General Ecology: Lecture 14
November 21, 2003
Topic: Community Organization I: Biodiversity
I.
II.
III.
IV.
Importance of biodiversity (from the perspective of ecosystems…)
A.
There need to be, at minimum, at least one species in a community that
performs each essential ecological role
B.
But most communities have far >1. Why is this apparent redundancy
important?
Measuring biodiversity
A.
Specific measures of biodiversity
1.
Species richness (number of species)
2.
Species richness combined with heterogeneity (relative abundance of
species)
B.
Patterns of biodiversity for groups of organisms
1.
Types of species distributions seen (2 examples)
a)
Hollow curve [Fig. 22.1]

This type of distribution indicates that most of the species are
rare in the community, but that there a very few dominant
species that have most of the individuals.
b)
Log-normal distribution [Fig. 22.2]

This type of distribution indicates that there are a few species
that are extremely rare or extremely abundant, but most species
of a particular group exist at intermediate densities in the
community.
Some examples of diversity gradients
A.
General trend: decrease in biodiversity from the equator to the poles
1.
By habitat
2.
Gradients in North American tree species [Fig. 22.5]
3.
Other latitudinal trends
4.
Aquatic ecosystems
B.
Other gradients of biodiversity
1.
Topographic relief/fronts of abrupt change: Mountains ranges show both
higher species diversity and abrupt changes in diversity, and there are other
“hot spots” such as kelp holdfasts
2.
East-west trends
3.
Coast vs. interior [Example: carnivorous marsupials, large—[Fig. 22.8b]
4.
Peninsular lows
Factors that might cause diversity gradients: Note that some of these ideas mirror our
discussion of diversity in coral reef fishes …
A.
History factor
1.
Biotas in warm, humid tropical areas are likely to evolve and diversify more
rapidly than in polar regions
a)
Constant, favorable environment year-round that allows for faster
generation times and thus faster rates of evolution

B.
C.
D.
E.
Tropical copepods with shorter generations and more per year.
Why?
2.
Tropical biotas have had more time to diversify
a)
Freedom from climatic disasters, specifically from glaciation
b)
This assumes that diversity increases through time. Some evidence to
support this…

Lake Baikal …

Plant species in fossil record [Fig. 22.12]
3.
Difficult/impossible factor to test…
Spatial heterogeneity
1.
By what mechanisms could spatial heterogeneity lead to increased
biodiversity?
2.
Two components of diversity should be examined
a)
Within-habitat diversity

Examine diversity on small scales, within a single habitat

Example: Land birds
b)
Between-habitat diversity (# of different habitats)

Examine diversity on larger scales,encompass multiple habitats

Land birds: Ecuador vs. New England
3.
If spatial heterogeneity is important to animal species, the number of animal
species should be correlated with the number of plant species
a)
Example: Animal diversity vs. tree species richness [Fig. 22.13]

Does this really provide evidence?
4.
Spatial heterogeneity not likely for similar gradients in ocean communities.
5.
General conclusions (text)
6.
How to measure spatial heterogeneity? One alternative: rugosity
Competition
1.
Niche breadth and overlap [Fig. 22.14]
2.
How do you decide which “resource axes” are relevant within the
description of a niche?
3.
Potential of niche diversification to explain plant vs. animal diversity
a)
Animals have more potential niches
4.
Evidence for niche breadth hypothesis (Roughgarden, 1986) [Fig. 22.15]
Predation
1.
Keystone predators help maintain diversity
a)
Pisaster ochraceus as a keystone predator in intertidal (Bob Paine)
b)
Can this help explain increased biodiversity in tropics?
2.
Janzen-Connell model for high tropical-forest diversity [Fig. 22.16]
a)
Some support for their model: Ocotea whitei saplings [Fig. 22.17]
b)
What key factors might be responsible for this effect?
Climate and climatic variability
1.
More favorable and stable climates allow for evolution of finer
specialization and adaptations than areas with erratic climates smaller
niches and thus more species/habitat
a)
Note similarity to the “history” hypothesis…
b)
F.
Severe and unpredictable climates would then have low levels of
diversity
2.
Species richness-energy model
a)
Species richness limited by available energy [Fig. 22.18]

Correlation with annual/potential evapotranspiration used as
measures of energy
b)
Species richness limited by productivity

Data?

Importance of growing season?
3.
Coral reefs: A special case for particular climatic conditions?
Disturbance/non-equilibrium: Similar argument as we discussed for reef fish.
1.
Disturbance prevents equilibrium from occurring (see previous lecture…)
2.
Intermediate disturbance hypothesis
a)
Mechanism
b)
In some ecosystems, “intermediate” disturbance can decrease
diversity

Aquatic invertebrates [Fig. 22.22]

Diversity in prairie grasslands decreased with fires
Study Questions
1.
Explain the importance of biodiversity from an ecological perspective. Discuss especially
the potential importance of redundancy, where more than one organism fulfills similar
ecological roles in an ecosystem.
2.
What do biodiversity indices such as the Shannon-Weiner diversity index take into
account besides simply the number of species?
3.
Explain, in words, the difference between having a “hollow curve” distribution vs. having
a “log normal” distribution for a particular group of organisms.
4.
Does biodiversity tend to increase or decrease when moving from the equator to the poles?
Provide specific examples of this trend for both terrestrial and aquatic organisms.
5.
Describe some additional types of diversity gradients seen.
6.
List the six key explanations for patterns in biodiversity seen.
7.
According to the “History” hypothesis, what are the two key reasons that groups of
organisms in warm, tropical climates should diversify at a faster rate than temperate or
polar climates? Provide one piece of evidence used by scientist to support the History
hypothesis. In principle, would the veracity of this hypothesis be relatively easy to find
evidence for/disprove compared to the other hypotheses?
8.
According to the History hypothesis, how might frequent climatic disasters serve to reduce
biodiversity?
9.
By what mechanisms could spatial heterogeneity lead to increased biodiversity?
10.
What two components of diversity need to be examined when considering spatial
heterogeneity as an explanation for biodiversity?
11.
If spatial heterogeneity is important to animal species, the number of animal species might
be correlated with the number of plant species. Is this idea supported by evidence? Is this
the best/only way of measuring spatial heterogeneity? Describe another method.
12.
Explain how niche width and niche overlap could affect the level of biodiversity (see Fig.
22.14). Which of these hypotheses does Roughgarden’s data support (i.e. how did the
niches of the lizard species change when there was one species vs. several)?
13.
Why is niche diversification thought to be a better explanation for plant diversity than
animal diversity?
14.
By what mechanisms could predation serve to increase biodiversity (two mechanisms)?
15.
Describe/explain the Janzen-Connell model for high tropical-forest diversity compared to
temperate-forest diversity.
16.
Describe two general mechanisms whereby climate and climatic diversity could account
for differences in biodiversity.
17.
What special aspects of climate are responsible for the existence of coral reefs? How is
coral reef existence/biomass related to biodiversity?
18.
Explain how disturbance can promote relatively high levels of species diversity. What
levels of disturbance are optimal in promoting high species diversity?
19.
Provide examples where even low levels disturbance apparently reduce biodiversity.