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).