Differences in woody vegetation are unrelated to use by African elephants (Loxodonta africana) in Mkhuze Game Reserve, South Africa

The reintroduction of African elephants into fenced game reserves throughout South Africa has presented managers with several challenges. Although elephants are a natural part of southern African ecosystems, their confinement to fenced protected areas in South Africa has exacerbated their potential to impact their habitats negatively. However, many studies investigating the impact of elephants have failed to control for the effects of other browsers on the vegetative community. In this study, we used location data on an elephant herd to delineate high-use and low-use areas. This paired design allowed us to minimize confounding factors that could explain differences in the structure, diversity and utilization of woody species. We found little evidence to suggest elephant-mediated change in, or selection for, the structure or diversity of woody species; however, our results suggest that elephants may be altering the composition of species by preferentially using areas with higher canopy diversity and by enhancing sapling recruitment. Although stripping of bark was higher in high-use areas, there was no evidence of differential mortality of tree species. Therefore, in our study area, and over the current time scale, elephants are having a negligible impact on the vegetative community.     

Termites of a South African savanna

Summary Cellulose toilet roll baits were exposed in three different subhabitats at Nylsvley Reserve, northern Transvaal, and respective attacks by up to eight species of termites are described and illustrated. In broad-leaved savanna attack was principally by Microcerotermes, a slow feeder that attacked rolls throughout the year, finding about half the rolls exposed each season. In Acacia savanna attack was largely by Microtermes, a fast feeder active only at wetter times of year. In open, grassy areas there was relatively little attack, except sporadically by Macrotermes which consumed whole rolls within a day or two. Bush-fires delayed initial attack on rolls, perhaps because reduced surface litter gave less protection to soil. The amount of each roll eaten during the trials is shown in histograms; it was consistently greater in Acacia than in broad-leaved savanna. The soil-feeding termites Aganotermes and Promirotermes were attracted during the wet season to the undersides of rolls, perhaps as a source of organic-rich soil (or possibly of water).     

Fire induced stem death in an African acacia is not caused by canopy scorching

Abstract The death of smaller stems of trees due to fire is widespread in savannas. There are currently two hypotheses as to how tree stems avoid stem death; by (i) growing tall and enabling the terminal buds to escape being scorched; and (ii) growing a larger stem diameter and thus being buffered against the heat of the fire. Laboratory‐based tests of these hypotheses on one savanna tree species, Acacia karroo Haynes, support the contention that the important parameter is stem diameter. In addition, anatomical evidence of heat impacts to xylem suggests that damage to the xylem of a stem may play a mechanistic role in causing stem death.     

Symbiotic acacia ants drive nesting behavior by birds in an African savanna

Mutualisms between plants and ants are common features of tropical ecosystems around the globe and can have cascading effects on interactions with the ecological communities in which they occur. In an African savanna, we assessed whether acacia ants influence nest site selection by tree-nesting birds. Birds selected nest sites in trees inhabited by ant species that vigorously defend against browsing mammals. Future research could address the extent to which hatching and fledging rates depend on the species of ant symbiont, and why ants tolerate nesting birds but no other tree associates (especially insects). Abstract in Swahili is available with online material.     

Assessment of contrafreeloading preferences in giraffe (Giraffa camelopardalis)

Contrafreeloading is an intriguing phenomenon in which animals will work to obtain resources, such as food, when the same resource is simultaneously freely available. Multiple hypotheses exist for why animals might choose to contrafreeload. In this study, we assessed preferences for contrafreeloading in giraffe at the Bronx Zoo to determine whether they actually preferred to contrafreeload or were simply demonstrating a willingness to contrafreeload. Food was presented in a range of distributions between an easily accessed feeding device and a more challenging one and the giraffes’ feeding behavior at these two types of feeding devices was recorded. As the experiments progressed, more giraffe used these more challenging feeders. There was significant individual variation in the expression of preference for contrafreeloading and willingness to contrafreeload. Individual, phase of the experiment, and an interaction between these factors were significant predictors of challenge feeder use. Three foraging strategies emerged among the giraffe that we termed “freeloaders,” “contrafreeloaders,” and “opportunists.” The results of this study demonstrate that multiple indices of preference are necessary when assessing contrafreeloading behavior, and that giraffe are affected to different degrees by the factors that stimulate contrafreeloading. These results may shed light on why different individuals use complex feeding enrichment devices to varying extents.     

Reproductive life history of Thornicroft’s giraffe in Zambia

Knowledge of the reproductive life history of giraffe in the wild is sparse. Giraffe have two fairly unusual reproductive patterns among large mammals: they can become pregnant while lactating, and calf mortality is extremely high. Longitudinal records are largely absent, so tracking reproductive parameters tends to combine information from captive and field studies. In this study, we examine longitudinal data obtained over a 33‐year period in one population of Thornicroft’s giraffe in order to chart their reproductive careers. We found that age at first parturition was 6.4 years, or slightly later than in captivity. Giraffe bred throughout the year, with cows producing offspring on average every 677.7 days. About half of the calves died before one year of age, but death of a calf did not reduce interbirth interval. We conclude that the lifetime reproductive success of giraffe is more dependent on longevity and calf survivorship than on reproductive rate.     

Failure of a chemical spray to significantly reduce stereotypic licking in a captive giraffe

The repetitive licking of nonfood substrates is a common stereotypic behavior in captive giraffes. In this study we attempted to reduce stereotypic licking in a Masai giraffe (Giraffa camelopardalis tippelskirchi) by applying a bitter chemical (marketed as a taste deterrent to prevent the unwanted licking and chewing of substrates) to the areas of the fence licked most frequently by the giraffe. We hypothesized that this treatment would reduce stereotypic licking. However, there were no significant overall changes in stereotypic licking following treatment with the chemical spray. Although licking of the treated area was observed to decrease, the behavior increased in nontreated areas. These results suggest that the underlying motivation driving the behavior was not affected by the aversive stimulus. Because stereotypic licking in giraffes may be based on a motivation to use their tongues in foraging, more effective techniques for reducing stereotypic fence‐licking in giraffes may include increasing feeding duration by increasing the quantity, processing time, or distribution range of food. Zoo Biol 22:601–607, 2003. © 2003 Wiley‐Liss, Inc.     

Leadership of herd progressions in the Thornicroft's giraffe of Zambia

In cohesive social groups, travel progressions are often led by dominant or older individuals, but the leadership traits of individuals residing in flexible social systems are poorly known. Giraffe reside in herds characterized by fission–fusion dynamics frequently mediated by kinship. We analyzed 41 years (1971–2012) of longitudinal data collected from a community of Thornicroft's giraffe (Giraffa camelopardalis thornicrofti) living around South Luangwa National Park, Zambia, to assess the characteristics of herd leaders. Movement of giraffe in a single file progression was not associated with either season or time of day, but progressions were significantly more likely to occur when giraffe traveled in open areas. The oldest female in a herd was significantly more likely to be at the front position than expected, occupying the leadership niche on 79% of observations. We reason that matriarchal leadership in giraffe, as in African elephants, Loxodonta africana, is associated with resource learning. Giraffe societies are constructed on a heretofore unrecognized foundation that integrates relatedness and familiarity with matriarchal leadership in herd movement.     

Insect outbreaks alter nutrient dynamics in a southern African savanna: patchy defoliation of Colophospermum mopane savanna by Imbrasia belina larvae

Invertebrate herbivore outbreaks have important impacts on system biogeochemical cycling, but these effects have been poorly documented in African savanna ecosystems. In semi‐arid African savannas, outbreaks of the lepidopteran Imbrasia belina (mopane worm) affect discrete patches of the dominant Colophospermum mopane trees; larvae may completely defoliate trees for up to six weeks during each of the early and late growing seasons. We studied the impact of mopane worm outbreaks on the availability of nitrogen (N), phosphorus (P), and potassium (K) within mopane savanna by quantifying major nutrient pools in defoliated and non‐defoliated savanna patches, including leaves, leaf litter, worm frass, and the soil beneath trees. Within an outbreak area, approximately 44 percent of trees were infested, supporting ~29,000 worms/ha, leading to ~640 kg/ha dry weight frass deposition at 1.4 g of frass/day‐individual (fourth or fifth instar), compared with an average 1645 kg/ha dry weight of leaf on trees most of which should be deposited by litterfall at the end of the growing season. Frass had twofold higher P, 10 percent higher K, but equivalent N content than litter. Taking frass and litter deposition together, the N, P, and K contents added due to the outbreak event at our study site were 0.88, 5.8, and 2.8 times those measured in non‐outbreak patches, a pattern which was reflected in the nutrient contents of soil surfaces beneath defoliated trees. Invertebrate herbivory appears to be an important driver for mopane savanna but has been largely neglected.     

Heterogeneity in African savanna elephant distributions and their impacts on trees in Kruger National Park,South Africa

1. Though elephants are a major cause of savanna tree mortality and threaten vulnerable tree species, managing their impact remains difficult, in part because relatively little is known about how elephant impacts are distributed throughout space.
2. This is exacerbated by uncertainty about what determines the distribution of elephants themselves, as well as whether the distribution of elephants is even informative for understanding the distribution of their impacts.
3. To better understand the factors that underlie elephant impacts, we modeled elephant distributions and their damage to trees with respect to soil properties, water availability, and vegetation in Kruger National Park, South Africa, using structural equation modeling.
4. We found that bull elephants and mixed herds differed markedly in their distributions, with bull elephants concentrating in sparsely treed basaltic sites close to artificial waterholes and mixed herds aggregating around permanent rivers, particularly in areas with little grass.
5. Surprisingly, we also found that the distribution of elephant impacts, while highly heterogeneous, was largely unrelated to the distribution of elephants themselves, with damage concentrated instead in densely treed areas and particularly on basaltic soils.
6. Results underscore the importance of surface water for elephants but suggest that elephant water dependence operates together with other landscape factors, particularly vegetation community composition and historical management interventions, to influence elephant distributions.

     

The impact of giraffe, rhino and elephant on the habitat of a black rhino sanctuary in Kenya

The habitat in an enclosed black rhino sanctuary, the Sweetwaters Game Reserve in Kenya, is being altered as populations of elephant, giraffe and black rhino increase. Height‐specific browse impact data were recorded for 1075 trees of the dominant species, the whistling thorn, Acacia drepanolobium. Rhinos and elephants browsed 18% of these trees in 1 year, including 5% that were killed or removed. The remaining trees were subjected to high levels of giraffe browse and low rainfall and grew by only 7.5 cm in a year. A mathematical model has been constructed that predicts how the number of trees ha^?1 will change with time under different browsing impacts. The model compares recruitment rate with removal rate and estimates that the number of trees ha^?1 will fall by 2% per year under the current browsing impact of black rhino (0.27 per km²), elephant (1.1 per km²) and giraffe (1.9 per km²). In 7 years, if the rhino and elephant populations continue to increase at the current rates, tree density will be falling by 5% per year and nearly one‐third of the trees will have been removed. These conditions are unsustainable and will result in habitat change and may affect rhino breeding. Several ways of alleviating the problem are discussed.     

Dominant ants can control assemblage species richness in a South African savanna

1. Competition is considered a key factor structuring many communities, and has been described as the 'hallmark' of ant ecology. Dominant species are thought to play a key role structuring local ant assemblages through competitive exclusion. 2. However, while there have been many studies demonstrating competitive exclusion and consequently reduced richness at baits, it is not clear whether such regulation of 'momentary' diversity at clumped food resources can scale up to the regulation of richness at the site or assemblage level. 3. In this study, ant assemblages were sampled in three different savanna habitats in South Africa using both baiting and pitfall trapping. 4. As has been found in previous studies, there was a unimodal relationship between dominant ants and species richness at baits, with high abundances of dominant ants regulating species richness through competitive exclusion. Analysis of pitfall samples revealed strong convergence in pattern, and results from null model co-occurrence analyses supported the findings. 5. The importance of competition in structuring local ant assemblages was, however, only apparent at one of the three savanna habitats suggesting that a full range of extreme environments is needed to produce the full unimodal relationship at the assemblage level. 6. Although the relative importance of competition varied with habitat type, the study demonstrated that in some habitats, dominant ants can control species richness at the assemblage level.     

Condensed tannins deter feeding by browsing ruminants in a South African savanna

Summary The palatability of 14 species of woody plant was assessed for three species of browsing ruminant, namely kudus, impalas and goats. Results show that palatability was most clearly related to leaf contents of condensed tannins. The effect was a threshold one, with all plants containing more than 5% condensed tannins being rejected as food during the wet season period. In contrast palatability was not influenced by concentrations of protein-precipitating polyphenols, and only weakly related to contents of nitrogen, phosphorus, cations, fibre components and other secondary metabolites. Insect herbivory shows a different pattern. These findings support the hypotheses that (i) condensed tannins function to protect plant cell walls against microbial attack; (ii) hydrolyzable tannins function to inactivate the digestive enzymes of insect herbivores. Large mammalian herbivores are influenced by condensed tannins due to their dependance upon microbial fermentation of plant cell walls for part of their energy needs.     

Bush encroachment under three contrasting land-use practices in a mesic South African savanna

This study determined the effects of land-use practice had on the rate and extent of bush encroachment in a mesic savanna in KwaZulu-Natal, South Africa. Changes in woody cover were measured for 1 km² sites in areas under communal, commercial and conservation land-use systems for the period between 1937 and 2000. Land users from each area were interviewed to gain the histories of each area and to determine how the changes in woody cover had impacted them and whether anything was being done to counteract the spread of trees and shrubs on their land. Bush encroachment occurred across all three of the land-use types in the 67-year period between 1937 and 2000. The results showed that land-use practice had enormous impacts on the process of bush encroachment. The communal site showed a decrease in grass (21%) and tree (5%) cover and an increase in shrub cover (13%). At the commercial site, there was a considerable decrease in grass cover (46%) and moderate increase in shrub cover (10%) and a massive increase in tree cover (36%). The area under conservation showed a substantial decrease in grass cover (47%), a slight decrease in shrub cover (19%) and a massive increase in tree cover (66%). The perceived causes of these changes were fairly similar amongst the different land users. The changes were mostly not perceived to be a problem for the communal land users. The main advantages mentioned were increased woody resources for building and firewood and increased browse availability. The commercial and conservation land users perceived the changes to have significant negative connotations including the loss of grazing land and biodiversity and secondary invasion of encroached areas by alien plant species. Despite these perceptions, very little has been done to combat bush encroachment in the commercial and conservation land use systems.     

Landscape genetics of the key African acacia species Senegalia mellifera (Vahl)- the importance of the Kenyan Rift Valley

Acacias across Africa have enormous ecological and economic importance, yet their population genetics are poorly studied. We used seven microsatellite loci to investigate spatial genetic structure and to identify potential ecological and geographic barriers to dispersal in the widespread acacia, Senegalia (Acacia) mellifera. We quantified variation among 791 individuals from 28 sampling locations, examining patterns at two spatial scales: (i) across Kenya including the Rift Valley, and (ii) for a local subset of 11 neighbouring locations on Mpala Ranch in the Laikipia plateau. Our analyses recognize that siblings can often be included in samples used to measure population genetic structure, violating fundamental assumptions made by these analyses. To address this potential problem, we maximized genetic independence of samples by creating a sibship-controlled data set that included only one member of each sibship and compared the results obtained with the full data set. Patterns of genetic structure and barriers to gene flow were essentially similar when the two data sets were analysed. Five well-defined geographic regions were identified across Kenya within which gene flow was localized, with the two strongest barriers to dispersal splitting the Laikipia Plateau of central Kenya from the Western and Eastern Rift Valley. At a smaller scale, in the absence of geographic features, regional habitat gradients appear to restrict gene flow significantly. We discuss the implications of our results for the management of this highly exploited species.     

Characterization of 16 microsatellite marker loci in the Maasai giraffe (Giraffa camelopardalis tippelskirchi)

Sixteen polymorphic microsatellite markers with an average allele size of k = 4.3 are identified from a genomic plasmid library constructed for giraffe (Giraffa camelopardalis ssp.). Primer sequences and marker data are reported in tabular form. The markers were screened in a population of 25 Maasai giraffe (G. c. tippelskirchi) collected near the Athi River, Kenya. The average observed heterozygosity for each marker was 0.36 with an average expected heterozygosity of 0.535. Hardy–Weinberg deviations are reported from this population, which is suspected to be inbred. The markers will be used to screen the captive giraffe population for subspecific or hybrid classification.     

Ecology of spiders inhabiting abandoned mammal burrows in South African savanna

Summary We marked the sites chosen by 338 foundress queens of two desert ant species (Veromessor pergandei and Myrmecocystus flaviceps) and monitored changes in the spacing of both species and the foraging activity at V. pergandei young nests. Although the long established colonies of both species tend intraspecifically toward regular dispersion, queens of both species were intraspecifically clumped. After 3 months, when the first workers emerged, the young colonies (reduced to a total of 42 colonies) were randomly spaced intraspecifically. We also followed the spatial patter of queens with respect to established colonies of both species. Queens founded nests away from the nests of all established colonies on the site. After three months, the young colonies were dispersed away from conspecifics only. During June through August 1986, we censused the number of foragers at the surviving V. pergandei nests. Young colonies that were more active also tended to be far from established conspecifics in July and August. There was no correlation of forating activity with distance to heterospecific established colonies in any month. These results indicate that established conspecifics may reduce the survivorship of young ant colonies.