The Critical Zone

Lesson 10 Introduction


The variety of life on Earth exists as a myriad of microscopic and macroscopic forms. This biological diversity, or biodiversity, can be described at scales ranging from planetary, biome, or ecosystem to site-specific. Biodiversity includes genetic variation within species, the variety of species in an area, and the variety of habitats within a landscape. Roughly 1,750,000 species have been described and formally named, though estimates of the total biodiversity of life on Earth range from 5 to 30 million, with some estimates as high as 100,000,000 total species.

Most of terrestrial life reproduces, feeds, lives, and dies in the Critical Zone (see "Soil Biodiversity" at Wikipedia for a brief introduction to soil biodiversity topics). Soil is intimately linked to biodiversity: soil provides the substrate through which much of the terrestrial biosphere interacts with the Critical Zone and its other component "spheres," and biota exert a significant influence on soil formation and Critical Zone processes. For example, plants absorb atmospheric carbon dioxide and store it in roots—eventually root respiration adds carbon dioxide to the soil atmosphere, changing, and at times controlling, weathering rates and other chemical processes within the soil and Critical Zone. A primary function of roots is to absorb water and nutrients, activity that directly influences the hydrosphere by drawing in soil moisture and dissolved constituents. This influences recharge rates and the chemistry of the soil and groundwater system. Roots also anchor plants within the Critical Zone and physically erode rocks by penetration and wedging, thus influencing interaction with the lithosphere.

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Roots visible at approximately 40' feet beneath the ground surface in a limestone cave in central Pennsylvania. Scale note: the root portion suspended in air is approximately 16" long.
Credit: Tim White, 2007.
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Roots enveloping sandstone boulder near Clear Creek State Park, northwestern Pennsylvania. Notice that root spurs and rootlets penetrate and are separating preexisting bedding plane and weathering disparities in the rock.
Credit: Tim White, 2008.

But it is yet more complicated than that! Each species or organism may have a unique role in an ecosystem and therefore may interact with the Critical Zone in a different way. Furthermore, the interactions between organisms can be unique to species or unique to a specific habitat or ecosystem. Let's think about plants again. Cellulose, a potential source of energy, is tough and insoluble to most organisms. But termites (and grazing animals) contain microorganisms in their digestive tracts that convert cellulose to sugars usable by them and their hosts. Thriving termites in turn clear the landscape of dead plant litter, recycle contained nutrients back to the soil, and enhance soil porosity and permeability through construction of their subterranean chambers. An overall decrease in soil productivity and fertility has been observed in the absence of termites; thus, if not for those gut flora, a substantially different, and perhaps less fecund, ecosystem might exist.

Most cultures have recognized the importance of biodiversity for boosting overall ecosystem productivity and resilience to disasters. Yet continued destruction of habitat and species by human society is leading to what some biologists and paleontologists call the “Sixth Extinction,” perhaps the most rapid and destructive of extinctions in Earth history. Imagine total global human destructiveness perhaps exceeding the comet impact that occurred at the end of the Cretaceous Period that finished off the dinosaurs!

In Lesson 10, we will consider the various means by which biologists and other natural scientists classify life on Earth. From there, we will launch into concepts of ecology, examining some of the interactions and processes between organisms and their environment—this examination will gradually focus in on the Critical Zone and soil processes that involve biota. We will complete Lesson 10 with an online forum to discuss the merits of placing a financial value on ecosystem processes. In Lesson 11, we will take a closer look at various expansive ecosystems of Earth and consider the relationship between them and the rest of the Critical Zone.

Watch this!

Learn about some interesting examples of the role of life in the Critical Zone.

Video: Biota in the Critical Zone (11:44)

Click here for a transcript of the Biota in the Critical Zone Video.

DR. TIM WHITE: Greetings. Behind me, you can see the Nittany Valley, State College, and the Penn State campus. And I just want to point out to you that you can see a mix of land uses, including the town, and forest, and agricultural fields. Prior to European colonization of North America, this region would have been entirely forested, including that valley behind me.

In 2007, when I began teaching EARTH 530, I didn't make an introductory video for unit six on the biosphere, and thus here we are in 2014 making that video. Much has happened in the interim. For example, Earth's population has grown by 500 million people to over 7 billion total people living on earth.

Much of society has begun to recognize the degradational effects of human activities on the landscape. Let's consider an example. You likely recall that I am a geologist. Thus, this example I will provide of humanity's effect on the critical zones is from the geological literature-- a study from 2005 by Bruce Wilkinson.

Look at the graph in front of you, and notice that, on either one of the y-axes, is described annual loss of sediment and soil, either measured in tons or in meters per million years. On the x-axis, you can see time from the present on the right, to 2000 years before present on the left. The solid black line crossing the graph shows a long term average natural erosion rate, calculated by various means, through geologic time.

What you should notice, then, is that in the open diamond is shown this cumulative effect of erosion by human processes. Early on in humanity's history, this is primarily by agriculture. And as you move from the left to the right of the diagram, following those open diamonds, you'll see that the rate increases. And that increases as a function of agricultural activities, as well as industrial and construction activities.

The point of this graph is that, if you compare this open diamond at present to the solid black line crossing the graph, showing natural erosion rates, you should notice that there's an orders of magnitude shift in humanity's erosive effect to values that now exceed long term natural rates by as much as 50 times. I want to stress what I just said. The combined effects of humanity at present now erode and transport more solid material on Earth's surface then all natural processes combined.

Our 7.1 billion people move more dirt than all wind, streams, rivers, and glaciers combined. Thus, in this unit six, when we consider biotic effects on the critical zone, we cannot ignore humanity. Throughout the semester, you've been exposed to various topics associated with human degradation of the critical zone. In lesson 12, you will focus on this topic to provide some context. In unit six, we will consider topics of biodiversity and ecological processes, and ways in which landscape scale characterization of biotic attributes may be considered.

Life on earth can be classified into six kingdoms. Of these kingdoms, most people are familiar with the plant and animal kingdoms. Kingdoms are further subdivided, and the finest level of that subdivision is species. To give you some perspective on numbers, some estimates suggest that 8.7 million species exist on earth. And that means that up to 90% of life on Earth has yet to be discovered.

Each organism may play a unique role in its ecosystem, and therefore may impact the critical zone in a unique manner. Let's consider some examples. We've hiked about two miles on this glorious July day to this clearing in the Rothrock State Forest. The clearing is here due to a combination of human and invasive species. But the continued presence of the clearing-- the maintenance of it, if you will-- is due to colonization by Allegheny mound ants.

In the background here, you can see at least three ant mounds. And those mounds range in size from three to four feet in diameter, and two to two and 1/2 feet high. Let's have a closer look. The Allegheny mound ants are known to create mounds up 10 feet in diameter and five feet high, with tunnels that extend to similar depths below ground.

In the process of building the mounds, the ants churn the soil, thus introducing fresh mineral matter from depth, to the more intensive weathering environment at the surface. Their tunnels increase soil porosity, thereby providing pathways for water and gas transfer deeper into the soil than otherwise would be possible. The ants also inject formic acid into nearby vegetation, does thus it and easing expansion of the colony.

Here you can see a halo of dead blueberry branches surrounding this active colony. Here you can see an abandoned colony that's six feet in diameter and up to 2 and 1/2 feet high. We can tell it's abandoned because it's now vegetated by mosses and grass. The resident black bear population likes to eat the ants and their eggs, which may explain the presence of this crater in the top of the once thriving colony.

The sound you hear is from hundreds of thousands of cicadas that have erupted from the landscape during these few weeks approaching summer solstice, another example of life interacting with the critical zone. The cicadas emerge from soil burrows, churning the soil, and introducing mineral matter from depth to the surface. Notice the pen for scale. The burrows also increase soil porosity, thereby providing pathways for water and gas transferred deeper in the soil profile than might otherwise be possible. The combination of molted larval exoskeletons and dead adult cicadas may also periodically introduce a new source of organic matter to the surface litter of the forest floor, and [INAUDIBLE] of the forest soil.

The first example was from the animal kingdom. And now I'd like to consider what, for most of you, is going to be a less obvious effect of the plant kingdom. If a tree falls in the woods, does it matter?

Here beside me, you can see a fallen deciduous tree. And you can also see that the root mass of that fallen tree has incorporated large pieces of rock, and sediment, and soil-- what geologists call tree throw. In this process, the tree rips up the substrate and churns the soil, aerating an otherwise subterranean realm, and setting off a cascade of changing geochemical and ecological processes.

In a typical Appalachian Mountains setting, this process can entirely overturn a landscape in a few thousand years. On a slope, the effect can be relatively dramatic. In the Appalachian mountain region, a typical hill slope, away from the erosive effects of a stream, erodes primarily by gravitational processes that geologists call creep.

Creep is the slow, downward slide of loose soil mantle on top of a more compacted and less mobile zone of soil and rock. Detailed studies of the rates of sediment and soil movement and erosion by tree throw demonstrate that tree throw erodes at a rate that is at least two orders of magnitude greater than creep. Thus, in temperate regions of earth, like the Appalachian Mountains, trees are the dominant erosive force on the landscape, other than humans.

I hope these examples have demonstrated that the biosphere interacts with the critical zone in unique ways that may not be immediately obvious just to most people. As you study the biosphere in unit six and consider the critical zone near your home or school, remember this complexity and value it-- a topic we will also consider in this unit six.