We want to examine energy transformations in a simple pond ecosystem in which th

Question

We want to examine energy transformations in a simple pond ecosystem in which the only producers are phytoplankton, the only herbivores are zooplankton (crustaceans), the primary predators are fly larvae (Chaoborus), and the secondary predators are sunfish. We could diagram this as a linear grazing food chain:

sun --> phytoplankton --> crustaceans -->Chaoborus--> sunfish Assume that total solar inputs average 1,000,000 kcal/m2/yr, and the following efficiencies are true:

photosynthetic efficiency of phytoplankton = 2%

plant production efficiency = 80%

exploitation efficiency of zooplankton = 70%

ecological efficiency of zooplankton = 10%

ecological efficiency of Chaoborus = 20%

ecological efficiency of sunfish = 15%

What is the annual NPP of this ecosystem? What is the energy consumption (ingestion) rate of zooplankton?
What is the secondary productivity of the zooplankton? (note: you don’t need to use the ingestion rate to calculate this) What is the secondary productivity of the predacious fly larvae?

What is the secondary productivity of the sunfish?

Can you show how to work these problems?

Explanation / Answer

An ecosystem consists of the biological community that occurs in some locale, and the physical and chemical factors that make up its non-living or abiotic environment. There are many examples of ecosystems -- a pond, a forest, an estuary, a grassland. The boundaries are not fixed in any objective way, although sometimes they seem obvious, as with the shoreline of a small pond. Usually the boundaries of an ecosystem are chosen for practical reasons having to do with the goals of the particular study. The study of ecosystems mainly consists of the study of certain processes that link the living, or biotic, components to the non-living, or abiotic, components. Energy transformations and biogeochemical cycling are the two main processes that comprise the field of ecosystem ecology. A functional group is a biological category composed of organisms that perform mostly the same kind of function in the system; for example, all the photosynthetic plants or primary producers form a functional group. Membership in the functional group does not depend very much on who the actual players (species) happen to be, only on what function they perform in the ecosystem. Energy enters the biological system as light energy, or photons, is transformed into chemical energy in organic molecules by cellular processes including photosynthesis and respiration, and ultimately is converted to heat energy. This energy is dissipated, meaning it is lost to the system as heat; once it is lost it cannot be recycled. Without the continued input of solar energy, biological systems would quickly shut down. Thus the Earth is an open system with respect to energy. Elements such as carbon, nitrogen, or phosphorus enter living organisms in a variety of ways. Plants obtain elements from the surrounding atmosphere, water, or soils. Animals may also obtain elements directly from the physical environment, but usually they obtain these mainly as a consequence of consuming other organisms. These materials are transformed biochemically within the bodies of organisms, but sooner or later, due to excretion or decomposition, they are returned to an inorganic state (that is, inorganic material such as carbon, nitrogen, and phosphorus, instead of those elements being bound up in organic matter). Often bacteria complete this process, through the process called decomposition or mineralization. During decomposition these materials are not destroyed or lost, so the Earth is a closed system with respect to elements (with the exception of a meteorite entering the system now and then). The elements are cycled endlessly between their biotic and abiotic states within ecosystems. Those elements whose supply tends to limit biological activity are called nutrients. The transformations of energy in an ecosystem begin first with the input of energy from the sun. Energy from the sun is captured by the process of photosynthesis. Carbon dioxide is combined with hydrogen (derived from the splitting of   water molecules) to produce carbohydrates (CHO). Energy is stored in the high energy bonds of adenosine triphosphate, or ATP.

For the cycle arranged : sun --> phytoplankton --> crustaceans -->Chaoborus--> sunfish

With the following values:

solar inputs average 1,000,000 kcal/m2/yr

photosynthetic efficiency of phytoplankton = 2%

plant production efficiency = 80%

exploitation efficiency of zooplankton = 70%

ecological efficiency of zooplankton = 10%

ecological efficiency of Chaoborus = 20%

ecological efficiency of sunfish = 15%

Gross Primary Production, GPP, is the total amount of CO2 that is fixed by the plant in photosynthesis.

* Respiration, R, is the amount of CO2 that is lost from an organism or system from metabolic activity. Respiration can be further divided into components that reflect the source of the CO2.

Rp =Respiration by Plants

Rh = Respiration by Heterotrophs

Rd = Respiration by Decomposers (the microbes)

* Net Primary Production, NPP, is the net amount of primary production after the costs of plant respiration are included. Therefore, NPP = GPP – R

* Net Ecosystem Production, NEP, is the net amount of primary production after the costs of respiration by plants, heterotrophs, and decomposers are all included. Therefore, NEP = GPP - (Rp + Rh + Rd)

Therefore NPP = 432 g Cm-2yr-1

Ingestion rate = 8 g C ind.-1 day-1

The secondary production in : = dN/dt = rN (K-N) / K =  106/yr

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