ATLSS Crayfish Index Model: 
			  Basic Model Description

                               Nobel Hendrix
		    School of Aquatic Fisheries Science
			 University of Washington
			        Seattle, WA
			   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).
  Model Development
  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 
     species modeled.
         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
  each subregion.
  Literature Cited:
  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
  *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?

Return to ATLSS SESI Model Documentation Page
Return to ATLSS Home Page
Return to ATLSS Initial CERP Update Home Page