High reproductive efficiency as an adaptive strategy in competitive environments

1. Reproductive efficiency (the efficiency of conversion of resources from vegetative tissue to reproductive output) is a central to our understanding of reproductive allocation and the evolution of reproductive strategies in plants. Plant strategy theory predicts that reproductive efficiency should decrease under competition. Short-lived semelparous species are not predicted to evolve under competition and therefore should not express adaptive responses to the presence of competitors. Long-lived iteroparous species are predicted to delay reproduction in favour of growth and resource acquisition in the presence of competitors. I use life-history theory to advance a prediction that reproductive efficiency increases under competition in both short-lived semelparous and potentially longer-lived iteroparous species. 2. Contrary to the predictions of plant strategy theory, short-lived semelparous species are frequently observed to live in highly competitive environments. Further, iteroparous species under intense competition may die long before they reach competitive dominance or an optimal size for reproduction. 3. I surveyed the literature for studies on plant species including measurements of vegetative and reproductive allocation in high and low (or no) competition treatments. 4. Across species, relative reproductive efficiency (reproductive efficiency under high competition/reproductive efficiency under low competition) significantly increased with increasing competition intensity. 5. Patterns of allocation to reproduction under competition support the existence of a competitive annual strategy and a reproductive perennial strategy. Under these strategies, short-lived semelparous species and long-lived iteroparous species express high reproductive efficiency under competition as an adaptation to high neighbour density. In addition, some species also expressed patterns of allocation to reproduction consistent with plant strategy theories. 6. Under this interpretation, I predict that competitive strategies, where plants delay reproduction in competitive environments to gain competitive superiority, are favoured not under intense competition but under modest competition. Including a life-history interpretation in reproductive efficiency under competition provides a much needed predictive framework for strategies of reproduction observed across species.