I think that the article (that these sentences were taken from) was talking about an population in equilibrium with its environment. I think that the first sentence is incorrect. It should be "when food was added to these plots (food effect) and predators were excluded (no predator effect):" This type of information is presented better as a table, with food on one axis and predation on the other. It would be easier to understand that way. (and the author might have picked up their mistake) If you added food (but no predators), the population grew to a new equilibrium. If you added food and predators were not excluded, the population stayed the same. no added food and excluded predators, the population stayed the same.

My interpretation is as follows: "When food was added to these plots(no food effect-->food effect) and predators were excluded(no predator effect) from an experimental area, hare numbers increased tenfold and stayed there—the cycle was lost. However, the cycle was NOT retained if either of the factors was allowed to operate alone: if predators were excluded but food was not added (food effect alone--> no effect), or if food was added in the presence of predators (predator effect alone -->both effect). If you added food (but no predators), the population grew RAPIDLY(for example, tenfold). If you added food and predators were not excluded, the population stayed the same. No added food and excluded predators, the population stayed the same. If you DO NOT added food and predators WERE excluded, the population declined.

My interpretation is as follows: "When food was added to these plots(no food effect-->food effect) and predators were excluded(no predator effect) from an experimental area, hare numbers increased tenfold and stayed there—the cycle was lost. However, the cycle was NOT retained if either of the factors was allowed to operate alone: if predators were excluded but food was not added (food effect alone--> no effect), or if food was added in the presence of predators (predator effect alone -->both effect). " Please refer to the table.(Mr Michael suggested to make a table.) predator O X food O equilibrium grow X decline equilibrium If you added food (but no predators), the population grew RAPIDLY(for example, tenfold). If you added food and predators were not excluded, the population stayed the same. No added food and excluded predators, the population stayed the same. If you DO NOT added food and predators WERE excluded, the population declined.