Abstract
Some large-seeded plants lack effective seed dispersal agents when they are introduced as ornamental plants to new areas, but can rapidly colonize a landscape if seed dispersal functions are restored. We examined whether Gopherus polyphemus (Gopher Tortoise) facilitated the spread of Chrysobalanus icaco (Cocoplum; Chrysobalanaceae) over a 14-year period in a suburban nature preserve (in Jupiter, FL, USA) by: (i) comparing germination patterns among gut-passed, hand-depulped and whole fruit treatments, and (ii) testing hypotheses about environmental predictors of the spatial distribution of C. icaco, including information about G. polyphemus movement pathways and burrow locations. While we did not find a significant difference in the total proportion of C. icaco seeds that germinated in each treatment, time to event analysis revealed that seeds that were found in faeces germinated significantly earlier than seeds that were hand-depulped or that were planted as whole fruits, supporting a lone scarification effect. Point process modeling revealed that the density of C. icaco bushes was higher near G. polyphemus movement pathways and was lower inside Serenoa repens (Saw Palmetto) patches, supporting a positive effect of tortoise movement patterns on plant distributions. The density of C. icaco increased from west to east, consistent with westward dispersal from the four founder bushes on the east side of the study area. After removal of outliers, we also detected a negative association between C. icaco spatial density and G. polyphemus burrow density that was presumably explained by the fact that seeds defecated deep within burrows were unlikely to germinate and establish without secondary movement. The results suggest that G. polyphemus contributed to the rapid dispersal of C. icaco by scatter dispersal of seeds (via faeces) in areas where tortoises were active and that movement pathways provided suitable conditions for colonization. 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.
Highlights
How seed dispersal by animals affects the spatial distribution of plants is a major topic in the ecological literature due to its fundamental importance to the structure of almost all natural ecosystems (Nathan and Muller-Landau 2000; Wang and Smith 2002; Schupp et al 2010)
We examined whether Gopherus polyphemus (Gopher Tortoise) facilitated the spread of Chrysobalanus icaco (Cocoplum; Chrysobalanaceae) over a 14-year period in a suburban nature preserve by: (i) comparing germination patterns among gut-passed, hand-depulped and whole fruit treatments, and (ii) testing hypotheses about environmental predictors of the spatial distribution of C. icaco, including information about G. polyphemus movement pathways and burrow locations
While we did not find a significant difference in the total proportion of C. icaco seeds that germinated in each treatment, time to event analysis revealed that seeds that were found in faeces germinated significantly earlier than seeds that were hand-depulped or that were planted as whole fruits, supporting a lone scarification effect
Summary
How seed dispersal by animals affects the spatial distribution of plants is a major topic in the ecological literature due to its fundamental importance to the structure of almost all natural ecosystems (Nathan and Muller-Landau 2000; Wang and Smith 2002; Schupp et al 2010). Seeds can be transported by animals externally (= epizoochory) or ingested and later excreted (= endozoochory; Traveset et al 2007). Ingestion of seeds by animals can have varying effects on seed germination (Traveset et al 2007). Some animals may damage seeds during mastication and gut passage resulting in seed death or reduced viability (Herrera 2002; Traveset and Verdú 2002). The effect of ingestion on seed germination is affected by seed characteristics. The size, thickness or permeability of the seed coat can contribute to the success or failure of gut passage (Traveset and Verdú 2002; Milotić and Hoffmann 2016). Different combinations of seed characteristics and animal gastrointestinal properties can make the outcome of endozoochory dependent on the individual plant and/or animal species (Izhaki and Safriel 1990; Traveset et al 2001b, 2007; Fedriani and Delibes 2009)
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