Casparus J. Crous

Exploring the mesofilter as a novel operational scale in conservation planning

Summary 1. Increased emphasis is being placed on developing effective biodiversity conservation tools for practical conservation planning. The mesofilter is such a biodiversity planning tool, but has yet to be fully explored to appreciate its effectiveness. The key premise of the mesofilter is that ecosystems contain certain physical elements that are specifically associated with a diversity of species. Identifying such mesofilters could therefore complement existing conservation planning tools such as coarse and fine filters. 2. To explore the value of the mesofilter as an operational scale in conservation planning, we studied 18 remnant patches of endangered montane grassland in KwaZulu-Natal, South Africa, using the physical landscape feature of patch rockiness as an abiotic surrogate for biodiversity. The objective was to determine whether the mesofilter of rockiness can predict variation in species richness and composition for three dominant grassland taxa (plants, butterflies and grasshoppers) at the landscape scale. 3. Variable levels of rockiness had significant interactions with all three focal taxa. Higher species richness of all taxa was closely associated with higher levels of rockiness in a patch. The rocky mesofilter only predicted significant differences in species composition for butterflies. Elevation was also important, possibly another mesofilter for plants and grasshoppers in this landscape. 4. Synthesis and applications. The results indicate that the use of an abiotic surrogate such as rockiness can predict biodiversity value across multiple taxa. The mesofilter is therefore a valuable surrogacy and congruency tool for practical biodiversity conservation across this landscape and would likely have similar value if explored elsewhere. It also has value in the design and management of protected areas.

Drought-tolerance of an invasive alien tree, Acacia mearnsii and two native competitors in fynbos riparian ecotones

Invasive alien plants (IAPs) have successfully invaded many riparian zones in South Africa, especially Australian Acacia spp. which are prevalent along riverbanks in the south-western Cape of South Africa. This Mediterranean-type climate region is predicted to endure severe future water shortages under likely scenarios of increased population growth and climate change, and IAPs aggravate this problem due to their profligate water use. Acacia mearnsii competes aggressively with native species, however, it remains unclear what physiological advantage the species has over co-occurring native species under the predicted reduced streamflow scenarios. A mechanistic approach was used to investigate how key native fynbos riparian woody tree species compare in vulnerability to drought-induced cavitation against A. mearnsii by comparing findings from three Mediterranean-type fynbos river systems that differ in streamflow. A. mearnsii showed lower water potential at 50% hydraulic conductivity loss (P50 values) compared to native species at certain sites, an indication of drought-tolerance. This suggests it is likely to persist under future drier conditions and it therefore remains a top priority for control. The native Brabejum stellatifolium had consistently higher water potentials across all sites than the other studied species, and is a potentially valuable species for restoration of south-western Cape riparian zones. Consistency in the shapes of species vulnerability curves across sites illustrated a species-specific hydraulic response to different water availability, strengthening the argument that this approach to distinguish site-level drought-tolerance between trees is a practical technique, with great application in understanding future geographic distribution under climate change, and potential for use in restoration research. Additionally, streamflow was an inaccurate predictor of species drought-tolerance along these riparian systems.

Tree invasions and biosecurity: eco-evolutionary dynamics of hitchhiking fungi

Non-native plants reach novel environments with hidden fungal hitchhikers. Some remain on their hosts, often contributing to invasiveness while others move beyond and naturalise and some cause epidemics on native trees. Among the three main types of fungal associations, pathotroph, symbiotroph or saprotroph, there are many categories reflecting either the part of the plant with which the fungus forms an association, the type of association, or the type of disease symptom it causes. The likelihood of non-native fungi forming novel host associations in a new environment is dependent upon the type of association.

Root resistance to cavitation is accurately measured using a centrifuge technique.

Plants transport water under negative pressure and this makes their xylem vulnerable to cavitation. Among plant organs, root xylem is often highly vulnerable to cavitation due to water stress. The use of centrifuge methods to study organs, such as roots, that have long vessels are hypothesized to produce erroneous estimates of cavitation resistance due to the presence of open vessels through measured samples. The assumption that roots have long vessels may be premature since data for root vessel length are sparse; moreover, recent studies have not supported the existence of a long-vessel artifact for stems when a standard centrifuge technique was used. We examined resistance to cavitation estimated using a standard centrifuge technique and compared these values with native embolism measurements for roots of seven woody species grown in a common garden. For one species we also measured vulnerability using single-vessel air injection. We found excellent agreement between root native embolism and the levels of embolism measured using a centrifuge technique, and with air-seeding estimates from single-vessel injection. Estimates of cavitation resistance measured from centrifuge curves were biologically meaningful and were correlated with field minimum water potentials, vessel diameter (VD), maximum xylem-specific conductivity (Ksmax) and vessel length. Roots did not have unusually long vessels compared with stems; moreover, root vessel length was not correlated to VD or to the vessel length of stems. These results suggest that root cavitation resistance can be accurately and efficiently measured using a standard centrifuge method and that roots are highly vulnerable to cavitation. The role of root cavitation resistance in determining drought tolerance of woody species deserves further study, particularly in the context of climate change.

Evolutionary dynamics of tree invasions : complementing the unified framework for biological invasions

Evolution greatly impacts the outcomes of biological invasions. In our review, we review such evolutionary processes, with an emphasis on tree invasions, and place them in the context of a unified framework for biological invasions. The processes and mechanisms described are pre-introduction evolutionary history, sampling effect, founder effect, genotype-by-environment interactions, admixture, hybridization, polyploidization, rapid evolution, epigenetics, and second-genomes. By understanding the mechanisms underlying invasion success, researchers will be better equipped to predict, understand, and manage biological invasions.

Grasshopper assemblage response to surface rockiness in Afro-montane grasslands

Grasshoppers are often an important functional component of ecosystems, and many species show high levels of endemism. Evidence exists that percentage surface rock cover within a landscape can predict diversity of grasshopper species. Nevertheless, the reason why grasshopper species are responding to rocky landscapes has not been established. Here, we explore whether grasshoppers are responding to physical rockiness per se, or rather to specific correlates of higher surface rock exposure within a landscape. We also determine if this response varies between grasshopper taxonomic groups. We sampled grasshoppers in Afro‐montane grasslands in KwaZulu‐Natal, South Africa, and recorded 10 environmental variables. We explored the influence of these variables on grasshopper community composition and grasshopper family composition. We also determined the vegetation characteristics, which significantly correlate with percentage surface rock cover in this landscape (geophyte richness, perennial grass richness and vegetation density), and then measured the similarity of species composition across these correlates. Overall, grasshopper assemblage composition, as well as familial composition, responded strongly to an elevation gradient, and not to the correlates of surface rock cover. In turn, the higher species richness in such areas is more likely a function of the significant vegetation correlates of higher surface rock cover. Across taxonomic groups, there are specialist species within each group which are associated with environmental conditions related to surface rockiness through its underlying correlates. Rock exposure across this grassland landscape is therefore an important contributor to grasshopper dispersion patterns, and has important implications for conservation planning for this taxon.

Wood anatomical traits as a measure of plant responses to water availability: invasive Acacia mearnsii De Wild. compared with native tree species in fynbos riparian ecotones, South Africa

Riparian ecotones in the fynbos biome of South Africa are heavily invaded by woody invasive alien species, which are known to reduce water supply to downstream environments. To explore whether variation in species-specific functional traits pertaining to drought-tolerance exist, we investigated wood anatomical traits of key native riparian species and the invasive Acacia mearnsii across different water availability proxies. Wood density, vessel resistance against implosion, vessel lumen diameter and vessel wall thickness were measured. Wood density varied significantly between species, with A. mearnsii having denser wood at sites in rivers with high discharge. As higher wood density is indicative of increased drought tolerance and typical of drier sites, this counter-intuitive finding suggests that increased wood density was more closely related to midday water stress, than streamflow quantity per se. Wood density was positively correlated with vessel resistance against implosion. Higher wood density may also be evidence that A. mearnsii is more resistant against drought-induced cavitation than the studied native species. The observed plastic response of A. mearnsii anatomical traits to variable water availability indicates the ability of this species to persist under various environmental conditions. A possible non-causal relationship between wood anatomy and drought tolerance in these riparian systems is discussed.

Endophytic Botryosphaeriaceae, including five new species, associated with mangrove trees in South Africa

Little is known regarding the fungi, especially fungal pathogens, associated with mangroves in Africa. This includes fungi in the Botryosphaeriaceae that comprise numerous opportunistic, stress-associated pathogens often associated with trees affected by environmental and anthropogenically generated stresses, such as those affecting mangroves. We investigated the occurrence of endophytic Botryosphaeriaceae along the entire distribution of mangroves in South Africa. Asymptomatic branches were collected from ten localities and six mangrove species. Isolates resembling species of Botryosphaeriaceae were identified based on multi-gene sequence data of the internal transcribed spacer regions (ITS), including the 5.8S nrRNA, the beta-tubulin (tub2), partial translation elongation factor 1-alpha (tef1-α), and DNA-directed RNA polymerase II second largest subunit (rpb2) gene regions. Inoculation trials were conducted on healthy branches of Avicennia marina and Bruguiera gymnorrhiza to evaluate the potential pathogenicity of the collected species. Fourteen species in the Botryosphaeriaceae belonging to four genera, Botryosphaeria, Diplodia, Lasiodiplodia, and Neofusicoccum were collected, including five new species. Neofusicoccum was the most prevalent genus followed by Lasiodiplodia, with species of Diplodia and Botryosphaeria being the least frequent. The inoculation studies revealed that one of the new species, Lasiodiplodia avicenniae is highly pathogenic to A. marina and could pose a threat to the health of these trees.

Ecological disequilibrium drives insect pest and pathogen accumulation in non-native trees

Non-native plant invasions represent ecological disequilibrium situations, that is, the immediate breakdown of historically co-evolved interactions once introduced into novel environments. Globalization has led to increased transfer in organisms between countries. Native enemies also undergo host expansions onto non-native plants. Ecological disequilibrium conditions can thus be expected to change unpredictably over time. By retrospectively analysing the insect pest and pathogen accumulation on established non-native trees in South Africa, we review the eco-evolutionary background that may help to explain and predict variation in ecological disequilibrium conditions in non-native trees.

Rockiness determines meso-scale conservation of butterflies in Afro-montane grassland

Understanding how and why certain species respond to various habitat resources can optimize conservation strategies. Furthermore, behaviour can contribute significantly to predicting the presence or absence of a species under certain habitat conditions. There is a measurable interaction between higher percentage rock exposure in a landscape and butterfly species richness and composition in montane grasslands. Here, we attempt to explain this interaction by measuring the behavioural responses of montane butterfly species to rock cover. The butterfly assemblage was observed across three increasing levels of rockiness in the landscape. At sites within each of these rockiness categories, we also sampled the different behavioural traits of the different species. We determined whether there were significant differences in behavioural traits among this assemblage in response to rockiness. We also identified the specific species which were responsible for driving differential behavioural responses under varying rock exposure in a landscape. The rockiest areas had significantly more behavioural events, and these behaviours were more often associated with direct utilization of rocks, and related to agonistic interaction. Certain butterfly species therefore use rocks as a utility habitat resource, in different ways, highlighting the importance of the resource-based habitat concept for conservation. As such, for butterfly conservation in these montane grasslands, emphasis is placed on including a rocky gradient in protected areas and conservation landscape designs.