Close Encounters

Express delivery – seed dispersal by gopher tortoise

Not known for their speed, how effective are tortoises as seed dispersal agents?

Seed dispersal is a critically important ecosystem service for almost all natural ecosystems. Most vertebrates have been identified as seed dispersers, though the majority of well-known plant dispersers are birds and mammals, not reptiles. Animals can transport seed either externally (epizoochory) or ingest and later excrete (endozoochory). Ingestion of seeds by animals can have varying effects on seed germination, with seed damage sometimes occurring during chewing and digestion. For this reason, endozoochory by Chelonians (turtles and tortoises) may provide distinct advantages over other animals. Turtles and tortoises lack teeth and so have a tendency to swallow their food whole, reducing the chance of seed damage. There has been a recent boom in interest in the use of tortoises as seed disperser substitutes (especially on oceanic islands) to restore ecosystem functions that were lost due to the extinction of large herbivores, including native tortoise species.

Gopher tortoise T159 (“Grumpy Gertrude”) on 27 September 2016, feeding on cocoplum fruit from one of the original four founder plants in the Abacoa Greenway. Image credit: Hanish et al.

In their new study published in AoBP, Hanish et al. examined a case where four cocoplum bushes (Chrysobalanus icaco) were introduced to a suburban gopher tortoise (Gopherus polyphemus) refuge in Jupiter, Florida, USA for aesthetic purposes and subsequently spread throughout the area.  They found evidence that the gopher tortoise likely contributed to cocoplum expansion, by accelerating seed germination and by defecating seeds in areas, such as trails, that provided suitable locations for cocoplum establishment. The spread of C. icaco by G. polyphemus over a relatively short period of time provides a valuable window into the earliest stages of the colonization process and further supports the role of Chelonians as effective seed dispersal agents for large-seeded plants. The authors hope that additional studies that use similar methods of spatial analysis at other sites where C. icaco and G. polyphemus are syntopic would be helpful in determining if their results could be applied to other areas.

Researcher highlight

Carolyn Hanish (left) and Corey Anderson (right)

Corey Anderson was born In Los Angeles and raised in San Diego, California. He completed his BA in Integrative Biology at UC Berkeley, his PhD in Evolution, Ecology, and Population Biology at Washington University in St. Louis, and was a postdoctoral research associate in The Biodesign Instiute at Arizona State University. He is currently an Associate Professor of Biology at Valdosta State University, GA, USA.
Corey is a spatial data scientist that is interested in the geographic, ecological, and microevolutionary processes that drive the spatial dispersion of genes, individuals, and species in landscapes. Although his primary domain is the biological sciences (and his work is often geared towards conservation), he is generally interested in statistical geography and the analysis of spatially dependent data sets, as well as the development of software tools for conducting spatial analysis.

Carolyn Hanish completed her undergraduate studies at the University of Nebraska-Lincoln, her master’s in environmental science with Jon Moore at Florida Atlantic University and has now returned to Nebraska to complete a doctorate in veterinary medicine. The data presented in the AoBP paper above was part of Carolyn’s master’s thesis where she worked with John Moore, who has has been conducting a long-term behavioural/ecological study of the gopher tortoise at the Abacoa Greenway in South Florida for over 20 years. Carolyn and Corey met at the 2016 Joint Meeting of Ichthyologists and Herpetologists in New Orleans, and that’s what led to their collaboration.