rebalance 123 o one erbivorous fish, one mollusc species, one crustacean species, and one echinoderm species(Population 2); and the sole predator species, milkfish(Population 3) The interrelationships among the species are presented in Figure 1 crustacean species specles algae Population I among On this basis, we can establish a Volterra predator-prey model with three populations [Shan and Tang 2007 ]. Let the number of the ith population be t(t). If we do not take into consideration the restrictions of natural resources, the algae species of Population 1 growing in isolation will follow an exponential growthlaw with relative growthrate T1, so that i(t)=r101 However, species of Population 2 feeding on the alga species will decrease the growth rate of the algae, so the revised model of the alga species is 1(t)=x1(r1-1x2) where the proportionality coefficient A1 reflects the feeding capability of species in Population 2 for the alga species Assume that the death rate of the species in Population I is r2 when existing in isolation; then i2(t)=-T2at2, so based on the foodweb we con clude that 2(+)=x2(-T2+A2x1), where the proportionality coefficient A2 reflects the support capability of the alga species for Population 2-which in turn provide food for the milkfish. The milkfish reduce the growth rate of the species in Population 2, so we must subtract their feeding effect to get 2()=2(-T2+A2x1-az3) Likewise, the model for the milkfish is 示a(+)=x3(-T3+g3x2)Rebalancing 123 "* one herbivorous fish, one mollusc species, one crustacean species, and one echinoderm species (Population 2); and "* the sole predator species, milkfish (Population 3). The interrelationships among the species are presented in Figure 1. Population IIi herbivorous mollusc crustacean eci Populat fsh species sp cies ion ii Population I Figure 1. Interrelationships among three populations. On this basis, we can establish a Volterra predator-prey model with three populations [Shan and Tang 2007]. Let the number of the ith population be xi(t). If we do not take into consideration the restrictions of natural resources, the algae species of Population 1 growing in isolation will follow an exponential growth law with relative growth rate rl, so that ib(t) = rixi. However, species of Population 2 feeding on the alga species will decrease the growth rate of the algae, so the revised model of the alga species is WO= xi(r, -. A1X 2 ), where the proportionality coefficient A1 reflects the feeding capability of species in Population 2 for the alga species. Assume that the death rate of the species in Population II is r 2 when existing in isolation; then i2 (t) = -r 2x 2, so based on the foodweb we con￾dude that :i() X2 (-r 2 + 1\2X1), where the proportionality coefficient A2 reflects the support capability of the alga species for Population 2-which in turn provide food for the milkfish. The milkfish reduce the growth rate of the species in Population 2, so we must subtract their feeding effect to get .2(t) = X2 (-r 2 + A2X1 - IX). Likewise, the model for the milkfish is b3(t) = X3 (-r3 + A3X2)