Molles: Ecology 2nd Ed.

PRIMARY PRODUCTION AND ENERGY FLOW

Chapter 18

Molles: Ecology 2nd Ed.

Chapter Concepts

• Terrestrial Primary Production limited by temperature and moisture

• Aquatic Primary Production limited by nutrient availability

• Consumers can influence rates of primary production in terrestrial and aquatic ecosystems

• Energy losses limit the number of trophic levels found in ecosystems

Molles: Ecology 2nd Ed.

Fundamental Concepts

• Primary Production• Fixation of energy by autotrophs in an

ecosystem

• Or, conversion of inorganic energy (light) into organic forms (chemical potential energy)

Molles: Ecology 2nd Ed.

Rate of Primary Production

• Amount of energy fixed during period of time Gross PP – Total amount of energy fixed Net PP – Amount of energy leftover after

autotrophs have met their metabolic needs (subtract plant respiration)

This is amount left over for consumption by herbivores

Molles: Ecology 2nd Ed.

Fundamental Concepts

• Trophic Level Position in food web determined by

number of energy transfers from primary producers to current level:

Primary producers = first level Primary consumers = second level Secondary consumers = third level Tertiary consumers = fourth level

Molles: Ecology 2nd Ed.

Tertiary consumerpredator

Secondary consumerpredator

Primary consumergrazer

Primary producerplant autotroph

Molles: Ecology 2nd Ed.

Evapotranspiration and Terrestrial Primary Production

• Rosenzweig (1968) Estimated influence of moisture and

temperature on rates of primary production Plot annual net primary production and

annual actual evapotranspiration (AET)

Molles: Ecology 2nd Ed.

Plot annual net primary production and annual actual evapotranspiration (AET)

AET – annual amount of water that evaporates and transpires

Cold dry ecosystems tend to have low AET Fig 18.2

Molles: Ecology 2nd Ed.

Evapotranspiration and Terrestrial Primary Production

Positive relationship between net primary production and AET

Sala found east-west variation in primary production correlated with rainfall

Molles: Ecology 2nd Ed.

So?

• Rainfall or, rainfall and temperature Predict net primary productivity in

terrestrial ecosystems

QuickTime™ and aGIF decompressorare needed to see this picture.

Molles: Ecology 2nd Ed.

Soil Fertility and Terrestrial Primary Production

• Soil fertility is important too!• Shaver and Chapin

Arctic net primary production twice as high on fertilized plots compared to unfertilized plots

Molles: Ecology 2nd Ed.

• Bowman N is main nutrient limiting net primary

production in a dry tundra meadow; N and P jointly limit production in a wet

meadow

Molles: Ecology 2nd Ed.

Fig 18.5

Molles: Ecology 2nd Ed.

Patterns of Aquatic Primary Production

• Several studies found quantitative relationship between phosphorus and phytoplankton biomass

• – nutrient availability controls rate of primary production in freshwater ecosystems

Molles: Ecology 2nd Ed.

Figure 18.9

Molles: Ecology 2nd Ed.

Global Patterns of Marine Primary Production

• Highest primary production by marine phytoplankton – in areas with higher levels of nutrient

availability• = continental margins

Nutrient run-off from land Sediment disturbance

• Open ocean tends to be nutrient poor Vertical mixing main nutrient source

A Comparative Analysis of Annual Primary Production (Kcal / m2 /yr)

0.29

(29%)

0.44

(44%)

0.61

(61%)

0.62

(62%)

NPP / GPP

0.0186

(1.86%)

0.0055

(0.58%)

0.0058

(0.58%)

0.0117

(1.17%)

GPP / Total Solar Radiation

13,0005,0007,50015,200Net Primary Production

32,0006,4004,7009,200Respiration

45,00011,40012,20024,400Gross Primary Production

2,415,0002,091,0002,091,0002,091,000Total Solar Radiation

Mature Tropical

Rain Forest

Middle-Aged

Oak Forest

Young Pine

Plantation

Alfalfa

Field

Summary of Comparisons of Ecosystem Primary Production

• Efficiency of primary production is reduced by drought or freezing-induced dormant seasons (sun light going to waste)

• Efficiency of primary production is reduced by high temperatures that increase respiration w/o increasing photosynthesis.

• Efficiency of primary production is reduced by large amounts of non-photosynthetic plant biomass (increase respiration w/o increasing photosynthesis).

Association b/t Plant Biomass and Primary Production

0.02500Ocean Upwelling Zone

4700Savanna (Grass + Trees)

20800Boreal Conifer Forest

22000Algal Beds & Coral Reefs

152500Swamps & Marshes

442000Tropical Rain Forest

PlantBiomass

Net Primary ProductionEcosystem

}

}

(There isn’t one)

Molles: Ecology 2nd Ed.

Consumer Influences

• Bottom-Up Controls Influences of physical and chemical factors

of an ecosystem• Top-Down Controls

Influences of consumers

Molles: Ecology 2nd Ed.

Lake Primary Production

• Carpenter et al. (1985) Piscivores + planktivorous fish can cause

significant deviations in primary productivity• Carpenter and Kitchell (1988)

Influence of consumers on lake primary productivity propagate through food webs

= Trophic Cascade Hypothesis

Molles: Ecology 2nd Ed.

Fig 18.11

Molles: Ecology 2nd Ed.

Fig. 18.13

Molles: Ecology 2nd Ed.

Carpenter + Kitchell

• Whole lake experiments• N. Wisconsin / UP Michigan

Molles: Ecology 2nd Ed.

UNDERC

• Three lakes: Peter, Paul, Tuesday• 1985: reciprocal transplants of fish

Molles: Ecology 2nd Ed.

Carpenter and Kitchell experiment

• Three experimental lakes Two w/bass One winterkill lake; Tuesday – no bass

Many planktivore fishes = minnows

Molles: Ecology 2nd Ed.

• Experiment: Remove 90% bass from Peter lake Put into Paul lake Remove 90% planktivores from Paul lake Put into Peter lake Third lake = control (Tuesday)

Molles: Ecology 2nd Ed.

Lake Primary Production

• Carpenter and Kitchell Fewer planktivorous fish led to reduced rates

of primary production No planktivorous minnows resulted in more

predator invertebrates Abundant, large herbivorous zooplankton

increased, phytoplankton biomass and primary productivity

Molles: Ecology 2nd Ed.

Fig 18.13Summary

Molles: Ecology 2nd Ed.

Primary Production in the Serengeti

25,000 km2 grassland Millions of large mammals still present

Molles: Ecology 2nd Ed.

McNaughton (1985)

Serengeti grazers consume average of 66% of annual primary production

Rate of primary production in Serengeti positively correlated with rainfall quantity

Molles: Ecology 2nd Ed.

Primary Production in the Serengeti

• Found that grazers can increase primary production Increased growth rate =

Compensatory Growth is plant response Lower respiration rate due to lower

biomass Reduced self-shading Improved water balance due to

reduced leaf area

Molles: Ecology 2nd Ed.

Fig 18.14

Molles: Ecology 2nd Ed.

Primary Production in the Serengeti

• In addition, McNaughton found compensatory growth highest at intermediate grazing intensities Light grazing insufficient to produce

compensatory growth Heavy grazing reduces plant’s capacity to

recover

Molles: Ecology 2nd Ed.

Fig. 18.15

Molles: Ecology 2nd Ed.

Trophic Dynamic View of Ecosystems

• Lindeman (1942) Ecosystem concept fundamental to study

of energy transfer within an ecosystem Suggested grouping organisms within an

ecosystem into trophic levels Each feeds on level immediately below

As energy is transferred from one trophic level to another, energy is degraded

Molles: Ecology 2nd Ed.

Trophic Dynamic View of Ecosystems

As energy is transferred from one trophic level to another, energy is degraded:

Limited assimilation Consumer respiration Heat production

Energy quality decreases with each successive trophic level

– Pyramid-shaped energy distribution

Molles: Ecology 2nd Ed.

Fig. 18.16

Trophic Pyramids

Why are big, fierce animals(top carnivores) rare ?

Because there is notenough energy to supportlarge populations at thehighest trophic levels.

Bio-magnification

Human Trophic Pyramid

10 1010 7

10 6

How Will We Feed a Growing Human Population ?

After 30 Years of Increase, Grain Production per Person Has Stabilized

Green RevolutionAgricultural production increases faster than human population

Agricultural productionjust keeping pace withhuman population growth

What was the “Green Revolution”

• Genetic improvement of crop species to increase grain yields and resistance to pests, disease, wind damage.

• Increased use of fertilization, herbicide, pesticide, and irrigation.

All the “easy” gains in production have been obtained.Further gains will come slowly, if at all.

Negative effects of fertilization, pesticides and irrigationare beginning to affect production

The human population continues to grow

Acreage of Productive Farmland Is Declining In the USA

• From 1992 – 1997 six million acres of farm land were converted to developed use.

• We lost farm and ranch land 51% faster in the 1990’s than in the 1980’s.

• Prime agricultural land is being converted 30% faster than non-prime rural lands.

• We must depend more on food production from marginal land that requires more irrigation and fertilization.

“Farming On the Edge” (American Farmland Trust)

Losing Farm Land (USA)

25% of U.S. food supplycomes from the CaliforniaCentral Valley

Losing Farm Land (Indiana)

•Wasteful land use is the problem, not growth itself.

Losing Farm Land (Indiana)

•Wasteful land use is the problem, not growth itself.

•From 1982-1997 the U.S. population grew by 17%, while urbanized land grew by 47%.

Losing Farm Land (Indiana)

•Wasteful land use is the problem, not growth itself.

•From 1982-1997 the U.S. population grew by 17%, while urbanized land grew by 47%.

•Over the past 20 years, the acreage per person for new housing almost doubled .

Losing Farm Land (Indiana)

•Wasteful land use is the problem, not growth itself.

•From 1982-1997 the U.S. population grew by 17%, while urbanized land grew by 47%.

•Over the past 20 years, the acreage per person for new housing almost doubled .

•Since 1994, 10+ acre housing lots have accounted for 55% of land developed.

Energy Flow In the Human Food Chain

Energy In Corn Grain (8.2 million Kcal)

Energy Assimilated by Feedlot Cattle (7.0 million Kcal)

Energy in the Body of Mature Feedlot Cattle (1.2 million Kcal)

Energy in Meat from Feedlot Cattle (0.4 million Kcal)

5% of Energy in Corn is Available In Beef

Not assimilated by cow

Respiration

Waste in meat processing

Maybe humans shouldjust eat the corn

Meat Production Continues to Rise

All Endothermic Animals

More Efficient Meat Production

???Farm-grown Fish

2Chicken

4Pork

7Beef

Pounds of Grain per Pound of Meat ProducedType of Meat

How to Feed a Growing Human Population With a Finite or Shrinking Agricultural Land Base

• Increase the proportion of calories in the diet provided by grains.

• Increase the proportion of protein in the diet provided by energy-efficient animals (poultry and fish).

• Protect prime farm land from development.

The End