It would seem to me that there are also strategies involving social cooperation or quick adaptation through genetic changes.
In social species the cooperation of older generations improves the fitness of younger generations and encourages the proliferation of genes that support the ability to contribute to intraspecies cooperation later in life. If a grandparent is able to protect a grandchild and help them have children of their own then these traits may be passed on at a higher rate.
Compare this to the exact opposite strategy where grandparents are actually competing with grandchildren for limited resources. In this case, removal of the grandparents from the population results in stronger selection of new traits and may actually help speed up the process of adaptation.
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