Environmental Engineering Reference
In-Depth Information
Atmospheric CO
2
concentration
Climatic
warming
Carbon
sequestration
Photosynthesis
Acclimatization
Vegetation
growth
Nutrient
availability
Respiration
Positive
Negative
FIGURE 11.7
Both positive and negative feedbacks to climate, via atmospheric carbon dioxide, are possible
with warming of a tallgrass prairie. An enhancement of root and soil respiration and a positive feedback to cli-
mate could result from stimulation of soil microbial activity associated with higher temperatures. This positive
feedback would be weakened if there is acclimatization such that soil (root-plus-microbial) respiration does not
continue to increase with increased warming. In contrast, if greater microbial activity leads to enhanced nutrient
availability and plant growth, this could result in greater carbon sequestration in plants and soil—and a negative
feedback to carbon dioxide concentrations in the atmosphere. (From
Luo et al. 2001
.)
sediment moving in rivers, but the overall sediment export to the ocean has decreased
because of impoundments (
Syvitski et al. 2005
). We release toxins into air, water, and soil,
and these reach every ecosystem on the planet (
Weathers and Ponette-Gonz´lez 2011
).
We introduce, decimate, and even extirpate species in most ecosystems, radically change
disturbance regimes in ecosystems around the world, and now, we are altering the pla-
net's climate. As a result, it is hardly surprising that human population density (or some
other measure of human activity) often now appears on the x-axis of graphs in ecosystem
science. These human-mediated changes have altered the abiotic and biotic controls on
ecosystems, and in turn, the structure and function of ecosystems around the world. As a
result, we continuously need to expand our understanding of the structure, function, and
controls on the world's ecosystems.
COMPLICATIONS
One of the reasons that we have offered a brief list of useful generalizations rather than
a comprehensive theory of ecosystem control is that the concept of control turns out to be
slippery. For instance, suppose we ask the question about what controls primary production
in a local lake. If bottle bioassays show that phosphorus limits algal production, we might
conclude that phosphorus inputs to the lake control primary production, which would
