Ecosystems Reading: Freeman Chapter 54
An ecosystem is the unit composed of all the living things in a single place at a given time, in addition to, the important non-living components of the system. • Nonliving components include sunlight, rainfall, silica and clay particles in the soil, the air, the water in the soil, etc. • Thus, an ecosystem encompasses all aspects of a biological community, in addition to factors such as rates of CO2 uptake, rates of nitrogen fixation from the atmosphere, precipitation, seasonal flooding and its effects on nutrients, etc.
Ecosystems vary in size. Like communities, small ecosystems are “stacked” within larger ones, and the boundaries are sometimes diffuse. • The biosphere the largest and most encompassing ecosystem we know-it encompasses all the plants and animals on Earth.
Energy and Biomass • Much of ecosystems ecology concerns itself with the flow of energy and biomass. • Nutrient cycling and energy flow are common to all biological communities. • These phenomena are both a consequence, and a function of biological communities. • The complex matrix of interactions among members of a community expends energy, as well as passing it from one member to the next through trophic interactions. • Likewise, biomass is constantly recycled through production, predation, herbivory, and decomposition.
Energy • The sun is the ultimate energy source for almost every ecosystem on earth. • Hydothermal vent communities are a partial exception-(they rely on geothermal energy, but still depend upon oxygen fixed by photosynthetic organisms). • Energy enters ecosystems via photosynthesis (or, in a few exotic excosystems, chemosynthesis). • Organisms that bring energy into an ecosystem are called producers. • Producers include green plants, algae, cyanobacteria, etc..anything that can make its own energy from nonliving components of the environment.
Organisms continuously use energy. • All metabolic processes consume energy in some way, and in each reaction, much of it is effectively “wasted”… • ..this is one reason why rapid metabolism makes us homeothermic-the waste heat from metabolic processes, mostly as molecular motion, warms our bodies. • Ultimately, all biological energy radiates into the environment as infrared light (a by-product of respiration). • Much energy is lost every time it passes from one trophic level to the next. • Energy does not recycle. • it must be continually replenished from the sun.
Autotrophs fix their own energy from inorganic sources. • Autotrophs are the producers in an ecosystem. • Heterotrophs depend upon energy and carbon fixed by some other organism • they are consumers, detritivores, or decomposers. • (A mixotroph is gets its energy from inorganic sources, but relies of organic sources of carbon.)
A food web is a schematic diagram that describes the patterns of energy flow in an ecosystem • Every instance of predation, herbivory, and parasitism is a trophic interaction that moves energy from one organism to another. • Decomposition is also a trophic interaction that uses up the energy left over in dead bodies of organisms. • A food chain is one path through a food web, from bottom to top. • Because energy is lost at each step, food chains have a limited number of links.
Matter • Unlike energy, matter recycles through ecosystems. • Atoms of every biologically important element constantly recycle through ecosystems, into the abiotic component of the biosphere, and back into living systems. • Elements are passed from one organism to another via trophic interactions, or are taken directly from the environment. • Via the process of decomposition, each element ultimately becomes nonliving, and has the potential to re-enter the biosphere again. • Thus, each element has its own biogeochemical cycle-these can take days, years, or eons, depending upon the element and the circumstances.
Biomass • Biomass can be defined as the weight of living matter (usually measured in dry weight per unit area). • A pyramid of biomass is a figure that quantifies the relative amounts of living biomass found at each trophic level. • In most ecosystems, the amount of biomass found in each trophic level decreases progressively as one moves from the bottom to the top of the food chain.
Pyramid of biomass for a pond. (Source: Data from Whittaker, R.H. 1961. Experiments with radiophosphorus tracer in aquarium microcosms. Ecological Monographs 31:157-188).
Primary consumers eat producers. • They generally possess significantly less biomass than producers. • Plants have evolved numerous mechanisms to protect their tissues from consumption by herbivores and pathogens • In most ecosystems only a small amount of producer biomass is eaten. • Significant losses of biomass occur because of digestive inefficiencies, and return of CO2 to the atmosphere via respiration. • Assimilation efficiencies for most terrestrial herbivores range from 20 to 60 percent. Some invertebrates do better than that..some do not. • A very large proportion of the assimilated biomass is lost through the process of respiration, so only a small amount of the biomass is available to the next level.
Secondary consumers consume primary consumers. • Tertiary consumers consume secondary consumers, and so forth. • Not all organisms at one level are eaten, because of defensive mechanisms-and predation is only one way to die. • Defensive adaptations include the ability to fly and run, body armor, quills and protective spines, and camouflage. • In general, carnivores have higher assimilation efficiencies than herbivores. These range from 50 to 90 percent. • Only a small fraction of the assimilated energy becomes carnivore biomass because of the metabolic energy needs of body maintenance, growth, reproduction, and locomotion.
Most food chains have at most four or five trophic levels. • The amount of biomass found at each trophic level is small relative to amount found at the next lowest level. • This is because less energy is available to successive consumers. http://www.bioquip.com
Decomposers, scavengers, saprophytes, and detritivores are organisms that eat dead organic matter. • Detritivores eat the dead bodies of living things, such as carrion, leaf litter, etc.. • “Scavenger”s are animals that eat dead animals. • Decomposers are microscopic organisms that break down organic compounds into nonliving, inorganic precursors. • Saprophytes are organisms that feed on dead organic matter, this term is usually applied to fungi or bacteria, but there are plant saprophytes as well
Primary Productivity • Primary productivity is the amount of biomass produced through photosynthesis per unit area and time by producers. • It is usually expressed in units of energy (e.g., joules /m2 day) or in units of dry organic matter (e.g., kg /m2 year). • Globally, primary production amounts to 243 billion metric tons of dry plant biomass per year. • The total energy fixed by plants in a community through photosynthesis is referred to as gross primary productivity (GPP).
Net vs. Gross Primary Productivity • Most gross primary productivity is used via respiration by the producers themselves. • Subtracting respiration from gross primary production gives net primary productivity (NPP) • NPP represents the rate of production of biomass that is available for consumption (herbivory) by heterotrophic organisms (bacteria, fungi, and animals). It is also easier to measure, because it tends to accumulate over time.
Problem: • A plot of Panicum sp. grass has a gross primary productivity of 10,700 kcal/m2year. The grass respire approximately 9,100 kcal/m2year. • What is the net primary productivity?
Answer: • 10,700kcal/m2year - 9,100 kcal/m2year=1600kcal/m2year. • Problem: • The field is 10m x 10m. Over the course of one year, what is the total net primary productivity for the field?
Answer: • 100m2 x 1600kcal/m2year=1.6x105kcal/year. • Problem: • If Panicum grass has an energy value of 6kcal/gram, and all of the primary productivity were to accumulate as biomass, how much biomass (expressed as dry weight) will have accumulated in the field over the course of 1 year?
Answer: • (1.6x105kcal/year x 1 year)/(6kcal/gram)=2.67x104 grams or 267kilograms. • Problem: • Suppose herbivores (wild mules) eat ALL this biomass, and assimilate 10%. The respiration of the mule is 15kcal/kilogram day. • Would this field be sufficient to support a 150 kilogram mule?
Answer: • The mule would assimilate (1.6x105kcal/year x 10%)=1.6x104kcal/year. • Over the course of the year, the mule would require 15kcal/kilogram day x 365 days x 150 kilograms=8.21x105kcal. • The field is not nearly enough. This is why large herbivores move around so much.
Communities Differ in their Productivity • Globally, patterns of primary productivity vary both spatially and temporally. • The least productive ecosystems are limited by heat energy, nutrients and water like the deserts and the polar tundra. • The most productive ecosystems have high temperatures, plenty of water and lots of available soil nitrogen.
Productivity is high in areas of oceanic upwelling-oceanic producers, which include diatoms, dinoflagellates, cryptomonads, and other algae-require nutrients
Nutrient Cycling • Each biologically important element has nutrient cycle. • A nutrient cycle is the path of an element from one organism to another, and from organisms into the nonliving part of the biosphere and back. • Nutrient cycles are sometimes referred to as biogeochemical cycles, reflecting the fact that chemicals are cycled between biological organisms, and between organisms and the geologic (physical) environment.
C, H, O, N • Carbon, hydrogen, oxygen, and nitrogen make up most of the biological molecules found in living organisms. These elements are passed from organism to organism by chemical conversion processes, which occur in food webs. • They are also converted from non-living forms to living forms by photosynthesis and nitrogen fixation, and from living forms to non-living forms through cellular respiration.
Reservoirs • The non-living forms of carbon, hydrogen, oxygen, and nitrogen form huge reservoirs in the physical environment. For instance, nitrogen makes up 78% of the atmosphere as N2, and hydrogen comes from water. • In ecosystems ecology, a reservoir is a supply of a biologically meaningful element that is not easily obtainable by living organisms. • Elements can have multiple reservoirs
Carbon • Most of the material substances that make up living organisms consist of organic compounds of carbon. In contrast, carbon is relatively scarce in the nonliving part of the Earth. • Carbon exists in the non-living environment as carbon dioxide in the atmosphere, dissolved carbon dioxide (HCO3-, etc.) in the ocean, and as carbonates in the Earth’s crust. • It is also locked in fossil deposits, and embedded in the ocean floor as deposits of methane anhydride.
Carbon cycles between the living and nonliving components of the biosphere. • The most important reservoir for carbon is the atmosphere: • Although CO2 makes up less than one percent of the atmosphere, it is very important to the biosphere. • Much of the carbon in your body was part of the atmosphere, some of it relatively recently. • When you decompose, it will return to the atmosphere.
Carbon Fixation • Fixation, in this sense, means capture and conversion to a biologically useful form. • Eg., water does not need to be “fixed”, neither does sodium, but carbon and nitrogen do. • CO2 is fixed by plants during photosynthesis. • Photosynthesis converts atmospheric CO2 into organic carbohydrates by combining them with water, also from the nonliving part of the biosphere. • This process requires the input of specific light photons, which plants capture with the pigment chlorophyll. • Once fixed by plants, CO2 is passed up the food chain by trophic interactions such as herbivory and predation.
Respiration • Most organisms, including plants, respire. • Respiration liberates carbon back into the atmosphere and provides energy to the organism. • CO2 enters the atmospheric reservoir. • If it is not eaten and respired, or decomposed, organic carbon may become buried and enter a carbon reservoir in the soil, or ultimately fossilize.
Carbon that is "fixed" can also return to the atmosphere if the plant material is burned, either naturally, or through human activities. • Even ancient plant and animal material that contains carbon that was fixed millions of years ago can be returned to the atmosphere by burning fossil fuels. • Carbon can also be recycled back into the atmosphere through volcanic activity. • As a tectonic plate goes underneath a continent, superheated oceanic material upgasses through geological vents and reenters the atmosphere.
Carbon, Global Warming, Anthropogenic Climate Change • CO2 has a crucial role in the climate of the Earth because it is quite transparent to light at the visible wavelengths, and relatively opaque to infrared light. • Gasses with this property are called greenhouse gasses, because they tend to trap heat, forcing a higher equilibrium temperature. • Methane, and CFC’s are also greenhouse gasses, but CO2 is the most important because it occurs at higher concentrations. • Geological periods of low CO2 concentration (such as the present) are strongly correlated with low global temperatures, higher CO2 is strongly correlated with higher global temperatures. • Additionally, sudden increases in CO2 can be linked to a sudden warming of the climate. • Such an event occurred in the Miocene, 15 million years ago.
There is very solid evidence that CO2 concentrations have increased significantly over the course of the last 150 years. • This is partially due to the burning of fossil fuels, and partially due to deforestation. • By cutting and burning of forests, the carbon that once was locked in the trees is released into the atmosphere. • Huge stores of fossilized carbon are present within the Earth’s crust, much of it buried and fossilized during the Carboniferous period, 200million years ago. • Liberation of these stores into the atmosphere has the potential to dramatically change the climate of the Earth. • Evidence is mounting that these higher CO2 levels have already affected the climate of the Earth.
Some possible effects: • Higher temps, especially in the high latitudes • Drier continental interiors • More unpredictable weather patterns, with more extreme storms, and extreme heat events • The potential for tropical diseases to enter higher latitudes and higher elevations • The potential for currently farmable areas to become too dry to farm • The potential to interfere with oceanic thermohaline circulation, and cause conditions in Europe and Eastern North America to become very cold. • The potential to interfere with oceanic productivity through changes in Ph • The potential for increases in sea level.
N • N is one of the most common elements that form biological molecules. • It is a major component of amino acids, also a primary constituent of nucleic acids.
The major reservoir for nitrogen is the atmosphere • N2 makes up 78% of the Earth's atmosphere. • The majority of living organisms are not able to use it in that form. • N2 contains a triple bond between the atoms, it is a very stable molecule and therefore, biologically inert. • A large amount of energy is required to break the triple bond. • lightning is responsible for converting some of the atmospheric nitrogen into forms that organisms can use. • The process of converting atmospheric nitrogen into forms that organisms can use is called nitrogen fixation.
Although most organisms are not able to convert nitrogen, there are a few that are able to "fix" atmospheric nitrogen. • Some free-living soil bacteria as well as some blue-green bacteria have the ability to convert nitrogen into ammonia. • Nitrogen is also fixed by symbiotic bacteria that live in and among the root cells of several types of plants, most notably, the legume plants such as beans, peanuts, and peas. Other plants such as alfalfa, locust, and alders also have root nodules.
There are a few that are able to "fix" atmospheric nitrogen. • These include bacteria in the genus Rhizobium and Bradyrhyzobium, and also some cyanobacteria, such as Anabaena and Nostoc, • This process, which is energetically expensive, converts nitrogen into ammonia. • Other bacteria convert ammonia to nitrates through nitrification. • Most plants use nitrogen in the form of nitrates, though ammonia is also useful. • Nitrogen fixing bacteria frequently live in mutualistic symbiosis with plants, notably legumes. • Thus, legumes can be disproportionately important to the ecology of a plant community.
Once nitrogen is absorbed by plants and built into the plant molecules, the nitrogen can be passed to consumers and to decomposer organisms through the food chain. • Nitrogen can be mineralized and converted to organic compounds that enter the soil or water upon their death, or enter as waste through their digestive tracts. • These decomposed nitrogen compounds - ammonia, nitrite, and nitrates, then become available for other plants to absorb and recycle. This process is called ammonification. • Alternatively, other bacteria, known as "denitrifiers," convert nitrites and nitrates in the soil to N2O and N2, which returns to the reservoir in the atmosphere. This process, which completes the nitrogen cycle, is called denitrification.
Certain bacteria convert ammonia to nitrates through nitrification. Most plants use nitrogen in the form of nitrates. • Once nitrogen is absorbed by plants and built into the plant molecules, the nitrogen can be passed to consumers and to decomposer organisms through the food chain.
Water • The water cycle is one of the most important processes to living organisms on Earth. • Water that has evaporated into the atmosphere condenses and falls as precipitation. • This precipitation will either run off as surface water and collect as streams or rivers, or it can seep into the ground and collect in huge underground rock formations called aquifers, that act much like sponges. • The water eventually flows from lakes or streams down into the oceans, where it can reside for long periods of time, or get evaporated back up into the atmosphere as water vapor, which collects as clouds.
A portion of the water absorbed into the ground is taken up by plants, which use the water to transport minerals internally as well as to take part in the photosynthetic process. • Some of this water is transferred to animals that feed on plants; from there, water can cycle within the food web of an ecosystem. • Water can be given off to the atmosphere by plant leaves through transpiration, or by animals through respiration, perspiration, or excretion.