Environmental Engineering Reference
In-Depth Information
omitting key observations or measurements. Not surprisingly, careful natural history stud-
ies (such as Forbes' “The Lake as a Microcosm,” discussed later) were important precur-
sors to modern ecosystem science. Although these forerunners of ecosystem science often
included speculation about ecosystem processes, they did not have the technical means to
easily measure such functions as net ecosystem productivity or nutrient cycling, or to
quantify trophic transfers.
Long-Term Studies
Long-term studies (i.e., those lasting for more than 3 to 5 years—the length of most
grants or the time it takes to earn a PhD!) are relatively rare in ecology as a whole.
However, long-term studies are especially good at providing insight into slow processes
(e.g., changes associated with forest succession), subtle changes (e.g., changing chemistry
of precipitation), rare events (e.g., effects of hurricanes or insect outbreaks), or processes
controlled by multiple interacting factors (e.g., fish recruitment;
Likens 1989
;
Lindenmayer
and Likens 2010
; and see the Long-Term Ecological Research Program (LTER) of the
National Science Foundation,
http://www.lternet.edu
). Sometimes long-term understand-
ing can be obtained by short-term analyses of materials that record history, such as soil or
sediment cores, otoliths (fish ear-stones), or written historical records. For example, analy-
sis of pollen, diatoms, pigments, geochemistry, and minerals in lake sediment can reveal
the history of terrestrial vegetation, phytoplankton, soils, and lake level—in short, the his-
tory of the development of the linkages between terrestrial and aquatic ecosystems. It is
from long-term studies or their surrogates that scientists have documented climatic, atmo-
spheric, geochemical, and organismal changes over decades to billions of years.
Cross-Ecosystem Comparison
Comparative studies have served two important roles in ecosystem science. Most sim-
ply, scientists often have measured some variable associated with ecosystem structure or
function across a series of ecosystems to identify typical values of that variable, show how
it varies among types of ecosystems, and generate hypotheses about what factors might
TABLE 1.1
Strengths and limitations of approaches to understanding ecosystems. Natural history
observations and understanding underpin all of these approaches.
Approach
Some Strengths
Some Limitations
Theory
Flexibility of scale; integration; deduction of
testable ideas
Cannot develop without linkage to
observation, experiment
Long-term
observation
Temporal context; detection of trends and surprises;
test hypotheses about temporal variation
Potentially site specific, difficult to
determine cause
Comparison Spatial or inter-ecosystem context; detection of spatial
pattern; test hypotheses about spatial variation
Difficult to predict temporal change or
response to perturbation
Ecosystem
experiment
Measure ecosystem response to perturbation; test
hypothesis about controls and management of
ecosystem processes
Potentially site specific; potentially difficult
to rule out some explanations; hard to do
After
Carpenter (1998)
.
