SPECIATION
AND VULNERABILITY OF ISLANDS AND ISLAND CONTINENTS.
1. Continental
drifting made islands of continents from breakup of Pangaea; thus small continental islands formed (Madagascar, New
Zealand, Sri Lanka, Australia).
2. Each continent
developed plants and mammals that diverged with time since isolation and at the
same time converged functionally in 'niche' categories based on similar ways of
making a living on each continent.
3. Continental
Islands and oceanic islands are vulnerable to extinctions by arrival of humans.
4. Theory of island
biogeography predicts more species on large islands, but with reduced gene
pool. They drift and are selected to
fill empty niches.
5. Species evolving on islands are endemics. Endemism is
higher on older islands.
6. Extinction is
forever and most severe on islands.
Mauritius-Reunion-Rodriguez, West Indies, Hawaii, &
Galapagos
B. Formation of
Island Continents: Australia,
New Zealand, and Madagascar. and
Biogeography
Fig. 1. and 2
200 mya
As Pangaea broke up
200 million years ago in the height of the dinosaur era, it broke into a
northern LAURASIA= N. Am. +Europe+Asia. and Gondwanaland in S. Hemisphere.
135 mya
N.Am. separates from S. Europe + Africa 180 mya
a. India crosses from
Africa to slam into China.
b. East Gondwana
becomes Antarctica
c. West Gondwana
become S. Am. and Africa. 100 mya
65 mya
a. S.Am separates from Africa 100 mya.
b. Australia
begins to separate form Antarctica
49 mya, from S.Am. 100-110 mya.
c. New Zealand separates from Australia 80 mya
d. Madagascar and India separate from Africa
100 mya
Flora Affinity of
Tropical Rainforest Angiosperms is proportional to distance from each other in
Pangaea and Gondwana and time since they separated. Most are Pantropical, that is, found in all continents,
reflecting the Origin of the families in the late mesozoic and early Cenozoic
(65mya).
Continents became Islands as they separated
100 mya S. Am, Africa and Australia.
Each radiated from a different sock of Late Mesozoic mammals into
similar forms or niches by CONVERGENT EVOLUTION in the Cenozoic.
`
|
N.America S. America Australia |
|
|
|
Wolf none Tasmanian Wolf=marsupial |
|
|
|
Camel Llama,Alpaca none |
|
Bobcat Ocelot Marsupial cat |
|
Mouse Marsupial
mouse |
|
|
New Zealand = no
mammals, Parrot=burrowing rabbit.
Also Faunas of
mammals DIVERGED as function of time of separation. Holarctic fauna separated 49 mya, but rejoined many times by ice
bridges and low water land bridges.
D. Take home: FAUNA ON CONTINENTS SEPARATED
PHYLOGENTICALLY FROM TIME WHEN THE PLATES DRIFTED APART FROM PANGAEA
200mya. MANY FORMS CONVERGED ON
HERBIVORE and CARNIVORE NICHES
Similarities..
CASE STUDY 1 MADAGASCAR. Radiation of primitive Lemurs from 100 mya
separation from Africa. Primitive
placental early primate.
Vulnerability of
Islands and Island Continents--Drift, Founders and Adaptive radiation in a less
competitive community.
MADAGASCAR Island
size of Texas 420 km of E. coast of AFRICA.
More than 200,000 spp
of plants and animals make it 5 th most diverse country in world.
Severed by plate
tectonics form Africa about 100 mya in Cretaceous.
Species radiated from
primitive stocks on the island, now 80% ENDEMIC. Mammals and birds had not advanced too far. Primitive LEMURS
radiated as did birds.
Man arrives only 2000
yrs before present. Population now
10,000,000, most desperately poor not concerned with environmental
conservation, but subsistence farming.
They have cleared 80% of the forests.
Extinct = Madagascar
Serpent Eagle-eats venomous snakes, lizards, and frogs caught from diving
attack, alive. Extinct as forests >
550 m (1800 ft) have been cut down.
Elephant Bird, extinct 1700. Known
since time of Arabian stories of Sinbad and Aladdin with stories from visitors
of the giant Roc. First brought back by European Traders in 1300's by Dutch, Portuguese. and French seamen. Eggs 3 ft in circumference with a fluid
content of 2 imperial gallons = 200 chicken eggs. The Elephant bird lived in primeval wilderness of
Madagascar. It was 10 ft or 3 m tall,
weighted 504 kg or 1,100 lbs and was the largest bird that ever lived on
earth. It had massive legs armed with
taloned claw. Body covered with
bristling, hair like feathers. Vestial
Wings. Serpent like neck and bead.
Originated 60 mya, before there were many predators. There were once 6
spp. of elephant birds, now fossils only.
Isolated on
Madagascar, the elephant bird made to the time after man reached the island and
began to prey on it and its eggs. By 1658 the Dieur Etienne de Flacourt,
Director of the French East India Company wrote that it lived only in the S. of
the island, and laid its eggs in secret places, but not secret enough. It was extinct by 1700.
I. Island
biogeography: FORMATION OF ISLANDS and SOURCE OF BIOTA.
Two Kinds of
Islands:
1. Continental
Islands torn off of an
existing plate: Madagascar, New Zealand
made of old rock.
a. Cenozoic Rock (< 65mya): Faeroes, Bermuda, Madiera off Morocco, St. Helena off S.W. Africa.
b.
Cretaceous Rock (>65mya
): Bahamas, Cape Verde (W.Africa),
Fernando de Noronha off N.E. Brazil.
2. Oceanic
Islands formed by volcanic eruption on bottom of the sea building up
sea mount that breaks surface as island.
A. Usually young less 20 mya = Atlantic =
Azores, Ascension Tristan de Cunha and Bouvet from N.to S. down the mid
Atlantic ridge.
B. Some volcanic islands formed from HOT SPOTS under ocean crust that are
stationary, erupting as new islands as the crust moves over the hot spot. Pacific Chains = Hawaiian Islands youngest =
Hawaii, Oldest = Kauai
= Christmas Island Ridge ending at the
Tuamotu Islands.
= Austral Ridge
= Cocos Ridge = Galapagos Islands.
Continental Islands
lose species after being isolated and then may evolve some novel ones. Continental Island broke off from the
continent take a piece of the landscape and some of the species pool and their
natural genetic variation.
New volcanic islands
are ‘born’ with no animals and plants. Biodiversity increases by immigration
and by adaptive radiation into endemic species and genera.
How do New Islands
become colonized? Propagules,
colonists, founders = airborne, water washed, bird vector.
1. Bird Vectors--on feathers, ducks
feet, and more likely in gut = 49-77%
of angiosperms.
2. Seaborne on rafts of vegetation,
rafts of volcanic pumice = 12-77%
3. Airborne = 0-24%.
II. ISLAND
THEORY OF BIOGEOGRAPHY.
Species of plants and animals on islands
represent an equilibrium between new
arrivals (immigration) and local extinctions. Thus the species pool on an island does not change in time if it
is in equilibrium.
The number of species
is greater on large islands than small islands. The number of species on the mainland continent is much greater
than on any island. Thus islands have a
depauperate flora and fauna compared to equal areas of mainland.
A. Island Species
Richness is a function of area. Why?
Immigration of new species is a function of the target size from the mainland.
B. As a result islands have fewer species than mainland. Small
islands have fewer species than large islands.
C. Few colonists means small gene pool or small founder population.
D. In absence of all species in all niches,
newly arrived species and mutants are
selected directionally for their ability
to invade empty niches.
E . Many niches
become filled adaptive radiation of from stocks of whatever colonists happened to
arrive.
F. The degree of ENDEMISM is higher in older islands.
G. Loss
of species at time of human contact or settlement. Most pronounced on small
islands.
Deliberate over hunting.
Colonization of domestic animals and
plants.
Case Study:
GALAPAGOS ISLANDS 1000 km from
Ecuador = 13 large and 6 small islands.
AREA = 8000 KM^2
AGE = 5,000,000 yrs oldest island.
Long distance
dispersal required for colonization. Thus survivors are over represented
compared to mainland.
1. Plants over
represented = ferns, grasses, sunflower, pigweed, bean, sedge, lichen and
mosses. = small seeds or spores = Airborne
dispersal Mainland Ecuador as 20,000
species. Galapagos contains 500-575 species, of which 42% of the plant taxa are endemic (250 spp) that likely
radiated from 110 original colonists + 190 species introduced by recent human
activity.
2. Rafting of vegetation on sea = snakes, rice rate, reptiles well suited need little water
or food.\
3. Transport by
Birds = seeds, feces,
regurgitated pellets.
SPECIATION in animals over represented =
mockingbirds, giant tortoises, lava lizards, and land snails Bulimulus. Two land and on marine
iguana. 1 Galapagos hawk. Giant
tortoises = 14 races or subspecies on 9 islands. Origin San Cristobal.
Of
plants 34% of species are ENDEMIC, 250 species which arose from 110 species
of colonists. This is less than 95%
endemic on Hawaii chain (more distant).
Alternanthera, Scalesia, Opuntia, Chamaesyce and Mollugo have developed > = 8 taxa
from 1 progenitor to reach islands.
Hazards to endemics
are hunting and competition with
European animals: dogs, cats, goats, rats, pigs, guava plants and quinine tree.
III. LOSS OF SPECIES WITH HUMAN CONTACT.
A .Communities of
Island and Island Continents are more susceptible to eutherian mammals and
dicotolous plants, which drive local endemics to extinction.
B. Greatest loss of
species is occurring on islands and island continents.
Small populations on
islands are always at risk of going extinct, because deaths > births.
The few species
arriving find themselves in an ecological void. Adaptive Radiation is possible with resulting speciation as reproductive barriers arise.
Proportion of
endemics is normally directly related to time since isolation.
LAND BIRD SPECIES on
Madagascar, Jamaica, Hispaniola, Sao Thome 20% are endemic.
The number of
endemics species on an island must be dependent on the islands age, area and distance from mainland. The proportion Endemics goes up as island
area increases on isolated archipelagos around the world. WHY?
Probability of radiation on small islands without gene flow is remote.
III. DOMINANCE BY
REPTILES over MAMMALS on islands is
a function of Population Energetics.
Cost of being warm
blooded requires more food/gm of animal.
a. Small mammals use less
resources than large mammals.
b. Reptiles use much less energy than any warm-blooded mammal. Hence on smaller, energy restricted islands, reptiles began to proliferate at cost of mammals. Longevity and size allow reptiles to live through food poor years and live to reproduce. Hence a selection of giantinsm
Galapagos: Tortoises
Land and Sea Iguanas
Most herbivores, not predators.