An ecosystem consists of the biological
community that occurs in some locale, and the physical and chemical factors
that make up its non-living or abiotic environment. There are many examples
of ecosystems -- a pond, a forest, an estuary, a grassland. The boundaries
are not fixed in any objective way, although sometimes they seem obvious,
as with the shoreline of a small pond. Usually the boundaries of an ecosystem
are chosen for practical reasons having to do with the goals of the particular
study.
The study of ecosystems mainly consists
of the study of certain processes that link the living, or biotic, components
to the non-living, or abiotic, components. Energy transformations
and biogeochemical cycling are the main processes that comprise
the field of ecosystem ecology. As we learned earlier, ecology generally
is defined as the interactions of organisms with one another and with the
environment in which they occur. We can study ecology at the level of the
individual, the population, the community, and the ecosystem.
Studies of individuals
are concerned mostly about physiology, reproduction, development or behavior,
and studies of populations usually focus on the habitat and
resource needs of individual species, their group behaviors, population
growth, and what limits their abundance or causes extinction. Studies of
communities
examine how populations of many species interact with one another,
such as predators and their prey, or competitors that share common needs
or resources.
In ecosystem ecology
we put all of this together and, insofar as we can, we try to understand
how the system operates as a whole. This means that, rather than worrying
mainly about particular species, we try to focus on major functional aspects
of the system. These functional aspects include such things
as the amount of energy that is produced by photosynthesis, how energy
or materials flow along the many steps in a food chain, or what controls
the rate of decomposition of materials or the rate at which nutrients are
recycled in the system.
Components of an Ecosystem
You are already familiar with the parts
of an ecosystem. You have learned about climate and soils from past lectures.
From this course and from general knowledge, you have a basic understanding
of the diversity of plants and animals, and how plants and animals and
microbes obtain water, nutrients, and food. We can clarify the parts of
an ecosystem by listing them under the headings "abiotic" and "biotic".
|
ABIOTIC
COMPONENTS
|
BIOTIC
COMPONENTS
|
| Sunlight |
Primary
producers |
| Temperature |
Herbivores |
| Precipitation |
Carnivores |
| Water
or moisture |
Omnivores |
| Soil
or water chemistry (e.g., P, NH4+) |
Detritivores |
| etc. |
etc. |
|
All
of these vary over space/time
|
By and large, this set of environmental
factors is important almost everywhere, in all ecosystems.
Usually, biological communities include
the "functional groupings" shown above. A functional group
is a biological category composed of organisms that perform mostly the
same kind of function in the system; for example, all the photosynthetic
plants or primary producers form a functional group. Membership in the
functional group does not depend very much on who the actual players (species)
happen to be, only on what function they perform in the ecosystem.
Processes of Ecosystems
This figure with the plants, zebra,
lion, and so forth illustrates the two main ideas about how ecosystems
function: ecosystems have energy flows and ecosystems
cycle materials. These two processes are linked, but they are not
quite the same (see Figure 1).
Figure 1. Energy flows and material
cycles.
Energy enters the biological system
as light energy, or photons, is transformed into chemical energy in organic
molecules by cellular processes including photosynthesis and respiration,
and ultimately is converted to heat energy. This energy is dissipated,
meaning it is lost to the system as heat; once it is lost it cannot be
recycled. Without the continued input of solar energy, biological
systems would quickly shut down. Thus the earth is an open system
with respect to energy.
Elements such as carbon, nitrogen,
or phosphorus enter living organisms in a variety of ways. Plants obtain
elements from the surrounding atmosphere, water, or soils. Animals may
also obtain elements directly from the physical environment, but usually
they obtain these mainly as a consequence of consuming other organisms.
These materials are transformed biochemically within the bodies of organisms,
but sooner or later, due to excretion or decomposition, they are returned
to an inorganic state. Often bacteria complete this process, through the
process called decomposition or mineralization (see previous lecture on
microbes).
During decomposition these materials
are not destroyed or lost, so the earth is a closed system
with respect to elements (with the exception of a meteorite entering the
system now and then). The elements are cycled endlessly between their biotic
and abiotic states within ecosystems. Those elements whose supply tends
to limit biological activity are called nutrients.
The Transformation of Energy
The transformations of energy in
an ecosystem begin first with the input of energy from the sun. Energy
from the sun is captured by the process of photosynthesis. Carbon dioxide
is combined with hydrogen (derived from the splitting of water molecules)
to produce carbohydrates (CHO). Energy is stored in the high energy bonds
of adenosine triphosphate, or ATP (see lecture on photosynthesis).
The prophet Isaah said "all flesh
is grass", earning him the title of first ecologist, because virtually
all energy available to organisms originates in plants. Because it is the
first step in the production of energy for living things, it is called
primary
production (click here for a
primer on photosynthesis). Herbivores obtain their energy by consuming
plants or plant products, carnivores eat herbivores, and
detritivores consume the droppings and carcasses of us all.
Figure 2 portrays a simple food chain,
in which energy from the sun, captured by plant photosynthesis, flows from
trophic
level to trophic level via the food chain. A trophic
level is composed of organisms that make a living in the same way, that
is they are all primary producers (plants), primary
consumers (herbivores) or secondary consumers (carnivores).
Dead tissue and waste products are produced at all levels. Scavengers,
detritivores, and decomposers collectively account for the use of all such
"waste" -- consumers of carcasses and fallen leaves may be other animals,
such as crows and beetles, but ultimately it is the microbes that finish
the job of decomposition. Not surprisingly, the amount of primary production
varies a great deal from place to place, due to differences in the amount
of solar radiation and the availability of nutrients and water.
For reasons that we will explore
more fully in subsequent lectures, energy transfer through the food
chain is inefficient. This means that less energy is available
at the herbivore level than at the primary producer level, less yet at
the carnivore level, and so on. The result is a pyramid of energy, with
important implications for understanding the quantity of life that can
be supported.
Usually when we think of food chains
we visualize green plants, herbivores, and so on. These are referred to
as grazer food chains, because living plants are directly
consumed. In many circumstances the principal energy input is not green
plants but dead organic matter. These are called detritus food chains.
Examples include the forest floor or a woodland stream in a forested area,
a salt marsh, and most obviously, the ocean floor in very deep areas where
all sunlight is extinguished 1000's of meters above. In subsequent lectures
we shall return to these important issues concerning energy flow.
Finally, although we have been
talking about food chains, in reality the organization of biological systems
is much more complicated than can be represented by a simple "chain". There
are many food links and chains in an ecosystem, and we refer to all of
these linkages as a food web. Food webs can be very complicated,
where it appears that "everything is connected to everything else",
and it is important to understand what are the most important linkages
in any particular food web.
Biogeochemistry
How can we study which of these linkages
in a food web are most important? One obvious way is to study the flow
of energy or the cycling of elements. For example, the cycling of elements
is controlled in part by organisms, which store or transform elements,
and in part by the chemistry and geology of the natural world. The term
Biogeochemistry
is defined as the study of how living systems influence, and are controlled
by, the geology and chemistry of the earth. Thus biogeochemistry encompasses
many aspects of the abiotic and biotic world that we live in.
There are several main principles
and tools that biogeochemists use to study earth systems. Most
of the major environmental problems that we face in our world toady can
be analyzed using biogeochemical principles and tools. These problems include
global warming, acid rain, environmental pollution, and increasing greenhouse
gases. The principles and tools that we use can be broken down into 3 major
components: element ratios, mass balance, and element cycling.
1. Element ratios
In biological systems, we refer to
important elements as "conservative". These elements are
often nutrients. By "conservative" we mean that an organism can change
only slightly the amount of these elements in their tissues if they are
to remain in good health. It is easiest to think of these conservative
elements in relation to other important elements in the organism. For example,
in healthy algae the elements C, N, P, and Fe have the following ratio,
called the Redfield ratio after the oceanographer who discovered
it:
C : N : P : Fe = 106 : 16 : 1
: 0.01
Once we
know these ratios, we can compare them to the ratios that we measure in
a sample of algae to determine if the algae are lacking in one of these
limiting nutrients.
2. Mass Balance
Another important tool that
biogeochemists use is a simple mass balance equation to describe the state
of a system. The system could be a snake, a tree, a lake, or the entire
globe. Using a mass balance approach we can determine whether the system
is changing and how fast it is changing. The equation is:
NET CHANGE = INPUT + OUTPUT +
INTERNAL CHANGE
In this equation the net change in
the system from one time period to another is determined by what the inputs
are, what the outputs are, and what the internal change in the system was.
The example given in class is of the acidification of a lake, considering
the inputs and outputs and internal change of acid in the lake.
3. Element Cycling
Element cycling describes where and
how fast elements move in a system. There are two general classes of systems
that we can analyze, as mentioned above: closed and open systems.
A closed system refers
to a system where the inputs and outputs are negligible compared to the
internal changes. Examples of such systems would include a bottle, or our
entire globe. There are two ways we can describe the cycling of materials
within this closed system, either by looking at the rate of movement or
at the pathways of movement.
-
Rate = number of cycles /
time * as rate increases, productivity increases
-
Pathways-important because
of different reactions that may occur
In an open system there are
inputs and outputs as well as the internal cycling. Thus we can describe
the rates of movement and the pathways, just as we did for the closed system,
but we can also define a new concept called the residence time.
The residence time indicates how long on average an element remains within
the system before leaving the system.
-
Rate
-
Pathways
-
Residence time, Rt
Rt = total amount of matter
/ output rate of matter
(Note that the "units" in this
calculation must cancel properly)
Controls on Ecosystem Function
Now that we have learned something
about how ecosystems are put together and how materials and energy flow
through ecosystems, we can better address the question of "what controls
ecosystem function"? There are two dominant theories of the control of
ecosystems. The first, called bottom-up control, states that it
is the nutrient supply to the primary producers that ultimately controls
how ecosystems function. If the nutrient supply is increased, the resulting
increase in production of autotrophs is propagated through the food web
and all of the other trophic levels will respond to the increased availability
of food (energy and materials will cycle faster).
The second theory, called
top-down
control, states that predation and grazing by higher trophic levels
on lower trophic levels ultimately controls ecosystem function. For example,
if you have an increase in predators, that increase will result in fewer
grazers, and that decrease in grazers will result in turn in more primary
producers because fewer of them are being eaten by the grazers. Thus the
control of population numbers and overall productivity "cascades" from
the top levels of the food chain down to the bottom trophic levels.
So, which theory is correct? Well,
as is often the case when there is a clear dichotomy to choose from, the
answer lies somewhere in the middle. There is evidence from many ecosystem
studies that BOTH controls are operating to some degree, but that NEITHER
control is complete. For example, the "top-down" effect is often very strong
at trophic levels near to the top predators, but the control weakens as
you move further down the food chain. Similarly, the "bottom-up" effect
of adding nutrients usually stimulates primary production, but the stimulation
of secondary production further up the food chain is less strong or is
absent.
Thus we find that both of these controls
are operating in any system at any time, and we must understand the relative
importance of each control in order to help us to predict how an ecosystem
will behave or change under different circumstances, such as in the face
of a changing climate.
The Geography of Ecosystems
There are many different ecosystems:
rain forests and tundra, coral reefs and ponds, grasslands and deserts.
Climate differences from place to place largely determine the types of
ecosystems we see. How terrestrial ecosystems appear to us is influenced
mainly by the dominant vegetation.
The word "biome" is used to describe
a major vegetation type such as tropical rain forest, grassland, tundra,
etc., extending over a large geographic area (Figure 3). It is never used
for aquatic systems, such as ponds or coral reefs. It always refers to
a vegetation category that is dominant over a very large geographic scale,
and so is somewhat broader than an ecosystem.
Figure 3: The distribution
of biomes.
We can draw upon previous lectures
to remember that temperature and rainfall patterns for a region are distinctive.
Every place on earth gets the same total number of hours of sunlight each
year, but not the same amount of heat. The sun's rays strike low latitudes
directly but high latitudes obliquely. This uneven distribution of heat
sets up not just temperature differences, but global wind and ocean currents
that in turn have a great deal to do with where rainfall occurs. Add in
the cooling effects of elevation and the effects of land masses on temperature
and rainfall, and we get a complicated global pattern of climate.
A schematic view of the earth shows
that, complicated though climate may be, many aspects are predictable (Figure
4). High solar energy striking near the equator ensures nearly constant
high temperatures and high rates of evaporation and plant transpiration.
Warm air rises, cools, and sheds its moisture, creating just the conditions
for a tropical rain forest. Contrast the stable temperature but varying
rainfall of a site in Panama with the relatively constant precipitation
but seasonally changing temperature of a site in New York State. Every
location has a rainfall- temperature graph that is typical of a broader
region.
Figure 4. Climate patterns
affect biome distributions.
We can draw upon plant physiology
to know that certain plants are distinctive of certain climates, creating
the vegetation appearance that we call biomes. Note how well the distribution
of biomes plots on the distribution of climates (Figure 5). Note also that
some climates are impossible, at least on our planet. High precipitation
is not possible at low temperatures -- there is not enough solar energy
to power the water cycle, and most water is frozen and thus biologically
unavailable throughout the year. The high tundra is as much a desert as
is the Sahara.
Figure 5. The distribution
of biomes related to temperature and precipitation.
Summary
-
Ecosystems are made up of abiotic (non-living,
environmental)
and biotic components, and these basic components are important to nearly all
types of ecosystems. Ecosystem Ecology looks at energy transformations and biogeochemical
cycling within ecosystems.
- Energy is continually input into an
ecosystem in the form of light energy, and some energy is lost with each
transfer to a higher trophic level. Nutrients, on the other hand, are recycled
within an ecosystem, and their supply normally limits biological activity.
So, "energy flows, elements cycle".
- Energy is moved through an ecosystem
via a food web, which is made up of interlocking food chains. Energy is
first captured by photosynthesis (primary production). The amount of primary
production determines the amount of energy available to higher trophic
levels.
- The study of how chemical elements cycle
through an ecosystem is termed biogeochemistry. A biogeochemical cycle
can be expressed as a set of stores (pools) and transfers, and can be studied
using the concepts of "stoichiometry", "mass balance", and "residence time".
- Ecosystem function is controlled mainly
by two processes, "top-down" and "bottom-up" controls.
- A biome is a major vegetation type extending
over a large area. Biome distributions are determined largely by temperature
and precipitation patterns on the Earth's surface.
Review and Self Test
-
Review
of main terms and concepts in this lecture.
-
Self-Test for this lecture.
Suggested Readings:
-
Borman, F.H. and G.E. Likens. 1970.
"The nutrient cycles of an ecosystem." Scientific American, October
1970, pp 92-101.
-
Wessells, N.K. and J.L. Hopson. 1988.
Biology. New York: Random House. Ch. 44.
All materials © the Regents of
the University of Michigan unless noted otherwise.
