Food Chains and Food Webs
The sequence of organisms, each of which is a food source for the next, is called a food chain, and determines how energy moves through the ecosystem. As each organism uses the high-quality chemical energy in its food, this energy is converted into low-quality heat that flows into the environment (2nd energy law).
Each organism is assigned a feeding, or trophic level. Producers belong to the first trophic level, primary consumers to the second, secondary consumers to the third, and so on. Detritivores come from all trophic levels.
Real ecosystems are more complex and organisms form a network of feeding relationships called a food web.
In a grazing food web, energy flows from plants to herbivores (grazers), through many carnivores ending with decomposers (deep ocean). In detrial food webs, organic waste matter (detritus) is the major food source, and energy flows from plants to decomposers and detritivores (forests, streams, marshes).
A pyramid of numbers is an estimate of the numbers of organisms at each trophic level. The shape is a pyramid because there are typically many producers, to less primary consumers and even less secondary consumers. Each trophic level contains a certain amount of biomass, the weight of all organic matter contained in its organisms.
Biomass is transferred from one trophic level to the next, with some usable P4 energy lost in each transfer. The percentage of usable energy transferred to the next level varies from 5% to 20% (a loss of 80-95%), but 10% is usually used in calculation (90% loss). The pyramid of energy flow shows that the more trophic levels, the greater the accumulative loss of usable energy.
Earth can support more people if they eat at lower trophic levels (grains instead of grain eaters). The large loss of energy also explains why food chains rarely have more than four or five trophic levels, and why top carnivores are few in number and the first to suffer when systems are disrupted (eagles, tigers, sharks).
Productivity of Producers
The gross primary productivity is the rate at which producers capture and store chemical energy as biomass (photosynthesis to make plant material). The net primary productivity is what is left after producers use some of their biomass for their own respiration; available for use as food for consumers.
Net primary productivity is reported as the energy output in a given area over a given time (kilocalories per square meter per year). It is the "income" or rate at which energy is stored in new biomass. Earth 's total net primary productivity is the upper limit determining the planet's carrying capacity for all species.
An estimated 59% of Earth's annual net primary productivity takes place on land and the remaining 41% in oceans and other aquatic systems. Estuaries, swamps and marshes, and tropical rain forests are highly productive, open ocean, tundra (arctic grasslands), and desert are the least productive.
In tropical forests most nutrients are stored in vegetation rather than in the soil. When the trees are cleared the nutrient-poor soils are rapidly depleted of their nutrients by frequent rains and by growing crops. Thus food crops can be grown only for a short time without massive applications of commercial fertilizers. Humans now use or waste about 27% of the world's potential net primary productivity (40% for land systems).
MATTER CYCLING IN ECOSYSTEMS
Carbon is the basic building ofthe carbohydrates, fats, proteins, nucleic acids such as DNA and RNA, and other organic compounds necessary for life. The carbon cycle is based on carbon dioxide gas, which makes up almost 0.036% by volume of the troposphere and is also dissolved in water.
Producers absorb CO2·from the atmosphere or water and use photosymthesis to convert it into complex carbohydrates. Then the cells in oxygen- consuming producers and consumers carry out aerobic respiration, which breakdown glucose and athers complex organic compouncis and convert the carbon back to CO2 in the atmosphere or in water for reuse by producers. This linkage between photosynthesis in producers and aerobic respiration in producers and consumers circulates carbon in the ecosphere and is a major part of the global carbon cycle.
Carbon dioxide is the key component of nature's thermostat. If the carbon cycle removes too much CO2 from the atmosphere, Earth will cool; if the cycle generates too much, Earth will get hotter. Some carbon lies deep in the earth in fossil fuels-coal, pretroleum, and natural gas-and is released to the atmosphere as carbon dioxide only when these fuel extracted and burned. CO2 also enters the atmosphere from aerobic respiration and from volcanic eruptions, which free carbon from rocks deep in the earth's crust.
The oceans also play a major role in regulating the level of carbon diolride in the atmosphere. Some CO2 stay in the sea, some is removed by photosynthesizing producers. As water warms, more dissolved CO2 returns to the atmosphere. In marine ecosystems some organisms take up dissolved CO2 molecules from ocean water and form slightly soluble carbonate compounds such as calcium carbonate (CaCO4 ) to build shells and the skeletons of marine organisms. In fact, most of the earth's carbon - 10,000 times that in the total mass of all life on Earth-is stored in the ocean floor sediments and on the continents. This carbon reenters the cycle very slowly when some of the sediments dissolve and from dissolved CO2 gas that can enter the atmosphere. Geologic processes can also bring bottom sediments to the surface, exposing the carbonate rock to chamical attack by oxygen and conversion to CO2 gas. We have disturebed the carbon cycle in two ways that add more carbon dioxide to the atmosphere than oceans and plants can remove:
NITROGEN CYCLE: BACTERIA IN ACTION
Organisms need nitrogen to make proteins, DNA, RNA, and other nitrogen-containing organic compounds. The nitrogen gas (N2) that makes up 78% ofthe volume of the troposphere cannot be used directly as a nutrient by multicellular plants or animals. Bacteria convert nitrogen gas into water-soluble compounds, which are taken up by plant roots as part of the nitrogen cycle.
The conversion of atmospheric nitrogen gas into other chemical forms useful to plants is called nitrogen fixation. Animals get their nitrogen by eating plants or plant-eating animals.
We intervene in the nitrogen cycle in the following ways:
Phosphorus, is an essential nutrient of both plants and animals. It is a part of DNA molecules, which carry genetic information; other molecules, which store chemical energy for use by organisms in cellular respiration; certain fats in the membranes that encase plant and animal cells; and animal bones and teeth.
Phosphorus moves through water, Earth's crust, and living organisms in the phosphorus cycle.
Phosphorus released by the slow breakdown or weathering, of phosphate rock deposits is dissolved in soil water and taken up by plant roots. Most soils contain little phosphorus. Thus phosphorus is the limiting factor for plant growth in many soils and aquatic ecosystems.
Animals get their phosphorus by eating producers or animals that have eaten producers. Animal wastes and the decay products of dead animals and producers return much of this phosphorus to the soil, to streams, and eventually to the ocean bottom as deposits of phosphate rock.
Some phosphate returns to the land as guano-the phosphate-rich manure of fish eating birds such as pelicans and cormorants. We intervene in the phosphorus cycle chiefly in two ways.
Sulfur circulates through the ecosphere in the sulfur cycle. Much of the earth's sulfur is tied up underground.
However sulfur also enters the atmosphere from several natural sources. Hydrogen sulfide (H2S), a colorless, highly poisonous gas with a "rotten egg" smell, comes from active volcanoes and from the breakdown of organic matter in swamps, bogs, and tidal flats caused by decomposers that don't use oxygen (anaerobic decomposers).
In the atmosphere sulfur dioxide reacts with oxygen to produce sulfur trioxide gas (SO3), which in reacts with water vapor to sulfuric acid (H2SO4). These droplets of sulfuric acid and particles of sulfate salts fall to Earth as components of acid deposition, which can harm trees and aquatic life.
About a third of all sulfur, including 99% of the sulfur dioxide, that reaches the atmosphere comes from human activities. We intervene in the atmospheric phase of the sulfur cycle in two ways:
The main processes in this water recycling and purifying cycle are evaporation, transpiration, condensation, precipitation, infiltration, percolation, and runoffdownslope back to the sea to begin the cycle again.
The water cycle is powered by energy from the sun and by gravity. About 84% of the moisture comes from the oceans, and the rest from land. The amount of moisture in the atmosphere at any one time is the equivalent of only about 25 millimeters (1 inch) of rainfall spread over Earth's entire surface. However, because this moisture is renewed the average annual rainfall over Earth as a whole is 1 meter (40 inches), although actual rainfall varies widely in different places.
The amount of water vapor air can hold depends on its temperature, with warm air capable of holding more water vapor than cold air. Absolute humidity is air (usually expressed as grams of water per kilogram of air). Relative humidity is a measure (expressed in percentages) of the amount of water vapor in a certain mass of air compared with the maximum amount it could hold at that temperature. For example, a relative humidity of 60% at 27o C (80o F means that each kilogram (or other mass unit) of air has 60% of the water vapor it could hold at that temperature.
Winds and air masses transport this water vapor over various parts of the earth's surface, often over long distances. Falling temperatures cause the water vapor to condense into tiny droplets taht form clouds or fog. For precipitation to occur, air must contain condensation nuclei-tiny particles on which droplets of water vapor can collect. Volcanic ash, soil chtst, smoke, sea salts, and particulate matter emitted by factories coal- burning power plants, and vechicles are sources of such particles. The temperature at which condensation occurs for agiven·amount of water vapor is called the dew point.
It takes millions of tiny water droplets adhering to condensation nucleus to produce a drop or rain or a snowflake that will fall to the earth's surface. About 77% of this precipitation falls back into the sea, and the rest over land
Some of the fresh water returning to the earth's surface as precipitation becomes locked in glaciers. Much of it collects in piddles and ditches, and runs off into nearby lakes and into streams, which carry water back to the ocearns, completing the cycle.
Besides replenishing streams and lakes, surface water running ofllthe land also causes soil erosion which moves various nutriems through portions of other biogeochemical cycles. Water is the medium that carries rock debris from the mountainsinto valleys and eventually down to the ocean floor.
It is the primary sculptor of Earth's landscape. Also, acidic rainwater reacts chemically with metallic minerals on Earth's surface, forming water-soluble metallic salts that are carried by rivers to the sea. This is one of the reasons the ocean is so salty.
Much of the water returning to the land seeps into or infiltrates surface soil layers. Some percolates downward into the ground, dissolving minerals from porous rocks on the way. There this mineral-laden water is stored as grounctwater in the pores and cracks of rocks. This underground water, like surface water, flows downhill and seeps out into streams and lakes or comes out in springs. Eventually this water evaporates or reaches the sea to begin the cycle again. The average circulation rate of underground water in the hydrologic cycle is extremely slow (300-4,600 years) compared with that of water in lakes (13 years), in streams (13 days), and in the atmosphere (9 days). The turnover time for water in the ocean is about 37,000 years.
Withdrawing large quantities of fresh water from streams, lakes, and underground sources. In heavily populated or heavily irrigated areas withdrawals have led to groundwater depletion or intrusion of ocean salt water into underground water supplies.
Clearing vegetation from land for agriculture, mining, roads, construction, and other activities.
Are Ecosystems Completely Self-Contained?
Most ecosystems exchange water and nutrients With nearby ecosystems. Mature ecosystems, like old-growth forests, are close to being closed systems because they gain and lose small amounts of matter. Immature communities (young forest, grassland) cycle matter inefficiently, and lose it (erosion).
Through rain, soil and nutrients can be carried as sediment or runoff into another land or aquatic ecosystem. Wind can also blow away topsoil and deposit gaseous molecules released by one ecosystem over others, where they can be absorbed. The nutrients lost by one ecosystem must enter another.
Case Study:Effects of Deforestation on Chemical Cycling atthe Hubbard Brook Experimental Forest
Mature, undisturbed ecosystems recycle nutrients with only small losses into the air and water runoff. How do human activities affect the nutrient cycles in ecosystems?
Hubbard Brook Experimental Forest in New Hampshire is a mature forest consisting of valleys (watersheds), each drained only by a small creek. Bedrock impenetrable to water is close to the surface and prevents seepage from one ecosystem to another.
Researchers designed a controlled experiment to compare loss of water and nutrients from an uncut forest (control system) to one that was stripped of trees (experimental system). V-shaped concrete dams were built across creeks to allow for measurement of water volume and nutrients leaving each ecosystem.
For several years, amounts of water that entered and left an undisturbed forest, and the amount of dissolved nutrients, were measured. The baseline data showed that the amount of nutrients leaving was roughly equal to those entering through rain and snow. Most nutrients were being recycled within the ecosystem.
Next, all trees were cut down inone valley, were left where they fell, and the area was sprayed with herbicides to prevent regrowth. It was studied for three years.
Water runoff in the deforested valley increased 30-40%, eroded soil and carried nutriems out ofthe ecosystem (6-8 times the uncut forest). So much nitrogen as nitrate was lost that the water flowing out was unsafe to drink, and the over- fertilized stream below became overpopulated with cyanobacteria and algae. However, after a few years, vegetation grew back and nitrate levels returned to normal.
Other experiments showed that nutrient losses could be reduced if a forest was cut in horizontal strips. The remaining strips of standing trees helped absorb some of the water and reduce erosion and loss of nutrients.