Biology 115, Lecture 23: Ecology I

Spring 1999, T. Irving; revised 04/2000 A.Howard

Goals of this section:

Students should:

Understand concept of succession and climax communities

Understand components of ecosystems and concept of a niche

Understand what is meant by food chains and food webs

Understand how energy flows through ecosystems and does not cycle

Understand concept of ecological pyramid

Understand difference between human and natural ecosystems

Why Study Ecology?

• News is full of concerns such as toxic waste elimination, ozone layer depletion, global warming.
• All can be linked to human activities and have potentially serious consequences for human life on the planet
• These concerns all fall within the purview of academic discipline called ecology
• Study of genetics leads to a deeper appreciation of evolution and human health and welfare.
• Understanding of the basic biological principles of Ecology may lead to a deeper appreciation and true understanding of societal concerns.
• Can also lead to a richer and deeper appreciation of nature

Useful Definitions:

Ecology is sometimes used as a synonym for "environment" but it is not the same thing

• Population
  Members of the same species occupying a common geographic area
• Communities
  Populations of many species within a common geographic area
• Ecosystems
  Communities (biotic) and inorganic environment
• Biosphere
  Sum of all the world's ecosystems
• Ecology
  Branch of biology which studies interrelationships in communities and ecosystems
• Habitat
  Place where an organism can be found e.g. under rocks, sea cliffs
• Ecological Niche
  The "profession" or role of an organism in a community including its interactions with the physical environment

Succession

• Communities of plants and animals will replace each other in a predictable sequence in time until a stable or climax community is established
• Climax communities are typical of a particular region
• Climax communities are stable because there is a balance between predation and parasitism, the amount of energy that enters and the matter that cycles is "appropriate."
• Eastern US a hardwood forest, Midwest tall grass prairie, Southwest semi-desert
• Primary succession is the sequence of events by which originally bare rock becomes capable of sustaining many organisms
• Secondary succession is observed when climax communities have been disturbed and is allowed to re-establish; e.g. abandoned farmland

Energy and Energy Flow

• No conversion from one energy state or from one form to another is ever 100% efficient
• Energy is always lost as heat (2nd law of thermodynamics)
• True for organisms, organic processes, biosphere as a whole
• All processes in the biosphere requires conversion of energy from one from to another
• Eventually all energy which enters biosphere (photosynthesis) will be dissipated as heat
• Biosphere must have a constant external source of energy to make up the loss of heat with each conversion
• The ultimate energy source is the Sun

Sun's energy combines with elements from the earth's crust to form biological molecules

• Organisms in the biosphere use assemble pre-existing elements in simple forms into complex biological macromolecules (DNA, RNA, Proteins, Lipids etc.)
• Elements originated in the Earth's crust
• There are no new sources of elements
• Elements must recycle through the biosphere

What happens to the food eaten by an organism?

• Some goes to keeping the organism alive (breathing, movement etc.)
• Some is lost as heat (2nd Law Thermodynamics)
• Some not used as all (feces)
• Only about 10% becomes "new organism"
• Principles can be applied to an entire ecosystem

More Definitions

• Producers
  • Green plants, algae, some bacteria
  • Synthesize complex organic compounds from simple inorganic ones using energy from sunlight (i.e. make their own food)
• Consumers
  • Animals mostly
  • Get food by eating other organisms (Are any plants consumers?)
  • Primary consumers eat producers (herbivores)
  • Secondary consumers eat primary consumers
  • Tertiary consumers eat secondary consumers
  • Secondary and tertiary consumers are both carnivores
  • Omnivores (like us) eat producers and consumers
• Decomposers
  • Insects, fungi and many bacteria
  • Break down dead organic material and "eat" that
  • Dead material becomes simpler inorganic forms for re-use by producers

Energy Flow and "Energy Pyramids"

• Sun ---> producers ---> primary consumers--> secondary consumers --> tertiary consumers --> decomposers --> outer space eventually
• At each step energy is lost as heat and some as dead tissue and feces some of which gets used by decomposers
• Thus at each step from producers to tertiary consumers, there is decreasing energy that can be converted to biomass (live tissue) at each level
• This can be described by an "energy pyramid" which shows the energy stored as biomass at each "trophic" (feeding) level
• As you go up the pyramid the number of individuals become fewer because there is less total energy available for biomass
• Secondary and tertiary consumers tend to get larger as well.
• Each ecosystem will have very few large carnivores (lions, cougars, foxes, owls etc.) and comparatively many primary consumers (insects, mice, rabbits etc.)

Energy Pyramid and Humans

• Humans have the ability to live as either Primary consumers (vegetarians) or as a mixture of primary, secondary, and tertiary consumers (omnivorous)
• Given a finite agricultural potential, fewer people can be supported as secondary and tertiary consumers than as primary consumers
• If we were all vegetarians, many more people could be supported worldwide
• Would that be a good thing?

Biomagnification

• As compounds enter at lowest trophic levels in an ecosystem that are not broken down or eliminated from organisms, they accumulate in the organism that consumes them.
• Such compounds will accumulate to successively higher amounts as you go to higher levels in the energy pyramid
• e.g. DDT ( a pesticide) has caused severe reproductive problems by accumulating the bodies of secondary consumers like eagles and brown pelicans
• Particularly serious for larger animals of a particular species
• Larger because they are older so have been accumulating a particular compound for a longer time and thus to higher levels
• Natural Resources people say you should limit your intake of fish from Lake Michigan, especially larger ones.

Food Webs

• In very simple communities there might be a linear relationship of "who eats whom" or a "food chain"
• In more complex communities/ecosystems the interrelationships between all species can be complex
• The overall relationship of who eats whom is called a food web
• Many organisms are both primary and secondary consumers
• Most have more than one food source
• As one goes to warmer climates, natural communities have increasing numbers of species (not entirely clear why) and food webs become increasingly complex
• The more complex a community, the more stable it is and less prone to large population swings

Monocultures

• In North America, we tend to grow our crops in monocultures i.e. only one species (and usually only one variety) of plants
• This is an artificial simplification of an ecosystem
• Inherently unstable
• Example:
  • Sawflies grow in wheat
  • If left unchecked can lower yields
  • Because of monoculture, natural predators of sawflies do not exist since alternative sources of food for the predators do not exist
  • Can only keep sawflies in check with heavy use of pesticides
  • Can reduce this type of problem by making the fields a more complex community by planting different crops close to each other and close to non-crop" fringe areas".

Human Ecosystem vs. Natural Ecosystem

• Natural ecosystems tend to be stable
• Populations tend to stay at roughly the same levels because of a balance between predation/parasitism and reproductive capacity
• Cycling of materials and energy use tends to be efficient
• If it wasn't efficient, the ecosystem would be replaced by a more efficient one
• Human populations are steadily increasing
• No effective predators except other people
• Parasitism (disease) is being steadily reduced as far as reproductive capacity concerned.
• Human food supplies steadily increasing because of more "efficient" agriculture
• Efficiency defined in terms of yield per acre but not in energy use

Humans have created two ecosystems

• The rural ecosystem ("the country")
• The rural ecosystem replaces diverse climax natural ecosystems with monocultures
• Maintaining the monoculture requires input of energy, largely from fossil fuels
• Also requires input of raw materials e.g. potash for fertilizers, wood and steel for barns etc.
• Gasoline for tractors, chemical fertilizers, pesticides
• The urban ecosystem (the city)
• Is not self sufficient
• Depends on the rural ecosystem for its food source and as a place to deposit wastes

Human activities are very wasteful

• Require a lot of energy (fuels) and a lot of raw materials (metal, wood, petroleum)
• Intensive processing of raw materials inevitably results in environmental pollution
• Pollutants defined as compounds in the environment that wouldn't naturally be there

Biodiversity

• Human activities are well on their way to producing mass extinctions not unlike the disaster that killed off the dinosaurs
• We are part of the overall food web and each member of the food web that is eliminated makes the entire web or community weaker/prone to large disruptions
• The human ecosystem is dependent on natural ecosystems to provide resources and absorb wastes.
• We are loading up the natural ecosystems with increasing waste heat, sewage, agricultural runoff, pesticides and solid wastes.
• Removal of natural ecosystems (e.g. logging tropical rainforests, replacing grasslands with agricultural monocultures) reduces the amount of biomass in natural ecosystems available
• Reduction of natural ecosystems results in overloading of those that remain.

Towards more "natural" human ecosystems

• Stability of natural ecosystems vs. unstable human ecosystems would suggest that humans should stop working against nature and try working more in harmony with the natural environment
• Try and achieve "zero population growth"
• Adopt more "natural" agricultural practices
• Conserve raw materials and energy
• Recycle non-fuel raw materials
• Reduce reliance on fossil fuels making more use of renewable energy resources
• e.g. burning methane from feedlot animal wastes, ethanol from plant wastes, finding clever ways to utilize waste heat
• Overall goal to make human use of energy and resources more "cyclical"