Fighting extinction with excrement!

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“An understanding of the natural world and what’s in it is a source of not only a great curiosity but great fulfillment…” – Sir David Attenborough

Can Gopher tortoises help save an endangered plant species? A chance observation of a Gopher tortoise (Gopherus polyphemus) eating a freshly fallen Florida Ziziphus (Ziziphus celata) fruit at Bok Tower Gardens (BTG) inspired the investigation of such an inquiry.

Florida Ziziphus is a state and federally endangered shrub found nowhere else in the world other than the Lake Wales Ridge in Central Florida. The results of agriculture and urban sprawl, has led to the upland habitat that is home to this shrub to be severely degraded and reduced. The species is self-incompatible, meaning that mating only occurs between individuals that are genetically different from one another. This poses a severe concern considering that many of the remaining natural populations consist of a single clone. Observations have shown individual plants will flower profusely and pollinators are abundant; however, almost no fruit is produced, and seedlings have never been observed in the wild.

While this data is concerning, fear not! The Center for Plant Conservation National Collection planting beds located behind the scenes at Bok Tower Gardens, over 400 Florida Ziziphus plants are growing. Unlike their natural counterparts, our genetically robust “captive” population is able to interbreed and set fruit. In fact, the vast majority of Florida Ziziphus fruit is produced at Bok Tower Gardens! Our planting beds provide sanctuary for the species, and is an ideal place to study the seed biology.

Over the years, the Rare Plant Conservation Program has worked to optimize seed germination methods. We provide Archbold Biological Station with seedlings, used to create genetically diverse populations on protected public lands. However, based on greenhouse research at Bok Tower Gardens, Florida Ziziphus can exhibit a very low germination rate. When trying to produce a large number of seedlings for population introductions, low germination presents a serious obstacle. The fate of seeds produced by these introduced populations is also concerning. As scientists, we often ask ourselves will the seeds be able to germinate in the wild. We contemplate all the factors that may influence seed germination, and devise experiments to test such factors. Our research had led us to look for a natural seed disperser… cue the Gopher tortoise.

A factor that may explain the poor germination could be hidden in the structure of the fruit. Suspected to contain substances that inhibit germination, the edible fleshy fruit of the Florida Ziziphus has a woody layer surrounding the seed may act as a physical barrier preventing seedling emergence. Seed digestion by wildlife may increase germination by removing inhibitors, or by mechanical/chemical scarification of that woody layer. The Florida Ziziphus fruits ripen and fall to the ground at maturity, which is consistent with other species that rely on reptiles to disperse their seeds.

Known to rely on fruits as a food source, Gopher tortoises share the same habitat as Florida Ziziphus. Therefore, historically it is possible that Gopher tortoises will have a role in the dispersal and germination of Florida Ziziphus; however, this phenomenon needs further investigation.

Awarded a grant in 2017 by the Association of Zoological Horticulture, the Center for Plant Conservation at Bok Tower Gardens initiated exciting research into the interaction of Gopher tortoise digestion on seed viability. Over a period of six weeks, freshly collected Florida Ziziphus fruits were fed to three resident Gopher tortoise’s at Circle B Bar Reserve in Lakeland. The Gopher tortoise scat was examined in our laboratory, and seeds were excavated from the scat by staff and a dedicated volunteer. A germination trial on digested and un-digested seeds was conducted and the results indicated a statistically significant difference in germination rate, with digested seeds demonstrating an enhanced rate of germination compared to undigested seeds.

The exact mechanism behind this increase is not known, however the results from this project suggest that Gopher Tortoises may help support seed germination in the wild, and will certainly help guide future propagation efforts.

Article Written by Whitney Costner

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