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.
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.
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.