Environmental Engineering Reference
In-Depth Information
only one experimental unit or environmental condition to be studied (i.e., the
Everglades, Great Salt Lake marshes), interest is only in the experimental units
being studied (see BACI study below), or the scale of the study is so large that
replication is impractical. As an extreme example, it is not possible to use
replication to test hypotheses related to global climate change because Earth
cannot be replicated.
1.8
Impact Studies
In wetland ecology and management, biologists are frequently interested in the
effects of impacts to wetlands. Impacts can be natural or human-made, planned or
unplanned, but cause a change in the system state of the wetland. Impacts can be
natural disasters such as hurricanes, extensive prolonged drought, or designed for
management to improve ecological conditions (e.g., removal of invasive species,
restoration of historical hydrology). An
impact assessment study
includes a design
common to wetland studies known as a BACI (before-after/control-impact) design
(Green
1979
). Principally, these types of study designs are the result of some sort of
natural or anthropogenic disturbance. The majority of wetland studies include some
sort of measurement or modeling of the effects of disturbance on the abiotic and
biotic components of the ecosystem. In wetland ecosystems, disturbance is com-
mon and, for many wetland types essential to ecological function, differing primar-
ily in degree of disturbance (i.e., short-term flood, multi-year drought, hurricane
effects that last decades). Furthermore, included in definitions of wetland, both
ecological and legal, are references to disturbance that must occur prior to the
system being declared a wetland. For example, coastal marshes are affected daily
by the predictable disturbance of tides that raise and lower water depths and adjust
salinity levels in the marshes. In many inland, geographically isolated wetlands
such as prairie potholes and playa wetlands, fluctuations between wet and dry states
are fundamental to the function of these systems.
When conducting an impact study, the type of disturbance will greatly influence
the development of a study design. There are three primary categories of distur-
bance - pulse, press, and those affecting temporal variation (Bender et al.
1984
;
Underwood
1994
). A
pulse disturbance
is not sustained beyond initial disturbance,
but effects persist beyond cessation of the disturbance (e.g., fire, hurricane). A
press
disturbance
persists beyond the initial event (e.g., flood, drought, invasive spe-
cies). A
temporal variance disturbance
results in increasing or decreasing
amplitudes (i.e., variance) around a constant mean on some sort of meaningful
temporal scale. Documenting a temporal variance disturbance is difficult and
requires long-term investigation or system monitoring. For example, some wetlands
require precipitation runoff events to flood; however, future climate change may
increase variation of precipitation between years or years represented by extreme
precipitation events can change over time while average annual precipitation
remains
relatively constant. Therefore,
species adapted to the historical
