In these last two lectures of the semester, I wish to concentrate on human ecology, especially the impacts that humans have had on natural ecosystems.
Human impacts on natural ecosystems and the biosphere have intensified as the human population has grown in size -- a logical place to start our discussion is with human population growth.
Currently, the world human population stands at just over 6 billion people and is increasing at a rate of 1.6% per year -- may not sound like much of growth rate, but consider these facts:
1. at this rate of growth, the world’s population will double in only about 47 years.
2. this growth rate means that about 90 million additional people are being added to the world’s population each year = the population of Mexico.
3. the world’s population presently grows by about 250,000 people per day.
Given the current rate of growth, the world’s population will reach 7 billion by 2010 and nearly 8.4 billion by 2025.
It is revealing to look at the historical pattern of world population growth -- use a slightly different measure of population growth = doubling time.
Doubling time = number of years required for a population to double in size given its current rate of growth -- NOTE: small D.T. means rapid growth.
|
DATE |
EST. WORLD POP. |
DOUBLING TIME (YRS.) |
|
8,000 B.C. |
5 million |
1 million |
|
1650 A.D. |
500 million |
1500 |
|
1850 |
1 billion |
200 |
|
1930 |
2 billion |
80 |
|
1975 |
4 billion |
45 |
Notice that as the world's population has gotten larger it has grown faster and faster -- typical characteristic of exponential population growth.
Here’s what the past 300 years of human population growth looks like and some projections for the future.
Two events in human history have contributed to this pattern of exponential growth-- both these events significantly reduced death rates:
1. Agricultural Revolution -- ca. 8,000 years ago -- human population shifted from a hunter-gatherer mode of life to an agricultural one -- more stable way of life -- reduced risk of starvation.
2. Medical Revolution -- medical advances of this century that have greatly reduced death rates -- healthy people have healthy babies, etc. -- victories over tuberculosis, smallpox, cholera, malaria -- between 1940-1950, death rates declined 46% in Puerto Rico, 43% in China and 23% in Japan.
A doubling time of 47 years means that energy production, construction of houses, schools, etc., food production will also have to double every 45 years just for us to keep up -- even assuming we can keep up, the quality of life will be eroded due to various problems stemming from crowding, pollution, etc.
Recall our earlier discussion of population growth -- one of the fundamental laws of population growth is that no population can continue to grow exponentially forever -- there are limits to growth and the environment imposes a carrying capacity on all populations -- as carrying capacity is approached, death rates go up to equal birth rates and population growth stops.
A reasonable question to ask is: what is the carrying capacity of the world’s population? This is a difficult question to answer.
Some would argue that we are already approaching carrying capacity citing the fact that it is estimated that 1 billion of the world’s people today (ca. 20%) are malnourished or starving.
On the other hand, optimists argue that as of yet undiscovered technologies will allow us to "sidestep" the various kinds of limits that determine carrying capacity -- this has happened in the past -- e.g. agriculture, energy, medicine.
A central question is whether or not we want the environment to set the carrying capacity by bringing death rates up to match birth rates ("death rate solution") or whether we as a species through family planning, education, etc. can bring the birth rate down to meet the death rate ("birth rate solution") -- the latter certainly seems more humane.
Obviously, as the world’s population continues to grow, more natural resources, many of which are nonrenewable, will be removed and more wastes (e.g. pollution) will be produced -- these activities are having and will continue to have significant impacts on the ecosystems around the world -- some effects have been on and will continue to be on local ecosystems local, while others will be global -- let's look at some of these impacts.
Recall my diagram of a model ecosystem --- trophic levels connected by energy flow and cycling of nutrients -- perturbation of one level affects all others --- our use of pesticides really illustrates the connectedness of ecosystems and how disturbance at one level can influence the entire ecosystem
Pesticides -- chemicals to kill and thereby "control" undesired plants (e.g. weeds) and animals (e.g. insects) -- used by man on agricultural ecosystems -- notice that in normal ecosystems, "pests" are controlled by their natural predators and competitors -- however, in agricultural ecosystems, these natural competitors and/or predators have been eliminated, thus making it necessary to use pesticides to control them.
Types of pesticides:
1. chlorinated hydrocarbons -- e.g. DDT, Dieldrin -- notice that these are petroleum products.
2. organophosphates -- e.g. malathion, parathion.
Pesticides have several characteristics that cause them to damage ecosystems:
1. high stability -- chemically stable and tend to remain in ecosystems for long periods of time before they are broken down into less toxic compounds.
2. nonspecificity -- pesticides are generally toxic to a wide range of organisms in addition to the "target organism."
3. high mobility -- even with careful use, pesticides can be cycled through ecosystems much like nutrients.
4. biological magnification -- small concentrations administer to low trophic levels can be magnified to high concentrations in higher trophic levels -- concentrated in tissues -- also seen in heavy metals such as mercury.
Example: DDT applied to corn field
--- runoff from rain carries DDT into nearby lake and there it is
passed from trophic level to trophic level like regular nutrients
&emdash;- unlike regular nutrients, their concentrations are
magnified and they are stored in fatty tissue -&emdash; here are some
real data:
|
Trophic Level |
DDT Concentrations (ppm) |
|
Primary producers (phytoplankton) |
0.04 |
|
Primary onsumers (planktivorous fish) |
0.23 |
|
Secondary consumers (carnivorous fish) |
2.07 |
|
Tertiary consumers (fish-eating birds) |
13.8 |
High levels of DDT interferes with reproduction of large birds -- produce thin-shelled eggs that crack during incubation -- DDT has caused reproductive failures in eagles, ospreys, falcons, pelicans.
Alternatives to pesticides: biological control.
1. parasites -- e.g. Australian prickly pear and cactoblastosus
2. sterilization of males -- reproductive failure of pests.
3. pheromone traps -- attract pests to traps using chemical sex attractants.
Land use by humans is drastically altering natural ecosystems --
Deforestation -- forests around the world being destroyed for lumber, for fuel (1/4 earth population uses wood for fuel) and for agriculture -- deforestation is primarily occurring today in developing countries -- tropical rainforests are disappearing at an alarming rate -- each week a area of forest the size of Delaware is destroyed.
Recall that tropical rainforests are the most diverse of all ecosystems in terms of species diversity -- estimated that 1/2 of the world's species live in tropical rainforests-- destruction of forest impacts on all organisms that live there -- increasing extinction rates.
What happens when tropical rainforest is cleared for agriculture (e.g. "slash-and-burn" agriculture)? Soils are nutrient poor and only are productive for a couple of years -- removing vegetation also increases soil erosion because of the high rainfall these areas receive -- results in soil loss and increased sediments in watersheds -- may have negative impact on aquatic systems such as rivers, lakes and estuaries.
Desertification -- disruption of semi-arid grasslands due to overgrazing of livestock-- overgrazing results in removal of vegetation and exposing of soil to wind and water erosion -- grasslands become unusable desert -- 350,000 mi.2 of once usable grazing land along southern edge of the Sahara has been converted to desert in the past 50 years.
Western U.S. -- semiarid grassland -- much public land leased to livestock ranchers -- BLM reports that 70% of western rangeland is in poor or bad condition -- high rates of topsoil erosion due to overgrazing are increasing.
Pollution -- waste products of human activities are released into the environment with some serious consequences -- pollution effects our air, soil, and water.
Air Pollution -- stems primarily from combustion of fossil fuels and the release into the atmosphere of waste products such as particulates, HC, CO2, CO, NO, NO2, SO3 -- source may be industrial, autos, etc.
Photochemical Smog -- HC and NO react in presence of sunlight to produce ozone and PAN (peroxylacetyl nitrate) -- breathing ozone results in respiratory distress, headaches -- PAN damages plants -- local atmospheric conditions can make smog deadly -- in 1963, 400 people died in New York City when a thermal inversion trapped smog.
Acid rain -- sulfur and nitrogen oxides released into air are converted into sulfuric and nitric acids which return to earth in rainfall -- pH of rainfall normally about 5.6 -- rain falling now with pH 4.6 in central U.S. -- effects of acid rain include corrosion of metal and stonework, increased loss of soil nutrients through leaching -- most importantly, pH of lakes altered resulting "death" of the entire lakes across NE U.S. and Europe.
Greenhouse effect and Global Warming -- stems from increases of global carbon dioxide concentrations in the atmosphere -- 2 events important: increased carbon dioxide released by combustion of fossil fuels and decreased uptake of carbon dioxide by trees due to deforestation.
Prior to the Industrial Revolution, global CO2 was in a state of equilibrium -- releases of CO2 into the atmosphere by respiration and decomposition were balanced by removal of CO2 by photosynthesis -- estimated that CO2 concentrations in the atmosphere were about 270 ppm in 1850.
Since the Industrial Revolution, CO2 concentrations in the atmosphere have been increasing due to the combustion of fossil fuels and the burning of enormous quantities of wood removed by deforestation.
CO2 concentrations are presently about 350 ppm and increasing -- increased by 11% in the last 30 years -- at current rates, it is estimated that global CO2 concentrations will have doubled from the pre-Industrial Revolution levels by 2075.
What are the likely consequences of increasing CO2 concentrations in the atmosphere?
One prediction might be that more will allow increased photosynthesis and productivity by the earth’s vegetation -- in experimental chambers, it has been shown that plants do respond to elevated by CO2 -- however, increased plant production also means increased respiration and the release of CO2 back into the atmosphere.
One consequence of elevated atmospheric CO2 is global warming -- CO2 and water vapor in the earth’s atmosphere absorb heat from sunlight -- elevated CO2 means more heat will be absorbed and the result will be an increase in global temperature -- the greenhouse effect.
Because of the complexity of the biosphere, it is difficult to accurately predict the effects of global warming -- subject to much debate by scientists.
Predicted that global temperatures may increase by 3oC-4oC by the end of the next century given the current rates at which CO2 concentrations are increasing -- an increase of only 1.3oC would make the earth warmer than at any time in the last 100,000 years.
Worst case scenario -- warming would be greatest near the poles, resulting in the release of water into the oceans by the polar ice caps -- models predict 300 foot rise in sea level -- flood coastal areas 100 miles in land -- New York, Los Angeles, Miami (and San Diego!) would be under water.
Another prediction is that global warming will drastically alter global weather patterns, especially patterns of precipitation -- important agricultural regions of the world such as the U.S. could become semi-arid -- impacts on food production.
Water -- important resource that we are polluting and using up at an alarming rate -- water for drinking, irrigation, industrial purposes and energy production.
Much of our drinking and irrigation water comes from underground aquifers -- recharged by rainfall and snowmelt -- very slow process -- water being removed faster than aquifers are recharging (= groundwater mining) -- e.g. in Tucson, water table has dropped 110' in 10 years and its population continues to grow.
As aquifers empty, ground above them dries and settles (= subsidence) -- structural damage to canals, buildings and underground pipes.
Reservoirs -- water also captured behind dams for use -- used for flood control, hydroelectric power, irrigation, drinking.
Disadvantages include the fact that reservoirs have limited lifetimes because they silt up -- water loss due to evaporation may be 10%-25% of capacity in arid regions (e.g. Lake Mead) -- ecosystems below dam are also impacted -- do not receive same amount of water or silt as before (e.g. Grand Canyon below Glen Canyon Dam and Lake Powell).
Both surface and underground water supplies are being contaminated by a wide variety of pollutants from human activities that are health hazards and disrupt natural ecosystems.
Cultural eutrophication -- inputs into lakes of nitrates and phosphates from detergents, fertilizers and sewage results in "algal blooms" -- algae die, sink and decompose -- oxygen used up and fish die -- more decomposition and less oxygen.
Other pollutants include pesticides (runoff), industrial chemical wastes, and radioactive wastes from mines, etc.
Next time: Conservation Biology (Last Lecture!!!!)