Effect of Prosopis (mesquite) invasion and clearing on vegetation cover in semi-arid Nama Karoo rangeland,South Africa

The Nama-Karoo is the largest of the three biomes that comprise the semi-arid Karoo-Namib ecoregion of southern Africa. Large tracts of Nama-Karoo rangeland have been invaded by alien leguminous trees of the genus Prosopis. We assessed the impact of Prosopis invasion and clearing on vegetation cover in heavily grazed Nama-Karoo rangeland on two sheep farms in the Beaufort West district of the Western Cape province of South Africa. Our results suggest that, below critical threshold cover levels, Prosopis trees do not affect indigenous vegetation and add to overall rangeland vegetation cover. However, once Prosopis cover exceeds critical thresholds, the trees begin to negatively impact on indigenous vegetation (especially grasses) and overall vegetation cover. In addition, our data suggest that vegetation cover recovers after Prosopis clearing, being temporarily higher than pre-invasion levels possibly as a result of enhanced post-invasion soil fertility. Lastly, our results suggest that Prosopis trees reduce rangeland erosion potential at low cover and increase it after they exceed critical threshold cover. Information about thresholds of Prosopis impact can be used to direct clearing efforts for maximum benefit when resources are limited.     

Small‐scale reseeding trials in arid rangeland: Effects of rainfall,clearing and grazing on seedling survival

Abstract

Seeds of four indigenous plant species were sown in 1990 in small‐scale field trials in an arid, non‐seasonal rainfall region of the southern Karoo, South Africa. Seeds of a winter annual Tetragonia echinata (Aizoaceae) showed innate dormancy but those of three perennial shrubs Pteronia empetrifolia, P. pollens and Osteospermum sinuatum (all Asteraceae) emerged only after the first substantial autumn rain. In good rangeland, seedling growth and survival was better in clearings than in undisturbed vegetation. In poor, continuously‐grazed rangeland, sheep had more influence on growth and survival of O. sinuatum seedlings than did vegetation clearing. These findings should be tested by means of large‐scale trials replicated in time and space.     

Inselbergs persist as islands of diversity in a heavily grazed rangeland mosaic at the nexus of three arid biomes

Inselbergs are regarded as ‘islands of diversity’ due to the high number of plant species present and level of endemism. They also act as natural sources of fodder for livestock and thus risk becoming homogenised in a heavily grazed rangeland. The aim of this study was to compare plant diversity on inselbergs and matrices in heavily grazed sections of three arid biomes of South Africa’s west. The inselbergs are scattered in a matrix of rangeland vegetation in the Desert, Nama-Karoo and Succulent Karoo biomes where pastoral herding is the main land use. Plant composition, cover and growth forms were quantified in plots and along transects in 21 inselberg and matrix sites. Inselbergs had more diverse plant and growth forms and greater cover than adjacent matrices. Defoliated plants on inselbergs are able to recover due to more mesic conditions and because inselbergs are largely grazed during drier periods, species are able to flower and set seed during winter. Inselbergs are also less accessible to livestock due to their unique geomorphology. Therefore, inselbergs are able to persist as islands of diversity and warrant inclusion into conservation and rangeland farming management plans since climate change will result in biome boundary shifts.     

Species Richness and Abundance of Clinid Fish (Teleostei; Clinidae) in Intertidal Rock Pools

Factors affecting species richness and abundance of clinid fish(Fam. Clinidae)in 19 intertidal rock pools near Muizenberg, South Africa, were investigated. Some measure of cover is the most important predictor of clinid species richness, abundance and biomass. Intraspecific partitioning of habitat by Clinus superciliosus was observed, and apparently occurs in C. cottoides as well. Fish were observed to move between pools during high tides, and some individuals showed fidelity to particular pools. There is some evidence for suggesting that C. superciliosus might be a colonizing species.     

Exploring the invasion of rangelands by Acacia mearnsii (black wattle): biophysical characteristics and management implications

Australian acacias have spread to many parts of the world. In South Africa, species such as A. mearnsii and dealbata are invasive. Consequently, more effort has focused on their clearing. In a context of increasing clearing costs, it is crucial to develop innovative ways of managing invasions. Our aim was to understand the biophysical properties of A. mearnsii in grasslands as they relate to grass production and to explore management implications. Aboveground biomass (AGB) of A. mearnsii was determined using a published allometric equation in invaded grasslands of the northern Eastern Cape, South Africa. The relationships among the A. mearnsii leaf area index (LAI), normalised difference vegetation index (NDVI) and AGB were investigated. The influence of A. mearnsii LAI and terrain slope on grass cover was also investigated. Strong linear relationships between NDVI, LAI and AGB were developed. Acacia mearnsii canopy adversely impacted grass production more than terrain slope (p < 0.05) and when LAI approached 2.1, grass cover dropped to below 10% in infested areas. Reducing A. mearnsii canopy could promote grass production while encouraging carbon sequestration. Given the high AGB and clearing costs, it may be prudent to adopt the ‘novel ecosystems’ approach in managing infested landscapes.     

Long-Term Effects of Weed Control With Picloram Along a Gradient of Spotted Knapweed Invasion

Broadleaf herbicides are commonly used in rangelands to suppress exotic weeds and release native communities from negative impacts of invasion. However, few studies have comprehensively evaluated treatment effects on differing community components across a gradient of initial invasion levels. We conducted a 6-yr experiment within grasslands of western Montana to measure local-scale effects of a broadcast application of picloram on 1) cover of the target invader, spotted knapweed (Centaurea stoebe L.), 2) prevalence of native functional groups, and 3) the secondary invader cheatgrass (Bromus tectorum L.) at differing initial levels of knapweed invasion. Treatment effectively suppressed knapweed, with cover in treated vs. control plots reduced by >60% in the sixth posttreatment year. Treatment also appeared to alleviate knapweed’s impacts on native perennial grasses, but only at the highest initial level of invasion, where cover of this group increased by >30% in treated vs. control plots to equal levels associated with noninvaded plots. In some cases, treatment appeared to exacerbate knapweed’s impacts on native forbs. At the no-invasion level, perennial forb cover declined by >20% in treated vs. control plots to match values associated with moderate or high levels of invasion, but these treatment effects were minimal at the latter invasion levels. Across initial invasion levels, species richness of perennial and/or annual forbs declined by >20% in treated vs. control plots. Treatment also promoted increases in cheatgrass cover, although differences between treated and control plots were relatively small by the sixth posttreatment year. Overall, effects of picloram application depended on initial levels of knapweed invasion, largely due to the varying strength of release effects. Selective treatment of invaded patches vs. broadcast applications would reduce side effects of broadleaf herbicide application and increase compatibility with other management measures designed to improve rangeland conditions and restore grassland communities.     

Bunchgrass Root Abundances and Their Relationship to Resistance and Resilience of Burned Shrub-Steppe Landscape

Invasion of exotic annual grasses (EAG) and increased wildfire have led to an emphasis on managing rangeland plant communities for resistance to invasion and resilience to disturbances. In sagebrush steppe and similar rangelands, perennial bunchgrasses and particularly their roots are hypothesized to be primary contributors to resistance and resilience. We asked how bunchgrass root abundance relates to annual grass invasion and aboveground indicators of bunchgrass vigor that are more readily measured, such as plant height. We used a standardized US Department of Agriculture protocol for root measurement in 445 excavations made in 2016 ? 2018 across a topographically and ecologically varied region of sagebrush steppe burned in the 2015 Soda fire in the Northern Great Basin, United States. Nearly all (99%) bunchgrasses, including seedlings, had deeper roots than the surrounding annual grasses (mean depth of annuals = 6.8 ± 3.3 cm), and 88% of seedlings remained rooted in response to the “tug test” (uprooting resistance to ~ 1 kg of upward pull on shoot), with smaller plants (mean height and basal diameters < 20 cm and < 2 cm, respectively) more likely to fail the test regardless of their root abundance. Lateral roots of bunchgrasses were scarcer in larger basal gaps (interspace between perennials) but were surprisingly not directly related to cover of surrounding EAG. However, EAG cover increased with the size of basal gaps and decreased with greater basal diameter of bunchgrass (in addition to prefire EAG abundance), albeit with a low r². These results provide some support for 1) the importance of basal gaps and bunchgrass diameters as indicators of both vulnerability to annual grass invasion and bunchgrass root abundance and 2) the need for more detailed methods for root measurement than used here in order to substantiate their usefulness in understanding rangeland resistance and resilience.     

Livestock grazing has minimal effect on the species richness and diversity of semi-arid shrubland in the Western Little Karoo,South Africa

The Succulent Karoo, one of two arid biodiversity hotspots in the world, is known for its high plant species richness, but little is known about the influence of topography and how it mediates the potentially deleterious effects of grazing. Changes in vegetation species composition, cover and species diversity were examined along piosphere gradients on northerly slopes, bottomlands and low-lying plains on 45 farms. Landscapes differed in the plant speciesand life-form composition, species richness and cover, with the more species-rich, heterogeneous grassy northerly slopes distinct from the plains and bottomlands, which were dominated by Pteronia pallens, Psilocaulon junceum and Drosanthenum spp. with more annual and ephemeral species. Overall, species richness declined linearly with decreasing rangeland condition. A weak grazing effect was detected only on the north-facing slopes, where shrubs and grass decreased with intense grazing. Years of overgrazing have resulted in the widespread dominance of P. pallens, especially on the plains and bottomlands, rendering them insensitive to grazing. The current condition of the northern slopes should be maintained and managed such that palatable species can spread to more degraded areas of the landscape.     

Patchiness in semi‐arid dwarf shrublands: Evidence from satellite‐derived indices for elevated CO2 assimilation rates on a geochemical mound in the Karoo National Park,South Africa

Abstract

Satellite‐derived vegetation indices were used to identify a geochemical mound of higher active greenness in the Karoo National Park, Beaufort West, South Africa. We determined whether this mound was occupied by plants with higher CO₂ assimilation rate. Plant cover on and off the mound was determined. Three woody species with high cover were selected for further investigation. Two deep‐rooted species, Rhigozum obovatum Burch. and Eriocephalus ericoides (L.f.) Druce, showed greater net CO₂ assimilation rates on the mound. Net CO₂ assimilation rates for the third species, Pentzia incana (Thunb.) Kuntze were similar both on and off the mound. In an attempt to find a mechanistic basis for the elevated CO₂ assimilation rates, the relationships between soil factors, foliar nutrients and CO₂ assimilation capacity were also examined. Our results suggest that the elevated net CO₂ assimilation was not mediated via improved soil or plant water relations on the actively greening mound, nor through a difference in nitrogen levels in the soil or plant material, but possibly by way of the higher sub‐soil phosphorus levels measured from the geochemical mound. Genotype and cover cannot alone be used for rangeland condition assessment since localized elevated soil nutrient status (patchiness) contributes to greater photosynthetic carbon gain which may confer superior browsing responses to plants occurring on these mounds.     

Interactions of grazing and rainfall on vegetation at Grootfontein in the eastern Karoo§

Rainfall and grazing are primary drivers of vegetation composition in the Nama-Karoo. Increased rainfall increases grassiness, to where Nama-Karoo transitions to grassland. Severe grazing treatments (e.g. continuous or summer-only) increase abundance of grazing-tolerant dwarf shrubs and annual grasses, and decrease perennial grasses. Grootfontein, which is ecotonal between the Nama-Karoo and Grassland Biomes, houses long-term grazing trials. The area has experienced higher-than-average rainfall in recent decades. Plant basal cover data from the 1960s and 2010s allow several hypotheses to be addressed: (1) historical severe grazing (until 1985) will limit subsequent grassiness (grazing legacy effect); (2) severe grazing will preclude increases in grassiness, independent of rainfall (herbivore trap effect); and (3) historically leniently-grazed sites will transition to grassland with increased rainfall (biome shift effect). Rainfall was lower from 1957–1966 (350 mm) than from 2003–2012 (490 mm). The grazing legacy effect was supported based on the abundance of Aristida diffusa, despite all sites becoming much grassier. The herbivore trap effect was not supported. The biome shift effect was supported in that shifts to grassland sometimes occurred. Results suggest that increasing rainfall has prompted a shift to much increased grassiness and decreased abundance of dwarf shrubs, and that grazing had a smaller secondary effect.     

Long-Term Effects of Aeration and Fire on Invasion of Exotic Grasses in Mixed-Brush Plant Communities

Invasion of exotic grasses into grasslands dominated by native plants changes fire cycles and reduces biodiversity. Brush management practices that create soil disturbance, such as aeration, may potentially result in invasion of exotic grasses and replacement of native vegetation. We tested the hypothesis that a long-term effect of aeration and prescribed burning is an increase in exotic grasses. The study was conducted at the Chaparral Wildlife Management Area in the western south Texas plains where four treatments were evaluated: aeration, warm-season burn, aeration followed by a warm-season burn, and no treatment (control). The experimental design was a randomized complete block with four replicates. We estimated percentage canopy cover of exotic grasses, native grasses, forbs, litter, bare ground, and woody and succulent plants in 2007. There was a multivariate main effect among treatments for the dependent variables absolute canopy cover of exotic grasses, native grasses, forbs, litter, and bare ground (Wilks's Lambda F_15,179.84 = 2.78, P = 0.001). Variables that contributed to the significant overall effect included litter (F_3,69 = 4.32, P = 0.008) and native grasses (F_3,69 = 6.11, P = 0.001). The multivariate main effect of treatment was significant (Wilks's Lambda F_9,180.25 = 2.04, P = 0.038) for the relative canopy cover of herbaceous species. Relative cover of exotic grasses was 31% higher (P = 0.024) in control than in the prescribed burn treatment. Native grasses relative cover was 30% higher (P = 0.003) in prescribed burn than in the control treatment. We did not detect differences among treatments in the percentage of total woody and succulent plants canopy cover (P = 0.083). Under the environmental conditions at the time of the study, aeration and/or prescribed burning do not increase exotic grasses.     

A preliminary survey of viruses in pasture grasses in South Africa

Little work has been done in South Africa on the incidence of viruses in pasture grasses. The aim of this work was to carry out a survey of virus infections of some of the economically‐important pasture grasses in South Africa. Twelve winter and summer pasture grass species and one cereal forage crop species were collected from six different regions in early, mid‐ and late winter and summer. Plants were tested for virus(es) by means of symptomatology, transmission, dot‐blot immunoassays, Ouchterlony tests and electron microscopy. Serological tests indicate that Avena sativa from Roodeplaat and Bromus unioloides from Potchefstroom, Cedara and Nooitgedacht are positive for maize dwarf mosaic virus (MDMV). Lolium multiflorum from Cedara appears to be infected with two viruses, brome mosaic virus (BMV) and a potyvirus‐like agent, possibly ryegrass mosaic virus (RMV).     

Communal farmers’ ecological knowledge and perceptions of grasses in the central Eastern Cape province,South Africa: similarities with field studies and scientific knowledge and their implications

The local ecological knowledge (LEK) and perceptions of grasses were investigated in six semi-arid communal areas in the central Eastern Cape province, South Africa, and compared with field studies and scientific knowledge. Of the total of 21 grasses identified in the field survey, communal farmers had vernacular names only for 11 species. Respondents rated Digitaria eriantha and Pennisetum clandestinum to have the highest (p < 0.05) grazing value. Respondents’ ratings of Eragrostis obtusa and Sporobolus africanus as having similar (moderate) grazing values to Themeda triandra is against the established knowledge. Respondents’ ratings of T. triandra and S. africanus as having higher (p = 0.01) ecological value than the other grasses concur with the scientific knowledge, whereas their nomination of E. obtusa and D. eriantha as providing less ecological benefits than Aristida conjesta disagrees with some ecologists. The study hinted at the complementarity and discrepancies between LEK and scientific knowledge that need ratification. The respondents’ lack of detailed knowledge is suggestive of a gradual eroding of LEK among the local communities that will adversely impact the local people’s ability to implement sustainable rangeland utilisation practice. The current study also recognised farmers’ familiarity as well as training needs that would expound their knowledge and management of grasses.     

Monitoring Geomorphic and Traditional Post-Mine Reclamation using Digital Imagery: Vegetative Heterogeneity and Sampling Efficiency

Geomorphic reclamation creates variable topography and surface architecture, including rolling hillslopes and drainages. In contrast, traditional methods of reclamation result in landscapes susceptible to erosion due to steep, linear gradients. Geomorphic approaches to surface mine reclamation are relatively new, and hypotheses suggest the use of geomorphic principles in reclamation will improve vegetation outcomes relative to traditional methods. Topographic variability created by geomorphic reclamation likely results in more environmental heterogeneity, which should correlate with greater plant diversity. We examined revegetation outcomes of traditional and geomorphic reclamation on two reclaimed surface mines in Wyoming using nadir image sampling. Functional group diversity and measures of cover were compared between reclamation methods and undisturbed rangeland. Geomorphic reclamation supported greater total richness and greater native functional group richness relative to traditional reclamation. Native species cover on geomorphic reclamation, particularly for native perennial grasses, was either similar to undisturbed rangeland or greater than undisturbed rangeland and traditional reclamation. Reclamation shrub cover differed significantly from undisturbed sites, but was greater in geomorphic treatments. Results of nadir image analysis are compared to line-point intercept data from the same locations and outcomes are discussed in light of different reclamation techniques and sampling methods. Significant differences in cover categories were observed between nadir image and line-point intercept methods, however both methods revealed similar patterns between study sites.     

A description of the life stages of Echinoparyphium elegans (Trematoda: Echinostomatidae)

South Africa urgently requires a national strategic plan for the conservation of the country's biodiversity. The formulation of such a plan would be relatively easy if centres (hotspots) of richness, endemism and rarity were congruent, both within and among many different taxa, if these hotspots captured a large proportion of the total species, and if hotspots fell within existing reserves. The investigation of six vertebrate taxa (vizfreshwater fish, frogs, tortoises and terrapins, snakes, birds, and various mammal orders) at a national scale reveals that hotspots are not coincident within taxa. Centres of richness are concentrated in the north-eastern areas of the country, whereas endemism is concentrated in the south-west, and centres of rare and threatened {Red Data Book) species can be in either area. Paired comparisons among taxa reveal greatly varying proportional overlaps of species richness hotspots (0-72%). The proportion of total species falling in hotspots ranges from low (66% for fish) to high (92% for birds). Hotspots are thus not an efficient method of siting representative (ideal) reserves. In order to design a more representative reserve system to protect all vertebrate species, a complementarity algorithm was applied to all taxa separately, and then to all taxa combined. The combined analysis yielded more efficient results {66 reserves are required to represent all 1074 species at least once) than the separate analyses (97 reserves). Many of these representative reserves coincide with both hotspots and existing reserves, and over 85% of the hotspots of most taxa coincide with existing reserves, thus South Africa's vertebrate fauna could be more effectively protected with only moderate acquisition of new, well-sited reserves. A biome analysis reveals that these reserves will also have to incorporate areas of those biomes that are currently inadequately protected, viz grassland, Succulent Karoo and Nama-Karoo. The methods outlined in this paper should be applied to as many other taxa as possible, in order to aid the formulation of a national strategic plan for biodiversity conservation.     

Invasion of a South African Anguilla mossambica (Anguillidae) opulation by the alien gill worm Pseudodactylogyrus anguillae (Monogenea)

The parasitic gill monogenean Pseudodactylogyrus anguillae is alien to Africa. In an investigation of 227 longfin eel, Anguilla mossambica, and 26 mottled eel, Anguilla marmorata, sampled from four river systems in the Eastern Cape, South Africa, this parasite was only present on the gills of A. mossambica sampled from the Great Fish River system. In the Great Fish River, it infected 73.2% of the sampled population at a mean intensity of 63.8±34.3 parasites per fish. High prevalence and intensity are indicative of a well-established alien invasive parasitic species. Results showing the absence of P. anguillae from the co-occurring but less abundant A. marmorata indicate that this species may be less susceptible to P. anguillae infection.     

Patterns of distribution and current protection status of the Carnivora,Chiroptera and Insectivora in South Africa

Geographic patterns of species richness and endemism in three mammalian orders (Chiroptera, Insectivora and Carnivora) were studied in relation to the biomes and existing protected areas of greater South Africa (including Lesotho and Swaziland). Locality data for 21500 specimens representing 124 species were analysed with a geographical information system. Species richness of Chiroptera is high in the savanna biome, particularly in the north-east of the country, owing to the marginal intrusion of 14 tropical species. Endemism in Chiroptera is low, however, with only two endemic species in the fynbos and Karoo biomes. The Carnivora display less biome specificity and endemism than the Chiroptera. Whereas the north-eastern savannas have the highest species richness, the transition between the Nama-Karoo and grassland biomes is an important southern African centre of endemism for the Carnivora, particularly the smaller species. In addition to being an important centre for species richness in the Carnivora and Chiroptera, the Kruger National Park is also particularly important for Red Dala Book species in both orders. The Insectivora display both high species richness and endemism. Species richness of the Insectivora is greatest in the mesic south-east of the country, whereas endemism is most pronounced in the forest and grassland biomes. Differences in biome specificity and endemism between these orders reflect not only phylogenetic divergence, but also variation in body size, vagility and life-history strategies. Most of South Africa's endemics are small mammals and many of them are listed in the Red Data Book. Distributions, life-history strategies and trends in man-induced habitat degradation were used to re-evaluale the protection status of the 124 species. We conclude that at least 11 endemic species are not adequately protected by existing publicly owned protected areas and consequently identify several areas which need to be added to the existing protected area system.     

Ventenata and Other Coexisting Exotic Annual Grass Control and Plant Community Response to Increasing Imazapic Application Rates

Ventenata (Ventenata dubia [Leers] Coss.) is an exotic annual grass that can invade intermountain rangeland plant communities, where it can form monotypic stands, degrade wildlife habitat, and reduce livestock forage. There is limited information on ventenata control in rangelands as it has only recently been identified as a substantial problem. Imazapic is a pre-emergent herbicide commonly used to control other exotic annual grasses and, therefore, is likely to control ventenata in rangelands. We evaluated five application rates of imazapic (0 − 175 g ae ∙ ha^− 1) on ventenata and other exotic annual grass control and plant community response at two rangeland sites in 2 yr (2014 and 2015). Imazapic reduced exotic annual grass (largely ventenata) cover and density, with greater control with increasing imazapic rates. Exotic annual grass density at the highest levels of control (82%−94%) was 184 − 299 plants ∙ m^− 2 the first yr after imazapic application. Exotic annual grasses fully recovered in the second or third yr after imazapic application. Bare ground generally increased with imazapic application. However, density of perennial vegetation (grasses and forbs) did not vary among treatments. Perennial vegetation cover generally did not increase with imazapic control of ventenata and other exotic annual grasses. Imazapic can control ventenata; however, even at the highest rates, control was not enough to shift the dominance from exotic annual species to perennial species. Integrating other treatments with imazapic application may be a strategy to improve ventenata control and increase perennial vegetation and will require further investigation. The difficulty and likely expense of achieving substantial and lasting control of ventenata suggest, similar to other exotic annual grasses, that preventing ventenata invasion and dominance should be a high management priority.     

Digestive efficiency of indigenous and invasive avian species fed fruit of invasive alien plants in South Africa

Many highly invasive plant species produce fleshy fruit that are consumed and dispersed by frugivorous birds. However, little is known about assimilation efficiency of invasive fruit by indigenous and invasive avian species. We investigated whether indigenous Knysna (Tauraco corythaix) and Purple-crested (Gallirex porphyreolophus) Turacos and invasive alien Rose-ringed Parakeets (Psittacula krameri) met their energy demands when fed fruits of four fleshy- fruited invasive alien plant species: Solanum mauritianum, Cinnamomum camphora, Psidium guajava and Morus alba. Birds were fed single-fruit diets for two consecutive days and energetic parameters were calculated for all fruit diets. Our results showed that generally both invasive and indigenous avian species managed to gain their daily energy requirements from fruits of the four respective invasive plants, suggesting that they can meet their energetic demands by feeding on them only. The exception was P. krameri, which did not feed on S. mauritianum fruit. These findings may explain why fruits of invasive alien plants are attractive to most avian frugivores and highlight the role of avian frugivores in their dispersal.