Rain forest understorey ferns facilitate tree seedling survival under animal non‐trophic stress

Questions Although forest ecosystems harbour many animal species and animal non‐trophic effects are as ubiquitous as trophic effects, few studies have examined animal non‐trophic effects on plant–plant interactions. Can animal non‐trophic behaviour influence plant–plant interactions and thus, change the net effect of interactions, especially those between understorey vegetation and tree seedlings? How do the species traits of understorey vegetation and animals contribute to their interactions with seedlings? Location The M ain R ange N ational P ark, southeast Q ueensland, A ustralia. Methods Seedling survival related to the cover of understorey vegetation (mainly the fern L astreopsis decomposita ) was monitored in a 0.5‐m wide and 200‐m long transect for 2 yrs. Ten per cent of the transect was caged to estimate the effects of non‐trophic disturbances from two pheasant‐size, ground‐dwelling birds ( M enura alberti and A lectura lathami ) for 1 yr. Two hundred plastic tags (5 × 5 cm) were placed in the transect to quantify bird disturbance and litter input. Results The negative effects of the fern understorey on seedlings were increased light deficits, greater risk of herbivory and wilting or rotting; the positive effects were reduced uprooting and litter smothering due to abiotic and bird disturbances. Notably, the exclusion of bird activity changed the net effect of the fern understorey from positive to neutral, and seedling survival was positively correlated with fern cover. Conclusions The net effect of plant‐plant interactions is subject to change when additional species are involved. In addition to trophic effects, non‐trophic effects are such that they can change interactions between plants. A conceptual model of species traits contributing to interactions is presented.