CHANGE AND STABILITY IN ECOSYSTEMS

CYCLING OF MATTER

The cycling of matter in an ecosystem is highly dependant on organic substances in living things.

Organic substances contain atoms of carbon and hydrogen (sometimes oxygen and nitrogen). The macromolecules that make up living things (carbohydrates, lipids, proteins and nucleic acids) are all examples of organic molecules.

Matter that does not contain carbon and hydrogen are considered inorganic substances. For example carbon dioxide, water and ammonia.

CYCLING OF ORGANIC MATTER

You are made up of billions upon billions of atoms that have been recycled throughout the universe. In essence you are as old as the universe. Most importantly the organic matter that once existed on Earth millions of years ago as part of an exotic plant, may in fact be a part of you right now.

Along with organisms being organic, the food that they eat to nourish their bodies is also organic. This is a major part of the cycling of organic matter. Another process involved in the cycling of matter is decay. After an organism dies, the stored organic material will eventually break down. This happens in part by the help of other organisms called decomposers. Decomposers break down organic matter into small inorganic molecules that pass into soil or water to become part of the living world. Examples of decomposers are, bacteria, fungi and worms.

Nature has been recycling forever, and very effectively.

THE CARBON CYCLE

The burning of fossil fuels in homes, cars and industries releases CO₂ into the atmosphere. Plants absorb the CO₂; photosynthesis allows them to use sunlight to break apart the CO₂; the O₂ is released into the air while the C collects in the plant's tissues as it grows. (The presence of C in plants is why they burn.) When animals eat plants, the C goes into them. Next, respiration occurring inside the animal combines the C it obtained from the plant tissues with O₂ it inhales from the atmosphere. When the animal exhales, it releases CO₂ back into the atmosphere. When an animal dies, it's C goes into the soil where decomposers turn it into CO₂.

Carbon is stored on our planet in locations called sinks. The major sinks are found:

1. as organic molecules in living and dead organisms found in the biosphere;

2. as the gas carbon dioxide in the atmosphere;

3. as organic matter in soils;

4. in the lithosphere as fossil fuels and sedimentary rocks deposits such as limestone, dolomite and chalk; and

5. in the oceans as dissolved atmospheric carbon dioxide and as calcium carbonate shells in marine organisms.

Carbon cycle 

Estimated major stores of carbon on the Earth.

Sink	Amount in Billions of Metric Tons
Atmosphere	578 (as of 1700) - 766 (as of 1999)
Soil Organic Matter	1500 to 1600
Ocean	38,000 to 40,000
Marine Sediments and Sedimentary Rocks	66,000,000 to 100,000,000
Terrestrial Plants	540 to 610
Fossil Fuel Deposits	4000

CARBON DIOXIDE AND THE GREENHOUSE EFFECT

For an excellent animation explaining the greenhouse effect refer to:

http://earthguide.ucsd.edu/earthguide/diagrams/greenhouse/

Since the Industrial Revolution, humans have greatly increased the quantity of carbon dioxide found in the Earth's atmosphere and oceans. Atmospheric levels have increased by over 30 %, from about 275 parts per million (ppm) in the early 1700s to just over 365 PPM today.

Scientists estimate that future atmospheric levels of carbon dioxide could reach an amount between 450 to 600 PPM by the year 2100. The major sources of this gas due to human activities include fossil fuel combustion and the modification of natural plant cover found in grassland, woodland, and forested ecosystems.

Emissions from fossil fuel combustion account for about 65 % of the additional carbon dioxide currently found in the Earth's atmosphere. The other 35 % is derived from deforestation and the conversion of natural ecosystems into agricultural systems. Researchers have shown that natural ecosystems can store between 20 to 100 times more carbon dioxide than agricultural land-use types.

HW p65 # 1-7

THE NITROGEN CYCLE

Nitrogen gas is the major component of our atmosphere, making up about 78% of it.

When certain soil bacteria absorb the N₂, they "fix" it, i.e., change it into NH₄⁺ (ammonium ion).

Lightning flashes also fix N₂ into NH₄⁺. As the NH₄⁺ builds up in the soil, other bacteria called nitrifying bacteria change the NH₄⁺ into the NO₂^- (nitrite) ion.

A second group of bacteria then change the NO₂^- into the NO₃^- (nitrate) ion. With the creation of the NO₃^- ion, the plants have nitrogen in a form they can use.

When grazing animals eat the plants, they absorb the NO₃^- ions. When they die, bacteria use the process of ammonification to change the N compounds in their tissues back into the NH₄⁺ ions that gather in the soil.

These NH₄⁺ ions will be recycled into the NO₃^- ions again. (And happens to these NO₃^- ions again?)

The cycle is completed when denitrifying bacteria change unused NO₃^- ions back into N₂ gas that goes up into the atmosphere.

refer to worksheet: the nitrogen cycle

HW p.69 # 1-4, 6,8,9,11, 13

THE WATER CYCLE

A picture is worth one thousand words!!!!

For a terrific animation on the water cycle go to:

http://earthguide.ucsd.edu/earthguide/diagrams/watercycle/

PESTICIDES

A pesticide is a chemical that kills organisms some consider "pests". Weeds, insects, fungi and rodents are common examples of pests. Someone labels an organism a pest when it interferes with their work or quality of life to an "unacceptable" degree.

Sometimes this label may seem arbitrary, e.g., certain weeds are beautiful plants except in a lawn, but there are real dangers posed by some pests.

Diseases and food shortages are the results of the activities of some insects and rodents.

First generation pesticides were a few chemicals that occurred in soils, rocks or plants.

Second generation pesticides are made by the thousands in laboratories. An improvement is their faster rate of decomposition but a downside is that some, but not all, target specific types of pests.

Have you ever accidentally sprayed bug repellent on your lips? Remember the burning and numbness? Should it have affected you or just the mosquitoes? And sometimes, pesticides get into areas where they are not wanted. What about spraying pesticides on weeds along the edge of the garden but wind drift carries some onto a bed of expensive roses and they all die?

Q. What organisms are killed by: a) insecticides b) herbicides c) fungicides d) bactericides?

A major problem with pesticides is bioamplification. This occurs when pesticide molecules are passed up the food chain as things get eaten.

At each new trophic level, the consumers accumulate more and more of the pesticides in their tissues because they eat so many prey from the lower level. As the concentration level increases, at some point the consumer will begin to suffer - perhaps reduced fertility, cancers, early death, metabolism changes, and deformations.

Will this issue become important to us only when we eat something with a large amount of accumulated pesticide? Also, what about the few organisms that survive the application of pesticide? Will they become resistant super-pests and multiply?

Q. Imagine you are sprayed with just a bit of pesticide and now have a "pesticide card". You get in your lab group. A predator comes along and eats all of you, collecting your cards. How much pesticide is in that person now? A predator from a higher trophic level eats all those people and collects all their cards. How much harmful chemical is in that organism now?

HW: in class case study 2.2 Complete a-s

THE FACTORS AFFECTING POPULATION GROWTH

There are 5 major trends in population growth:

1. slowly increasing population growth
2. rapidly increasing population growth
3. slowly decreasing population growth
4. rapidly decreasing population growth
5. stable population growth

The chart below describes the possible factors associated with these trends:

Increasing	Population	Decreasing	Population	Stable Population
Slowly	Rapidly	Slowly	Rapidly
Only a small number of organisms are sexually mature and able to reproduce Organisms may be adjusting to a new environment	Many organisms have reached sexual maturity and can reproduce (\higher birthrate) Organisms have adjusted to the conditions around them and have found food, water, and shelter for survival More immigration Good climate	Increased predators or toxicity in environment or lack of food (\higher death rate) Lower birthrate More emigration Climate has not been particularly favourable	Combination of several factors (such as the ones mentioned ¬ here) have occurred A serious disease has plagued the species (or its main food supply) Catastrophe in climate i.e. – drought, forest fire, etc. Over hunting	Carrying capacity has been reached The number of deaths equals the number of births Population is steady
“I.S.”	“I.R.”	“D.S.”	“D.R.”	“S”

Population Growth

A population is a group of individuals of the same species living in the same geographic area. The study of factors that affect growth, stability, and decline of populations is population dynamics. All populations undergo three distinct phases of their life cycle:

1. growth
2. stability
3. decline

Population growth occurs when available resources exceed the number of individuals able to exploit them. Reproduction is rapid, and death rates are low, producing a net increase in the population size.

Population stability is often proceeded by a "crash" since the growing population eventually outstrips its available resources. Stability is usually the longest phase of a population's life cycle.

Decline is the decrease in the number of individuals in a population, and eventually leads to population extinction.

Population Growth Patterns (yeast lab)

Nearly all populations will tend to grow exponentially as long as there are resources available. Most populations have the potential to expand at an exponential rate, since reproduction is generally a multiplicative process. Two of the most basic factors that affect the rate of population growth are the birth rate, and the death rate. The intrinsic rate of increase is the birth rate minus the death rate.

How can we express all of the above information as a mathematical formula?

Population growth = (births + immigration) – (deaths + emigration)

Two modes of population growth. The Exponential curve (also known as a J-curve) occurs when there is no limit to population size. The Logistic curve (also known as an S-curve) shows the effect of a limiting factor (in this case the carrying capacity of the environment).

Open and Closed Populations

Most natural ecosystem contain 4 factors (natality, mortality, immigration, emigration) acting on the population of an organism. This is called an open population.

Closed population – These are populations that would exist in laboratory environments or other controlled environments. Immigration and emigration are factors that may be controlled and manipulated. In other words, a population does not have external populations affecting its growth.

LIMITS ON POPULATIONS AND CARRYING CAPACITY

Carrying capacity is the largest population of a species that an environment can support.

4 FACTORS DETRMINE CARRYING CAPACITY

1. materials and energy – limited resources
2. food chains – the biomass of trophic levels below (prey) and above (predators) affect the biomass of any trophic level
3. competition – among members of the same species and between different species
4. density – the need for space, population density is the number of individuals living in an area

We could also represent the factors that limit populations in terms of biotic and abiotic factors. (see table 1 p. 78)

Biotic Potential – is the maximum number of offspring that a species could produce, if resources were unlimited

HW. Population Graphs worksheet (in class assignment)

DENSITY DEPENDENT AND INDPENDENT FACTORS

Density-dependent factors - in significance as a population grows

e.g. overcrowding causes increased spread of disease which limits a population.

Density-independent factors – affect a population but are not based on the amount of crowding in a population

e.g. forest fires can limit a population regardless of how many individuals there are.

 Refer to table 2 on p 79 on density dependent and independent factors

HW. p. 80 # 1,3,4

COMMUNITY INTERACTIONS

These interactions also affect population growth.

Symbiosis: all populations in a community must share natural resources therefore numerous interactions occur among different species.

Three Types of Symbiotic relationships

1. Mutualism: both organisms benefit e.g. aphids and ants, termites and protozoa
2. Commensalisms: one-sided relationship (benefits one organism; neutral to other e.g. suckerfish and shark, orchids and tree (support for orchid)
3. Parasitism: parasite, an organism that obtains its nutrients from the tissues of other living organism; called hosts e.g. athletes foot, mosquitoes, mistletoe on trees

Competition: two populations, needing the same resources, interacting in a way that affects their growth, perhaps even their survival (both species are harmed)

Predation: one species benefits/the other dies

Predator (owls) -consumer Prey (mice/young rabbits) – consumed

If we were to graph this relationship we would see that the graph has peaks and valleys

Predator prey relationships dry lab