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Lecture 3

BIOL 121 Lecture Notes - Lecture 3: Intermediate Disturbance Hypothesis, Species Richness, Primary Production

Course Code
BIOL 121
Carol Pollock

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Biol 121 225
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, caves
-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
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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 predators/parasites/scavengers
-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
Consumer -eats living organisms
-herbivores are consumers that eat plants; carnivores are consumers that eat
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