Pond Ecosystem Analysis
Pond are complex ecosystems supporting fully aquatic, semi aquatic, and
even terrestrial (land dwelling) organisms. They are typically higher in biodiversity
(number of different species) than terrestrial (land) ecosystems due to the
availability of a constant water source. The amount of water in a pond can change
over time and therefore result in succession of the pond (see section 1.8 of the
Environmental science text book.) The pond from the field study has the
characteristics of an emergent plant stage with some areas succeeding into a marshy
stage based on the increased vegetation in those areas. The purpose of this study is
to determine the overall ecological health of the pond based on observations and
collection of both biotic and abiotic data.
When discussing the health of an ecosystem we are really talking about its
ecological stability. Stability is defined as how likely it is for the food web of that
system to resist a drastic decrease in populations and biodiversity. Biodiversity is
another aspect of ecosystem stability. Biodiversity is defined the number of species
in an ecosystem. Typically the larger number of organisms in an ecosystem allows
for a greater variety of organisms in a food web. This therefore increases the
number of feeding choices at each tropic level making food less of a limiting factor. It
also creates stability by reducing impact that a loss of one species would have on the
food web.
If there is one species that is vital to the functions of an ecosystem and the
stability of its community, it is termed a keystone species. Understand that many
food webs may have keystone groups. An example of a keystone group would be
algae in a pond or decomposing bacteria that are found in virtually every food web.
These are considered groups because they are comprised of multiple species
opposed to only one. A producer keystone is the most obvious example because it is
the base of the food chain but this can also occur at the predator level (see case
study 1.4 –The case of the missing hawk) The presence of a keystone species does
not make a food web unstable. If the population of that species is very stable, for
example a single species of grass over a large expanse of prairie that has been there
for centuries, the food web based on that grass species is very stable.
In summary more species allows for a more stable or healthy ecosystem.
There is an exception to this rule. The relative populations of the species in a web
also factors in its stability. For example, if you have 20 species that are all mostly in
endanger of being wiped out it is less stable then a web with 10 species that are all
high with few limiting factors. In order to analyze the health of an ecosystem you
should look at the number of species in the food web. In addition you should also
analyze the relative abundance of those populations in relationship to one another
and how important they are in supporting the food web. Finally determine if there is
a keystone species and if that species has a stable population.
You now understand how the removal of a species can disrupt an ecosystem,
but what about the addition of a species? Introduced species are having greater
impacts on ecosystems annually. An Introduced (exotic) species is a species that is
not native to an area. It was been brought in (introduced) either intentionally or
accidentally by humans. Many introduced species have become important parts of
human society such as livestock, pets, and many food crops. There are now many
introduced species that exhibit invasive characteristics. An invasive species, which
is most often introduced, is one that out competes native species for key resources
such as food, water, cover, space, light, etc. It could also become a predator to native
species directly reducing their populations. These species could therefore become
limiting factors for several species in a food web. This could cause a decrease in the
population of these species thus disrupting the stability of the food web.
Abiotic factors also impact the stability or health of an ecosystem. In a pond
ecosystem, physical factors such as heat, light, pH, dissolved oxygen, and nutrient
levels (nitrogen) can greatly affect the diversity. Dissolved oxygen in water is
essential to all truly aquatic organism such as fish but not as much of a factor to air
breathing animals such as turtles. Temperature affects dissolved oxygen in that
warmer water holds less oxygen than cooler water. When a pond starts to dry up
and fill in with more vegetation, the pond becomes shallower and therefore heats
faster than a deeper body of water. More organisms die and decompose as pond
succession continues, the decomposing bacteria consume great amounts of
dissolved oxygen through the respiration process. Therefore ponds in later stages of
succession have less dissolved oxygen and few truly aquatic organisms. Lack of
useable nitrogen and other nutrients are a major limiting factor to plant growth
such as algae in a pond. Nitrogen levels greater than 0.2 parts per million usually
indicate some form of nutrient pollution. The pollution source could be excess
fertilizers from lawns, farms, or golf course. The nutrient boost could cause
excessive algae growth in the pond. As the algae dies it decomposes using up
additional dissolved oxygen in the water. These factors could therefore have an
effect on the diversity and stability of the food web.