projects > across trophic level system simulation (atlss) > cape sable seaside sparrows > abstract
Individual Based Spatially Explicit Model of the Cape Sable Seaside Sparrow Population in the Florida Everglades
M. Philip Nott
The Cape Sable seaside sparrow (Ammodramus maritima mirabilis) is an ecologically isolated subspecies of the seaside sparrow. Recent surveys estimate its population at fewer than 6,000 individuals, and its range to be restricted to the extreme southern part of the Florida Peninsula, almost entirely within the boundaries of the Everglades National Park and Big Cypress National Preserve. The sparrow breeds in marl prairies typified by dense stands of graminoid species usually below 1 meter in height and naturally inundated by freshwater part of the year. As water levels recede during the dry season in late winter and spring, the sparrows establish territories and start nesting in the grass. Pairs may produce up to three broods if their nesting sites remain dry. If water levels do not recede early enough in spring, nesting may be delayed and if reflooding occurs during the nesting season, eggs or nestlings may be lost.
Declines have occurred in the sparrow population across its entire range, probably due to higher water levels in recent years. Because the current range of the sparrow is limited to a few hundred square kilometers and because it is subject to flooding and fires, the population is highly vulnerable. Changes to the hydrology of the southern Everglades, planned as part of an Everglades restoration project, could increase the water levels in parts of the sparrows range and inadvertently increase the risk to the reproductive success of the sparrow in certain areas. Figure 1 shows some of these risks in the "western" habitat area of the Cape Sable seaside sparrow. It is critical to predict how serious these risks are.
A model for the Cape Sable sparrow subpopulation in the nesting area northwest of Shark Slough has been developed and has the following features:
The above conceptual model has been implemented as a Monte Carlo simulation model called SIMSPAR (M.P. Nott, Ph.D. dissertation, 1998). The model has been applied to a main subpopulation of the Cape Sable seaside sparrows on the western side of Shark Slough in the Everglades. The size of this subpopulation has reached as high as 3,000 sparrows, and the model is capable of keeping track of the locations and breeding status of all these birds during the breeding season. The model increases the age of an individual each day and updates its status according to movement and behavior rules. The core of the model is a simple flow of decisions and actions that affect individuals in relation to abiotic factors and other individuals. At each step the model updates the breeding status and tracks associations between individuals. Figure 2 shows some of the management questions to which this model is being applied.
Subsequently, to address the question of uncertainty in model parameters affecting results, a sensitivity analysis was performed on SIMSPAR. The sensitivity analysis was done in the following way.
The sensitivity analyses have been extended to determine the relative contributions of each parameter to the overall response using generalized linear modeling. It is also possible that the model will be used to explore the evolutionary consequences of inherited dispersal behaviors.
Another objective of this work will be to design and edit web-based documentation of the model and the assumptions behind the mechanics. Such a design would mimic the experience of an individual bird as it proceeds through the model and the outcome would depend upon the course of action and status at any stage. This objective has also been extended to provide web-based documentation understandable at various levels of technical ability.
The most sensitive parameters and the biological or biophysical realities they correspond to are as follows:
These parameters all have a significant effect on population trajectories. However, it appears that dispersal behavior is a very crucial factor in population persistence, especially when linked to habitat degradation. Finding new, recently available, or restored habitat is crucial if small populations are to recover. An important question is: What is the impact of imposing an observed dispersal-distance distribution on a deterministic or simpler population model when the actual dispersal-distance distribution resulting may change as the result of an evolutionary stable strategy? We need much more information concerning age and sex-specific dispersal behaviors in this species and how these might change dependent upon the hydrologic conditions experienced over a number of years. Only by utilizing an individual based approach can we explore the limits of these relations and guide field studies.
(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report (PDF, 8.7 MB))
|U.S. Department of the Interior, U.S. Geological Survey, Center for Coastal Geology
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Last updated: 04 September, 2013 @ 02:08 PM (KP)