Apple Snail
Habitat Suitability Index (HSI)
Empirical basis and model assumptions:
Several types of habitat have been identified as suitable
habitat for apple snails. These are,
with their FGAP numbers: Freshwater
marsh (29,30), Typha (34), Spartina (35), Muhlenbergia (33, 39), Eleocharis
(31), Open_water (0).
*
Habitat types other
than those listed are excluded (the HSI index is set to zero).
The Apple Snail HSI
measures potential production of apple snail recruits through a year. The HSI makes the assumption that there are
mature apple snails present in a particular year and it projects the fraction
of potential recruits from those adults.
It does this by calculating (1) interruptions in oviposition due to
drydowns, (2) losses of recent hatchlings due to drydowns and (3) losses of
eggs due to submersion by high water.
*
Thus the HSI projects
the fraction of potential recruitment that is realized in a given year.
The pattern of the
fractions of apple snail eggs produced through the year typically looks like
that shown below, with the peak in April, such that the fractions add to
1.0. Uninterrupted oviposition and
complete survival of all eggs would lead to an HSI of 1.0. Interruption of oviposition by drydowns, as
well as losses of eggs and hatchlings, leads to a lower HSI. The index considers only the effects within
the given year on production of apple snail recruits during that year, all of
which will be negative.
Now, we adjust this
index to lower values to reflect the occurrence of drydowns.
The most important
effect of a drydown (meant here as water depths less than 10 cm) is to
interrupt reproduction. Egg production
will stop when water depths fall to that level
*
If there is a drydown
of some period of time, for example 1/2- month, the basic effect is assumed to
be the elimination of egg production during that 1/2-month, which will effect
the histogram shown in the figure above by halving the histogram height for
that month. We assume that the
reduction in reproduction is not compensated for by a greater rate of
production following the drydown.
A drydown may also
cause mortality of recently hatched snails, at a rate that depends on the
length of the drydown,
*
Mortality is applied
to recently hatched snails at a rate that depends on the length of a drydown.
If a drydown exceeds a month, it is assumed to kill all snails that were less
than a month old during that period.
An increase in water
level after eggs are laid but before they hatch may cause mortality of eggs in
the pre-hatching stage.
*
If the water level
goes up by a certain amount, by more than 20 cm, the eggs produced during the
preceding 20 days are destroyed.
Selected references:
Darby, P.C., PL. Valentine Darby, R.F. Bennetts, J.D. Croop,
H.F. Percival, and W.M. Kitchens.
1997. Ecological studies of
apple snails (Pomacea paludosa, Say).
Final Report prepared for South Florida Water Management District and
St. Johns River Water Management District.
Contract # E-6609, Florida Cooperative Fish and Wildlife Research Unit, Gainesville, Florida.
Hanning, G.W. 1978. Aspects of reproduction in Pomacea
paludosa (Mesogastropoda: Pilidae).
M.S. Thesis. Florida State
Univ., Tallahassee 119 pp.
Little, C. 1968.
Aestivation and ionic regulation of two species of Pomacea (Gastropoda, Prociobranchia). Journal of Experimental Biology. 48: 569-585.
Flow Chart for Construction of Apple Snail
Habitat Suitability Index
The flow
chart shows the steps in computing an index value for a cell:
Variables
of index computation (top box):
The ideal potential eggs production factor (ProdFactor) is shown
for each month. Several FGAP types of
habitat are listed have been identified as suitable habitat for apple snail
reproduction.
Cycle through days of
year to determine breeding conditions:
The model cycles through the breeding season, from March 1
through October 31, to compute the fraction of potential recruits that are
realized in a given year. The fractions
of eggs potentially produced in each of the eight reproductive months are shown
in the table at the bottom of the flow chart.
Egg production does not occur on a particular day if the water level is
below 10 cm.
If water levels fall to 0.0 or below 0.0 for a 30-day
period, then all hatchlings produced from eggs in the preceding month (before
the start of the 30-day drydown) are assumed to die. That is, if, on day i, water depth in the cell has been 0 or <
0 for 30 continuous days, then all of the egg production for days i-60 to i-31
is assumed to be lost. This simply
means that recent hatchlings (less than a month old) die in a dry period of a
month or more.
If the water depth stays at 0.0 or below 0.0 for an
additional 30 days, then all hatchlings produced for two months preceding the
start of the 60-day drydown are assumed to die. That is, if, on day i, water depth in the cell has been 0 or <
0 for 30 continuous days, then all of the egg production for days i-90 to i-61
is assumed to be lost. This simply
means that somewhat older hatchlings (less than 2 months old) die in a dry
period of two months or more.
If, on any day the water level is at least 20 cm greater
than all days during the preceding month, then it is assumed that all eggs
produced during the preceding 30-day period die.