ecosystems power point

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Dynamics of Ecosystems

Chapter 57

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Biogeochemical Cycles• Ecosystem: includes all the organisms

that live in a particular place, plus the abiotic environment in which they live and interact

• Biological processing of matter: cycling of atoms in the environment and in living organisms

• Biogeochemical cycles: chemicals moving through ecosystems; biotic and abiotic

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Biogeochemical Cycles• Biogeochemical cycles usually cross the

boundaries of ecosystem– One ecosystem might import or export

chemicals to another• Carbon is a major constituent of the bodies

of organisms:– ~20% of weight of human body is carbon– Makes up 0.03% volume of the

atmosphere; 750 billion metric tons

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Biogeochemical Cycles• The carbon cycle

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Biogeochemical Cycles• Carbon fixation: metabolic reactions

that make nongaseous compounds from gaseous ones

• In aquatic systems inorganic carbon is present in water as dissolved CO2 and as HCO3

- ions

• CO2 is used by algae and aquatic plants for photosynthesis

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Biogeochemical Cycles• Methane producers

– Microbes that break down organic compounds by anaerobic cellular respiration provide an additional dimension to the carbon cycle

– Methanogens: produce methane (CH4)

– Wetland ecosystems are a source of CH4

– CH4 is oxidized to CO2, but can remain as CH4 for a long time

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Biogeochemical Cycles• Over time, globally, the carbon cycle may

proceed faster in one direction• This can cause large consequences if

continued for many years• Earth’s present preserves of coal, and other

fossil fuels were built up over geological time

• Human burning of fossil fuels is creating large imbalances in the carbon cycle

• The concentration of CO2 in the atmosphere is going up year by year

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Biogeochemical Cycles• Water Cycle

– All life depends on the presence of water– 60% of the adult human body weight is

water– Amount of water available determines the

nature and abundance of organisms present

– It can be synthesized and broken down• Synthesized during cellular respiration• Broken down during photosynthesis

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Biogeochemical Cycles• Basic water cycle

– Liquid water from the Earth’s surface evaporates into the atmosphere

– Occurs directly from the surfaces of oceans, lakes, and rivers

– Terrestrial ecosystems: 90% of evaporation is through plants

– Water in the atmosphere is a gas– Cools and falls to the surface as

precipitation

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Biogeochemical Cycles• Groundwater: under ground water

– Aquifers: permeable, underground layers of rock, sand, and gravel saturated with water

– Important reservoir : 95% fresh water used in United States

– Two subparts: • Upper layers constitute the water

table• Lower layer can be tapped by wells

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Biogeochemical Cycles• Water cycle

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Biogeochemical Cycles• Changes in the supply of water to an

ecosystem can radically alter the nature of the ecosystem

• Deforestation disrupts the local water cycle

• Water that falls as rain drains away

• Tropical rain forest semiarid desert

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Biogeochemical Cycles• Nitrogen Cycle

– Nitrogen is a component of all proteins and nucleic acids

– Usually the element in shortest supply– Atmosphere is 78% nitrogen– Availability

• Most plants and animals can not use N2 (gas)

• Use instead NH3, and NO3-

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Biogeochemical Cycles• Nitrogen fixation: synthesis of nitrogen

containing compounds from N2

– Nitrification: N2 --> NH3 --> NO3-

– Denitrification: NO3- --> N2

– Both processes are carried out by microbes: free or living on plant roots

– Nitrogenous wastes and fertilizer use radically alter the global nitrogen cycle

– Humans have doubled the rate of transfer of N2 in usable forms into soils and water

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Biogeochemical Cycles• Nitrogen Cycle

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Biogeochemical Cycles• Phosphorus cycle

– Phosphorus is required by all organisms• Occurs in nucleic acids, membranes,

ATP– No significant gas form– Exists as PO4

3- in ecosystems– Plants and algae use free inorganic

phosphorus, animals eat plants to obtain their phosphorus

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Biogeochemical Cycles• Phosphorus cycle

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Biogeochemical Cycles• Limiting nutrient: weak link in an

ecosystem; shortest supply relative to the needs of organisms

• Iron is the limiting nutrient for algal populations

• Nitrogen and phosphorus can also be limiting nutrients for both terrestrial and aquatic ecosystems

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Biogeochemical Cycles

Every year millions of metric tons of iron-rich dust is carried by the trade winds, from the Sahara Desert, across the globe to as far as the Pacific Ocean

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Biogeochemical Cycles• Biogeochemical cycling in a forest

ecosystem-- Hubbard Brook Experiment• Undisturbed forests are efficient at

retaining nutrients• Disturbed (cut trees down) amount of

water runoff increased by 40%– Loss of Ca; increased nine fold– Loss of Phosphorus did not increase – Loss of NO3

-; 53kg/hectare/yr

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Biogeochemical Cycles

The Hubbard Brook Experiment38-acre watershed. Orange curve shows nitrate concentration in the runoff water from the deforested watershed. Green curve shows the nitrate concentration in runoff from an undisturbed watershed

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Flow of Energy in Ecosystems• Energy is never recycled• Energy exists as;

– Light– Chemical-bond energy– Motion– Heat

• First Law of Thermodynamics: energy is neither created nor destroyed; it changes forms

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• Organisms cannot convert heat to any of the other forms of energy

• Second Law of Thermodynamics: whenever organisms use chemical-bond or light energy some is converted to heat (entropy)

• Earth functions as an open system for energy

• Sun our major source of energy

Flow of Energy in Ecosystems

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• Earth’s incoming and outgoing flows of radiant energy must be equal for global temperatures to stay constant

• Human activities are changing the composition of the atmosphere

• Greenhouse effect: heat accumulating on Earth, causing global warming


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• Trophic levels: which level an organism “feeds” at

• Autotrophs: “self-feeders” synthesize the organic compounds of their bodies from inorganic precursors– Photoautotrophs: light as energy source– Chemoautotrophs: energy from

inorganic oxidation reactions • prokaryotic


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• Heterotrophs: cannot synthesize organic compounds from inorganic precursors; – animals that eat plants and other

animals;– fungi that use dead and decaying

matter (detritivores)


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• Trophic levels– Primary producers: autotrophs– Consumers: heterotrophs

• Herbivores: first consumer level• Primary carnivores: eat herbivores• Secondary carnivores: eat primary

carnivores or herbivores• Detritivores: eat decaying matter

–Decomposers: microbes that break up dead matter


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Trophic levels within an ecosystem

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• Productivity: the rate at which the organisms in the trophic level collectively synthesize new organic matter

• Primary productivity: productivity of the primary producers

• Respiration: rate at which primary producers break down organic compounds


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• Gross primary productivity (GPP): raw rate at which primary producers synthesize new organic matter

• Net primary productivity (NPP): is the GPP less the respiration of the primary producers

• Secondary productivity: productivity of a heterotroph trophic level


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• Standing crop biomass: chief static property of a population or trophic level; the amount of organic matter present at a particular time

• Fraction of incoming solar radiant energy captured by producers is ~ 1%/year

• Used to make chemical-bond energy• Break bonds in ATP for metabolic

processes


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• 50% of chemical-bond energy is not assimilated and is egested in feces

• 33% of ingested energy is used for cellular respiration

• 17% ingested energy is converted into insect biomass


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Ecosystem productivity per year

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• Limits on top carnivores: exponential decline of chemical-bond energy limits the lengths of trophic chains and the numbers of top carnivores an ecosystem can support– Little energy– Large carnivores– Longest chains occur in the oceans– Top carnivore populations are small


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Flow of energy through the trophic levels of Cayuga Lake


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• Trophic level interactions– Trophic cascade: process by which

effects exerted at an upper level flow down to influence two or more lower levels

– Top-down effects: when effects flow down

– Bottom-up effects: when effect flows up through a trophic chain


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Top-down effects in a simple trophic cascade in a New Zealand stream


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Top-down effects in a four-level trophic cascade. Stream enclosures with and

without large carnivorous fish

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Trophic cascade in a large-scale ecosystem


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• Human removal of carnivores produces top-down effects– Aldo Leopold: posited effects– Over fishing of cod - 10% their

previous numbers– Jaguars and mountain lions absent

on Barro Colorado Island– Smaller predators become abundant


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• To predict bottom-up effects must take into account life history of the organisms

• When primary productivity is low, producer populations cannot support herbivore populations

• As primary productivity increases, herbivore populations increase

• Increased herbivore populations lead to carnivore populations increasing


44Bottom up effects


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Biodiversity and Stability

David Tilman: species richness may increase stability of an ecosystem

• Plots with more species showed less year-to-year variation in biomass

• Drought: decline in biomass negatively related to species richness

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• Tilman’s conclusion not accepted by all ecologists

• Critics question the validity and relevance: – When more species are added to a

plot the greater the probability that one species will be highly productive

– Plots would have to exhibit “over yielding”


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• Species richness is influenced by ecosystem characteristics– Primary productivity– Habitat heterogeneity

• Accommodate more species– Climatic factors

• More species might be expected to coexist in seasonal environment


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Factors that affect species richness


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• Tropical regions have the highest diversity– Species diversity cline: biogeographic

gradient in number of species correlated with latitude • Reported for plants and animals

– Evolutionary age of tropical regions– Increased productivity– Stability/constancy of conditions– Predation– Spatial heterogeneity


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Latitudinal cline in species richness


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Island Biogeography• Robert MacArthur and Edward O. Wilson

proposed that species-area relationship was a result of the effect of geographic area and isolation– Islands have a tendency to accumulate

more and more species through dispersion

– Rate of colonization must decrease as the pool of potential colonizing species becomes depleted

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Island Biogeography• The rate of extinction should increase--the

more species on an island• At some point extinctions and colonizations

should be equal• MacArthur and Wilson equilibrium

model: island species richness is a dynamic equilibrium between colonization and extinctionIsland size and distance from the mainland would affect colonization and extinction

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• The equilibrium model is still being tested by Wilson and Simberloff

• Long-term experimental field studies are suggesting that the situation is more complicated than first believed

Equilibrium model

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