Ecology 4 Ecosystem Ecology

 Ecosystems are the sum of all the organisms living in an area plus all the abiotic factors with which they interact. A lake is a dynamic ecosystem. Sit quietly beside a lake and closely observe: sunlight reflects off the surface of the water, small rings form where fish snatch insects at the surface, a stream flows into the lake delivering nutrients.

Trophic Levels show feeding relationships in an ecosystem. Here are food chains from two different ecosystems, indicating the trophic levels of each organism.

 The trophic level that ultimately supports all others consists of autotrophs, also called primary producers.  Most autotrophs are photosynthetic organisms that use light energy to synthesize sugars and other organic compounds.

 Heterotrophs either directly or indirectly depend on the primary producers for their energy needs and organic compounds needed to build their own bodies. Prairie Dog Juvenile Hawk

 Because herbivores consume plants (producers), they are called primary consumers.  They get their energy and nutrients directly from plants.  These animals are in the 2 nd trophic level.

 Animals that eat other animals are called carnivores.  When an animal eats an herbivore (a primary consumer), they are called secondary consumers.  They are feeding at the 3 rd trophic level.

 Omnivores eat both plant material and other animals.  When they are feeding on plants, they are primary consumers  When they are feeding on other animals, they are secondary consumers. Most songbirds such as this Black-capped Chickadee are omnivores, eating seeds from plants and insects.

 Animals that hunt and kill other animals for food and are rarely (if ever) hunted and killed themselves are called top carnivores.  They feed as tertiary or even quaternary consumers, depending on what they are eating. The Fossa is the largest carnivore on the island of Madagascar. It is a top carnivore in this ecosystem.

 Another important group of heterotrophs are the detritivores (also called decomposers).  These are consumers who get their energy from detritus, nonliving material such as the remains of dead organisms, feces, fallen leaves, and wood.  Many detritivores are eaten by secondary and tertiary consumers. Two important groups of detritivores are the bacteria and fungi. These release enzymes that digest organic material, eventually returning nutrients to the soil.

 The amount of light converted to chemical energy by autotrophs during a given time period is an ecosystem’s primary productivity.  This sets the “spending limit” for all organisms in that ecosystem. Which ecosystem has a greater primary productivity?

 The total primary productivity in an ecosystem is called its’ Gross Primary Productivity.  Not all of this production is stored as organic material by primary producers, however.  They will use some of the energy for their own metabolism. Net Primary Productivity is equal to the Gross Primary Production minus the energy used for respiration by the plants. NPP=GPP-R

 Limiting Factors in an ecosystem are those things that can limit Gross Primary Productivity.  In most ecosystems, the two things that control this are light and nutrients.  A limiting nutrient is the element that must be added for productivity to increase.

 Question:Which nutrient limits phytoplankton production along the coast of Long Island?  Pollution from duck farms concentrated near Moriches Bay adds both nitrogen (N) and phosphorus (P) to the coastal water off Long Island, New York.  To determine which nutrient limits phytoplankton growth in this area, John Ryther and William Dunstan of the Woods Hope Oceanographic Institution, cultured phytoplankton with water collected from several sites.

Fig Atlantic Ocean Moriches Bay Shinnecock Bay Long Island Great South Bay A B C D E F G EXPERIMENT Ammoniu m enriched Phosphate enriched Unenriched control RESULTS A B C D E F G Collection site Phytoplankton density (millions of cells per mL)

 Ryther and Dunstan collected water from sites labeled A-G on the map. They added either ammonium (NH 4 + ) or phosphate (PO 4 3- )  Results: The addition of ammonium caused heavy phytoplankton growth in the cultures, but the addition of phosphorus did not.  Conclusion: Since adding phosphorus, which was already in rich supply, had no affect on phytoplankton growth, whereas adding nitrogen increased it dramatically, nitrogen is the nutrient that limits phytoplankton growth in this ecosystem.

 Nutrient limitation is common in freshwater lakes.  Sewage and fertilizer runoff from farms and yards add large amounts of nutrients to lakes.  Cyanobacteria and algae grow rapidly in response to these added nutrients, eventually leading to a process that reduces oxygen. This is called eutrophication.

 Much of the material made by primary producers is not used by consumers.  When a caterpiller feeds on a leaf, only about 33 j out of 400 j (or 1/6 of the energy in the leaf) is used in the growth of the caterpiller’s body. This is the only available energy for the bird that eats the caterpiller.

 Trophic efficiency is the % of production transferred from one trophic level to the next.  It is generally about 10%  This means that 90% of the energy available in one trophic level is not transferred to the next level. Only about 0.1% of the energy fixed by photosynthesis flows all the way to the owl. This is why most food webs only have about 4 or 5 levels.

 Although most ecosystems receive plenty of solar energy, chemical elements are available only in limited amounts.  New matter only comes rarely from meteorites!  Life on Earth therefore depends on the recycling of essential chemical elements.

And Transpiration

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