Spatial patterns as long transients in submersed-floating plant competition with biocontrol

Abstract

Abstract A cellular automata model was developed and parameterized to test the effectiveness of application of biological control insects to water hyacinth (Pontederia crassipes), which is an invasive floating plant species in many parts of the world and outcompetes many submersed native aquatic species in southern Florida. In the model, P. crassipes is allowed to compete with Nuttall’s waterweed (Elodea nuttallii). In the absence of biocontrol acting on the P. crassipes, E. nuttallii excluded P. crassipes at low concentrations of the limiting nutrient (nitrogen), and the reverse occurred at high nutrient concentrations. At intermediate values, alternative stable states could occur; either P. crassipes alone or a mixture of the two species. When the biocontrol agent, the weevil Neochetina eichhorniae, was applied in the model, there was initially a rapid reduction of the P. crassipes, However, over time a regular striped pattern emerged of moving spatially alternating stripes of P. crassipes and E. nuttallii. This pattern of moving stripes persisted over thousands of days, but in some simulations the pattern was suddenly replaced by an irregular temporally varying pattern that lasted indefinitely. Thus, the striped patterns is an example of a long transient. The irregular spatio-temporal pattern that replaces it appears to be permanent, though that has not yet been established. Model parameters were varied to study effects of plant growth rate, nutrient concentration and nutrient diffusion rate on the dynamics of the system.