ch23 - Eco

Chapter 23: Biodiversity
Robert E. Ricklefs
The Economy of Nature, Fifth Edition
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
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:
Greenland, we find 56 species
New York state, 105 species
Guatemala, 469 species
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
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
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
structure may be
more important
than primary
productivity in
Sonoran Desert of
Baja California
structure may
be more
important than
in determining
Salt marsh in
== inverse
and species
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
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
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
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
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
Evidence from 7 continental areas and
islands of various sizes in the Caribbean
basin illustrate the process of ecological
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
reflects the
species niche
and total
niche space
horizontal: an
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
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
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
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
• 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
As species diversity increases: total variety of morphology
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
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
Equilibrium Theory of Island
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
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
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 (+
Equilibrium Theory in
Continental Communities
Processes responsible for diversity in continental
communities differ from those in island
the principal source of new species on continents is
the rate of speciation might level off if opportunities for
diversification were restricted by increasing diversity
the extinction curve might look like that for islands
regardless of the shapes of these curves, an
equilibrium number of species for continents, Š,
should be expected
Probability of extinction per species increases at higher diversity if competitive exclusion
increased with diversity and decrease if mutualistic relationships and alternative
pathways of energy flow buffered processes that lead to extinction
Rate of speciation would increase if diversity led to greater speciation and a higher
probability of reproductive isolation of subpopulations
Regardless of the particular shape of the immigration, speciation,
and extinction curves
Many biologically reasonable models can define an equilibrium level
of diversity
In every case
Increased rates of speciation
Decreased rates of extinction
Or both
Lead to greater equilibrium species diversity
So: question now becomes: what are the factors responsible for
variation in rates of species appearance and disappearance in
different places
Explanations for High Tree
Species Diversity
Diversity of plants affects diversity of animals in
a straightforward manner:
thus the most rigorous tests for general explanations
of diversity lie in their application to plants!
“Why are there so many kinds of trees in the
tropics?” has many plausible answers...
Environmental heterogeneity
Gaps from disturbances
Herbivores and pathogens
Slow competitive exclusion
Environmental Heterogeneity
Many ecologists have argued that diversity of
trees varies in relation to heterogeneity of the
since tropical trees are specialized to certain soil and
climatic conditions, perhaps greater specialization (or
a greater distinction among habitats) explains the
greater diversity of tropical trees
this argument seems unlikely to account for tenfold
(or more) differences in diversity in tropical versus
temperate forests
Disturbance & Gap Dynamics
Some ecologists have related the high
diversity of tropical rain forests to habitat
heterogeneity created by disturbance:
the intermediate disturbance
hypothesis states that, with a moderate
level of disturbance, a community becomes a
mosaic of habitat patches at different stages
of succession:
such patches contain the full variety of species
characteristic of the sere
Disturbance & Gap Dynamics
 For disturbance and gap dynamics to account for higher
diversity in tropical forests one might expect:
differences in the level of disturbance
more heterogeneity between forests gaps and the rest of the
 Death rates of individual forest trees do not differ
significantly between temperate and tropical
 Studies of gap capture in tropical forests indicate that
gap specialists are no more likely to establish
themselves in gaps than shade-tolerant species!
Death rate of canopy trees between tropical and
 Hubbell suggested: even though species of trees may be specialized
differently for germination sties (shade vs sun, eg), the species that
actually invade a particular gap depend more on the vagaries
(unexpected changes) of recruitment than on particular ecological
conditions in the gap
 Thus: level of competition for germination sites is reduced by
dispersal limitation (i.e. not all species to get to germination sites to
compete effectively)
 But: recruitment is partly due to high species diversity and low
average density of species in tropical forest
 Thus: suggests that as tropical forests become more diverse (for
some reason), competitive exclusion declines. Thus: biological
diversity self-accelerating
Herbivore and Pathogen
Daniel Janzen has proposed that herbivory could
promote high diversity in tropical forests:
herbivores in tropical forests feed so efficiently on
abundant species that they depress their populations,
permitting less common species to grow in their
place, thereby maintaining high diversity
Note: herbivores eat what is most abundant rather
than a particular individual tree species
‘pest pressure’ hypothesis
Herbivore and Pathogen
 The “pest pressure” hypothesis predicts that seedlings
should be less likely to establish successfully near adults
of the same species:
this prediction has been tested with generally supportive results
 Also consistent with the “pest pressure” hypothesis,
tropical monoculture plantations have been
 Ultimately, however, too few data exist to support the
idea that latitudinal differences in pest pressure exist
and, if they do, whether they are sufficient to explain
differences in tree species diversity!
Reduced Competitive
Steve Hubbell has proposed that pest pressure
and recruitment limitation reduce consequences
of interspecific competition for tropical trees:
new species invading a community are likely to
remain indefinitely
large populations of trees in extensive areas of
tropical forest would be relatively immune to
thus, latitudinal gradients in species diversity would
reflect regional species production more than the
ability of species to coexist in local communities
The Bottom Line?
 Many competing hypotheses have been developed to
account for larger numbers of tree species in tropical
 How well Hubbell’s model explains patterns in diversity
of trees remains to be seen. However, his ideas argue
for the potential importance of large-scale processes.
 If rates of species production have been high
throughout the tropics and local processes have failed to
remove new species, then diversity has been
accumulating in tropical forests for tens of millions of
Summary 1
Species diversity in tropical ecosystems tends to
greatly exceed that at higher latitudes.
Patterns of diversity can vary at local (alpha)
and regional (gamma) levels; species turnover is
called beta diversity.
Local communities are assembled from the
regional species pool. The processes that
determine local community composition are
called species sorting.
Summary 2
Ecological release provides evidence that
competition for resources structures
communities and limits diversity.
Diversity can be affected by differences in total
niche space, niche breadth of individual species,
and niche overlap between species.
Equilibrium theories of diversity focus on
processes that add and remove species from
Summary 3
Differences in numbers of species on islands
focus on equilibria between rates of immigration
and extinction.
Several explanations for high diversity of
tropical trees focus on environmental
heterogeneity, gap dynamics, pest pressure,
and reduced competitive exclusion.