ATLSS Crayfish Index Model:
Basic Model Description
School of Aquatic Fisheries Science
University of Washington
Jane Comiskey, Lou Gross
The Institute for Environmental Modeling
University of Tennessee
Knoxville, TN 37996-1610
(Copyright University of Tennessee -- 1998)
The ATLSS Crayfish Index Model incorporates information about
crayfish habitat preferences and hydrologically-driven aspects
of crayfish ecology to assess the relative impacts of hydrologic
scenarios proposed for Everglades Restoration on the occurrence
potential for two species of crayfish.
Statement of Limitations
In addition to factors included in the Crayfish Index Model -- water
regime and vegetative cover -- crayfish population dynamics are also
potentially affected by other factors such as predation, water
temperature, salinity, nutrient levels, turbidity, presence of
contaminants, and presence of exotic plants and animals. The extent
to which factors not included in the model affect survival and
distribution of crayfish limits the validity of relative predictions
from our simulations. The time scales at which evaluation of
alternative scenarios are evaluated are likely too short to encompass
some long-term changes in habitat quality. Stabilized hydrologic
regimes may result in a slow degradation of habitat that may be
overlooked at the time scales evaluated with this model. In addition,
verification of this model's performance has been limited by the
scarcity of crayfish population measurements over the model area for
the calibration period and the lumping together in pre-2000 studies
of the two crayfish species now known to inhabit the Everglades. Some
parameter values are "best guess" approximations, for which data are
currently lacking. A spatially explicit full demographic model for
crayfish is currently under development as part of the ATLSS project.
Five-hundred species of crayfish occur in North America, of which
fifty species in six genera are found in Florida (Franz and Franz 1990).
However, only four species of crayfish occur in the southern half of
Florida, and only two live in the Everglades. Until recently, crayfish
studies in the Everglades reported a single species, the endemic
Procambarus alleni( the Everglades crayfish), ascribing different
behavior patterns in sloughs vs. wet prairies to the plasticity of the
species (Hobbs 1942, Kushlan and Kushlan 1979, Franz and Franz 1990,
Jordan 1996). A recent investigation found that a second species,
Procambarus fallax, is also present (Hendrix and Loftus 2000). Abundance
of P. fallax was found to be highest in long-hydroperiod sites, while
P. alleni dominated in short hydroperiod marshes.
P. alleni individuals survive the dry season in a semi-resting state in
underground burrows. Mating typically occurs in the autumn and females
carry eggs while still underground. At the start of the rainy season,
young crayfish repopulate the flooded marshes, feeding on algae and
small invertebrates. The reproductive timing of P. alleni makes it one
of the first abundant prey early in the Everglades' wet season.
Crayfish are short-lived and thrive when high levels of reproduction
are possible. Potential for P. alleni reproduction is increased by
slow water turnover times, seasonally fluctuating water tables, high
levels of algal production, complex vegetative stands, and rich
substrates. Investigators have reported that fluctuating rather than
consistently high water levels are necessary to sustain high population
levels of crayfish in the Everglades (Kushlan and Kushlan 1979), but
interpretation of past population measurements is complicated by the
fact that P. alleni and P. fallax individuals, which have very
different environmental requirements, were mistakenly considered as one
Crayfish have been identified as key food web components in the
wetlands of South Florida and possible indicator species for wetland
integrity. Crayfish are omnivorous, consuming snails, insect larvae,
worms, tadpoles, dead organisms, and a significant amount of
vegetation. They are in turn an important constituent in the diets of
wading birds, young alligators, fishes, raccoons, snakes, and pig
frogs, linking primary production with higher trophic levels. In
addition to their recognized importance as food for wading birds,
contributing half the diet of white ibis (Kushlan and Kushlan 1975),
crayfish also constitute about half of the diet of raccoons in South
Florida, which are a major food source for the endangered Florida
panther in some parts of its range. The underground burrows of P.
alleni may be used as refuges by mosquitofish during dry periods.
While the biota of South Florida, including crayfish, have adapted to
the natural cycle of hydration and drydown, the timing and extent of
these periods have been altered by managed water flows supported by an
extensive system of canals, levees, and pumps. Health and abundance
of crayfish and other invertebrates are affected by management
decisions related to hydroperiod, aquatic weed control, and nutrient
Differences in habitat and hydrologic affinities for the two species
modeled are reflected in patterns of crayfish densities. Conditions
which favor one species typically are sub-optimal for the other.
Crayfish density and biomass estimates are generally higher for wet
prairies, where P. alleni predominates, than for slough habitats, where
P. fallax are more commonly found. P. alleni tends to occupy more
complex habitats that provide more food resources and refuge from
predators (e.g. higher plant biomass, higher stem density). Plant
biomass is positively correlated with P. alleni densities in wet
prairies, but not with densities of P. fallax in sloughs, while water
depth is generally negatively correlated with P. fallax densities in
sloughs, but not with densities of P. alleni in wet prairies.
Densities of P. fallax, associated with slough habitats, decreases
with increasing depth and prolonged hydroperiod, due in part to
increased predation from fish (Hendrix 2000).
The ATLSS Crayfish Index Model incorporates several landscape map
layers. Habitat information is provided by the Florida Gap Analysis
(FGAP) vegetation map (USGS 2000).
For predictive simulations, projected daily water level for each cell
is provided by the South Florida Water Management Hydrology Model for
a 31-year period, based on historical weather patterns (1965-1995) but
reflecting proposed modifications to water delivery schedules and
infrastructure (Fennema et al., 1994). The ATLSS high resolution
hydrology model is used to translate the SFWM Model water depths at
a 2-mi scale of resolution to finer resolutions needed by our models.
Separate indices are computed for P. alleni and P. fallax, since their
habitat and hydrologic affinities differ markedly. Our Crayfish
Indices (CFI) takes on values between 0 and 1 for each landscape grid
cell: 0 is unsuitable, while values between 0 and 1 represent
increasing degrees of suitability from marginal to optimal. Several
aspects of the hydrologic regime at different temporal scales are used
to define suitable conditions for crayfish production:
1. Hydroperiod factor: Hydroperiod for the current year. We consider
relative habitat quality of a landscape grid cell to be unsuitable
during a year when the hydroperiod is less than 2 months.
2. Drydown factor: Pattern of repeated drying events. Cells inundated
fewer than 335 days (eleven month hydroperiod) in a given year are
considered to have experienced a significant drying event for that
year (0 in drying history columns of table below). The pattern of
drying events over a three year period is used to assess the
relative suitability of each landscape cell for the two Procambarus
Drying history P. alleni P. fallax
yr-2 yr-1 yr index index
0 0 0 1.0 0.2
1 0 0 0.8 0.4
0 1 0 0.4 0.6
0 0 1 0.6 0.4
1 1 0 0.8 0.6
1 0 1 0.6 0.8
0 1 1 0.4 0.6
1 1 1 0.2 1.0
Habitat factor: In addition, 500-m x 500-m cells are considered to be
unsuitable habitat for crayfish if any of the following are true:
(1) more than 60% of their constituent finer resolution 30-m x 30-m
pixels consist of avoided habitats;
(2) greater than 15% are agricultural types; or
(3) greater than 1% are urban types.
Results for each species are presented in the standard ATLSS 3-panel
color-coded map format for comparing alternative and base hydrologic
scenarios. Tables will display indices by year and by subregions
within the model area, along with difference values computed within
DeAngelis, D.L., W.F. Loftus, J.C. Trexler, and R.E. Ulanowicz. 1997.
Modeling fish dynamics and effects of stress in a hydrologically
pulsed ecosystem. Journal of Aquatic Ecosystem Stress and Recovery
Fennema, R.J., C.J. Neidrauer, R.A. Johnson, T.K. MacVicar, and W.A.
Perkins. 1994. A computer model to simulate natural Everglades
hydrology. p. 249-289. IN S.M. Davis and J.C. Ogden( eds.) Everglades:
the Ecosystem and Its Restoration. St. Lucie Press, Delray Beach, FL,
Franz, R. and S.E. Franz. 1990. A review of the Florida crayfish
fauna, with comments on nomenclature, distribution, and conservation.
Florida Scientist 53:286-296.
Hobbs, H.H.,III. Trophic Relationships of North American Freshwater
Crayfishes and Shrimps. 1994. Milwaukee Public Museum, Milwaukee,
Hendrix, A.N. and W.F. Loftus. 2000. Distribution and relative abundance
of the crayfishes Procambarus alleni (Faxon) and P. fallax (Hagen)
in southern Florida. Wetlands 20(1).
Hobbs, H.H.,Jr. 1942. The crayfishes of Florida. University of Florida
Biological Science Series 3(2):v+1-179.
Jordan, F., K.J. Babbitt, C.C. McIvor, S.J. Miller. 1996. Spatial
ecology of the crayfish, Procambarus alleni , in a Florida wetland
mosaic. Wetlands 16: 134 - 142.
Kushlan, J.A. and M.S. Kushlan. 1979. Observations on crayfish in the
Everglades Crustaceana, Supplement 5: 115-120.
Kushlan, J.A. and M.S. Kushlan. 1975. Food of the white ibis in
southern Florida. Florida Field Natural., 3:31-38.
Loftus, W.F., J.D. Chapman, and R. Conrow. 1990. Hydroperiod effects
on Everglade marsh food webs, with relation to marsh restoration
efforts. p. 1-22. IN G. Larson and M. Soukup (eds) Fisheries and
coastal wetlands research: proceedings of the 1986 conference on
science in national parks, Fort Collins, CO.
Momot, W. T., H. Gowing & P. D. Jones 1978. The dynamics of crayfish
and their role in ecosystems. American Midland Naturalist 99:10-35.
U.S. Geological Survey, Biological Resources Division [USGS-BRD].
2000. Classification of 1993/1994 Landsat TM Imagery. Florida
Cooperative Fish and Wildlife Research unit, University of Florida,
Gainesville, Florida http://www.wec.ufl.edu/coop/gap.
*CFI values for P. alleni are highest for very dry cells. Adding
the < 2-month hydroperiod cut-off helped, but still the highest values
(5's) are found mostly in Big Cypress. Since this index may be interpreted
as standing crop, would it make sense to scale the results by the
number of months a cell is inundated, if in fact the highest year-round
biomasses are found in wet prairie?
*Do we need to consider water depth, in addition to hydroperiod, as
a factor in computing index values, esp. for P. fallax, since papers
report that densities of crayfish in sloughs decrease with increasing
water depth (dilution, predation)?
*Looking at the AltD13R4 vs. F2050, the Alt is generally better than
F2050 for P. fallax, but worse for P. alleni. The East Panhandle subarea
(western sparrow area) and northern WCA3A/southern WCA2B are exceptions
for both species. In addition to the separate indices for P. alleni and
P. fallax, should we compute a weighted composite index?
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