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) opposed to terrestrial systems 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 from the standpoint of reducing the loss of a species in the food
web. If one species is the only member of a feeding niche, such as only one producer,
and it is eliminated, the entire food web could collapse since all organisms in that
web are either directly or indirectly dependent upon that one producer species. If
you have 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. Keystone species can occupy any niche not only a producer. 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 would 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 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 latter 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.