Researcher: Gabriella TerenRegion: Linyanti, Botswana
BackgroundThe extremely high population of elephants in Botswana and the seasonal concentration of large herds along perennial rivers such as the Chobe and Linyanti Rivers make northern Botswana the ideal environment to study the effects elephants have on riparian ecosystems, not only to provide information on the disturbance effect of elephants in Botswana, but the results can be extrapolated out to other parts of Southern Africa where elephants are in lower, but increasing densities and management questions are being raised. Elephants are agents of tree mortality and concern has been raised about the vegetation changes created by them. The argument that elephants reduce habitat for other species and consequently biodiversity has been used to justify culling programmes to reduce elephant numbers.
Effective methods of measuring biodiversity are urgently needed to monitor changes in the Linyanti area. For conservation of biodiversity in areas subject to elephant disturbance to be successful it is necessary to understand the effects that elephants have on regeneration, composition and structure of woodlands. Currently there is a lack of knowledge on how elephants affect biodiversity in the context of little ‘robust documentation of the degree, extent and duration of the effects of elephant-induced habitat change’. This study investigated the changes in woody plant biodiversity through elephant disturbance in a woodland subject to exceptionally high elephant concentrations.
We know from several surveys that the vegetation along the Linyanti River is visibly changing with regards to both composition and structure, in particular species abundance with losses of 60% of Acacia erioloba, and 50% of Acacia nigrescens and Terminalia sericea in the large tree size class. Stuart Bell (2003) compared replicate aerial photographs of the area used in the previous study and those taken in 2001, nine years later. He found that the disappearance rate of canopy trees (taller than 5m) averaged at 2% per year, or 15.5% over the nine year period. Also that several species (Acacia erioloba, A. nigrescens, Peltophorum africanum,and Terminalia prunioides) had disappeared from the canopy altogether, although there were specimens in the smaller size classes. The woodland has therefore been shown to be undergoing slow, but progressive decline. However, what was observed was that the shrub layer appears to be rapidly increasing in density from 1992-2001. This apparent incongruity has raised the hypothesis that this woody shrub layer is increasing in response to the carbon fertilisation effect, brought about by recent climate change. This effect is where increasing atmospheric CO2 concentration increases photosynthetic rates and therefore growth of woody species, particularly where water is a non limiting-resource. Environmental Change has been expected to change whole ecosystem structural and compositional types through disturbance driven mortality of some vegetation assemblages, followed by the growth of new assemblages. This change is what is being observed along the Linyanti, and therefore climate change as a driver of these changes in addition to elephant disturbance should be considered.
ObjectivesThe project aims to assess the changes in the vegetation of the Linyanti riverfront, locate the drivers of this change (elephants and/or climate change) and attempt to establish what these changes mean for heterogeneity and ultimately biodiversity of the area.
This study built on the findings of two previous studies on the Linyanti woodland where high resolution (1:10 000) aerial photographs were taken (in 1992 and 2001) and covered a 40km stretch of woodland along the river. The first fieldwork period was devoted to establishing ground control points for the GIS, preliminary testing of sampling methods and data collection, as well as pollen collection. After that the 1992 and 2001 photos were analysed for changes, before the 2008 photos were taken in July. From July till the next fieldwork period in November, the 2008 photos are being analysed and compared to the previous photos. The main fieldwork period will then take place. The third year will encompass analysis of data collected from the field and diversity changes in patches.
The project focuses on four core areas: The long-term woodland dynamics of the Linyanti ecosystem as a baseline; the spatio-temporal dynamics of elephant disturbance on canopy trees; the causes and effects of shrub encroachment in the woodland; the regeneration dynamics of the woodland and future projections; and the overall consequences of compositional and structural change for biodiversity of the woodland.
Advances in Geographic Information System (GIS) technology now enables detailed analysis of the vegetation changes in the aerial photographs from 1992 and 2001. Using GIS and other spatial statistical programmes, as well as fieldwork the following questions were addressed:
What is the % change in the shrub layer, other size classes of trees, and non-woody plants?
What species are changing, and associated changes in species richness, and taxonomic value (resource value for elephants and other herbivores, rarity etc).
In terms of possible drivers of the change, there are two hypotheses: Hypothesis 1: Elephants are the main drivers of change. Hypothesis 2: Climate change is the main driver of change. Fieldwork as well as GIS analysis was used to answer these two.
This is a rare opportunity to document possible climate change effects upon an ecosystem at present and will also add to the knowledge base of climate change in Southern Africa. It will also highlight the advantages and possibilities of using geographical information systems and the latest technologies in addressing ecological issues. The final outcome should enable planning of a future management scenario for biodiversity regarding elephant disturbance and the effects of climate change in northern Botswana.
The Trust funded the aerial photographs in 2008.
Annual Report 2011
This project assesses the effect of concentrated elephant impact on biodiversity, incorporating the paradigm that savannahs are non-equilibrium systems, by looking at the system over large spatial and long temporal scales. The advantage of this study is the existence of high resolution (1:10 000) aerial photographs taken in 1992 and 2001 covering 50km of riparian woodland and 18 years of data. 2010 saw an opportunity to replace problematic 2008 aerial photographs with new images, sponsored in part by the Trust. LiDAR (Light Detection and Ranging) data was acquired at the same time; a laser is used to create a super-accurate 3D image, and as the laser penetrates the canopy, this includes shrubs underneath trees – a vital component of savannah structure.
Extensive fieldwork was completed in December 2009, with over 10 000 trees, shrubs and seedlings identified, measured and GPSed. Adding the the historical 1992 and 2001 aerial photographs, over 30 000 individual dead canopy trees have been mapped across the 50km woodland, with identification of dead trees in the 2010 images underway.
Results so far show that acacias have largely disappeared from the canopy tree layer through debarking by elephants. Whilst seedlings of some canopy trees are found, these do not survive to replace parent trees, and conversion from woodland to shrubland has occurred. A single shrub species, Combretum mossambicense, has annexed the woodland, comprising over 50% of the total number of individuals taller than 2.5m. As such this indigenous shrub is behaving as an invasive and is apparently more of a threat to biodiversity than the decline in canopy trees.
Preliminary investigations using OBIA (Object Based Image Analysis, where artificial intelligence creates a tree from of a group of pixels) point to exciting results showing where tree canopy cover declines, where shrub canopy cover increases, and the intensity of these changes, pointing to possible causes.
This research is primed to effectively increase our understanding of long-term, spatially explicit savannah dynamics, driven by disturbance, and climate (both long-term, and anthropogenic-induced change) at the scale needed for regional planning and biodiversity management.
In May 2010 we flew a LiDAR and aerial photo survey of the woodland, with support from the Wilderness Trust, Wilderness Safaris and University of the Witwatersrand. This completes 18 years of high resolution photography showing actual changes across the entire woodland.
The LiDAR (Light Detection And Ranging) process is where laser pulses are emitted from a sensor aboard an aeroplane and return to the sensor. The time taken for the laser to return determines the height and a vertical profile, and after processing, a 3D image is created. The LiDAR survey occurs at the same time as aerial photographs are taken, and we are able to match the laser points to the aerial photograph.
A preliminary example of the 2010 LiDAR output below shows the laser point clouds of the ground and (from l to r): a termite hill, a shrub under the canopy of a tree, and a tall tree. This method is very good at surveying sub-canopy vegetation and will help determining if shrubs establish in open areas or under trees.