3. The Biosphere and the Carbon Cycle

Photosynthetic organisms use this energy trapped in glucose to make other organic (carbon-based) molecules… like lipids, proteins, and DNA.  Other organisms eat these “producers” to make their own molecules, and they grow and get bigger.  So, LOTS of carbon that would otherwise be in the atmosphere is now present in LIVING, carbon-based life forms!  There is more carbon in living systems, now, than in the atmosphere. And when they die, sometimes they are trapped under sediments and fossilized.  Over the long history of the Earth, lots of living things and their carbon have been converted to coal and other fossil fuels.

The concentration of these gases has changed over earth history, as a consequence of changes to the living world.  Previous to the evolution of oxygenic photosynthesis approximately 2 billion years ago, there was only trace amounts of O₂ in the atmosphere and dissolved in the oceans.  There were still photosynthetic producers that formed the foundation of ecosystems, but these were the Sulphur Bacteria that used H₂S as a source of electrons instead of water… so oxygen was note released as a waste product.  But around 2 billion years ago, a dramatic change occurred: iron dissolved in the oceans (eroded from the landscape by cation displacement) was OXIDIZED and precipitated out of solution as “banded iron formations” and “red beds”.  This demonstrates that O₂ was present in the oceans—produced by oxygenic photosynthesis. 
About 400 million years ago, the amount of oxygen in the atmosphere began to increase – largely as a function of the evolution of true plants on land.  About 350 million years ago, we see a spike in atmospheric oxygen levels.  This was a consequence of an imbalance between photosynthesis adding O₂ and exceeding the respiration of bacterial and fungal decomposers consuming O₂.  Respiration dropped because organic material—largely the forests of tree-like club mosses growing in coastal swamps—was covered by sediment and protected from decomposers.  So, oxygen accumulated (photosynthesis > respiration).  That organic material, buried under layers of sediment, eventually metamorphosed into our coal, oil, and natural gas deposits: “fossil” fuels.  So, changes in life had an effect on the atmosphere once again.

About 65 million years ago, a meteor struck the Yucatan peninsula and caused a mass extinction event that wiped out the dinosaurs. This is relfected in changes in the atmosphere, as well. The impact causes massive amounts of dirt to fly into the atmosphere, and causes worldwide wildfires that also threw up ash and soot into the atmosphere. This global cloud--an "ecological winter"--reduced sunlight so much that ecosytems collapsed. With less light there was less photosynthesis - and a drop in global oxygen levels. With less plant growth--primary productivity--the largest, most food-demanding animals (dinosaurs) died out.

Today, these effects can be seen in the annual cycles of CO₂ and oxygen levels in the atmosphere.  Most of the land mass on our planet is on the northern hemisphere.  so, from March-September, photosynthesis exceeds respiration at a planetary scale and O₂ increases while CO₂ drops.  From October to February, even though this is the southern hemisphere’s summer, global photosynthesis < respiration and CO₂ levels rise and O₂ levels drop.  The earth breathes.
And, of course, as we cut forests, we reduce the amount of CO₂ that is absorbed by plants each year, and reduce the amount of O₂ produced.  So, CO₂ levels oscillate and rise, while O₂ levels oscillate and drop. CO₂ is increasing, as well, as a consequence of burning fossil fuels—returning the CO₂ absorbed by plants 350 million years ago back to air.

Conclusion:  The Earth is a LIVING PLANET.  Life doesn’t just live on Earth.  It has transformed Earth, and it continues to maintain these odd conditions.  Recent human activity is changing these relationships that have evolved over billions of years.

IV. The Biosphere

The biosphere is a critical component of the Earth system--when it changes, the way the Earth works changes, too, reflected by obvious changes in atmospheric composition. If we are concerned about these changes to the atmosphere and the global warming, sea level rise, increased fires and storms that follow, we might want to protect and sustain the biosphere so it can do its job. Well, if we want to sustain it, and maintain its function, we have to know how it works and why it works the way it does. What philosophical approach seems relevant here? How might we learn why it works the way it does? Reductionism dictates that we should describe what it is made of--that might help us understand how it works, and help us maintain these functions.

A. What is the BIOSPHERE?

The biosphere is ALL LIVING THINGS: you, me, the trees outside, the squrrels, the mushrooms, the fish and the algae. It is the 2 million different species of living things that compose the known living world. The periodic table has only 92 naturally occurring elements; the biosphere has over 2 million species, interacting in complex ecosystems. We have just begun to understand how this DIVERSITY affects how the biosphere works. But you cannot appreciate the biosphere without appreciating these parts--what they are and how they work. This is the domain of BIOLOGY... Not environmental science and not sustainability science. If you want to sustain .the biosphere, then you must understand and study the biosphere. That is what BIOLOGY does, at levels from cells to the BIOSPHERE.

Of the 2 million species we have given a name to (like Homos sapiens or Drosophila m.elanogaster), about HALF--fully 1 million--are insects. Another 25% are other invertebrate species: other arthropods like crustaceans, and other phyla like the molluscs, segmented worms (Annelida), flat worms (Platyhelminthes), round worms (Nematoda), Cnidarians (corals and jellyfish), sponges, Echinoderms (starfish), and many more. Only 5% of the species are vertebrates: fish, amphibians, reptiles, birds, and mammals--the things most people think of as 'animals'! 20% of the named species are plants, and there are small fractions of fungi, protists, and bacteria. Aside from the insects (which are terrestrial), most animal species are marine organisms. We have probably found most the big stuff that lives with us on land; most of what is unnamed is marine and small. A quick walk through the insects shows us that they are diverse because there are differnt species that live in very differnt ways--they have different ecologies and contribute to their ecosystems in differnt ways. Unfortunately, alhtough we have named 1 million species, we know NOTHING about most of them. We collected them, and have yet to study how they fit into this extraordinary "machine" of the biosphere.

B. What does Biodiversity DO?

But, as a result of describing this biodiversity, we have begun to understand what biodiversity DOES (reductionism).

1. Diversity increases productivity

'Productivity' is the amount of biomass produced... typically, we measure 'primary productivity', which is the amount of plant biomass produced. Usually, this is standardized per unit area (m2) and per unit time (year). This 'primary productivity' is the energy harvest from sunlight and trapped in chemcial bonds in plant molecules: glucose, cellulose, and all the plant proteins, carbohydrates, lipids, etc. In other words, 'primary productivity' is the food upon which all other life forms--including our own--depends. More diverse systems can be more productive than less diverse systems--even less diverse systems containing only the most productive single species. This occurs because of niche compatability and positive effects. Let's consider an example:

Consider a set of agricultural plants that vary in productivity, like corn, squash, and beans. Per unit area, under ideal growing conditions and densities, corn is the most productive species. So, you might conclude that, to produce the most food, you should plant your entire field just in corn. How can adding less productive species increase total food production over a monoculture of the most productive species? This is how.

- Niche complementarity: If you plant less corn, you may be able to plant squash at the base of each corn plant. The squash and corn use different nutrients, and won't compete as strongly as two corn plants placed this lose together. So, although you get less corn, the amount of squash produced can over-compensate for this lower corn production, because they don't have the same ecological requirements... their 'niches' are complementary ('fitting together', not 'overlapping').

- Positive effects: Now, suppose you add beans to the mix. Beans are legumes--and they have bacteria in root nodules that 'fix' nitrogen. Although our atmosphere is 78% nitrogen gas, N2, the atoms are held together by the strongest bond in nature: the triple bond. No animals or plants have enzymes that can break this bond, and thus make allow the nitrogen to bond to other atoms like hydrogen, carbon, or oxygen... as it needs to to make DNA and amino acids (in proteins). So, all animals and plants must get their nitrogen from their food or the decmposed nutrients in the soil. There are a few bacteria, however, that can break N2, and link N to H and O ('fixing' it to H or O), making it available to plants as a soil nutrient. Thus, these nitrogen-fixing bacteria 'fertilize' soils. They infect legumes and grow, feeding the legume but also raising the level of available nitrogen in the soil. SO! By planting beans, the corn grows a little better and you get more corn. The squash grows a little better and youget more squash. And, you get the beans!

So, by adding 'less productive' crops to the system, you actually get MORE food than the monculture of the most productive plant. When we look at the productivity of entire ecosystems, the productivity correlates with diversity: the most diverse terrestrial systems (rainforests) and marine systems (coral reefs and algal beds) are also the most productive. So, diversity is FUNCTIONALLY IMPORTANT to how the biosphere works.

2. Diverse ecosystems provide economically valuable services

The biosphere cycles energy and matter through the Earth system, and thus cleans our air and cleans our water, in addition to providing food (above). In addition, ecosystems protect our manmade systems from variation in nature: they act as storm breaks, they absorb excess water in coastal storm surges, they reduce erosion, they moderate the climate. In a 1997 article, Costanza and coauthors quantified th economic value of these services. It exceeded the GNP of all countries, together. In other words, if we had to replace these services...like constructing desalinization plants to make fresh water... it would take more money than all our economies generate. The biosphere is doing these things for free.

3. Aesthetic value

Finally, the biosphere provides the diverse contexts in which our diverse cultures have evolved. We draw sybols from nature to communicate ideas. Indeed, even how we think is a function of our environment. A rich and diverse nature enriches our culture and our lives.

C. Why Preserve Biodiversity?

So, diversity does some really important things. But do we really need to protect all of it? There are 350,000 species of beetles; does each species do something uniquely important? Probably not; there is alot of redundancy in nature, and nature can respond to changes like the loss of a species. Over the last 200 years, we have hundreds of examples of species that went extinct, yet the biosphere has not collapsed. The problem is this: although we have given names to 2 million species, we know nothing about most of them. Ecology is a young science, and we don't really know how the parts of an ecosystem--the species--all fit together to cause the system to work (reductionism). So, we don't know which species are more important than others. We are 'tinkering around' with these systems, killing off this species and that one indiscriminantly, without knowing what we are doing. As Aldo Leopold, a famous ecologist of the 20th century said, "the first rule of the tinkerer is to save all the pieces." If you are tinkering around with a toaster and you want to have any hope of putting it back together again so it works, you better keep all the pieces.

D. How is this Diversity Produced?

To preserve diversity, we must understand how it is produced and maintained. Charles Darwin was the first to conclude that diversity is produced through the process of radiatinal evolution: species 'split' over time, giving rise to multiple new species. As he said at the close of Origin of Species, "There is grandeur in this view of life, with its several powers, having been originally breathed a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved." This course will now examine how this happens.

Study Questions:

1. Describe biogenic limestone formation.

2. Outline the process of photosynthesis, and state the two ways it affects the composition of the atmosphere.

3. What evidence do we have that oxygen was rare in the early atmosphere, and began to accumulate about 2.5 billion years ago?

4. Why did oxygen rise 350 million years ago?

5. Why did oxygen drop 65 million years ago?

6. How and why do CO2 and O2 levels vary over the course of a year?

7. How and why is CO2 increasing and O2 decreasing from decade to decade? Describe two reasons.

8. How many species have been identified by science, and what fractions are comprised by insects, other invertebrates, plants, and vertebrates?

9. Describe the two ways that biodiversity can increase productivity.

10. What are ecosystem services?

11. What is a 'keystone species' (look it up somewhere)? Describe how sea otters act as keystone species.