Transcript Biogeography

Biogeography
Species
Richness and
the Extinction
Crisis
How many species are there?
 Species
richness –
the total number of
species in an area.

5-10 million is best
guess, but may be
anywhere from 3100 million.
Biogeographic Patterns –
Latitudinal Gradient
 The
number of species is greatest near the
equator and decreases as you move toward
the poles.
Latitudinal Gradient Raises
these Questions:
 What
determines how many species live
in a particular area?
 Why do some regions have more species
than others?
 Is there any limit to the number of species
Earth can support?
Biogeographic Realms &
Regions
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
Different portions of
the globe frequently
have unique biotas.
A rainforest in South
America has species
more closely related
to those found in
South American
prairies than to
rainforest species in
Africa.
This became known
as Buffon's Law.
Biogeographic Realms &
Regions
 What
determines which species are found
in a particular region?
 Why are some species widely distributed
while others have more restricted ranges?
 How does a species' evolutionary history
affect its current distribution?
Biogeographers consider two distinct
perspectives to answer all these questions:
 The
first is an ecological perspective,
concerned with how short-term
interactions among organisms and the
physical environment affect a species'
current distribution.
 The second is an historic perspective that
focuses on how processes
like speciation, extinction and dispersal af
fect taxa and biotas.
Sampling Species Richness
 How
do we
determine the total
number of species
in the area.


Sampling effort
Species
accumulation
curve
Why do some areas have
greater species diversity?
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Hypothesis #1
The number of species on any wall is determined
by the number of of species in the regional
species pool.
Each time space in a site opens up, it may be
settled by any colonist arriving from the regional
species pool, the size of which is determined by
rates of speciation and extinction.
Why do some areas have
greater species diversity?
 Hypothesis
#2
 The number of species on any wall is
determined by local species interactions.
 Competition for space and other
resources, predator/prey dynamics, etc.,
drive diversity by determining which
species can survive at any given site and
whether or not a new species can
successfully colonize it.
Hypotheses lead to Predictions


In contrast, if species
interactions limit the species
that can live in a particular
place, then as local species
richness increases, species
interactions will become more
and more intense until,
eventually, new species are
unable to colonize the site.
Strong interspecific interactions
will place an upper limit on
alpha diversity, regardless of
the size of the regional species
pool.
Species Turnover
 If
you compared two communities within
a region, you might find they have few
species in common.
 This turnover in species from one site to
the next is also a form of diversity.
 Biogeographers describe this as beta
diversity.
Generalists vs Specialists
 Generalist
species that can reach and
occupy just about any habitat type will
see any region as one large continuous
area full of suitable habitat.
 In contrast, specialist species that have
very narrow niches will see the same area
as divided into many distinct habitats,
only some of which they can occupy
Biogeography and
Conservation Biology
 One
practical reason to
study Earth's biodiversity is
so we can judge whether
there is really an
extinction crisis
happening now.
Human-Related Causes of
Extinction
 Hunting,
introduced species, and
anthropogenic habitat degradation are
big threats to diversity now, just as they
were in the past, although technological
advances have enabled people to clear
forests more quickly and hunt animals
more efficiently than their prehistoric
ancestors.
Why Protect Biodiversity?
 Protect
the "genetic library" of natural
ecosystems.

Earth's biodiversity has already provided
humanity with food, medicines, and many
other resources.
Why Protect Biodiversity?
 Ecosystems
provide a broad set of
services that we rely on for our welfare.

These ecosystem services include things like
climate regulation, water purification, flood
control, crop pollination, and soil
generation and maintenance.
Why Protect Biodiversity?
 People
have an aesthetic and ethical
obligation to protect our planet and the
only other living species known in the
universe—even if only so future
generations can marvel at the wonderful
diversity of life on Earth.
Ecological
Biogeography
Species-Area
Curves
 Bigger
islands
tend to have
more species.
 Easiest to see
pattern if data is
log-transformed.
“Islands”
 From
an organism's
perspective, an island is
simply an area of suitable
habitat surrounded by an
inhospitable landscape
matrix.

With this definition there
are any number of
habitats that could be
islands, such as caves,
desert oases, and the cool
habitats of mountain tops.
Island Biogeography
 An
island's species richness is determined
by three distinct processes:
 Immigration
 Extinction
 Evolution
McArthur & Wilson
 Equilibrium
theory of island biogeography,
focused on islands that varied in size and
distance from a mainland.

They restricted their analysis to ecological
time scales, ignoring evolution.
Immigration Rate
 Immigration


rate is affected by:
Size of the island
Distance from the mainland
Extinction Rate
 The
extinction rate will be affected by the
availability of resources.

If more species than can be supported by
the available resources have immigrated to
the island, there will be a high extinction
rate.
Equilibrium Theory of Island
Biogeography
 MacArthur
and
Wilson plotted an
island's immigration
and extinction rate
curves on the same
set of axes and
made two important
predictions about an
island's biota.
Equilibrium Theory of Island
Biogeography

The intersection of the two curves, which
occurs when immigration rate equals
extinction rate, predicts
the equilibrium number of species, S*.

When the number of species on an island is
greater than S*, the extinction rate exceeds the
immigration rate, and the number of species on
the island will decrease. Conversely, when the
number of species is smaller than S*, immigration
exceeds extinction and the number of species
will increase.
Equilibrium Theory of Island
Biogeography
 The
intersection of the two curves also
predicts the rate at which new species
replace extinct species, reflecting the
dynamic nature of equilibrium on the
island.

There is one theoretical value of S*, but the
composition of S* is always changing. It
changes at a rate that MacArthur and
Wilson called the equilibrium turnover rate,
symbolized as T*.
Support for Turnover
 Natural
experiments supported the
concept that species turnover.
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Krakatau
Channel Islands
 Manipulative
field experiments also
supported turnover.

Florida Keys mangrove islands
Mangrove Experiments
 Showed
the
affects of size
and distance
from the
mainland.
Complicating Factors


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Many communities seldom, if ever, attain an
equilibrium number of species.
Both natural and
anthropogenic disturbances like hurricanes,
volcanic eruptions, and clear-cutting can
remove species and open up habitat.
Evolution can add species to either the island
community or to the mainland species pool.
Complicating Factors
 Islands
are not homogenous patches
sitting within a featureless seascape. Both
islands and their surroundings can have
complex environments which can alter
immigration and extinction rates
Complicating Factors
 Species
are not interchangeable.
Whether or not a species successfully
colonizes an island or later goes extinct is
affected by its inherent biology and by
interactions with other members of the
community.
Dispersal
 For
a species to colonize an island,
individuals of that species must move
there.
 Movement of individuals between
different islands or other patches of good
habitat is known as dispersal.
 Barriers to dispersal prevent organisms
from immigrating.
Types of Barriers
 Types
of barriers that species might need
to cross in order to successfully move from
one patch to another:
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Corridors are routes through a landscape
that all species can cross.
Filters are routes that only some species can
cross.
Sweepstakes routes are those that are
nearly impossible to cross except during
rare and unpredictable circumstances.
Crossing Barriers
 Biogeographers
recognize three means
by which a barrier might be crossed:
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Jump dispersal occurs when an organism
leaps a barrier in a single bound.
Diffusion occurs when organisms slowly
percolate through a relatively hospitable
matrix.
Secular migration occurs when populations
move so slowly from one area to another
that they evolve en route.
Dispersal Example
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Some 40-45 million years ago, the
ancestor species to camels and llamas
(family Camelidae) originated in North
America.
About 3 million years ago some of these
animals migrated west across the land
bridge from Alaska to Siberia, and from
there dispersed across Asia and into
Africa (see the arrows on the right).
When South America crashed into North
America at Panama, other individuals
migrated south along the newly formed
corridor of land.
In this way, the original ancestor species
in North America dispersed to four
different continents.
Species-Area Curves
 Both
island and
mainland speciesarea curves tend
to show the same
general pattern.
Larger areas
contain more
species.
Species-Habitat Diversity
Hypothesis
 As
you expand
sampling area, you
will encounter
different habitats,
and thus different
sets of species. This
is known as the
species-habitat
diversity
hypothesis.
Island Biogeography &
Conservation Biology

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An early question conservation biologists asked
was: Given the ability to set aside a certain
amount of land as a reserve, is it better to target
single large pieces of land or several small
pieces?
Because extinction rates are lower in larger areas,
some ecologists argued that making the largest
reserve possible would keep the most species from
going extinct.
Others argued that several smaller reserves might
capture more habitat diversity, as well as protect
against major disturbances (e.g., what if a fire
burned down all of a single large reserve?).
Historical
Biogeography
Evidence of Major Changes
 Fossils
of aquatic
animals, such as
large sharks, in
Kansas indicate
that the Earth has
changed
dramatically over
time.
Plate Tectonics
 Such
huge changes
in geography
happen because
Earth's continents
slide across the
surface of the planet
in a process known
as plate tectonics.
https://youtu.be/Cm5giPd5Uro
Realms & Regions
 Earth's
terrestrial
biota can be
divided into 12
biogeographic
realms that reflect
the different
evolutionary
histories of land
masses on different
tectonic plates.
Allopatric Speciation

Allopatric speciation begins when populations of
the same species become geographically
isolated. Over time, natural selection and drift
cause them to diverge evolutionarily until they are
distinct, reproductively isolated species. This may
occur as a result of two distinct processes:


A vicariance event occurs when a population is
physically split by geologic or geomorphic
processes.
A founder event occurs when a new population is
founded by a small number of individuals
dispersing from their ancestral range.
A Brief History of Frogs
 If
you could time-travel back onto the
Madagascar-India-Seychelles fragment
100 million years ago, you would see frogs
that remind you of today's American
bullfrog and leopard frog species.
 What frog history led them from a single
species on a fragment of land off Africa
to a nearly worldwide distribution today?
A Brief History of Frogs
 The
breakup of the southern continent,
Gondwana, created smaller, isolated
populations on the new land masses.
 As the land masses separated, ocean
water filled the gap.

Animals like frogs could not cross the gaps.
 This
is an example of vicariance.
A Brief History of Frogs

According to the model of allopatric
speciation, once two populations are
physically isolated, the evolutionary processes
of mutation, natural selection, and genetic
drift would cause mating behavior, feeding
strategies, and habitat use to become
different between the populations.

Over long enough time spans, new,
reproductively isolated species were likely to
evolve.
Jump Dispersal
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
Geologically-caused vicariance events are
one way for populations to become isolated.
A different way is through jump dispersal. If
individuals from the one population cross a
barrier (such as a stretch of ocean) to a new,
unoccupied habitat, the new population may
be isolated enough to eventually evolve into
a new species.

Founder Event
A Brief History of Frogs
 We
can make
predictions about
the pattern of
speciation we
would expect from
dispersal
Sympatric Speciation
 Sympatric
speciation occurs
when a population
splits into two and
eventually
speciates without a
physical
separation.
The Great American Biotic
Interchange

Approximately 3
million years ago
the Isthmus of
Panama emerged
from the sea,
connecting North
and South America.
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Route for mammals
to cross from one
continent to
another.
Human Accelerated Dispersal
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
Although dispersal and
colonization of new
habitats is a fundamental
biogeographic process,
humans have altered
dispersal rates for many
species.
Intentionally and
accidentally, people
bring species with them
as they move from place
to place.
Introduced Species - Impacts
 Introduced
species often lead to
extinction of native species and loss of
biodiversity.
 Because many of the same species are
introduced over and over again, many
areas of Earth are losing their distinctive
nature.
Latitudinal Gradients
 Latitudinal
gradients are perhaps the bestknown biogeographical pattern.


Species diversity tends to be high near the
equator and lower near the poles for many
taxa.
These patterns are well-correlated with a
number of climatic variables but are
incompletely understood.
Biodiversity Hotspots &
Conservation

Conservation biologists have attempted to identify
hotspots that contain a large number of endemic
species that are threatened by human activities.
Focusing conservation activities on these hotspots
may be an effective use of limited conservation
resources.
Global Patterns
in Physical
Conditions
Global Patterns in Physical
Conditions
 Climate
predicts not only the number of
species likely to occur in a place, but also
the physical appearance of those
species.
Differential Energy Input from
the Sun
 Because
the Earth
is round, the
intensity of solar
radiation is lower in
the poles than near
the Equator.
The Tilt and Orbit of the Earth
Creates Seasons
 The
tilt of the Earth's axis relative to its orbital
plane creates seasonality.
Atmospheric Circulation
 Differential
heating
from the Sun
across the globe
creates air and
water currents,
such as Hadley
cells.
Bands of Wet and Dry
 Large-scale
air
circulation patterns
tend to create a
band of wet areas
near the Equator
and deserts to the
north and south of
the Equator.
Terrestrial Biomes


Differences in
climate and
geography give rise
to a variety of
biomes around the
world.
These patterns
determine a region's
climate and, to a
large degree, what
species can thrive
there.
Temperate Deciduous Forest
 Temperate
deciduous forests receive rain
year-round.
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
Cold winters and hot, humid summers.
Animals may migrate, hibernate, or survive on
scarce available food or stored fat through the
winter.
Coniferous Forest
 Coniferous
forests, or taiga, are common in
the northern hemisphere.


Evergreens dominant
Colder, less rain than temperate forests.
Coniferous Forest
 Mammals
that
inhabit coniferous
forests include
deer, moose, elk,
snowshoe hares,
wolves, foxes,
lynxes, weasels,
bears.
 Adapted
for
long, snowy
winters.
Tropical Forest
 Tropical
rain forests receive lots of rain and
are generally warm year-round.


Stratified
Diverse
Tropical Forest
– insectivorous birds and bats fly
above the canopy.
 Canopy
 Fruit
bats, canopy birds, and mammals live in the
canopy eating leaves & fruit.
 Middle
zones are home to arboreal
mammals (monkeys, sloths), birds, bats,
insects, amphibians.
 Climbing
animals move along the tree trunks
feeding at all levels.
 Ground
level contains larger mammals
(capybara, paca, agouti, pigs) as well as a
variety of reptiles and amphibians.
Tropical Forest
 Nutrients
in a tropical forest are tied up in
living organisms.

Soil is poor.
 Slash
and burn agriculture involves removing
vegetation to grow crops – but the soil is so
poor that the fields must be moved often.
Grassland
 Temperate
grasslands receive
seasonal precipitation and have cold
winters and hot summers.
 Prairie
Grassland
 Grasses
and
herds of large
grazing mammals
are dominant.
 Jackrabbits,
prairie dogs, and
ground squirrels
are common.
 Predators include
coyotes, cougars,
bobcats, raptors,
badgers, and
ferrets.
Grassland
 Savannas
are tropical grasslands with
seasonal rainfall.
Grassland
 Chaparral


receives highly seasonal rainfall.
Shrubs and small trees are common.
Adaptations to fire.
Tundra
 Tundra
has a permanently frozen layer
of soil called permafrost that prevents
water infiltration.
 Very
cold, short growing season.
 Little rain
Tundra
 Tundra
is often covered with
bogs, marshes, or ponds.
 Grasses, sedges, and lichens
may be common.
 Lemmings, caribou, musk-oxen,
arctic foxes, arctic hares,
ptarmigans and other migratory
birds.
Desert
have very low precipitation – less
than 30 cm/yr.
 Deserts
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
Variable temperatures.
Animals often nocturnal and live in burrows.
Reptiles and small mammals are common.
Establishing Conservation
Priorities
 Understanding
the distribution of the
Earth's flora and fauna across the globe
can greatly assist conservation efforts, but
naming and mapping biomes and
ecoregions is not enough.
 To make the best-informed decisions
possible, conservation biologists want to
know more about these places, including
the threats that they face and the species
they contain.