Chapter 54: EcosystemsOverview: Ecosystems, Energy, and Matter- An ecosystem consists of all the organisms living in a community as well as all the abiotic factorswith which they interact.- The dynamics of an ecosystem involve two processes that cannot be fully described by population or community processes and phenomena: energy flow and chemical cycling.- Energy enters most ecosystems in the form of sunlight. It is converted to chemical energy by autotrophs, passed to heterotrophs in the organic compounds of food, and dissipated as heat.- Chemical elements are cycled among abiotic and biotic components of the ecosystem.- Energy, unlike matter, cannot be recycled. An ecosystem must be powered by a continuous influx of energy from an external source, usually the sun.- Energy flows through ecosystems, while matter cycles within them.Concept 54.1: Ecosystem ecology emphasizes energy flow and chemical cycling- Ecosystem ecologists view ecosystems as transformers of energy and processors of matter.- We can follow the transformation of energy by grouping the species in a community into trophic levels of feeding relationships.Ecosystems and Physical Laws- The law of conservation of energy states that energy cannot be created or destroyed but only transformed. Plants and other photosynthetic organisms convert solar energy to chemical energy, but the total amount of energy does not change. The total amount of energy stored in organic molecules plus the amounts reflected and dissipated as heat must equal the total solar energy intercepted by the plant.- The second law of thermodynamics states that some energy is lost as heat in any conversion process. We can measure the efficiency of ecological energy conversions.- Chemical elements are continually recycled. A carbon or nitrogen atom moves from one trophic level to another and eventually to the decomposers and back again.Trophic Relationships- The tropic level that ultimately supports all other consists of autotrophs, also called the primary producers of the ecosystem. Most autotrophs are photosynthetic organisms that use light energy to synthesize sugars and other organic compounds, which they then use as fuel for cellular respiration and as building material for growth.  Plants, algae, and photosynthetic prokaryotes are the biosphere’s main autotrophs. - Heterotrophs are at trophic levels above the primary producers and depend on their photosynthetic output. Herbivores that eat primary producers are called primary consumers. Carnivores that eat herbivores are called secondary consumers. Carnivores that eat secondary producers are called tertiary consumers.- Another important group of heterotrophs consists of the detritvores. ◦ Detritivores, or decomposers, are consumers the get their energy from detritus, nonliving organic material such as the remains of dead organisms, feces, fallen leaves, and wood. Detritivores play an important role in material cycling.Decomposition- The organisms that feed as detritivores form a major link between the primary producers and the consumers in an ecosystem.- Detritivores play an important role in making chemical elements available to producers. Detritivores decompose organic material and transfer chemical elements in inorganic forms to abiotic reservoirs such as soil, water, and air.1- Producers then recycle these elements into organic compounds.- Decomposition by prokaryotes and fungi accounts for most of the conversion of organic materialsfrom all tropic levels to inorganic compounds usable by primary producers, thereby closing the loop of an ecosystem’s chemical loop. Concept 54.2: Physical and chemical factors limit primary production in ecosystems- The amount of light energy converted to chemical energy by autotrophs in a given time period is an ecosystem’s primary production.- This photosynthetic product is the starting point for studies of ecosystem metabolism and energy flowEcosystem’s Energy Budget- Most primary producers use light energy to synthesize organic molecules, which can be broken down to produce ATP.- The amount of photosynthetic production sets the spending limit of the entire ecosystem.- A global energy budget can be analyzed. Every day, Earth is bombarded by approximately 1023 joules of solar radiation. The intensity of solar energy striking Earth varies with latitude, with the tropics receiving the greatest input. Most of this radiation is scattered, absorbed, or reflected by the atmosphere. Much of the solar radiation that reaches Earth’s surface lands on bare ground or bodies of water that either absorb or reflect the energy. Only a small fraction actually strikes algae, photosynthetic prokaryotes, or plants, and only some of this is of wavelengths suitable for photosynthesis. Of the visible light that reaches photosynthetic organisms, only about 1% is converted to chemical energy. Although this is a small amount, primary producers produce about 170 billion tons of organic material per year.Gross and New Primary Production- Total primary production in an ecosystem is known as that ecosystem’s gross primary production (GPP) – the amnt of light energy that is converted to chem energy by photosynthesis per unit time. ◦ Not all of this production is stored as organic material in the growing plants, because the plants use some of the molecules as fuel in their own cellular respiration.- Net primary production (NPP) is equal to gross primary production minus the energy used by the primary producers for respiration (R):◦ NPP = GPP − R- To ecologists, net primary production is the key measurement, because it represents the storage ofchemical energy that is available to consumers in the ecosystem.- Primary production can be expressed as energy per unit area per unit time, or as biomass of vegetation added to the ecosystem per unit area per unit time. This should not be confused with the total biomass of photosynthetic autotrophs present in a given time, which is called the standing crop. Primary production is the amount of new biomass added in a given period of time. Although a forest has a large standing cross biomass, its primary production may actually be less than that of some grasslands, which do not accumulate vegetation because animals consume the plants rapidly.- Different ecosystems differ greatly in their production as well as in their contribution to the total production of the