Ferns as facilitators of community recovery following biotic upheaval

Abstract

The competitive success of ferns has been foundational to hypotheses about terrestrial recolonization following biotic upheaval, from wildfires to the Cretaceous-Paleogene asteroid impact (66 million years ago). Rapid fern recolonization in primary successional environments has been hypothesized to be driven by ferns' high spore production and wind dispersal, with an emphasis on their competitive advantages as so-called disaster taxa. We propose that a competition-based view of ferns is outdated and in need of reexamination in light of growing research documenting the importance of positive interactions (i.e., facilitation) between ferns and other species. Here, we integrate fossil and modern perspectives on fern ecology to propose that ferns act as facilitators of community assemblage following biotic upheaval by stabilizing substrates, enhancing soil properties, and mediating competition. Our reframing of ferns as facilitators has broad implications for both community ecology and ecosystem recovery dynamics, because of ferns' global distribution and habitat diversity.

Keywords: ecosystem recovery; facilitation; interdisciplinary science; paleontology; plant ecology.

Figure 1.

Time-calibrated fern phylogeny adapted from Shen and colleagues (2018). Major extinction events are shown as boxes centered around the Permian–Triassic (251 million years ago; in purple), Triassic–Jurassic (201.3 million years ago; in pink), Cretaceous–Paleogene (66 million years ago; in pink), and Eocene–Oligocene (33.9 million years ago; in purple). The dominance of ferns varies following these extinction events, with true fern spikes occurring during the Triassic–Jurassic and Cretaceous–Paleogene recoveries (the pink boxes), whereas the Permian–Triassic and Eocene–Oligocene events (the purple boxes) experienced a radiation but not dominance of ferns, and lack true fern spore spikes. Numbers in circles indicate corresponding lineages and representative leaves and sporangium with the same numbers.

Figure 2.

Examples of fern preservation within the fossil record as either compression macrofossils (a) from the Paleocene (Cladophlebis sp.; DMNH EPI.51030) or microfossils (b) such as fern spores (Barrington et al. 2020). Within modern ecosystems ferns recolonize heavily disturbed landscapes such as the tephra of El Chichón in Southern Mexico (c). Photograph: RA Spicer, used with permission; Thomas and Cleal (2022).

Figure 3.

Map visualizing global fern species (class Polypodiopsida) richness using all known herbarium collection occurrences (after 1950; GBIF.Org 2), produced using geographic information system (GIS) mapping software ArcGIS Desktop Advanced 10.6 (ESRI 2018). Georeferenced data sets were spatially joined with a regular grid of 100,000 square kilometer cells (100 × 100 kilometers). The conjoined data set was extracted from the GIS and reconfigured to calculate and map the distinct species count per unique grid cell.

Figure 4.

Conceptual figure showing fern facilitation before and after a biotic upheaval. Each column (a–e) shows how a theoretical community may respond at the level of species abundance (top) and the realized niche (bottom). In the top panels, time is on the x-axis, and species abundance is shown on the y-axis. Before the biotic upheaval, ferns (fern icons and green dotted line) are dominant on the landscape relative to seed plants (the black icons) with natural variability (a). Following the K–Pg (represented by the asteroid icon; b), fern and seed plant abundance dropped significantly, with widespread extinctions. However, because the habitat was ameliorated by fern facilitation, species began to reestablish (c–e; black dotted line). The presence of other species eventually decreased fern abundance, restricting them on the landscape (e). The black dotted line differs from the solid black line in panels (a) and (b) because of assumed differences in pre- and post-impact communities; in addition, it represents the positive influence ferns have on outside species that fosters both seed plant establishment and expands the available niche space available for ferns themselves. These changes in the environment are also shown in realized two-dimensional idealized niche space in the bottom panels with niche variable 1 on the x-axis and niche variable 2 on the y-axis. Non-fern plant communities are shown in various black circles and ferns are shown in green dotted circles with overlapping circles representing theorized overlap in niche space. Prior to the K–Pg ferns were limited in their realized niche space (a), minimally occurring on the landscape during the disturbance (b) but recolonized post-impact (c–e). Ferns were able to expand their niches because of a lack of competition (c) but eventually were restricted again on the landscape (d–e), highlighting the dynamic nature of community recovery.