Ecosystems (Ecology is the study of ecosystems)
ECOSYSTEMS ARE OPEN SYSTEMS
Ecosystem – The interactions between all of the biotic components (the community) and the abiotic
component (the habitat). The biotic components affect the abiotic components (oxygen in the
atmosphere is a result of photosynthesis; the water cycle is strongly affected by transpiration in plants,
etc.), and vice versa. The two (biotic and abiotic) are 100% interdependent.
Community (biotic) – ALL of the life forms within the system, from microbes to plants, animals, and
everything else… all means all. Producers are autotrophic… they make their own food (most are
photoautotrophs, a few are chemoautotrophs). Consumers get their energy molecules form producers
(herterotrophs). Decomposers are a sub-set of consumers… they recycle nutrients. Detriovores eat dead
stuff, and they are sometimes considered decomposers, but the ‘real’ decomposers are bacteria and fungi.
Habitat (abiotic) – temperature, moisture, minerals in the soil/water, atmospheric gasses, sunlight
exposure, etc…
ENERGY FLOWS THROUGH ECOSYSTEMS….MATTER IS RECYCLED.
Energy flow through ecosystems. Most ecosystems are based on solar energy (photosynthesis). The
photosynthesizers are PRODUCERS. Everything else gets its energy from another carbon source
besides making their own carbon-based fuel from CO2. As energy moves from one trophic level to the
next, most (90%) is lost. This can be attributed to the energy organisms use as they live, and to the 2 nd
law of thermodynamics. The lost energy leaves the planet as heat and contributes to the entropy of the
universe. The sun is still working, so ideally, the energy in = the energy out. This might be out of
balance at the moment.
TROPHIC LEVELS (Energy levels)
The producers form the base trophic level of all ecosystems. Only 10% of the energy collected by
producers is passed to the next level, primary consumers. Only 10% of the energy in primary consumers
makes its way to the secondary consumers, and so on. From this we can calculate and/or infer the
following:
1. There is a limit to the number of trophic levels (about 4-10, with most systems having about
5).
2. The total biomass energy of at each level is decreased by 90% as it moves to the next level.
3. There are fewer individuals at each increasing level (especially in the consumers), thus…
4. Top level consumers are the fewest in number.
5. Tropic levels can be represented by an “energy pyramid” with top-level consumers at the top
level… duh.
Feeding (energy acquisition) can be viewed as either a food chain or a food web. Food webs more
accurately portray the complex feeding interactions in an ecosystem. Humans are top-level consumers.
We are at the top of the food chain. This is a good thing in my opinion.
Some species are more significant in ecosystems than others. Think of a house of cards… some cards
can be removed but the house will stand. Others are essentially holding the house of cards up. The most
critical species are called “keystone species.” When they go, the ecosystem may collapse. Humans are
NOT keystone species.
DIVERSITY is measured by the number of species in an ecosystem. As a general rule, the greater
the diversity, the more stable the system.
Succession: Like everything else, ecosystems evolve… they change over time. Especially when
something alters the ecosystem in a dramatic way.
When an ecosystem experiences a catastrophic event (fire, flood, volcanic eruption, etc..) many species
may be lost. This creates a “biological void.” Species rush in to fill the vacated niches. The recovery of
severely damaged ecosystem is called secondary succession. ‘Secondary’ because not everything was
killed off. Primary succession occurs when an ecosystem is obliterated and completely sterilized. The
eruption of Krakatoa in the early 1800s made an island disappear…. Poof. Magma rising from
underwater then created a new island that was completely devoid of life. This set the stage for primary
succession. Primary succession is rare. Secondary succession is common.
When an ecosystem evolves to a point where all of the interactions are in balance and sustainable, we
say the ecosystem has reached its climax state.
Species Interactions: The relationship between any two (or more) species in an ecosystem can usually
be identified. Here are the major examples:
1. Herbivory – an animal eats a plant.
2. Predatory/prey – an animal eats another animal
3. Symbiosis – a long-term, intimate relationship.
a. Parasitism – parasite/host
b. Mutualism – both partners benefit
c. Commensalism – one partner benefits, the other is unaffected. (Moss grows on trees…
the moss benefits from getting into the light, the treat is neither helped nor harmed.)
Niche: An organism’s niche is defined as the sum of its interactions with the biotic and abiotic
components of an ecosystem. Any organism’s niche is limited by competition from other organisms.
This is called competitive exclusion. All organisms compete in some way. You might even think of
competition as a category of interaction, but it is common to all organisms in one way or another.
Population growth: Since all organisms produce more offspring than can possibly survive owing to
competition, predation, herbivory, disease, etc… We can model population growth mathematically. If an
organism is placed in a habitat with (theoretically) unlimited resources and no competition, it’s growth
will be exponential. We call this exponential growth. At some point, the population size becomes selflimiting owing to competition – even from within. Exponential growth begins to level out. This is
logical. Therefore it is called logistic growth. If we look at the population growth curves of two species
in a predator/prey relationship, we see that it is cyclic. Human population growth is currently
exponential. Exponential growth is NEVER sustainable. The carrying capacity for any organism is the
population that the ecosystem can support indefinitely. It is a topic of debate, but many scientists
calculate that human population (currently between 6 and 7 billion) has already passed its carrying
capacity.
Cycles: Matter cycles, and cycles matter. You know about the oxygen/carbon dioxide cycle. You need
to also understand the nitrogen cycle and the water cycle (we have alluded to these earlier). All of these
go through a phase that involves atmospheric gases. The phosphorous cycle does not involve an
atmospheric stage. Phosphorous cycles through water and soil.