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Chapter 26: Biodiversity,
Extinction, and Conservation
1
Biodiversity
The tropics are a storehouse of biological diversity
unrivalled anywhere on the planet:
this fact was brought to light by the work of the great
explorer-naturalists of the nineteenth century -Darwin, Bates, Wallace, and others
estimates of global biological diversity range from 10 to
30 million species or more:
most of these species occur in the tropics (many are small
insects!)
thus far, fewer than 2 million species have been catalogued
worldwide
Tropical Biodiversity
Within most groups of organisms, numbers of species
increase markedly toward the equator:
consider the ants:
at 60oN, we might find 10 species in a small region
at 40oN, between 50 and 100 species
within 20o of the equator, between 100 and 200 species
consider breeding birds:
in Greenland, we find 56 species
in New York state, 105 species
in Guatemala, 469 species
in Colombia, 1,395 species
Contour lines on the map indicate the # of species of nearshore and continental-shelf bivalves found at
locations w/ in the contour intervals
Why so many in the Tropics – and so
few toward the poles?
Biologists hold two views on the subject of high
biodiversity in the tropics:
diversity increases without limit over time:
tropical habitats, being older than temperate and arctic
habitats, have had time to accumulate more species
But with integration of population ecology into
community theory… diversity reaches an
equilibrium at which factors adding species balance
factors removing species:
factors adding species weigh more heavily, or factors
removing species weigh less heavily, in the tropics
Patterns of Diversity
Large-scale patterns of diversity vary on a regional
basis according to several key factors:
these factors can be seen to operate on the number of
mammals and other animals in 150 x 150 mile blocks
in North America:
suitability of physical conditions (fewer amphibians in the xeric
west)
heterogeneity of habitats (more mammals in the western
mountains than in eastern North America)
isolation from centers of dispersal (fewer mammals as one
moves down the Baja California peninsula)
Vegetation structure determines local
diversity.
The number of bird species tends to increase
somewhat with increasing productivity within a
temperate zone region:
however, the principal determinant of bird diversity
seems to be structural diversity of vegetation:
6 species in grasslands (areas of 5-20 ha)
14 species in shrublands
24 species in floodplain forests
the MacArthurs quantified this observation by relating
bird diversity to foliage height diversity in the 1960s
Marshes are very productive but are structurally
uniform and have relatively few species of plants
Desert vegetation is less productive than marsh
vegetation but its greater variety of structure makes
room for more diversity
== inverse relationship between productivity and
species diversity
Vegetation
structure may be
more important
than primary
productivity in
determining
diversity
Sonoran Desert of
Baja California

Vegetation
structure may
be more
important than
primary
productivity
in determining
diversity
Salt marsh in
MA
== inverse
relationship
between
productivity
and species
diversity
Birds. Birds.
Structural complexity and diversity have always gone
together for bird-watchers and naturalists
 the MacArthurs quantified this observation by relating
bird diversity to foliage height diversity in the 1960s
 Plotted diversity of birds observed in different habitats
according to diversity in foliage height, a measure of the
structural complexity of the vegetation
Diversity is correlated with overall
energy input
Potential evapotranspiration (PET) is a good
predictor of diversity over large regions:
PET is the amount of water that could be evaporated
from the soil and transpired by plants under prevailing
conditions of temperature and humidity:
this index integrates temperature and solar radiation (energy
input) for a given system
Why this relationship holds is poorly understood:
sharing of more energy by more species?
larger populations less likely to go extinct?
Diversity has both regional and local
components.
Diversity can be measured at a variety of spatial levels:
local diversity (alpha diversity) is the # of species in a
small area of homogeneous habitat
regional diversity (gamma diversity) is the total # of
species observed in all habitats within a barrier-free
geographic area
Beta diversity measures turnover in species
composition.
Consider two extremes of compositional
segregation by habitat:
if each species occurred in each habitat with a
region, local diversity would equal regional diversity
if each habitat had a unique biota, then regional
diversity would be the sum of all local diversities
Beta diversity measures turnover in species
composition from one habitat to the next
within a region.
Computation of Beta Diversity
One measure of beta diversity is the # of habitats
within a region divided by the average number of
habitats occupied per species:
thus, regional diversity = local diversity x beta diversity
consider the island of St. Lucia, West Indies:
9 habitats (grassland, scrub, lowland forest, mangroves, etc.)
15.2 species of birds/habitat (local diversity)
each species occupies on average 4.15 of the 9 habitats
beta diversity = 9 habitats/4.15 habitats = 2.17
regional diversity = 15.2 species x 2.17 = 33 species
Local communities contain a subset of the
regional species pool.
The species that occur within a region are referred
to as its species pool:
each local community is a subset of the species pool
what determines whether a given member of the species
pool can be a member of a given community?
the species must be able to tolerate the conditions of the
environment and find suitable resources (these conditions
must fall within the fundamental niche of the species)
the species must also be able to persist in the face of
competitors, predators and parasites (where the species is
successful in this respect constitutes the realized niche)
Species Sorting
The membership of a species within a local community is
determined:
partly by its adaptations to conditions and resources
partly by competitive and other interactions with species
Thus species from the regional pool are sorted into
different communities based on their adaptations and
interactions, a process called species sorting:
this process may be demonstrated experimentally (read ecologists
in the field)
Ecological Release
For a given range of habitats, species sorting (and beta
diversity) should be greatest where the regional species pool
is largest:
when the species pool is smaller (perhaps because of
geographic isolation)
competition should be relaxed
species should expand into habitats normally filled by other
species, a process called ecological release
Ecological release
Comparing levels of diversity in islands and
neighboring continental regions
Islands:
fewer species
island species have greater densities
Island species expand into habitats normally filled by
other species on the mainland
Collectively referred to as: ecological release
Evidence for Ecological Release
Evidence from 7 continental areas and islands of
various sizes in the Caribbean basin illustrate the
process of ecological release:
as regional numbers of birds increased:
habitat breadth and local abundance decreased
local diversity and turnover of species between habitats
(beta diversity) increased
Diversity & Niche Relationships
A niche represents the range of conditions and
resource qualities within which an individual or species
can survive and reproduce:
the niche is multidimensional
overlap of niches of two species determines how strongly
the two species might compete with each other
Diversity & Niche Relationships
Every community can be thought of as having a
total niche space within which the niches of all
species must fit:
adding or removing species may result in compression
or expansion of the realized niches of other species
communities with different numbers of species may
differ with respect to:
total community niche space
degree of niche overlap among species
niche breadths of individual species
Diversity reflects the
relationship between
species niche and
total community
niche space
horizontal: an ecological
resource that defines the
total niche space (eg:
average size of prey items);
height of curve – intensity
of use of resource
How could more species be added to a
community?
(how to move from condition A)
A community could accommodate more species by:
an increase in total niche space (with no change in breadth or overlap)
(condition B)
Niche space refers to variety of resources and not amount of resources
an increase in niche overlap (with no change in breadth or total space)
(condition C)
Average productivity of each species would decline due to increased sharing of resources –
all things equal
a decrease in niche breadth (with no change in total space or overlap)
(condition D)
Average productivity would decline since each species would have access to a narrower
range of resources
Each curve represents
a species’ niche
horizontal: an ecological
resource that defines the
total niche space (eg:
average size of prey items);
height of curve – intensity
of use of resource
Competition, Diversity & the Niche
intense competition leads to exclusion of species from
the community
Thus many ecologists have argued that in communities
with high diversity, competition must be weak: (condition
D: narrower niche + reduced niche overlap)
what mechanisms might lead to reduced interspecific
competition?
greater ecological specialization (narrower niches)
greater resource availability (greater niche space)
reduced resource demand (smaller populations)
intensified predation (populations below carrying capacity)
Are there more ecological roles
in the tropics?
More ecological roles in the tropics could be the
result of greater niche space: (condition B)
greater niche space could result from increase in the
number of niche axes as well as the length of each:
increase in bird species in the tropics is related to nectarfeeding and insectivory from fixed perches (both rare in
temperate zone)
tropics are rich in mammals primarily because of the number
of flying mammals (bats), less common at higher latitudes
Nonflying mammals =ly diverse in tropics and temperate
epiphytes and lianas are tropical plant forms generally absent
or uncommon in forests at higher latitudes
Thus: variation in species diversity is generally paralleled by variation
in the functional (or niche) diversity of species
One way to assess niche diversity: use the morphology of a species as
an indicator of its ecological role
In other words: assume that differences in morphology among
related species reveal different ways of life
Example: size of prey captured varies in relation to body size of the
consumer
As species diversity increases: total variety of morphology increases
Species diversity parallels niche
diversity.
Evidence for increasing species diversity with increasing
niche diversity comes from a study of morphological
diversity in bats: (compared bat communities in temperature and tropical
localities)
this diversity was determined using ratios (morphological axes)
reflecting type and location of prey utilized and flight
characteristics:
the less diverse bat communities in Canada (all small insectivores) had
relatively limited variation along these morphological axes
the more diverse bat communities of Cameroon, tropical West Africa,
occupied much greater range of morphological space
Species diversity parallels niche
diversity.
Evidence for increasing species diversity with
increasing niche diversity also comes from a
study of fish in the Rio Tamesi drainage of east
central Mexico:
headwater springs and small streams had relatively
few fish representing few niches
farther downstream, additional species were added,
increasing the diversity of niches
lower reaches added still more species and diversity
of niches
Predation and Diversity
Diversity generally appears to increase with higher
productivity:
higher productivity results in more energy reaching
higher trophic levels, thus supporting larger populations
of predators:
increased predation pressure should reduce competition
among prey and permit more prey to coexist
increased predation should also promote diversification in
mechanisms of prey escape
Evidence for Predation Effects on
Diversity
Do predators play an indirect role in promoting diversity
among prey?
when predators are removed from a community, a common
consequence is loss of prey species
the variety of color patterns and resting positions among moths is
much higher in diverse tropical communities than it is in
temperate latitudes
Predators may play an important role in shaping niche
relationships and regulating diversity.
Diversity of adaptations for escaping predators is high
among moths in the Tropics (moths from the Amazon basin in
Ecuador)
Equilibrium Theories of Diversity: how do we
explain patterns of diversity?
Most ecologists now believe that diversity achieves
an equilibrium value at which processes that add
species and those that subtract species balance
each other:
species are added by:
production of new species
movements of individuals between habitats and regions
species are removed by:
competitive exclusion, efficient predation, bad luck
equilibrium concepts can be applied to islands...
Each type of community has an equilibrium number of species, often
referred to as the saturation number
Similar to how a habitat has a carrying capacity for the population of
a particular species
This view helps explain what was known about species diversity
within local habitats and its places at least part of the problem of
species diversity within the domain of ecology
Diversity on Islands
In the 1960s, Robert MacArthur and Edward
Wilson developed their famous equilibrium
theory of island biogeography:
the number of species on an island balances regional
processes governing immigration (colonization) against
local process governing extinction
Equilibrium Theory of Island
Biogeography
Consider a small offshore island:
addition of species results from immigration from
other islands or from a landmass:
the rate of arrival of new species is a declining function of
the number of species already on the island (explain?)
removal of species results from extinction:
the rate of extinction is an increasing function of the
number of species already on the island
where immigration and extinction curves cross is
the equilibrium number of species, Š – steady-state
number of species
Equilibrium theory of island biogeography balances
immigration against extinction
More on Island Biogeography
Immigration and extinction rate probably do not
vary in strict proportion to the # of potential
colonists and # of species on the island
The immigration and extinction functions are
curved, not straight:
Thus: the immigration rate initially decreases rapidly
because the best colonizers reach the island first
the extinction curve increases more rapidly with high
numbers of species on the island because smaller
populations are more susceptible to extinction
More on Island Biogeography
Extinction curves should be higher for smaller
islands than for larger ones:
smaller islands generally have smaller populations
for any number of species:
thus smaller islands should have fewer species
Immigration curves should be lower for distant
islands than for nearer ones:
more distant islands are less likely to be colonized:
thus more distant islands should have fewer species
These predictions have been verified for islands throuhgout the
world
Species richness increases with island size and decreases with distance from
colonization source
Equilibrium theory of island
biogeography
Also predicts that if some disaster reduced the
diversity of a particular island, new
colonists/immigrants would – over time –
restore diversity to its pre-disturbance
equilibrium
Experiment: (1) counted the # of arthropod
species on each of 4 small mangrove islands in
Florida; (2) removed all the insects by
fumigating the islands; (3) islands censused at
regular intervals for a year
Can also apply an equilibrium view of diversity to assemblages of species on continents
Main difference:
Continental regions: new species likely to form within the region (+ immigration)