Biodiversity studies on forest canopies often have narrow arthropod taxonomic focus, or refer to a single species of tree. In response, and to better understand the wide range of drivers of arthropod ...diversity in tree canopies, we conducted a large-scale, multi-taxon study which (a) included effect of immediate surroundings of an individual tree on plant physiological features, and (b), how these features affect compositional and functional arthropod diversity, in a warm, southern Afro-temperate forest. We found that tree species differed significantly in plant physiological features and arthropod diversity patterns. Surprisingly, we found negative correlation between surrounding canopy cover, and both foliar carbon and arthropod diversity in host trees, regardless of tree species. Subtle, tree intraspecific variation in physiological features correlated significantly with arthropod diversity measures, but direction and strength of correlations differed among tree species. These findings illustrate great complexity in how canopy arthropods respond to specific tree species, to immediate surroundings of host trees, and to tree physiological features. We conclude that in natural forests, loss of even one tree species, as well as homogenization of the crown layer and/or human-induced environmental change, could lead to profound and unpredictable canopy arthropod biodiversity responses, threatening forest integrity.
Tree canopies in forests host exceptional levels of biodiversity. However ecologists are still faced with vast regional gaps in need of fundamental information on the nature of tree canopy diversity. ...The southern Afrotemperate forests at the tip of Africa are ancient, existing today as a biogeographical string of natural remnants from long-past climate events. Their canopy arthropod fauna is scarcely known, but could provide insights into global canopy diversity patterns. As part of the broader Afromontane archipelago and southern hemispheric forests in general, they form part of the last and largest global gaps in canopy science. Here we ask how the canopy beetle diversity of these forests qualitatively and quantitatively compare with other forests around the world. We show that beetle species richness and richness per m
2
are closer to temperate than tropical canopies. Estimates for beetle richness most closely matched the latitudinal equivalent Chilean temperate rainforests. However, feeding guild structure of beetles are closer in resemblance to tropical than temperate forest canopies, in line with expectations regarding the region’s paleo-history. The most diverse beetle families, Curculionidae, Staphylinidae and Chrysomelidae, were proportionately similar to both temperate and tropical forest canopies. We discuss the importance of latitude, paleo-history and forest size when considering canopy fauna in a global context.
Communities are mostly composed of rare species; yet, the factors that determine their patterns of occurrence remain obscure. Theory predicts that, in contrast with common species, the occurrence of ...rare species will be poorly correlated with environmental variables (niches) and more affected by stochasticity (ecological drift), but how this pattern varies across different trophic groups is still poorly understood. Here, we compared the ability of environmental variables (bottom–up biotic niches) to predict the occurrence of plant‐dwelling arthropods across different abundance classes in the Cape Floristic Region of South Africa. We compared three trophic groups, including 104 herbivorous hemipteran, 171 parasitoid wasp and 84 spider species, totalling 4511 individuals in 48 quadrats. To quantify bottom–up biotic niches, we studied the influences of species composition of plants on hemipterans, and of plants and hemipterans on spiders and wasps. We compared the observed strength of the correlation between rare species and their niches with expectations that were generated by repeatedly rarefying abundant species. A large proportion of arthropod species were very rare, i.e. with only one or two individuals (49–55%). Although rarefying abundant species greatly decreased the correlation with bottom–up biotic niches, bottom–up biotic niches generally better predicted the occurrence of rarefied abundant species than very rare ones, suggesting a greater influence of drift on very rare arthropods. That is, (very) rare arthropods are distributed more randomly than rarefied abundant species. Nevertheless, trophic groups differed in the details of their response to bottom–up biotic niches. Plant species composition was a better predictor of rarefied abundant than truly rare hemipterans. In contrast, the importance of bottom–up biotic niches among abundance classes varied less visibly in spiders and wasps. Our study thus suggests that the importance of niches in structuring arthropod communities depends on species rarity and trophic group.
Fungi in the genera
Knoxdaviesia
and
Sporothrix
dominate fungal communities within
Protea
flowerheads and seed cones (infructescences). Despite apparently similar ecologies, they show strong host ...recurrence and often occupy the same individual infructescence. Differences in host chemistry explain their host consistency, but the factors that allow co-occupancy of multiple species within individual infructescences are unknown.
Sporothrix splendens
and
K. proteae
often grow on different senescent tissue types within infructescences of their
P. repens
host, indicating that substrate-related differences aid their co-occupancy.
Sporothrix phasma
and
K. capensis
grow on the same tissues of
P. neriifolia
suggesting neutral competitive abilities. Here we test the hypothesis that differences in host-tissues dictate competitive abilities of these fungi and explain their co-occupancy of this spatially restricted niche. Media were prepared from infructescence bases, bracts, seeds, or pollen presenters of
P. neriifolia
and
P. repens
. As expected,
K. capensis
was unable to grow on seeds whilst
S. phasma
could. As hypothesised,
K. capensis
and
S. phasma
had equal competitive abilities on pollen presenters, appearing to explain their co-occupancy of this resource. Growth of
K. proteae
was significantly enhanced on pollen presenters while that of
S. splendens
was the same as the control.
Knoxdavesia proteae
grew significantly faster than
S. splendens
on all tissue types. Despite this,
S. splendens
was a superior competitor on all tissue types. For
K. proteae
to co-occupy infructescences with
S. splendens
for extended periods
,
it likely needs to colonize pollen presenters before the arrival of
S. splendens
.
Protea
flowers host saprobic
Knoxdaviesia
and
Sporothrix
fungi that are dispersed by pollinating insects and birds. Different
Protea
species contain sympatric populations of different fungal species. ...For example,
P. repens
host
S. splendens
and
K. proteae
, while
P. neriifolia
host
K. capensis
and
S. phasma
. Even though all fungi can grow vigorously on alternative hosts and they share the same spore vector species, they rarely colonise alternative hosts. We investigated the role of fungal differential competitive abilities on their usual and alternative hosts to explain their host exclusivity. In a de Wit replacement series experiment,
S. splendens
outcompeted and later overgrew all other fungi on media prepared from its usual and alternative hosts. Host exclusivity of
S. splendens
on
P. repens
may therefore be maintained by restricted movement of spore vectors rather than weaker competitive abilities on alternative hosts. On their preferred hosts,
S. splendens
and
S. phasma
rapidly overgrew
Knoxdavesia
species with which they do not usually share a host, explaining host exclusivity of the
Knoxdavesia
species.
Knoxdaviesia proteae
likely only persist on
P. repens
with sympatric
S. splendens
if it colonizes flowers earlier, in a different area, or completes its life cycle before being overgrown. On their usual
P. neriifolia
host,
K. capensis
and
S. phasma
had neutralistic interactions and
S. phasma
could not overgrow
K. capensis
, explaining their co-existence. Host exclusivity of saprobic
Protea
-associated
Knoxdaviesia
and
Sporothrix
may therefore be maintained by both the activities of spore vectors and differential competitive abilities on different hosts, but the influence of other competing microbes and micro-niche differentiation cannot be excluded.
Southern Africa's grassland biodiversity is threatened by habitat transformation such as commercial forestry. Ecological networks (ENs) have been instigated to alleviate the pressure of habitat ...transformation on local biodiversity. ENs are large scale webs of corridors and patches of natural vegetation criss-crossing production landscapes that can simulate conditions in protected areas (PAs). Many ENs have lost many native large mammal species, which have been replaced by domestic livestock to retain natural grazing dynamics, which could have an impact on the long-term value of ENs for insects. Here we compared dung beetle, butterfly and grasshopper diversity in ENs across a landscape mosaic of timber plantations, where 1) wild megaherbivores were maintained, 2) in ENs where these herbivores were replaced by livestock and, 3) in a nearby World Heritage PA which retained its natural complement of megaherbivores. Sites in the PA far from any plantation were similar in composition to those in the wild grazed EN. Presence of the wild grazers improved the alpha- and beta-diversity of all focal insect taxa when compared to domestic grazing. Furthermore, species composition shows significant differences between the two grazing systems indicating that an assemblage of native large mammals facilitates insect diversity conservation. We support the maintenance or introduction of large native mammals in ENs or similar conservation areas in production landscapes to simulate the ecological conditions and natural heterogeneity in nearby PAs.
The polyphagous shot hole borer (PSHB) beetle is a recent invader in South Africa. Together with its fungal symbiont, Fusarium euwallaceae, it can rapidly kill highly susceptible host plants. Its ...impact is most profound in urban areas, but it has also been found infesting impor tant forestry, agricultural crop and native species. Since its first detection in 2012, PSHB has spread to all but one province in the country. The beetle-fungus complex has several biological traits that enhance its anthropogenically mediated dispersal, establishment and survival in novel environments - factors that have likely facilitated its rapid spread across the country. We review the history of the PSHB invasion in South Africa, its taxonomic status and the reasons for its rapid spread. We highlight its potential impact and challenges for its management. Finally, we provide an updated distribution map and list of confirmed host plants in South Africa. Of the 130 plant species identified as hosts, 48 of these (19 indigenous and 29 introduced) are reproductive hosts able to maintain breeding PSHB populations. These reproductive hosts may succumb to beetle infestations and act as 'pest-amplifiers'. The economic impact on urban forests, plantation forestry and agricultural crops may be severe, but the ecological impact of PSHB invasion in native ecosystems should not be underestimated.Significance: • We provide an updated host list and distribution map for South Africa of the globally significant tree pest, the polyphagous shot hole borer (PSHB, Euwallacea fornicatus). The South African PSHB invasion represents the largest outbreak of this pest in its global invaded range. PSHB was confirmed to infest 130 plant species in urban, agricultural, and native ecosystems in South Africa, including 44 previously unreported hosts. Impact in South Africa is in its infancy but will likely be substantial to local economes and ecosystems. Mitigation has proven difficult, but numerous research projects have been initiated throughout the country.
Teratosphaeria destructans
is one of the most aggressive foliar pathogens of
Eucalyptus
. The biological factors underpinning
T. destructans
infections, which include shoot and leaf blight on young ...trees, have never been interrogated. Thus, the means by which the pathogen modifies its host environment to overcome host defences remain unknown. By applying transcriptome sequencing, the aim of this study was to compare gene expression in a South African isolate of
T. destructans
grown on nitrogen-deficient and complete media. This made it possible to identify upregulated genes in a nitrogen-starved environment, often linked to the pathogenicity of the fungus. The results support the hypothesis that nitrogen starvation in
T. destructans
likely mirrors an
in planta
genetic response
.
This is because 45% of genes that were highly upregulated under nitrogen starvation have previously been reported to be associated with infection in other pathogen systems. These included several CAZymes, fungal effector proteins, peptidases, kinases, toxins, lipases and proteins associated with detoxification of toxic compounds. Twenty-five secondary metabolites were identified and expressed in both nitrogen-deficient and complete conditions. Additionally, the most highly expressed genes in both growth conditions had pathogenicity-related functions. This study highlights the large number of expressed genes associated with pathogenicity and overcoming plant defences. As such, the generated baseline knowledge regarding pathogenicity and aggressiveness in
T. destructans
is a valuable reference for future
in planta
work.
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