INTRODUCTION
Large vertebrates are being extirpated across the tropics, which affects
the myriad tree species that interact with these animals (Kurten 2013;
Dirzo et al. 2014). Such defaunation can affect plants in many
ways. When trees are left without seed dispersers, for example, they may
suffer population declines (Brodie et al. 2009; Culot et
al. 2017; Rogers et al. 2017). Because many trees dispersed by
large vertebrates are themselves large or have dense wood, defaunation
may even induce shifts in tree species composition that reduce the
aboveground biomass of tropical forests, with implications for the
global carbon cycle (Brodie & Gibbs 2009; Bello et al. 2015;
Osuri et al. 2016; Peres et al. 2016). Furthermore, if
hunting removes large predators, granivore populations could increase,
leading to reduced seed survival (Galetti et al. 2015; Rosin &
Poulsen 2016). However, many tropical trees experience very strong
conspecific density dependence (Harms et al. 2000; Peters 2003;
Comita et al. 2010; Terborgh 2012), which implies that lower
survival at early stages (e.g. through reduced seed dispersal or
enhanced seed predation) could potentially be offset at the population
level by ameliorated density dependence. Moreover, many of the hunted
vertebrates are potent seed predators (Roldán & Simonetti 2001; Donattiet al. 2009) or trample seedlings (Rosin et al. 2017), so
removing these animals could benefit regeneration in certain plant
species. It is critical, therefore, to assess how defaunation affects
not just seed dispersal or seedling survival, but the entire life cycle
of tropical trees.
Most previous studies on how defaunation affects trees (particularly
those focusing on forest carbon impacts) have focused almost exclusively
on reduced seed dispersal. These studies often simulate community
composition in defaunated forests by ‘removing’ tree species that are
large vertebrate-dispersed (Peres et al. 2016; Chanthorn et
al. 2019) or that have large seeds (Bello et al. 2015; Osuriet al. 2016), and show that this can result in substantial
reductions in aboveground biomass (i.e. carbon storage). Looking at
empirical evidence from defaunated forests, though, the patterns are
less clear. Populations of a tree species that significantly contributed
to carbon stocks were indeed declining in defaunated forests in the
Brazilian Atlantic Forest (Culot et al. 2017), but
hunting-induced dispersal limitation appeared to have no impact on total
tree biomass in Malaysian Borneo (Harrison et al. 2013).
The best way to predict the effects of defaunation on tree species is to
conduct population-level, whole-life-cycle analyses. But such analyses
are very resource-intensive, precluding the evaluation of all tropical
tree species over any conservation-relevant time frame. Therefore, it is
important to try to ascertain whether we can predict a prioriwhich tree species might be susceptible to defaunation, for example
based on their phenotypic traits. Defaunation responses could
potentially vary with morphological traits such as seed size, which may
affect seed predation (Mendoza & Dirzo 2007), or with ecological traits
such as dispersal mode, which could affect susceptibility to disperser
loss (Peres et al. 2016). However, given the multiple effects of
defaunation on plants at different life stages, what matters is how all
of the impacts combine to influence overall population dynamics
(Harrison et al. 2013) and whether this varies with life history
characteristics. Demographic rates for tropical trees tend to be
correlated with physical traits (Poorter et al. 2008) that are
easier to measure and collect. If we could use combinations of physical
traits to ascertain a given tree species’ susceptibility to
defaunation-induced population decline, we could better predict the
changing species composition of defaunated tropical forests.
Here we synthesized data on tropical tree populations, the multiple
impacts of defaunation across the plant life cycle, and tree
morphological and demographic traits to assess whether we can predict
how trees with different traits vary in their responses to
hunting-induced population declines. We used density-dependent
demographic models and Monte Carlo simulations that incorporate data on
all facets of tree life history, defaunation effects, and the (often
substantial) uncertainty in these factors. Specifically, our objective
was to determine whether any traits or trait combinations were
associated with tree susceptibility to a range of different defaunation
effects.