Dean E. Pearson 1, Yvette K. Ortega 1, and Lukasz Dylewski 2
1 USDA Forest Service, Rocky Mountain Research Station, Missoula, Montana, USA
2 Poznań University of Life Sciences. Poznań, Poland
Scientists have long recognized that rodents and other small mammals are such voracious seed predators that they could affect plant abundance in natural systems. However, many plant species produce so many more seeds than are necessary to replace and expand their populations that seed predation may have negligible effects on plant populations and communities.
Most studies of seed predation have simply set out seeds and measured seed removal – a method which can quantify seed predation rates but fails to demonstrate their effects on plant populations. Only in the past few decades have enough seed sowing experiments been conducted around the world to assess the global effects of small mammal seed predation on plant communities.
Paired seed cages allowing (left) or excluding (right) rodent access to native plant seeds demonstrate rodent effects on blanket flowers. USDA Forest Service photos by Dean Pearson.
Thirty years ago, an elegant field experiment in the desert southwestern U.S. demonstrated that selective seed predation by rodents could restructure the local plant community. Excluding rodents that selectively consumed large seeds resulted in a dramatic shift in the plant community from a classic open desert shrubland to a system dominated by large-seeded grasses previously suppressed by rodent seed predation (including some invasive plants).
A recent meta-analysis collating global results from seed predation studies found that small mammals may structure plant communities in similar ways around the world, with an interesting twist. In systems where seeds are relatively small, like deserts and grasslands, small mammal seed predation suppresses the largest-seeded plant species, but in systems where seed sizes are much larger, like tropical forests, these predators tend to suppress relatively smaller-seeded species.
These results indicate not only that small mammal seed predation is important in determining the relative abundance of plant species in natural communities around the world, but also that it can have opposing effects on plant species composition in different ecosystems. These findings have important ramifications for understanding natural plant communities, biological invasions, and for restoring plant communities.
Implications for ecological restoration
These new findings have important implications for ecological restoration, particularly in the context of a recent global meta-analysis evaluating the efficacy of invasive plant control for restoration (Pearson et al 2016). This meta-analysis found that successful control of targeted invasive plants most often resulted in “secondary invasion” by other invasive/noxious plants, rather than recovery of native plant communities - a result corroborated by similar studies around the globe (Reid et al. 2009, Kettenring and Adams 2011). These findings emphasize the need for active restoration management such as reseeding following invasive plant control in order to restore native plant communities, but how do restoration practitioners get their valuable native seeds past the powerful rodent seed predation barrier?
Recent studies show that applying modern seed coating technologies to attach plant-based secondary compounds like capsaicin and neem oil to seeds can strongly deter rodent seed predators (Pearson et al. 2018, Taylor et al. 2020). Some rodent deterrents such as activated carbon may even protect seeds from herbicides and facilitate water and nutrient uptake (see Taylor et al. 2020 and Madsen et al. 2014), thereby helping to overcome abiotic constraints to establishment as well. Importantly, field experiments demonstrate that the effect of deterring rodent seed predation using these coatings not only increases plant recruitment, but it increases recruitment rates sufficiently to offset the additional costs of seed coating applications (Pearson et al. 2018). Hope is on the horizon in the development of new seed technologies for advancing ecological restoration!
Featured Publications
Dylewski, Lukasz ; Ortega, Yvette K. ; Bogdziewics, Michal ; Pearson, Dean E. , 2020. Seed size predicts global effects of small mammal seed predation on plant recruitment
Pearson, Dean E. ; Valliant, Morgan ; Carlson, Chris ; Thelen, Giles C. ; Ortega, Yvette K. ; Orrock, John L. ; Madsen, Matthew D. , 2018. Spicing up restoration: Can chili peppers improve restoration seeding by reducing seed predation?
Larios, Loralee ; Pearson, Dean E. ; Maron, John L. , 2017. Incorporating the effects of generalist seed predators into plant community theory
Pearson, Dean E. ; Icasatti, Nadia S. ; Hierro, Jose L. ; Bird, Benjamin J. , 2014. Are local filters blind to provenance? Ant seed predation suppresses exotic plants more than natives
Pearson, Dean E. ; Hierro, J. L. ; Chiuffo, M. ; Villarreal, D. , 2014. Rodent seed predation as a biotic filter influencing exotic plant abundance and distribution
Pearson, Dean E. ; Callaway, Ragan M. ; Maron, John L. , 2011. Biotic resistance via granivory: Establishment by invasive, naturalized, and native asters reflects generalist preference
Additional references
Kettenring, K. M., & Adams, C. R. (2011). Lessons learned from invasive plant control experiments: a systematic review and meta‐analysis. Journal of applied ecology, 48(4), 970-979.
Madsen MD, Davies KW, Mummey DL, Svejcar TJ (2014) Improving restoration of exotic annual grass-invaded rangelands through activated carbon seed enhancement technologies. Rangeland Ecology & Management 67:61–67.
Pearson, D. E., Ortega, Y. K., Runyon, J. B., & Butler, J. L. (2016). Secondary invasion: the bane of weed management. Biological Conservation, 197, 8-17.
Pearson, D. E., Valliant, M., Carlson, C., Thelen, G. C., Ortega, Y. K., Orrock, J. L., & Madsen, M. D. (2019). Spicing up restoration: can chili peppers improve restoration seeding by reducing seed predation? Restoration Ecology, 27(2), 254-260.
Reid, A. M., Morin, L., Downey, P. O., French, K., & Virtue, J. G. (2009). Does invasive plant management aid the restoration of natural ecosystems?. Biological Conservation, 142(10), 2342-2349.
Taylor, J. B., Cass, K. L., Armond, D. N., Madsen, M. D., Pearson, D. E., & St. Clair, S. B. (2020). Deterring rodent seed‐predation using seed‐coating technologies. Restoration Ecology.