Everglades and Slough Crayfish
Habitat Suitability Indices (HSI)
Empirical basis and model assumptions:
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, the Everglades crayfish (Procambarus alleni) and the slough crayfish (Procambarus fallax).
The index for crayfish is computed at year's end from 3 factors:
(1) hydroperiod for the current year
(2) pattern of drydowns over the past 3 years
(3) a static habitat factor, which measures the % of 30-m cells in each 500-m cell that is of suitable FGAP habitat type.
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. 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.
Separate indices are computed for P. alleni and P. fallax, since their habitat and hydrologic affinities differ markedly.
The flow chart shows the steps in computing an index value for a cell:
Suitable habitat types and hydroperiods
The habitats that are generally unsuitable for the crayfish are listed at the top. Computation of the crayfish indices proceeds on all suitable habitat cells. Habitat information is provided by the Florida Gap Analysis (FGAP) vegetation map (USGS 2000).
The habitat factor, HSI, is set to zero if the 500-m cell contains greater than any of the following percentages (based on classification of the 30-m pixels within each 500-m cell): 1% urban, 15% agricultural, or 60% other unsuitable types. For all other cells, if percent Muhlenbergia grass > 60%, then HSI = 1 for P. alleni and 0.85 for P. fallax; If percent Muhlenbergia grass < 60%, then HSI = 0.85 for P. alleni and 1 for P. fallax.
Cycle through days to determine suitability conditions
Hydroperiod factor. The model tracks hydrologic condition through a whole year. For any year for which the hydroperiod is less than 60 days, the SESI is set to 0.
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. The table below lists all 9 possible combinations of the previous three years in terms of whether they had (1) or did not have (0) a drydown.
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
The total index value for the crayfish, is a product of the hydroperiod factor, habitat factor, and drydown factor.
For more information, see the original model description.