Analysis and simulations of fragmentation patterns in the everglades

Abstract

Sawgrass (Cladium jamaicense) communities of Water Conservation Area 2A (WCA-2A a 43 281 ha Northern Everglades impoundment, are being invaded by cattail (Typha spp.). Results from analyses suggest that the yearly invasion rate of cattails has increased from 1% in 1973 to 4% by 1987. The total area of the landscape impacted by cattail increased from <5% (2054 ha) in 1973 to more than one-third of the landscape (16 017 ha) by 1991. The landscape also became more fragmented. The number of sawgrass patches increased from 173 in 1973 to 5709 by 1991. During the same period the lacunarity index of saw grass changed from 2.8 to 3.9. The effect of agricultural phosphorus (P) runoff and water depth (D) on invasion of cattail were expressed as, Prob(pe) = 1/(1 + αe(-βP)) + ζD/P. The threshold for accelerated cattail invasion was estimated at ≃650 mg/kg soil total phosphorus. Cattail dispersal was mostly spatially dependent. For a given year, the probabilities of sawgrass changing to cattail based on the number of 1-8 adjacent cattail cells (20 x 20 m) were calculated to be Prob(ad) = [0.049 0.052 0.061 0.065 0.069 0.072 0.076 0.094]. The probabilities Prob(pe) and Prob(ad) were used as Markov chain probabilities in a spatial model to simulate vegetation dynamics. The simulated landscape matched the actual landscape with an overall accuracy of 72.8% and predicted that cattail would invade 50% of WCA-2A in another 6-10 yr if the driving forces remain unchanged.