Question Key
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What key features do we use to distinguish terrestrial biomes? Aquatic
biomes? What types of variation are we highlighting in our definitions
of particular biomes, both terrestrial and aquatic?
Terrestrial biomes are
primarily distinguished by their average annual precipitation and
average temperature. So, tropical rain forests have high precipitation
and also high temperature. In contrast, high temperature and low
precipitation will give you hot desert. Temperature tends to be the axis
defining whether a biome is polar, temperate, or tropical in nature.
Within a given temperature range, the amount of precipitation defines
what type of specific biome is found (desert, grassland, deciduous
forest, rain forest).
Aquatic biomes are defined
somewhat differently. One major axis is freshwater versus marine
(saltwater), defined by the salinity of the environment (with brackish
waters being an intermediate zone). Another axis is related to the depth
of the water, related to light penetration: the depth to which light
penetrates is known as the photic zone, the aphotic zone being the
deeper waters where sunlight does not reach. In addition, along a
shoreline, we identify the intertidal zone (which is exposed during low
tide but submerged during high tide), the neritic zone (constantly
submerged but light penetrates), and then the benthic zone (submerged,
no light penetration). Temperature is correlated with position along
these axes, as is the availability of oxygen such that the benthic zone
is the coldest and has the least oxygen.
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What are the essential differences between the movement of carbon,
nitrogen, and phosphorus in ecosystems? Organize the info so it’s easy
to visually compare them.
Carbon (C)
atmosphere: circulates
globally where it’s available to photosynthesizers who fix it (C enters
the biosphere).
hydrosphere: enters at
air-water interface, increased C increases ocean
acidity.
lithosphere: C enters
from marine deposits (remnants of dead organisms) and from soil organic
matter (decomposing organisms). This can be a long-term storage for C
(except for when we burn fossil fuels).
biosphere: C enters when
photosynthesizers fix it, contribute to their own biomass. Consumers eat
producers, C moves through the biosphere. Exits biosphere after
decomposition of organisms, enters soil organic matter.
Nitrogen (N)
atmosphere: circulates
globally, abundant in atmosphere but N2 is not available for most life
forms to incorporate so largely unavailable to biosphere. Burning fossil
fuels and denitrifying bacteria add N2 to the
atmosphere.
hydrosphere: inorganic N
circulates in water but is largely unavailable to biosphere in this form
(except for N-fixing microbes). Bioavailable N enters the hydrosphere
from eutrophication (runoff of fertilizers or other nutrient rich waste
flows) and naturally from leaching out of soils.
lithosphere: can enter
via soil organic matter.
biosphere: can be fixed
from atmosphere by nitrogen-fixing bacteria, producing ammonia/ammonium
(or nitrates from nitrifying bacteria) that can be taken up by plants
and incorporated into biomass. Over-use of fertilizers (N and P) can
lead to eutrophication in aquatic ecosystems. Eutrophication can also
occur naturally in lakes and ponds as they age.
Phosphorus (P)
atmosphere: No
atmospheric circulation (movement of P is local, not
global).
hydrosphere: circulates
in water, highly available to biosphere but relatively limited in most
ecosystems. Use of P in fertilizers has caused increased P inputs in
water systems from runoff. Also enters hydrosphere from weathering of
rocks (primary storage of P).
lithosphere: primary
storage of P on planet. Human mining from rock deposits for use as
fertilizers has increased the availability of P in the biosphere.
Typically, P would cycle slowly through the lithosphere into the
hydrosphere and biosphere. Mining has increased circulating P in
hydrosphere and biosphere.
biosphere: inorganic P
is taken up by plants and microbes from soil or water, incorporated into
biomass. P is typically a limiting nutrient in ecosystems such that
P-rich fertilizers increase the productivity of terrestrial and aquatic
ecosystems. Over-use of fertilizers (N and P) can lead to eutrophication
in aquatic ecosystems. Eutrophication can also occur naturally in lakes
and ponds as they age.
What is the role of decomposition in biogeochemical cycles?
Decomposition, carried out
primarily by fungi and bacteria/archaea, is critical for breaking down
the biomass of dead organisms into less complex molecule. Many of the
breakdown products can then be recycled back into the biosphere, once
they have been converted into usable forms. Thus, some fraction of the
biomass produced in a system will be recycled by decomposers and
immediately re-enter the biosphere after decomposition. Without
decomposers, a great deal of the nutrients in dead biomass would likely
end up in the lithosphere as organic matter accumulated but was not able
to be used by other organisms.
How is anthropogenic disruption of nutrient cycles similar to, and
different from, the processes that occur in ecological succession?
If you recall the slides
from the lecture on Ecological Succession, the availability of different
nutrients varies as succession proceeds. Early on, few nutrients are
available and not in great quantity; later larger concentrations of
nutrients are available thanks to growth and subsequent decomposition.
Anthropogenic disruptions that impact nutrient cycles might include
things like deforestation, eutrophication, over-harvesting. Like
succession, anthropogenic disruptions can alter the availability of
nutrients, but in larger ways than is true of something like succession
in an old field and typically faster than in primary succession. As a
result, anthropogenic disruption can act like a disturbance, suddenly
making conditions favorable for r-selected taxa. In situations like
eutrophication, this leads to explosive over-growth such that various
resources (oxygen) are depleted so greatly as to render the habitat
unlivable for a period of time. However, some research shows that with
time, ecosystems can be resilient to anthropogenic disruption, such as
the experimentally deforested watershed that showed nutrient levels
returning to pre-clearance levels within about a decade.
Weekly Reflection. Consider this week’s material and reply to one or
more of the following prompts:
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What was confusing or interesting to you about this week’s material?
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Did you have any key insights while studying this material?
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Does anything from this week’s material particularly stick with you?
When you are finished, check your responses on the key for PS14.
Remember to sign the Honor Code on your assignment.