Frugivory and seed dispersal by chelonians: a review and synthesis

Abstract

In recent years, it has become clear that frugivory and seed dispersal (FSD) by turtles and tortoises is much more common than previously thought. We here review published and unpublished records of chelonian FSD, and assess the role of chelonians as seed dispersers, from individual species to the community level. We first discuss the distribution of chelonian FSD and the characteristics of the fruit and/or seed species eaten and dispersed by chelonians. We then use the seed dispersal efficiency framework to explore the quantitative and qualitative components of seed dispersal by tortoises and turtles, embarking on a journey from when the fruits and/or seeds are consumed, to when and where they are deposited, and assess how efficient chelonians are as seed dispersers. We finally discuss chelonian FSD in the context of communities and of chelonians as megafauna. A substantial proportion of the world's aquatic and terrestrial turtles and a major part of testudinid tortoises (71 species in 12 families) include fruits and/or seeds in their diet; fruits of at least 588 plant species in 121 families are ingested and/or dispersed by chelonians. For some chelonians, overall or in certain seasons, fruit may even form the largest part of their diet. Contrary to seed dispersal by lizards, the other major reptilian frugivores, chelonian FSD is not an island phenomenon in terms of geographic distribution. Nevertheless, on islands tortoises are often among the largest native terrestrial vertebrates – or were until humans arrived. We synthesise our knowledge of chelonian FSD, and discuss the relevance of our findings for conservation and restoration, especially in relation to rewilding with large and giant tortoises.

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“…Several recent studies have also highlighted the potential for tortoises to facilitate germination and dispersal of exotic fleshy fruited plants with large seeds, such as Mimusops coriacea (Monkey’s Apple; Waibel et al 2013 ). Although long gut retention times in tortoises have generally been thought to facilitate dispersal and germination of large-seeded fruits, there may be diminishing returns with increasing body size in giant tortoises due to prolonged passage times that may increase the probability of seeds being damaged or destroyed ( Waibel et al 2013 ; Falcón et al 2020 ).…”

Frugivory and seed dispersal by chelonians: A review and synthesis

“…This turtle is very abundant, practices "windfall" feeding in water under riparian fig trees, and emerges on riverbanks and defecates seeds while on land along shorelines in optimal growing locations for these plants. Tortoises may also be especially important for the dispersal of largeseeded plant species on islands (Heleno et al 2011;Falcón 2018), which has important implications at the ecosystem level (see section on chelonians as megafaunal seed dispersers).…”

“…For example, turtle and tortoise species may respond strongly to precipitation, and their activity and movements decrease with increasingly dry periods (Luiselli, 2005;Falcón et al, 2018). In addition, increasing droughts can affect the habitats of chelonians (Haverkamp et al, 2017), and potentially reduce shade availability, which is an important resource for thermoregulation (Merton et al, 1976;Moulherat et al, 2014).…”

“…In addition, increasing droughts can affect the habitats of chelonians (Haverkamp et al, 2017), and potentially reduce shade availability, which is an important resource for thermoregulation (Merton et al, 1976;Moulherat et al, 2014). Moreover, increasing temperatures have been shown to decrease the activity of chelonians, and they may be particularly vulnerable to increases in air temperature in terms of thermoregulation (Lambert, 1981;McMaster & Downs, 2013;Falcón et al, 2018). Thus, the magnitude and outcome of chelonian FSD is very likely to be negatively affected by climate change.…”

“…Networks are qualitative (i.e., the strength of the interactions are not considered) and the size of the boxes represent the number of interactions for each frugivore (top; organised from largest to smallest) and each plant (bottom) present in the community. Networks drawn from data available inDonatti et al (2011; a),Falcón (2018;, andHeleno et al (2013;.…”