Events

Introducing Events into the Virtual Ecosystem

You can specify a number of events to occur during the lifetime of the virtual ecosystem. These are used in numerical experiments designed to clarify the cause of an emergent property of the ecosystem.

Events are also used to describe natural or man-made events, and actions taken to deal with unwanted consequences. The Virtual Ecosystem reveals how the emergent properties would respond if those events occured and that action were taken. The consequences are emergent properties of the virtual ecosystem.

Specifying an event

You specify each event using the Events tool in VEW Designer.

1. Start date and duration
2. Depth range
3. Ecosystem properties
4. Nature of the change.

The properties

External
You can specify an exogenous variable, such as atmospheric partical pressure of carbon dioxide, or wind speed.

Physical
You can specify a physical variable, such as temperature or salinity.

Chemical
You can specify a change in an existing chemical (such as nitrate). Oy you can introduce a new chemical into the model, and specify when and at what depth it will appear in the water and with what concentration.

Biological
You can specify the immigration or emigration of an existing species. Or you can change the trophic closure criteria (to trigger a trophic cascade response). Or you can add new species to the model, and specify when they will arrive (e.g. in a release of ballast water, or eggs being laid by an exogenous fi sh).

Nature of the change

Whatever the variable, you can specify how it will be changed by the event. For example, the existing value might be increased or decreased by a specifi ed amount. Or you may wish to multiply the existing value by sum amount (e.g. double the phosphate concentration). The multiplicative changes are useful for defi ning actions to be taken to remedy problems (see "What-if Prediction").

Climate change

Woods & Barkmann (1993) investigated the response of the ecosystem to the increase in atmospheric carbon dioxide over the next fi fty years according to the IPCC Business-as-usual scenario. They found that the enhanced greenhouse effect caused a progressive decrease in the annual maximum depth of the mixed layer. That reduced the re-entrainment of nitrogen from the seasonal thermocline, so that primary production decreased each year. Zooplankton grazing effi ciency rose to compensate for this loss. So the fl ux of dead phytoplankton to the deep ocean declined even faster than primary production. The demand for carbon dioxide from the atmosphere declined with primary production. It was predicted that the atmospheric concentration of carbon dioxide would therefore rise faster than if the ecosystem were inactive. This postive feedback is called the “plankton multiplier”.

Pollution

...(2001) showed that diurnally migrating plankton contribute substantially to the vertical transport of chemicals in the seasonal thermocline. The dissolved chemical is first taken up by phytoplankton, which are eaten by zooplankton. Their gut passage time is typically an hour or so, during which time they migrate tens of metres, before defacating. Bacteria then extract the chemical from these faecal pellets as they are tumbled by turbulence in the mixed layer, or sink slowly through laminar flow in the thermocline. The chemical retained in the body of the zooplankton rises in concentration above that in the water. Some of the herbivorous zooplankton are eaten by carnivores, so taking the concentrated chemical to the next trophic level. They in turn release faecal pellets with the high concentration of the chemical.
The net transport effected by this chain of events can be predicted for various injection scenarios, and for different assumptions about the phytoplankton uptake rate.

Fisheries

Adult fish cannot yet be included in a virtual ecosystem because we have no primitive equations for the way they learn new behaviour and select different behaviour patterns from their repertoire. So fish are treated as external. Their interaction with the ecosystem is an exogenous process described as an event in the scenario. One example is a fish laying eggs. When the eggs hatch, the fish larvae can be treated as plankton until they grow to a critical size (perhaps one cm in length). The mortality of the fish larvae can be simulated as an emergent property of the virtual ecosystem. In that way we use the power of virtual ecology to test theories of fi sheries recruitment.

Disease

...(1999) investigated the response of the ecosystem to infection by bacteria and viruses that were taken up by the plankton, which then became diseased, increasing mortality and/or decreasing reproduction. Model equations were added for the processes by which infection passed between individual plankton. The nature of the epidemic in the whole population was an emergent property of the virtual ecosystem.