Abstract
The staghorn fern ( Platycerium bifurcatum , Cav. Polypodiaceae) is an epiphyte that lives in cooperative colonies whose members work together to build a communal water and nutrient store. Previous research has shown that staghorn colonies are sometimes invaded by Zealandia pustulata (G.Forst. Polypodiaceae), a related species that causes sharp declines in the density and reproductive output of P. bifurcatum colonies. Here, we investigate how invasion of staghorn nests affects the epiphytic distribution, growth and reproductive output of Z. pustulata along an elevational gradient on Lord Howe Island. Results showed that P. bifurcatum decreased in abundance with elevation, while solitary epiphytic Z. pustulata plants increased in abundance with elevation. The distribution of Z. pustulata plants inhabiting P. bifurcatum nests peaked at intermediate elevations, suggesting that the invasion of P. bifurcatum nests alters the distribution of Z. pustulata . Invasion of P. bifurcatum nests also affected frond production in Z. pustulata . The proportion of reproductive fronds increased with height above ground and decreased with elevation, regardless of growth habit, but there was a relative increase in the reproductive proportion for Z. pustulata plants inhabiting P. bifurcatum nests. Hence, rates of reproduction were higher in invasive individuals, suggesting increased fitness. Overall results illustrate two species of epiphytes interact in a way that is similar to parasitism, altering the distribution of the nest invader.