4.2.2 Focal taxa
Broadly equivalent numbers of studies focused on birds and mammals (74 and 80 respectively). In total 29% of species were studied more than once, and the most heavily studied species were frugivores that are known to be readily caught and tagged, and those commonly found in research hotspots (Biro, 2013; Rosenthal et al, 2017). These frequently studied species suggest that frugivore tracking studies focus on key long-distance dispersers, including larger-bodied animals that can disperse larger seeds across greater distances (African bush elephant & Asian elephant) and animals that have the ability to fly and connect fragmented landscapes (Little yellow-shouldered bat and Seba’s short-tailed bat).
Bats were the largest group of mammal species studied, studied in 40 articles or half of the mammal studies. The high number of bat studies represents the high diversity of frugivorous bats and their importance in long distance dispersal (Muscarella & Fleming, 2007). However, only four studies calculated seed dispersal distances, potentially due to tag weight limitations. Many bat species are too small to carry tracking devices (O’Mara et al, 2014; van Harten et al, 2019), and those that are not, are often constrained to just a few weeks of data collection because of common tag attachment techniques e.g. surgical glue. Whilst this method is widely used, tags rarely remain attached for longer than 4 weeks (O’Mara et al, 2014), and therefore only provide a limited snapshot of a species movement capabilities. Bats provide important links among forest fragments due to their mobility (Estrada & Coates-Estrada, 2002), and with the advent of smaller tags (Dressler et al, 2016), future studies could better explore their role in seed dispersal.
Four species of reptiles were tracked across these studies including the yellow-footed tortoise (Chelonoidis denticulatus ), Lilford’s wall lizard (Podarcis lilfordi ), Eyed lizard (Timon lepidus)and Southeast Asian box turtle (Cuora amboinensis ), suggesting that reptiles are currently being underrepresented in biologging studies in terms of their potential role as seed dispersers This is of particular importance for island habitats where reptiles, predominately lizards and tortoises, often occur disproportionately compared to other species. These habitats are often species-poor in terms of diversity, meaning reptiles become some of the only seed dispersers around (Olesen & Valido, 2003). In particular, giant tortoises are thought to fill traditional megaherbivore roles on islands and are noted as ecosystem engineers (Blake et al, 2012; Falcon et al, 2020). Therefore, future seed dispersal studies should be encouraged to quantify the role that reptiles play as seed dispersers.
Body mass is clearly instrumental in predicting which tracking method is more likely to be used in frugivore studies; globally, 72% of bird species and 55% of mammal species weigh less than 100g (Wilman et al, 2014), which is the minimum body mass for a 5g tag (typical for commercial GPS tags; Altobelli et al, 2022). In our review the median body mass was 83.4g and 192.8g for birds and mammals, respectively, suggesting that larger animals in general have a significantly increased probability of GPS tags being deployed compared to smaller animals, irrelevant of taxa. Whilst radio transmitters can weigh as little as 0.2g (Naef-Daenzer et al, 2005) and offer a low-cost alternative to GPS tags, there are trade-offs with the quality of data collected (Gottwald et al, 2019). Radio telemetry often results in low temporal and spatial resolution due to infrequent location fixes and the required intensive labour in collecting these fixes (Harris et al, 1990; Ryan et al, 2004; Alexander & Maritz, 2015).
Since miniaturisation and technological advances have reduced the size and weight of GPS tracking technology, we would expect frugivores to be tagged with GPS tags more frequently in recent years, but we did not see a clear pattern. For mammals, we found that in later years there was an increased probability of GPS tags being deployed, but body mass did not have a significant effect. We observed the opposite effect with bird species, with increased body mass there was an increased probability of GPS tags being deployed on birds, but an interaction between body mass and years shows a significant effect on the increased probability of GPS tag being deployed for bird species.
The pattern seen here suggests to us that the technological advances made concerning increased storage and remote download capabilities of GPS tags (Kays et al, 2015) have led to the increase of studies focusing on large, frugivorous bird species that have migrational behaviour or extensive home ranges where previous use of radio transmitters would have been ineffective (Hallworth & Marra, 2015; Lenz et al, 2015).