Harsh environmental conditions promote cooperative behavior in an epiphytic fern

Abstract

Harsh, unpredictable environments are known to favor cooperative groups in animals. Whether plants exhibit similar relationships is unknown. Staghorn ferns (Platycerium bifurcatum, Polypodiaceae) are epiphytes that form cooperative groups which build communal water and nutrient 'nests' at the tops of trees, a habitat characterized by water and nutrient stress. We conducted field observations to test whether staghorn ferns continue to live in large, reproductively active groups after they become dislodged from the canopy and fall to the forest floor, where they are less limited by water and nutrient deprivation. To rule out the potentially confounding effects of light limitation on the forest floor, we also conducted a multi-year glasshouse experiment where we transplanted individual plants into soil and onto vertically oriented boards under standardized light conditions. Results from field observations showed that dislodged colonies formed smaller groups that reproduced less than epiphytic colonies. Results from the glasshouse experiment showed that even when growing in full sun, terrestrial individuals tended to remain solitary, while epiphytic individuals tended to recruit new individuals into colonies. Results also showed that plants growing in potting soil and exposed to full sunlight sporulated more heavily than plants growing epiphytically. However, localities that are characterized by both elevated soil and light resources are generally not available to staghorn ferns in the wild, perhaps with the exception of large, epiphytic colonies with well-developed nests at the top of tree canopies. Overall results indicate that the harsh environmental conditions at the tops of trees trigger the formation of colonies in staghorn ferns, similarly to group living animals.

Keywords: Platycerium; Sociality; behavior; colonial living; cooperation.

Figure 1.

(a) Staghorn fern colony (Platycerium bifurcatum) growing epiphytically on Lord Howe Island. (b) Staghorn fern colony growing terrestrially on Lord Howe Island. (c) Experimental (glasshouse) colony growing epiphytically. (d) Experimental (glasshouse) colony growing terrestrially.

Figure 2.

(Top) differences in the density of individuals within colonies growing epiphytically (orange points) and after becoming dislodged on the forest floor (green points). (Bottom) differences in the rate of spore production between colonies growing epiphytically (orange points) and after becoming dislodged on the forest floor (green points).

Figure 3.

Rates of colony development (top graph) and spore production (bottom graph) in 10 staghorn fern individuals planted epiphytically (on vertically oriented boards, orange symbols) and 10 individuals planted terrestrially (in soil, green symbols). When exposed to standardized (full sun) conditions terrestrial colonies tend to remain solitary, while those growing epiphytically, without access to elevated water and nutrients, begin to form colonies. Plants exposed to high light, water, and nutrient conditions (conditions which are generally not available innature) begin to reproduce via spores.