DISCUSSION
To our knowledge, this is the first study to experimentally unravel
cascading effects of tree functional diversity, through trophic chains,
on a biogeochemical process through time. Our concept and findings have
added a new dimension to the “afterlife effects” of functional traits
by pinpointing the cascading effects of variation in deadwood quality
among tree species through a “brown” food chain, with important
consequences for both forest carbon turnover and animal populations.
While, without the involvement of invertebrate decomposers wood
decomposition rates were predicted by initial values along the WES, when
termites participated in the decomposition process they actively altered
the species’ ranking in terms of the WES through time and moved in
abundance and activity from initially high WES species to initially
medium WES species. These findings have an interesting parallel with a
recent study that found a modulating effect of invertebrate consumption
on leaf mass loss rankings along an axis of initial litter quality,
i.e., the leaf economics spectrum, over time (Guo et al. 2019).
However, in that study the modulation was due to a single outbreak by
detritivorous moth larvae, while in this study the decomposing
invertebrates (i.e., termites) were present in considerable abundance
through time in the studied forests, but changed their relative
abundance and contribution to decomposition among tree species as
decomposition progressed. As a consequence, the temporally stable
overall consumption of deadwood by termites should help to sustain
deadwood turnover in a forest of high functional diversity. In turn,
negative feedback by pangolins (Fig. 1) should contribute to net carbon
sequestration via controlling the termite populations, but we could not
carry out manipulation of pangolins to quantify this feedback.
Interactive effects of top-down and
bottom-up forces in regulating food web structure have been shown
(Faithfull et al. 2011; Leibold et al. 1997). However, in
our system the positive bottom-up control of termite populations by
deadwood seemed to strongly outweigh the negative top-down control by
pangolins, as the termite population was still larger in PT with than in
TT without pangolins. To overcome predation pressure, prey communities
may show compensatory population growth via faster regeneration (Crawley
1992). This can provide an explanation why the PT termite population did
not decline with a significant increase in predator population
(evidenced by pangolin burrow density). In a functionally diverse
forest, with the wood resource complementarity as shown in our study,
the basal resource constraint is likely relatively weak through time,
allowing prey (termites) to reach high population density in spite of
predator (pangolin) control. If hunting and trafficking of pangolins
ceased completely (e.g., in response to pangolins’ suspected role in
transmission of the 2020 Covid-19 virus to humans), and pangolin
populations were allowed to regain their natural population size across
the entire region, the contributions of top-down versus bottom-up
control of termite populations and their effect on wood decomposition
and the forest carbon budget would be easier to study.
New experiments are needed to enrich our conceptual model with empirical
evidence, beyond the specific food chain studied here. On the one hand,
other termite predators in the world (e.g., other pangolin species;
aardvarks in Africa; some armadillo species in the Americas) are also in
dramatic population decline. We need to know whether, besides halting
their persecution by people, improving their habitat by promoting plant
functional diversity and, thereby, their staple food will help to
recover or sustain the populations of these species in general. Some
ants are also known to have termites
as a staple foods (Buczkowski & Bennett 2007), but whether or how they
compete for termites with vertebrate termite feeders such as pangolins,
or are themselves eaten and thereby controlled top-down by these
vertebrates, and thereby indirectly affect wood decomposition, are
intriguing questions for further investigation.
Moreover, terrestrial decomposition is integrated in multiple food
chains in the food web. Thus, to better understand the generality of the
positive deadwood turnover feedback studied here, we need to identify
the interactions between other wood (or leaf, root) decomposing
invertebrates or predators of other detritivores and plant functional
trait diversity. Although termites are the dominant species in
warm-climate wood decomposition, other invertebrates such as beetles
tend to control the animal contribution to decomposition in temperate
ecosystems; either directly through feeding or by facilitating other
decomposers such as fungi or similar-sized animals to invade the
deadwood by tunneling, feeding, nesting or intended microbe cultivation
(Ulysen 2016; Zuo et al. 2014). In our experiment we found a wide
variety of deadwood beetles and millipedes; although their abundance is
far less than that of termites, how (much) they affect wood
decomposition, directly or by affecting other decomposers such as fungi
and termites, needs in-depth study. Importantly, we need to study how
these different decomposers, through time (e.g., decay stages, seasons),
interact with each other and with the trait diversity of their basal
resource via their decomposing function. Multiple harvests and tracking
animal population dynamics (Schwarzmüller et al. 2015) in such
experiments can provide further insights into time effects of
decomposers, and their suppression by predators, as influenced by
resource availability.
Taken together, our findings strongly point to high wood functional
diversity helping to provision sufficient food resource in terms of
deadwood quantity and quality for termites to maintain their population
through time. In turn, this larger and stable termite population seemed
to have increased the density of pangolins as indicated by the four-fold
increase in density of their burrows. Thus, our results strongly suggest
that woody plant functional (trait) diversity can support a (threatened)
mammal species at the top of the food chain through the detritivore
subsystem. We have to make the caveat that the causality of wood
functional diversity supporting the pangolin population in our
experiment cannot be proven and would need further, perhaps manipulative
study. The fact that we added deadwood in our research plots may by
itself have stimulated both termites and pangolins and, in theory,
year-to-year changes in environmental conditions could also have
interfered with termite and pangolin populations. Still, our results
strongly suggest that, if only initially high, medium or low quality
wood had been added in the experiment, the termite populations would not
have increased as much as they did with the broader resource economic
spectrum (also compared to the initial forest community) incubated in
our forest plots over the time span of the experiment. And neither would
the number of pangolin burrows in the PT site have quadrupled. Moreover,
although indirect evidence, the (living) tree community in PT, with the
larger overall termite density, had a greater wood trait diversity than
in TT (Table 1, unweighted WES range, community-weighted WES variance),
which likely translates into a wider overall range in deadwood quality
in PT. Thus, our results are consistent with the hypothesis that woody
plant functional (trait) diversity drives an intriguing interplay of
dead wood quality, its decomposition and termites through time, thereby
even supporting a threatened mammal at the top of the food chain. These
findings should invite further studies, in other green or brown food
webs, of how plant functional diversity can cascade dynamically through
different trophic levels all the way to the top of food chains.