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What is an Ecosystem

Technically an ecosystem is the biotic and abiotic elements of the environment at some location (a lake, a forest, the Amazon, the Pacific Ocean).  The biotic component is all the living organisms living there and the abiotic component is the non-living stuff such as the atmosphere, water, soil, etc.

Ecosystem ecology is the study of the interaction between the abiotic and biotic parts of the ecosystem.  It is different from community ecology which is the study of interactions of species with one another.  Community ecology ‘cares’ about who is eating who, who is competing with who for resources, which species have mutually beneficial relationships and so on.

Ecosystem biologists have more of a big picture (or very small picture depending on your perspective)orientation than other ecologists.  They tend to group species into broader categories depending on the role they play in the ecosystem.  At the broadest level you can break organisms down into three groups: producers, consumers, and decomposers.  Producers (plants and other organisms capable of photosynthesis) capture energy from the abiotic environment and incorporate it into the biotic component of the ecosystem.  Consumers and decomposers extract energy from producers and each other by consuming living things (consumers) or formerly living things (decomposers).

Why We Should We Care About Ecosystems

Two big practical reasons.

1. ‘Natural’ ecosystems rely on the same basic set of processes as human society and both systems operate under the same physical constraints.  We need matter and energy to run our society – ecosystems also need matter and energy.

1. We rely on ecosystems one way or another – for most of what we need.

The fundamental rule of ecosystems is that energy flows and matter cycles.  The earth gets a continual input of energy from the sun.  That energy eventually radiates out into space as heat.  The earth is an open system with regard to energy.  In contrast the earth is, for all practical intents and purposes, a closed system for matter.  Just as for the planet as a whole, energy moves into and eventually out of ecosystems.  In contrast matter cycles through ecosystems and is continuously reused.  Therefore ecosystems can be described by a) the rate and pathways of energy movement and b) the rate and pathways of what are formally called biogeochemical cycles but which are often referred to as nutrient cycles.

Energy Flow

Between 1-2% of the solar energy reaching the earth is taken up into living organisms through the process of photosynthesis.  The rest of the solar energy is either reflected back into space as light or absorbed as heat by the atmosphere, water, soil, etc.  Of the energy an organism takes in, about 10% of it is used to build (i.e. make more biological structures) and 90% of it is used to run the organism.  As a side note – this is why eating plants is much more environmentally friendly than eating animals.  If you eat a cow, 90% of the cow’s energy input is used to run the cow.  If you eat the plants then the 90% used to run the cow can be used to run you!  The 90% used to do biological work (run organisms) ends up radiating out into the environment as heat.  So each time energy moves from one place to another in an ecosystem 90% ends up leaving the ecosystem.

Nutrient Cycles

The movement of matter through ecosystems can be described through a number of pathways known as nutrient cycles.  The name is somewhat deceptive as they are not simple circular pathways but have lots of branching points.  However they are cyclic in that there is no input or output.  In various forms the nutrient moves from one ‘pool’ in the cycle to another ‘pool’  These cycles are known as the water cycle, nitrogen cycle, carbon cycle, phosphorus cycle and so on.  The four mentioned are the largest in terms of volume.  They all differ from one another in important ways that I will omit for the sake of brevity.

Two of these cycles have biological bottlenecks.  The carbon cycle requires photosynthesis to continue and the nitrogen cycle requires nitrogen fixing bacteria to continue.

The Carbon Cycle

Carbon atoms make up the skeletal structure of the vast majority of all biological molecules.  Carbon enters the biotic component of an ecosystem through photosynthesis.  CO2 is removed from the atmosphere (one pool in the carbon cycle) and the energy from sunlight is utilized to build molecules containing multiple carbon atoms.  Some of the light energy ends up in the chemical bonds of these molecules.  So the carbon ends up in a new pool, the producers, in the form of a variety of organic molecules.

This biologically key process provides both carbon for the producer to build bits of itself and a stable transportable source of energy to do the work of building.  A consumer or decomposer that takes in these molecules gets the same thing.  When these molecules are broken down through cellular respiration the energy is made available for biological work (running the organism) and the carbon is released into the environment in the form of CO2. The CO2 is only available to organisms capable of photosynthesis.  In short we need producers but they don’t need us.  Producers do need decomposers – otherwise the carbon in things that die would not be available to them.

Short and long term cycling

The water cycle is a very rapid cycle.  You drink in water, you lose it in a variety of ways.  It evaporates up into the atmosphere and it comes down as rain.  Water in the oceans and in fresh water is moving in and out of living things all the time.

But what about water in aquifers?  Some water sinks deep into the ground where it is not available to living things.  Water that falls as precipitation over Antarctica may remain frozen for centuries (or at least that was true in the past).

In many cases matter (nutrients) moves out of pools where it is accessible to organisms into pools where it may remain inaccessible for long periods of time.  Aquifers and polar ice are examples of this.

Over the history of the earth long term changes affect the availability of nutrients which affect the organisms in the ecosystem which in term can affect the environment.  The evolution of photosynthesis caused CO2 to be made available to organisms and O2 to be released into the atmosphere.  The result was a dramatic and permanent change in the environment which completely changed the hospitality of the earth to different kinds of life.

Other changes have happened as well.  Over time dead organic matter has ended up being buried under sediments and has not decomposed.  Under high pressure these tissues are converted to fossil fuels.  The carbon and the energy in the dead organisms does not cycle back into the biotic part of the ecosystem, nor is the energy lost as heat.  Instead it goes into long term storage.

Human Impacts on the Carbon Cycle

Humans have had a large number of major impacts on ecosystems.  Many of them can be categorized in two ways:  greatly accelerating the movement of matter and energy through ecosystems and simplifying ecosystems and diverting the energy and matter so that it more directly serves our purposes.

Fossil fuels are enormously convenient.  The are portable and energy can be easily extracted from them to do all kinds of useful things.  It is not surprising that we use them extensively.  Energy harvested from sunlight millions of years ago that was sitting underground not being used seems like a great deal.

From an ecosystem perspective the most immediate effect of fossil fuel burning is that formerly inaccessible carbon re-enters the active part of the carbon cycle in the form of CO2.  The additional CO2 being added is about 2% of the total CO2 entering the atmosphere.  To phrase it a different way, about 50 times as much CO2 enters the atmosphere through biological processes as does through use of fossil fuels.

This doesn’t sound like much but you have to remember that it is an extra 2% per year and that the size of the CO2 pool in the atmosphere depends on both output and input.  If the input goes up and the output stays the same then the CO2 concentration will increase.

So why don’t plants just grow faster if they have more CO2 (food)?  Experiments have been done for quite a while that simulate a high carbon atmosphere and look for a response in plant growth – generally the results have not been encouraging.  One problem is that plants don’t just need carbon and light to grow – they need water, nitrogen, phosphorus, and a host of other substances.  Their growth will be limited by whichever is in the shortest supply relative to plant needs.  An analogous situation would be if you were baking cookies.  Bringing in a lot more flour is not going to help you make more cookies if what you are short of is sugar.

Ecosystem Services

The concentration of CO2 in the atmosphere is still very small relative to the concentration of oxygen.  If it weren’t for some side effects then there wouldn’t be a problem.

The two big side effects are the changing the rate at which heat escapes the atmosphere and ocean acidification.  Increasing the CO2 concentration slows the rate of heat loss.  As the CO2 concentration in the atmosphere increases, more CO2 disolves into the water where it forms carbonic acid.  The water pH decreases.

Rather than just thinking of these things as undesirable side-effects we should be thinking about the impact of our actions on ecosystem services.  An ecosystem moves matter and energy around and provides them to the species living in that ecosystem.  It also provides an environment in which the species can exist.  For example shark living in the Gulf of Mexico receives services in the form of prey (energy and matter), O2 dissolved in water which its gills can take up, an environment with a suitable temperature and pH, and so on.  These ‘services’, which the shark requires to survive, are products of the ecosystem.

Similarly we rely on ecosystems for our survival.  Current ecosystems are providing us with an atmosphere, a supply of water, food, etc.  Past ecosystems are providing us with energy run our societies and to increase our food production.

Humans have been systematically out-competing all other species on the planet.  Every year a larger and larger supply of energy and nutrients moving through the world’s ecosystems is being diverted to human use.

We also move nutrients around at what are ecologically and evolutionarily ridiculously fast rates.  The sequestered carbon was lost from active cycling over hundreds of millions of years.  It is being returned in centuries.  These changes are too dramatic for most biological systems.

The problem is that our activities have placed a premium on some ecosystem services and ignored others.  We pour energy, water. and other nutrients into food production but ignore the negative effects of our actions on other services such as nutrient movement or soil stabilization.  We rely on functioning ecosystems for the air we breathe and the water we drink but we don’t really consider these services – they are just always there.  Like the rest of living world we rely on energy to do things and we put a lot of effort into obtaining energy.  But we also need an atmosphere with a live-able temperature range and an ocean with a live-able pH.-

The End