• The percentage of energy transferred from one trophic level to the next varies between 5% and 20% and is called the ecological
efficiency.
An average figure of
10% is often used.
This ten percent law states that the total energy content of a trophic level in an ecosystem is only about one-tenth that of the preceding level.
100 J
Plant material consumed by caterpillar
200 J
33 J
Feces Growth
67 J
Cellular respiration

‣ Energy flow into and out of each trophic level in a food chain can be represented on a diagram using arrows of different sizes to represent the different amounts of energy lost from particular levels.
• The energy available to each trophic level will always equal the amount entering that trophic level, minus total losses to that level.

•

•
energy content of individuals at each level.
• This information can be presented as an ecological pyramid.
• The base of each pyramid represents the producers and the subsequent trophic levels are added on top in their ‘feeding sequence’.

‣ Various types of pyramid are used to describe different aspects of an ecosystem’s trophic structure:
Pyramid of numbers
Pyramids of numbers: In which the size of each tier is proportional to the number of individuals present at each trophic level.
Pyramids of biomass: Each tier represents the total dry weight of organisms at each trophic level.
Pyramids of energy (production):
The size of each tier is proportional to the production
(e.g. in kJ) of each trophic level.
Pyramid of biomass
Pyramid of energy

• In a typical pyramid of numbers, the number of individuals supported by the ecosystem at successive trophic levels declines progressively.
• This reflects the fact that the smaller biomass of top level consumers tends to be concentrated in a relatively small number of large animals.
• There are some exceptions. In some forests a few producers (of a very large size) may support a larger number of consumers, and the pyramid is inverted. This also occurs in plant/parasite food webs.
Forest Grassland

•
In pyramids of biomass, dry weight is usually used as the measure of mass because the water content of organisms varies.
• Organism size is taken into account so meaningful comparisons of different trophic levels are possible.
• Biomass pyramids may be inverted in some systems (e.g. in some plankton communities) because the algal (producer) biomass at any one time is low, but the algae are reproducing rapidly and have a high productivity.
A Florida bog community The English Channel

• Pyramids of energy (or production) are often very similar in appearance to pyramids of biomass.
• The energy content at each trophic level is generally comparable to the biomass because similar amounts of dry biomass tend to have about the same energy content.
• This example illustrates the similarity between pyramids of biomass (gm -2 ) and energy (kJ) in a freshwater lake community. During warm months, when algal turnover time is short, pyramids of energy and biomass may be inverted.
Zooplankton (C1)

‣ Carbon cycles between the Burning fossil fuels living (biotic) and non-living
(abiotic) environments.
Gaseous carbon is fixed in the process of photosynthesis and returned to the atmosphere in
respiration.
Carbon may remain locked up in biotic or abiotic systems for long periods of time, e.g. in the wood of trees or in fossil
fuels such as coal or oil.
Humans have disturbed the balance of the carbon cycle through activities such as combustion and deforestation.
Petroleum

‣ Nitrogen cycles between the biotic and abiotic environments. Bacteria play an important role in this transfer.
Nitrogen-fixing bacteria are able to fix atmospheric nitrogen.
Nitrifying bacteria convert ammonia to nitrite, and nitrite to nitrate.
Denitrifying bacteria return fixed nitrogen to the atmosphere.
• Atmospheric fixation also occurs as a result of lightning discharges.
‣ Humans intervene in the nitrogen cycle by producing and applying nitrogen fertilizers.

‣ The ability of some bacterial species to fix atmospheric nitrogen or convert it between states is important to agriculture.
Nitrogen-fixing species include Rhizobium, which lives in a root symbiosis with leguminous plants. Legumes, such as clover, beans, and peas, are commonly planted as part of crop rotation to restore soil nitrogen.
Root nodules in Acacia
Nitrifying bacteria include Nitrosomonas and
Nitrobacter. These bacteria convert ammonia to forms of nitrogen available to plants.
Nodule close-up
NH
3
NO
2
-
Nitrosomonas Nitrobacter
NO
3
-

• Phosphorus cycling is very slow and tends to be local; in aquatic and terrestrial ecosystems, it cycles through food webs.
Phosphorous is lost from ecosystems through run-off, precipitation, and sedimentation.
A very small amount of phosphorus returns to the land as
guano (manure, typically of fisheating birds). Weathering and
phosphatizing bacteria return phosphorus to the soil.
Human activity can result in excess phosphorus entering water ways and is a major contributor to
eutrophication.
Deposition as guano…
Loss via sedimentation…
Fertilizer production

Guano deposits

• Sulfur is an essential component of proteins and is important in determining the acidity of precipitation, surface water, and soil.
• Sulfur circulates through the biosphere as: hydrogen sulfide (H
2
S) sulfur dioxide (SO
2
) sulfate (SO
4
2) elemental sulfur (S)
• Human activity releases large quantities of sulfur through: combustion of sulfur-containing coal and oil, refining petroleum, smelting, and other industrial processes
Sulfur in petrol
Molecular bridges in proteins
Elemental sulfur

SO
2 from combustible fossil fuels
Sulfates in the atmosphere (SO
4
2)
Acid precipitation
SO
2 and sulfates from volcanoes, hot springs and biogenic activity
Sulfur in living organisms
Mining
Sulfur in fossil fuels
Uplifting in groundwater and and weathering
Decomposition and other processing
Sulfates in soil(SO
4
2)
Microorganisms
Uptake by plants
Reduced sulfur
(H
2
S)
Inorganic sulfur
Sedimentation of sulfides and sulfates
Sulfates in water (SO
4
2)
Iron sulfides in deep soil and sediments Organic deposition

• The hydrological (water) cycle, collects, purifies, and distributes the Earth’s water.
Over the oceans, evaporation exceeds precipitation. This results in a net movement of water vapor over the land.
Precipitation
On land, precipitation exceeds evaporation. Some precipitation becomes locked up in snow and ice for varying lengths of time.
Most water forms surface and groundwater systems that flow back to the sea.
Rivers and streams

Transport overland : net movement of water vapor by wind
Condensation conversion of gaseous water vapor into liquid water
Precipitation
(rain, sleet, hail, snow, fog)
Rain clouds
Precipitatio n to land
Evaporation from inland lakes and rivers
Transpiration Evaporatio n from the land
Surface runoff
(rapid)
Water locked up in snow and ice
Infiltration : movement of water into soil
Lakes
Percolation : downward flow of water
Transpiration from plants
Rivers
Aquifers: groundwater storage areas
Groundwater movement (slow)
Precipitation
Precipitation over the ocean
Evaporation
Evaporation from the ocean
Ocean storage
97% of total water

• Humans intervene in the water cycle by utilizing the resource for their own needs.
• Water is used for consumption, municipal use, in agriculture, in power generation, and for industrial manufacturing.
• Industry is the greatest withdrawer of water but some of this is returned.
Agriculture is the greatest water consumer.
• Using water often results in its contamination. The supply of potable
(drinkable) water is one of the most pressing of the world’s problems.
Hydroelectric power generation…
Irrigation…
Washing, drinking, bathing…

‣ An ecosystem’s stability refers to its apparently unchanging nature over time.
• Components of ecosystem stability include inertia (the ability to resist disturbance) and resilience (the ability to recover from external disturbance).
The diversity of ecosystems at low latitudes
(nearer the equator) is generally higher than at higher latitudes (nearer the poles). This photograph shows a forest in Hawaii.

Short-term
Supercell thunderstorms
Communities
Local communities and populations
Medium-term Long-term
Very longterm
10 4
Mountain building
10 3
10 2
Drainage basins, soil landscapes 10
1
10 5
Hill slopes, flood plains, glacial moraines and alluvial fans
10 4
10 3
10 2
10
Individual organisms
1
0.1
Time (years log
10 scale)

Short-term Medium-term Long-term
Very longterm
Phylum
Global circulation
Size of Earth
10 8
Class
Cyclones and anticyclones
Fronts
Zonobiomes
Biomes
Order
Family Tectonic plates form
Genus Large tectonic plate movements
Species
10 7
10 6
Hurricanes/ cyclones
10 5
Squall lines
Communities
Major tectonic movements
10 4
Time (years log
10 scale)

‣ Ecological theory suggests that all species in an ecosystem contribute to ecosystem function.
• Species loss past a certain point is likely to be detrimental to the functioning of the ecosystem and on its ability to resist change
(its stability).
• Ecosystem stability is closely linked with biodiversity but it is not clear what level of biodiversity is required to guard against ecosystem dysfunction.
Species play different roles in ecosystems

• Species whose influences on ecological communities are greater than would be expected on the basis of their abundance are called key
(keystone) species.
They are more influential in ecosystem stability than other species because of their pivotal role in some ecosystem function such as nutrient cycling.
• Elephants are a key species, and can alter the entire structure of the vegetation in those areas into which they migrate.
Their pattern of grazing on taller plant species promotes a predominance of lower growing grasses with small leaves.
Elephants knock down and consume taller trees and shrubs...
… allowing lower growing species to predominate

• Termites are key species. They are among the few larger soil organisms able to break down plant cellulose.
They shift large quantities of soil and plant matter and have a profound effect on the rates of nutrient processing in tropical environments.
• Pisaster ochraceous, the ochre star, is also a key species. It feeds on mussels along the coasts of North America.
If it is removed, mussels dominate, crowding out most algal species and leading to a decrease in the number of herbivore species.
Termite hill, Australia
The ochre star, California

•
A low diversity system varies more consistently with environmental variation.
A high diversity system is buffered against major fluctuations.

• Monocultures (single species crops), are low species diversity
systems. They are vulnerable to disease, pests, and disturbance.
• In contrast, natural ecosystems may appear homogeneous, e.g. grasslands, but contain many species which vary in their predominance seasonally.
Although natural grasslands may be easily disturbed, e.g. by burning, they are very resilient and usually recover quickly from disturbance.
Monoculture
Savanna

• The biodiversity of ecosystems at low latitudes is generally higher than that at high latitudes, where climates are harsher, niches are broader, and systems may be dependent on a small number of keystone species.
• Tropical rainforests are amongst the highest diversity ecosystems on Earth. They are generally quite resistant to disturbance, but once once degraded they have little ability to recover.
Deforestation of tropical rainforest

• One of the best ways to determine the health of an ecosystem is to measure the variety of organisms living in it.
• Diversity indices attempt to quantify the degree of diversity and identify indicators for environmental stress or degradation.
Natural, unaltered headwater streams are generally high in diversity
Artificially managed and channelled rivers are generally low in diversity

• Most indices of diversity are easy to use and widely used in ecology.
• Diversity indices include Simpson’s
Index for finite populations and the complement of Simpson’s Index for an infinite population (below).
The index ranges from 0 to almost 1
High diversity stream community
DI = 1 – p i
2 p i
2 = Ni/N : the proportion of species i in the population
Ni = the number of individuals of each species in the sample
N = the total number of individuals of all species in the sample
Low diversity stream community

‣
•
Past community
Community composition changes with time
Present community
Future community
Some species in the past community were out-competed or did not tolerate altered abiotic conditions.
The present community modifies such abiotic factors as:
• Light intensity and quality
• Wind speed and direction
• Air temperature and humidity
• Soil composition and water content
Changing conditions in the present community will allow new species to become established.
These will make up the future community .

• A succession (or sere) proceeds in
seral stages, until the formation of a climax community, which is stable until further disturbance.
• Early successional (or pioneer) communities are characterized by:
Simple structure, with a small number of species interactions
Broad niches
Low species diversity
Pioneer community, Hawaii
Broad niches

• In contrast to early successional communities, climax communities typically show:
Complex structure, with a large number of species interactions
Narrow niches
High species diversity
Climax community, Hawaii
Large number of species interactions

‣ Primary succession refers to colonization of a region where there is no pre-existing community.
Examples include:
Newly emerged coral atolls, volcanic islands
Newly formed glacial moraines
Islands where the previous community has been extinguished by a volcanic eruption
Hawaii: Local plants are able to rapidly recolonize barren areas

‣
A classical sequence of colonization begins with lichens, mosses, and liverworts, progresses to ferns, grasses, shrubs, and culminates in a climax community of mature forest.
In reality, this scenario is rare.
Bare rock and lichens
Mosses and liverworts
Grasses and herbaceous plants
Shrubs and fast growing trees
Mature, slow growing trees

• Secondary succession occurs where an existing community has been cleared by a disturbance that does not involve complete soil loss.
• Such disturbance events include cyclone damage,
forest fires and hillside slips.
Cyclone
Forest fire

• Because there is still soil present, the ecosystem recovery tends to be more rapid than primary succession, although the time scale depends on the species involved and on climatic and
edaphic (soil) factors.
Bare land
Grasses and herbaceous plants
Shrubs and small trees
Pioneer community
(annual grasses)
Young fast growing trees
Mature forest

•
•
Grassland and heathland in lowland Britain are plagioclimaxes.

‣ The reduced sunlight beneath large canopy trees impedes the growth of the saplings below.
When a large tree falls, a crucial hole opens in the canopy, allowing sunlight to reach the saplings below.
‣ The forest regeneration following the loss of a predominant canopy tree is called gap regeneration.
‣ Gap regeneration is an example of secondary succession.

‣ Gap regeneration is an important process in established forests in temperate and tropical regions.
‣ Gaps are the sites of greatest understorey regeneration and species recruitment.
‣ The creation of a gap allows more light to penetrate the canopy and alters other factors that affect regeneration, exposing mineral soils and altering nutrient and moisture regimes.

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