The dynamics of nitrogen movement in an Arctic salt marsh in response to goose herbivory: a parameterized model with alternate stable states

Abstract

Summary LPBN is a parameterized simulation model of flows of nitrogen (N) in an ecosystem of cyanobacteria, grass and grazers, based on the N dynamics of a grazed Puccinellia lawn in an intertidal marsh on Hudson Bay. This system shows two alternate stable states: (a) lawns that either support a foraging population of lesser snow geese, or are not grazed by geese; and (b) exposed saline sediments that support little or no vegetation. The model represents the flow of N from cyanobacterial fixation, the major N input into the system, to the geese that migrate in autumn; those that do not return represent the major N output from the system. We have modelled N fixation, the transformations of N in the soil, plant growth, lawn regeneration, and goose grazing and grubbing. The model simulates steady‐state flows of N similar to those observed in the field at zero and at moderate goose density, and it also simulates the transition to the state of zero plant biomass, a consequence of increased grubbing at high goose density. The simulated steady‐state flows are found to be more sensitive to changes in the parameters that describe N fixation and goose biology, than to similar changes in the parameters describing plant biology. Because the model shows the alternate stable states and the transition between them, with values for the state variables that are consistent with field data, we conclude that N dynamics are crucial in determining the stability of the real salt marsh‐goose system. The determining factor is the loss of the input of N from fixation when lawn area is reduced because the rate of goose grubbing exceeds that of plant re‐establishment.