Apparent competition from ferns in a forest. To determine whether the effect of ferns was due to competition or apparent competition, the researchers used fencing to exclude rodents and manipulated the presence of ferns. (a) Traps caught very few rodents inside fenced plots, but they caught many rodents in unfenced plots, especially when ferns were present. (b) Few seeds of beech trees were removed from fenced plots, but many seeds were removed from unfenced plots, especially when ferns were present. (c) Similarly, sugar maple seedlings experienced low rates of mortality in fenced plots, but high rates of mortality in unfenced plots in which ferns remained in place. Error bars are standard errors.

Data from A. Royo and W. C. Carson, Direct and indirect effects of a dense understory on tree seedling recruitment in temperate forests: Habitat-mediated predation versus competition, Canadian Journal of Forest Research 38 (2008): 1634–1645.