Download LECTURE 18 BIODIVERSITY

BIO 300
Ecology
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LECTURE 18
BIODIVERSITY
I. Accounting for Species Diversity (pp 296 -301)
A. Accounting for Species in the Community
1. The simplest approach –a list of species,
2. Species richness- # of species in a sample of a standard size or # of species/unit area.
3. Performance curve. As n increases, the value of the mean should stabilize near theoretical pop. mean.
4. Log-normal curve. Arises in all communities in which the total number of species is large.
The log of species abundance versus # of species in each abundance interval → Bell-shaped curve.
B. Species abundance
1. Accounting for species, or the adequacy of sampling, is accomplished by generating a species:area curve.
a. The Cumulative number of species is plotted on the Y-axis and the number of areas sampled, or the increase
in the size of the area sampled, is plotted on the X-axis.
2. Log-normal curve indicates that in an area, there is not necessarily species evenness, or species equitability.
a. Species evenness is the distribution of individuals among species- Evenness is maximized when all
species have the same # of individuals.
C. Concept of assessing species diversity.
1. Species diversity- species richness and evenness combined. It is species richness weighted by species
evenness.
2. For example:
a) Community A has 5 species with uneven #'s in each. Community B has 4 species, but even #'s in each.
b) Community A has greater richness, but Community B is more diverse, and diversity is the measure of
population heterogeneity of a community.
c) Although different, richness and diversity may be correlated, either positively or negatively.
D. Assumptions or simplifications made in calculating species diversity.
1. All of the individuals of any species are equal.
2. All species are equally different- either morphologically or niche breadth.
a. Therefore, one needs to calculate diversity differently for different strata.
3. Expressed using the same units, so basis for comparison is equal- e.g. # individuals, biomass, productivity.
E. What does diversity signify? (pp 317-320)
1. In the 1960’s, most ecologists believed that the greater the species diversity and the accompanying web of
feeding and biotic interactions in an ecosystem, the greater its stability.
2. According to this hypothesis, an ecosystem with a diversity of species and feeding paths has more ways to
respond to most environmental stresses because it does not have “all its eggs in one basket.”
3. There has been some evidence to support this concept.
⇒ Small islands, which are lower in species richness, are much more vulnerable to invading species than are
continents.
⇒ Outbreaks of pests are most often found in simple communities on cultivated land or land disturbed by
humans.
⇒ Tropical rain forests do not have insect outbreaks like those of common temperate forests.
⇒ Pesticides have caused outbreaks by the elimination of predators and parasites from the insect community
of crop plants.
⇒ Polar regions have low species diversity compared with the tropics; one characteristic of polar populations
is that they are more likely to fluctuate widely than are tropical populations.
4. Evidence that diversity does not create stability.
⇒ The fluctuations of microtine rodents (lemmings, voles and so on) are as pronounced in relatively complex
temperate ecosystems as they are in simple Arctic environments.
⇒ Rain forests seem particularly susceptible to man-made perturbations, yet these are very complex systems.
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⇒ Agricultural systems may suffer insect outbreaks not because of their simple nature, but because the
individual components have no coevolutionary history whatsoever, in complete contrast to the long
associations evident in natural systems.
Part of the problem is that ecologists don’t agree on how to define stability and diversity.
A more precise definition can be utilized by recognizing that stability has two separate components; resistance
and resilience.
a. Resistance is the ability of a system to withstand or resist variation (or disturbance). A forest stand is
relatively resistant. It can withstand such environmental disturbances as sharp temperature changes,
drought, and insect outbreaks because the system is able to draw on energy reserves and stored nutrients.
b. Resilience is the ability of a system to return to normal following disturbance. If the forest is disturbed
by fire or logging, its return to original condition will be slow.
Ecologists have demonstrated for only a few precisely defined systems that
diversity → complexity → stability.
II. Patterns of Species Diversity
A. The variety of life existing worldwide emphasizes diversity on a global scale.
1. The diversity of species is greater at lower latitudes and at lower elevations in mountainous country.
2. Conversely, species diversity is lowest in high latitudes, the Polar Regions.
3. The number of species of nesting birds, mammals, fish, lizards and trees all decrease latitudinally from the
tropics to the polar regions.
4. From east to west in North America the number of species of breeding land birds, and mammals increases.
5. Much of the east to west gradient in vertebrate species richness can be correlated with annual potential
evapotranspiration.
a) The species richness of trees can be linked to actual potential evapotranspiration.
6. Mountain regions generally support more species than flatlands because of topographic diversity.
III. Mechanisms of Species Diversity- Island Biogeography (pp. 304-310 + p. 311)
A. Non-equilibrium Theory of Island Biogeography
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Islands contain low species richness (i.e. fewer species than the mainland) because they have not had sufficient
time to accumulate species via immigration. They have not yet reached equilibrium.
The Equilibrium Theory of Island Biogeography
The lower number of species on an island is not the result of insufficient time, but due to an equilibrium
situation that is peculiar to island alone.
2. On any island, species immigrate from the nearest mainland.
a. Species that can disperse readily will colonize sooner than those that can't. (E.g. insects, spiders and birds
vs. animals and plants with heavy seeds.)
3. As more and more species arrive on the island, the rate of new species arriving declines.
a. This is because there are fewer species from the mainland that have yet to colonize the island. Most likely
species of low dispersal ability.
b. When all species from the mainland have arrived, or will arrive, the immigration rate will drop to zero.
4. As more and more species arrive on the island, the extinction rate should increase.
a. This is because competition for limited resources becomes more intense as more species arrive. Not all
will survive.
C. Determining the Equillibrium number of species on islands.
1. The intersection of immigration and extinction rates.
a. Not necessarily straight lines.
2. Colonization curves of islands.
a. Simberloff and Wilson survey all the species of arthropods (insects) on several mangrove islands off the
Florida coast. Some islands have been fumigated.
b. Over time the number of species present on the islands levels off to numbers near those that were present
pre-fumigation.
c. However, the types of species make up the total changed. Again, confirm that islands don't contain the full
compliment of the mainland.
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Bottom line - there is something fundamentally different about islands that they will never have the same
species richness as islands.
D. Far and Near Islands
1. Far islands should have lower immigration rates since probability of a colonist making it there is lower.
a. Once there, extinction rates should not be different.
2. Large islands should have a greater immigration rate since they present a larger target and increase the chances
of a colonist encountering them.
a. Once there, large islands have more resources, so extinction rates should be lower.
IV. More Mechanisms of Species Diversity (pp.310-315)
1. The history factor has two components; evolutionary time hypothesis and the ecological time hypothesis.
a. The evolutionary time hypothesis relates diversity to the age of the community. The older the community,
the more chance for species to evolve, ergo the more diversified it should be.
b. The ecological time hypothesis is based largely on the need for species to disperse into unoccupied areas of
suitable habitat. This requires that an area be stable for sufficient time.
2. Climatic stability hypothesis. Closely linked with the history hypothesis.
a. The more stable the climate, (one that does not change with the seasons) the greater the species richness.
b. Closely related is the Climatic predictability model. The climate can vary, but must do so in a
predictable manner so that specie can adapt.
3. The primary productivity model (The energy model). For regions of roughly equal area, the energy flux per
unit area should be the major determinant of species determinant. Dependent upon evapotranspiration
4. Also the Spatial heterogeneity model, Predation, Competition, Biotic Interactions, Disturbance, and
Geographic Area models.