Do spatially homogenizing and heterogenizing processes affect transitions between alternative stable states? : abstract

It is well established now that ecosystems can have alternative stable states, where under similar conditions, two possible states exist. Systems can switch between these two states as a result of
external drivers. Such dynamics are well understood for spatially homogeneous systems and increasingly so for spatially heterogeneous systems. When systems are spatially heterogeneous, sudden shifts between states can occur in local homogeneous patches but are averaged out over larger
spatial area causing a more gradual response. Several spatial processes affect the spatial pattern of ecosystems. In savannas, a system also thought to have alternative stable states of trees and grasses,
fires can maintain spatial patterns of woody vegetation, while dispersion by plants homogenize these
spatial patterns. It is unclear how these two processes interact and affect possible transitions between
alternative states in savannas. We modelled fire and plant dispersion in savannas and show how the interaction between the size of fire events and the rate of dispersion of plants creates spatial patterns in savannas under certain conditions. When dispersion is high, and the size of fire patches is small the spatial pattern becomes more homogeneous . We then show how systems with high and low heterogeneity, induced by these processes, respond differently to increases in grazing, an external driver. We find that when the spatial heterogeneity is low discontinuous responses occur, and that when spatial heterogeneity is high more gradual responses occur to this external driver.