Changes between Version 3 and Version 4 of EwEugLinkingMediationAndTimeForcingFunctionsToTrophicInteractionRates
 Timestamp:
 20101124 00:05:06 (10 years ago)
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EwEugLinkingMediationAndTimeForcingFunctionsToTrophicInteractionRates
v3 v4 8 8 To understand how effects of habitat changes as represented through time forcing functions, and mediation effects as expressed through mediation functions of abundances of other organisms, are likely to affect trophic flow rates, we need to be a bit more careful about the ''a,,ij,,'' parameter. In particular, we need to recognize that for most trophic interactions, predators search for prey only over restricted spatial foraging arenas, and hence ''V,,ij,,'' is distributed only over such areas rather than at random over the whole system. Suppose the (practically unmeasurable) restricted area where foraging by j on prey i takes place is ''A,,ij,,'' per unit total model area. Suppose that while in this area, each unit of predator abundance (per ''P,,j,,'') searches an effective area ''a*,,ij,,''for prey. On average, each such area searched should result in capture of ''V,,ij,,/A,,ij,,'' prey, since this ratio is prey density in the arena area. In other words, the flow rate could be modelled more precisely (if we could measure ''A,,ij,,'') as 9 9 10 flow rate =''a *,,ij,,/A,,ij,, . V,,ij,, .P,,j,,''10 flow rate =''a ^.^ ,,ij,,/A,,ij,, ^.^ V,,ij,, ^.^ P,,j,,'' 11 11 12 12 i.e., the basic Ecosim equation's aij can be interpreted as ''a,,ij,,=a*,,ij,,/A,,ij,,''. Expressed this way, we see that time forcing and/or mediation effects can influence the flow rate in at least three quite distinct ways: … … 14 14 1. by altering the effective search rate ''a*,,ij,,'' of the predator, for example by using a turbidity time forcing function or a mediation function of algal biomass that reduces ''a*,,ij,,'' at high algal biomass; 15 15 1. by altering the area ''A,,ij,,''over which vulnerable prey and/or predators are distributed, for example by a mediation effect where macrophyte or seagrass biomass limits the foraging area usable by small predatory fish, so increases in those plant biomasses should be represented as causing increases in ''A,,ij,,'' for all prey'' i'' of the small fish as predator ''j''. Another example would be restriction of ''A'',,''ij'',, for feeding on small fishes by pelagic birds caused by large pelagic fishes, which drive small fishes nearer to the surface where they are more available to the birds. 16 1. by altering the [EwEugVulnerabilitiesInEcosim vulnerability exchange rates]''v,,ij,,'' that determine (along with ''a*,,ij,,/A,,ij,,'') ''V,,ij,,'' from total prey biomass ''B,,i,,'' (the basic equation for ''V'' from ''B'' is ''V,,ij,,''='' v,,ij,, ∙B,,i,,/(v,,ij,,+v',,ij,,+a*,,ij,,/A,,ij,,P,,j,,)''. For example, if small fish respond to increased large plant biomass by occupying a larger area, the mixing rate (''vij'') of planktonic food organisms into that larger area will increase as well.16 1. by altering the [EwEugVulnerabilitiesInEcosim vulnerability exchange rates]''v,,ij,,'' that determine (along with ''a*,,ij,,/A,,ij,,'') ''V,,ij,,'' from total prey biomass ''B,,i,,'' (the basic equation for ''V'' from ''B'' is ''V,,ij,,''='' v,,ij,, ^.^ B,,i,,/(v,,ij,,+v',,ij,,+a*,,ij,,/A,,ij,,P,,j,,)''. For example, if small fish respond to increased large plant biomass by occupying a larger area, the mixing rate (''vij'') of planktonic food organisms into that larger area will increase as well. 17 17 18 18 Until recently, Ecosim only allowed users to apply a single time forcing function to each trophic flow rate (each'' i,j''), and only as a multiplier (with base value 1.0) on the rate of effective search ''a,,ij,,'' (Case 1 above). Likewise, users could only apply a single mediation function to each flow rate, as a multiplier on vij (Case 3 above). So, for example, users could model how increases in large plant biomass affect exchange rates of food organisms into areas adjacent to where small fish hide, but not how increases in ''A,,ij,,'' imply less severe intraspecific competition (reduced effect of ''P,,j,,'' on ''V,,ij,,'') and also reduced predation rates on ''j'' when it is spread over a larger area relative to its predators. Further, they could not represent such multiple impacts as reduced foraging efficiency (lower ''a*,,ij,,'') associated with increased algal abundance, or reduction in area occupied by macrophytes due to shading by the algae.