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1Université Polytechnique de Bobo-Dioulasso, 01 B.P. 1091 Bobo-Dioulasso 01 Faso ; 2 Ecole Nationale Supérieure des Sciences et Techniques Agronomiques de Kétou & 3Laboratoire d’Ecologie Appliquée de la Faculté des Sciences Agronomiques, 4Département de Biologie Végétale, Faculté des Sciences et Techniques Université d’Abomey-Calavi, BP 526 Cotonou, Bénin. 5Université de Ouagadougou, 03 B.P. 7021 Ouagadougou, Burkina Faso
Corresponding author email: [email protected] , or [email protected] Original submitted in on 18 th July 2012. Published online at www.m.elewa.org on 31 st October 2012.
Objectives : This study aims to 1) assess the woody vegetation structure and plant diversity in the area used by elephants for several years and 2) the effect of selected environmental variables (palatable woody density, distance to water ponds and to paths, vegetation and the soil types) on the pattern of the damages created by elephants in habitats they utilized. Methodology and Results : We used 100 plots of 30 m radius laid out in area used by elephants to assessed diameter and height class distribution within each vegetation type, and characterized phytodiversity using species richness, Shannon-Weaver index, and Smith and Wilson’s evenness. We searched for environmental variables to better explain the pattern of damaged tree. We reported an overall stability state of the vegetation. Disturbances affected recruitment in higher height class. The diversity values are low indicating the insignificant effect of elephant on the phytodiversity. Attraction of elephant to woody vegetation may depend mainly on the availability of woody plant they feed on. Conclusions and application of findings : These results highlight the interactions between elephants and vegetation and the main factors that drive their damaging behaviour towards tree species. They should guide park managers in making suitable habitat in terms of space and food for wildlife using preferred plant species to enrich and assist recruitment in gaps and degraded areas in natural habitats.
Disturbance by herbivores occurs in most of the contribution of elephants to adult tree destruction natural ecosystems and plays a vital role in and to natural habitat changes. The interactions determining species richness and the structure of between elephant and vegetation have often been plant and animal communities (Moloney and Levin, studied in a broad context (Cumming, 1997; 1996; Olofsson et al. , 2001; Grellmann 2002). Osborn, 2002). Most of these studies were carried Profound disturbances can reduce plant diversity, out in Eastern and Southern Africa and showed alter vegetation structure and eliminate animal that the physiognomy and species composition of species (Petraitis et al. , 1989; Pickett and White, African savannah was highly determined by 1985). Most opinions emphasize on the elephant (Barnes et al. , 1994; Osborn, 2002; 4223
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Loarie et al. , 2009). In West Africa, the size of related to the W Biosphere Reserve (Bénin side) is suitable habitats for elephant is dramatically recognized to concentrate a large population of declining due to human activities (habitat elephants in the WAP. Despite this abundance of fragmentation). Nevertheless, there is noticeable elephants in the Djona Hunting Zone, there is little engagement of West African wildlife managers to scientific information on the influence of these assess these threats and to take action towards animals on the vegetation structure and floristic conserving elephant species for future generations diversity. This study aims to 1) assess the woody (CMS, 2011). Elephant studies in West Africa were vegetation structure and plant diversity in this area occasional and almost only focused on population used by elephant for several years and 2) assess dynamics. The last global census in the sub-region the effect of selected environmental variables has shown that more than 50% of West African (palatable woody density, distance to water ponds elephant was attached to the complex W-Arly- and to paths, vegetation and the soil types) on the Pendjari (WAP) parks (Blanc et al. , 2004). Within pattern of the damages created by elephants in the WAP protected areas, the Djona Hunting Zone habitats they utilized. e The Djona Hunting Zone (DHZ) (115200 W Biosphere Reserve of Benin between the latitudes ha) is located in the north of Bénin in the department of 11°20’-11°60’N and longitudes 2°50’-3°20’E Alibori (Dauzan, 1991; Burini, 2004). It belongs to the (ECOPAS, 2005).).
(a) 4224
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(b) e : Location of the Djona Hunting Zone in Bénin (a)(West Africa) and the vegetation sampling plots within the DHZ (b)
The mean annual rainfall (from 1980 to 2009) in this region is 968.4 ± 148.2 mm. The rainfall regime is unimodal with one rainy from May to October and the second a dry season from November to April. The elephant distribution shows a seasonal pattern that goes with the damage on the woody vegetation within the study area. The annual mean temperature for the same period was 33.5°C for maxima and 21.75°C for the minima. The geomorphology of the DHZ is crystalline plateau (Tehou and Sinsin, 2000
The soils of the study site are ferruginous and leached the same pasture by ungulates from both sides. The on crystalline rocks. The Alibori River is the only one main vegetation corresponds to the Sudano-sahelian important watercourse in this area receiving water from transition savannahs with shrub savannahs as the its tributaries. In addition, there are few water ponds dominant vegetation type (White, 1983; Adjanohoun et scattered within the protected area that all together al. , 1989; Adomou, 2006). The most representative provide drinking water to the wildlife; as major source of woody plant species include: Acacia sieberiana drinking water in the area, it attracts the livestock (Leguminosae-Caesalpinioideae), Combretum spp coming from the villages around DHZ. Thus, (Combretaceae), Piliostigma thonningii (Leguminosae- competition occurs on foraging resources between wild Caesalpinioideae), Terminalia avicennioides animals and livestock (Francisco, 2009). There is (Combretaceae), Vitellaria paradoxa (Sapotaceae), potential risk of disease contamination due to the use of Detarium microcarpum (Leguminosae-
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Caesalpinioideae), Burkea africana (Leguminosae- Caesalpinioideae), and Balanites aegyptiaca The tree-density of the stands (N), i.e. the average (Zygophyllaceae); Crossopteryx febrifuga (Rubiaceae), number of trees per plot was computed in trees/ha as: Strychnos spinosa (Loganiaceae) and Gardenia N = (1) erubescens (Rubiaceae) e n is the overall number of trees in the plot, and s the e We first interviewed wildlife guards, area (s = 0.283 ha). foresters and local committees working for the The basal area of the stand (G), i.e. the sum of the conservation and the management of the DHZ to get cross-sectional area at 1.3 m above the ground level of the exact location of the habitats where elephants occur all trees in a plot, expressed in m 2/ha: at different periods of the year. We then conducted a G = quick field control on the indicated habitats and di is the DBH (in cm) of the i-th tree of the plot; s = proceeded on the selection of the research sites. The 0.283 ha. signs of habitat use by elephants were remaining foods, The mean diameter of the trees (D, in cm), i.e. the damaged trees (broken, debarked, uprooted), dung, diameter of the tree with the mean basal area in the tracks, etc. stand: e Three habitat (vegetation) types were targeted: shrub and tree savannahs, woodlands, D = and riparian forests. 100 circular plots of 30 m radius were laid out in each habitat type for floristic inventory where n is the number of trees recorded in the plot, and of damaged trees. Circular plot has the advantage to di the diameter of the i-th tree in cm. overcome the boundary effect and thus is less vulnerable to errors (Lackmann, 2011). The size of the The Lorey’s mean height (H, in meters), i.e. the plot (30 m radius or 2828.57 m 2) was adopted taking average height of all trees found in the plot, weighted into account the minimum area covered by a herd of by their basal area (Philip, 2002), was computed as elephants eating in a given habitat. We recorded follows: variables such as vegetation type ( cf. classification of Yangambi), soil texture (visual appreciation), land and clay content, vegetation burning index that consider H = with g i = visible burning impact on tree barks, signs of elephant gi and hi are the basal area (in m 2/ha) and the total activities (dung, tracks, broken or fallen trees etc.). height (in m) of tree i. Within each circular plot, all trees with diameter at The following ecological parameters were assessed breast height (DBH) ≥ 10 cm were tagged and using the Evenness program measured: individuals were counted, DBH was taken (http://www.nateko.lu.se/personal/benjamin.smith/softw using a diameter-tape and tree height was estimated are): using clinometers (SUUNTO). The floristic list was - The species richness (S) is the number of made for each plot and the plant nomenclature followed species recorded in the whole stand. the Analytical Flora of Benin (Akoègninou et al. , 2006). - The Shannon’s Diversity Index (H’, in bits) is In order to assess the effect of selected environmental computed using the following formula: variables on the elephant damage patterns we counted the number of damaged trees within each 30 m-radius H’ = - pi circular plot. Damaged trees included those that were where broken, fallen, uprooted, debarked etc. We used the S = total number of species in the community (species expertise of experienced elephant hunters to richness) ; distinguish tree damaged by elephants from damages Pi = abundance of the ith species expressed as a due to abiotic factor such as wind or storm. We also proportion of total number of trees inventoried in a measured distances from habitats exploited by given vegetation type (habitat). Values of the index elephants with signs of damages to water ponds as well usually lie between 1.5 and 3.5, although in exceptional as distances to paths using ribbon. cases, the value can exceed 4.5 (very high diversity). * The Simpson's dominance index (D’) was calculated 4226
et al e e e ee e e e e e using the formula: maximum likelihood method (Johnson and Kotz, 1970). D’ = The log-linear analysis was performed in SAS (SAS Inc., 1999) in each case to test the adequacy of the Where pi is the relative abundance based on number of observed structure to the Weibull distribution. The individuals per species. D ranges from 0 to 1 in case of considered model described by is as follows: complete dominance.
* The Evenness index of Smith and Wilson (Evar) was used as a measure of equitability (Smith & Wilson, Log Frequency = F + F_ Class + F_ Adjustment + 1996; Biaou, 2009). It is independent from species F = mean frequency of the classes; richness and has equal sensitivity to rare and abundant F_ Class = non-randomly gap linked to the differences in species. Evar ranges from 0 to 1, with 1 indicating frequency between classes; equal abundance of all species and values close to 0 F_ Adjustment = non-randomly gap linked to differences indicating dominance of one or few species. between observed and theoretical frequencies; is the error of the model. The hypothesis of adequacy e e e e e between both distributions is accepted if the probability e e e e value of the test is higher than 0.05. The study of the vertical and horizontal When c < 1 the distribution shows an inverse J-shaped structures of the vegetation exploited by elephant that characterises natural multispecific stands; c = 1 for population is crucial for the understanding of ecosystem decreasing exponential distribution characteristic of functioning. This study can help to make clear the effect disturbed stand on the brink of extinction. of elephant’s disturbances on the vegetation structure. When 1 < c < 3.6, the distribution is qualified as To establish the stem diameter structure of vegetation asymmetric positive often founded indicating types (woodlands, shrub and tree savannahs, and monospecific stands dominated by young individuals of riparian forest) exploited by elephants, all the recorded small diameter. When c = 3.6, the distribution is trees were grouped into diameter classes of 10 cm in symmetric showing a bell-shape of normal distribution order to obtain enough diameter classes (theoretically characterising monospecific stands or natural stands at least 10). This allows the adjustment of Weibull that experienced selective logging; and if c > 3.6, the theoretical distribution to the observed shape distribution is negative asymmetric and the stand is (Rondeux, 1999). The tree densities were assessed for characterised by a monospecific stand dominated by diameter classes. For the height structure, classes with old individuals (Husch et al ., 2003). 2 m amplitude were considered. The observed different By applying the Weibull equation to trees of DBH ≥ 10 diameter structures were adjusted to the 3-parameter- cm, it should be pinpointed out those mature shrubs Weibull distribution because of its flexibility (Johnson such as Piliostigma thonningii , Annona senegalensis , and Kotz, 1970; Burk and Newberry, 1984), whose Crossopteryx febrifuga are overlooked and thus are not density function, f is expressed for a tree-diameter x as taken into account in the interpretation of the results. follows: e e e e e e e e − c x − a c 1 x − a c e e : The generalized linear model f (x) = exp − with Poisson distribution function was used to explain b b b the damage tree pattern (Crawley, 2007). Because of the overdispersion found (Residual deviance/degree of freedom >> 1), we chose the negative binomial where x = tree diameter; distribution that allowed us to fit the best model. From a = 10 cm for the diameter structure and 2 m for the the full model including a set of environmental variables height structure; ("palatable woody density", distances from water b = scale parameter linked to the central value of pounds and paths, vegetation and soil types) with two diameters and heights; ways interactions, and using Aiccmodavg package for c = shape parameter of the structure. stepwise deletion, we ended up with the best model including each factor effect without any interaction. For each identified group, diameters of trees were used to estimate the parameters b and c based on the 4227
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e e e The showed an inverse "J" shape, characteristic of dendrometric and ecological characteristics of the multispecific plant communities (figure 2) with c-value vegetation types used by elephants are presented in of the Weibull distribution smaller than or close to 1, table 1. The mean basal area was higher in the riparian characteristic of multispecific or uneven-aged forest than in the open habitats (woodlands and communities. The 10–20 cm dbh classes were the best savannahs). Values of tree density, mean diameter, represented whereas individuals with dbh > 40 cm dbh Lorey’s height are similar among the three vegetation were scarce regardless the vegetation type. We found types (p > 0.05). a good adjustment of the observed diameter distribution e e e e The observed to the Weibull theoretical distribution (p > 0.05). diameter structure for the three vegetation types
e Vegetation structure and the quality of habitats utilised by elephants in the Djona Hunting Zone e e ee e e e ± e ± e ± Tree density (n/ha) 176.13 ± 17.6 167.97 ± 78.12 248 ± 93.38 0.09 Mean diameter (cm) 22.79 ± 5.2 21.95 ± 5.15 26.33 ± 5.74 0.17 Lorey's height (m) 10.58 ± 2.35 9.85 ± 2.34 11.95 ± 1.26 0.07 Basal area (m²) 7.76 ± 4.63 6.55 ± 3.86 13.22 ± 6.03 0.02* Species richness 6 6.27 6 Shannon wiener index 1.45 1.54 1.42 Simpson dominance index 0.30 0.28 0.31 Smith and Wilson evenness 0.69 0.74 0.57 *p<0.05; **p<0.01
The height distribution patterns exhibited a bell-shaped the woodlands, and [6-16] for the riparian forest. The distribution for all vegetation types. Individuals having a log-linear analysis (computed for each height structure) height ranging from 4 to 13 m (or [4-13]) were the most indicated a good adjustment of the observed represented in the shrub and tree savannahs, [4-17] for distribution to Weibull distribution (p > 0.05).
180 40 Shrub and tree savannah Shrub and tree savannah 160
140 Shape 0.8571 30 Shape 2.149 Scale 8.253 120 Scale 6.550 Thresh 10 Thresh 2 N 1099 100 N 1099 20 80 3-Parameter Weibull 3-Parameter Weibull Percent Percent
60
40 10
20
0 0 15 25 35 45 55 65 3 5 7 9 11 13 15 17 19 21 Diameter class (cm) Height class (m) 4228
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70 25 Woodland Woodland 60 Shape 1.021 Scale 13.13 20 Shape 2.112 50 Thresh 10 Scale 8.463 Thresh 2 N 539 40 15 N 539 3-Parameter Weibull Percent 30
Percent 3-Parameter Weibull 10 20
5 10
0 0 15 25 35 45 55 65 75 85 3 5 7 9 11 13 15 17 19 21 23 25 Diameter class (cm) Height class (m)
30 90 Riparian forest
80 Riparian forest 25
70 Shape 0.9682 Shape 2.137 20 60 Scale 11.42 Scale 8.085 Thresh 10 Thresh 2 50 N 119 N 119 15 Percent 40 Percent 3-Parameter Weibull 3-Parameter Weibull 30 10
20 5 10
0 0 15 25 35 45 55 65 75 85 95 105 3 5 7 9 11 13 15 17 19 21 23 25 Diameter class (cm) Height class (m) e Diameter and height structure within the three vegetation types e e e e Table 2 soil types. The best model gave in output Null synthesises the generalised linear model performed on deviance: 192.21with 99 as degrees of freedom; the following set of environmental factors: "palatable Residual deviance: 99.17 with 89 as degrees of woody density", the distance to water ponds, the freedom; AIC: 629.36 distance to paths/roads, the vegetation types, and the
e Generalised linear model explaining the pattern of damaged trees by environmental variables. e e e e Intercept 2.31E+00 2e -16 *** Palatable woody density 1.27E -03 0.01 ** Distance to water -1.32E -04 0.07 Distance to path/road -6.13E -05 0.00 *** Riparian -0.37 0.11 Shrub 0.04 0.77 Woodland 0.06 0.65 Clay -Sandy soil 0.24 0.29 La teritic soil 0.67 7.70e -05 *** Slimy -sand soil 0.61 9.58e -05 *** *** p < 0.001 4229
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The environmental variables such as the "palatable (palatable woody density) was most influenced by woody density", the distance to roads and the soil types accessibility to habitats (distance to paths/roads) and significantly influenced the model (p<0.05) showing that by the soil types. the intensity of damages caused by elephants e e e e e e found a preference of elephants for adult trees, which e The structure of the vegetation may entail switching from stem and leaf browsing to exploited by elephants in the Djona Hunting Zone bark stripping as height increases beyond 4 m (Barnes, (DHZ) showed a high number of small trees while the 1983; Smallie and O’Connor, 2000). This can explain number of large trees is low. This natural distribution is the utilization of larger (DBH > 55 cm) and tall trees we found in stable ecosystem (Gaoué, 2000; Kossou, reported with the present study. 2007; Ouédraogo et al. , 2008; Senzota et al. , 2009). We reported in this study that old and high trees (height Thus, the vegetation of DHZ presents visible stability > 21 m) were rare and were more frequent in the although it has been utilized for many years by riparian forest than in savannahs and woodlands. elephants (Thiombiano et al. , 2005). This can be Individuals between 7 and 11 m high were well regarded as an adaptation of natural ecosystems to represented in almost all the vegetation types that were animal disturbances. Pfeiffer (1989) found that elephant exploited by elephants. This can be because the tree showed an amazing ecological adaptation. Adult and uprooting, the branch breaking and the browsing big trees are more prone to being damaged by regime did not allow the recruitment of individuals elephants, and the natural trade-off was ruled by the towards higher classes. Therefore, elephant seems to recruitment of individuals from the natural regeneration. contribute to the creation and the maintenance Despite the trampling and browsing of young trees, savannah ecosystems (Valeix et al. , 2011; de Beer et elephant did not affect the recruitment ability of trees al. , 2006). In the DHZ, the reported elephant effects are and the demographic structure of woody communities. likely to provide excellent services for grassland Several factors account for tree vulnerability to elephant establishment because the dominant trees are not very disturbance: exposure to mutilation, thickness of the large and tall to develop extended and thick shadow bark, weakness of the root system (shallow root) and detrimental to grasses that necessitate light for their substrate instability (O’coonor et al. , 2007). Bell (1985) establishment. This situation that favour grasses is worked in East Africa on elephant’s damages on beneficial to ungulates and this regarded as an vegetation and confirmed these factors but stressed on ecosystem service. The species diversity in habitats the preference for specific plants such as Adansonia used by elephant in the Djona Hunting Zone (DHZ) is and Acacia that were identified like cause of habitat’s low compared with the value found by Natta (2003) in vulnerability to elephants’ disturbances. The author the Sudanian zone. Elephant disturbances probably reported with the same study that when these species contributed to this low woody plant diversity in the study dominate one habitat, the impact of elephants can be area. Negative effects of herbivores on phytodiversity so pronounced to induce a conversion to grassland were reported by Mligo and Lyarum (2008) in a when fire intervenes to accelerate and deepened the situation where both livestock and wild mammals fed on disturbances. Previous studies have found that vegetation what is likely to occur in the study zone elephants prefer lowland forest habitats (Kinnaird et al. , where there are permanent conflicts between cropping 2003; Hedges et al. , 2005; Azad, 2006; Pradhan & activities and herds breeding, all this coupled with the Wegge, 2007) where nutritious foliage is abundant presence of water sources situated in the protected showing a direct link of the disturbances of elephants area leading to constant attraction of livestock to the with the need of food resources. This finding same pastures and water points with wildlife. However, demonstrated why elephants affects more habitats some studies presented elephants as excellent agents where exist the plants they prefer. Belayneh and that contribute to increasing plant diversity (Western, Demissew (2011) reported the preference of elephants 1989; Rao et al. , 1990; Botha et al. , 2002). Therefore, for fruit trees and according to Kalemera (1989); plants the impact of elephants on the vegetation and shorter than 1m tend to be ignored. Other workers have phytodiversity remains a controversial debate. 4230
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e e e e e hunting and touristic activities use roads. These roads e Our results revealed that the type of the seemed to trigger the behaviour of elephants towards vegetation could significantly influence damages branch breaking, uprooting and debarking of trees caused by elephants through their feeding behaviour. alongside the roads. We also found that tree damages Habitats that are rich in large and tall trees seem to be were remarkable on lateritic soils that are colonised by more exposed to elephant damages. Contrary to the population of Detarium microcarpum which fruits are results found by several authors (De Villiers and Kok, preferred by elephants. The clay soil is not preferred by 1988; Owen-Smith, 1988; Ben-Shahar, 1993), distance elephants because of its instability that makes it difficult to water was not the only factor that drives the elephant to walk. The heavy body of this large mammal seems to or disturbance on a given habitat. This study reports the trigger his feeding behaviour and habitat selection. It is distance to roads as negatively correlated with the also common to come across elephant tracks following intensity of the damages caused by elephants. This roads over relatively long distance in a park or to implies that elephant movements were not influenced observe a group of elephants avoiding a rocky terrain by human presence since park managers for patrols, (Pers. observation). We are grateful to the African Wildlife Foundation fellowship, which financed this research. Adjanohoun E, Adjakidjè V, Ahyi MRA, Aké Assi L, Measurements, models and management. J. Akoègninou A, d'Almeida J, Apovo F, Boufef Appl. Ecol. 20: 521- 540. K, Chadaré F, Cusset G, Dramane K, Eyme J, Belayneh A, Demissew S, 2011. Diversity and Gassita J-N, Gbaguidi N, Goudoté E, Guinko Population Structure of Woody Species S, Houngnon P, Issa LO, Kéita A, Kiniffo HV, Browsed by Elephants in Babile Elephant Kone Bamba D, Sanctuary, eastern Ethiopia: an implication for Musampa Nseyya A, Saadou N, Sodogandji Th, de conservation. Ee-JRIF Vol 3, No 1 2011 – Souza S, Tchabi A, Zinsou Dossa C, Zohoun Agriculture and Forestry issue: 20-32. T 1989. Contribution aux études Bell RHV,1985. Elephants and woodland - a reply. ethnobotaniques et floristiques en République Pachyderm 5: 17-18. Populaire du Bénin. ACCT, Paris, 895p. Ben-Shahar R, 1993. Patterns of elephant damage to Adomou AC, Sinsin B, van der Maesen LJG, 2006. vegetation in Northern Botswana. Biological Phytosociological and chorological Conservation , 65: 249–256. approaches to Phytogeography: A study at Biaou SSH, 2009. Tree recruitment in West African dry meso-scale in Benin. Systematics and woodlands: The interactive effects of climate, Geography of Plants, 76: 155-178. soil, fire and grazing . PhD thesis, Wageningen Akoègninou A, Van der Burg WJ, Van der Maesen University, Wageningen, The Netherlands. LJG., Adjakidjè V, Essou JP, Sinsin B, Blanc JJ, Barnes R FW, Craig CG, Dublin HT, Thouless Yédomanhan H, 2006. Flore analytique du CR, Douglas-Hamilton I and Blake and Bénin. Backhuys publishers, Wageningen, Hedges, 2004. Sinking the flagship: the case 1034 p. of forest elephants in Asia and Africa, Azad M, 2006. Mammal diversity and conservation in a Conservation Biology , 18: 1191–1202. secondary forest in Peninsular Malaysia. Botha J, Witkowski ETF and Shackleton CM, 2002. A Biodiversity and Conservation, 15, 1013– comparison of anthropogenic and elephant 1025. disturbance on Acacia xanthophloea (fever Barnes RFW., Barnes LK, Kapela BE, 1994. The long- tree) populations in the Lowveld, South Africa. term impact of elephant browsing on Baobab Koedoe , 45(1): 9–18. trees at Msembe, Ruaha National Park, Burini F, 2004. “Le carte partecipative: strumento di Tanzania. African Journal of Ecology ,177-184. recupero dell’identità Africana”, dans: E. Casti, Barnes RFW, 1983. Effects of elephant browsing on M. Corona (dir), Luoghi e Identità , geografie e woodlands in a Tanzanian National Park: letterature a confronto , Bergamo University Press, Bergamo, pp. 185-214. 4231
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