Download Traits underpinning desiccation resistance explain distribution

Oecologia (2013) 172:667–677
DOI 10.1007/s00442-012-2541-3
PHYSIOLOGICAL ECOLOGY - ORIGINAL RESEARCH
Traits underpinning desiccation resistance explain distribution
patterns of terrestrial isopods
André T. C. Dias • Eveline J. Krab • Janine Mariën •
Martin Zimmer • Johannes H. C. Cornelissen • Jacintha Ellers
David A. Wardle • Matty P. Berg
•
Received: 19 July 2012 / Accepted: 19 November 2012 / Published online: 7 December 2012
Ó Springer-Verlag Berlin Heidelberg 2012
Abstract Predicted changes in soil water availability
regimes with climate and land-use change will impact the
community of functionally important soil organisms, such
as macro-detritivores. Identifying and quantifying the
functional traits that underlie interspecific differences in
desiccation resistance will enhance our ability to predict
both macro-detritivore community responses to changing
water regimes and the consequences of the associated
species shifts for organic matter turnover. Using path
analysis, we tested (1) how interspecific differences in
desiccation resistance among 22 northwestern European
terrestrial isopod species could be explained by three
underlying traits measured under standard laboratory conditions, namely, body ventral surface area, water loss rate
and fatal water loss; (2) whether these relationships were
robust to contrasting experimental conditions and to the
Communicated by Matthias Schaefer.
Electronic supplementary material The online version of this
article (doi:10.1007/s00442-012-2541-3) contains supplementary
material, which is available to authorized users.
A. T. C. Dias (&) E. J. Krab J. Mariën J. H. C. Cornelissen J. Ellers M. P. Berg
Department of Ecological Science, Faculty of Earth and Life
Sciences, VU University Amsterdam, De Boelelaan 1085,
1081 HV Amsterdam, The Netherlands
e-mail: [email protected]; [email protected]
M. Zimmer
FB Organismische Biologie: Ökologie, Biodiversität
& Evolution der Tiere, Paris-Lodron-Universität,
Hellbrunner Str. 34, 5020 Salzburg, Austria
D. A. Wardle
Department of Forest Ecology and Management,
Swedish University of Agricultural Sciences,
90183 Umeå, Sweden
phylogenetic relatedness effects being excluded; (3) whether desiccation resistance and hypothesized underlying
traits could explain species distribution patterns in relation
to site water availability. Water loss rate and (secondarily)
fatal water loss together explained 90 % of the interspecific
variation in desiccation resistance. Our path model indicated that body surface area affects desiccation resistance
only indirectly via changes in water loss rate. Our results
also show that soil moisture determines isopod species
distributions by filtering them according to traits underpinning desiccation resistance. These findings reveal that it
is possible to use functional traits measured under standard
conditions to predict soil biota responses to water availability in the field over broad spatial scales. Taken together,
our results demonstrate an increasing need to generate
mechanistic models to predict the effect of global changes
on functionally important organisms.
Keywords Detritivores Drought Functional traits Isopoda Soil moisture Water loss rate
Introduction
Climate and land-use change can greatly alter the structure
and functioning of soil communities (Emmerling 1995;
Bardgett and Wardle 2010; Moron-Rios et al. 2010; Blankinship et al. 2011). Soil detritivores are especially sensitive
to changes in soil water availability, which can promote
significant shifts in their abundance, vertical stratification,
species richness and composition (Maraldo and Holmstrup
2010; Moron-Rios et al. 2010; Blankinship et al. 2011;
Makkonen et al. 2011) and in turn alter essential soil processes (Briones et al. 2009). It is therefore expected that
intensification of changes in precipitation patterns resulting
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from climate change and artificial regulation of groundwater
resources due to land-use change will strongly influence the
abundance, community composition and functioning of
these organisms. Macro-detritivores, such as isopods, millipedes and earthworms, are important regulators of the
turnover of dead plant material (Lavelle 1997), with significant consequences for soil fertility, primary productivity
and carbon turnover (Wardle 2002). Whether or not soil
invertebrate communities will decline in abundance due to
global changes (Barrett et al. 2008; David and Handa 2010;
Blankinship et al. 2011) or how their functional composition
will change has important implications for determining the
overall impact of global changes on soil processes.
The huge diversity of macro-detritivores (Schmalfuss
2003; Sierwald and Bond 2007) poses a challenge to ecologists with respect to synthesizing and applying current
knowledge on species autecology to predict how changes in
soil hydrological regimes influence their community structure. It has been proposed that ecological generality will
improve with a shift from a nomenclatural approach focusing
on species number and identity to a more functional
approach focusing on species’ functional traits (McGill et al.
2006). Quantification of the variation in functional traits
directly links organism performance to environmental conditions (Violle et al. 2007) and facilitates recognition of
generalities in species responses to spatial or temporal variation therein (McGill et al. 2006). The functional approach
has been mainly developed by plant ecologists, and some
recent studies have successfully used functional traits to
explain how animals respond to disturbance (Moretti et al.
2009; Langlands et al. 2011). However, the importance of
functional traits for predicting responses of detritivore
communities to changes in either macro- or micro-climate
has not yet been studied.
Climate change is expected to increase the frequency and
intensity of both drought periods and extreme precipitation
events (IPCC 2008), subjecting detritivore communities to a
greater temporal variability of soil water availability. This
variability is expected to have a strong impact on soil communities as both drought and flooding are considered to be
important stress factors for detritivores (Plum 2005; David
and Handa 2010). Here, we focus on resistance to desiccation, which varies greatly among macro-detritivore species
(Haacker 1968; White and Zar 1968). Desiccation resistance
measured under standard laboratory conditions has been
suggested to be a useful trait for predicting detritivore
responses to decreases in water availability, since it can
explain abundance patterns in dry habitats (David and Handa
2010) and microhabitat preferences (Edney 1951; White and
Zar 1968). However, progress in using this trait from published studies has been hampered by the lack of comparative
studies and standard protocols for measuring desiccation
resistance, as well as by a poor understanding of the
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mechanisms that underpin interspecific variation. Identifying and quantifying the functional traits that underlie interspecific differences in desiccation resistance will enable
more general inferences about the response of detritivore
species to changes in water availability. In the study reported
here, we used terrestrial isopods as a model system to
investigate whether desiccation resistance and the traits
underpinning this resistance can explain species distribution
patterns along moisture gradients.
Terrestrial isopods are important members of detritivore
communities, with more than 3,500 species distributed
worldwide (Schmalfuss 2003). Over evolutionary time, the
colonization of contrasting terrestrial habitats in which soil
water regimes range from semi-aquatic to desert conditions
has led to considerable variation among isopod species in
desiccation resistance (Warburg 1993; Schmalfuss 2003).
However, little is known about which traits are strong
candidates as causal drivers of desiccation resistance. Data
are currently available for only a few species (Warburg
1993), and there have been even fewer explicit species
comparisons (Edney 1951; White and Zar 1968). A study
by Tsai et al. (1998) on two Ligia species demonstrated
that intraspecific variation in desiccation resistance for the
two species was explained primarily by water loss rate,
which in turn is determined by body size (Fig. 1a). Fatal
water loss (i.e. maximum water loss tolerance) also
explained part of the variation in desiccation resistance but
was independent of body size. In our study we asked
whether these traits also explain interspecific differences in
desiccation resistance. We specifically tested the following
hypotheses: (1) interspecific variation in desiccation resistance is explained by underlying traits such as body surface
area, water loss rate and fatal water loss (as shown in the
path model in Fig. 1a), (2) the results of this model do not
change qualitatively when measurements are made at
contrasting water availabilities or when phylogenetic
relatedness effects are excluded and (3) desiccation resistance and the traits underlying this resistance can explain
species distribution over soil moisture gradients. To test
these hypotheses, we measured desiccation resistance and
hypothesized underlying traits for 22 dominant northwestern European terrestrial isopod species belonging to
nine different families. We then used our trait measurements to explain species distribution patterns in relation to
site moisture level using a previously published database.
Materials and methods
Species collection and traits measurements
Adult individuals of 22 terrestrial isopod species (suborder
Oniscidea) from nine families naturally occurring in the
Oecologia (2013) 172:667–677
669
(a)
A
WL
R
F
(b)
A
-0.89**
WL
-0.93**
2
R
R2=0.80
X = 1.13; P = 0.77
GFI = 0.97
R2=0.90 AGFI = 0.91
0.15*
F
(c)
2
-0.53**
A
WL
-0.67**
R2=0.28
R
R2=0.57
X = 1.29; P = 0.73
GFI = 0.97
AGFI = 0.89
0.36*
F
Fig. 1 Path diagrams showing: a hypothesized causal relationships
between traits explaining interspecific variability in desiccation
resistance in terrestrial isopods (based on Tsai et al. 1998), b fitted
model with species trait attributes and c with phylogenetically
independent contrasts. A Surface area, WL water loss rate, F fatal
water loss, R desiccation resistance. Trait attributes were logtransformed to improve normality and linearity. Path coefficients,
goodness-of-fit index (GFI) and adjusted goodness-of-fit index
(AGFI) are shown for each model (*P \ 0.05, **P \ 0.01). Direct
effect of surface area on desiccation resistance was not significant
whether species trait attributes or phylogenetically independent
contrasts were used. See ESM 6 for alternative models including
the direct effect of surface area
Netherlands, comprising 71 % of the 30 indigenous Dutch
species (Berg et al. 2008), were collected. The collected
individuals were kept in plastic pots containing moist
plaster of Paris and litter from the site of origin. Animals
were stored in a climate room maintained at 15 °C, 75 %
air relative humidity (RH) and 12:12 light:dark ratio for at
least 1 week and a maximum of 6 weeks before starting the
measurements, allowing them to acclimate to the experimental conditions. Because different species were collected
during five field trips to distinct locations from March to
September 2010 [see Electronic Supplementary Material
(ESM) 1], we used site controls to evaluate the degree to
which the variation in trait values was due to betweenspecies differences versus within-species differences
among sites. For this purpose, we collected the most
abundant species, Porcellio scaber, in each site as a site
control. We also collected P. scaber from the garden of the
Hortus Botanicus at VU University in Amsterdam
(52:33°N, 4:86°E) on every field collecting day to serve as
a time control. We are aware that using only one species as
a control could influence conclusions about trait variability
across sites and time because this partly depends on which
species we select, but P. scaber was the only species that
occurred at sufficient densities in all sites visited.
To standardize the initial conditions before measuring
desiccation resistance, individuals from each species were
kept isolated in small cylinders (diameter 2 cm, height
3 cm) with moist plaster of Paris in the bottom and no food
for 3 days. These individual cylinders were placed in a
closed glass box (20 9 30 9 25 cm) on top of a of 10-cm
layer of moist sand that covered a water-saturated plaster of
Paris element, ensuring constant humid conditions close to
100 % RH. This procedure allowed the animals to
replenish any possible water deficit, so measurements
started when they had approximately the maximal possible
body water content. During this period, animals also
evacuated most of their gut content which prevented defecation during the measurements and therefore changes in
mass not related to water loss. This starvation period did
not affect the survival of the isopods. For the more abundant species, two or three individuals were kept in the glass
box (approximately 100 % RH) after the starvation period;
none of these animals died before the end of the desiccation
resistance measurements.
Isopods were exposed to relatively dry but realistic
conditions, i.e. 85 % RH, to record their water loss rate,
fatal water loss and survival time. This humidity level was
chosen to represent a moderate stress condition, just below
the threshold above which isopods are able to absorb water
vapour (91–93 % RH; Wright and Machin 1990). The
humidity level was acquired using a glycerol–water solution with a specific gravity of 1.11 or 42 % volume (White
and Zar 1968). Falcon tubes (50 ml) were filled with 20 ml
of glycerol solution. In each tube, a platform made of
plastic mesh (width 2 mm) was placed about 1 cm above
the solution surface; the chamber containing the animal to
be measured was then placed on this platform. The bottom
of the plastic open-top chambers was made of mesh (width
0.5 mm), allowing airflow between the animal and the
glycerol–water solution. Before we started the measurements, the chambers were acclimatized inside the Falcon
tubes with the glycerol–water solution overnight to ensure
no changes of mass during the experiment due to the
adherence of water on the chamber walls.
We measured 6–13 individuals per species depending on
the number of animals collected in the field. At the start of
each measurement, we placed a single individual in the
open-top chamber to record their initial fresh mass (to the
nearest 1 lg; model Supermicro; Sartorius AG, Göttingen,
Germany) and immediately returned the chamber with the
animal to the Falcon tube. This procedure allowed us to
record changes in the isopod’s mass with minimal
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disturbance. We re-weighed the isopods at regular intervals, the length of which varied from 15 min to 3 h
depending on the species. Before weighing the isopods, we
checked if they were alive. Using tweezers, we gently
flipped the isopods onto their back. Individuals that were
unable to flip back to the normal position and did not
present leg and antenna movements were considered to be
dead. All animals were followed until they died. Dead
isopods were weighed and immediately frozen at -20 °C
for further analyses. A schematic overview of the experimental setup can be found in the ESM 2.
The average survival time (hours) was used as an estimate of desiccation resistance (i.e. the capacity of an
organism to withstand dry conditions). Water loss rate was
calculated by the slope of the linear regression between
water mass and time and was expressed as the proportion
of initial water content that was lost per hour. The relationship between water mass and time was linear for all
species with R2 varying from 0.75 to 0.99. Fatal water loss
was expressed as the proportion of the initial water content
that was lost at the time of death (see ESM 3 for details on
the calculation of traits). For individuals that died overnight, we used the median of the values of the last measurement in the afternoon and the first in the morning to
calculate the above-mentioned traits.
To measure body size, we removed isopods from the
freezer and left them to thaw for 5 min, after which we
took pictures of their ventral surface using a stereomicroscope (Leica model Wild M8; Leica Microsystems,
Wetzlar, Germany) equipped with a digital camera (Leica
model DC 200). We measured the length and width (to the
nearest 0.1 mm) of each individual using the software Axio
Vision 4.8 and calculated the ventral surface area using the
formula of the ellipse area. We are aware that this measurement does not encompass the total surface area where
transpiration occurs, but it does cover the largest part of the
transpiring surface (Warburg 1993). Moreover, as the
morphology of isopods is relatively similar among species,
this measurement should be a good surrogate of the ‘‘real’’
surface area. The high power of this variable on explaining
water loss rate (see ‘‘Results’’) confirms that. We then
oven-dried the isopods at 60 °C until constant mass and
recorded the dry mass.
Additionally, to test if our results were consistent under
different experimental conditions, we compared water loss
rate values measured at 85 and 36 % RH. The measurements at more extreme drought conditions, 36 % RH,
15 °C and 12:12 light:dark ratio had been performed in
2005 for 24 species comprising 20 species in common with
the sampling in 2010. Most of the species had been collected from the same sites in 2005 as in 2010 (ESM 1). The
measurements under 36 % RH had followed the same
protocol as described above, except that we carried out the
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measurements only until the animals lost about 30 % of the
initial water content. This resulted in much faster experiments (\1 day).
Species distribution
We searched the literature for data on the distribution of
terrestrial isopod species that fulfilled two sampling criteria: (1) sampling should not be biased toward a given group
of species (e.g. by using pitfalls), and (2) sampling should
provide information on water availability for the species
records. We found only one data set with such characteristics comprising species in common with our own data set.
Vilisics et al. (2007) compiled information on 126 site
records describing isopod communities in nine major
geographic regions across Hungary. Field sampling was
carried out by direct search (20–40 min per site) where
species presences were recorded. Sampling sites were
classified into three moisture levels—dry, moist and wet.
Sites were assigned to these categories according to the
authors’ expert knowledge of the local environmental
conditions. We are aware that this is only an approximate
categorization of water availability. However, we believe
that the classification of the sites into three levels of water
availability should provide sufficient resolution to test
whether this environmental factor can filter species
according to their trait values. Future studies, with more
detailed measurements of water availability, could improve
this approach through providing a more complete
description of the relationship between water availability
and traits underlying desiccation resistance.
Statistical analyses
One-way analysis of variance (ANOVA) was used to test
for interspecific differences in surface area, water loss rate,
fatal water loss and desiccation resistance. Box–Cox
transformation was performed using the car package in R
2.12 to improve data normality and homogeneity of variances. Because the control populations of P. scaber and
other pilot experiments did not show any difference
between males and females for the traits that we considered, sex was not considered as a factor in the statistical
analyses.
We used path analysis to test the hypothesis about the
causal relationships between species traits. We hypothesized that desiccation resistance is determined by water
loss rate and fatal water loss and that water loss rate is
determined by surface area (Fig. 1a). We also tested an
alternative model that included a direct effect of surface
area on desiccation resistance (i.e. due to mechanisms not
specified in the model). Maximum likelihood was used to
estimate path coefficients in R 2.0.1 and the SEM package.
Oecologia (2013) 172:667–677
The chi-square statistic (v2) was used to test if the observed
covariance matrix differed from that predicted by the
model. A significant difference between matrices indicates
a low probability that the observed data was generated by
the hypothesized causal model and the model should
therefore be rejected. We used the goodness-of-fit index
(GFI) and adjusted goodness-of-fit index (AGFI) to evaluate the fit between observed covariance matrix and that
predicted by the models. We considered values of [0.9 to
be a very good fit and values of C0.8 to be a good fit.
To test whether relationships between traits were consistent when phylogenetic effects were excluded from the
analysis, we constructed a phylogenetic tree based on 18S
rRNA and used phylogenetically independent contrasts
(Felsenstein 1985) to re-run the same models as described
above. The final phylogeny, the details on the molecular
work and on how the tree was constructed can be found in
ESM 4. We first tested for the phylogenetic signal of the traits
by comparing the variance of the standardized contrasts
(VarCont) in the phylogeny with the variance of contrasts
generated by a randomization test (i.e. trait values swapped
across the tips of the tree; Blomberg and Garland 2002;
Webb et al. 2008). We then calculated phylogenetically
independent contrasts (PIC) for the four studied traits and reran the path models. Since our phylogeny was not completely
resolved we followed the approach described by Pagel
(1992) to deal with polytomies. The phylogenetic signal and
PIC were calculated using the software Phylocom (Webb
et al. 2008).
Weighted least square regressions were used to investigate the relationship between the data on traits derived from
our measurements and the species distribution data across
the three moisture levels derived from Vilisics et al. (2007).
Within each of the moisture levels we calculated the relative
frequency of each species present, i.e. the number of
occurrences of a given species divided by the total number of
occurrences for all species within that moisture level. This
represents how frequently each species (and its respective
trait values) is observed within each level. We used this
relative frequency to weigh the regression between trait
values and moisture level. To evaluate which part of the
gradient had filtered the trait values, we also used correlation
analysis to test for relationships between trait values and the
frequency of species across the moisture levels, where frequency at each moisture level was calculated as the number
of occurrences of a given species at that level divided by its
total number of occurrences at all three levels. This provides
a relative measure of how often a given species is found at
each moisture level. Using species as replicates, we then
determined the Pearson correlation coefficient between
species trait values and species relative frequency for each of
the moisture classes. Correlation was tested by re-sampling
techniques.
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Results
Ventral surface area, water loss rate, desiccation resistance
and fatal water loss differed significantly among species
(ANOVA, F21,200 = 295, P \ 0.001; F21,200 = 226,
P \ 0.001; F21,200 = 151, P \ 0.001; F21,200 = 2.88,
P \ 0.001, respectively; Fig. 2). The first three abovementioned traits showed great variation among species,
while fatal water loss showed a much lower variation with
values of around 0.47 ± 0.05 [mean ± standard deviation
(SD), proportion] of the initial water content. The controls
also showed significant differences among samples in
surface area and water loss rate, but not in desiccation
resistance or fatal water loss (ANOVA, F8,76 = 19.9, P \
0.001; F8,67 = 2.28, P = 0.03; F8,67 = 1.31, P = 0.26;
F8,67 = 0.16, P = 0.99, respectively; see ESM 5).
Although significant, differences among controls in surface
area and water loss rate showed much lower magnitude
than the differences between species. These two traits
showed a variation of one order of magnitude among
species (Fig. 2), while among control populations, surface
area varied only from 11.0 to 38.6 mm2 and water loss rate
varied from 0.007 to 0.012 mg mg-1 h-1. These results
indicate that the interspecific variation in these isopods is
substantially greater than intraspecific variability.
Our path model showed a strong fit between observed
data and model predictions (Fig. 1b), corroborating the
hypothesis that interspecific variation in desiccation resistance is explained by water loss rate and fatal water loss,
with water loss rate as the main driver of desiccation
resistance. The accepted model explained 90 and 80 % of
the variation in desiccation resistance and water loss rate,
respectively. The direct effect of surface area on desiccation resistance was not statistically significant (P = 0.80;
see ESM 6 for alternative model), indicating that surface
area affects desiccation resistance only indirectly, via
changes in water loss rate. The structure of the model
indicates that it is possible to use water loss rate and,
indirectly, surface area as surrogates for desiccation resistance in isopods.
All of the studied traits showed a significant phylogenetic signal (surface area: VarCont = 2.675, P = 0.002;
water loss rate: VarCont = 0.006, P \ 0.001; desiccation
resistance: VarCont = 3.002, P = 0.001; fatal water loss:
VarCont = 0.009, P = 0.007). However, the path model
using PIC for traits also showed a strong fit between the
observed data and model predictions (Fig. 1c), indicating
that the hypothesized relationships between traits are not
only caused by a shared evolutionary history. However, the
model using traits’ PICs explained a lower percentage of
the variation of desiccation resistance and water loss rate:
57 and 28 %, respectively. Again, the direct effect of
surface area on desiccation resistance was not statistically
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Oecologia (2013) 172:667–677
Water loss rate
(mg mg hr-1)
Water loss rate
(mg mg-1 hr-1)
Surface area (m
mm2)
672
Relative humidity (%)
a oceanica
Ligia
Oniscus asellus
adillidium vulgare
Arma
Porccellio spinicornis
Porccellio scaber
helipus rathkii
Trach
adillidium opacum
Arma
ma caelatum
Elum
adillidium pictum
Arma
oscia muscorum
Philo
dium hypnorum
Ligid
cellium conspersum
Porc
adillidium pulchellum
Arma
adillidium album
Arma
Andrroniscus dentiger
yarthrus hoffmannseggi
Platy
honiscus pusillus
Trich
Hapllophthalmus danicus
oniscus patiencei
Mikto
honiscoides sarsi
Trich
honiscoides albidus
Trich
atrichoniscoides leydigii
Meta
Fatal water loss
(proportion)
Desiccation resiistance (hr)
Fig. 3 Reaction norm of water loss rate measured at 36 and 85 % RH
(both at 15 °C) for 20 isopod species. Lines link the average values of
water loss rate for the same species measured at different relative
humidities
Fig. 2 Trait attributes [mean ± standard deviation (SD, bars)] for 22
terrestrial isopod species. The four traits analysed are: ventral surface
area, water loss rate as the proportion of initial water content lost per
hour, desiccation resistance at 85 % relative humidity (RH) and fatal
water loss as the proportion of initial water content that was lost at the
time of death. Species are sorted by surface area
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significant (P = 0.54; see ESM 6 for an alternative model).
More details on the results of the phylogenetic analyses can
be found in ESM 4.
All species showed increased water loss rate at 36 %
RH when compared to measurements made at 85 % RH
(values are given in ESM 1) with few changes in the
rank order of species (i.e. few lines crossing in
Fig. 3). This is also indicated by a high correlation
between the two measurements (rSpearman = 0.94; n = 20;
P \ 0.001).
The relationship between desiccation resistance and
both water loss rate and surface area were best described by
a power function (Fig. 4). The very good fit of the equation
for water loss rate indicates that this is a better surrogate of
desiccation resistance than surface area. The residuals of
the equation for surface area were not similar along the
observed range of surface area values. Residuals are particularly high for species larger than 10 mm2. This increase
in residuals can be a consequence of the way surface area
was measured because the error of the measurement is
likely to increase with the square root of the length and
width measurements.
Our data set had 11 species in common with Vilisics
et al. (2007) for the measurements made under 36 % RH
and eight for the measurements under 85 % RH; we
therefore present the results using traits measured under
36 % RH. Site moisture level showed a positive
Fig. 4 Power functions
describing the relationship of
desiccation resistance to water
loss rate (F1,21 = 2,207,
P \ 0.001) and body surface
area (F1,21 = 68.8, P \ 0.001)
among 22 terrestrial isopod
species, with each point
representing the average value
for a single species
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Desiccation resistance (hr)
Oecologia (2013) 172:667–677
y = 0.415 x
-1.043
2
R = 0.99
y = 1.629 x
0.997
2
R = 0.75
Surface area
Water loss rate
(mm2)
(mg mg-1 hr-1)
Discussion
This is the first study to comprehensively reveal the functional traits underlying macro-detritivore responses to
in situ soil water availability by showing relationships
between standardized measurements of morpho-physiological traits and species distribution patterns across a soil
moisture gradient. We revealed that differences in body
water loss rate serves as the main mechanism behind
interspecific variation in isopod desiccation resistance. Our
findings contribute to the increasing need to generate
empirically tested, mechanistic predictions on the effect of
global changes, especially impacts of extreme weather
events, on functionally important organisms such as
detritivores.
Functional traits underpinning desiccation resistance
The physiological ability of an organism to show resistance
to unfavourable conditions can be determined by physiological avoidance and tolerance strategies. In our study,
Log10(1 + water loss rate)
0.15
Log10(1 + surface area)
relationship with water loss rate (F1,24 = 6.8, P = 0.015,
R2 = 0.19) and a negative relationship with body surface
area (F1,24 = 5.0, P = 0.035, R2 = 0.14) but only when
weighting for relative frequency within moisture levels
was performed (Fig. 5, solid lines). Moreover, species
water loss rate was negatively correlated with frequency
on dry sites (r = -0.52, P = 0.039) and positively with
frequency on wet sites (r = 0.83, P = 0.002). While
species body surface area was negatively correlated with
frequency in wet sites (r = -0.67, P = 0.021), frequency
in moist sites did not show a significant correlation with
any of the traits. The traits measured at 85 % RH showed
the same patterns described above, although relationships
were not significant (ESM 7), possibly due to the lower
number of species with available data.
a
b
0.10
0.05
1.5
1.0
0.5
b
a
Dry
Moist
Wet
Fig. 5 Relationship between site moisture level and two isopod traits,
i.e. water loss rate and body surface area. The Y-axis value for each
point represents the average trait value for a single species. The area
of the symbols is proportional to the relative frequency of the species
at a given moisture level (i.e. number of occurrences of the species at
a moisture level divided by the total number of occurrences at this
level). Relationships between traits and moisture levels were significant when weighting traits by the relative frequency of species (solid
lines), but not when simple regression was used (dashed lines).
Letters indicate two species that are specifically mentioned in the
‘‘Discussion’’: a Haplophthalmus mengii, b H. danicus
water loss rate (avoidance) and fatal water loss (tolerance)
explained 90 % of the interspecific variation in desiccation
resistance, with water loss rate being the main factor
explaining such variation. The low interspecific variation in
fatal water loss found here is a common pattern in
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Oecologia (2013) 172:667–677
arthropods in general, for example, 40–54 % for ants
(Hood and Tschinkel 1990) and 42–52 % for weevils
(Chown 1993). This low interspecific variation also corroborates the notion that morpho-physiological avoidance
strategies (i.e. adaptations that influence water loss rate) are
the main mechanism determining desiccation resistance
among terrestrial arthropods (Villani et al. 1999). However,
physiological tolerance might play an important role in
determining differences in desiccation resistance between
soil invertebrate taxa. Earthworms, for example, can show
much higher values for fatal water loss (70–75 %; Roots
1956). Therefore, future studies should quantify the relative importance of water loss rate and fatal water loss in
determining differences in desiccation resistance among
contrasting detritivore taxa.
Body size affected desiccation resistance only indirectly—through changes in the water loss rate. The surface
area to volume ratio, which varies allometrically with body
size, has been suggested as an important factor influencing
soil arthropod water balances (Villani et al. 1999). The
cuticle of isopods is more permeable than that of most terrestrial arthropods, and transpiration across the integument,
especially across the ventral surface, is a major source of
water loss for isopods even at relatively high humidity
(Warburg 1989, 1993). We observed that the water loss rate
was highly constant over time for all species at both values of
RH, both before and after the animals died (ESM 3). This
observations suggests that water loss is a passive process in
terrestrial isopods and that a larger body size could therefore
contribute significantly to reduce water loss rate simply by
reducing the surface area to volume ratio.
Although body size explained 80 % of the variation in
water loss rate, other morphological adaptations are also
important to reduce water loss rate. For instance, Ligidium
hypnorum and Androniscus dentiger, two species without
pleopodal lungs, have much higher water loss rates and,
consequently, lower desiccation resistance than species
with pleopodal lungs and similar body size (such as Armadillidium album and A. pulchellum; Fig. 2). The evolution of pleopodal lungs was an important step in the
colonization of terrestrial habitats (Schmidt and Wägele
2001) because the pleopods are the main site of water loss
(Warburg 1993). As different adaptations can affect water
loss rate, a considerable amount of variation in this trait
cannot be explained by body size alone. Consequently, the
use of body size as an indirect surrogate to desiccation
resistance can lead to erroneous conclusions, especially
when species of similar size are compared.
negligible compared to the interspecific variability, leading
to consistent and meaningful species ranking. However,
this assumption has rarely been tested (Albert et al. 2010;
Hulshof and Swenson 2010). Although our site control
populations of P. scaber showed significant differences in
surface area and water loss rate, the magnitude of this
intraspecific variability was much lower than that of the
interspecific variation. Also, the tight correspondence of
species ranking based on measurements of water loss rate
made in 2005 and in 2010 indicates that species ranking
was robust when different populations were sampled.
However, intraspecific variability should not be neglected.
We were able to explain differences between species using
the same traits as those used by Tsai et al. (1998) to explain
intraspecific variability for two Ligia species. This result
suggests that water loss rate can be used as a surrogate for
desiccation resistance at the individual level, irrespective
of the species under study. The variability between individuals of the same species is essential to the process of
natural selection, and whether soil invertebrates are able to
adapt to climate and land-use change remains largely
untested. The few studies investigating the presence of
adaptive variation in desiccation resistance for soil invertebrates provide results that are both positive (Holmstrup
and Loeschcke 2003; Bahrndorff et al. 2006) and negative
(Maraldo et al. 2008; Maraldo et al. 2009). More detailed
studies on the intraspecific variability of desiccation
resistance could reveal its potential importance to the
adaptation of detritivores to environmental changes.
Trait variation among versus within species
Our results show that species traits measured under standardized laboratory conditions can be used to predict desiccation resistance at different experimental or natural
conditions. The fact that traits measured in individuals
When working with species average trait values, it is
generally assumed that the intraspecific variability is
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Phylogenetic signal
We showed that desiccation resistance has a strong phylogenetic signal that reflects the evolutionary history of
colonization of terrestrial habitats, with ancestral species
showing lower desiccation resistance and derived species a
higher resistance. All other functional traits that we measured also showed strong phylogenetic signals. However,
the acceptance of the path model using phylogenetically
independent contrasts indicates that the relationships
between the traits could not be explained by phylogeny
alone, suggesting that these relationships do not only reflect
ancient divergences (as revealed through a strong phylogenetic signal) but have evolved repeatedly in the course of
phylogenetic history as a result of convergent evolution.
Applying the functional approach to macro-detritivore
communities
Oecologia (2013) 172:667–677
collected in The Netherlands could explain the distribution
pattern of species in Hungary indicates that this approach
can be applied to broad spatial scales. Animal ecologists
have applied habitat preferences as surrogates for traits to
explain species distributions (e.g. Purse et al. 2012).
However, this approach has been challenged as species
interactions can lead to a mismatch between species
occurrences and their physiological optima (Ellers et al.
2010). We argue that the use of morpho-physiological
traits, as we adopted here, is a better approach as it provides a direct, mechanistic link between organism performance and environmental conditions.
Still, a part of the variation of species distribution could
not be explained by traits underpinning desiccation resistance, possibly due to two main reasons. First, the data set
is based on species presence and absence only. Environmental filters can have a stronger impact on the relative
abundance than on species presence because even under
unfavourable conditions species may persist with low
abundances. Second, other factors, which may or may not
be related to soil water availability, can affect species
distributions. These limitations can be exemplified by
Haplophthalmus mengii, the species showing lower surface
area and higher water loss rate in Fig. 5. This species was
found at a low frequency at the three moisture levels, but it
is expected to have higher abundances on wet sites through
being hygrophilous (Berg et al. 2008; Gregory 2009). In
this case, adding data on relative abundance of species
could improve the explanatory power of the studied traits.
Additionally, this species inhabits clayish soils with high
water holding capacity (Berg et al. 2008; Gregory 2009),
which can buffer variations in water availability. In contrast, the congeneric species H. danicus typically inhabits
soils with a high organic matter content that are moist but
well drained (Berg et al. 2008; Gregory 2009), and it was
only present in sites classified as moist and wet (Fig. 5).
Determining how soil type preference is related to water
availability or other requirements (i.e. nutrients, shelter)
will be an important step towards a better understanding of
detritivore species distribution.
Another potential problem with using pure physiological
traits to explain animal responses to environmental conditions is that changes in behaviour can obscure relationships
between traits and the environment. For example, isopods
with high water loss rates and small body size are able to
move deeper into the soil through small cracks, thereby
minimizing the risk of exposure to dry conditions. However, when changes in water availability regimes are strong
enough to reduce the availability of suitable refuges, our
results indicate that macro-detritivore communities differ
considerably in terms of desiccation resistance.
Recently, functional traits have been successfully used
to explain community structure and species responses to
675
environmental change for different animal groups (Moretti
et al. 2009; Schamp et al. 2010; Langlands et al. 2011;
Makkonen et al. 2011; Wiescher et al. 2012). However,
there is still no consensus on what the most important traits
are and what is the best way to measure them. Standardization of trait measurements is a pressing issue for the full
implementation of a functional trait approach. For example, the standardization of measurements of plant functional traits (Cornelissen et al. 2003) has enabled analyses
of trait relationships on broad spatial scales (Diaz et al.
2004; Wright et al. 2004). Nowadays, trait values are
available in several plant databases, including a huge global database (Kattge et al. 2011). However, animal ecologists have not yet reached a similar agreement on standard
protocols for trait measurements. For soil fauna, the
experimental conditions that we used (85 % RH, 15 °C)
represent a moderate stress condition in the temperate
region of Europe (David and Vannier 2001; Holmstrup and
Loeschcke 2003; Kaersgaard et al. 2004). We suggest that
this experimental set could be applied to a vast number of
groups of soil organisms, including similar-sized animals,
such as millipedes, as well as very small animals, such as
springtails and mites.
Ecological implications and extensions of this study
The differences in desiccation resistance between terrestrial isopod species have important ecological implications.
The negative relationship between site moisture level and
body size suggests that a reduction in soil water availability
will probably filter out—or at least reduce the relative
abundance of—small species. It is still not clear if and how
the variation in desiccation resistance and body size is
related to the effects of isopods on soil processes. The mass
ratio hypothesis (Grime 1998) states that the effect of a
given species on ecosystem processes is proportional to its
relative abundance in the community and has received
increasing empirical support (Diaz et al. 2007; Quested
et al. 2007; Mokany et al. 2008), although exceptions have
also been reported (Wardle et al. 2008; Peltzer et al. 2009).
Therefore, changes in community functional composition
due to species filtering can potentially have strong impacts
on ecosystem processes. If desiccation resistance is correlated with the effects of isopod species on soil processes,
the directional changes in desiccation resistance of the
isopod community due to environmental filtering is
expected to have consequences for ecosystem processes
(Suding et al. 2008; Webb et al. 2010). Nonetheless, the
loss of species with particular trait attributes will decrease
functional diversity with possible detrimental effects on
ecosystem functions (Heemsbergen et al. 2004).
Here, we identified traits related to the response of
isopods to decrease in soil water availability. Future studies
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676
should investigate whether our findings for isopods are
consistent with those of other detritivore groups and whether water loss rate can be used to predict species desiccation resistance more generally across detritivore groups.
The main challenge, however, will be to test the importance of such responses for driving species composition
and its consequent impact on soil organic matter turnover
processes.
Acknowledgments We thank Rudo Verweij for assistance in the
laboratory, Herman Verhoef for constructive discussion and two
anonymous reviewers for constructive comments and suggestions.
A.T.C. Dias was financed by NWO postdoctoral grant no. NWO/
819.01.017.
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