Chapter 54: Ecosystems Football.

Key Concepts Ecosystem ecology emphasizes energy flow and chemical cycling. Physical and chemical factors limit primary production in ecosystems. Energy transfers between trophic levels is usually less than 20% efficient. Biological and geochemical processes move nutrients between organic and inorganic parts of the ecosystem. The human population is disrupting geochemical cycles throughout the biosphere.

Ecosystems, Energy, and Matter An ecosystem consists of all the organisms living in a community as well as the abiotic factors with which they interact. Ecosystem ecologists group species into trophic levels to follow energy transformations and chemical movements in an ecosystem. Energy flows from sun to primary producers (photoautotrophs) to primary consumers (herbivores) to secondary consumers (carnivores) and so on. Detritivores (decomposers) consume detritus composed of organic waste. Decomposition returns necessary chemicals to the soil to be absorbed by the roots of producers. Energy is dispersed as heat because of the inefficiencies of living organisms’ metabolic processes.

Physical and Chemical Limitations Primary production is the amount of light energy converted into chemical energy during a period of time – photosynthetic output of an ecosystem’s autotrophs. Net primary production (NPP) is the ecosystem’s gross primary production (GPP) minus the energy used by plants in their own cellular respiration (R). Standing crop is the total biomass of photosynthetic organisms in an ecosystem. Limiting nutrient is the element that must added in order for production to increase in a particular area, most often nitrogen or phosphorus. Actual evapotranspiration is the annual amount of water evaporated from a landscape and transpired by plants. Ecosystems usually show a positive correlation between actual evapotranspiration and primary production.

Inefficiencies in Energy Transfer The rate at which consumers in an ecosystem produce new biomass from their food is called secondary production. Production efficiency is equal to the net secondary production divided by the assimilation of primary production. This represents the proportion of assimilated food and energy that is used for net secondary production and is a measure of efficiency of energy transformation. Trophic efficiency is the percentage of the energy of one trophic level that makes it to the next level, usually ranging from five to twenty percent. The biomass pyramid explains the progressive loss of energy along the food chain that severely limits the overall biomass of top level carnivores that any ecosystem can support. The pyramid of numbers illustrates that higher trophic levels contain smaller numbers of individuals, resulting from the larger size of these animals and the greatly decreased energy availability illustrated by the pyramid of production.

Biological and Geochemical Processes Chemical elements are passed between abiotic and biotic components of ecosystems through biogeochemical cycles. While and individual organism is alive, much of its chemical stock is rotated continuously as nutrients are assimilated and waste products released. The decomposition replenishes the pools of inorganic nutrients that plants and other autotrophs use to build organic matter. The major abiotic processes are the water cycle, the carbon cycle, nitrogen fixation, ammonification, and the phosphorus cycle.

The Water Cycle Water cycle involves evaporation, precipitation, and transpiration, with a net flow of water evaporating by solar energy from the oceans, moving as water vapor to the land where it precipitates, and returning to the oceans through runoff and groundwater.

Carbon Cycle In the carbon cycle, plants take CO2 from the atmosphere for photosynthesis, and organisms release it in cellular respiration. Fossil fuel combustion is increasing atmospheric CO2. Decomposition adds carbon to the ground.

Nitrogen Fixation Plants require nitrogen in the form of NH4+, or NO3-. Animals obtain nitrogen in organic form from plants or other animals. The major pathway for nitrogen to enter an ecosystem is via nitrogen fixation. Soil bacteria and symbiotic bacteria in root nodules fix nitrogen into ammonium, and, surprisingly, by LIGHTNING!!!!!!!!!!!! Bacterial and fungal decomposers, in a process called ammonification, break down organic compounds and return ammonium to the soil.

Phosphorus Cycle Weathering of rocks gradually adds phosphorus in the form of phosphate into the soil and is absorbed by producers and incorporated into biological molecules. This is then transferred to consumers and returned to the soil via decomposition. Phosphorus does not move through the atmosphere because there are no phosphorus-containing gases.

Disruption as a Result of Humans Harvesting of crops removes nutrients that would otherwise recycle to the soil. After depleting the organic and inorganic reserves of nutrients, crops require the addition of synthetic fertilizers. The addition of nitrogen fertilizers, increased legume cultivation, and burning have doubled Earth’s supply of fixed nitrogen. Nitrogen that exceeds the critical load, the amount of added nutrient that can be absorbed by plants without damaging the ecosystem, can contaminate groundwater, degrade lakes and rivers, and drain into the ocean. Burning of coal and other fossil fuels, as well as wood, releases oxides of sulfur and nitrogen, which forms sulfuric and nitric acid in the atmosphere. The acids return to the Earth as acid precipitation. Nutrients leached from soils as acid precipitation changes soil chemistry, and forests have been damaged. Fish populations and lakes have declined and community compositions have changed.

Toxins and Atmospheric CO2 Humans release a huge variety of toxic chemicals into the environment. Organisms absorb these toxins from food and water and may retain them within their tissues and fat. In a process known as biological magnification, the concentration of such compounds increase in each successive trophic level. Many toxic chemicals dumped into ecosystems are nonbiodegradable; others, such as mercury, may become more harmful as they react with other environmental factors. The concentration of CO2 in the atmosphere has been increasing since the industrial revolution as a result of the burning of fossil fuel and the burning of wood. If C3 plants become able to outcompete C4 plants with the increase in CO2, species composition in natural and agricultural communities may be altered.

The Greenhouse Effect Through a phenomenon known as the Greenhouse Effect, CO2 and water vapor in the atmosphere absorb infrared radiation reflected from Earth and re-reflect it back to Earth, causing an increase in temperature. Scientists use various models to estimate the extent and consequences of increasing CO2 levels. Ecologists study the effects of vegetation of previous global warming trends to try to predict the impact of increasing temperatures A layer of ozone molecules (O3) in the lower stratosphere absorbs damaging ultraviolet radiation. This layer has been gradually thinning since 1975, largely as a result of the accumulation of breakdown products of chlorofluorocarbons in the atmosphere. The dangers of ozone depletion may include increased incidents of skin cancer and cataracts and unpredictable effects on phytoplankton, crops, and natural ecosystems.