When relative allocation depends on total resource acquisition: implication for the analysis of trade-offs

A central tenet of evolutionary biology states that life-history traits are linked via trade-offs, as classically exemplified by the van Noordwijk and de Jong model. This model, however, assumes that the relative resource allocation to a biological function varies independently of the total resource acquisition. Based on current empirical evidence, we first explored the dependency between the total resource acquisition and the relative resource allocation to reproduction and showed that such dependency is the rule rather than the exception. We then derived the expression of the covariance between traits when the assumption of independence is relaxed and used simulations to quantify the importance of such dependency on the detection of trade-offs between current reproduction and future survival. We found that the dependency between the total energy acquisition and the relative allocation to reproduction can influence the probability to detect trade-offs between survival and reproduction. As a general rule, a negative dependency between the total energy acquisition and the relative allocation to reproduction should lead to a higher probability of detecting a trade-off in species with a fast pace of life, whereas a positive dependency should lead to a higher probability of detecting a trade-off in species with a slow pace of life. In addition to confirming the importance of resource variation to reveal trade-offs, our finding demonstrates that the covariance between resource allocation and resource acquisition is generally not null and also plays a fundamental role in the detection of trade-offs.