BSc (Aust Env Studies), BSc (App Maths and Stats), PhD
Director, Atmospheric Environment Research Centre
Senior Lecturer, Griffith University
Contact details for Dr Roger Cropp
Research interests
My research interests focus on the dynamics of plankton ecosystems in the oceans. Plankton produce about half of the organic matter on Earth and about half the oxygen in the atmosphere, and underpin all marine fisheries. They are also important contributors to climate, as they mediate the transfer between the ocean and the atmosphere of some important gases such as carbon dioxide and dimethyl sulphide. They are further implicated in climate change as they may be fertilized by iron in aeolian dust blown off the continents in times of lower rainfall and deposited into the ocean. Plankton ecosystems are also central to the uptake and incorporation into marine food webs of persistent organic pollutants that are delivered to remote and otherwise pristine environments via the atmosphere.These interests all require that the dynamical properties of marine plankton systems are well understood. Plankton functional type models are being developed to simulate the contribution of plankton to fisheries and important biogeochemical processes in the oceans. I am interested in the mechanisms of competition that allow (or more commonly preclude) competing functional types from coexisting in computer simulations of these processes. Currently, my research on ecosystem dynamics is focussed on the coexistence properties of complex plankton ecosystem models that may be determined a priori.
This figure shows the dynamics of two phytoplankton functional types (P1 and P2 ) that compete for a single limiting and conserved resource and that are predated upon by a single zooplankton (Z). The red and green trajectories show different starting points, that end up at the same solution. In this case the two phytoplankton functional types coexist in a limit cycle, where the populations of P2 and Z exhibit large oscillations (bloom and bust cycles) and P2 has an almost constant population. The vector fields on the faces show that this property is consistent throughout the model’s state space. However, such coexistence is rare, though ubiquitous, in the parameter space of this model. An order of magnitude more common is competitive exclusion, as shown in the following figure.
In this case, either P1 or P2 will go extinct depending on where in the state space the model is started. Again, the red and green trajectories show different starting points, but in this case either P2 goes extinct (red trajectory) or P1 goes extinct (green trajectory). A much more common model is one for which only one competitor survives, and the survivor is determined when the model is constructed. This type of model is approximately 100 times as common as the equivalent model in which both competitors coexist. Plankton functional type models that maintain competing functional types extant in computer simulations are therefore very rare, and it is difficult to parameterise models to have this property. As each functional type is included to perform a specific biogeochemical role, coexistence is an essential property for these models. Our recent work is focused on developing heuristics for model construction that ensure coexistence properties.
Publications