__Everglades and Slough Crayfish__

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__Habitat Suitability Indices (HSI)__

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**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. allen*i 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.

__Flow Chart for Construction of Everglades and Slough Crayfish
Suitability Indices (HSI)
__

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).

__Habitat factor__

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

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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.