|
 Populations
within an ecosystem are constantly increasing and decreasing (population
fluctuations and cycles) in numbers based on the amount of food,
clean water available, weather and natural disasters that have occurred.
The numbers within
a species in an ecosystem population change over time. From time to
time new species appear while others disappear, depending upon the
resources available. Carrying capacity describes the ability of the
biotic and
abiotic resources in an ecosystem to support life.
Within the environment of an ecosystem, organisms interact constantly
with both the biotic (living) and abiotic (nonliving) elements.  In
an ecosystem different species of plants and animals interact with
one another. For example, honey bees seek food from flowering plants
and also pollinate the flowers as they go from plant to plant.
If
the amount of resources in an ecosystem changes dramatically for some
reason, then the carrying capacity changes and the plants and
animals either adapt, move, or die out. For example, the northern areas
of Alaska and Canada can sustain large numbers of birds during the
summer due to the quantity of summer plants and animals available as
food sources. As Fall begins, the carrying capacity of these northern
climates decreases, causing the birds to fly south looking for areas
where food is available. Another example of a change in carrying capacity
can be illustrated in the southwestern U.S. Here there are geologic
signs that as little as a 1000 years ago the land in northern New Mexico
was at a lower elevation or there was a lot more water available from
rain and in the rivers. American Indians in that area farmed crops
and the animals and plants present were similar to what is found in
fertile river valleys. The area of northern New Mexico shows signs
of a great uplift. After the uplift the land became a much drier climate.
This change in the climate dramatically changed the carrying capacity
of the land for the types of plants and animals that could survive.
Population Fluctuations, Cycles, and Carrying Capacity
In
an ecosystem, population fluctuations and cycles are natural and
constant as populations are dynamic. The number of organisms of a
species in an ecosystem changes naturally due to birth rates, death
rates, numbers of predators, amount of food, clean water, shelter
available, and other environmental factors. If the number of trees
and brushes in a forest increases there becomes more food and shelter
available. The size of small animals and bird populations might also
increase over a number of years. Local predators will have more food
so their population numbers might increase as well.
The
size of the population may decrease, at some point, due to over population
of a species, disease, lack of food, water, cover for breeding and
shelter, or extreme weather (prolonged drought or severe temperatures). During a period of low rainfall, the number of trees,
brushes, and grasses available might decline, making it more difficult
for small animals to find food and shelter. Limited shelter enables
predators to identify and catch these food sources more easily. Population
cycles have been documented in the grouse and hare populations.
L.
Keith and Associates (Meadows and Keith, 1968; Keith and Windberg, 1978)
studied snowshoe hare populations in a particular area through
two periods of decline and one period of increase. The study took 15
years and showed that the full population cycle for the woody browse
(trees and brush), grouse, snowshoe hare, and the lynx was about 10 years.
These cycles can be interrupted by natural disasters or any dramatic
change in the land or climate.
Bottomland Hardwood Forests of the Southeastern United States
One
ecosystem that has dramatically decreased and been altered over the
past 200 years is the bottomland hardwood forest of North America.
These forests
are found along the banks of most large rivers in the southeastern and south
central U.S. where annual flooding occurs during the late winter or spring
of each year. This flooding provides needed water and nutrients to the trees
and other vegetation, which in turn become habitats for large numbers of species
of animals.
Bottomland
hardwood forests are an important renewable forest resource as well as a wetland.
These forests are ecologically significant as a habitat for thousands
of plants and animals (as spawning grounds, nesting areas and food resources)
that help make up the great diversity of life on the North American continent.
The bottomland hardwood forests are home to: hardwood trees, smaller vegetation,
insects, fish, amphibians, reptiles, deer, birds (native and migrating) and many
other animals.
Population
sizes in ecosystems, such as bottomland hardwood forests, have been fluctuating
and sustaining themselves for thousands of years. In the past, population
size has been effected by changes in the weather, land and natural disaster.
Humans have been developing these forest areas and changing the resources needed
by plants and animals. This has resulted in a decrease in the carrying capacity
of these bottomland hardwood forests. Of the 25 million original acres of forested
wetlands or bottomland hardwood forests in the Mississippi River alluvial floodplain,
more than 80% of the these acres have already been cleared and drained, leaving
only about 5 million acres of bottomland hardwood forest as a habitat for many
native species of animals and plants. Most of the 20 million acres has been put
into croplands, while some of it has been used for housing and roadways. |
