BIO 262 Productivity and Energy Flow + Nutrient Cycling

Biomass

The weight of an organism's living tissues, measured per unit area

Ecosystem

Biological community plus all of the factors influencing that community

Trophic level

an organism's position in a food web, based on the number of transfers of energy from primary producers to that level (producers to herbivores, 1 level; producers to carnivores, 2 levels).

- Most food chains have 3-5 levels.

Gross primary productivity (GPP)

Total amount of energy fixed by all producers(plants, algae, and other autotrophs) in an ecosystem

Net primary productivity (NPP)

GPP minus the energy required for the producer's energetic needs (respiration, etc.)

The ratio of _______ primary productivity to gross primary productivity gives an indication of the cost of keeping the organism going, with large ratios indicative of relatively few costs(e.g., algae, ~50%) and smaller ratios associated with many costs (e.g., complex plants such as trees, ~10%)

net

Secondary productivity

The rate at which an ecosystem's consumers convert the energy of their food into their own new biomass.

- Secondary productivity is dependent, in part, on the efficiency of transfer of energy between trophic levels.

Energy flow

A. Energy enters most ecosystems in the form of sunlight. It is then converted to chemical energy by producers, passed to consumers in the organic compounds of food, and dissipated in the form of heat

B. Energy is lost from ecosystems primarily as waste heat, also substantial within-trophic-level losses

Factors affecting productivity

I. Moisture

II. Temperature

III. Nutrients

a. Liebig's law of the minimum: nutrient limitation to plant growth can be traced to a single limiting nutrient.

Trophic efficiency

A. Definition: the transfer of energy up trophic levels, e.g., the ratio of secondary productivity to primary productivity consumed

I. Generally range from 5% to 20%; that is, only 5% to 20% of primary producer biomass consumed is converted into new consumer biomass

II. Inefficiencies arise not just due to the second law of thermodynamics but because of inefficiencies in digestion (i.e., not everything is assimilated; much food becomes...shall we say...poop)

Ways of depicting trophic efficiency

I. Pyramid of numbers (measured in number/area)

II. Pyramid of biomass (measured in mass/area)

III. Pyramid of productivity (measured in mass/area/time)

Ways of depicting trophic efficiency

I. Pyramid of numbers (measured in number/area)

a. The total numbers of individual organisms tend to decline as one goes up trophic levels. This decline is a consequence of ecological efficiencies being less than 100%

b. A consequence of the pyramid of numbers is that top predator numbers tend to be small, thus making top predators both slow to evolve (also because they tend to be long-lived and have long generation times) and relatively easy to drive to extinction

Ways of depicting trophic efficiency

II. Pyramid of biomass (measured in mass/area)

a. Biomass associated with the primary producer is placed on the bottom with blocks associated with trophic levels stacked one upon the other

b. Just as with pyramids of productivity, biomass pyramids can show dramatically decreasing biomass with increasing trophic levels

Ways of depicting trophic efficiency

III. Pyramid of productivity (measured in mass/area/time)

a. Productivity consumed is compared to productivity acquired, going up trophic levels, e.g., each level represents a drop of net productivity of approximately 90% (95% to 80%)

Can pyramids ever be inverted?

I. In open ocean systems, biomass pyramid appears inverted - why?

II. Key is the very rapid growth of lower trophic-level organisms; in terrestrial systems, producers are long-lived and inverted pyramids do not occur.

Case FOR being a vegetarian

I. Trophic efficiency: total energy required to support a carnivore is approx. 10x that of a vegetarian (carnivores eat 2 trophic levels above producers, while herbivores only eat 1 level above)

II. Ecological impact: because of the trophic efficiency argument, takes approx. 10x the amount of resources (land, etc) to support carnivores than it does to support vegetarians. Thus, vegetarians have a smaller ecological impact than do carnivores

Case AGAINST being a vegetarian

I. Transfer efficiencies: Less efficient to turn producer biomass into consumer biomass (approx. 10% transfer efficiency) than it is to turn consumer biomass into biomass of another consumer (approx 15-20%). In other words, we have a (net) gain of 10 calories from consuming 100 calories of vegetables, but a (net) gain of 15-20calories from consuming 100 calories of beef.

Nutrient cycling

Definition: the use, transformation, movement, and reuse of nutrients in ecosystems. Cycling of energy and the cycling of elements in ecosystems are fundamentally different: 20.3

A. Chemical elements are reused repeatedly, whereas energy flows through an ecosystem only once

B. Ecosystems consist of a series of linked compartments, and elements can move among them at different rates

Phosphorus cycle

A. Phosphorus is essential for the formation of ATP, RNA, DNA, and phospholipid molecules. Phosphorus may limit productivity:

I. In aquatic systems, sediments act as a phosphorus sink unless oxygen-depleted

II. In soils, phosphorus is only readily available between pH of 6 and 7

Where's phosphorus found?

I. Found primarily in mineral deposits and marine sediments

II. No substantial atmospheric pool

III. Phosphorus weathered from rocks eventually enter the oceans, where it becomes phosphate-rich sedimentary rock

Phosphorus cycle Mean time per cycle?

thousands of years

Nitrogen cycle

A. Nitrogen is essential for the formation of amino acids, nucleic acids, chlorophyll, hemoglobin, etc. May limit primary production in marine and terrestrial environments

Where's nitrogen found?

I. Major atmospheric pool in the form of molecular nitrogen, N2

II. Also stored in sediments.

Biological sources of nitrogen

I. Cyanobacteria

II. Free-living soil bacteria

III. N-fixing bacterial symbionts of legumes

Processes in the Nitrogen Cycle

I. Nitrogen Fixation

II. Ammonification

III. Nitrification

IV. Denitrification

Nitrogen fixation

Process by which N2 is reduced to ammonia, NH3 (can occur biologically or as a result of lightning strikes)

Ammonification

process in which nitrogen is released as ammonium, NH4+(occurs primarily during decomposition by fungi and bacteria)

a. Dissimilation of N is carried out by all organisms: the initial step is breakdown of proteins into constituent amino acids by hydrolysis; carbon (not nitrogen) in amino acids is then oxidized releasing ammonia (NH3)

Nitrification

process by which bacteria convert ammonium to nitrate, NO3-

a. Oxidation of ammonia to nitrite (NO2-) carried out by Nitrosomonas in soil and Nitrosococcus in oceans

b. Nitrite oxidized to nitrate (NO3-) by Nitrobacter in soil and Nitrococcusin oceans

Denitrification

process in which nitrate is converted to molecular nitrogen, N2, under anaerobic (without oxygen) conditions

a. Carried out by heterotrophic bacteria in waterlogged, anaerobic soils, oxygen-depleted sediments, and bottom waters in aquatic ecosystems

Carbon cycle

A. Carbon is essential part of organic molecules and a variety of atmospheric compounds

Where's carbon found?

Atmosphere, sediments, organisms, etc

Photosynthesis removes ___ from the atmosphere, while respiration by primary producers and consumers returns carbon to the atmosphere in the form of CO2

CO2

In aquatic ecosystems, CO2 must first dissolve in ______ before being used; while in water, carbonate (CO3-) can precipitate out and be buried in sediments

I. Turnover of these sediments is far slower than those associated with assimilation/dissimilation or ocean-atmosphere exchange

II. Carbonate sediments are the single largest compartment of carbon on planet

water

Carbon cycle mean cycle time?

Varies as a function of whether carbon is or is not stored in sediments

Factors affecting nutrient loss in ecosystems

I. Decomposition

II. Disturbance

Decomposition

breakdown of organic matter accompanied by the release of CO2

I. Abiotic factors affecting decomposition rates

a. Moisture: Positively correlated with decomposition

b. Temperature: Positively correlated with decomposition

c. Soil fertility: Positively correlated with decomposition

II. Biotic factors affecting decomposition rates

a. Species richness often increases decomposition rates

b. Burrowing organisms (e.g., pocket gophers, earthworms) increase decomposition by bringing nutrient-poor soil to the surface

c. Quality of leaf litter (plant detritus): Lignin to nitrogen ratio is negatively correlated with decomposition

Disturbance

Positively correlated with nutrient loss

I. Clearcutting at Hubbard Brook Experimental Forest increased nutrient loss into stream basins

II. In aquatic systems, flooding and/or high-flow events cause most nutrient loss