Changes between Version 7 and Version 8 of EwEugVulnerabilitiesInEcosim

2014-02-06 06:42:23 (5 years ago)

Comment added.


  • EwEugVulnerabilitiesInEcosim

    v7 v8  
    2929'''Figure 3.3''' Simulation of flow between available (''V,,i,,'') and unavailable (''B,,i,, - V,,i,,'') prey biomass in Ecosim. ''a,,ij,,'' is the predator search rate for prey ''i'', ''v'' is the exchange rate between the vulnerable and not-vulnerable state. Fast equilibrium between the two prey states implies ''V,,i,,'' = ''vB,,i,,'' / (''2v'' + ''aB,,j,,''). Based on Walters et al. (1997). 
     31==== Comment by jeroens on 2/6/2014 6:42:23 AM ==== 
     32'''Note from Carl Walters about vulnerabilities and the EwE user interface''' 
     34The numbers that appear in the ecosim interface are vulnerability exchange "multipliers", not the rates.  If the ecopath base predation rate for a link is Mij=Qij/Bi (base flow rate of biomass to the predator divided by prey biomass), the vulnerability exchange rate (vij) for that link is set to the interface multiplier kij times Mij, i.e. the input multiplier kij is the ratio vij/Mij so vij=kijMij, i.e. the maximum predation mortality rate at very high predator density divided by the ecopath base rate.  Very high multipliers imply that the foraging arena biomass flow rate equation Qij=aijvijBiBj/(2vij+aijBj) approaches the mass-action (Lotka-Volterra rate Qij=aijBiBj.  Note of course that for most models, the aij and vij are further modified over time to account for changes in predator and prey foraging times, possibly handling times (type II responses), and other user-defined forcing factors. 
     36You need to be really careful with the multiplier settings, because for exploited species these multipliers reflect both ecological limits caused by prey and predator behavior, and also how much the ecopath base predator biomass is below the natural level that would be achieved if fishing were stopped.  To allow an overexploited species to increase greatly following reduction in fishing, you need to set the kij values quite high so that the predators can consume maximum amounts of prey far larger than they do at the ecopath base biomass. 
     38We use the kij interface option rather than allowing users to directly enter vijs for two reasons: (1) in order for the initial ecopath biomasses to be near equilibrium, vij must be set to at least Mij (otherwise the model predicts initial consumption flows Qij below the ecopath base values), and (2) it is much easier for most users to think predation rate limits by thinking about how much larger Mij might ever be than its initial ecopath base value.  So if we didn't use the multiplier parameterization, users would have to go through messy independent analyses in order to provide reasonable direct settings for the vij. 
     40It is always difficult to specify reasonable kij values for ecological linkages involving non-exploited species; typically we must resort to thinking about the fine-scale spatial structure of the trophic interaction, in particular whether only a small proportion of the prey are exposed to predation at any moment e.g. as occurs with fish feeding on benthic invertebrates that are mostly hidden from sight at any moment.  For exploited species, we can often obtain better kij estimates by noting that kij represents not only the upper limit on how much food these species can get, but also how much higher their percapita consumption rates can get when they are less abundant, i.e. how strong their compensatory response is to reduction by fishing; very low kij values imply very strong compensatory responses, i.e. big increases in Qij/Bj=aijvijBi/(2vij+aijBj) as Bj decreases.  Note that ecosim allows users to specify in low kij cases whether the predator just realizes higher per-capita consumption rates Qij/Bj as Bj decreases, or instead translates the potential increase in food intake rate into reduced foraging time and hence reduced vulnerability to its predators (actually a more common response, especially in juvenile fish, than increased growth rate).