Left: Grass taming a sandy area. Right: Grass dominating a semi-arid climate

Other angiosperms formed symbiotic relationships with bacteria in their roots to turn Nitrogen in the air into usable fertilizer. This is known to help other surrounding plants thrive, and has been used by farmers to benefit crops. The concept of crop rotation takes advantage of one species’ assistance of another, using nitrogen-fixing plants like alfalfa or clover to enrich soil for grass crops like wheat or corn.

A recent study has shown that blooming plants also humidified entire regions to make continental rainforests possible. Angiosperms, with advanced root and circulatory systems, are so good at finding water and pumping it into themselves that evaporation from their leaves humidified the air and generated new rains. Those rains invited millions of species to move in. Ferns have about a fifth of the leaf veins that blooming plants like ficus trees and avocados have, so weren’t able to humidify the air in the same way. But when ferns adapted the ability to grow in shady places beneath trees or cling to rainforest tree trunks themselves, they benefited greatly from the expanding biosphere, and developed their own ways to contribute to it. Again, it was a new evolutionary development that allowed places like the Amazon Rainforest to cover half a continent with green.

Clouds form over the Amazon Rainforest. Water evaporates from tree leaves, facilitating
cloud formation, at a faster rate than even the warm ocean does.

Even what we see as “primitive” kinds of species continue to evolve. Mosses don’t have roots or the ability to store water, but by developing the ability to survive freezing, they were able to begin growing on mountain tops and arctic tundra between rocks and logs, provided the area is moist with melting snow. Moss is known to be a pioneer species itself since it does not need soil, but produces soil when it dies and decays, allowing other plants to grow.

The way a species can make room for other species to thrive – as well as our understanding of it – is increasingly complex. Also increasing is the hostility of environments that diverse ecosystems cover with life. There is life beneath the ice in Antarctic, in abandoned nuclear reactors, and covering undersea volcanic vents. Animal species have long been learning to thrive in human environments, turning urban parks and sewers into thriving ecosystems.

If we see human beings as part of this system, greenhouses and irrigated farms are examples of one life form improving things for another. When there’s snow on the ground outside, the potted plants in my bedroom are certainly glad to have their human-built, adapted environment. Sure, ecology leaves room for the idea that one species can royally screw up an ecosystem for many others – even driving itself to extinction by overpopulating itself or overtaxing its food source – and doesn’t suggest that every species’ impact on the environment is “good.” (Evolution doesn’t see things in terms of “good” or “bad,” just working and not working.)

But humans are part of the principle nonetheless; we bring water to deserts and pass through tunnels blasted through solid rock. If we are able to seed Earth’s life on artificial space stations, the moon, or even other planets, isn’t this, too, an example of Earthly life facilitating its own expansion and filling inhospitable areas?

Expansion and adaptation will continue with or without human influence. It is easy to predict that evolutionary trends that have led to today’s green planet will only continue into the future. If a complex plant evolved to grow in ice or snow, it would turn huge tracts of arctic land into shrubland. As more desert species evolve, a landscape that gets only five inches of rain a year doesn’t seem so dry. Maybe the brown areas on the Earth we see in satellite images will be filled with shades of green in 50 million years.

Perhaps scientists will someday be able to use this concept to determine how long life has existed on a planet as humans begin to explore space. A planet that seems hospitable but only has life forms growing in specific areas is new, and a planet utterly covered in organisms has been experiencing the processes of life for billions and billions of years. Either way, it is obvious that this planet is experiencing somewhat of an unlimited expansion of its own, and that the natural world has its own way of unconsciously creating and organizing itself towards growth.


1 Comment »

  1. These are beautiful pictures!

    Comment by seanshawn — January 3, 2010 @ 9:24 pm | Reply

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