Ecosystem engineers as selective agents: the effects of leaf litter on emergence time and early growth in Impatiens capensis.

By physically modifying the abiotic environment, ecosystem engineers can have dramatic effects on the distribution and abundance of species in a community. However, ecosystem engineering can also change the selective environment and evolutionary dynamics of affected species, although this remains relatively understudied. Here, we examine the potential for an ecosystem engineer - oak trees - to affect the evolutionary dynamics of the herbaceous, understory annual, Impatiens capensis, through leaf litter deposition. Using a quantitative genetic experimental approach, we found that: (i) the presence of leaf litter significantly affected a suite of germination, growth and phenological traits in I. capensis; (ii) I. capensis does not exhibit performance trade-offs across litter and bare soil environments in the form of negative across-environment genetic correlations; (iii) the presence or absence of leaf litter significantly alters the pattern of natural selection germination timing and hypocotyl length; and (iv) the frequency of leaf litter environments can dramatically change which combinations of hypocotyl length lead to highest mean fitness across both bare soil and leaf litter environments. More generally, our results demonstrate the potential for ecosystem engineers to alter both the ecological and the evolutionary dynamics of the species they affect.