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CAS Workshop on Ecosystem Succession Theory and Practice of Ecological Restoration Ecological effects of phosphorus enrichment in
Florida everglades wetland: An integrated study driven by regulation
and restoration[1] Miao Shili (South
Florida Water Management District, 3301 Gun Club Road, West Palm
Beach, FL, USA) Abstract:
Over the past 30 years, scientists worldwide have been facing
challenges of prioritizing research to solve real-world problems due
to severe habitat segmentation, widespread pollution, and worldwide
destruction caused by non-indigenous species. As a result, diverse
disciplines driven by these crises emerged, including global change
ecology, restoration ecology, conservation biology, biological
invasion, urban ecology, and landscape ecology. Ecologists,
particularly restoration ecologists, are gradually aware that
publication is not the only final product of their research.
Providing sound scientific basis for regulation, policy, adaptive
management, and ecosystem restoration is equally important. It is
imperative for ecologists to conduct integrated studies by linking
science and restoration, so that ecosystem integrity can be managed,
protected, and/or restored in effective ways. Ecological restoration
provides inspiring and precedent opportunities for ecologists to
conduct large-scale projects under natural and human-altered
conditions. This generates likelihood to test basic ecological
principles and theories, to capture surprises and/or failures, and
to suggest new theories. My presentation shall highlight an
integrated case study to illustrate how ecology contributes to
establish a numerical phosphorus (P) criterion of surface water in
Everglades wetland mandated by Florida law. I will specifically
focus on plant research of the study to elucidate how basic
ecological approach yields pragmatic and effective ways to manage
and restore the Everglades vegetation. The Everglades, a subtropical
freshwater wetland, originally covered most of south Florida and was
approximately 10,000 km2. Historically, it was a
P-limited, oligotropic ecosystem characterized by distinct wet and
dry seasons, occasionally prolonged drought or flood, frequently
lighting-induced fire, winter frosts and hurricanes. These natural
events created a spatial and temporal dynamic Everglades wetland
ecosystem consisting of sawgrass (Cladium
jamaicense Crantz) marsh mixed with tree islands, wet prairies,
and sloughs. Yet, the unique Everglades landscape has been disturbed
significantly by decades of human land use with intensive
agriculture, flood control, and urban development. As a result, over
50% of the original Everglades was lost and the remaining has
experienced adverse changes in soil and water chemistry, hydrology,
and vegetation dominance. Facing the
danger of losing the entire Everglades, America's treasurer, US
Federal Government launched a lawsuit in 1988 to against the State
of Florida and South Florida Water Management District (SFWMD) for
allegedly not enforcing water quality laws. In 1994 Florida
legislature passed the most comprehensive restoration plan ever, The
Everglades Forever Act. The law states that "in no case
shall nutrient concentrations of a body of water be altered so as to
cause an imbalance in natural populations of aquatic flora and
fauna" and requires to establish a
numeric water quality criterion for P. Since then, extensive
ecological research in support of the P criterion have been
conducted by various government agencies and universities. The research program launched by
SFWMD in 1994 was aimed to: 1) determine
whether P is a primary driving force responsible for the degradation
of Everglades; 2) quantify P threshold value of surface water
discharged into the Everglades that does not cause the imbalance; 3)
explore the best technologies to remove P from enriched runoff; and
4) provide basis for managing and restoring the Everglades
ecological integrity. The program was characterized by
three-pronged studies at different organization levels: field
transect monitoring along nutrient gradients (landscape level);
field P dosing experiments (community level); and greenhouse
experiments (individual level). One important feature of the study
was the reference site approach for the P criterion development.
Distinct P gradient in the field and long-term nature of
P-enrichment and impacts are rationales for using this common
approach to measure the attainment of ecological integrity.
Reference site approach allows the actual ecological system be
studied along an existing P gradient at a full scale and avoid the
difficulties of trying to replicate the natural system on a small
scale. However, this approach cannot separate the measured
biological responses to P levels from responses to other parameters
rather than P along the gradient. To overcome this disadvantage, a
hypothesis-tested experimental approach was applied either in field
mesocosms or in greenhouse where only P levels were altered.
The primary hypothesis was that P was the primary factor
resulting in biological degradation observed along the gradient.
Another feature of the integrated study was careful selection of
sensitive indicators (early-warning signals of ecosystem state
change) at several trophic levels. Theoretic framework of ecosystem
ecology suggests that all ecosystems respond to gradually changing
conditions such as nutrient loading. However, the responses to
nutrient loads vary greatly with trophic levels in the scale of
sensitivity and time. Thus,
it is critical to select biotic indicators that can predict ecological
processes and ecological effects at both landscape and individual
organism levels to monitor early changes before ecosystem
state shifts occur. Finally, the program
is characterized by incorporating natural spatial variability and
long-term ecological trend. Nine years after the initiation of the
study, not only numerous peer-reviewed ecological publications were
generated, but also a recommendation of 10 ppb of surface water P
was made, and the strategies to implement the most expensive
ecosystem restoration ever in the United States were recommended. The contrast state
change in ecosystem is usually due to a shift in dominance among
organisms with different life forms and/or life history
characteristics, which was usually triggered by P enrichment. The replacement of historical native
dominant sawgrass by cattail, a former minor species, in the
Everglades is such an example. Fighting with vegetation expansion
caused by previously ecological restrained species, it is essential
to compare life history characteristics and population dynamics of
invasive species in their new range (nutrient-enriched) versus those
in historic habitats (nutrient unenriched) to understand mechanisms
underlying the species invasion and vegetation replacement. Approaches
from plant community ecology, population biology, and ecophysiology
were combined to investigate the responses of structure, function,
and resource allocation of sawgrass and cattail communities along P
gradients. Then, hypotheses related to why and how sawgrass were
replaced by cattail were formed and tested in a series of field and
greenhouse experiments. The central hypothesis was that sawgrass and
cattail developed contrast life history characteristics (including
nutrient uptake, use, release, reproductive and vegetative
propagation, and resource allocation) and adapted different P
environments. P enrichment not only enhanced cattail seed production
and seedling establishment to colonize new habitat, but also
increased its vegetation rhizome growth to expand locally. These
studies suggest that P enrichment is a primary driving force
responsible for cattail expansion and other factors that modify
hydrology and create gaps in the landscape play important role as
well. Key words:
phosphorus enrichment ; the Everglades ; reference site ; vegetation
replacement ; cattail expansion
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