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\begin{document}

\title{Soil invertebrates are the key drivers of litter decomposition in
tropical forests}



\author[1]{Xiaoyi Zeng}%
\author[1]{Huilin  Gao}%
\author[1]{Runxi  Wang}%
\author[1]{Bartosz Machjer}%
\author[2]{Joel Woon}%
\author[3]{Cheng Wenda}%
\author[4]{Paul Eggleton}%
\author[5]{Hannah Griffiths}%
\author[1]{Louise Ashton}%
\affil[1]{The University of Hong Kong}%
\affil[2]{University of Liverpool}%
\affil[3]{Sun Yat-sen University - Shenzhen Campus}%
\affil[4]{The Natural History Museum}%
\affil[5]{University of Bristol}%


\vspace{-1em}



  
  \date{May 24, 2024}


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\selectlanguage{english}
\begin{abstract}
Forest litter decomposition is an essential component of global carbon
and nutrient turnover. Soil invertebrates play important roles in litter
decomposition, but the regional pattern of their effects is poorly
understood. We examined 476 case studies across 93 sites and performed a
meta-analysis to estimate regional effects of invertebrates on forest
litter decomposition. We then assessed how invertebrate diversity,
climate and soil pH drive regional variations in invertebrate-mediated
decomposition. We found that (1) invertebrate contributions to litter
decomposition are 1.5 times higher in tropical forests than forests
outside the tropics, with an overall contribution of 31\% to global
forest litter decomposition; and (2) invertebrate diversity,
particularly of termites, together with warm, humid and acidic
environments in the tropics are positively associated with forest litter
decomposition by soil invertebrates. Our results demonstrate the
significant difference in invertebrate effects on mediating forest
litter decomposition between the tropics and elsewhere. We demonstrate,
also, the significance of termites in driving litter mass loss in the
tropics. These results are particularly pertinent in the tropics where
climate change and human disturbance threaten soil invertebrate
biodiversity and the ecosystem services it provides.%
\end{abstract}%



\sloppy


\textbf{INTRODUCTION}

Plant litter is important in the formation of soil organic matter in
terrestrial ecosystems (Pan \emph{et al.} 2011). Decomposition of plant
litter is fundamental in maintaining soil health, carbon sequestration
and plant productivity (H\selectlanguage{ngerman}ättenschwiler \emph{et al.} 2005). Litter
decomposition is primarily caried out by microbes (fungi and bacteria),
microfauna (e.g. nematodes), mesofauna (e.g. collembola and mites) and
macrofauna (e.g. termites, earthworms, isopods and millipedes) (Lehmann
\& Kleber 2015). These organisms break down plant materials to small
particles, mineral salts, carbon dioxide and water which are then used
by plants and soil organisms for nutrients and energy (Swift \emph{et
al.} 1979; Griffiths \emph{et al.}2021a). Generally in ecology microbes
are considered the major decomposers since they can produce cellulase
and lignases that degrade structural polysaccharides in plant litter
(i.e. cellulose and lignin) (Papanikolaou et al. 2010; Pausas \& Bond
2020). Another important component of the soil biome is the
invertebrates whose effects on litter decomposition are often
underestimated (Filser \emph{et al.} 2016; Pausas \& Bond 2020).
However, micro-organisms are the only soil biota currently included in
global biogeochemistry models (Cotrufo \emph{et al.} 2010; Wieder
\emph{et al.} 2015). This omission limits our ability to model global
nutrient and carbon budgets, which are crucial for understanding the
links between climate change and ecosystem services (Grandy \emph{et
al.} 2016; Bishop \emph{et al.} 2021).

\par\null

Recently there has been an increase in understanding of invertebrate
decomposer assemblages and how they affect carbon cycling and nutrient
availability (Chang \& Lai 2018; Shelomi \emph{et al.} 2019;
Joly\emph{et al.} 2020). Studies have shown decomposer invertebrates can
increase litter mass loss and nutrient release either in partnership
with symbiotic microbes or using endogenous cellulases (Eggleton \&
Tayasu 2001; Ashton \emph{et al.} 2019; Griffiths \emph{et al.} 2019).
Earthworms, beetles, isopods and millipedes can produce extracellular
cellulases in partnership with gut symbionts (Pauchet \emph{et al.}2010;
Ni'matuzahroh \emph{et al.} 2022). In addition and apart from gut
symbionts, some of these groups are also capable of producing endogenous
cellulases to digest dead organic matter through invertebrate-enzymatic
decomposition without the support of microbes (Pauchet \emph{et
al.}2010; Griffiths \emph{et al.} 2021a).

In addition to affecting litter decomposition directly through the
breakdown of organic material, soil invertebrates are important
bioturbators that enhance microbial decomposition by reshaping soil
physical and chemical properties. Termites and earthworms, for example,
build huge networks of soil galleries that promote water infiltration
and transportation (Jouquet \emph{et al.} 2011; Hoeffner \emph{et
al.}2019). These water-filled pores and water films are ideal habitats
for aquatic microbiota, such as protists, nematodes, bacteria and fungi
(Hoeffner \emph{et al.} 2019). Foraging and nesting behaviors of
invertebrates improve soil nutrient heterogeneity, creating hotspots of
essential plant-nutrients such as nitrogen and phosphorus (Griffiths
\emph{et al.} 2018; Ashton \emph{et al.} 2019; Phillips \emph{et al.}
2021). Invertebrate faeces influence soil nutrients and ambient
microbial communities (David 2014). The wide-ranging ways in which
invertebrates influence soil properties are complex, with ecosystem-wide
effects mediating microbial decomposition and plant growth (Swift
\emph{et al.} 1979; Adejuyigbe \emph{et al.}2006). Although a growing
body of work highlights the importance of soil invertebrates in
decomposition, there remains large gaps in our understanding of the
biogeography of soil invertebrate contributions to carbon and nutrient
cycling.

The ecological functions carried out by soil invertebrates vary across
regions, such as the tropics and non-tropics (Brussaard \emph{et
al.}2012; Phillips \emph{et al.} 2021; Kass \emph{et al.} 2022).
Evolutionary history and habitat have led to regional differences in
soil faunal taxa (van den Hoogen\emph{et al.} 2020). In tropical
forests, macrofauna (e.g. termites, earthworms, isopods and millepedes)
dominate the decomposer invertebrate assemblages (Swift \emph{et al.}
1979; Lavelle \emph{et al.} 2022). The termites are especially important
for decomposition yet are a group that is largely absent from temperate
regions. Termites remove large quantities of dead organic material from
the forest floor (Bignell \& Eggleton 2000). In forests outside the
tropics (e.g. temperate, subtropical, and boreal) where microbes are
accepted as the dominant decomposers, the mesofauna (e.g. mites and
collembolas) and macrofauna (e.g. earthworms, isopods and millepedes)
are also important contributors to litter decomposition (Anderson 1978;
Korboulewsky\emph{et al.} 2016; Hed\selectlanguage{polish}ě\selectlanguage{english}nec \emph{et al.} 2022). The quality
and palatability of plant-derived resources affect soil invertebrate
diversity (Cebrian 1999; Kurokawa \emph{et al.} 2010; Bastida \emph{et
al.} 2020). Plant materials with a high proportion of nitrogen relative
to structural polysaccharides (e.g. low C:N or lignin:N ratios) usually
shows high palatability and decomposability (Swift \emph{et al.} 1979;
Kurokawa \& Nakashizuka 2008). In addition, most invertebrates are
sensitive to climate. They are usually more active in warm or humid
biomes such as tropical forests, limited by cold and dry conditions
(Garc\selectlanguage{ngerman}ía-Palacios \emph{et al.} 2013). As most published observations are
from non-tropical regions (Xu \emph{et al.} 2020; McCary \& Schmitz
2021), current models and their conceptual framework may not assess
invertebrate effects on decomposition accurately. Accordingly, an
understanding of the regional differences in invertebrate-mediated
decomposition and how specific decomposer invertebrates, climate, and
soil mediate regional differences are important for us to estimate
invertebrate effects on biogeochemistry.

\par\null

We have performed a meta-analysis to compare the effects of soil
invertebrate on leaf litter decomposition in forests contrasting the
tropics and the non-tropics. We then analyzed whether and how
invertebrate diversity, litter traits, climate, and soil pH influence
the global patterns of soil invertebrate-mediated decomposition. Using
this approach, we address whether soil invertebrate contributions to
forest leaf litter decomposition vary across regions. Based on previous
work on soil invertebrate-mediated decomposition, we hypothesize that
(1) soil invertebrate effects on forest litter decomposition will be
significantly higher in tropical forests than non-tropical forests; (2)
high termite diversity, warm and humid environments will be the major
drivers of high tropical invertebrate contributions to forest litter
decomposition.

\textbf{MATERIALS AND METHODS}

\textbf{Data collection}

We synthesized studies related to soil invertebrate effects on forest
leaf litter decomposition and searched articles published in the Web of
Science, Elsevier ScienceDirect, SpringerLink, and Wiley. The keywords
used for the literature searches were (`decomposition' OR `breakdown' OR
`degradation' OR `decay') AND (`leaf' OR `foliar' OR `litter') AND
(`forest' OR `wood land') AND (`litterbag' OR `naphthalene' OR `mesh
size') AND (`soil animal' OR `soil fauna' OR `soil invertebrate' OR
`soil detritivore'). We initially selected 2278 contributions based on
the keyword list. Studies were included in our database when fulfilling
all three of the following requirements:

(1) conducted leaf litter decay experiments in natural forests;

(2) used graded mesh sizes (fine (mesh size \selectlanguage{english}[?] 1 mm) vs coarse (mesh size
\textgreater{}1 mm)) or chemical agents (dose vs control) to establish
treatments including invertebrate inclusion and exclusion under the same
abiotic and biotic conditions; and,

(3) measured litter decomposition in terms of mass loss, mass remaining,
or decomposition rate over a known decay duration.

\textbf{Data extraction}

For invertebrate exclusion and/or inclusion treatments of each article,
we recorded sample sizes (n), means of mass loss or decomposition rates,
and standard deviations (SD) from tables directly or extracted data from
figures by performing Web-PlotDigitizer (Burda \emph{et al.} 2017).
Standard errors (SE) reported in the original articles were converted
into SD using the formula \(\mathbf{S}\mathbf{D=SE\times\sqrt{}n}\). Means of mass loss were
converted into annual decomposition rates using the negative exponential
decomposition equation described by Olson (1963). Other information we
recorded from the original articles include latitude, longitude, biome,
mean annual temperature (MAT, degC), mean annual precipitation (MAP, mm
yr\textsuperscript{-1}), soil pH, litter traits (carbon (C), nitrogen
(N), C:N ratio, lignin:N ratio), duration of decomposition, and the
method to exclude invertebrates (physical vs. chemical).

\par\null

All sites were classified into geographic groups for testing regional
variations. First, we grouped sites into `the tropics' and `the
non-tropics' based on biomes as stated in the original articles which we
further checked by spatial coordinates. Specifically, tropical wet and
dry forests were grouped into `the tropics' (96\% belonged to tropical
wet forests); other forest biomes were grouped into `the non-tropics'.
Biomes are powerful biogeographic units for studying large-scale
patterns of carbon and energy fluxes (Yi \emph{et al.} 2010; Mucina
2019). Our classification of biomes followed Dinerstein \emph{et
al.}(2017). Fig. 1 was plotted using ArcGIS (version 10.2, ESRI, 2020).
We also assigned sites into zoogeographic realms to explore potential
biogeographic effects (e.g. dispersal and evolutionary histories).
Zoogeographic information of each observation followed Holt \emph{et
al.} (2013) which is based on vertebrates but is generally pertinent to
the assessment of invertebrate distributions (Liria \emph{et al.} 2021).

To explore potential moderators of regional variation of the effects of
invertebrate son decomposition, we tested several potential explanatory
factors: termite diversity (a decomposer group the diversity of which is
different in the tropics and non-tropics), litter traits (C, N, C:N and
lignin:N ratios), climate and soil pH. Termite diversity values were
extracted from a corresponding prediction model. The diversity
predictions are estimated from a model which was \selectlanguage{english}'trained\selectlanguage{english}' using
alpha-diversity values from 700 sites (Woon et al., in preparation). We
acknowledge that species diversity and richness do not always confer
higher contribution to ecosystem services compared with functional
diversity, but, currently, this is the best proxy we have to identify
global patterns of species distribution of the group. Where data were
absent from focal studies we obtained missing litter quality data from
the TRY plant trait database (Kattge \emph{et al.} 2020), missing soil
pH data from the Harmonized World Soil Database
(https://www.fao.org/soils-portal/en/, resolution = 5\selectlanguage{english}'), and missing
climate data (mean annual temperature, MAT and mean annual
precipitation, MAP) from the Worldclim database
(http://www.worldclim.org/, resolution = 5\selectlanguage{english}').

\textbf{Statistical analysis}

We first predicted the relative contributions of soil invertebrates and
microbes across absolute latitude using weighted least square models
controlled for the random effects of references. The relative
contribution of microbes in each case was calculated as one minus the
invertebrate contribution. We then used a natural log-transformed
response ratio (LRR) to estimate invertebrate effect size of each
observation (Hedges \emph{et al.} 1999), \emph{viz} :

\(L\text{RR}\ \)=\(\ln{(K}_{c}\)/\(K_{f})\)

where \(K_{c}\) and \(K_{f}\) are the mean decay rates
under invertebrate inclusion and exclusion treatments,
respectively.\(LRR>0\) indicates that soil invertebrates
contribute positively to forest litter decomposition. The within-study
variance (\(v_{i}\)) of each effect size was calculated as:

\begin{equation}
v_{i}=\frac{S_{c}^{2}}{n_{c}K_{c}^{2}}\ +\ \frac{S_{f}^{2}}{n_{f}K_{f}^{2}}\nonumber \\
 \end{equation}

where \(n_{c}\) and \(n_{f}\) are the sample sizes of
invertebrate inclusion and exclusion treatments, respectively, and
\(S_{c}\) and\(S_{f}\) are the standard deviations
of invertebrate inclusion and exclusion treatments. We calculated the
effect size and \(v_{i}\) using the `escalc' function in the R
package `metafor' (Xu \emph{et al.} 2020). We estimated
missing\(S_{c}\) and \(S_{f}\) values using random
number simulation (10000 repetitions) and estimated the
missing\(\ v_{i}\) using the `impute\_SD' function in the
`metagear' package (Bracken \& Sinclair 1992). Invertebrate
contributions (\%) to forest leaf litter decomposition were calculated
as:

Invertebrate contribution (\%) = {[}1 -- 1/exp (\(L\text{RR}\)){]}
\selectlanguage{ngerman}× 100\%

In our meta-data, a single reference usually reported multiple
observations, which means the observations are nested in the reference.
This nested data structure may cause non-independent response variables.
Thus, we applied an inverse variance-weighted hierarchical
random-effects model (rma.mv) with a random part (\textasciitilde{}
1\textbar{} reference / observation) to estimate the weighted mean
effect size (LRR\textsubscript{++}) with 95\% confidence intervals
(Viechtbauer 2010). Confidence intervals not crossing zero indicate
significant mean effect sizes. We first estimated the mean invertebrate
effect sizes at spatial scales and then performed a driving factor
analysis to assess the relationships between moderators and invertebrate
effect sizes. For categorical moderators (i.e., region, biome, and
realm), we used the hierarchical model to calculate the mean effect
sizes at different levels and compared them by employing multiple
comparisons using the `multcomp\selectlanguage{english}' package (Bretz \emph{et al.} 2010). For
continuous moderators (i.e. termite diversity, earthworm richness,
microbial biomass carbon, MAT, MAP, and soil pH), we used mixed-effects
meta-regression to assess the relationships between effect sizes and
moderators. We also tested the effects of decomposition duration and
protocol (mesh vs. chemical) of invertebrate exclusion on invertebrate
effect sizes.

We used a Q-statistic to evaluate the heterogeneity of effect sizes,
which is based on a chi-squared test. Total heterogeneity (Qt) can be
divided into the variance explained by the moderators (Qm) and the
residual error variance (Qe). A significant Qm (P \textless{} 0.05)
indicates that the moderator significantly influences effect sizes
(Viechtbauer 2010). Publication bias arises from a preponderance of
articles presenting `favorable\selectlanguage{english}' results which can impact the reliability
of our assessment. We tested the possibility of publication bias using a
funnel plot and performed Egger's regression test to examine,
quantitatively, the funnel symmetry (Su \emph{et al.} 2021). A p value
greater than 0.05 for Egger's test indicates that the result is less
affected by publication bias. All analyses were performed in R 4.2.0.

\textbf{RESULTS}

\textbf{Meta-data in this study}

A total of 62 articles, encompassing 476 observations across 93 study
sites met our criteria (for detailed references see supporting material
S1). Eighty-five per cent of case studies used graded mesh sizes to
exclude soil invertebrates, with 15\% using chemical agents.
Observations were distributed across all continents on Earth except
Antarctica: Asia (37\%), Europe (32\%), South America (24\%), North
America (5\%), and Australia and Africa (2\% for both) (Fig. 1). For
litter traits and environmental variables, 36\% of leaf litter traits
(C, N, C: N and lignin:N ratios), 50\% of soil pH, 90\% of mean annual
temperature (MAT, degC), and 91\% of mean annual precipitation (MAP, mm
yr\textsuperscript{-1}) were taken from the original articles.

\textbf{Spatial distribution of soil invertebrate effects on forest leaf
litter decomposition}

Forest litter decomposition by invertebrates was the highest in the
tropics and decreased with increasing absolute latitude (Fig. 2a).
Although overall microorganism effect (69\% on average) on forest litter
decomposition twice that of invertebrate effect (31\% on average), the
contribution of invertebrates to litter decomposition in the tropics
approaches that of microorganisms. In boreal forests, the invertebrate
contribution is low, with microorganisms acting as the major decomposers
(Fig. 2a). Overall, invertebrates had significantly positive effects on
forest litter decomposition, leading to a 31\% increase in decomposition
where there was invertebrate access (LRR\textsubscript{++} = 0.368, Qt =
37687.144, df = 475, P \textless{} 0.001) (Fig. 2b, Table S1). Soil
invertebrates increased litter decomposition significantly by an average
of 40\% in tropical forests (LRR\textsubscript{++} = 0.513, df = 161, P
\textless{} 0.001) compared with 26\% in non-tropical forests
(LRR\textsubscript{++} = 0.303, df = 311, P \textless{} 0.001),
respectively (Fig. 2b, Tables S1, S2). Tropical invertebrates
contributed 1.5 times more to forest litter decomposition \emph{in
situ}than did non-tropical invertebrates (Fig. 2b, Table S2).

Across different forest biomes, soil invertebrates contributed
significantly to litter decomposition in all forest biomes, increasing
decomposition by 43\% in tropical wet forests (LRR\textsubscript{++} =
0.554, P \textless{} 0.001), 28\% in evergreen broadleaf forests
(LRR\textsubscript{++} = 0.323, P = 0.001), 26\% in deciduous forests
(LRR\textsubscript{++}= 0.303, P \textless{} 0.001), 25\% in coniferous
forests (LRR\textsubscript{++} = 0.293, P \textless{} 0.001), and 24\%
in cold or dry woodlands (LRR\textsubscript{++} = 0.274, P \textless{}
0.001) (Fig. 2b). Compared with non-tropical forests, tropical wet
forests held the highest invertebrate-mediated litter decomposition
(Table S2). Among forests in different zoogeographic realms measured, we
found the higher soil invertebrate effects on decomposition in
Neotropical forests (48\%, LRR\textsubscript{++} = 0.656, P \textless{}
0.001) compared with those in the Nearctic (29\%, LRR\textsubscript{++}
= 0.338, P = 0.001), the Palearctic (25\%, LRR\textsubscript{++} =
0.292, P \textless{} 0.001), and the Sino-Japanese forests (22\%,
LRR\textsubscript{++} = 0.247, P \textless{} 0.001), respectively (Fig.
3, Table S2). There were no significant differences in invertebrate
effects in the Neotropical forests compared with forests in the Oriental
(29\%, LRR\textsubscript{++} = 0.344, P \textless{} 0.001), Panamanian
(46\%, LRR\textsubscript{++} = 0.625, P \textless{} 0.001), and
Australian (35\%, LRR\textsubscript{++} = 0.426, P = 0.004) data,
respectively (Table S2).

\textbf{Drivers of soil invertebrate effects on forest leaf litter
decomposition}

Mixed-effect meta regression showed a significantly positive
relationship between termite diversity and invertebrate effect sizes (Qm
= 17.842, P \textless{} 0.001, df = 446, Fig. 4a). For litter traits,
our meta regressions showed significantly negative relationships between
invertebrate effect sizes and C: N ratios in litter (Qm = 5.475, P =
0.019, df = 266, Fig. 4b) and lignin: N ratios (Qm = 5.634, P = 0.018,
df = 153, Fig. 4c), whereas no significant relationships were observed
between effect sizes and litter C, N, P contents (as percentage of dry
litter weight) (Table S1). In terms of climatic variables, there were
significant positive relationships between invertebrate effect sizes and
mean annual temperature (Qm = 8.133, P = 0.004, df = 474, Fig. 4d) and
mean annual precipitation (Qm = 25.672, P \textless{} 0.001, df = 475,
Fig. 4e). We also found that soil pH negatively correlated with
invertebrate effects on forest litter decomposition (Qm = 11.226, P =
0.001, df = 450, Fig. 4f).

In our dataset, the decomposition durations of 73\% of observations are
\selectlanguage{english}[?] 1 year, and the remaining 23\% are from 1 to 2 years. Mixed-effect
meta regression showed that soil invertebrate effects weakened with the
increasing of decomposition duration (Qm = 5.942, P = 0.015, Table S1),
indicating soil invertebrates contribute differentially to the early
stages of leaf litter decomposition (Fig. S1a). We did not find a
significant impact of faunal exclusion protocol (chemical vs. physical)
on invertebrate effects (Table S1). The reliability of our results was
supported by Egger's test for funnel plot asymmetry (z = 0.8708, P =
0.3838), showing that invertebrate effect sizes were not affected by
publication bias (Fig. S2).

\textbf{DISCUSSION}

\textbf{Soil invertebrates contribute to forest litter decomposition
across the globe}

Soil invertebrates make considerable contributions to the decomposition
of forest litter. Our synthesis shows 31\% of litter decomposition,
globally, is mediated by invertebrates. Our estimate is in line with the
previous estimates ranging from 27\% to 35\% of invertebrate effects on
decomposition (Kampichler \& Bruckner 2009; Garcia-Palacios \emph{et
al.} 2013; Xu \emph{et al.} 2020). Based on the overall estimates, our
analysis further shows a regional pattern of invertebrate effects on
litter decomposition and particular invertebrate groups driven the
regional difference. We have shown that the invertebrate contribution to
litter decomposition in tropical forests is 1.5 times higher than in
forests outside the tropics. This latitudinal pattern was observed
consistently across all zoogeographic realms except the Ethiopian
(Afrotropical) and all biomes. The pattern is supported both
historically and ecologically. The presence of particular decomposer
groups (e.g. termites) may explain the strong effects of soil
invertebrate on decomposition in the tropics. The so-called `tropical
conservatism hypothesis\selectlanguage{english}' suggests that, in the past, when the Earth was
warmer, tropical lineages had wider distributions. Long-term cooling
since the late Eocene has restricted distributions of tropical fauna and
driven their extinctions in now non-tropical regions (Wiens \& Donoghue
2004; Guenard \emph{et al.} 2015). This history over deep time may well
underpin current ecological functions. Major biogeographic differences
in soil invertebrate species pools must be considered when discussing
the major drivers of decomposition globally. Plant community and
environmental conditions also drive soil invertebrate activities and
abundance in tropical forests. Diverse plant communities create rich
food resources for soil invertebrates and is the basis for maintaining
their abundance and activities (Cebrian 1999; Kurokawa \emph{et al.}
2010; Lavelle \emph{et al.} 2022). Humid and warm environment in
tropical regions are generally favorable for the activities of specific
decomposer invertebrates, which are crucial for their corresponding
ecological functions, which includes litter decomposition (Tan \emph{et
al.} 2020).

\textbf{Drivers mediate regional variation in soil invertebrate effects
on forest litter decomposition}

Soil invertebrate fauna

We have shown that termites are important decomposers responsible for a
significant component of mass loss of litter. Termites specializing in
the decomposition of dead plant material and are diverse in the tropics
but not in temperate regions (Lavelle \emph{et al.} 2022). The
Termitidae, for example, are litter-feeders with high diversity in
tropical forests (Eggleton \& Tayasu 2001), well adapted to breaking
down organic matter (Hogan \emph{et al.} 1988; Bignell 2019). Many
termite species are able to produce endogenous cellulase predominantly
in the midgut enabling them to digest cellulose (Hogan \emph{et al.}
1988). They also produce extracellular enzymes that digest cell wall
polysaccharides cooperatively with symbiotic protozoa (e.g.
Kalotermitidae) or microbial symbionts (e.g. the Termitidae) (Eggleton
\& Tayasu 2001). Our results provide further evidence that termites are
key decomposers in the tropics, shaping nutrient turnover with
ecosystem-wide implications for carbon flux and soil modification
(Ashton \emph{et al.}2019; Griffiths \emph{et al.} 2021b). Understanding
the links among termite biodiversity, ecosystem processes and
environmental change such as climate and land use change is key for
understanding biogeochemical cycles.

Climatic variables

We demonstrated that soil invertebrate contributions to forest litter
decomposition are positively related to temperature and precipitation.
Climate can moderate faunal decomposition through its impacts upon
invertebrate activities (Thakur \emph{et al.} 2018). Low temperature or
water-limited conditions in subtropical, temperate, and boreal forests
limit soil invertebrate activities and their decomposing effects (David
\& Handa 2010; Castanho \emph{et al.} 2012). The behaviour of some
invertebrate decomposers are directly sensitive to temperature. Foraging
termites, for example, show increasing discovery rates with increasing
temperature (Zanne \emph{et al.} 2022). For precipitation, many soil
organisms are sensitive to low soil moisture. Protozoa, nematodes and
mites, for instance, live within water droplets or films and are
directly affected by water potential (Hoeffner \emph{et al.} 2019). Some
soft-bodied meso- or macro-invertebrates (e.g. earthworms) are also
susceptible to moisture stress (Swift \emph{et al.} 1979). In contrast,
invertebrates that mainly occupy the litter layer or air-filled soil
spaces (e.g. termites, isopods, millipedes and beetles), have lower
water sensitivity and are more resistant to desiccation (Swift \emph{et
al.} 1979; Berg \& McClaugherty 2020). Foraging discovery rates and
activities of termites may even be enhanced with increasing drought,
enhancing their ecological importance in extreme droughts (Ashton
\emph{et al.} 2019). Any loss of decomposer organisms driven by climate
change, therefore, may lead to a decrease in future litter decomposition
(Wall \emph{et al.} 2010), but the relative magnitude of termite impacts
on biogeochemistry may increase at the Earth's surface with increasing
tropicalization (Zanne \emph{et al.}2022).

Soil pH

Soil pH shows a negative relationship with invertebrate effects on
forest litter decomposition. Microbiota within water films, were clearly
the most sensitive to soil pH (Tao \emph{et al.} 2019). Tropical soils
are usually acidic due to high leaching intensity driven by high
precipitation (Zhang \emph{et al.} 2019), with pH levels ranging from
3.8 to 6.2 in our meta-data. In the tropics, litter-feeders with high
acidity tolerance, such as termites, can reach high abundance levels and
so have a competitive advantage over microorganisms, especially bacteria
(Lavelle \emph{et al.} 1995). Reduced vegetation cover caused by
deforestation leads to soil acidification as lack of organic matter and
root systems lead to loss of soil nutrients (Birhanu \emph{et al.}2016).
Increased land-use intensity with concomitant loss of tree cover,
therefore, threatens soil biodiversity not only through habitat loss but
also by changing soil properties by acidification. This negative effect
may undermine the ecological functions of both microbial and faunal
decomposers, while some highly adapted invertebrates, such as invasive
earthworms and termites, may be responsible for more litter
decomposition in disturbed areas (Liu \& Zou 2002; Huang \emph{et
al.}2020).

\textbf{FUTURE WORK}

Including regional variations in soil invertebrate effects on
biogeochemistry to conceptual and predicting models. The temperate bias
of studies examining the role of invertebrates in decomposition means
that much of the raw data for existing biogeochemical models are from
non-tropical zones, which may result in an underestimation of soil
invertebrate effects on carbon flux and nutrient turnover. Moreover,
invertebrate biodiversity has been under tremendous pressure, with
knock-on effects on soil health, plant growth and climate regulation.
The tropics represent 36\% of earth's landmass and support the majority
of aboveground plant biomass (Crowther \emph{et al.} 2019). The regional
differences in invertebrate functions we have reported suggests that the
loss of invertebrate biodiversity could be particularly catastrophic in
the Tropics.

More empirical studies needed in Africa and Australia. As with many
meta-analyses in ecology, only a small number of studies have occurred
in Australia and Africa, where large areas of tropical dry forests and
rainforests are located (Pan \emph{et al.} 2013). We have repeatedly
stressed the relative importance of the termites in driving tropical
decomposition. Termites are diverse in both Australia and Africa.
Africa, indeed, is the evolutionary cradle of termites and hosts the
highest number of termite species (Buitenwerf \emph{et al.} 2011;
Bignell 2019). The lack of observations in these areas suggests that we
may have underestimated tropical invertebrate effects on forest litter
decomposition. We urge, extensive data collection in the tropics,
especially in Africa, as essential for future assessments of global
carbon cycling and nutrient turnover.

Detailed information on litter traits and environmental variables are
needed. We suggest that detailed measurements of leaf litter traits need
to be included in further studies of litter decomposition. We advocate
for the establishment of a global dataset of leaf litter traits. No such
comprehensive global dataset for leaf litter traits exists. Contrasts
with living leaves (for which global data bases do exist (Kattge\emph{et
al.} 2020)) will be informative. In addition, information on plant
communities, plot-based microclimates and soil properties should
accompany future studies since they are important factors driving the
ecological functions of soil invertebrate. Collecting such data at a
local scales is essential (Bradford \emph{et al.} 2014).

Protocol used to exclude soil invertebrates. To date, most studies have
used physical methods (i.e. litterbags and mesocosms) to exclude soil
invertebrates in field experiments, accounting for 85\% of observations
in our meta-data. Sixteen percent of observations in our meta-data
showed negative effects of invertebrates on litter mass loss. These
negative faunal effects on decomposition have not been well tested.
Possible explanations would be non-target effects of chemicals (e.g.
naphthalene) on both invertebrates and microbes (Lan \emph{et al.}2020).
Within the observations of negative faunal effects in our meta-data,
62\% of observations were from the use of naphthalene to exclude
invertebrates. The inclusion of fungivores such as Collembola and
Nematoda may affect microbial decomposition negatively (Johnson\emph{et
al.} 2005) in cold or dry regions where invertebrate activity is
constrained by temperature and water availability (Wall \emph{et
al.}2008). Our analysis shows an insignificant relationship between
exclusion method and invertebrate effects on forest litter
decomposition. The potential non-target effects of chemicals such as
naphthalene on soil organisms and environments (Lan \emph{et al.} 2020),
however, leads us to advocate use of physical exclusion methods
(Lan\emph{et al.} 2019), particularly in highly stochastic tropical
rainforests and in regions with high precipitation.

\textbf{CONCLUSION}

We have demonstrated conclusively that soil invertebrates are important
components of global forest leaf litter decomposition, especially in the
tropics, from both biogeographical and ecological perspectives. Termites
in particular are major decomposers mediating the regional variation in
animal-driven decomposition. Soil invertebrates are experiencing
remarkable diversity loss and functional changes driven by multiple
disturbances including climate change, habitat loss and environmental
pollution (Cifuentes-Croquevielle \emph{et al.} 2020). Potentially, this
loss will decrease the release of carbon and plant-available nutrients
from dead organic matter significantly with cascading effects on plant
nutrient uptake and soil health (Handa\emph{et al.} 2014; Griffiths
\emph{et al.} 2021a). Future climate change and other human disturbance
has the potential to reshape the relative contributions of soil
invertebrates and microbes in global litter decomposition with unknown
but undoubtedly serious consequences.

\textbf{ACKNOWLEDGEMENTS}

We gratefully acknowledge Dr. Timothy Bonebrake, Dr. Mike Boyle and Dr.
Roger Kitching for helpful comments on the manuscript.

\textbf{COMPETING INTERESTS}

The authors declare that they have no known competing \selectlanguage{english}financial interests
or personal relationships that could have in\selectlanguage{english}fluenced the work reported in
this article.

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\textbf{Figure 1} Global distribution of forest leaf litter
decomposition experiment used in this study. The map indicates a total
of 476 observations at 93 sites across the world superimposed on the
background of biome patterns.

\textbf{Figure 2} Soil invertebrate contributions to forest litter
decomposition across regions. \textbf{(a)} Relative contributions of
soil invertebrates (blue) and microorganisms (grey) to forest litter
decomposition against absolute latitude. \textbf{(b)} Effect sizes of
soil invertebrates on forest litter decomposition at global, regional,
and biome scales. The errors represent 95\% confidence intervals. The
numbers of observations are in the brackets. Positive mean effect sizes
indicate soil invertebrates significantly contribute to forest litter
decomposition.

\textbf{Figure 3} Soil invertebrate effect sizes on forest leaf litter
decomposition across zoogeographic realms. The numbers of observation
are in the brackets. Realms with \selectlanguage{english}[?] 5 observations are included. Colors
are identical in forest plot and map, realms with observations less than
5 are indicated by the grey color. The errors represent 95\% confidence
intervals.

\textbf{Figure 4} Influence of\textbf{(a)} termite diversity,
\textbf{(b)} litter C: N ratio,\textbf{(c)} litter lignin: N ratio,
\textbf{(d)} mean annual temperature (MAT), \textbf{(e)} mean annual
precipitation (MAP), and\textbf{(f)} soil pH on invertebrate effect
sizes determined using mixed-effect meta regressions. Point sizes
represent the relative weights (log) of corresponding observations.
Significant correlations (P \textless{} 0.05) are shown with solid
regression lines with 95\% confidence intervals.\selectlanguage{english}
\begin{figure}[H]
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