Extinction
-estimated species extinction is 100-1000X greater than speciation
-genetic variation is being lost even in domesticated species such as wheat,
corn, rice, chicken, cattle, and pigs Ex. 97% of vegetable varieties once
cultivated are now extinct
-word extinct has many nuances:
1) extinct -no member of the species remains alive anywhere in the world
Ex. Montverde golden toad
2) locally extinct or extirpated-no longer found in an area that individuals
once inhabited Ex. gray wolf occurred throughout North America but is
locally extinct in many states
3) ecologically extinct -numbers of the populations are reduced to a point
where its effects on other species in the community are negligible Ex.
tigers
7.1 Bachman’s warbler became extinct as a result of tropical deforestation in its wintering grounds
4) extinct in the wild –individuals of a species remain alive only in captivity
Ex. Franklin tree is only found in cultivation
5) globally extinct –extinct in the wild throughout the world
Ex. Montverde golden toad and the Franklin tree also fit this
definition
Past rates of extinctions
-diversity of species has been increasing since life first originated. This has
not been a steady increase. There have been times where there were high
rates of speciation followed by periods with little change or episodes of
mass extinction
7.2 The number of families of marine organisms has been increasing over geological time
7.3 During each of five episodes of natural mass extinction a large percentage of gradually
increasing groups disappeared
Human-caused extinction—The Sixth Extinction Episode:
-elimination of 74-86% megafauna (> 100 lbs) from Australia, North and
South America by hunting
-reduction of natural ecosystem size to make way for domestic animals
-extinction rates during the past 2000 years mostly involve land vertebrates,
especially birds,and mammals
• Since 1600, 77 species (1.6%) of mammals and 129 species of birds
(1.3%) have gone extinct with the majority of extinctions occurring over
the last 150 years
Table 7.1
Figure 7.4 Rate of extinction of birds during 25 year intervals:
-rates of bird extinctions doubled after 1750 and tripled after 1850.
-apparent decline in extinction rates is due to current practice of not declaring a
species extinct until decades after they can no longer be found
-many others are committed
to extinction due to habitat
destruction
Extinction rates will remain high in the coming century because of the
large numbers of threatened species.
Table 7.2
St. Helena ebony is
reduced to two
individuals on the
cliff side of a south
Atlantic Island. Will
likely go extinct in wild
but will persist in
cultivation
Extinction rates on islands
-islands have some of the highest extinction rates
-rates are often increased when competitors, predators, and diseases are
introduced by human colonists and visitors Ex Box 7.1 with data from the
Hawaiian islands on birds
-species extinction rates peak soon after humans occupy an island and then
decline after the most vulnerable species are eliminated Figure 7.6
7.6 Relationship between recently extinct or currently endangered bird species and length of time
non-European peoples have occupied an island group
-islands often have large numbers of endemic species and they are particularly
vulnerable to extinction
-island plant species often have large percentages of endemics and many are
threatened through habitat destruction Table 7.3
Studies of species on islands has led to the development of a model that allows
one to predict numbers of species
Island biogeography model (MacArthur and Wilson, 1967)
-central observation is species-area relationship. Islands with large areas have
more species than islands with smaller areas
Figure 7.8
7.8 The number of species on an island can be predicted from the area of an island
Species-area relationship can be summarized by the empirical formula:
S = CAZ
S=number of species on an island or habitat fragment
C=constant dependent on the type of island and species. It is less where the
quality of the environment is poorer and the total number of organisms is
smaller. It also decreases with increased isolation of the island.
A=area of the island
Z=exponent that determines the slope of a curve where species numbers are
plotted against island area. The m-value in y=mx+b.
Z-values are typically about 0.25 with range of 0.15-0.35.
Ex. C = 1, Z=0.25 for raptorial birds (birds of prey) on an island. How many
species would you expect to find on islands of a. 10, b. 100, c. 1000, and
d. 10,000 km2?
-tenfold increase in island size does not result in a tenfold increase in the
number of species (increase is about a factor of two)
MacArthur and Wilson found that the number of species on
an unoccupied island will increase over time, since more species
will be arriving or evolving than are going extinct, until the rates
of extinction and immigration are balanced
Figure 7.9
-extinction rates will be lower on large islands than on small
islands because larger islands have greater habitat diversity,
a greater number of populations, and present a larger target
for immigrants (green and orange lines on graph)
-immigration of new species will be higher for islands near the
mainland than for islands further from the mainland because
mainland species are able to disperse to near islands more
easily (blue and red lines on graph)
7.9 Island biogeography model. Blue & red = immigration rates; green & gold = extinction rates
Species-area relationships can be used to predict the number and percentage
of species that would become extinct if habitats were destroyed.
-calculation assumes that reducing island habitat would result in the island
being able to support the number of species found on a smaller island
-this holds true for islands and habitat islands (fragments)
-model predicts that when 50% of an island is destroyed, about 10% of the
species occurring on the island will be eliminated; 90% of an island is
destroyed, about 50% of the species occurring on the island will be
eliminated; 99% of an island is destroyed, about 75% of the species
occurring on the island will be eliminated.
Figure 7.10
7.10 According to the island biogeography model, the number of species present in an area
increases asymptotically to a maximum value
7.11 Ringtail possums in habitat fragments less than 10ha go extinct in under 10 years.
Extinction Rates in Water
-Extinction rates in oceans appear to be low but this is likely an
underestimate as these areas are often poorly known and/or marine
species may be able to adapt to disturbance well
-Only four marine mammals (Carribean monk seal, Sea mink, Stellar's
sea cow, Japanese sea lion), five birds, four mollusks (limpet) are
known to have gone extinct in oceans.
Large numbers of aquatic/wetland animals are in danger of extinction in
USA
Figure 7.7
7.7 Dams, irrigation systems, polluted runoff, introduced species, and habitat destruction threaten
aquatic species like fish and mussel species shown below:
In addition to global extinctions on mainlands, islands, and aquatic
ecosystems, a series of local extinctions or extirpations are occurring
across the range of many species in all types of habitats developed by
people
Ex. Chelone field work
American Burying Beetle Figure 7.11
-estimated that five million populations are lost per year in tropical rain
forests (13,500 populations/day)
Chelone obliqua L., a threatened wetland species in many states has
had numerous local extinctions based on fieldwork by Nelson (1995)
7.112 The American burying beetle is now found only in four isolated populations