ATLSS:  Spatially Explicit Species Index (SESI) Models




Landscape level breeding potential indices (BPI) and foraging potential indices (FPI) have been created for particular Everglades species as part of the Across Trophic Level System Simulation (ATLSS) Program.  These are referred to under the general name of spatially-explicit species index (SESI) models, and have been developed as an attempt to provide a basis for quantifying  assessment of the effects of different water regulation plans on these species in the context of Everglades restoration.


SESI models are similar to Habitat Suitability Index (HSI) models both in that population response is predicted by a set of relationships between a given species and its environment and in that habitat indices are quantified by an index value.  Thus, SESI values range from 0.0 to 1.0, with 1.0 indicating the most favorable conditions.  SESI models differ from most HSI models in the following ways.  (1) They are flexible in that they can focus either on one particular part of the life cycle, such as breeding or foraging, or on the life cycles as a whole.  (2) They incorporate temporal changes in the environment, and can change from year to year, reflecting the unique conditions of any particular year.  Thus, for example, short-term variations in hydrology are incorporated in the index for a particular year (3) They are based on a 'landscape structure' which, once established, can be used to model the responses of any species in the system. (4) They provide a landscape index map rather than just a single index or set of indices.


For SESI models, the landscape is divided into equal‑sized spatial cells or pixels (500 x 500 m), each pixel having a suite of values that correspond to the parameters included in the model. Suitability of each pixel for the particular index is determined by combining a set of rules. Generally, these rules are of two types. (1) There are binary (0/1 or yes/no) rules which invoke known or estimated limits on the suitability of habitat or environmental conditions concerning a species.  (2) There are quantitative rules ‑ whereby having met the basic requirements of a species the habitat parameters are given a value that reflects their relative potential for breeding, foraging, or both.  These rules are then combined to create an overall index value between 0 and 1.


The primary output of a SESI model is a visual representation of the landscape with color‑coded values assigned to each pixel.  One way to use the model is to compare different management scenarios.  For example, the application of these models for assessing Everglades restoration plans involves comparing a baseline scenario that assumes no restoration with an 'alternative' management plan.  The scenarios differ only in hydrologic variables, primarily water depths across the landscape.  Hydrological data for the base scenario assumes no changes to the current water management practices.  The alternative scenarios incorporate proposed changes to management activities.  The predicted effects of one scenario could be compared to those of the second by simply subtracting the index value for each pixel calculated under base conditions from the value for the same pixel calculated under the alternative scenario.  In practice three maps are generated, representing the index values under the base scenario, the values under the alternative scenario, and the difference between the two.  For the difference map the values range from ‑1 to 1. 


This type of methodology represents that of relative assessment.  Relative assessment of the suitability of habitat under alternative management plans produces meaningful results, even when knowledge of ecological details are insufficient to assess habitat suitability of a pixel in an absolute sense (DeAngelis et al. 1998 Ecosystems 1:64-75, Curnutt et al., 2000 Ecological Applications 10:1849-1860).  The emphasis of this approach, therefore, is on comparing the spatial pattern of differences between two alternative management plans for suitability, in terms of whether a cell is good for foraging, breeding, or maintaining the overall life cycle of the population. 


For predictive simulations of the SESI model, 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. 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.


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