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As the need to better understand the ecology of hotspots of endemism intensifies, the insurance hypothesis is drawing increasing attention from policy-makers and scenario-planners. The hypothesis states that biodiversity increases ecosystem stability. When species numbers fluctuate, there is potential for further perturbation, loss of function and increased opportunity for invasive species to fill vacated niches. Southern Africa is predicted to be disproportionately impacted by global change, and high altitude systems as foci of endemism are particularly vulnerable to warming. Using ants, a group key to ecosystem function, we assess effects of temperature, season, aspect, vegetation and soil conditions on montane ant species richness, stability of ant community composition, and stability of ant species richness across an altitude gradient. Over six consecutive years of bi-annual sampling, we gathered one of the largest standardized data sets to date. We showed for the first time that stability of ant species richness decreases with increasing altitude, whilst compositional similarity of ant communities is higher with increasing altitude. Findings reveal more similar, species-poor, less stable ant communities at high altitude at the same sites over time.
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Original Research Article
Stability of Afromontane ant diversity decreases across an
elevation gradient
Grant S. Joseph
, Mulalo M. Muluvhahothe
, Colleen L. Seymour
Thinandavha C. Munyai
, Tom R. Bishop
, Stefan H. Foord
SARChI-Chair on Biodiversity Value and Change, Department of Zoology, School of Mathematical and Natural Science, University of
Venda, Private Bag X5050, Thohoyandou, 0950, South Africa
Percy FitzPatrick Institute of African Ornithology, DST/NRF Centre of Excellence, Department of Biological Sciences, University of Cape
Town, Rondebosch, 7701, South Africa
South African National Biodiversity Institute, Kirstenbosch Research Centre, Private Bag X7, Claremont, 7735, South Africa
School of Life Science, College of Agriculture, Engineering and Science, University of KwaZulu-Natal, Private Bag X01, Scottsville, 3209,
South Africa
Department of Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool, UK
Department of Zoology &Entomology, University of Pretoria, Pretoria, 0002, South Africa
article info
Article history:
Received 24 November 2018
Received in revised form 11 March 2019
Accepted 11 March 2019
Ant compositional similarity
Ant species richness
Climate change
Ecological community thresholds
Altitude gradient
Insurance hypothesis
As the need to better understand the ecology of hotspots of endemism intensies, the
insurance hypothesis is drawing increasing attention from policy-makers and scenario-
planners. The hypothesis states that biodiversity increases ecosystem stability. When
species numbers uctuate, there is potential for further perturbation, loss of function and
increased opportunity for invasive species to ll vacated niches. Southern Africa is pre-
dicted to be disproportionately impacted by global change, and high altitude systems as
foci of endemism are particularly vulnerable to warming. Using ants, a group key to
ecosystem function, we assess effects of temperature, season, aspect, vegetation and soil
conditions on montane ant species richness, stability of ant community composition, and
stability of ant species richness across an altitude gradient. Over six consecutive years of
bi-annual sampling, we gathered one of the largest standardized data sets to date. We
showed for the rst time that stability of ant species richness decreases with increasing
altitude, whilst compositional similarity of ant communities is higher with increasing
altitude. Findings reveal more similar, species-poor, less stable ant communities at high
altitude at the same sites over time.
©2019 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND
license (
1. Introduction
Global anthropogenic change is shifting distributions of vertebrate and invertebrate assemblages at a range of scales, and
the nal outcome is likely to be deleterious to biodiversity and ecosystem function (Joseph et al., 2018a., Davis and Vincent,
*Corresponding author. University Of Cape Town, FitzPatrick Institute of African Ornithology, DST/NRF Centre of Excellence, Cape Town, 7701, South
E-mail addresses: (G.S. Joseph), (M.M. Muluvhahothe), (C.L. Seymour), caswell. (T.C. Munyai), (T.R. Bishop), (S.H. Foord).
Contents lists available at ScienceDirect
Global Ecology and Conservation
journal homepage:
2351-9894/©2019 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (
Global Ecology and Conservation 17 (2019) e00596
2017;Duffy et al., 2015;García-Palacios et al., 2018). Ecological stability is therefore important, and has become a focal topic in
ecological circles, as the impacts of global change to ecosystem stability through altered species richness and composition
patterns become better appreciated (García-Palacios et al., 2018;Hautier et al., 2015).
The insurance hypothesis postulates that species richness can increase ecosystem stability. The hypothesis has received
wide research attention (Doak et al., 1998;García-Palacios et al., 2018;Isbell et al., 2009;Lehman and Tilman, 2000;Loreau
et al., 2001;Pennekamp et al., 2018;Yachi and Loreau, 1999), and is emerging as a key issue politically, as impacts to
biodiversity threaten ecosystem services and mutualistic networks (e.g., Intergovernmental Science-Policy Platform on
Biodiversity and Ecosystem Services; Isbell et al., 2017;Simba et al., 2018). Stability, the inverse of variation over time, has long
been recognised as a fundamental ecosystem property that offers insights into biodiversity and ecosystem processes (Doak
et al., 1998). The insurance hypothesis posits that species respond to environmental perturbations in different ways, uc-
tuating in their abundance and contribution to ecosystem functioning under different conditions. In a system with high
species richness, these different species responses are more likely to complement each other, conferring overall stability, than
in a low species richness system. Species richness thus acts as a buffer against environmental perturbation (García-Palacios
et al., 2018;Loreau et al., 2001;Tilman, 1999).
Mountain-inhabiting ectotherm assemblages across elevational gradients can be particularity vulnerable (García-Robledo
et al., 2016), as the exibility of adaptive traits that inuence organism tness and ecosystem function become tested by the
rapid changes in thermal regimes that are predicted to occur in these regions (Modiba et al., 2017;Petchey and Gaston, 2006;
Salas-Lopez et al., 2017). Mountain-dwelling communities need to adapt to specic temperature niches, microhabitats and
soils. Furthermore, landscape heterogeneity means that montane assemblages can be effectively isolated and surrounded by
an ecological matrix to which they are not well adapted (Bishop et al., 2017;Suggitt et al., 2011). The modication of cooler
microclimates (which can enhance ectotherm persistence; Duffy et al., 2015;G.S.Joseph et al., 2016), and microhabitat loss
(e.g. loss of shade provided by vegetation through herbivory; G. Joseph et al., 2018b;Minor et al., 2016;Suggitt et al., 2011) are
anticipated to impact community composition, species richness, and increase the likelihood of localised extinctions (Stocker
et al., 2013;Thomas et al., 2004).
As anthropogenic impacts to ecological systems intensify, identifying patterns and conditions that affect biodiversity over
time become increasingly relevant. Research in arid systems has revealed that climate can modulate the relationships be-
tween diversity and ecosystem stability, leading to climate dependency of the biodiversityeecosystem stability relationship,
and that species richness can play an important stabilizing role with increasing aridity (García-Palacios et al., 2018;
Pennekamp et al., 2018). Despite ample evidence of the biodiversity-stability relationship (García-Palacios et al., 2018;Loreau
et al., 2001;Pennekamp et al., 2018), and that global change is impacting ecosystem stability (Donohue et al., 2016), predictive
modelling efforts addressing ecosystem stability at regional and global scales require studies conducted over protracted
timescales (García-Palacios et al., 2018). For the most part, such studies do not exist.
Insects, amongst the most species-rich and functionally important of animals, have evolved specic traits and thermal
tolerances to suit their habitat, impacting distribution, tness and ultimately functioning of ecosystems dependent on their
services (Wilson, 1987). Amongst insects, ants (Hymenoptera: Formicidae) are critical to ecosystem processes at a range of
scales, and are indicators of environmental change (Tiede et al., 2017). Recent studies highlight that ant species richness can
decrease linearly, or with mid-elevational peaks, as altitude increases and temperatures decrease (Bishop et al., 2014;Yusah
et al., 2012). Despite the central role of ants to ecological systems, and the vulnerability of isolated montane communities to
environmental perturbation, few studies examine temporal trends in biodiversity, and to our knowledge no study has to date
addressed the stability of invertebrate diversity over time across an elevation gradient.
As maintaining species number can buffer and modulate an ecosystem's response to change, preventing niches from
becoming available to invasive species and restructuring communities (García-Palacios et al., 2018;Yachi and Loreau, 1999),
we test the implications of an altitudinal gradient for stability of ant species richness (stability
) and ant community
composition over time. We sampled ant communities in an Afrotropical mountain range recognised for its high endemism of
several taxa, the Soutpansberg, in southern Africa. The Vhembe biosphere planning group has recently proposed that all areas
in the Soutpansberg be proclaimed core conservation areas, prompting the South African government's Department of
Environmental Affairs, and the South African National Spatial Biodiversity Assessment (NSBA) to include the Soutpansberg
complex as a national priority area for conservation action (Depatment of Environmetal Affairs, 2018). We used an extensive
dataset collected biannually for six consecutive years, making it amongst the largest standardized, spatio-temporal inver-
tebrate community datasets in existence.
To date, research conrms the negative relationship between ant species richness and increasing altitude (Bishop et al.,
2017;Munyai and Foord, 2012;Szewczyk and McCain, 2016). Given that the presence of more species enhances stability,
we predicted decreased stability
at altitude. In the knowledge that endemic, altitude-adapted taxa often occur at higher
elevations, and that this holds for the Soutpansberg (Munyai and Foord, 2015), we expected increasing altitude to impact
community similarity through time, with community composition being more variable between years. As ant diversity de-
creases with altitude, we anticipated that species-poor communities at high altitude might be dominated by a small number
of similar species able to survive the environmental ltering of high altitude. To test these hypotheses, we asked:
(1) does species richness of ants vary with altitude, aspect and with variables of habitat structure and soils?
(2) does stability
change across altitude?
G.S. Joseph et al. / Global Ecology and Conservation 17 (2019) e005962
(3) is similarity of composition of ant communities constant across an altitudinal gradient?
(4) are there elevational thresholds that inuence occurrence of ant taxa across an altitudinal gradient?
2. Methods
2.1. Study site
Ants were sampled in a recognised southern African centre of endemism (Van Wyk and Smith, 2001), the Soutpansberg
Mountains within the Vhembe Biosphere Reserve. We sampled along an elevational transect beginning at 23
E, running in a north-south direction, starting at 800 m above sea level (a.s.l.) on the southern aspect, and
climbing to 1700 m a.s.l., before descending to 800 m a.s.l. on the northern aspect. The transect is characterised by sandstone,
erosion-resistant quartzite, conglomerate, basalt, and shale rocks (Mostert et al., 2008), experiencing summer rainfall, dry
winters and mean annual precipitation of about 450 mm (Mucina and Rutherford, 2006).
2.2. Ant sampling
Epigaeic ant sampling took place biannually for 6 years, from 2009 to 2015. Given that sampling can vary even at the same
time of year through uctuating environmental variables (e.g. rainfall events, res, temperature variability), we mitigated
against short-term temporal variation in foraging activity by (1) sampling at the same time each year, during January (wet
season) and September (dry season), (2) ensuring that sampling was not coupled to rainfall events, (3) only sampling on sites
not impacted by re, (4) sampling during 5 day periods in which full sun (no cloud) was present for an averageof over 6 h per
day. Sampling was done at eleven sites, spaced 200 vertical metres apart, for each elevational band. Each site contained four
replicates, spaced at least 300 horizontal metres apart to avoid pseudo-replication (McKillup, 2011). At each replicate, ten
pitfall traps (each ø 62 mm) were laid out in a grid composed of two parallel lines (2 5) with 10 m spacing between traps,
following Munyai and Foord (2012) and Bishop et al. (2014). Traps contained a 50% solution of propylene glycol and were left
open for ve days during each survey. Ants were identied to morphospecies or species when possible.
2.3. Environmental variables
Temperature was recorded in two replicates at each site. Within each replicate, one Thermocron iButton (Semiconductor
Corporation, Dallas/Maxin/Texas) was buried 1 cm below the soil to record temperature at hourly intervals. Mean, minimum
and maximum temperatures were calculated for wet and dry season for each year at each elevational band.
Vertical and horizontal habitats were quantied. During each survey, a 1 m
grid was placed over each pitfall trap, and
imaged to establish horizontal habitat structure by estimating percentage area covered by bare ground, vegetation, rock and
leaf litter. For vertical structure, a 1.5 m measuring rod was placed at four corners of the grid, 1.5 m from the pitfall trap. The
number of vegetation contacts on the rod was recorded along 25 cm intervals (0e25 cm, 25e50 cm, 50e75 cm, 75e100 cm,
100 e125 cm, 125e150 cm, >150 cm).
In January 2010, ten soil samples were taken randomly from each replicate using a soil auger, and analysed for particle size
composition (clay, sand, rock and silt), pH, conductivity, Carbon (C), Potassium (K), Sodium (Na), Calcium (Ca), Magnesium
(Mg), Phosphorus (P), and Nitrate (NO3) by BemLab, South Africa.
Principal component analysis (PCA) was performed to summarize the variation for vertical and horizontal habitat
structure respectively. The rst two principal coordinates explained 37% and 24% of variation for vertical (cumulative vari-
ation ¼61%) and 41% and 30% (cumulative variation ¼71%) for horizontal habitat structure. The rst principal component
axis for vertical habitat structure (PC1
) was positively correlated with sites harbouring intermediately
tall vegetation structure (50e75 cm, 75e100 cm, 100e125 cm), and negatively with habitats lacking vertical structure. The
second principal component (PC2
) was positively correlated with tall canopy cover (125e150 cm, 150þ) and
negatively with vertical vegetation below 25 cm and no canopy cover. The rst principal component axis for horizontal
habitat structure (PC1
) was positively correlated with bare ground and negatively correlated with
vegetation cover. The second principal component (PC2
) was positively correlated with leaf litter
presence and negatively to exposed rock.
For soil characteristics the rst two axes explained 46% and 15% of the variation. The rst principal component axis
) was positively correlated with acidic soil and negatively with basic soils. The second principal component axis
) was positively correlated with sandy soil and negatively with clay soil.
2.3.1. Statistical analysis
For each of the 528 ant communities in the dataset [i.e. 4 replicates per sampling site x 11 sites 6 years x 2 sea-
sons ¼528), we calculated species richness. Species richness was modelled using linear mixed effect models with Gaussian
distribution and replicates specied as random intercept to account for temporal pseudo-replication while all predictor
variables were included as xed effects, with various subsets of variables and in various combinations. Certain variables
G.S. Joseph et al. / Global Ecology and Conservation 17 (2019) e00596 3
(altitude, mean temperature, minimum temperature, and maximum temperature) were collinear, therefore none of the
candidate models contained more than one of these as the explanatory variable. For measures of vertical (vegetation height,
canopy cover) and horizontal (percentage bare ground, vegetation, rock, leaf litter) vegetation structure, and soil parameters
(particle size, pH, conductivity, and chemical composition), we used the values generated by the principal correspondence
analyses. Along with these, other predictor variables used in the model were aspect, year and season. The best model was
chosen using Akaike information criterion (AIC). Marginal R
(variation explained by effects only) and conditional R
(variation explained by xed and random effects) were calculated for the best random-intercept model (Nakagawa and
Schielzeth, 2013) in R programming environment version 3.5.0 (R Core Team, 2017).
Next we calculated stability
over time using mean species richness and its standard deviation, using each replicate set of
traps at each site (n ¼4 replicate x 11 sites ¼44). These were used to calculate the coefcient of variation, the inverse of which
yielded a measure of stability, following various authors (Isbell et al., 2009;Lehman and Tilman, 2000). Thus stability was
calculated as:
CV1¼mean ðXÞ
standard deviation x100
We calculated community similarity using two methods, Bray-Curtis similarity (1 minus the Bray-Curtis dissimilarity,
which quanties compositional similarity between different sites, based on abundance at each site), and Sorensen's index
(which uses presence and absence to discriminates as to whether a species is common at a site or not, giving greater emphasis
to species common to sites than to those found at only one site). Replicates from each site were lumped to yield the total
community caught at each site. We compared the similarity of community composition for each site from each year and
season with the composition of the community at that site for every other year and season, using the Vegan package in R
(Oksanen et al., 2016). We then assessed these similarity values for each of the sites with the similarity values for sites at other
altitudes and aspects, using a multiple linear regression.
To better understand which species drive community similarity at altitude, we used community abundance data,
exploring the directionality, magnitude and uncertainty of individual taxa threshold responses and community threshold
responses to an altitudinal gradient for each year separately using Threshold Indicator Taxa Analysis in TITAN2 package in R,
(Baker et al., 2015;Baker and King, 2010). Z scores are standardized against the mean and standard deviation of permuted
samples, and so emphasise degree of change, thus prioritising taxa with infrequent occurrence. We used untransformed
abundances on taxa occurring three or more times in the different sites over the entire period (2009e2015), with all par-
titions having at least three observations on both sides. TITAN also identies taxon-specic community thresholds. Boot-
strapping is used to estimate indicator reliability and purity as well as uncertainty around the location of individual taxa and
community change points. Standardized taxa responses increasing at the change point (zþ) are distinguished from those that
decrease (z-).
3. Results
3.1. Patterns of species richness
Our traps contained a total of 102496 ants representing 35 genera from 128 species. Altitude and mean temperature were
correlated (t ¼8.8, n ¼438, p <0.001). We therefore ran two sets of models, one using altitude, and the other using mean
temperature as one of a number of explanatory variables. We adopted an information theoretic approach, based on the bias-
corrected Akaike information criterion (AIC
) to choose the best model. The t of altitude (AIC
¼2377; variation explained
42%) to the data was better than mean temperature (AIC
¼2378; variation explained 39%). Species richness (1) declined
signicantly with increasing altitude and was lower (2) on south-facing slopes (3) at the end of the dry season, and (4) in 2009
than in other years. The decrease in richness with increasing altitude was more marked on northern aspects. Species richness
was signicantly greater for sites with sandy soils with lowclay content (Table 1a; Fig. 1.). An interaction between altitude and
southern aspect was observed, with richness declining with altitude on the northern aspect, but remaining almost constant
across altitudes on southern slopes.
3.2. Patterns of stability
Overall, stability
declined signicantly with altitude, and was signicantly lower on south-facing aspects. There was an
interaction between altitude and aspect, such that stability
decreased with increasing altitude on northern slopes, but
increased slightly on southern slopes (Fig. 2.).
also varied with vegetation, increasing with both axis 1 (more bare ground and little live vegetation cover) and
2 (increasing leaf litter with minimal exposed rock) of the PCA for horizontal vegetation structure, and with the second axis
for soils (sandy soils with low clay content; Table 1b).
G.S. Joseph et al. / Global Ecology and Conservation 17 (2019) e005964
3.2.1. Patterns of similarity
Using the Bray-Curtis index, overall for the system similarity increased with altitude and this was the case for both
northern and southern aspects. There was an interaction between aspect and altitude, with a more rapid increase in similarity
Table 1
Summary of the AIC
ebased model selection for variables explaining ant species richness (a), stability of ant species richness (b), compositional similarity of
ant communities using Bray-Curtis (c) and Sorensen's (d) indices. The change in AICc between the best model, the next best, and worstare reported. Marginal
), measuring variation explained by xed effects only, and conditional R
), measuring variation explained by both xed and random effects, are
Response variable Model AIC
(next best)
(a) ant species richness ~ elevation x aspect þseason þPC2
þyear 2377.4 3.2 101.4 0.30 0.42
Best Model:
North aspect: y ¼16.2 ( ±1.7) e0.004 ( ±0.001)elevation e1.5 ( ±0.4)wet þ8.0 ( ±1.8)PC2
þ1.6 ( ±0.6)year
3.3 ( ±0.6)year
þ2.4 ( ±0.6)year
þ4.5 ( ±0.6)year
þ3.1 ( ±0.8)
South aspect: y ¼7.5 ( ±1.7) þ0.001 ( ±0.001)elevation e1.5 ( ±0.4)wet þ8.0 ( ±1.8)PC2
þ1.6 ( ±0.6)year
3.3 ( ±0.6)year
þ2.4 ( ±0.6)year
þ4.5 ( ±0.6)year
þ3.1 ( ±0.8)
(b) stability of ant species richness ~ elevation x aspect þ
þPChor2 þPC2
115.5 2.6 12.2 0.47 e
Best Model
y¼7.7 ( ±1.0) e0.003 ( ±0.001)elevation e6.7 ( ±1.3)aspect þ39.5 ( ±17.0) PC1
þ42.2 ( ±13.7) PC2
þ1.9 ( ±0.9)PC2
(c) similarity: Bray-Curtis ~ elevation x aspect 694.9 25.3 187.3 0.23 e
Best Model:
North aspect: y ¼0.23 ( ±0.03) þ1.9 x 10
(±2.6 x 10
South aspect: y ¼-0.08 ( ±0.03) þ4.1 x 10
(±2.6 x 10
(d) similarity: Sorensen's ~ elevation x aspect 1691.1 32.2 214.5 0.21 e
Best Model:
North aspect: y ¼0.73 ( ±0.02) e8.3 x 10
(±2.2 x 10
South aspect: y ¼0.44 ( ±0.02) e6.7 x 10
(±2.2 x 10
Fig. 1. Ant species richness as a function of aspect and increasing altitude (a), aspect alone (b), season (c), sandy soils low in clay content (d).
G.S. Joseph et al. / Global Ecology and Conservation 17 (2019) e00596 5
on southern aspects. However, it is notable that at low elevations of 800 m similarity between communities was lower on the
southern than on the northern aspect, but that by 1700 m, similarity had become higher on the southern aspect (Table 1c;
Fig. 3a). With Sorensen's index, overall, similarity also increased with altitude, but similarity underwent a slight decrease on
the northern aspect with increasing altitude (Table 1d; Fig. 3b).
3.2.2. Threshold Indicator Taxa Analysis
Threshold Indicator Taxa Analysis cumulatively identied 37 individual ant taxa that declined in response to increasing
altitude, with an observed environmental change point occurring around 1200 m. For 20 species, a positive change point was
observed at 1400 m, and these species increased in response to increasing altitude (Fig. 4).
4. Discussion
We found that the stability of invertebrate richness, using Afrotropical montane ants as an example, decreased with
increasing altitude. This study is the rst to evaluate stability of species richness and composition along an elevational
gradient over time. Unsurprisingly, patterns of ant species richness echo previous studies (decreasing with higher altitude,
cooler south-facing slopes, and the dry season, conditions with lower forage availability, temperatures and humidity; Bishop
et al., 2014;Mauda et al., 2018;Yusah et al., 2012), emphasising that broad changes in temperature are a strong driver of ant
richness patterns (Bishop et al., 2017;H
olldobler and Wilson, 1994;Sanders et al., 2007).
In general, the presence of many species increases stability (García-Palacios et al., 2018;Loreau et al., 2001), and the
nding that stability
displayed a strong elevational pattern, with species number uctuating increasingly with altitude,
supported our hypothesis of deceasing stability
at higher elevations, in line with a decrease in ant species richness at
altitude (Bishop et al., 2014;Munyai and Foord, 2015). Stability
was lower on southern slopes, which are colder across
seasons, and receive less exposure to sunlight in the southern hemisphere. Although stability
remained lower than on
northern slopes, it increased marginally on southern slopes with increasing elevation. Vegetation structure was not shown to
Fig. 2. Stability of ant species richness decreased signicantly with increasing altitude on northern slopes, yet increased negligibly on southern slopes.
Fig. 3. The relationship between ant community similarity and altitude using Bray-Curtis (a) and Sorensen's (b) similarity indices.
G.S. Joseph et al. / Global Ecology and Conservation 17 (2019) e005966
be a driver of stability
in our study, but these southern slopes are known to receive more precipitation, and are charac-
terised by more woody vegetation, resulting in shading and cooler microclimates (Munyai and Foord, 2015). This in turn may
inuence stability
on southern slopes, but remains untested. It is more likely that species richness, which did not decrease
on southern slopes (also perhaps due to the vegetation and precipitation properties of this southern aspect; Fig. 3), exerts a
modulating effect, given the expectation that presence of more species can enhances stability (García-Palacios et al., 2018;
Loreau et al., 2001).
Bare ground with little intermediate vegetation structure increased stability
, as did increased leaf litter with minimal
exposed rock, perhaps as a consequence of additional habitat complexity minimising variation of species richness over time
(Mauda et al., 2018;Tiede et al., 2017). Stability
also increased with sandy soils with low clay content, a substrate known to
favour ant species richness (Mauda et al., 2018). Stability
, lower in the rst year, is interpreted in the context of both mean
annual temperature, which was lower in 2009 than in all other years (by at least 1
C), and minimum annual temperature,
which was >1
C higher in 2010, but 5
C higher by 2014e2015 (Appendix Fig.S1;Sanders et al., 2007).
Compositional similarity of ant communities was higher overall with increasing altitude regardless of the index used, but
the ndings for aspect were more complex. On both northern and southern slopes, the Bray-Cutis index showed that on both
northern and southern slopes, ant communities become more similar with higher altitude. As this index determines simi-
larity by using species abundance, ndings suggests that species that are common (altitude-adapted) at high elevations,
remain common as altitude increases, and that ant taxa poorly adapted to altitude disappear, or become less common with
increasing elevation. Sorensen's index revealed the same pattern for the southern slope, but that communities were slightly
less similar with increasing altitude on northern slopes. Given that Sorensen's index uses presence-absence, it is sensitive to
the appearance or disappearance of a given species, and results may reect a lower degree of environmental ltering on the
relatively warmer, more hospitable northern slopes. Furthermore, within the context of there being fewer species at
elevation, Sorensen's index can be expected to be sensitive to adding or taking away species, because the number of species at
altitude is low to start off with.
Given the thermophilic nature of ants (Bishop et al., 2017;H
olldobler and Wilson, 1994;Sanders et al., 2007), and studies
conrming that ability to tolerate cold temperatures at altitude can be important for ant distribution (Bishop et al., 2017), it is
likely that at higher altitude, the limited species that are best adapted to elevation come to dominate high altitude com-
munities, which gradually become more similar with increasing elevation. In summary, ndings reveal that there are few
species at higher relative to lower altitudes, and that these few species tend to be dominant at higher altitudes as only they
can persist in such conditions. Conversely, at lower altitudes, more species are able to persist in the relatively benign low-
elevation conditions, limiting opportunities for specic species to emerge as dominant over the protracted period of six years.
Fig. 4. Signicant ant indicator taxa identied in threshold indicator taxa analysis (TITAN), across an altitudinal gradient. Red symbols correspond to negative (z-)
indicator taxa, and denote taxa that decrease with increasing elevation, and blue correspond to positive (zþ) taxa, namely those that increase as altitude increase.
Symbols are in size proportional to z scores. Horizontal lines show 5th and 95th percentiles among 500 bootstrap replicates. (For interpretation of the references
to colour in this gure legend, the reader is referred to the Web version of this article.)
G.S. Joseph et al. / Global Ecology and Conservation 17 (2019) e00596 7
Threshold Indicator Taxa Analysis conrmed that there were 85% more indicator species that decreased with increasing
altitude, nearly double the small number of indicator species that increased with increasing altitude (Fig. 4). This facilitates
interpretation of similarity indices (which increased with increasing altitude), and stability
measures, if one considers that
not only do the threshold indicator taxa reveal a smaller species pool at altitude, but also reveal a threshold altitude, above
which the majority of indicator species are poorly represented. The majority of indicator ant taxa decrease consistently as
elevation increases to 1200 m. The small number of altitude-adapted indicator species dominate at altitudes above 1420 m,
and as altitudes declines below 1420 m, such species no longer dominate as they did at higher altitudes. Findings reveal that
these high altitude ant communities are (1) species poor (2) have lower stability
(and with low-species numbers, even
alteration of a few species can cause uctuation to richness over time), and are (3) more similar to one another by virtue of
there being only a limited suite of ant species able to tolerate high altitude (and the variables correlated with elevation, e.g.
lower minimum, mean and maximum temperatures, decreased humidity, differing soils and vegetation structure).
At a regional scale, currently within the Soutpansberg, a few altitude-adapted species appearto be holding the fortat high
elevation, living in communities that increasingly resemble one another as altitude increases. The new Vhembe biosphere
reserve zonation proposes that all areas above 1200 m in the Soutpansberg be proclaimed core conservation areas, and our
ndings identify 1200 m as the change point where the lower elevation species start falling out of assemblages, whilst al-
titudes approaching 1400 m become important for the high altitude species. At the scale of the Soutpansberg complex itself,
1200 m also coincides with the appearance of Afromontane forests on southern aspects, and 1400 m corresponds with a
switch to more open, Soutpansberg Mountain Sourveld habitat (Depatment of Environmetal Affairs, 2018;Mucina and
Rutherford, 2006).
Given that montane assemblages across elevational gradients are often dominated by rare or endemic species, they can be
particularly vulnerable to temperature changes (García-Robledo et al., 2016), so at the broadest scales, uctuation of species
numbers at higher elevations disproportionality places them at risk from global change. In southern Africa, temperatures are
anticipated to rise by up to 2.5
C over three decades (Davis and Vincent, 2017), potentially opening niches to thermophilic,
heat-adapted invertebrates. Although this study does not address the impact of global change, it can be speculated that with
the buffer that stability of species richness can confer already compromised at altitude, communities at higher elevation may
be at increased risk of invasion and restructuring as new niches form. Suitable microclimates and microhabitats may
modulate this (Duffy et al., 2015;G.S.Joseph et al., 2016), as was the case with e.g. sandy soils and leaf litter in this study, but
further research will be needed to determine whether the lowered stability
will allow thermophilic, low-altitude ants and
invasive species to restructure communities at high altitudes.
We thank the DST-NRF Centre of Excellence for Invasion Biology, through the South African Research Chairs Initiative Chair
on Biodiversity Value and Change in the Vhembe Biosphere Reserve, hosted by the University of Venda.
Appendix A. Supplementary data
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... This approach therefore has considerable potential for addressing the influence of both the successional stage and strata on ant communities. Ecological stability, the inverse of variation over time, is a fundamental ecosystem property that offers insights into biodiversity and ecosystem processes (Doak et al., 1998;Joseph et al., 2019;Tonkin et al., 2017). Critically, β-diversity can be decomposed into the turnover and nestedness components, which provide complementarity insights into community ecological stability: the turnover component reflects species replacement, while the nestedness component reflects species richness differences driven by a pattern of community subsetting (Baselga, 2010). ...
... This result is somewhat surprising, especially because there is a significant change in the richness and composition of ant species with the advance of the successional secondary vegetation (Marques et al., 2017;Neves et al., 2013). Studies carried out in savannas and mountain ecosystems have found that greater habitat complexity should minimize the variation of ant diversity over time (Joseph et al., 2019;Mauda et al., 2018;Tiede et al., 2017), which is contrary to our finding of high variation even in the late stage. However, it is important to note that tropical dry forests are strongly seasonal, and the ant fauna might have evolved to cope with natural variations under these habitat conditions. ...
en Ants are diverse and ecologically important organisms in tropical forests, where their spatiotemporal distribution can be highly complex. This complexity arises mainly from marked differences in microclimatic conditions and resource availability through space and time that is even more evident in highly seasonal environments, such as tropical dry forests. However, it is unclear how seasonality interacts with other factors that might shape temporal variation of ant composition (β‐diversity), like vertical strata and habitat disturbance. Our goal was to examine the potential influence of vertical stratification and the successional stage on the spatiotemporal variation of a tropical dry forest's ant species composition. We assessed whether species turnover or nestedness was the main component determining the spatiotemporal β‐diversity of ant communities across the canopy and litter strata. We sampled canopy and litter ants in ten plots, half in the early and half on the late stage of secondary succession at four times, twice in wet and twice in dry season. A high species turnover defined the spatiotemporal β‐diversity of canopy and litter ant communities across years and seasons in our focal dry forests. Importantly, the temporal ant species composition was much more stable in the canopy than in the litter. Moreover, we found that the ant community's temporal dynamics was consistently high across successional stages, not differing in the temporal β‐diversity between early and late succession. Our results provide valuable insights into the potential underlying causes of community assembly and spatiotemporal dynamics in seasonal habitats, like the highly threatened and diverse tropical dry forests. Abstract in Portuguese is available with online material. RESUMO pt As formigas são organismos diversos e ecologicamente importantes em florestas tropicais, onde sua distribuição espaço‐temporal pode ser altamente complexa. Essa complexidade se deve principalmente às diferenças marcantes nas condições microclimáticas e na disponibilidade de recursos no espaço e no tempo, o que é ainda mais evidente em ambientes altamente sazonais, como as florestas tropicais secas. No entanto, não está claro como a sazonalidade interage com outros fatores para a determinação da variação espacial e temporal da composição de comunidades de formigas (β‐diversidade), como por exemplo a estratificação vertical e distúrbios ecológicos. Nosso objetivo foi examinar a influência da estratificação vertical e do estágio sucessional na variação espacial e temporal da composição de espécies de formigas de uma floresta tropical seca. Mais especificamente, avaliamos qual é o principal componente determinante da β‐diversidade espacial e temporal (substituição ou aninhamento de espécies) de formigas de dossel e da serapilheira em distintos estágios de sucessão secundária. Para isso, amostramos formigas no dossel e na serapilheira em dez parcelas, metade em estágio inicial e a outra metade no estágio final da sucessão secundária. Uma elevada substituição de espécies definiu a β‐diversidade espacial e temporal das comunidades de formigas, tanto no dossel quanto na serapilheira, em ambos estágios de sucessão secundária. No entanto, a comunidade de formigas do dossel é muito mais estável no tempo, se comparada à comunidade que utiliza a serapilheira. Além disso, descobrimos que a dinâmica temporal da comunidade de formigas foi consistentemente alta em ambos os estágios de sucessão secundária, sem diferenças verificadas para a β‐diversidade temporal entre os estágios sucessionais. Nossos resultados fornecem informações valiosas sobre a assembleia e dinâmica de comunidades de formigas ao longo do espaço e do tempo em habitats sazonais, como as florestas tropicais secas, ambientes altamente diversos e ameaçados por pressões antrópicas.
... In general, warmer locations and time periods host greater ant abundance and species richness. These patterns can be seen at the local scale (Joseph et al., 2019), across elevational gradients (Bishop et al., 2014;Sanders et al., 2007), latitudinal gradients (Dunn et al., 2009;Economo et al., 2018;Gibb et al., 2015;Jenkins et al., 2011), and seasons (Andersen, 1983;Bishop et al., 2014). This link between temperature and community-level richness at a range of scales and contexts suggests that communities will support a greater diversity of ant species as temperatures rise. ...
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Ants (Hymenoptera: Formicidae) are one of the most dominant terrestrial organisms worldwide. They are hugely abundant, both in terms of sheer numbers and biomass, on every continent except Antarctica and are deeply embedded within a diversity of ecological networks and processes. Ants are also eusocial and colonial organisms-their lifecycle is built on the labor of sterile worker ants who support a small number of reproductive individuals. Given the climatic changes that our planet faces, we need to understand how various important taxonomic groups will respond; this includes the ants. In this review, we synthesize the available literature to tackle this question. The answer is complicated. The ant literature has focused on temperature, and we broadly understand the ways in which thermal changes may affect ant colonies, populations, and communities. In general, we expect that species living in the Tropics, and in thermally variable microhabitats, such as the canopy and leaf litter environments, will be negatively impacted by rising temperatures. Species living in the temperate zones and those able to thermally buffer their nests in the soil or behaviorally avoid higher temperatures, however, are likely to be unaffected or may even benefit from a changed climate. How ants will respond to changes to other abiotic drivers associated with climate change is largely unknown, as is the detail on how altered ant populations and communities will ramify through their wider ecological networks. We discuss how eusociality may allow ants to adapt to, or tolerate, climate change in ways that solitary organisms cannot and we identify key geographic and phylogenetic hotspots of climate vulnerability and resistance. We finish by emphasizing the key research questions that we need to address moving forward so that we may fully appreciate how this critical insect group will respond to the ongoing climate crisis.
... Such a discrepancy probably arises as result of lower taxonomic richness of alpine environments, since temporal stability at the community level cannot be achieved solely by higher asynchrony across species ('insurance effect'; Naeem & Li, 1997;Yachi & Loreau, 1999), but also by higher species diversity (the so-called 'portfolio effect ';Doak et al., 1998). The negative relationship between community stability and elevation provides support for the notion that species richness acts as a buffer against environmental perturbation, which is highly relevant in conservation terms (García-Palacios et al., 2018;Joseph et al., 2019;Tilman, 1999). ...
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Aim Mountains are biodiversity hotspots and are among the most sensitive ecosystems to ongoing global change being thus of conservation concern. Under this scenario, assessing how biological communities vary over time along elevational gradients and the relative effects of niche‐based deterministic processes and stochastic events in structuring assemblages is essential. Here, we examined how the temporal trends of bird communities vary with elevation over a 20 year‐period (1999–2018). We also tested for differences in temporal dynamics among habitat types (among‐community variability) and functional groups (within‐community variability). Taxon 97 species of common breeding birds. Location Swiss Alps. Methods We used abundance data from the Swiss breeding bird survey to compute different temporal dynamic metrics (temporal turnover, synchrony, rate of community change and community‐level of covariance among species). We also examined the relative contribution of deterministic and stochastic processes in community assembly using the Raup‐Crick method and the normalized stochasticity ratio. Results We found that, with greater elevation, temporal species turnover increased while the rate of overall community change over successive years decreased, suggesting that high‐elevation communities display more erratic dynamics with no clear trend. Despite this, we found a more deterministic assembly of alpine communities in comparison to those located at lower elevations. Deterministic processes had greater influence than stochastic processes on community assembly along the entire elevational gradient (80% of communities). Forest communities exhibited higher synchrony in comparison to the remaining habitats likely because they consisted of species with greater functional redundancy, whereas alpine communities were the least stable as a result of their low taxonomic richness (‘portfolio’ effect). Main conclusions Community‐level synchrony was overall positive supporting the idea that compensatory mechanisms are rare in natural biological communities. Our results suggest that rather than competition, the existence of differences in the ecological strategies of species may have a stabilizing effect on bird communities by weakening the concordance of species responses to fluctuations in environmental conditions (i.e. enhanced interspecific temporal asynchrony). This study provides evidence that, although species turnover in metacommunities is frequent, a high temporal turnover does not necessarily imply the overriding importance of stochastic processes.
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Blackflies are amongst the most abundant and diverse group of aquatic insects globally, being rarely absent from rivers, their distributions are largely driven by current velocity, conductivity and total dissolved solids (TDS). While research on blackflies has mainly focused on medical, veterinary and economic aspects, ecological studies in the Afrotropical Region are relatively scarce. This study aimed to determine the response of blackfly species to environmental drivers in the north-eastern tropical regions of South Africa. Generalized Linear mixed models and the Permutational Multivariate Analysis of Variance were fitted to model blackfly richness and abundance structure. The Threshold Indicator Taxa Analysis (TITAN) was used to analyze species-specific change along environmental gradients. A total of 1343 larvae, representing seven blackfly species in four sub-genera, were collected from 22 sites over a twelve-month period. Water temperature and flow velocity were the most important drivers of species richness. Conductivity and land cover had the strongest effect on community composition across species and sites. Species responded synchronously along flow velocity and water temperature gradients at 1.6 m.s−1 and between 20–22 °C, respectively. Simulium (Metomphalus) hargreavesi was identified at an indicator species for flow velocity, while three were indicator species for water temperature change. We demonstrated that the richness and relative abundance of blackflies are driven by different variables across the Luvuvhu catchment. The new information obtained from this study provides an insight on the importance of protecting and managing the natural environment in a high biodiversity and endemism area.
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High-altitude-adapted ectotherms can escape competition from dominant species by tolerating low temperatures at cooler elevations, but climate change is eroding such advantages. Studies evaluating broad-scale impacts of global change for high-altitude organisms often overlook the mitigating role of biotic factors. Yet, at fine spatial-scales, vegetation-associated microclimates provide refuges from climatic extremes. Using one of the largest standardised data sets collected to date, we tested how ant species composition and functional diversity (i.e., the range and value of species traits found within assemblages) respond to large-scale abiotic factors (altitude, aspect), and fine-scale factors (vegetation, soil structure) along an elevational gradient in tropical Africa. Altitude emerged as the principal factor explaining species composition. Analysis of nestedness and turnover components of beta diversity indicated that ant assemblages are specific to each elevation, so species are not filtered out but replaced with new species as elevation increases. Similarity of assemblages over time (assessed using beta decay) did not change significantly at low and mid elevations but declined at the highest elevations. Assemblages also differed between northern and southern mountain aspects, although at highest elevations, composition was restricted to a set of species found on both aspects. Functional diversity was not explained by large scale variables like elevation, but by factors associated with elevation that operate at fine scales (i.e., temperature and habitat structure). Our findings highlight the significance of fine-scale variables in predicting organisms’ responses to changing temperature, offering management possibilities that might dilute climate change impacts, and caution when predicting assemblage responses using climate models, alone.
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Losses and gains in species diversity affect ecological stability1–7 and the sustainability of ecosystem functions and services8–13. Experiments and models have revealed positive, negative and no effects of diversity on individual components of stability, such as temporal variability, resistance and resilience2,3,6,11,12,14. How these stability components covary remains poorly understood¹⁵. Similarly, the effects of diversity on overall ecosystem stability¹⁶, which is conceptually akin to ecosystem multifunctionality17,18, remain unknown. Here we studied communities of aquatic ciliates to understand how temporal variability, resistance and overall ecosystem stability responded to diversity (that is, species richness) in a large experiment involving 690 micro-ecosystems sampled 19 times over 40 days, resulting in 12,939 samplings. Species richness increased temporal stability but decreased resistance to warming. Thus, two stability components covaried negatively along the diversity gradient. Previous biodiversity manipulation studies rarely reported such negative covariation despite general predictions of the negative effects of diversity on individual stability components³. Integrating our findings with the ecosystem multifunctionality concept revealed hump- and U-shaped effects of diversity on overall ecosystem stability. That is, biodiversity can increase overall ecosystem stability when biodiversity is low, and decrease it when biodiversity is high, or the opposite with a U-shaped relationship. The effects of diversity on ecosystem multifunctionality would also be hump- or U-shaped if diversity had positive effects on some functions and negative effects on others. Linking the ecosystem multifunctionality concept and ecosystem stability can transform the perceived effects of diversity on ecological stability and may help to translate this science into policy-relevant information.
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ContextWith global change, microclimates become important refuges for temperature-sensitive, range-restricted organisms. In African savannas, woody vegetation on Macrotermes mounds create widely-dispersed microclimates significantly cooler than the surrounding matrix, which buffer against elevated temperatures at the finer scale of mounds, allowing species to persist at the landscape scale. Termite colonies cultivate symbiotic fungi to digest lignin, but the fungi require temperatures between 29 and 32 °C, which termites strive to maintain. Mound-associated vegetation is a hot-spot for elephant herbivory, so removal of woody species cover by elephants could influence mound-associated microclimates, impacting temperature regulation by termites. Objectives We explored the interaction between two prominent ecosystem engineers (termites and elephants) to ascertain whether elephant removal of mound woody cover affects (1) external mound-associated microclimate and (2) internal mound temperature. Methods We surveyed 44 mounds from three sites in Kruger National Park, South Africa, during an El Niño/Southern Oscillation-induced drought and heatwave, recording whether sub-canopy, external, mound-surface and internal mound temperatures varied with vegetation removal by elephant. ResultsElephant damage to mound-associated vegetation reduces the fine-scale microclimate effect provided by vegetation on Macrotermes mounds. Despite this, termites were able to regulate internal mound temperatures, whereas internal temperatures of abandoned mounds increased with elevated surface temperatures. Conclusions Termites can persist despite loss of mound-associated microclimates, but the loss likely increases energetic costs of mound thermoregulation. Since mound vegetation buffers against drought, loss of widely-dispersed, fine-scale microclimates could increase as megaherbivores remain constrained to protected areas, impacting climate-sensitive organisms and ecosystem function at a range of scales.
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Africa’s savannas are undergoing rapid conversion from rangelands into villages and croplands. Despite limited research, and evidence of deleterious effects to biodiversity, international organisations have earmarked this system for cropland. Invertebrates, and ants in particular, are sensitive indicators of habitat fragmentation, and contribute to ecosystem services at a range of scales. We investigated how rangelands, villages and croplands differ in ant species and functional diversity, and assemblage composition. We sampled ants using pitfall traps at 42 sites (14 replicates each in rangeland, cropland, and village) in northern South African savannas. We investigated the impact of landuse, season, and multiple soil and vegetation habitat variables on ant species diversity, assemblages and functional diversity. Rangelands had the greatest ant species richness, particularly in the wet season. Richness declined with increasing soil clay content. Ant assemblages were distinctly different between landuse types. Rangeland harboured the widest diversity of indicator species, and contained greatest functional diversity. Rangelands accommodated more scavengers, granivores, and plant-matter feeders than cropland, and representation of these groups varied with season. Ants play essential roles in soil nutrient cycling, plant and seedling recruitment, and impact other arthropods through predation and aphidoculous behaviour that in turn influences entire food webs. Thus, the reduced species richness, changes in assemblage composition and the loss of functional groups in ant assemblages found in cropland and villages is potentially problematic. Left unchallenged, these new forms of landuse threaten to characterise the entire African savanna system, impacting not only future ecological, but possibly also human wellbeing.
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Predators play a disproportionately positive role in ensuring integrity of food webs, influencing ecological processes and services upon which humans rely. Predators tend to be amongst the first species to be affected by anthropogenic disturbance, however. Spiders impact invertebrate population dynamics and stabilise food webs in natural and agricultural systems (potentially mitigating against crop pests and reduced yields). Africa’s savannas are undergoing continent-wide conversion from low-density rangelands to villages and croplands, as human populations burgeon. Despite limited research, and evidence of deleterious impacts to biodiversity, African savannas are earmarked by prominent international organisations for conversion to cropland. Given the key role of spiders in food webs, they can have beneficial impacts in agroecosystems. Furthermore, functional diversity (FD) reflects ecosystem pattern and processes better than species diversity, so we evaluated impacts of large-scale landuse change on both species richness and FD. We surveyed spiders using pitfall traps at 42 sites (14 replicates each in rangeland, cropland, and villages) in South African savannas, investigating effects of landuse, season, and habitat variables on spider species diversity and FD. Species richness was lowest in villages. FD was lowest in cropland, however, with reduced representation of traits associated with hunting of larger invertebrates. Furthermore, there were fewer specialists in croplands. These findings suggest that even when cropland does not impact species diversity, loss of FD can still occur. As savanna systems transform, impacts on invertebrate population dynamics may increase the possibility of a breakdown in pest control in natural and agricultural systems, given changes in FD of invertebrate predators.
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Biodiversity enhances many of nature’s benefits to people, including the regulation of climate and the production of wood in forests, livestock forage in grasslands and fish in aquatic ecosystems. Yet people are now driving the sixth mass extinc- tion event in Earth’s history. Human dependence and influence on biodiversity have mainly been studied separately and at contrasting scales of space and time, but new multiscale knowledge is beginning to link these relationships. Biodiversity loss substantially diminishes several ecosystem services by altering ecosystem functioning and stability, especially at the large temporal and spatial scales that are most relevant for policy and conservation.
The insurance hypothesis, stating that biodiversity can increase ecosystem stability, has received wide research and political attention. Recent experiments suggest that climate change can impact how plant diversity influences ecosystem stability, but most evidence of the biodiversity-stability relationship obtained to date comes from local studies performed under a limited set of climatic conditions. Here, we investigate how climate mediates the relationships between plant (taxonomical and functional) diversity and ecosystem stability across the globe. To do so, we coupled 14 years of temporal remote sensing measurements of plant biomass with field surveys of diversity in 123 dryland ecosystems from all continents except Antarctica. Across a wide range of climatic and soil conditions, plant species pools, and locations, we were able to explain 73% of variation in ecosystem stability, measured as the ratio of the temporal mean biomass to the SD. The positive role of plant diversity on ecosystem stability was as important as that of climatic and soil factors. However, we also found a strong climate dependency of the biodiversity-ecosystem stability relationship across our global aridity gradient. Our findings suggest that the diversity of leaf traits may drive ecosystem stability at low aridity levels, whereas species richness may have a greater stabilizing role under the most arid conditions evaluated. Our study highlights that to minimize variations in the temporal delivery of ecosystem services related to plant biomass, functional and taxonomic plant diversity should be particularly promoted under low and high aridity conditions, respectively.
As the Anthropocene advances, understanding the complex web of interactions between species has become a central theme in the maintenance of biodiversity, ecosystem functions, and agricultural systems. Plant-flower visitor networks yield insights into how natural vegetation supports crop pollination. Although crops themselves also support pollinators, the importance of spillover of flower-visiting pollinators from natural vegetation into croplands is increasingly appreciated. Natural vegetation not only provides forage and nesting sites, but can also support crop flower visitors when the crop is not flowering. We evaluated indirect effects between mango (the dominant tropical fruit crop globally) and wild plant species in neighbouring vegetation, and the factors determining these indirect effects, by constructing flower visitor networks. We constructed these networks for transects that included mango fields and neighbouring natural vegetation in north-eastern South Africa. Surveys were conducted before, during and after mango flowering, to allow evaluation of the importance of pollinator support when the crop was not in flower. Network analysis showed that potential indirect effects of other plant species on mango increased with flower abundance of those species, although this increase was less marked for species growing in natural vegetation. The cumulative (total, i.e. indirect effects summed) effect of natural vegetation on mango flower visitation was greater both during mango flowering and when it was not flowering. This is likely because of the greater plant diversity in natural systems, and because the combination of these species provided flowers over a protracted period. These positive indirect effects among plants flowering over extended periods of time have to date rarely been considered in crop pollination studies. Given the rapid expansion of high-intensity, high-yield monoculture plantings, such effects warrant further investigation.
Environmental stressors and changes in land use have led to rapid and dramatic species losses. As such, we need effective monitoring programs that alert us not only to biodiversity losses, but also to functional changes in species assemblages and associated ecosystem processes. Ants are important components of terrestrial food webs and a key group in food web interactions and numerous ecosystem processes. Their sensitive and rapid response to environmental changes suggests that they are a suitable indicator group for the monitoring of abiotic, biotic, and functional changes. We tested the suitability of the incidence (i.e. the sum of all species occurrences at 30 baits), species richness, and functional richness of ants as indicators of ecological responses to environmental change, forest degradation, and of the ecosystem process predation on herbivorous arthropods. We sampled data along an elevational gradient (1000–3000 m a.s.l.) and across seasons (wetter and drier period) in a montane rainforest in southern Ecuador. The incidence of ants declined with increasing elevation but did not change with forest degradation. Ant incidence was higher during the drier season. Species richness was highly correlated with incidence and showed comparable results. Functional richness also declined with increasing elevation and did not change with forest degradation. However, a null-model comparison revealed that the functional richness pattern did not differ from a pattern expected for ant assemblages with randomly distributed sets of traits across species. Predation on artificial caterpillars decreased along the elevational gradient; the pattern was not driven by elevation itself, but by ant incidence (or interchangeable by ant richness), which positively affected predation. In spite of lower ant incidence (or ant richness), predation was higher during the wetter season and did not change with forest degradation and ant functional richness. We used path analysis to disentangle the causal relationships of the environmental factors temperature (with elevation as a proxy), season, and habitat degradation with the incidence and functional richness of ants, and their consequences for predation. Our results would suggest that the forecasted global warming might support more active and species-rich ant assemblages, which in turn would mediate increased predation on herbivorous arthropods. However, this prediction should be made with reservation, as it assumes that the dispersal of ants keeps pace with the climatic changes as well as a one-dimensional relationship between ants and predation within a food-web that comprises species interactions of much higher complexity. Our results also suggested that degraded forests in our study area might provide suitable habitat for epigaeic, ground-dwelling ant assemblages that do not differ in incidence, species richness, functional richness, composition, or predation on arthropods from assemblages of primary forests. Most importantly, our results suggest that the occurrence and activity of ants are important drivers of ecosystem processes and that changes in the incidence and richness of ants can be used as effective indicators of responses to temperature changes and of predation within mega-diverse forest ecosystems.