The Ecosystem and how it relates to SustainabilityIf you want me again, look for me under your boot-soles." - Walt Whitman In this lesson, we will learn answers to the following questions:
IntroductionIn the previous lectures we have learned about the Earth and its environment, and we have learned about the diversity of life on the planet and about ecological interactions between species. Now we will combine these two basic components and consider how the environment and life interact in "ecosystems". But before that we should return to a topic introduced at the very start of class, which is that of sustainability and how we view it in terms of system science. Sustainability and System Science - The example of sustainability used at the start of class was to consider that I give everyone a dollar each time you come to class. The question was: Is that sustainable? In lecture we agreed that more information was needed to answer that question. For example, we needed to know how much money do I have, or the “stock” of money (e.g., if there were 100 students in class and I had a stock of $100, this would work once...). What if I spend money on other stuff like food? What is the "input" or renewal rate or "turnover time" of money in my bank account, compared to how fast I consume money? What if the class size grows because class popularity increases? Right away we see that this is a “system” that has a balance point in it that depends on many other parts of the “system”. Solving this problem is an example of“systems thinking”, and we need to learn how to apply that to science and to problems of sustainability. Scientific Concepts, applied to ecosystems and to sustainability. Working through this simple example illustrates how complex the issue of sustainability can become. However, what we also find is that in all such problems there is a common set of key scientific concepts and principles that we will learn to understand in this course – these concepts include the following (there will be more specific examples given later on): Standing Stock = the amount of material in a "pool", such as the amount of oil in the ground or greenhouse gases in the atmosphere. "Standing" refers to the amount at the current time (like what is the stock of trees standing in the forest right now). Mass Balance = asking the question of "do the numbers add up?" If I need $100 each class to give to students, but I only have $1, then the mass balance is off. We can also use a mass balance equation to determine how a system is changing over time (we will do this in a later lecture for heat-trapping gases in the atmosphere). Material Flux Rate = the input or output of material from a system, such as the amount of oil we pump out of the ground each year, or the amount of greenhouse gas we pump into the atmosphere each year by burning fossil fuels. Residence Time = the standing stock divided by the flux rate, which provides the average time that materials spent circulating in a pool - for example, the residence time of methane in the atmosphere is about 10 years. Negative and Positive Feedbacks = negative feedbacks tend to slow a process, while positive feedbacks tend to accelerate a process. For example, in a warming world the ice caps will melt, which reduces the Earth's albedo, we retain more of the sun's heat energy, and that accelerates warming which in turn melts more ice cap -- this is a positive feedback. What is an Ecosystem? 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. The two main processes that ecosystem scientists study are Energy transformations and biogeochemical cycling. 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 particular 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 (required for the production of new organic matter) are recycled in the system. Components of an Ecosystem
You are already familiar with the parts
of an ecosystem.
From this course and from general knowledge, you also 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".
By and large, this set of components and 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 EcosystemsThis 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 (that is, inorganic material such as carbon, nitrogen, and phosphorus, instead of those elements being bound up in organic matter). Often bacteria complete this process, through the process called decomposition or mineralization (see next 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 (the shorthand notation is "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" (this is a major take-home point of this lecture),
and it is important to understand what are the most important linkages
in any particular food web. The next question is how do we determine what the important processes or linkages are in food webs or ecosystems? Ecosystem scientists use several different tools, which can be described generally under the term "biogeochemistry". BiogeochemistryHow 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 (biology) 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 today 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. The ratio of number of atoms of these elements (referenced to 1 P atom) is as follows: 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 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.
(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. In earlier lectures this idea was also introduced and explained as a "trophic cascade".
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 toward the primary producers. 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 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 thus is somewhat broader geographically 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.
Review
of main terms and concepts in this lecture. All materials © the Regents of
the University of Michigan unless noted otherwise. |
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