Modeling the dynamics of the invasive tree, Melaleuca quinquenervia, in the Everglades, with and without biological control
Melaleuca quinquenervia is an invasive tree that has spread over the freshwater ecosystems of southern Florida, displacing native vegetation such as slash pine (Pinus elliottii), pond cypress (Taxodium ascendens), thus threatening native biodiversity. Suppression of Melaleuca appears to be progressing through the introduction of insect species. It is important to project the long-term effects of the biocontrol on suppression of Melaleuca. Our objective is to help provide a better understanding of the dynamics of the Melaleuca in native southern Florida plant communities, and the effect that the biocontrol is having on those dynamics. In addition, we are using modeling to project likely future changes in the plant communities over the next few decades. The individual based model, JABOWA, was used as a basis for developing a model to simulate successional processes occurring in areas of Everglades occupied by native species and Melaleuca, both in the absence and presence of the biological control agents. JABOWA simulated plant succession in a 0.1 hectare plot, given the characteristic of a number of plants and a set of environmental conditions. Five native species, slash pine, pond cypress, dahoon holly, sweet bay and Loblolly bay, were included in the model.
The JABOWA model was started with different initial conditions and parameters based on field data, mimicking stands dominated by young, intermediate, old and mixed-aged Melaleuca stands, to determine how the other species populations would behave after the application of biocontrol to the Melaleuca. In addition, we simulated different hydrological conditions to determine the effect of biocontrol on tree communities across a spectrum of environmental conditions. We simulated the function of biocontrol through a reduction in reproductive and growth rates of Melaleuca. Each of the simulation scenarios was run for 300 years and replicated 50 times. The model reproduced the regular oscillations observed in the real system. Our simulation results overall showed that the density and total basal area of Melaleuca decreased after the bio-control, in agreement with empirical data, and that native species tended to increase in density and total basal area. The results also showed that the biocontrol had a better effect if it was applied in the earlier stage of Melaleuca stand or if it was applied in the drier place. The species that we selected had different tolerances to water level depths; therefore, the future forest community combinations that emerged after biocontrol differed depending on the hydrologic environment.