Emma R. Neigel, University of Lethbridge, discusses her article, Dispersal limitation and seed predation drives rarity of a plant species at its range edge
Ecological theory proposes that three ‘filters’ determine which species are present in a community: (1) the suitability of abiotic conditions (e.g., soil moisture and temperature), (2) dispersal limitation – the ability of the species to get to a site, and (3) biotic interactions, either positive (i.e., mutualists) or negative (i.e., herbivores). It is often assumed that the abiotic conditions are the most important factor. However, the relative influence of each filter driving species rarity is infrequently quantified.
The wood-poppy (Stylophorum diphyllum; Papaveraceae) is an herbaceous woodland perennial with showy, four-petaled, yellow flowers. It produces hairy capsules that produce many seeds that are ant-dispersed. It is common throughout its central range in Kentucky and is even sold in the native plant nursery industry.
However, in Canada it is listed as endangered under the Species at Risk Act (SARA) and is known from only five populations in southern Ontario. Threats include habitat loss, recreation pressure (e.g., an increase in trail creation and access), and small populations with low recruitment. In addition, mice eat the seeds. Some botanists argue the wood-poppy has always been naturally rare in Canada. However, like many rare plant species, the cause of its rarity is unknown. We conducted a seed addition trial to disentangle the relative influence of the three ecological filters and to inform strategies for the species’ recovery.
We used a species distribution model (SDM) built by McCune (2019) based on broad-scale, abiotic factors (including surficial geology, annual precipitation, and temperature) to predict habitat suitability. We tested the role of dispersal limitation by carefully planting 4,050 seeds with tweezers into unoccupied, one-hectare (100 m × 100 m) sites that varied in their SDM-predicted habitat suitability. To test the role of biotic interactions, we excluded seed predators (i.e., mice) by caging half of the sub-plots. We also measured the microclimate conditions of each plot (including relative temperature, soil moisture, and canopy cover) and monitored seedling emergence and survival over two years (2021-2023).
Contrary to popular belief that species occurrences are less frequent at their range edge because of a scarcity of suitable habitat, we found habitat suitability did not predict seedling emergence or short-term survival. However, the SDM’s ability to predict long-term survival remains to be tested. Seedlings had significantly higher emergence rates with the protection of a cage (18.4% emergence in cages versus 5.1% in uncaged). Overall, dispersal limitation coupled with seed predation were the strongest predictors of seedling emergence, while microsite temperature was a significant predictor of short-term survival. Understanding causes for species rarity is an important step towards informing species recovery. Our findings suggest that the wood-poppy needs a helping hand to disperse to new sites and into the optimal microsite within those sites, as well as management actions that reduce seed predation. We propose this may be the case for many other rare plant species at their range edge.
We would like to thank our NSERC Alliance grant partners: The Nature Conservancy Canada (NCC), The Wilder Institute/ Calgary Zoo, Dr. Ryan Norris at the University of Guelph, and Kayanase – also a big shout-out to Nina Hunt! We dedicate our study to the late Dr. Jane Bowles, who led early recovery efforts for the wood-poppy.
For more on wood poppies and other research, see this video and this podcast.
