130 The UMAP Journal 30.2(2009) *+X1 99份的636合66-86e9969ee93 Figure 4. The numbers of species meeting the demands after adjustment. clear the reasons why the growth rate will decrease after the milkfish in- crease. Factors such as natural resources and environmental conditions restrict the growth of milkfish; and with their growth, the blocking effect will become greater and greater. The blocking effect is expressed in terms of the influence on the growth rate r of milkfish, making r decrease with the increase in the number a of milkfish. If we express r asr(=), afunction f it should be a decreasing function, so we have =r(am),(0)=霆 The simplest assumption of thatr(a)is a linear function: r()=r-s(r>0,s>0), where r is the intrinsic growth rate. To confirm the meaning of the coeffi- cient s, we introduce the maximum quantity m that is allowed by natural resources and environmental conditions, which we regard as the milkfish capacity. When a =am, then s will stop increasing, that is, the growthrate r will be 0. That occurs fors=r/am, so that we have (2) Another interpretation of (2)is that the growthrate(a)is in direct pro- portion to the unsaturated part of the milkfish capacity a=(am-s)/am,130 The UMAP Journal 30.2(2009) x10 0 5 10 15 Figure 4. The numbers of species meeting the demands after adjust•ment clear the reasons why the growth rate will decrease after the milkfish in￾crease. Factors such as natural resources and environmental conditions restrict the growth of millcfish; and with their growth, the blodking effect will become greater and greater. The blocIdng effect is expressed in terms of the influence on the growth rate T of milkfish, maldng T decrease with the increase in the number x of milkfish. If we express T as T(x), a function of x, it should be a decreasing function, so we have: & = r() (0) = xo. The simplest assumption of that Tr(X) is a linear function: r'(x) = r -- sx (,r > 0, s > 0), where T is the intrinsic growth rate. To cornfirm the meaning of the coeffi￾dient s, we introduce the maximum quantity x..• that is allowed by natural resources and environmental conditions, which we regard as the milkfish capacity. When x = x,,, then x will stop increasing, that is, the growth rate r(x) will be 0. That occurs for s r /x,,,, so that we have r(x) =r (1-- -- )• (2) Another interpretation of (2) is that the growth rate r(x) is in direct pro￾portion to the unsaturated part of the milkfish capacity x = (x. - x)/x7n