Modeling elephant-mediated cascading effects of water point closure
Hilbers, J.P. ; Langevelde, F. van; Prins, H.H.T. ; Grant, C.C. ; Peel, M. ; Coughenour, M.B. ; Knegt, H.J. de; Slotow, R. ; Smit, I. ; Kiker, G.A. ; Boer, W.F. de - \ 2015
Ecological Applications 25 (2015)2. - ISSN 1051-0761 - p. 402 - 415.
kruger-national-park - african savanna - south-africa - distribution patterns - wildlife management - sexual segregation - large herbivores - habitat use - landscape - systems
Wildlife management to reduce the impact of wildlife on their habitat can be done in several ways, among which removing animals (by either culling or translocation) is most often used. There are however alternative ways to control wildlife densities, such as opening or closing water points. The effects of these alternatives are poorly studied. In this paper, we focus on manipulating large herbivores through the closure of water points (WPs). Removal of artificial WPs has been suggested to change the distribution of African elephants, which occur in high densities in national parks in Southern Africa and are thought to have a destructive effect on the vegetation. Here, we modeled the long-term effects of different scenarios of WP closure on the spatial distribution of elephants, and consequential effects on the vegetation and other herbivores in Kruger National Park, South Africa. Using a dynamic ecosystem model, SAVANNA, scenarios were evaluated that varied in (1) availability of artificial WPs, (2) levels of natural water, and (3) elephant densities. Our modeling results showed that elephants can indirectly negatively affect the distributions of mesomixed feeders, mesobrowsers and some mesograzers under wet conditions. The closure of artificial WPs hardly had any effect during these natural wet conditions. Only under dry conditions the spatial distribution of both elephant bulls and cows changed when the availability of artificial water was severely reduced in the model. These changes in spatial distribution triggered changes in the spatial availability of woody biomass over the simulation period of 80 years and this led to changes in the rest of the herbivore community, resulting in increased densities of all herbivores, except for giraffe and steenbok, in areas close to rivers. The spatial distributions of elephant bulls and cows showed to be less affected by the closure of WPs than most of the other herbivore species. Our study contributes to ecologically informed decisions in wildlife management. The results from this modeling exercise imply that long-term effects of this intervention strategy should always be investigated at an ecosystem scale.
Selection and spatial Arrangement of rest sites within Northern tamandua (Tamandua mexicana) home ranges
Brown, D.D. ; Montgomery, R.A. ; Millspaugh, J.J. ; Jansen, P.A. ; Garzon-Lopez, C.X. ; Kays, R. - \ 2014
Journal of Zoology 293 (2014)3. - ISSN 0952-8369 - p. 160 - 170.
mammal myrmecophaga-tridactyla - resource selection - sleeping sites - utilization distributions - saguinus-fuscicollis - sexual segregation - habitat selection - predation risk - panthera-onca - tree holes
The distribution of suitable rest sites is considered to be a key determinant of spatial patterns in animal activity. However, it is not immediately evident which landscape features satisfy rest site requirements or how these sites are configured within the home range. We used Global Positioning System (GPS)/accelerometer telemetry to investigate rest site selection at the home-range scale for northern tamanduas Tamandua mexicana on Barro Colorado Island (BCI), Panama. We developed models specifying each tamandua as the individual experimental unit and averaged coefficients to produce population-level estimates. Tamanduas had on average 17.8 (±¿8.1) rest sites within their home range and used 1.36 (±¿0.51) on any given day. These rest sites tended to be located in the core of tamandua home ranges, with active locations associated with the periphery of the home range. Rest sites were positively associated with (1) a high density of Attalea butyracea palm trees; (2) elevation; (3) tall vegetation. There was a slight negative relationship between the distribution of rest sites and slope, and no apparent relationship between rest site selection and relative distance to forest canopy gaps. From focal animal observations, we identified that tamandua rest sites were typically located in trees (90%), with 25% (12 of 49) occurring in palms. We contend that northern tamanduas on BCI selected vegetated arboreal rest sites because of reduced likelihood of detection from terrestrial predators in these sites. Our models identified considerable individual variation in rest site selection, which suggests that the practice of pooling individuals and fitting models at an aggregate level may be inappropriate for certain types of habitat selection research.
Grazing ungulates select for grasses growing beneath trees in African savannas
Treydte, A.C. ; Beek, J.G.M. ; Perdok, A.A. ; Wieren, S.E. van - \ 2011
Mammalian Biology - Zeitschrift für Säugetierkunde 76 (2011)3. - ISSN 1616-5047 - p. 345 - 350.
mburo national-park - foraging behavior - mammalian herbivores - sexual segregation - woody vegetation - feeding patterns - quality - habitat - kenya - nutrients
In savannas, isolated large trees can form 'islands of fertility', referring to their elevated soil nutrients and their effect on light and water availability in their direct surroundings. Consequently, a quality difference between understorey grasses and open grassland can develop, creating patches of highly nutritious forage for grazing ungulates. Grass species composition beneath and outside of tree canopies was determined in a savanna system of Kruger National Park, South Africa. Direct observations were used to test whether grazing ungulates, i.e., impala, Burchell's zebra and blue wildebeest graze relatively more often beneath than outside of large tree canopies. Additionally, it was investigated whether they selected feeding locations according to allometric scaling and to their sex, and if feeding behaviour was influenced by weather conditions. Instantaneous scan sampling showed that ungulates preferred beneath-canopy grasses, independently of weather conditions. Grass species composition differed beneath and outside tree canopies, and beneath-canopy grasses exhibited more bite marks than outside canopy grasses. Blue wildebeest grazed least often beneath canopies compared to the other species. Females of impala were found to feed on beneath-canopy forage more often than males. Thus, large isolated trees strongly influenced the feeding behaviour of grazing ungulates. The current decline in large savanna trees should therefore be retarded and protective measures should be taken. © 2010 Deutsche Gesellschaft für Säugetierkunde.
Tree cover and biomass increase in a southern African savanna despite growing elephant population
Kalwij, J.M. ; Boer, W.F. de; Mucina, L. ; Prins, H.H.T. ; Skarpe, C. ; Winterbach, C. - \ 2010
Ecological Applications 20 (2010)1. - ISSN 1051-0761 - p. 222 - 233.
kruger-national-park - northern botswana - sexual segregation - large herbivores - watering points - colophospermum-mopane - mammalian herbivores - woody vegetation - fire - woodlands
The growing elephant populations in many parts of southern Africa raise concerns of a detrimental loss of trees, resulting in overall reduction of biodiversity and ecosystem functioning. Elephant distribution and density can be steered through artificial waterpoints (AWPs). However, this leaves resident vegetation no relief during dry seasons. We studied how the introduction of eight AWPs in 1996 affected the spatiotemporal tree-structure dynamics in central Chobe National Park, an unfenced savanna area in northern Botswana with a dry-season elephant density of ~3.34 individuals per square kilometer. We hypothesized that the impact of these AWPs amplified over time and expanded in space, resulting in a decrease in average tree density, tree height, and canopy volume. We measured height and canopy dimensions of all woody plants around eight artificial and two seasonal waterpoints for 172 plots in 1997, 2000, and 2008. Plots, consisting of 50 × 2 m transects for small trees (0.20–3.00 m tall) nested within 50 × 20 m transects for large trees (=3.0 m tall), were located at 100, 500, 1000, 2000, and 5000 m distance classes. A repeated-measures mixed-effect model showed that tree density, cover, and volume had increased over time throughout the area, caused by a combination of an increase of trees in lower size classes and a decrease in larger size classes. Our results indicate that the decrease of large trees can be attributed to a growing elephant population. Decrease or loss of particular tree size classes may have been caused by a loss of browser-preferred species while facilitating the competitiveness of less-preferred species. In spite of 12 years of artificial water supply and an annual elephant population growth of 6%, we found no evidence that the eight AWPs had a negative effect on tree biomass or tree structure. The decreasing large-tree component could be a remainder of a depleted but currently restoring elephant population