Galler-induced reduction of shoot growth and fruit production in the shrub Colliguaja integerrima (Euphorbiaceae)
Gonzales, W.L., P.P. Caballero & R. Medel. 2005. Galler-induced reduction of shoot growth and fruit production in the shrub Colliguaja integerrima (Euphorbiaceae). Revista Chilena de Historia Natural 78: 393-399.
We evaluated experimentally the effect of a gall-maker insect on vegetative and reproductive traits of the shrub... more
We evaluated experimentally the effect of a gall-maker insect on vegetative and reproductive traits of the shrub Colliguaja integerrima (Euphorbiaceae). We performed two experiments: (1) a chemical (insecticide) exclusion to prevent gall formation, and (2) a mechanical removal of new galls at the early stage to prevent gall growth. In the study area, galled shrubs were common (77 %). Because the pattern of insect attack may
influence plant fitness, the distribution of egg clusters deposited by the galler and the number of galls among shrubs were also evaluated. Fruit production was inversely associated with the number of galls, but did not correlate with shrub height, shrub cover, and number of shoots. Ungalled shoots were longer than galled shoots after 12 months initiated the experiment. Chemical exclusion produced a delayed positive effect on plant reproduction. Fruit production was higher in experimental than control branches after 24 months. Mechanical removal of galls increased fruit production in comparison to control branches in the next reproductive season. These results indicate that the galler reduces shoot growth, and has a delayed detrimental impact on fruit production of C. integerrima. Because the chance of finding new galls was higher on the previously infected shrubs, it is possible that reinfection processes account for the cumulative negative effects of the bud-galling insect on plant fitness.
DownloadThe impact of waterfowl herbivory on plant standing crop: a meta-analysis
by Kevin Wood
Wood, K.A., Stillman, R.A., Clarke, R.T., Daunt, F. & O'Hare, M.T. (2012). The impact of waterfowl herbivory on plant standing crop: a meta-analysis. Hydrobiologia, 686: 157-167.
Waterfowl can cause substantial reductions in plant standing crop, which may have ecological and economic... more Waterfowl can cause substantial reductions in plant standing crop, which may have ecological and economic consequences. However, what determines the magnitude of these reductions is not well understood. Using data from published studies, we derived the relationship between waterfowl density and reduction in plant standing crop. When waterfowl density was estimated as individuals ha−1 no significant relationship with reduction in plant standing crop was detected. However, when waterfowl density was estimated as kg ha−1 a significant, positive, linear relationship with reduction in plant standing crop was found. Whilst many previous studies have considered waterfowl species as homologous, despite large differences in body mass, our results suggest that species body mass is a key determinant of waterfowl impact on plant standing crop. To examine relative impacts of waterfowl groups based on species body mass, a measure of plant biomass reduction (R s) per bird per hectare was calculated for each group. Comparison of R s values indicated some differences in impact between different waterfowl groups, with swans having a greater per capita impact than smaller-bodied waterfowl groups. We present evidence that this difference is linked to disparities in individual body size and associated differences in intake rates, diet composition and energy requirements. Future research priorities are proposed, particularly the need for experiments that quantify the importance of factors that determine the magnitude of waterfowl impacts on plant standing crop.
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An individual-based model of swan-macrophyte conflicts on a chalk river
by Kevin Wood
Wood, K.A., Stillman, R.A., Daunt, F. & O’Hare, M.T. (2012). An individual-based model of swan-macrophyte conflicts on a chalk river. In: (Eds. Boon, P.J. & P.J. Raven) River Conservation and Management. Wiley-Blackwell, Chichester (UK): pp.339-343.
Foraging mute swans Cygnus olor can cause a substantial reduction of aquatic macrophyte biomass in chalk rivers in... more Foraging mute swans Cygnus olor can cause a substantial reduction of aquatic macrophyte biomass in chalk rivers in southern England. To reduce the adverse effects of this on ecology, hydrology and fisheries interests a river management strategy needs to be able to predict where and when grazing pressure will be greatest. To assess the suitability of mathematical modelling for this purpose, an individual-based model (IBM) was constructed and tested for swans grazing a 1.1km length of the River Frome and adjacent pasture land in Dorset, England. Comparison of predicted and observed data indicates that the model was good at estimating the number of swan days, macrophyte biomass depletion, swan feeding time and feeding patch use. The results indicate that IBMs used at the appropriate scale could help to reconcile river management conflicts involving swans.
Biased oviposition and biased survival together help resolve a fig-wasp conflict
by Douglas Yu
Hui Wang, Jo Ridley, Derek W. Dunn, Ruiwu Wang, James M. Cook, Douglas W. Yu. Manuscript
We report evidence that helps to resolve competing explanations for stability in the classic fig-wasp mutualism. Fig... more We report evidence that helps to resolve competing explanations for stability in the classic fig-wasp mutualism. Fig trees are pollinated by wasps that lay their eggs in the tree’s ovules. Each wasp larva then develops at the expense of a fig seed. Upon maturity, the female wasps collect pollen and disperse to a new tree, continuing the cycle. Fig fitness is increased by producing both seeds and wasps, whereas short-term wasp fitness increases only with more wasps, thereby leading to a conflict of interests. We show experimentally that wasps exploit the inner layers of ovules first (biased oviposition), which is consistent with optimal-foraging theory. As oviposition increases, seeds in the middle layer are replaced on a one-to-one basis by pollinator offspring. Finally, in the outer layer of ovules, seeds disappear but are only partially replaced by pollinator offspring, which suggests high wasp mortality, perhaps due to plant defences (biased survival). We propose that selection to increase growth rate in inner ovules trades off against survivorship in outer ovules, and since there are more wasp offspring in inner ovules, biased survival is maintained. Our results suggest that both biased oviposition and biased survivorship ensure seed production in figs, thereby stabilising the mutualism. Finally, we report evidence suggesting that fig trees direct more resources to figs with more seeds, and this may help balance seed and wasp production at the level of the tree. Because an equal allocation would appear to favour the fig’s interests, the fig appears to be in control of the mutualism.
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Trichomes on the leaves of Anomozamites villosus sp. nov. (Bennettitales) from the Daohugou beds (Middle Jurassic), Inner Mongolia, China: Mechanical defence against herbivorous arthropods
Reference:
Pott, C., McLoughlin, S., Wu Shunqing, Friis, E.E., 2012. Trichomes on the leaves of Anomozamites villosus sp. nov. (Bennettitales) from the Daohugou beds (Middle Jurassic), Inner Mongolia, China: Mechanical defence against herbivorous arthropods. Review of Palaeobotany and Palynology 169, 48–60.
The abaxial leaf surfaces of Anomozamites villosus sp. nov., a new species of bennettitalean foliage from the Middle... more The abaxial leaf surfaces of Anomozamites villosus sp. nov., a new species of bennettitalean foliage from the Middle Jurassic Daohugou beds of Inner Mongolia, China, are covered with soft, shaggy hairs that are loosely, but regularly distributed across the lamina. Selectively arranged, long, stiff or acerate hairs directed or bent towards the leaf apex, project on the abaxial side of the leaf from the rachis margins. Both types of hairs are interpreted predominantly as adaptations for passive defence against damage by arthropods (sap-sucking, leaf-chewing or oviposition) rather than being morphological responses to physical environmental parameters, e.g. xeromorphic features. As several of the leaves show evidence for high levels of leaf-margin feeding by phytophagous arthropods, these mechanisms have not been entirely effective and may have evolved to defend against only selected arthropods or behavioural strategies. These findings contribute to the reconstruction of ecological interactions in the exceptionally preserved Jurassic Daohugou biota of northern China, and provide a rare insight into the plant–arthropod co-evolutionary ‘arms race’.
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Seen by: Shifting species interactions in terrestrial dryland ecosystems under altered water availability and climate change
Species interactions play key roles in linking the responses of populations, communities, and ecosystems to... more
Species interactions play key roles in linking the responses of populations, communities, and ecosystems to environmental
change. For instance, species interactions are an important determinant of the complexity of changes in trophic biomass
with variation in resources. Water resources are a major driver of terrestrial ecology and climate change is expected to
greatly alter the distribution of this critical resource. While previous studies have documented strong effects of global
environmental change on species interactions in general, responses can vary from region to region. Dryland ecosystems
occupy more than one-third of the Earth’s land mass, are greatly affected by changes in water availability, and are
predicted to be hotspots of climate change. Thus, it is imperative to understand the effects of environmental change on
these globally significant ecosystems.
Here, we review studies of the responses of population-level plant-plant, plant-herbivore, and predator-prey
interactions to changes in water availability in dryland environments in order to develop new hypotheses and
predictions to guide future research. To help explain patterns of interaction outcomes, we developed a conceptual
model that views interaction outcomes as shifting between (1) competition and facilitation (plant-plant), (2) herbivory,
neutralism, or mutualism (plant-herbivore), or (3) neutralism and predation (predator-prey), as water availability crosses
physiological, behavioural, or population-density thresholds. We link our conceptual model to hypothetical scenarios
of current and future water availability to make testable predictions about the influence of changes in water availability
on species interactions. We also examine potential implications of our conceptual model for the relative importance of
top-down effects and the linearity of patterns of change in trophic biomass with changes in water availability. Finally,
we highlight key research needs and some possible broader impacts of our findings. Overall, we hope to stimulate and
guide future research that links changes in water availability to patterns of species interactions and the dynamics of
populations and communities in dryland ecosystems.
Inter- and intra-plant variations in nitrogen, tannins and shoot growth of Sclerocarya birrea browsed by elephants
Scogings, P.F., Taylor, R. & Ward, D. 2012. Inter- and intra-plant variations in nitrogen, tannins and growth of Sclerocarya birrea browsed by elephants. Plant Ecology 213: 483-491.
Impacts of elephants (Loxodonta africana africana) on woody vegetation has attracted substantial attention for... more Impacts of elephants (Loxodonta africana africana) on woody vegetation has attracted substantial attention for decades, but plant-level responses remain a gap in the understanding of savanna ecology. Marula (Sclerocarya birrea caffra) forms an important part of elephant diets. We investigated the relationships between browsing intensity and shoot/leaf size, nitrogen (N) and condensed tannin (CT) concentrations in upper and lower canopies of male and female marula individuals in Hluhluwe-iMfolozi Park, South Africa. Browsing intensity (54%) did not differ between sexes, suggesting no preference by elephants for either sex. Females had higher [CT] than males and tannin decreased with increasing browsing intensity in both sexes. In lightly or moderately browsed trees, [CT] was controlled by unmeasured factors such that within-tree impacts of browsing were more variable in lightly/moderately browsed than heavily browsed trees. There was little change in [N] up to 60% browsing intensity, but [N] increased dramatically at higher intensity. Shoots and leaves on broken branches in the lower canopy were larger (2.5 and 1.2 times, respectively) than those on unbroken branches in either upper or lower canopies. Chemical responses were systemic and potentially influence browsing among trees, while growth responses were strongly localised and potentially influence browsing within trees. Although marula trees are able to compensate vigorously for browsing at the scale of individual organs, trees may become progressively carbon-deficient and have their lives shortened if total plant growth is negatively affected by chronic browsing, e.g. near permanent water.
Responses of potted Acacia natalitia and Scutia myrtina saplings to type of nitrogen fertilizer and rate of application
Zharare, G. & Scogings P.F. 2011. Responses of potted Acacia natalitia and Scutia myrtina saplings to type of fertiliser and rate of application. African Journal of Agricultural Research 6: 5278-5290.
Two experiments were conducted to study the responses of two savanna trees species Acacia natalitia (deciduous legume)... more Two experiments were conducted to study the responses of two savanna trees species Acacia natalitia (deciduous legume) and Scutia myrtina (evergreen non-legume) to nitrogen (N). Experiment 1 tested the responses of the species to three N-source fertilizers, namely; limestone ammonium nitrate (LAN), a composite fertilizer NPK3:1:5(26) containing slow release nitrate and an “organic” fertilizer (Accelerator). Experiment 2 tested the responses of the species to five LAN rates equivalent to 0, 3.4, 5.7, 23.1 and 34.6 gN m-2. A. natalitia formed functional nodules with soil-born rhizobial species, and hence, responded less to applied N compared with the non-legume tree species S. myrtina. The source of applied N was inconsequential to the growth of A. natalita, but in S. myrtina, LAN produced the most positive growth response. The biomass of S. myrtina, responded positively to increasing LAN application over the entire range of rates tested, whereas that of A. natalitia showed an optimum at LAN rates equivalent to between 23.1 and 34.6g m-2 N. The stimulation of S. myrtina growth by N increased the allocation of dry matter (DM) to the shoots at the expense of the roots in an N rate-dependent manner, whereas A. natalitia generally allocated more DM to the roots.
Diet selection of Nguni goats in relation to season, chemistry and physical properties of browse in sub-humid subtropical savanna
Basha, N.A.D, Scogings, P.F., Dziba, L.E. & Nsahlai, I.V. 2012. Diet selection of Nguni goats in relation to season, chemistry and physical properties of browse in a sub-humid subtropical savanna. Small Ruminant Research 102: 163-171.
This study was conducted to determine the influence of plant chemical, physical and phenology properties on diet... more This study was conducted to determine the influence of plant chemical, physical and phenology properties on diet selection of Nguni goats during the dry, early wet and late wet seasons in savanna in South Africa. Diet composition was estimated by direct observation of two different adult Nguni goats randomly selected from a herd each day for 7–8 days in each season. Observations were made during active foraging periods for 2 h in the morning and 1.5 h in the afternoon. The duration of each feeding bout and the species of woody plant from which bites were cropped at each feeding station were recorded. Diet selection was determined from the relative duration of feeding. Diet preference of each species was expressed as an index calculated using the selection and relative abundance of woody species. Browse species consumed by goats were sampled and analysed to determine crude protein, neutral detergent fibre, acid detergent fibre, acid detergent lignin, condensed tannins, cellulose and hemicellulose. Diet selection varied among the three seasons. The five species most selected (utilised) by goats were Scutia myrtina, Acacia nilotica, Dichrostachys cinerea, Acacia natalitia and Chromolaena odorata. S. myrtina was the most selected species during the dry season while D. cinerea was the most selected in the wet seasons. S. myrtina was the most preferred (highest utilisation relative to availability) in the dry and early wet seasons while A. nilotica was most preferred in the late wet season. Spinescent species were generally selected more than non-spinescent species in all seasons, while fine-leaf and deciduous species were selected more than broad-leaf and evergreen ones in the wet seasons. However, preference for broad-leaf and evergreen species increased in the early wet season. Although plant chemistry varied across seasons, it did not explain the preference of goats for various plant species in this study. Instead, effects of chemistry were species-specific. In conclusion, this study demonstrates the importance of evergreen browse species as a source of fodder when deciduous species are unavailable.
Acacia karroo responses to early dormant season defoliation and debarking by goats in a semi-arid subtropical savanna
Scogings, P.F. & Macanda, M. 2005. Acacia karroo responses to early dormant season defoliation and debarking by goats in a semi-arid subtropical savanna. Plant Ecology 179:193-206
Research was conducted to investigate responses of Acacia karroo to severe defoliation, with or without debarking, in... more Research was conducted to investigate responses of Acacia karroo to severe defoliation, with or without debarking, in a semi-arid, subtropical savanna. A herd of 250 goats reduced available foliage by 87% in a 10.5-ha area of Acacia woodland over 3 weeks during the early dormant season (May). The goats debarked 37% of the trees. On average, bark was stripped less than halfway round a stem and 11% of the bark area was affected. Trees 1.6–2.5 m tall and 11–30 cm in basal circumference were most susceptible to debarking. A year later, there were no detectable effects of defoliation on the number of browseable shoots, internode length, shoot length, spine length, number of seed pods, neutral detergent fibre (NDF) or nitrogen (N). Defoliation tended to increase condensed tannin concentration (CT), but the effect was not significant (p > 0.05). The absence of goats during the growth season following defoliation allowed the trees to recover. Debarking had greater impacts on the trees than defoliation had. Debarking reduced the number of browseable shoots, while ringing at the base reduced CT and increased N. Debarking tended to reduce shoot and spine length, and halve the number of pods, but the effects were not significant (p > 0.05). Responses of basally ringed trees indicated rapid vertical growth when the trees were highly vulnerable to browsers and fires, suggesting strong selection for herbivore/fire tolerance. Among all treatments, excluding basal ringing, there were significant variations in NDF and CT between years. There was a positive relationship between N and NDF, and a negative relationship between N and CT, in 1 year, but not in the other. Among basally ringed trees, CT was negatively related to N, but NDF was not. The strong trade-off between N and CT in basally ringed trees probably indicates that large changes in root:shoot ratio are required to effect definite changes in defence.
Long-term impacts of goat browsing on bush-clump dynamics in a semi-arid subtropical savanna
Hester, A.J., Scogings, P.F. & Trollope, W.S.W. 2006. Long-term impacts of goat browsing on bush-clump dynamics in the False Thornveld of the Eastern Cape, South Africa. Plant Ecology 183:277-290
The effects of 16 years of continuous browsing by goats in a South African savanna at stocking rates intended for bush... more The effects of 16 years of continuous browsing by goats in a South African savanna at stocking rates intended for bush control were compared with plots unbrowsed for the same period of time. Differences in bush-clump density, structure and species composition were recorded. Bush-clump density did not differ between browsed and unbrowsed plots. Within individual bush-clumps, browsing was shown to impact more on structure than species composition, with smaller, shorter bush-clumps, containing fewer species but much greater stem-densities. Although species presence/absence was little affected by browsing, many species showed differences in abundance, growth and location within browsed and unbrowsed bush-clumps. Species reduced in abundance in browsed plots included Cussonia spicata, Ehretia rigida, Grewia occidentalis, Jasminum angulare and Senecio linifolius. Several species showed reduced growth in browsed plots, particularly those located at bush-clump edges. The relatively unpreferred Aloe ferox was a notable exception. Although browsing had little effect on the composition of the main clump founding species, emergents or late arrivals, there were twice as many single plants in browsed plots and emergence of several species was restricted to the middle of bush-clumps. Comparison of our findings with aerial photographic evidence and other literature suggest that browsing alone is unlikely to significantly reduce scrub cover, although it can clearly control further expansion. Combinations of fire and browsing, rather than one factor alone, are considered likely to act fastest and most effectively to significantly reduce or remove scrub cover altogether.
Rapid chemical responses of Acacia karroo to early dormant season defoliation in a semi-arid subtropical savanna
Scogings, P.F. 2005. Rapid chemical responses of Acacia karroo to early dormant season defoliation by goats browsing in a semi-arid subtropical savanna. Journal of Arid Environments 62:225-233
Research was conducted to investigate rapid chemical responses of Acacia karroo to severe defoliation in a semi-arid,... more Research was conducted to investigate rapid chemical responses of Acacia karroo to severe defoliation in a semi-arid, subtropical savanna. A flock of 250 goats reduced available foliage by 87% in a 10.5-ha area of Acacia woodland over 3 weeks during May (early dormant season). During the first 10 days, concentrations of nitrogen (N), neutral detergent fibre (NDF) and condensed tannin (CT) did not change. The concentration of potentially hepatotoxic alkaloids (PHAs) decreased. There were no immediately detectable effects of defoliation intensity or recovery time on any of the measured chemical variables. There were no relationships between N and NDF, CT or PHA. Given that the treatments were applied at the start of the dormant season, the metabolism of mature or senescing leaves may have precluded rapid increases in defence. Because a previous study under similar browsing conditions showed increased defence in A. karroo at the same time of year, it was concluded that intra-annual variations in rainfall, and the resulting variations in plant growth activity, lead to intra-annual variations in plant responses to defoliation in semi-arid subtropical savannas. Such variations need to be accounted for in future experiments aimed at investigating induced responses to browsing in savannas.
Effects of water, grass and N on responses of Acacia karroo seedlings to early wet season simulated browsing: Leaf N, fibre and tannin concentrations
Scogings, P.F. & Mopipi, K. 2008. Effects of water, grass and N on responses of Acacia karroo seedlings to early wet season simulated browsing: leaf N, fibre and tannin concentrations. Journal of Arid Environments 72:1666-1674
Bush encroachment is a phenomenon that affects semi-arid savannas world-wide. To apply effective browsing to control... more Bush encroachment is a phenomenon that affects semi-arid savannas world-wide. To apply effective browsing to control trees, the browse–browser interaction must be well understood. An important aspect is feedbacks between browsers and trees because browsers influence the nutritional value of plants. When resources are abundant, tannins, which are assumed to negatively affect browsers, are predicted to decrease following herbivory because defoliation is assumed to reduce C relative to N. In opposite conditions, defoliation is expected to increase tannins by increasing C relative to N. A split-plot experiment was conducted to investigate the effects of irrigation, fertiliser, grass cover and clipping on leaf N, neutral detergent fibre (NDF) and condensed tannin (CT) concentrations of Acacia karroo seedlings in a semi-arid savanna. Clipping reduced N of irrigated plants, and increased N of unirrigated plants, in the absence of grass cover. By reducing N, clipping in relatively high resource conditions resulted in reduced potential value of the seedlings as fodder. The reverse occurred in relatively low resource conditions through N elevation. The development of negative feedback in conditions that promote woody seedling encroachment would render bush control by browsing difficult.
Forage quality, twig diameter, and growth habit of woody species selected by black rhinoceros in the Great Fish River Reserve, South Africa
Ganqa, N.M. & Scogings, P.F. 2007. Forage quality, twig diameter, and growth habit of woody species selected by black rhinoceros in the Great Fish River Reserve, South Africa. Journal of Arid Environments 70:514-526
Rapid growth of the black rhino (Diceros bicornis) population in the Great Fish River Reserve (GFRR), South Africa,... more Rapid growth of the black rhino (Diceros bicornis) population in the Great Fish River Reserve (GFRR), South Africa, has potential to negatively impact the semi-arid thicket vegetation that occurs in the reserve. Woody plants presenting nutritious twigs to twig-biting browsers are assumed to be potentially at greater risk of losing biomass than those that do not and should be considered in management protocols. We investigated plant growth habits, twig diameter and forage quality of woody species commonly browsed by black rhino in the GFRR to see if any fit a high risk profile. Samples of twigs 2, 6, 10 and 14 mm in diameter, and the leaves thereon, were collected during the wet season to determine the dry mass (DM), crude protein (CP) and neutral detergent fibre (NDF) concentrations of leaves and wood, as well as leaf:stem ratios (LSR). CP and LSR decreased while NDF and DM increased with twig diameter. Rates of change appeared to be greater for evergreen than deciduous species. Black rhinos foraging in the GFRR are therefore expected to inflict greatest biomass losses on deciduous species, especially ones with many long shoots. We recommend that relationships among forage quality and growth habits of woody species need to be better understood for the management of black rhinos in semi-arid subtropical thicket.
Plant factors affecting diet selection by black rhino in the Great Fish River Reserve, South Africa
Ganqa, N.M., Scogings, P.F. & Raats, J.G. 2005. Plant factors affecting diet selection by black rhino in the Great Fish River Reserve, South Africa. South African Journal of Wildlife Research 35:77-83
Diet selection of black rhinoceros and forage quality factors affecting diet selection were investigated in the Great... more Diet selection of black rhinoceros and forage quality factors affecting diet selection were investigated in the Great Fish River Reserve. Plant species, number of bite marks, heights and diameters of bites, and diameter of the largest bite were recorded. Five woody species were sampled to quantify forage quality factors that could influence diet selection.Twigs 2, 6, 10 and 14 mm in diameter were collected from each species for determining the dry mass (DM), crude protein (CP) and neutral detergent fibre (NDF) of the leaves and wood of browseable material as well as leaf:stem ratio (LSR). Azima tetracantha and Plumbago auriculata were the most preferred woody plants in winter and summer. Euphorbia bothae was the most preferred succulent species in summer while in winter it was Jatropha capensis. Most browsing occurred below 1 m and most browsed twigs were <6 mm in diameter. Twig bites were larger during summer than winter. Forage quality factors varied across the plant species. Plant preference was significantly related to CP and LSR. Relationships between plant preference and CP were positive while the relationships between plant preference and LSR were negative. Within forage quality factors measured CP was the major factor affecting diet selection of woody plants.
Season and plant species influence foraging efficiency of Nguni goats in pens
Mkhize, N.R., Scogings, P.F., Dziba, L.E. & Nsahlai, I.V. 2011. Season and plant species influence foraging efficiency of Nguni goats in pens. African Journal of Range & Forage Science 28: 29-34
This study investigated the seasonal and plant species patterns of short-term intake rate (STIR) by Nguni goats fed... more This study investigated the seasonal and plant species patterns of short-term intake rate (STIR) by Nguni goats fed six common browse species in subhumid subtropical savannas. Six 2-year-old castrated Nguni goats weighing an average of 26 kg each were penned individually and maintained on a basal diet of ram, lamb and ewe pellets and grass hay. The STIR was influenced by season, with the highest mean STIR recorded during the dry season (mean, SEM: 0.18, 0.012 g DM s−1) and least during the late wet season (0.15, 0.009 g DM s−1). Broadleaf species had higher STIR than fine-leaf species (0.18, 0.052 g DM s−1 vs 0.14, 0.055 g DM s−1). Species that produce new leaves on new long shoots were consumed at higher rates than those that produce new leaves on short shoots (0.19, 0.054 g DM s−1 vs 0.15, 0.056 g DM s−1). STIR showed no clear patterns in relation to leaf chemistry. These results can be used to improve estimates of intake rates from free-foraging animals using direct observation methods based on time spent at a feeding station depending on the plant species and the season in which it is consumed.
Effects of moisture, nitrogen, grass competition and simulated browsing on the survival and growth of Acacia karroo seedlings
Mopipi, K., Trollope, W.S.W. & Scogings, P.F. 2009. Effects of moisture, nitrogen, grass competition and simulated browsing on the survival and growth of Acacia karroo seedlings. African Journal of Ecology 47, 680–687
The effects of irrigation, nitrogen fertilization, grass competition and clipping were investigated for one growing... more The effects of irrigation, nitrogen fertilization, grass competition and clipping were investigated for one growing season at the research farm of the University of Fort Hare in the Eastern Cape Province of South Africa. The aim of the experiment was to assess the short-term performance of Acacia karroo seedlings under different environmental conditions and the implications of such factors on the longterm recruitment of plant species in savanna rangelands. There were no significant treatment effects on the survival of A. karroo seedlings. Using stem length and basal diameter as growth parameters, it was observed that irrigation enhanced both variables, while nitrogen fertilization did not have any significant effects. Clipping, grass competition and their interaction greatly suppressed the growth of the seedlings. Clipping increased the mean stem length when they were irrigated and fertilized. Control and fertilized plants had the highest stem length in the absence of grass competition, while grass competition combined with clipping resulted in the lowest stem length in both irrigated and nonirrigated plants. It was concluded that in the presence of grass competition, controlled browsing could be a viable solution to the problem of bush encroachment in savanna rangelands.
Responses of woody vegetation to exclusion of large herbivores in semi‐arid savannas
Scogings, P.F., Johansson, T., Hjältén, J. & Kruger, J. 2012. Responses of woody vegetation to exclusion of large herbivores in semi‐arid savannas. Austral Ecology 37: 56-66.
The Nkuhlu large-scale long-term exclusion experiment in Kruger National Park was designed to study the long-term... more The Nkuhlu large-scale long-term exclusion experiment in Kruger National Park was designed to study the long-term effects of large herbivores on vegetation. One treatment excludes elephants, another excludes all herbivores larger than hares and another one comprises an open, control area.Vegetation monitoring was implemented in 2002 when a baseline survey was conducted prior to exclusion. Monitoring was repeated 5 years after exclusion. Data from the surveys were analysed to establish how structure and composition of woody vegetation had changed 5 years after herbivore exclusion.The analysis showed that neither plant assemblage nor mean vegetation height had changed significantly since exclusion. However, both species richness and density of woody plants increased 5 years after exclusion of all large herbivores, but not after the exclusion of elephants alone. One already common species, Dichrostachys cinerea, became more common after excluding all large herbivores compared with either no exclusion or elephant exclusion, possibly leading to competitive suppression of other species. Species other than D. cinerea tended to either increase or decrease in density, but the changes were insufficient to induce significant shifts in the overall assemblage of woody plants.The results indicate that after 5 years of exclusion, the combined assemblage of large herbivores, and not elephants alone, could induce changes in species richness and abundances of woody plants, but the effect was so far insufficient to induce measureable shifts in the assemblages of woody plants. It is possible that assemblages will change with time and increasing elephant numbers may amplify future changes.
Leaf chemistry of woody plants in relation to season, canopy retention and goat browsing in a semiarid subtropical savanna
Scogings, P.F., Dziba, L.E. & Gordon, I.J. 2004. Leaf chemistry of woody plants in relation to season, canopy retention and goat browsing in a semi-arid subtropical savanna. Austral Ecology 29:278-286.
It is assumed that the phytochemistry of browse species protects their biomass and nutrients against herbivory. In... more It is assumed that the phytochemistry of browse species protects their biomass and nutrients against herbivory. In this study we were primarily interested in (i) seasonal and phenology-related variations in leaf chemistry, and (ii) chemistry-related variations in the feeding behaviour of domestic goats. Such knowledge would guide management-orientated modelling of browse–browser interactions in seasonal, subtropical zones where goats are abundant. The browse species studied were typical of semiarid savannas in southern Africa: Grewia occidentalis L. (Tiliaceae), Scutia myrtina (Burm. f) Kurz (Rhamnaceae), Diospyros lycioides Desf. ssp. lycioides (Ebenaceae), Rhus longispina Eckl. and Zeyh. (Anacardiaceae), Ehretia rigida (Thunb.) Druce (Boraginaceae) and Acacia karroo Hayne (Mimosoideae). Nitrogen (N), phosphorus (P), condensed tannins (CT), protein-precipitating tannins (PPT), total phenols (TP), cellulose and lignin concentrations were estimated for each species during the late dormant and early growing seasons. N, P, CT and TP were elevated during the growth season, while cellulose, lignin and PPT decreased. Unlike cytoplasm contents, which varied seasonally, cell wall and vacuole contents varied both seasonally and among species. Except that seasonal variation in N of deciduous species was greater than that of evergreen species, leaf phenology was not related to variations in forage quality. Short-term intake rates were not related to primary metabolite concentrations, but were positively related to secondary metabolites. Elevated intake rates of putative defences were concluded to be side-effects of attempts by goats to increase nutrient intake rate, indicating tolerance of chemical defences. Without support for the hypothesis that chemical defences are correlated to canopy retention, we propose an alternative hypothesis, that defences are distributed among woody plants in semiarid, subtropical savannas according to shoot morphology because it affects the vulnerability of plant parts to browsers.
Stem growth of woody species at the Nkuhlu exclosures, Kruger National Park: 2006-2010
Scogings, P.F. 2011. Stem growth of woody species at the Nkuhlu Exclosures, Kruger National Park: 2006-2010. Koedoe 53 (1) doi:10.4102/ koedoe.v53i1.1035
An important aspect of managing African conservation areas involves understanding how large herbivores affect woody... more
An important aspect of managing African conservation areas involves understanding how large herbivores affect woody plant growth. Yet, data on growth rates of woody species in savannas are scarce, despite its critical importance for developing models to guide ecosystem management. What effect do browsing and season have on woody stem growth? Assuming no growth happens in the dry season, browsing should reduce stem growth in the wet season only. Secondly, do functional species groups differ in stem growth? For example, assuming fine-leaved, spiny species’ growth is not compromised by carbon-based chemical defences, they should grow faster than broad-leaved, chemically defended species. Dendrometers were fixed at 20 cm in height on the main stems of 244 random plants of six woody species in three plots (all large herbivores excluded, partial exclusion, and control) and observed from late 2006 to early 2010. Average monthly increment (AMI) per dendrometer and season (dry, wet)
was calculated and the interaction between plot and season tested per species, controlling for initial stem girth. AMIs of Combretum apiculatum, Dichrostachys cinerea and Grewia flavescens were zero in the dry season, whilst those of Acacia exuvialis, Acacia grandicornuta and Euclea divinorum were either positive or negative in the dry season. Wet-season AMI of D. cinerea and dry-season AMI of G. flavescens tended to be reduced by browser exclusion. Net AMI (sum of the seasonal AMIs) was tested per species, but results suggested that only D. cinerea tended to be affected by browser exclusion. The results also suggested that stem radial growth of some
fast-growing species is more prone to reduction by browser exclusion than the growth of other species, potentially reducing their competitiveness and increasing their risk of extirpation.
Finally, the usefulness of grouping woody species into simple functional groups (e.g. fine-leaved vs. broad-leaved) for ecosystem management purposes in savannas requires further
consideration.