Ecology-Lecture 22
Nov 28, 2013
Introduction
As the human population has grown, and our use of resources has increased, we have destroyed
the habitat of many species (outright destruction or through changes in the biological and
physical properties).
A biodiversity crisis has developed. The World Conservation Union lists 16,913 species as
threatened with extinction (1% of all species worldwide). This number is certainly an
underestimate, as only the best studied taxonomic groups have been assessed for their
conservation status. Conservation Biology
Conservation biology: This is the field of biology dedicated to reversing species declines. It can
be defined as the scientific study of phenomena that affect the maintenance, loss, and
restoration of biodiversity. Conservation biology is an integrative discipline that applies the
principles of ecology to the protection of biodiversity.
Stabilization of populations requires expertise from several biological disciplines, as well
as law, political science, and sociology (this integrative approach is characteristic of
conservation biology).
Diversity protection is important for both practical and moral reasons. Humans rely on natures
diversity. There are hundreds of domesticated species that sustain us (food, fuel, fiber). In
addition, we harvest wild species for medicine, building materials, spices and decorative items.
Beyond what we harvest, natural functioning of biological communities provides valuable
services to humans via ecosystem services (water purification, soil maintenance, crop
pollination, climate regulation, flood control). Natural communities and ecosystems are also
important for our emotional health (providing place for solace). Many people also believe that
we should protect species and their ecosystems for moral reasons (religious or spiritual beliefs
that other species have a right to exist just as we do).
Conservation biology is a value-based discipline. The scientific method calls for objectivity
(collection and interpretation of data without bias). However, science is not free of human
values, and it takes place within a larger social context.
Declining Biodiversity
Biodiversity is declining globally
Rates of extinction are difficult to measure because the number of species on Earth currently is
unknown. Most studies have estimated that there are between 5-10 million eukaryotic species
on earth, but there may actually be as few as 3 million or as many as 100 million.
Extinctions of barely known plant species continue throughout the tropics, despite our
decades long recognition of the problem. Through greater efforts to explore earths
ecosystems, ecologists are gaining knowledge of the worlds biota and tabulating new
species at a faster rate, but threats to those species are keeping pace with such gains in
our knowledge about them.
Many exploration voyages identified new endemic species (species that occur in
a particular geographic region and nowhere else) at one time, to find them
extinct a few years later (possibly as result of forest clearing)
Despite the uncertainty in species number, extinction rates can be estimated using several
indirect measures.
Extinction rates determined from the fossil record are used as background rates. We can
then compare current extinction rates to background extinction rates. For mammals and
birds, the background rate is one species every 200 years. This is equivalent to an
average species life span of 1 million to 10 million years. The current extinction rate for mammals and birds is one per year, equivalent to an average species life span of 10,000
years. Overall, the extinction rate in the twentieth century was 100 to 1,000 times
higher than the background rate.
Today, we use the species area relationship to estimate current extinction rates. This
involves the relationship between the number of endemic species and area. It allows us
to estimate the number of species that would be driven to extinction by a given amount
of habitat loss.
We also look at changes in the threat status of species (e.g., shift from endangered to
critically endangered).
Finally, we look at rates of population decline or range contraction of common species
(is the range in which they live declining?).
It is sometimes difficult to know when a species is definitely extinct. Many species are known
from a single specimen or location; the logistics of relocating them may be insurmountable.
However, declaring a species extinct can stimulate biologists’ search efforts.
A flora of Hawaiian plants (1990) listed many extinct species. 35 have since been
relocated, though only a few individuals. These extremely small populations cannot
serve the same ecological functions as larger populations.
Humans have always had a large impact on other species (although rapid increases in population
over the last century have quickened and intensified the effects).
Bones found on Pacific islands reveal the prehistoric extinction of up to 8,000 species of
birds following colonization by Polynesians. Most of the species were endemic. Some
entire guilds (species with the same method of feeding) went extinct, which must have
caused large community changes. Image (A) shows that prior to 3000 years ago, there
were a large number of species. However, fossils records indicate that this number
rapidly declined as soon a colonization began to occur. Figure (B) indicates the species
decline, classified by the feeding guild found on the island. It shows an extinction of 50%
or more of the frugivores (fruit feeding) and nectarivores (nectar feeding). This likely led
to drastic changes in the islands plant communities (role in maintaining endemic tree
populations).
Much research on extinction has focused on problems of small populations, which are
vulnerable to genetic, demographic, and environmental events. This can reduce the population growth rate and increase the rate of extinction. As a result, a cyclic chain of events may ensue in
which an already small population drops even further in size, thereby making it even more
vulnerable to genetic, demographic, and environmental events.
Extinction vortex: A small population declines even further and becomes ever more
vulnerable to processes that lead to extinction. There is a need to determine the causes
of population declines, to identify actions that could counteract problems before the
extinction vortex takes hold (otherwise, this pattern can doom a population to
extinction).
Research has also been focused on a spatial approach that tracks changes in species’ ranges. A
decline often moves through a population as a wave, retreating from all edges of a population
into its center (may be due to invasive species spreading through)
A study of 173 declining mammal species worldwide showed that, collectively, these
species had lost half of their range area over the past 100-200 years.
Example: The cheetah occupies only 56% of the land it once did.
When populations are lost from a community,
there are consequences for that species’
predators, prey, or mutualistic partners (not
just declining species).
The resulting changes at the community
level may bring about secondary
extinctions and ultimately affect
ecosystem function.
While food webs can be resistant to
species removal, the deletion of certain
species can trigger a cascade of
secondary extinctions. Generally, the
stronger a species’ interactions in the
food web, the greater the impact of its
removal.
In a study of plant–pollinator interaction webs, the effect of removing
pollinators depended on whether they were specialists or generalists. If
generalist pollinators (those that visited the most plant species) were removed
first, there was a more rapid reduction of pollinator visits to plants than if the
removal started with the most specialized pollinators.
The movement and introduction of species to
all parts of the globe has increased greatly over
the last century (largely as a result of human
expansion).The range expansion of some
species has coincided with range contraction
of many native species (native species are
going extinct as non native take over). These three images show that the
number of species that have become
established in the U.S has increased
about five fold over the past century.
Image (A) shows that plant pathogens
and terrestrial vertebrates have
increased, posing a threat to the native
species (American Elm tree). Image (B)
shows as increase in the number of
mollusks and fishes, largely from ballast
water. For example, the Round Goby
was introduced into the N.A Great Lakes
by ballast water, and it has outcompeted
native fish species causing their
populations to decline. Image (C) show
that there has been an increase in non
native plants and insects, such as the
Mediterranean fruit fly which causes
extensive damage to a wide range of
fruits and vegetables.
The greatest “losers” among native species tend to be specialists with adaptations that resulted
from evolution in a particular place (small ranges).
The “winners” tend to be generalists with less stringent habitat requirements (can live in a wide
variety of areas, so may leave if invasive species invade their habitat).
The spread of introduced species and native generalists, and the decline of native specialists, is
leading to taxonomic homogenization of Earth’s biota.Taxonomic homogenization is common in
terrestrial areas, but also in fresh waters.
Homogenization has been observed among the fresh water fishes of the U.S, largely as a
result of widespread introductions of game fishes.
A study quantified the homogenization of U.S fish faunas by examining the
change in the number of
species shared between
all possible pairs of the
48 states. On average,
pairs of states share 15
more species than they
did at the time of
European colonization.
The below graph shows
that the average increase in the number of shared species was 15. Historically, Arizona and Montana had
no species in common, but they now share 35.
Genetic homogenization is also occurring through hybridization between native and non-native
species (on genetic level).
Example: The California tiger salamander, a threatened endemic, has hybridized with
another species of tiger salamander introduced from the Midwest 50 years ago as fish
bait. The California tiger species is at risk of extinction through genetic swamping by the
introduced species.
Threats to Biodiversity
Primary threats to biodiversity include habitat loss, invasive species, overexploitation, pollution,
disease, and climate change.
Understanding the causes of biodiversity loss is the first step toward reversing them.
Multiple factors are likely to contribute to decline and extinction of a species.
The Pyrenean ibex was endemic to the Pyrenees. Its decline was due to hunting, climate
change, disease, and competition with domestic livestock and non-native ungulates
such as chamois. The last one was killed in 2000 by a falling tree.
Multiple causes of biodiversity loss are also apparent in higher taxonomic groups.
Over 1,100 mammal species are currently threatened with extinction.
Primary threats facing mammals are loss of habitat, hunting, accidental mortality, and
pollution. Some mammals are threatened by additional factors such as disease.
However, the relative importance of these factors differs between terrestrial and
marine mammals. Habitat Loss & Degradation=Largest Threats
The source of the biodiversity crisis is the scale of human impact on the planet. Earth has been
modified across 60% of its land surface and all marine ecosystems have been affected by
humans. There are areas of extreme influence (agricultural regions and coastal waters) and
areas of little influence (deserts and polar areas) but everywhere has been affected to some
extent.
Addressing the degradation, fragmentation, and loss of habitat is central to conservation work.
Habitat loss: conversion of an ecosystem to another use (urban development or
agriculture).
Habitat fragmentation: breaking up of once continuous habitat into a series of habitat
patches amid a human-dominated landscape.
Habitat degradation: changes that reduce quality of the habitat for many, but not all,
species.
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