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BIOL 121 Lecture Notes - Intermediate Disturbance Hypothesis, Primary Production, Species Richness

Course Code
BIOL 121
Carol Pollock

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Freeman 1217-20, 1222-30 Jan. 18, 10
Species richness
-is a simple count of how many species are present in a given community
Species diversity
-is a weighted measure that incorporates a species’ relative abundance as well as its
presence or absence
-hard to do on relatively small study plots
Why are some communities more
species rich than others?
-larger patches of habitat contain more species than do smaller patches large
areas contain more types of niches and therefore support higher number of species
Immigration and Extinction rates
and the theory of island
-immigration rates should decline as the number of species in an area increases
individuals that arrive are more likely to represent a species already present
-extinction rates should increase as species richness increases niche overlap and
competition becomes more intense
-the result is an equilibrium balance between arrival of new species and extinction
of existing ones
-immigration rates should be higher on large islands close to mainland because
immigrants are more likely to find large islands that are close to shore than small
ones that are far away
-extinction rates should be highest on small islands far from shore, because fewer
resources are available to support large populations and because fewer individuals
arrive to keep the population going
-therefore, species richness should be higher on larger islands than smaller islands
and on near shore islands versus remote islands
Why is this theory important? (3)
-relevant to wide variety of island-like habitats like alpine meadows, lakes, ponds,
-made specific predictions that could be tested
-help inform decisions about design of natural preserves
-in general, most-species rich reserves should be ones that are relatively large, and
located close to other relatively large habitat areas
Why does a strong latitudinal
gradient in species diversity exist?
(higher species richness near
equator, and decreasing numbers
going South or North)
-first, consider two fundamental principles to solve problem:
1) Causal mechanism must be abiotic because latitude is a physical phenomenon
produced by Earth’s shape
2) Species diversity of a particular area is the sum of four processes: speciation,
extinction, immigration, emigration (dispersal)
-thus, latitudinal gradient must be caused by abiotic factor that affects the rates of
these four things
-over 30 hypotheses have been proposed to explain latitudinal gradient:
1) High productivity in tropics promotes high diversity by increasing speciation rates
and decreasing extinction rates (productivity is the total amount of P/S per unit area
per year)
-idea is that increased biomass production supports more herbivores and thus more
Biol 121 225
Freeman 1217-20, 1222-30 Jan. 18, 10
-speciation rates should increase also when niche differentiation occurs w/n pop. of
herbivores, predators, parasites, scavengers
-however, there is some evidence that increased productivity can be coupled with
decrease in diversity
2) Energy hypothesis high temperatures increase species diversity by increasing
productivity and the likelihood that organisms can tolerate the physical conditions in
a region
3) Tropical regions have had more time for speciation to occur than have other
regions tropical regions did not have ‘ice age’
4) Intermediate disturbance hypothesis: Species diversity is much higher in mid-
successional communities than in pioneer or mature communities
-regions with moderate type, fq, and severity of disturbance should have high
species richness/diversity with intermediate disturbance levels, communities will
contain pioneering as well as late-successional species
Definition of ecosystem
-an ecosystem consists of the organisms that live in an area together with their
physical, or abiotic, environment
Primary producer/autotroph
-a primary producer is an autotroph (“self-feeder”), an organism that can synthesize
its own food from inorganic sources
-in most ecosystems, they use solar energy to get food via P/S
-but in some such as deep-sea hydrothermal vents, bacteria use chemical energy
contained in inorganic compounds like hydrogen gas, methane, H2S to make food
-primary producers form basis of ecosystem because they transform energy in
sunlight or inorganic compounds into chemical energy stored in sugars
-they use the chemical energy in two ways: most supports maintenance/respiration;
some makes growth and reproduction possible
Net primary productivity
-abbreviated NPP, it is the energy invested in new tissue
-it represents the amount of energy available to 2nd and 3rd components of an
ecosystem: consumers and decomposers
-eats living organisms
-herbivores are consumers that eat plants; carnivores are consumers that eat
-decomposers and detritivores obtain energy by feeding on the dead remains of
other organisms or waste products
Decomposer vs. detritivore
-both types of consumers obtain energy by feeding on dead organic material
(remains, waste)
-the decomposer such as certain fungi secrete enzymes that decompose organic
material, allowing them to then feed on the material
-the detritivore such as earthworms ingest detritus (decomposing organic matter)
first, and then it is further decomposed inside their bodies
NPP global distribution
-NPP dictates amount of energy available to consumers and decomposers
-global warming alters NPP worldwide
-terrestrial ecosystems with highest productivity are in the wet tropics
-marine productivity is highest along coastlines, and high near poles as it is in tropics
-oceanic zones have extremely low NPP
-even though ocean has low NPP, it is so vast that it is responsible for a large
Biol 121 225
Freeman 1217-20, 1222-30 Jan. 18, 10
percentage of Earth’s NPP
What limits productivity?
-overall productivity of terrestrial ecosystems limited by combination of T,
availability of water, and sunlight
-productivity of marine habitats is higher along coastlines than in deepwater due to
nutrient limitation
-trace elements like Zn, Fe, Mg are rare in open ocean but are important for enzyme
-net primary productivity results in biomass organic material that non-P/S
organisms can eat
-in every terrestrial and marine environment in world, chemical energy in primary
producers eventually moves to one of two types of organisms: primary consumers
or primary decomposers
-biomass represents chemical energy
Primary consumer
-is a herbivore organism that eats plants or algae or other photosynthetic cells
-only a small % of all plant tissue is consumed by herbivores tissues not consumed
eventually die
Primary decomposer
-dead animals and dead plant tissues (“plant litter”) are collectively known as
-detritus is consumed by a variety of primary decomposers, incl. bacteria, archaea,
Trophic level
-biomass represents chemical energy each time herbivore eats leaf, chemical
energy flows from primary producer to primary consumer/decomposer
-to describe energy flow, biologists identify distinct feeding levels in ecosystem
-organisms that obtain energy from same type of source are said to occupy the same
trophic (feeding) level
Food chain
-connects trophic levels in particular ecosystem describes energy flow between
trophic levels
-primary consumers are called the grazing food chain eat plants (herbivores)
-secondary consumer consumers that eat herbivores
-organisms at top trophic level in food chain are not killed and eaten by any other
organisms but enter the decomposer food chain when they die
-a decomposer food chain starts with primary decomposers and includes primary
consumers that usually specialize in eating primary decomposers
Food web
-most organisms eat more than one type of food (feed at several trophic levels),
resulting in more complex food webs
-food webs are a compact way of summarizing energy flows and documenting
complex trophic interactions occurring in communities
Energy loss
-all ecosystems lose biomass produced
-total biomass produced each year declines from lower trophic levels up to higher
-energy is lost from one trophic level to the next
-much of NPP produced each year is unavailable to herbivores, b/c in indigestible
substances like lignin in wood
-efficiency of energy transfer decreases at higher trophic levels
What limits the length of a food
Hypothesis 1: Energy Transfer
-as energy transferred up chain, large fraction of energy lost
-by time energy reaches top trophic level, may not be enough left to support
additional suite of consumers
-therefore there should be more trophic levels in ecosystems with high productivity
or higher efficiency of energy transfer than in ecosystems with lower productivity of