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ENV330 Module 2A AVP Transcript
Narrator: Let’s review some basic ecology. Levels of organization of nature, population dynamics,
ecosystems, ecological succession, and biogeochemical cycles.
Title: Levels of Organization of Nature
Slide Content: [an image depicting the levels of organization]
Narrator: Ecology is the study of the interdependencies among living organisms and between organisms
and their physical, that is, non-living, environment. Nature is made up of a hierarchy of nested systems,
each self-organizing and self-maintaining.
All organisms are made up of molecules which are made up of atoms. All matter, including the rocky
Earth, the oceans and the atmosphere, are made up of atoms. The complex molecules that make up
living things are also made up of atoms.
Those complex molecules, such as proteins, carbohydrates, fats and DNA, make up the structure of the
cells of which all organisms are comprised. All living things are made up of cells – the cell is the simplest
level of organization of nature which is alive, that is, which is self-organizing, self-maintaining, able to
reproduce itself, senses its environment and responds to it, takes in high quality energy and materials and
releases low quality energy and wastes.
In multicellular organisms, cells are organized into tissues. Different tissues form layers which make up
organs. Different organs together form organ systems. Each organism is made up of many integrated
Groups of organisms of the same species which live in the same place are called populations.
Populations of organisms of different types which live together in one place are called biological
Ecosystems are self-organizing, self-maintaining groups of interdependent plants, animals, micro-
organisms and their physical environment, through which energy flows and materials recycle. A lake
ecosystem would include all the populations of fish, plankton, larger plants, the mud on the bottom — with
its myriad creatures including soil bacteria, protozoans, worms, mussels, larval insects – the water in the
lake, the sunlight entering the lake, the heat leaving the lake, the mineral nutrients in the mud and all the
water, etc., etc.
Ecosystems can make up huge sections of continents called biomes defined by similar terrains and
climates. In North America, the Temperate Deciduous Forest Biome extends from the Atlantic Ocean to
the Mississippi and from the Canada to the Gulf of Mexico.
All of the biomes of Earth – all of the living systems of Earth – taken together, make up the Ecosphere or
Biosphere. The Ecosphere operates as if it is a single system with complex feedback loops. The
Ecosphere of Earth, is like a thin, moist membrane on the rocky crust of the Earth and including the
oceans and lower atmosphere, it is almost as if it is alive! The very atmosphere of the Earth is the joint
exhalations of all of the life on Earth! This is not true on any other planet that we have observed. It is a
living blue-green jewel in the vast abyss of space.
[image of a drop of water on a leaf]
Narrator: Ecosystems are self-organizing, self-maintaining groups of interdependent plants, animals and
microorganisms – and their physical environment, dirt, air, water, light, etc. – through which energy flows,
and materials recycle.
High quality energy, sunlight, enters and powers the ecosystem, but is degraded due to the 2
Energy into low quality “waste” heat energy. (The 2
law of energy or thermodynamics, remember, states
that whenever energy is converted from one form to another, the total energy quality must decline).
Living organisms must have a dependable supply of high quality energy in order to stay alive. Plants use
high quality, concentrated sunlight energy, and animals and decomposers use high quality concentrated
chemical energy (food). Plants convert the high quality sunlight energy into high quality chemical energy
(sugar and other complex organic molecules). Animals eat plants or other animals that have eaten
The matter of an ecosystem is recycled around and around and around. The molecules that make up
your body have been part of dinosaurs, trees, rocks, the air, the water — and will be parts of other
creatures and the air and the rocks again. With each breath we exhale CO2 which will be taken in by a
plant and used to make their body. And, with each breath we take in O2 released by a plant to be used by
us to break down the sugars that we’ve taken in, by eating plants and animals, into energy that powers
We are connected at a deep and basic level to all the plants, animals and microorganisms of Earth – and
to the rocks and air and water!
Title: Food Chains and The Pyramid of Energy
[image of a leaf, grasshopper, mice, a snake and an owl]
Narrator: A food chain is the linear flow of energy and materials through one pathway of a food web.
Each link in a food chain is called a trophic level. Heat is “lost” from the food chain at each step due to
Law of Energy. The energy conversions that take place in every cell of every organism have a
cost – “there ain’t no such thing as a free lunch”! That is, every time energy is converted from one form to
another, as happens each second in every living cell, the energy quality always must decline. High
quality concentrated chemical energy stored in the body as sugar or starch is converted to low quality
“waste” heat energy which leaves the body and therefore is unavailable to creatures further along the
food chain. You can’t eat heat!
Question: is energy lost from the universe in this process?
No. The laws of conservation of mass and energy tell us that the total amount of energy – and matter – in
the universe is constant. Energy and matter can be converted from one form to another but can neither
be created nor destroyed. That’s the law!
The Pyramid of Energy: The total amount of biologically useable energy declines by about 90% at each
step in a food chain – because of the 2
Law of Energy. Therefore, food chains are never more than
about 4-5 links long because so little useable energy is left as you move along it. At each step in a food
chain most of the energy is converted into low quality “waste” heat energy, which is lost to the food chain.
So, in a food chain comprised of sunlight, grass, grasshoppers, mice, snakes and hawks, there will be
very few hawks, not very many snakes, quite a few mice, and lots of grasshoppers!
Question: Why can more people be fed if they subsist on a vegetarian diet?
Answer: Because the food chain is shorter and so less energy is lost as waste heat due to the 2
Livestock convert about 90% of the grain fed to them into waste heat energy which is lost to the food
chain. If people are fed the grain directly, skipping the livestock, 10-20 times as many people can be fed!
Title: Food Webs
[image of sea lions on the beach]
Narrator: Food webs are much more complex than food chains. They involve dozens or hundreds or
even thousands of interconnections – each organism eating many different kinds of food and each in term
being preyed upon by several different predators. Think about coral reef ecosystems or tropical rain
forest ecosystems. In such complex, interdependent systems, disruptions of any part of the system
effects all parts of the system and can result in surprising consequences.
For example, let’s consider the Antarctic food web, where all sorts of creatures migrate to take advantage
of the huge and rapid growth of plankton during the Antarctic summer. If lots of Killer Whales are
successfully hunted by humans, fewer Leopard Seals would be eaten by Killer Whales, so their numbers
would go up and they would eat more Crabeater Seals, Elephant Seals and Squid, reducing their
numbers. Emperor Penguins would be dramatically affected since one of their main sources of food is
squid. They would eat more fish, allowing the zooplankton – tiny microscopic animals — to increase in
numbers, perhaps clouding the water and cutting down on the amount of light that the microscopic
phytoplankton – plants at the base of the food web – receive. Those effects would ripple through the
Title: Ecological Succession
[image of a field of wheat]
Narrator: Ecosystems normally undergo successive stages of development over time, one ecological
community replacing another. Each stage changes the physical and ecological environment, thus
creating conditions that allow the next stage to develop. Each Biome has characteristic stages of
Primary succession starts from bare rock with Pioneer Species such as lichens and mosses, and
progresses ultimately to the Climax stage of development such as a mature hardwood forest. It can take
centuries or thousands of years for a system to go through all the stages. The early stages from bare
rock to lichens and mosses, for example, take the most time and are the most fragile.
Secondary Ecological Succession starts with soil and some plants and other organisms already present,
such as an abandoned farmer’s field. It may take centuries for the land to restore itself. But, Nature is
resilient — if a system is not too severely degraded it can recover over time.
Succession is a back and forth process. Disturbances such as storms, fires, floods, Wallmart parking
lots, halt the successional process, pushing the ecosystem back to a previous stage. Large ecosystems
will be spotty with different sections at different stages of succession due to their histories of fire, storm or
other disturbances. This spottiness actually increases the biological diversity of the larger system,
making it healthier. Some animals and plants thrive in the transitional boundaries between areas at
different stages – the ecotones, as these transitional areas are termed. Thus, within a forest there may
be meadows of grass where a fire took out trees years ago. Deer and other creatures move back and
forth between the two habitats.
Title: Population Dynamics
[image of a timeline of population sizes]
Narrator: Ecosystems are comprised of populations of organisms of particular species. Populations go
through stages as they develop. At first, a new population will grow exponentially while resources are not
limiting – its rate of increase increasing, its doubling time shortening. The ability of populations to grow so
rapidly is called their Biotic Potential.
Each ecosystem has a set Carrying Capacity for each population, the point at which the natural resources
that the species depends upon becomes limiting. This could be food, water, space, waste processing
ability of the micro-organisms in the soil, or many other factors. The carrying capacity is the maximum
number of individuals which can be sustained without degrading the ecosystem.
Following the initial period of rapid exponential growth, there is a period of adjustment to the realities of
the natural world – the logistic phase. In this phase, populations come into balance with the carrying
capacity. If they overshoot the carrying capacity, thus degrading the environment, their numbers go down
if and until the rest of the environment recovers and then their numbers may increase again to stable
Sometimes populations so exceed their carrying capacities, and cause so much ecosystem degradation
that they completely crash to zero or to very low numbers. For example, when 26 reindeer (24 of them
female) were introduced to a small Bering Sea island of St. Paul in 1910, lichens, mosses, and other food
sources were plentiful. By 1935, the herd size had soared to 2,000, overshooting the island’s carrying
capacity. This led to a population crash, and the herd size plummeted to only 8 reindeer by 1950.
Populations sometimes never recover if the damage is bad enough.
How about the human population? Current UN median world population projections, assuming that by
2050 women will have 2.0 children per lifetime, is that we will reach 9.3 billion people by 2050. The world
population is currently over 6.5 billion.
What kind of growth has the human population experienced in the last 50 years?
What is the Carrying Capacity for Humans on Earth?
Are there any signs that we have exceeded it?
Is there any environmental degradation or resource depletion evident caused by the human population?
What will happen to the human population if it has drastically exceeded the Earth’s carrying capacity for
Answers: The carrying capacity of Earth for humans depends on our lifestyles. An Indian peasant farmer
uses far less resources and creates far less waste than you or I do. They have a very small ecological
footprint whereas you and I have rather large ecological footprints. This concept is known as the Cultural
Carrying Capacity. The Earth can support far more people living simply than it can people living high
consumption, high natural capital degradation lifestyles.
It has been calculated that a population of perhaps 40 billion peasants eating a bowl of rice a day could
be supported on an Earth totally cleared of all ecosystems and devoted solely to growing grain. Raise
your hand if you volunteer for that lifestyle!
Is there evidence that humans have overshot their carrying capacity? Plenty – degraded air, water, soil,
ecosystems, climate, oceans — the ecological footprint of humans today is far greater than the planet’s
What could happen? Human populations could crash just like any other organism that exceeds its
carrying capacity. Many past civilizations have declined and disappeared due to resource abuse and
degradation of their natural environment.
In this course you will learn about ways to stabilize the human population and avoid a human population
crash, and create sustainable societies.
Title: Predator-Prey Relationship
[image of a lion]
Narrator: Populations within an ecosystem which directly depend upon each other affect each other
dramatically. When there are many lions hunting zebra, the zebra population declines, providing less
food for the lions, and so the lion population declines. With less lions, less zebra are hunted and so the
zebra population increases providing more food for the lions and thus allowing the lion population to
increase. Thus their populations directly affect one another.
In addition to this predator-prey relationship both populations are also affected by other populations of
organisms in their food webs. Everything is interconnected, or as Naturalist John Muir said almost a
hundred years ago, “When we try to pick out anything by itself, we find it hitched to everything else in the
End of Presentation