Ecology Lectures Intro Biology 1102 Spring 2002

Transcription

1 Ecology Lectures Intro Biology 1102 Spring 2002 Ecology - Relevance to Today s Society Ecology is a relatively new science, only about 150 years old, and in reality, perhaps less than 100 in terms of doing experimental research. The early work was pretty much natural history and descriptive, not mechanistic. Since we only have three lectures left, I am going to concentrate on three major areas, all of them mostly applied ecology. They are: Ecosystem Ecology, Biodiversity, and Overpopulation. These are all inter-related, and cover only a small fraction of the material you should know, but these subjects are all critically important to society today, and will be so for the foreseeable future, thus they will affect you and your children as well. wrote: I think John Sawhill, former President of The Nature Conservancy, said it best when he In the end, our society will be defined not only by what we create, but by what we refuse to destroy. Ecosystem Ecology We can imagine an ecosystem as the sum total of all the creatures and the abiotic (non-living) portions of the landscape. This means that we are looking at the fluxes of elements, such as nutrients, and energy as affected by living organisms. Nutrients can be recycled, but not energy. Ecologists define ecosystems by their trophic levels: Base Level Autotrophs Plants (primary producers) Next Heterotrophs Animals that consume plants (primary consumers) Next Heterotrophs Secondary consumers Next Heterotrophs Tertiary consumers Detritivores Animals that consume dead material (Fungi, bacteria, dung beetles, etc.) Source of Energy The ultimate source of energy is the sun. About 1% of the sun s energy is used to convert sunlight into chemical energy through the process of photosynthesis. Some of the sun s energy is used to melt snow, move ocean currents, etc. Even with only 1% of the sun s energy, that is enough to produce 170 B metric tons of plant material per year. Annual Productivity of the Earth Plant productivity known as annual net primary productivity (ANPP). Total productivity would also include energy used in respiration, and is gross primary productivity. The relationship between the two is: ANPP = GPP + R But respiration releases energy off as heat and as CO2. What s left is the net primary

2 productivity. For most systems, ANPP is about 50-60% of GPP. Productivity by Ecosystem (g m -2 yr -1 ) Tropical Rainforests 2200 Savannah 900 Temperature Deciduous Forest 1200 Tundra 200 Coral Reefs 2500 Productivity a function of T, rainfall, nutrients, and seasonality. This represents the amount of energy stored in plant biomass each year. That is energy available to consumers. But as food moves up the trophic levels, energy is lost due to various processes, such as heat, indigestibility, feces, etc. What ends up in the consumer is only a small fraction of what is consumed. Raymond Lindemann was the first to work out these relationships, after studying food chains (actually webs) in a small pond, back in the 1940's. Here is an example of how energy is transferred from the leaves of a plant to a caterpillar: Energy Content in Joules Percent Caterpillar eats 48 gms of leaves % Amount consumed (edible parts) % Respiration uses 2/3 of absorbed energy 67 Remaining for growth 33 16% Generally, as food moves up trophic levels, approximately 90% of the energy is lost. That means that if we start with 100 J of energy, by the time it gets into a bird, only 1% of the energy of the original plant material remains. Obviously, this constrains how many food levels one can have - most food chains are 4-6 lengths long at most. See below: This also limits the number of higher order consumers - can t have as many lions as mice!! Number of organisms Trophic Level 3 Tertiary consumer 355,000 Secondary consumer 710,000 Primary consumer 5, 800,000 Primary producer Implications of Food Web and Trophic Energy Transfer Obviously, the more links food goes through, the less there is available for the next trophic level. If we eat lower down on the trophic chain, there will be more energy available. Thus, eating vegetarian will stretch the food over more people than eating meat. This has important implications for agriculture - raising cows for meat consumes a lot of energy and less is available for consumption by humans. China is now largest consumer of pork, and as a result, can no longer raise enough grain to feed itself, whereas before, when Chinese ate lower on the trophic scale, they were self-sufficient.

3 Elemental Cycling When ecosystems function to cycle elements, we often call this biogeochemical cycling. Plants, in fact, are the largest miners of minerals in the world. They routinely extract from the ground nearly 10X what humans do, and they do it in pure form every day!! mainly Some elements cycle as gases, such as SO2, CO2, and N. Others cycle primarily as solids, dissolved in water. component ecosystem Soil Major s of an include: Living Plants Dead Material Atmosphere Water We can better understand some of these cycles by studying individual elements. Let s look at three such cycles: C, N, and water. Carbon has a complex cycle - it can be taken out of the atmosphere by photosynthesis, stored in living biomass, or dissolved in the oceans, where some is deep-sixed to great depths. There, it may remain for many millions of years. Carbon is also released whenever fossil fuels are burned. See cycle below: Man s been altering the activities have C cycle. The

4 burning of fossil fuels, as well as the clearing of forests, is putting more C in the air than can be taken out by photosynthesis or dissolution in the ocean. Charles Keeling began measuring atmospheric carbon dioxide in the late 1950's in Mauna Loa, HI, and the measurements continue to today. We can see that it is rising rapidly, at about 2-3 ppm per year. The jiggles in the line reflect the seasonal differences in atmospheric carbon dioxide - going down in the summer, and up in the winter. What has been the historical trend? The graph below, obtained from ice core data (ice cores can trap bubbles of air and preserve them for thousands of years) shows that this increase is a fairly recent phenomenon, mainly associated with the industrial revolution. Now, you might wonder, so what? Is there any reason to be concerned about this rising carbon dioxide? Look at the next graph. It shows the correlation between changes in atmospheric carbon dioxide and global mean temperatures.

5 Here, it is very easy to see that whenever carbon dioxide goes up, so does global mean temperatures. Thus, carbon dioxide is the atmospheric gas of most concern with regards to rising global temperatures. However, other gases, such as methane, and nitrous oxides, also contribute to global warming, but carbon dioxide is the one of most concern. We see very clearly that humans are altering the global mean temperature and the cycling of C. Even if we were to stop putting C into the atmosphere today, the time lags of mixing are such that the concentration would continue to rise for several decades afterwards. The Nitrogen Cycle Nitrogen cycles similarly to C, but has several other pathways not available to C. For example, there are organisms, particular bacteria, that can fix N out of the atmosphere into forms used by plants. The main forms of N are: N=N or N 2, nitrate (NO 3 -), ammonium ion (NH 4 +), and urea. Plants can t fix N out of the air, but legumes have bacteria in their roots that can. Humans have been altering the N cycle by adding fertilizers to agricultural fields. Normally, growth in most ecosystems is strongly limited by N. Now, N is less limiting, and there is greater cycling than there used to be. In addition, NOx, added to the atmosphere by burning fossil fuels, contributes to acidic deposition. This is converted to nitric acid, which then falls to the ground, and contributes to acidification of lakes and streams, and forests. An diagram of the N cycle is shown on the next page.

6 N al rces of N include lightening, which can add nitrate to ecosystems. Bacteria can also release N by a process called denitrifying, which turns nitrate N or ammonium N back into N 2. When too much N enters an ecosystem, it causes N saturation. When more comes in than can be taken up by the plants, the only place for it to go, if not into the atmosphere, is the stream water. This can cause nitrates to build up in the water supply, which is toxic to humans. atur sou Water Cycle The global water cycle is fairly simple. The drawing below shows the major pathways. Water will most likely become the most limiting element constraining further growth of the human population. Plants can substantially alter the

7 global water cycle. In the Amazon, for instance, nearly 50% of the water that falls on the forest is transpired comes from the trees below. Eliminating the trees greatly reduces evaporation into the atmosphere, and can lead to drying out of the ecosystem. Humans currently use nearly 60% of all available freshwater. How will we deal with the situation when the population doubles, if it can? How To Study Ecosystems Hubbard Brook - Herb Bormann and Gene Likens (see overheads) Hubbard was one of the first great ecosystem studies, done by two ecologists, Herb Bormann from Yale, and Gene Likens from Cornell. They studied how nutrients and energy are moved through and around a forested ecosystem in New Hampshire. With a host of graduate students and collaborators, they followed nutrients entering, being stored, and leaving the Hubbard Brook watersheds, a USFS site. Their strategy was this: Nutrients Entering - Nutrients Stored = Nutrients Leaving (in stream water) So, they measured the nutrients coming in the precipitation, how much was being stored in the forest each year, and the concentration going out in the stream water. The idea was that since the forests were underlain by solid granite, no water could leach out except in the streams. So it was a tight system. What did they find? There were many findings. Of importance were: 1. ph of the rain coming in was acidic. It was Herb Bormann and colleagues who coined the term acid rain. 2. Forests were crucial to the cycling of nutrients. When trees were cut, the ecosystem could not retain nutrients. They were washed out in the stream water. 3. As forests mature, their ability to retain nutrients varies - young forests are good at it, but as they mature, ecosystems tend to get leaky - growth slows down, and uptake of nutrients does also. Comparison of Temperate and Tropical Rainforest Nutrient Cycles Tropical rainforests cover about 6% of the earth s surface (about the size of the lower 48 states in the US). But they may contain nearly 50% or more of the world s species. Very important ecosystems. But they are being chopped down at alarming rates - nearly a football field each second, or 1% per year. Or the equivalent of half of Florida per year. Some think the rates even higher, at 2% per year, or all of Florida per year. Ecuador, Venezuela, and Bolivia, as well as Brazil, have very high deforestation rates. Already, 14% of the Amazon has been destroyed in one way or another, and most of it will likely disappear in your lifetime. What was the historical use history of the Amazonian rainforest? How did it persist for so long without being destroyed? In large part, it was because it was used up at a rate that balanced it s ability to regenerate. Let s look at how indigenous peoples used the Amazon. Slash and Burn Indigenous peoples used to burn a small area, maybe 1-2 hectares in size. Then, they planted native crops, like manihot (a starchy crop). The burning turned the downed trees to ash, which acted like a fertilizer. This promoted crop growth for a few years. But then, due to the high rainfall, the

8 ash was leached out of the soil, and crop productivity dropped. Since the soils were low in nutrients to begin with, further production was fruitless. So, the natives moved to another portion of the forest, and repeated the process. This nomadic life was necessary in this ecosystem. They would not return to an area for at least a century or more. This gave the cutover forest time to recover it s nutrient capital. But today, due to the loss of available forest land, they are returning at less than 75 year intervals, and the forest has not had a chance to recover fully. This is slowly wearing down productivity in the forest. Farmers who raise cattle in the Amazon do so only by applying large amounts of fertilizers to the land -otherwise the forest would encroach back in, or, the pasture productivity without fertilizers would be so low as to not be able to sustain cattle on it. Thus, the plants in the Amazon are used to low nutrients in the soil - how do they deal with it? Faced with high rainfalls, one might suppose that all the nutrients would be leached out of the system. But the plants have many adaptations that retain the nutrients in their bodies, and keep them from being leached out. Comparison of Temperate and Tropical Nutrient Pools Temperate Tropical 40-60% nutrients in soil 20-30% in soil 60-40% nutrients in plants 70-80% in plants moderate rainfall moderate productivity low leaching rates Recycle rates slow to moderate Efficiency of recycling moderate very high rainfall very high productivity very high potential leaching rates recycle rates very high Efficiency of recycling very high Adaptations to reduce leaching: 1. roots above ground, called a root mat or aerial roots 2. high uptake efficiency - take up nutrients at low concentrations 3. drip tips on leaves - dries leaves out so nutrients don t leach from leaves 4. mycorrhizae - very efficient in tropics. Figure to left shows mycorrhizal roots Figure to right shows aerial roots