In reproduction organisms have evolved different strategies. Some (like the frogs) put their main energy into many children of whom only a few survive. Others (like the big mammals) produce only little offspring and put their energy in their feeding and protection. In accordance to a mathematical formula, the former strategy is called r selection, the latter one K selection.
Organisms with r selection are more adaptable to changing conditions whereas those with K selection are more suitable to a stable environment and are more vulnerable.
French: modèle évolutif K / r