Resources that are used towards survival cannot be simultaneously available to an organism for reproduction.
The principle of allocation is a basic concept in ecology. The principle of allocation also underlies some aspects of the concept of tradeoffs and typically is considered in terms of energy. As a result, organisms tend to be unable to be simultaneously exceptionally durable, such as in terms of maintaining homeostasis, and at the same time exceptionally fecund, at least in terms of a limited resource budget.
Note that these ideas are to some degree independent of that of the investment of resources towards obtaining more resources. Such strategies can provide additional resources that, on a per resource-unit basis, can be allocated towards increasing survival as well as an organism's reproductive potentials.
In monetary terms, a dollar spent on new shoes cannot simultaneously be spent on dinner. Alternatively, a dollar invested could reap additional dollars that in fact may be used towards purchasing these items.
An organism must survive in order to reproduce. Failure to invest adequately in whatever it takes to survive therefore can result in a failure to be reproductively successful. On the other hand, an organism that invests excessively in survival can end up having few resources available to devote to reproduction. The most fit organisms typically are those that best balance these conflicting demands.
A good example of this conflict, as defined by the principle of allocation, can be seen with iteroparous organisms – organisms that display more than one reproductive episode per lifetime. That is, with such organisms excessive investment in reproduction in one year can reduce an organism's potential to survive and/or reproduce in a subsequent year.