Preliminary: We are temporarily abandoning our text. It doesn't investigate the ultimate causes
of population decline as much (it does, however, do a good job with detecting population decline
and treating the problem). Some of what follows is covered in Chapter 8.
(I'm using a variety of sources here, particularly a text I considered for your class entitled
"Fundamentals of Conservation Biology"by Hunter (2nd edition), 2002, Blackwell
Science.)
Habitat loss.
Simply put, habitat refers to the environment an organism (or organisms) lives in;
specifically, the environment needed by an organism (or organisms) to survive.
Thus, habitat loss is the removal of that habitat from the availability to that organism.
Some simple examples:
In northern Virginia we live in one of the fastest growing communities in the
United States. But, one result of that has been the removal/destruction of vast
amounts of habitat.
George Mason University:
- In 1979, GMU consisted (approximately) of the four original
buildings, Thompson Hall, Robinson I, the Library, Lecture Hall,
Student Union I, a few residence halls (behind the Student Union)
a physical education building and a few odd trailers. There might
have been a few other buildings like parts of the physical plant, etc.
- In 2004 (25 years later), we have built up most of the campus.
Numerous new buildings (King Hall, Johnson Center, Patriot
Center, Center for the Performing Arts, Student Union II, Science
and Tech I & II, the "Arts(?)" buildings (not sure what the official
name is), many more Residence Halls, a second Library tower, the
Krachnow (sp.?) institute, aquatic fitness center, police building,
Abomination Hall, as well as more trailers).
- Deer (admittedly, not always desirable) used to be common on
campus. So were numerous species of snakes, lizards, and
salamanders. Numerous birds (e.g. pileated woodpeckers) were
often seen nesting on campus. Some things are still around, but if
current trends continue, soon it'll be reduced to squirrels and
crows.
- Note that not one animal was deliberately killed in all of this.
- Is this good? bad?
Other areas in Norther Virginia:
- How many people drive past a new housing development on their
way to work (or GMU)? All these areas used to provide habitat for
native (and introduced) organisms.
- Again, is this good? bad? What are the trade-offs?
Let's investigate this phenomenon a little more by looking at some causes:
Urbanization - the effect is obvious, pretty much what we discussed above.
There's no real need to dig into the details here.
Deforestation - Forests are being cut down for lumber, construction, and
agriculture. The latter can be rather insidious:
In the tropics, many rain forests are being felled to make room for
agriculture. Because of the poor soil, these plots don't last very long, and
farmers will move on to other areas, clearing these as well.
Some authors will argue that clear-cutting is not necessarily deforestation,
providing the land is allowed to recover naturally. A good point, but still,
that area is obviously disturbed for a while.
The consequences include the obvious impact on forest species (e.g.,
pandas, tigers, etc.), but also some others:
- a carbon sink is removed (those trees were host to a large amount
of carbon). It is true that if the wood is used for lumber, the carbon
may not make it back into the atmosphere right away, but often the
wood from clear cut areas is just burned.
- local climate can be influenced since a major source of
evaporation has been removed.
Estimates vary, but one source indicated an area the size of Switzerland is
being deforested every year. The tropics are the places hit the hardest.
Of particular concern is the removal of rain-forest. Some estimates are
that 50% of all species will be found in this environment, and we are
destroying rapidly (this is only an estimate - it is clear that rain forests are
diverse, but the extent of the diversity is unknown).
Desertification - Areas adjacent to deserts are being converted into desert (not
deliberately, but the process is ongoing.
A simple (small scale) example. An aid organization discovers that local
tribesmen must cover great distances to water holes to water their
livestock. To try to alleviate this, they provide funding for a well in the
areas used by the tribesmen. The result is twice as much livestock, with
the added effect that wide areas around both the original water hold and
the well are denuded due to heavy livestock use. Oops.
But the real problems are much more serious:
Agriculture in dry areas often causes loss of soil and eventual
desertification of the habitat.
Livestock in the same areas trample vegetation and uproot/eat
what's left. Goats are particularly bad that way and have been
implicated in many areas (similar to the example above, but on a
much larger scale).
Firewood collection - firewood is an important fuel in many parts
of the world. As populations increase, firewood is collected from
an ever widening area. This often involves the killing of what trees
do exist in drier climates.
Irrigation can cause salinization of the soil. As water evaporates in
dry climates, salt is left behind - this increases in concentration
until the soil is unusable.
Particularly in sub-Saharan Africa, vast areas bordering the North African
Deserts have been turned into desert. There are even dramatic pictures of
dunes encroaching on towns and villages, though usually the effects are
much more subtle. Usually it's not until years later that one finds that the
desert habitat has spread (don't forget - desert IS home for many plants
and animals, but this is not the way to "help" these animals.)
Estimates are that as much as 70% of the world's dryland agricultural
areas are threatened, and a total of 30% of the world's land surface.
Loss of wetlands - Wetlands are some of the most productive habitats. These are
often considered "useless" since it's hard to build or do agricultural activities in
these areas. The end result had been a vast effort of draining marshes, building
dikes and dredging to get rid of these areas.
Estimates are that since the late 1700's, about 53% of the world's wetlands
have been removed.
Wetlands host a unique array of species, are crucial in cleaning pollutants,
serve as temporary refuges for numerous species (the young of many fish
use these areas), and in terms of sheer biomass production are one of the
most productive areas around.
Recent laws in the U.S. have made these areas more difficult to develop.
(Though this is under assault).
A small example - the everglades have been adversely affected by a lack of
water flowing through the everglades. Much water has been diverted for
agriculture, and for draining marshes "upstream" from the everglades.
One problem with "protecting" wetlands is that the surrounding areas can
have a huge impact on the wetlands.
(Of course, much has been made of water shortages throughout the world -
we should keep this in mind as well.)
Fragmentation - This is simply the breaking up of habitat into smaller chunks.
There are numerous consequences of this:
Obviously, species that need large tracts of undisturbed habitat will be
adversely affected as roads and fields bisect their traditional ranges.
Edge effect - species that normally can not penetrate deep into (for
example), forests, suddenly find themselves able to exploit numerous areas
that they were off limits. Some species were driven to the edge of
extinction:
Kirtland's warbler - two problems, first, it was dependent on fire-
disturbed areas, second, with the increasing edge effect cow birds
have been able to decimate populations by gaining access to nests
they would not normally be able to reach.
(Cow birds are brood parasites - they lay their eggs in
another birds nest and let the other bird raise their young -
in the process the original young is often killed).
Recent reports show that this bird is recovering due to
increased burns and controlling for cow birds.
Isolation - different parts of a population can become isolated, thus
reducing genetic diversity in individual sub-populations. It may also make
it more difficult to find mates.
Migration patterns can be seriously disrupted.
In general, large "fragments" can support a more diverse population than
small populations. One reason for this can be explained by the familiar
Island biogeography theory by MacArthur and Wilson (you can review this
on you own if you wish).
See page 242 in your text for a nice summary.
A brief word on causes of fragmentation:
As usual, increasing human populations are fundamentally to
blame. Typically what happens is that a road is built into new
areas. This provides access for people, who come along and clear
small areas for living and/or farming. As populations grow, more
and more land is cleared, leading to greater fragmentation. As the
area becomes saturated, more roads are built....
A few concluding remarks.
Some authors differentiate between habitat loss and habitat degradation. The
latter is the case where the habitat is still suitable, but no longer ideal. For
instance, a road cutting across a wilderness area can prevent some animals from
crossing it (some studies show that even a dirt track through a forested area can
prevent some animals from crossing). This "degrades" the habitat. Obviously
there are various levels of degradation.
An interesting display is at:
http://www.co.fairfax.va.us/maps/timeMachine.htm#
(provides sequence of aerial photographs starting in 1937 up to the late 1990's for
several areas in the county).
Habitat loss is probably the single biggest factor causing the world-wide decline
of species. It is not at all obvious what can be done about this in the long term.
(The answer is not necessarily "create a reserve").