Plant Ecol (2009) 202:31–40 DOI 10.1007/s11258-009-9575-6
Mound building termites contribute to savanna vegetation heterogeneity Stein R. Moe Æ Ragnhild Mobæk Æ Anne Kjersti Narmo
Received: 28 June 2007 / Accepted: 13 January 2009 / Published online: 11 February 2009 Ó Springer Science+Business Media B.V. 2009
Abstract With biomass densities comparable to large ungulates and megaherbivores, termites play a key functional role in many tropical savanna ecosystems. This study focuses on vegetated termite mounds (termitaria) constructed by the Termitidae species Macrotermes herus. We studied how resource rich termitaria affect graminoid herbs (Poaceae and Cyperaceae), forbs and woody species composition and diversity. The density of termitaria explained 89% of the variation in dense thickets in the area. Fire tolerant Acacia species dominated the open savanna while fire sensitive species like Grewia spp. and the succulent Euphorbia candelabrum were restricted to termite mounds. Termitaria plots had four times the mean number of woody species and supported three times as many forb species as the adjacent savanna. For woody species, both the Shannon–Wiener index and the Shannon evenness index were higher on
S. R. Moe (&) R. Mobæk A. K. Narmo Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, ˚ s, Norway P.O. Box 5003, A e-mail:
[email protected] Present Address: R. Mobæk Department of Animal and Aquaculture Sciences, Norwegian University of Life Sciences, P.O. Box 5003, ˚ s, Norway A
temitaria than on the savanna. There were no differences for graminoid herbs, except for the Shannon evenness index which was higher on termitaria. Our results indicate that graminoid herb richness peaks at lower productivity levels than trees and forbs in savanna ecosystems, as also recently found in temperate areas. Keywords Biodiversity Macrotermes herus Species richness and productivity Termitaria Termitidae Uganda
Introduction Termites constitute an important group of the soil fauna, accounting for 40–60% of the total soil macrofauna biomass in many tropical ecosystems (Wood and Sands 1978). Termite standing biomass in African savannas has been estimated to be between 70 and 110 kg ha-1 (Wood and Sands 1978; Ferrar 1982; Wood et al. 1982). Thus, termite biomass are comparable to the biomass of ungulates (10–80 kg ha-1, Bell 1982) as well as to savanna megaherbivores (commonly \110 kg ha-1, Owen-Smith 1988), indicating that termites play a major ecological role in the functioning of savannas (Dangerfield et al. 1998). While savanna soils generally have a low reserve of weatherable minerals, especially calcium, magnesium and phosphorus (Jones and Wild 1975), termite
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activity directly or indirectly modifies the availability of nutrients for other organisms (Jones et al. 1994; Dangerfield et al. 1998). The importance of termites in the decomposition of plant material is well documented (Wood 1978; Abe 1980; Martius 1994; Mando and Brussaard 1999), and termites also contribute to carbon and nitrogen mineralization, nutrient availability and stimulation of microbial activity (Lobry de Bruyn and Conacher 1990; Martius 1994; Eggleton et al. 1996; Lawton et al. 1996; Lavelle et al. 1997; Okwakol 2000). The construction of large epigeal nests and extensive underground gallery systems has major effects on soil characteristics, especially soil physico-chemical structure and properties (Anderson and Wood 1984; Nutting et al. 1987; Dangerfield et al. 1998; Asawalam et al. 1999) as well as infiltration, water status and rainfall use efficiency (Watson 1969; Arshad 1982; Eldridge 1994; Konate` et al. 1999; Mando et al. 1999; Le`onard and Rajot 2001). Nest building activities, which are typical for higher termites (Termitidae), provide nutrient-enriched microhabitats within the savanna soils (Watson 1977; Arshad 1982; Pomeroy 1983; Nyamphene 1986; Donovan et al. 2001), which in turn may facilitate vegetation growth (Hesse 1955; Lal 1988; Watson 1976, 1977; Mando et al. 1999). This study focuses on vegetated termite mounds (termitaria) constructed by the Termitidae species Macrotermes herus. Macrotermes termites do not have symbiotic flagellates in their hindgut, and contrary to lower termites they cannot digest cellulose. Instead they grow gardens of fungi (Termitomyces) both outside and inside the mound and feed the fungi with plant material together with saliva and faeces (Lee and Wood 1971). The fungus decomposes cellulose and concentrates nitrogen, phosphorus and potassium into fungal tissue that the termites consume as part of their diet (Button et al. 1983). By transporting soil mineral particles vertically and horizontally during mound construction and by bringing surface organic material into the mound to feed the fungi, the Macrotermes species commonly enrich the nutrient status locally on and around mounds (Lee and Wood 1971). Since Macrotermes mounds often contain elevated levels of soil minerals (Lee and Wood 1971; Miedema and Van Vuure 1977; Maduakor et al. 1995; McCarthy et al. 1998) they increase plant nutrient availability and thereby influence plant spatial distribution and vegetation dynamics (Jones 1992;
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Konate` et al. 1999). Watson (1976, 1977) found that Macrotermes falciger mounds in oligotroph miombo woodland (Brachystegia spp. and Julbernardia spp.) in Zimbabwe were richer in plant nutrients than adjacent savanna soil and that this mound soil strongly stimulated plant growth. Probably due to this plant nutrient enrichment, the particular Macrotermes mound vegetation is found to be preferred browse by megaherbivores in miombo woodland in Zimbabwe (Loveridge and Moe 2003) and by ungulates in eutrophic savanna in Uganda (Mobæk et al. 2005). Although earlier savanna studies do indicate that termitaria support a denser plant cover and more diverse plant communities (Lee and Wood 1971; Loveridge and Moe 2003), to our knowledge there is no comprehensive study on how termitaria contribute to the diversity of graminoid herbs, forbs and woody species and consequently how termites act as agents of savanna plant heterogeneity. This study focuses on how M. herus activity affects savanna plant species composition and diversity. Specifically we tested whether termitaria support vegetation that is richer in species and denser than the surrounding savanna vegetation and whether plant community composition differs between termitaria and savanna vegetation.
Materials and methods Study site The study was conducted in Lake Mburo National Park, which is located in the southwest of Uganda (between 00°300 and 00°450 S, and 45°000 and 31°050 E). The park covers an area of *260 km2, and the altitude varies between 1,220 and 1,450 m a.s.l. (Bloesch 2002). Lake Mburo National Park lies in a rain shadow between Lake Victoria and the Ruwenzori Mountains, and receives an average of 800 mm of rain annually, with two distinct rainy seasons (February–June and October–December). The average recorded temperature is 27.5°C with daily variation ranging from 21.5 to 34.0°C. The dominant soil types are ferrasols, histosols, vertisols and leptosols (Bloesch 2002). The vegetation in Lake Mburo National Park is dry open Acacia savanna (Langdale-Brown et al. 1964), with the main graminoid herbs being
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Sporobolus pyramidalis, Chloris gayana, Loudetia kagerensis and Bracharia eminii. The dominant tree species is Acacia hockii. Other important Acacia species are Acacia gerrardii, Acacia sieberiana and Acacia polycantha (Bloesch 2002; Hoag et al. 1991). Taxonomy follows Palgrave (2002) and Katende et al. (1995) for woody plants, Tropin (1983) for forbs, and Haines and Lye (1983) and Phillips et al. (2003) for graminoid herbs (Poaceae and Cyperaceae). Tree cover varies from open areas with only few trees to woodland, and the more open parts of the park commonly support thickets associated with termitaria. The vegetated termitaria in the park are constructed by the termite genus Macrotermes, and the dominating termite species is the mound-building M. herus (Bakuneeta 1989). This study was conducted in open savanna in flat valley bottoms in the central and eastern part of the park. These valleys are characterized by numerous thicket-termitaria associations surrounded by open savanna (Hoag et al. 1991). The termitaria thickets consist of several species of trees, shrubs, climbers and forbs. S. pyramidalis and Themeda triandra make up the main grassland association in this area, while the dominating tree associations are mixed woodland composed of a variety of Acacia species and thickets where Rhus natalensis and Grewia spp. are common. The most numerous ungulate species in Lake Mburo National park are impala (Aepyceros melampus) and Burchell’s Zebra (Equus burchelli). Other common ungulates are eland (Taurotragus oryx), topi (Damascilus korrigum), waterbuck (Kobus ellipsiprymnus), bushbuck (Tragelaphus scriptus), warthog (Phacochoerus africanus) and the African buffalo (Syncerus caffer) (Rannestad et al. 2006). Sampling Twenty termite mounds (termitaria) and 20 savanna plots were selected randomly in different areas of the park. A table of random numbers was used to pick a random distance (0.1–1 km) on the park road network. After covering the random road distance, a random compass bearing (1–360°) was selected along with another random distance away from the road. The nearest mound from the end point was included in the survey. On each termitarium, the height of the mound and four radii (two longest radii and the two perpendicular shortest radii) were measured. Mound
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height varied from 0.5 to 2.8 m with a mean of 1.74 m (±0.59 SD) (termitaria n = 20). The average radius of mounds was 3.7 m (±0.95 SD), ranging from 2.1 to 6.4 m. By averaging the four mound radii, we estimated the mound area and determined the radius of the corresponding circular random savanna plot. Twenty random savanna plots were selected by choosing a random distance between 20 and 80 m from the mound in a random direction. Careful inspections ensured that the savanna plots were never closer than 20 m to any termitaria and that the distance from the mound to the savanna plot never exceeded 80 m. A minimum of 20 m was selected to avoid any immediate effect of the mound, and a maximum of 80 m was selected because distances longer than 80 m would increase the risk of being too close to adjacent mounds. Vegetation was recorded in June and July (forbs and woody species) and in February (graminoid herbs). All woody, forb and graminoid herb species were recorded both on mounds and in the random plots. At both mounds and random plots, woody plant stem diameter was recorded by using a slide calliper. Woody plants below 3 m height and a stem diameter less than 6.0 cm at the base (10 cm) were categorized as shrubs. Plants with non-woody stems with stem diameter less than 1.0 cm were categorized as forbs. The woody component of the vegetation was determined by systematically counting all individuals on the plots. Individual counting was done for some forb species while the percentage cover was used for others. Graminoid herb cover was estimated visually on a percentage scale. All the mounds in used in this study were M. herus mounds and active at the time of plant census. However, the mounds are frequently abandoned and re-colonised and during some periods other termite species may occupy the whole mound or only part of it (unpubl. data). The alpha- and beta-diversity was calculated for woody, forbs and graminoid herbs. Alpha-diversity or within-habitat diversity was defined as the total number of species on the plots, whereas beta-diversity or between-habitat diversity was defined as the difference in community composition between termitaria and savanna plots. We used the Jaccard index (Jaccard 1928) to calculate the beta-diversity. Shannon–Wiener index (Whittaker 1977) and Shannon evenness index (Gurevitch et al. 2002) were calculated
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for woody and graminoid herbs on termitaria and savanna plots. The Shannon–Wiener and Shannon evenness index were not calculated for forb species because these data comprised a mixture of individual counts and estimated percentages. In order to identify how termite density related to the spatial distribution of dense thickets found in the area, the total area occupied by termitaria and thickets was estimated from transect lines. Thickets were defined as closed patches of tall vegetation consisting of more than one plant species. A total of 17 transects each 500 by 50 m were located randomly in different parts of the open savanna. One person walked a straight line in a random compass bearing and using a GPS, while two persons counted the termitaria on each side. The number of termitaria with thickets (vegetated mounds), termitaria without thickets (bare mounds) and thickets without termitaria were recorded.
Results Woody species were present on all the 20 termitaria, and all of the mounds supported more than one species. In contrast, woody species occurred in 14 of the savanna plots and only 7 of these plots contained more than one species. We found that woody species alpha-diversity was more than four times higher on termitaria compared to savanna plots (paired t-test, t = 8.28, P \ 0.0001) (Table 1). The Shannon– Wiener index was significantly higher for woody species on termitaria than for woody species in the savanna plots (paired t-test, t = 8.54, P \ 0.0001). Also the Shannon evenness index was significantly
Table 1 Differences in number (mean ± SD) of plant species, densities and Shannon–Wiener index and Shannon evenness index between termitaria and savanna plots (paired t-tests, n1 and n2 = 20)
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higher for termitaria woody vegetation compared to the savanna plots (Paired t-test, t = 4.72, P = 0.001) (Table 1). Tree densities on termitaria were 25 times the densities in the savanna plots (paired t-test, t = 5.02, P \ 0.0001), while shrub densities were 6 times higher on termitaria compared to savanna plots (paired t-test, t = 6.77, P \ 0.0001). Forb species were present on all of the 20 termitaria and in 19 of the savanna plots. All of the mounds and 13 of the savanna plots supported more than one forb species. The forb species alphadiversity on termitaria was more than twice as high as in the savanna plots (paired t-test, t = 6.80, P \ 0.0001) (Table 1). There were no significant differences in number of graminoid herbs between termitaria and savanna plots (paired t-test, t = 5.97, P = 0.53) (Table 1), nor did the Shannon–Wiener index for graminoid herbs differ between savanna and termitaria (paired t-test, t = 0.17, P = 0.11). However, the graminoid herbs Shannon evenness index was significantly higher on termitaria compared to the savanna (paired t-test, t = 3.49, P = 0.0003) (Table 1). A total of 40 woody species were found on termitaria, while 14 woody species were recorded in the savanna plots. Eleven species were unique for the termitaria while only A. sieberiana and A. hockii were exclusively found on the savanna plots (Fig. 1a). The most frequent woody species on termitaria were Grewia mollis, Grewia similis, Maytenus heterophylla and R. natalensis while the savanna plots were dominated by Dichrostachys cinerea, A. sieberiana, Solanum incanum and A. gerrardii (Fig. 1a). Succulent species like Cissus quadrangularis, Cyphostemma adenocaule and Euphorbia candelabrum were
Termitaria Number of woody species
10.5 ± 3.9
Savanna plots 2.5 ± 2.0
P-value P \ 0.0001
Shannon–Wiener index for woody species
1.53 ± 0.51
0.25 ± 0.38
P \ 0.0001
Shannon evenness index for woody species
0.80 ± 0.13
0.30 ± 0.42
P = 0.0001
Density of trees (m-2)
0.25 ± 0.20
0.01 ± 0.02
P \ 0.0001
Density of shrubs (m-2) Number of forb species
0.56 ± 0.33 7.6 ± 2.4
0.09 ± 0.12 2.7 ± 2.2
P \ 0.0001 P \ 0.0001
Number of graminoid herbs
6.80 ± 2.19
7.10 ± 2.07
P = 0.53
Shannon–Wiener index for graminoid herbs
1.58 ± 0.33
1.40 ± 0.43
P = 0.14
Shannon evenness index for graminoid herbs
0.52 ± 0.13
0.38 ± 0.13
P = 0.004
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Fig. 1 Number of plots where a particular woody (a), forb (b) and graminoid herb (c) species was recorded on termitaria (n = 20) and in the savanna plots (n = 20). Only species present on more than 3 (woody and forb species) or 7 (graminoid herbs) termitaria or savanna plots are included in the figure
20
a)
15 10 5 0
. llis ilis ylla sis tata um eri osa nse sp rum ilis ulis nus ura sita sp. dea rdii rea ana ckii mo sim rophtalen aris can himp ent arie aris lab nob ed rica one ppo eae scoi erra cine beri ho a i ia e a e in sc om sc pp de lea ssa af ap o ac di g ys ie cia t s a a a n c ri s s ia i a ia h ew ew et n m m Gr Gr us h hus taec lanu belia aris dag C ia ca Te Ca hylu iscu lund Rub itari cac stac acia Ac r A ro c p ib os n R ae So m pp ma b a r o e A l t g a l o H H E C m r y ch A Ch ph iu Di Ma Ma Eu pis ir le T
Number of plots
20
b)
15 10 5 0
. . i i i ell nus sp aris ule aris um lius nti tata esis es lata per tus ans cta sp aris rch frica lina ngul noca gell ssim atifo gra den ngn liform ricu him gula adic arre ium line u b a me ra e fla ati n l ens vix bo bel au sc an e r era od sia o id ia f sm o a la m io st a nia us m ad ad us gr vo rag Co qu ma rag um stem B rler occ va fl grofi inu nec hori digo De ephr a a s a im no a sc In a roc ri m P sp m l y T u p s e B o P H Ja S Dy ss st As Oc ola A yth Ci pho S Er y C 20
c)
15 10 5 0
s i a a a n a a um tylo alis ym inica ens dula ndr nus nus ntha rata nsis yan iflor osa vata tidu edi at thrix ximdac amid mon yss umb pen tria pfia frica riza spe hire ga ong cem ia o s ni dbra com cno a a i s l a l d y a c b a u b l i a r a i t m on y ho a ec fi d s s a ex s or ia r ar re m ri p um od s p ria ria ia d nia me lus olu iari tis on hl itar stis ga yc tis hia ris nic Cyn bulu eta igita hiar rrhe The obo rob ach ros pog C Dig gro bild P ros Brac Chlo a a A o S D ac pa P ag or po Br rag ndro Er or Er E A Sp S Br Hy Sp
Termitaria
restricted to mounds. The Jaccard index between termitaria and savanna plots for woody species was 31.7%. The total number of forb species recorded on termitaria and in the savanna plots was 41 and 22, respectively. The most frequent forb species on termitaria were Pavonia burchellii, Asparagus
Savanna plots
africanus and Commelina sp. (Fig. 1b), while the most frequent forb species in the savanna plots were Ocimum gratissimum, Bidens grantii and Asparagus flagellaris. The Jaccard index between termitaria and savanna plots for forb species was 18.9%. Although some graminoid herbs dominated the termitaria while others dominated the savanna plots,
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only two graminoid herbs (Setaria homonyma and Pycreus nitidus) occurred exclusively on the termitaria while none of the graminoid herbs were exclusively associated with the savanna (Fig. 1c). Panicum maximum was the most abundant graminoid herb on termitaria, while T. triandra and Sporobolus staphianus dominated open savanna plots. In total, 28 graminoid herbs were recorded on termitaria and 30 species were found in the savanna plots. The Jaccard index between termitaria and savanna plots for graminoid herbs was 62.2%. There were no correlations between number of graminoid herbs and forb species, graminoid herbs and trees or forbs and trees, neither within the mounds nor within the savanna plots (Person Product Moment Correlations, all P [ 0.05). The only significant species group correlation was between number of graminoid herbs on the mounds and the number of graminoid herbs on corresponding savanna plots (Person Product Moment Correlation, r = 0.51, P = 0.02). The density of termitaria explained 89% of the variation in the density of thickets (R2 = 0. 89, P \ 0.001, transect n = 17) (Fig. 2). The total density of termitaria was 10.1 ha-1 (±3.4 SD) and the total density of thickets was 8.9 ha-1 (±3.3 SD). When looking at vegetated and bare mounds separately, the density of termitaria with thickets was 8.5 ha-1 (±2.8 SD) while the density of termitaria without thickets was 1.6 ha-1 (±1.3 SD). The density of thickets without termitaria was 0.5 ha-1 (±0.6 SD).
Number of thickets ha-1
18
R2 = 0.89 P < 0.001
16 14 12 10 8 6 4
4
6
8
10
12
Number of termitaria
14
16
18
ha-1
Fig. 2 The relationship between density of termitaria (ha-1) and density of thickets (ha-1) (R2 = 0.89, P \ 0.001, n = 17)
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Discussion Woody and forb species numbers, the Shannon– Wiener indexes and density were far higher on termitaria compared to savanna plots, while for graminoid herbs only Shannon evenness index was higher on termitaria. Mean number of woody species on termitaria was four times of the savanna areas while termitaria supported three times as many forb species. The fact that species richness for woody, forbs or graminoid herbs was not related indicates that the species richness of these plant species groups is determined by different resource constraints. Consistent with previous studies we found that woody species densities were significantly higher on termitaria compared to the savanna areas. Several studies have reported that termitaria support denser vegetation than surrounding savanna areas (Wild 1952; Arshad 1982; Pomeroy 1983; Loveridge and Moe 2004). In a study from miombo woodland in Zimbabwe, Macrotermes termitaria were found to have a higher number of tree species, twice the density of trees and almost three times the woody basal cover than the adjacent woodland (Loveridge and Moe 2004). In a vegetation restoration experiment from the Sahelian zone in West Africa, Mando et al. (1999) found that termite activity increased plant cover. According to Hesse (1955), crop plants also grow better on Macrotermes mounds than on soils of the surrounding savanna. Lal (1988) proposed that the improved growth on mounds is due to a combination of high water status and enhanced soil mineral contents. He suggested that termitaria are less leached than the surrounding soil and therefore contain relatively high amounts of soluble plant nutrients (Lal 1988). This is supported by Konate` et al. (1999), who found that the water storage capacity was higher in termite soil compared to surrounding soil in a West African savanna. It is well documented that termitaria are humid, well-drained patches containing high levels of important plant nutrients (e.g. Lee and Wood 1971; Lal 1988; Mahaney et al. 1999) and as such represent resource islands in relatively nutrient poor savanna environments (Frost 1996). Plant species richness generally follow a hump-shaped relationship, increasing with increasing productivity until it peaks and the diversity declines with a further productivity increase (e.g. Tilman 1988). The fact that the number of
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woody and forb species increased between savanna and termitaria indicates that, for these species groups, the species richness peaks at relatively high productivity levels. The species richness probably peaks at lower productivity levels for graminoid herbs since we found no difference between the resource poor savannas and the resource rich termitaria. In central Europe, species richness of grasslands and wetlands has been found to peak in relatively nutrient poor soils while forest species richness peaks in relatively nutrient rich soils (Cornwell and Grubb 2003). Similarly, when fertilizer was experimentally added to temperate grasslands the graminoid herbs richness was reduced (Crawley et al. 2005; Gross et al. 2005). Thus, our findings indicate that these relationships between soil nutrients and species richness may also be applicable to tropical savannas. The plant species community composition differed between termitaria and savanna vegetation. Although many woody and forb species grew on both mounds and in the savanna plots, many plant species were unique for termitaria, while only a few species occurred exclusively in the savanna plots. Succulent species were restricted to mounds. Succulent growth forms are commonly associated with high soil nutrient levels under arid conditions (Knight et al. 1989), and termitaria seems to be the only suitable sites for these nutrophilic species in Lake Mburo National Park. Furthermore, since succulents are highly sensitive to fire (Pe´rez-Garcı´a and Meave 2006), termitaria may provide protection from grass fires for these species. Individuals of R. natalensis, Embelia schimperi, Trilepisium madagascarensis and Hibiscus aponeura found in the savanna were all saplings (\50 cm), but on termitaria these species also survived to mature trees, indicating high seedling predation levels on the savanna. Dense termitaria vegetation may to some extent protect woody species from browsing animals. Furthermore, woody plants growing on nutrient rich sites have sufficient carbon and nutrient reserves to replace lost tissue and thus compensate for ungulate browsing (Bryant et al. 1983). The mounds in Lake Mburo National Park are elevated spots and accordingly vegetation on termitaria may be less subjected to ground fires (Lee and Wood 1971; Lind and Morrison 1974). Both A. sieberiana and D. cinerea, dominating the savanna
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plots, are well adapted to fire (Gillon 1983; Sabiiti and Wein 1987). When investigating seeds responses to fire in Acacia sieberiana in Queen Elizabeth National Park in Uganda, Sabiiti and Wein (1987) found that fire stimulated germination from the seed pool in the soil, and that the emergence of seedlings was higher on burned plots than on unburned plots. Acacias are also generally shade intolerant species with no seedling establishment under the canopies of mature trees (Owen-Smith 1988). For other woody species, fire may limit both tree recruitment and the progression of an individual tree from the seedling stage to adult (Higgins et al. 1999). The number of thickets in Lake Mburo National Park can almost exclusively be explained by the density of termite mounds. Expanses of thicket associations are found in the south-western part of the park, which is less disturbed by fire and grazing (Hoag et al. 1991). In a recent study, including our study area, Bloesch (2008) concluded that thickets on seasonally waterlogged plains were established on Macrotermitinae mounds because of fire protection, increased soil fertility and good drainage, while on stony hillsides thicket clump establishment was a function of the fire regime and the browsing intensity. Smaller termite mounds of more recent origin commonly lack vegetation. According to Bloesch (2008) recent mounds are effectively maintained and woody vegetation is only able to established in periods were the mounds are not active. In a study from Serengeti, Sharam et al. (2006) found that Croton thickets were associated with hilltops or riparian forests. In Serengeti, Euclea divinorum acted as a pioneer species facilitating thicket establishment. The establishment of Euclea divinorum was hampered by frequent fires, browsing and dense graminoid herbs cover (Sharam et al. 2006). In Lake Mburo National Park, Rhus spp. and Grewia spp. are suggested to be instrumental to thicket establishment (Hoeg et al. 1991; Bloesch 2002). While termite mounds benefit vegetation establishment because of fire protection, increased soil fertility and good drainage (Bloesch 2008), the vegetation also provide benefit to termites in providing detritus and dead wood. In conclusion, Macrotermes termitaria, occupying about 5% of the area in this savanna landscape, are important contributors to vegetation heterogeneity within the Lake Mburo National Park. Explaining
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89% of the variation in dense thickets, termitaria represent resource islands for many plant species found exclusively on termitaria. Fire tolerant Acacia species dominate the open savanna while fire sensitive species like Grewia spp. and succulents like E. candelabrum are restricted to mounds. While the Shannon–Wiener index and the Shannon evenness index on termitaria far exceed the adjacent savanna for woody and forb species, there were no such differences for graminoid herbs, except Shannon evenness index which was higher on termitaria. Our results support findings from temperate areas in that graminoid herb richness peaks at lower productivity levels than trees and forbs. Accordingly the highly productivity termitaria only contribute to a species richness increase of trees and forbs. Acknowledgements We thank the Uganda Wildlife Authority and the Uganda National Council for Science and Technology for permission to conduct this study. J. Colman and O. G. Støen commented on earlier drafts of this manuscript. Dr. C. Bakuneeta and D. K. Aleper assisted during the study. O. Wanyana Magyani and Prof. K. A. Lye helped in identifying plant species. We are grateful to the park staff for their hospitality and support. In particular, we would like to thank K. T. Anderson for assistance in the field. This study was supported by grants from Noragric at the Norwegian University of Life Sciences and by NUFU.
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