Profitability of prey determines the response of population abundances to enrichment

Theoretical and empirical evidence in a one-predator-two-prey system consis tently indicates a regular trend that the less profitable (therefore, less vulnerable) prey increases in abundance with enrichment. The response in th e abundance of the more profitable (more vulnerable) prey to enrichment has , however, remained unclear. Previous theoretical models have assumed the l ess profitable prey as inedible, though its actual profitability is unknown . Here, relaxing this assumption, we show that the response of the more pro fitable prey abundance to enrichment depends critically on the profitabilit y of the less profitable prey. Specifically, the more profitable prey incre ases in abundance with enrichment if the profitability of the less profitab le prey is lower than a critical value so that it cannot support the predat or population by itself even at high densities (in this case, the prey is r eferred to as 'unpalatable') and decreases otherwise. This establishes a mo re general rule which unifies the previous works and resolves the indetermi nacy on the response of the more profitable prey.