The importance of nutrient hot-spots in the conservation and management of large wild mammalian herbivores in semi-arid savannas

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The importance of nutrient hot-spots in the conservation and management of large wild mammalian herbivores in semi-arid savannas C.C. Grant*, M.C. Scholes School of Animal, Plant and Environmental Sciences, Private Bag 3, WITS 2050, South Africa

A R T I C L E I N F O

A B S T R A C T

Article history:

Extensive studies in the Serengeti showed a strong link between patchy herbivory and for-

Received 12 June 2005

age quality, and the factors that determine forage patch selection at different scales have

Received in revised form

been evaluated in various models. The purpose of this study was to examine to what extent

21 December 2005

favoured forage patches in the Lowveld of South Africa are determined by forage quality

Accepted 4 January 2006

and how important these are as forage resources. Understanding the factors that deter-

Available online 3 March 2006

mine herbivore distribution will provide insights into how the utilization of forage by herbivores affects and is influenced by the ecosystem and thereby improve our abilities to

Keywords:

conserve and manage these systems. The Kruger National Park is a large wildlife conserva-

Forage nutrients

tion area, with stratified rainfall and soil nutrient patterns. Tuft utilization and number of

Selective utilization

faecal deposits were used to determine favoured forage patches within these stratifica-

Forage quality

tions. On the low-nutrient granites, the use of grass tufts on the crests of hillslopes was

Herbivore conservation

about a quarter that on sodic sites, and a third of that of termite mounds. On the high-

Monitoring

nutrient basalts, utilization of crests was about a third of that of termite mounds and about

Herbivore nutrition

a quarter of the utilization of sodic sites in the wet and growth season, but towards the end

Scale

of the dry season all patches were utilized to a similar extent. On sodic sites, shorter grass

Kruger National Park

grazers accounted for almost ten times more faecal deposits than other species, although there was evidence of all the other large herbivore groups also utilizing these sites. The foliar nitrogen and phosphorus concentrations from termite mounds and sodic patches were up to twice that found on the crests in the wet season, especially on the granites, indicating that forage quality may indeed play an important role in determining favoured forage patches. Furthermore, only the patches on the sodic sites and termite mounds produced foliage of sufficient quality to support reproduction and maintenance of body condition. These patches thus form key resource areas that determine animal condition and hence dry season survival. These findings have important management consequences: firstly in predicting the number of animals that may be supported by an area; secondly, because these nutrient hot-spots are so intensely utilized by herbivores, they will be the first to show degradation, and monitoring programs should thus include these areas. Appropriate monitoring designs will detect degradation in these areas in time to take appropriate management actions that would avoid irreversible system changes.  2006 Elsevier Ltd. All rights reserved.

* Corresponding author: Present address: South African National Parks, Kruger National Park, Northern Plains Programme, Box 106, Skukuza 1350, South Africa. Tel.: +27 13 7354415/7355524. E-mail addresses: [email protected] (C.C. Grant), [email protected] (M.C. Scholes). 0006-3207/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2006.01.004

B I O L O G I C A L C O N S E RVAT I O N

1.

Introduction

Savannas make up 12.5% of the earth’s land area and are found in the drier tropical areas. They are heterogeneous landscapes characterised by a continuous layer of palatable and unpalatable grass species and a discontinuous tree canopy. The ecosystem function that structures these landscapes arises from the interaction of diverse processes that control net carbon assimilation and transpiration, water extraction and infiltration, nutrient cycling and retention, predation and herbivory (Holling et al., 2002). The effects of herbivory are influenced by the habitat and forage choices of herbivores. Optimal foraging strategies of herbivores thus influences the way herbivores alter and utilize the landscape, with dietary quality and the spatial distribution of foraging patches being important considerations (e.g., Owen-Smith and Novellie, 1982; Belovsky, 1984; Pyke, 1984; Senft et al., 2005; Adler et al., 2001). There is, however, some disagreement on the importance of the quantity and quality of forage material and different spatial scales at which it occurs (Wallace et al., 1995). Other factors that may influence the foraging decisions of herbivores are the distance to water and threats of predation, which have been discussed by various authors (Fryxell, 1995; Adler et al., 2001; Redfern et al., 2003) Soil and foliar nutrient status have both been implicated in site selection (McNaughton, 1988; McNaughton, 1990; Turner et al., 1997) as has biomass availability (Wallace et al., 1995; Bergman et al., 2001). In East Africa, patches of high utilization form grazing lawns which are high in soil and plant nutrients and are created and maintained by intense herbivory (McNaughton, 1984). However, comparisons between South and East Africa suggest that even though animal biomass is the key driver of nutrient cycling in East Africa, edaphic factors seem to play a more important role than animal numbers, in the Kruger National Park (KNP) in determining high nutrient patches (Naiman et al., 2003.) Observations in KNP indicated that certain parts of the landscape were favoured by herbivores, which lead to the question of the reason for selection of patches and their role in the foraging strategy of herbivores. Little information is available on the nutrient requirements of wild herbivores. According to the study by Prins and Beekman (1987), buffalo need 6.2% protein (1% N) in forage with excess energy for maintenance, and a crude protein content of 9% (1.4% N) for a balanced diet. The requirement for lactating cows is about 11.5% crude protein (1.8% N). According to Duncan (1990), 0.24% P in forage is required for maintenance of wild equids. A lactating mare requires between 11% and 13% crude protein (1.76–2.08% N) with a maintenance requirement of 8.5–10% protein (1.36–1.6% N). Sodium requirements have been established at 0.08–0.1% of dry matter for cattle (Morris, 1980). Very few studies have been done in southern Africa on relating patch utilization by animals to nutrient availability. This study attempts to contribute to the understanding of factors that determine patch selection by herbivores and the importance of these patches for the conservation and management of wild herbivores. This study examined the role of soil and foliar nutrients in determining herbivore foraging patterns as well as the rela-

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tionship between foraging patterns and forage quality, patch size, patch position and season. The study aimed at testing the following hypotheses: 1. Tufts on sodic sites and termite mounds are more intensely utilized by large mammalian herbivores than the surrounding crest areas. 2. Favoured foraging sites have higher foliar nutrient concentrations. 3. Higher foliar nutrient concentration will be associated with higher soil nutrient concentrations, although this study was not aimed at determining the origin of these nutrients. 4. In each patch, utilized tufts will have higher nutrient concentrations, especially nitrogen, than unutilized tufts. 5. Patches that provide quality forage are essential in maintaining large herbivore populations.

2.

Materials and methods

2.1.

Study site

2.1.1.

Region description

The 1.9 million ha of the Kruger National Park (KNP) is located along the north-eastern boundary of South Africa, and falls within the semi-arid savanna biome, in the Lowveld region (Low and Rebelo, 1998). The KNP is roughly bisected geologically and climatically: granite underlying the western half and basalt the eastern half and the southern section being relatively wetter than the north (Fig. 1). Herbivore distribution is determined by this variation on the park-wide scale, with higher herbivore densities on the eastern, nutrient-rich basaltic soils, than on the western nutrient-poor granitic soils (Naiman et al., 2003) (Table 1). Herbivore populations are not managed, unless a threshold of potential concern (Biggs and Rogers, 2003), defined by upper and lower population limits (Kshatriya et al., 2001), is exceeded. Grazing lawns as described in East Africa do not exist in the KNP, although sodic sites and termite mounds have been reported to be favoured foraging areas in this conservation area (Gertenbach, 1983; Naiman et al., 2003). Granites erode to form sandy soils, which are relatively low in nutrients compared to the nutrient-rich clay-rich soils derived from the Ecca shales and basalts (Venter et al., 2003). The granitic areas are undulating with well-developed catenas, sandy crests or rocky outcrops at the top of the catena, sloping down towards the more clay-rich footslopes and valley bottom. The same pattern is repeated on the much less prominent hillslopes of the basalts where open plains are common. The extent of crests is generally larger than that of the footslopes where the sodic sites are located. The area covered by the footslopes is much larger in the granitic landscapes than in the basaltic zones (Venter, 1990) (Table 1) (Fig. 1). The Olifants River indicates the position of the northern and southern division of the KNP. North of the river the climate is generally more arid with a mean annual rainfall between 300 and 500 mm and the vegetation is dominated by mopane shrubs (Colophospermum mopane). South of the river, the rainfall increases to a mean annual rainfall between 500

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1990). Soils from termite mounds are higher in nutrients and grasses growing on the mounds have been reported to be heavily utilized by herbivores (Dangerfield et al., 1998; Naiman et al., 2003; Venter et al., 2003). Termite mounds are more densely distributed on the granites than basalts and occur at an average density of about 111 km2 in the northern KNP (Meyer et al., 1999).

2.1.2.

Fig. 1 – The Kruger National Park depicting the location of the main geological unites with granites in the west and basalt in the east. Mean rainfall at rainfall stations indicate the declining rainfall from south to north. The division of the south and north is indicated by the location of the Olifants river.

and 700 mm and is dominated by Combretum and Acacia species (Fig. 1) (Table 1). These contrasting geological and rainfall areas have been classified into land systems with similar geological, geomorphic and climatic features (Venter et al., 2003). Venter (1990) describes sodic sites as open areas with low standing herbaceous biomass and are usually associated with the top end of footslopes, particularly on the undulating granitic landscape. Sodic sites are characterized by duplex soils associated with deflocculated clays, the A horizon is less than 15 cm, the pH is high and the exchangeable sodium percentage is between 15% and 20%. The area covered by sodic sites on the granites is larger than on the basalts, although the exact extent of the area covered by these patches has not yet been determined for the KNP. The large, often grass covered termite mounds, 47% of which are built by the genus Macrotermes , tend to occur where the soil is more sandy and are often positioned above the sodic sites on the hillslope (Meyer et al., 1999; Venter,

Site description

The study was undertaken at three spatial scales: the first being the entire KNP, stratified according to geology and rainfall. At this scale, four sampling sites were selected, with one in each of the main land systems in the lower and higher rainfall zones of the granites and basalts (Fig. 1). The land systems sampled thus represented 77.3% of the 1.9 million hectares of the KNP. The second was at a finer scale where a number of more intensive measurements were made in the higher rainfall land systems in the south. This part of the study focused on utilization and foliar nutrients in the three forage patch types to confirm the patterns found in the broader study. The third and finest forage patch scale consisted of the crest area representing a less utilized forage patch, and within a kilometre a sodic site and termite mound representing favoured, intensely utilized forage patches. Sodic patches were identified by their locality and the dominance of Euclea divinorum (Gertenbach, 1983). Only grass-covered mounds of Macrotermes spp. were sampled to represent forage patches on termite mounds. Water sources are fairly uniformly distributed throughout KNP (Redfern et al., 2003), and a permanent water supply was within 4 km walking distance of all sampling sites. The study covered two consecutive years. In the first year when the study was undertaken for the whole KNP, the rainfall was below average in the north, but close to average in the south with: northern granites 461 mm (83% of average); northern basalts: 368 mm (71% of average); southern granites: 573 mm (104% of average); southern basalts: 620 mm (83% of average). When the study was limited to the south in the second year, the rainfall was below average with: the southern granites 307 mm (60% of average) and the southern basalts 350 mm (56% of average). Samples were collected to cover three distinct seasons: the beginning of the growth season after the onset of the first rains (November in the broad scale study and January in the fine scale study in the south), the end of the growth season (March), and the end of the dormant season (October) in the broad scale study and September in the fine scale study in the south). The aerial annual herbivore counts are only accurate at a scale of 800 m. To determine herbivore distribution at a finer scale, tuft utilization and the distribution of faecal deposits were used.

2.2.

Utilization

In the broader scale study, tuft utilization was determined only in March, which is the end of the growth season. This period was chosen to give an indication of patches that were preferentially selected in the presence of abundant forage. Grass biomass can still increase after the end of the rainy season

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Table 1 – Characterization of the scales from land type to geology represented in the study (Venter et al., 2003) (also see Fig. 1) Landscape Geology Land system Mean annual rainfall (mm) Area covered by crest (ha) Area covered by footslope (ha) Herbivore biomass kg ha1a Crest soil Vegetation

Northern granites

Northern basalts

Southern granites

Southern basalts

Granite Phalaborwa 450–600

Basalt and Gabbro Letaba 450–500

Granite Skukuza 500–750

Basalt and Gabbro Satara 500–650

161,597

192,779

155,392

179,095

142,011

48,024

146,040

36,854

29.1 ± 3.18

23.0 ± 3.26

19.4 ± 1.37

34.6 ± 3.70

Shallow dystrophic sand and loam Combretum apiculatum and Colophospermum mopane

Calcareous clay

Sand and loam

Clay soils

Colophospermum mopane, shrub savanna

Combretum apiculatum

Acacia nigrescens and Sclerocarya birrea

Footslope soil Vegetation

Duplex soils Acacia gerrardi and Euclea divinorum

Combretum hereroense and Euclea divinorum

a (Naiman et al., 2003).

in March, before the onset of the cool dry season in May; however, tufts that have been utilized by herbivores toward the end of the growth season can still be identified during the dry season (work in progress Alard et al.). Utilized tufts were defined as those that showed removal of more than 15% of leaf material at a single point in a tuft, repeated utilization of the same tuft was thus not accounted for. Utilization was determined along four line transects of one meter in length each radiating from the point where soil samples were collected. Every tuft touching the line was classified as utilized or unutilized and the percentage utilized tufts was calculated from the total. Thus, utilization for each forage patch was estimated over 32 m line transect (4 m of line transects for each of the eight soil samples collected in a forage patch). In the more intensive study in the south, utilization of tufts on crests and sodic sites was recorded over an 800-m line transect, the tuft closest to each of four markers on a disk with a diameter of 500 mm, was classified as utilized or unutilized every 10 m (80 readings per transect). A disc pasture reading was also taken to calculate herbaceous biomass (Trollope, 1990). At each site, forage utilization was determined on eight termite mounds. On the granites, disc readings and utilisation counts were done at three positions on the termite mound (top, middle and foot), and on the basalts at two positions (top and foot), because of the much smaller size of termite mounds on the basalts. In this part of the study, utilization was determined at the end of March, end of September, and after the first rainfall of above 20 mm in the rainy season in the following January. Utilization in September is cumulative, reflecting tuft utilization since the end of the growing season (March), while the number of tufts utilized in January are only a reflection of utilization over the last two to four weeks depending on the rainfall.

2.3.

Foliar samples

In the broader scale study, foliar samples were collected from utilized tufts as much as possible, but on occasion, unutilised

tufts were sampled, around each one of the soil sampling points during March, October and November. Green and brown (senescing) leaves were sampled; proportions changing as the season progressed. For the more intensive southern study, representative foliage samples of utilized and unutilized tufts were collected separately over every 100 m distance along each 800 m transect. Thus, as far as possible, eight samples of utilized and eight of unutilized tufts were collected in each forage patch.

2.4.

Forage biomass

Forage biomass was determined for each forage patch by determining the average compressed height measured with a disc pasture meter every 10 m along the 800-m transect in the southern basalt and granites. The disc pasture meter has been calibrated for this vegetation type by Trollope (1990). Biomass was calculated using the following formula: p 1 ð X  2260Þ  3019 ¼ kg ha where X is the average disc height for the transect (Trollope, 1990).

2.5.

Animal distribution

In the intensive study in the south, the presence of different herbivore species on crests and sodic patches was determined by recording the number and species of faecal deposits in a 2 · 800 m belt transect in March, and September. Counts were not done in January due to the insect activity, which removed faecal deposits within hours of deposition, while counts in March were cumulative reflecting the presence of animals in the area over a period of about 1–2 months, and in September over about 7 months. This procedure had to be followed as it was not possible to remove deposits over an area of 1600 m2 and to identify exactly the same site for the follow up survey several months later.

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2.6.

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Soil samples

Soil samples were only collected in the broader KNP study. The three seasons sampled represented the end of the growth season (March), the beginning of summer with increasing temperatures (October) and the initial increase in available soil moisture after the first rains (November). On the crest and sodic patches, eight samples were collected per transect at 10 m intervals. Three termite mounds were sampled at the top, middle and foot on the granites, and on the top and foot of four termite mounds on the basalts. Nitrogen (N) mineralization rates in the soil were determined for the three seasons using paired 30 · 5 cm stainless steel cores for an estimate of in situ N mineralization rates. The first core was removed immediately and the second core was incubated for between 14 and 35 days to allow the accumulation of ammonium and nitrate as a result of microbial action. Cores were covered to avoid leaching due to rainfall and a ventilation hole allowed samples to remain aerobic. Results from cores that were found open or water-logged after the incubation period were not included in the statistical analysis.

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more intensive southern study, the difference in utilization and disk height was examined in a multiple ANOVA (Statgraphics plus version 7) with forage patch and season as factors. The ratio utilized was normalised using the arcsine square root transformation. The number of faecal deposits were calculated for each 20 m2 (10 · 2 m) block of the 800-m transect. Forage biomass was calculated using the biomass formula at intervals of 10 m. These results were compared in a multiple ANOVA (Statgraphics plus version 7) with a log transformation of the counts (in the case of faecal deposits) and forage patch and season as factors. To examine the species contribution to the faecal deposits, species were grouped as follows: bulk grazers – species that require quantity rather than quality forage consisted of zebra (Equus burchellii), buffalo (Syncerus caffer) and white rhino (Ceratotherium simum); concentrate grazers (species that are more dependent on quality than quantity) were impala (Aepyceros melampus) and blue wildebeest (Connochaetes taurinus); browsers were kudu (Tragelaphus strepsiceros) and giraffe (Giraffa camelopardalis).

3.2. 2.7.

Soil and foliar nutrients

Analysis of samples

Soil net N mineralization rates were measured using a field incubation technique as outlined by (Anderson and Ingram, 1993) and (Okalebo et al., 1993). After collection from the field, each soil core was well mixed and a 10-g sub-sample extracted overnight with 20 ml of 0.5 M potassium sulphate (K2SO4). The supernatant was analysed for nitrate and ammonium using colorimetric methods (Anderson and Ingram, 1993). Net N mineralization rates were calculated from ammonium and nitrate concentrations at the beginning and end of incubation. Results were corrected for soil moisture and expressed on a dry mass basis. Soil available Phosphorus (P) was determined according to the resin bag method (Sibbesen, 1978) in soil samples collected at the beginning and end of the growth season to estimate P available for forage growth. Resin bags were activated with 0.5 M sodium bicarbonate. These resin bags were used to extract P from 4 g soil samples suspended in 20 ml of distilled water. The extracted P was released from the resin bag with a 0.5 M hydrochloric acid solution, and P concentrations were determined using a colorimetric procedure (Murphy and Riley, 1962). Total N and extractable P and Sodium (Na) were determined in three representative, combined soil samples collected in October. Foliar samples from both studies were analyzed for total P, Na and N by the Institute for Soil, Climate and Water in Pretoria, using standard procedures (Handbook of Soil, Climate and Water 2001).

3.

Data analyses

3.1.

Utilization

Due to the nested arrangement of the study sites, the differences in the percentage of tufts utilized in the three forage patches of the four land systems in the broad-scale study was examined using a nested ANOVA (Genstat 5), with forage patch nested in land type, which was nested in geology. In the

In the broader study, soil and foliar nutrients were examined separately in a nested ANOVA (Genstat 5) with forage patch nested in land type, nested in geology. The effect of season was examined separately in a simple ANOVA (Statgraphics plus version 7). In the more intensive study, a multiple ANOVA (Statgraphics plus version 7) with forage patch and season as factors was used to examine differences in foliar nutrients.

4.

Results

4.1.

Utilization of forage sites

Forage patches differed significantly in the percentage of utilized tufts in the land systems examined in the broad scale study (F = 6.78; p < 0.001, n = 93). By the end of the growing season in March, 62.6% and 66.6% of sodic sites and termite mounds were utilized respectively on the southern basalts, and 39.1% and 35.8%, respectively, on the southern granites. This was much higher than the utilization of the crests of 11.1% on the basalts and 11.9% on the granites of the south. In the north utilization patterns were similar but much lower, the highest being 23.9% on the sodic sites of the granites. In the more intensive study in the south, sodic sites were the most favoured forage patches on the granites, with significantly higher utilization than the other patches in March and September (F = 29.6, p < 0.0001, n = 227) (Fig. 2a). On the basalts, the preferences were more variable, with a strong selection for termite mounds in January, selection for sodic sites in March and no specific preference for any particular forage patch in September (Fig. 2b).

4.2.

Forage biomass

The highest forage biomass was recorded in the southern basalts at the end of the growing season in March (F = 10.2, p < 0.0001, n = 227). The biomass on the sodic sites was kept

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a

431

b

Fig. 2 – The average percentage of tufts utilized in each of the three foraging patches over the three seasons, recorded during the more intensive study in the southern granites (a) and basalts (b).

low by utilization throughout the year, both on the granites (Fig. 3a) and the basalts (Fig. 3b). In both geological areas, biomass tended to be as high on termite mounds as it was on crests, in spite of the heavier utilization on termite mounds.

4.3.

Animal presence as indicated by faecal deposits

Animal presence as indicated by their faecal deposits differed significantly in the different forage patches examined (F = 2.27, n = 132, p = 0.03). In the broad study, at the end of the growth season, the highest number of deposits was found on the granites was on sodic sites, both in the north (25.8 ± 3.6 100 m2) and the south (22.1 ± 4.4 100 m2). The number of deposits on the granite crests was much lower: 0.26 (±6.5) 100 m2 in the north and 1.56 (±5.5) 100 m2 in south. Even in the nutrient-rich basalts, faecal deposits were significantly higher in the sodic sites of the basalts, north (7.8 ± 3.6 100 m2) and south (11.5 ± 4.4 100 m2) than the deposits on the crests, with 1.88 (±5.5) 100 m2 in the north and 4.22 (±4.4) 100 m2 in the south. The more intensive study in the south done at the end of the growth season (March) and the end of the dry season (September), confirmed these results. Dung deposition was significantly higher at the end of the dry season (10.5 ± 1.1 100 m2) than it was at the end of the growth season (5.1 ± 1.1 100 m2) (F = 29.6, n = 67, p < 0.0001). The sodic sites

a

on the granites (17.4 ± 1.5 deposits 100 m2) had a significantly higher number of faecal deposits than sodic sites of the basalts (9.9 ± 2.2 deposits 100 m2), with the sodic sites showing significantly more faecal deposits than the crests, both on the granites (1.8 ± 1.5 deposits 100 m2) and the basalts (2.1 ± 1.5 deposits 100 m2) (F = 23.5, n = 67, p < 0.0001). There was a significant difference in the utilization of forage patches by the three groups of species examined (as indicated by faecal deposits) (F = 8.91, n = 316, p < 0.0001). The concentrate feeders, (impala and blue wildebeest which formed 56% of the animals in KNP in the 2002 census), showed a preference for sodic sites. Bulk feeders (31% of KNP herbivores) were more prevalent in the basalts, utilizing both crests and sodic sites, while browsers (5.9% of KNP herbivores) showed no preference for any of the forage patches (Fig. 4). The tuft utilization and faecal deposit data thus support the first hypothesis that sodic sites and termite mounds are preferentially utilized by grazers.

4.4.

Foliar nutrient concentrations in forage patches

The sodic sites and termite mounds tended to support foliage with higher N concentrations than the crests in the different land systems of the broad scale study (F = 4.25, n = 110, p < 0.001) (Fig. 5a and b). Phosphorus concentrations did not

b

Fig. 3 – The calculated standing forage biomass in kg ha1 in each of the three foraging patches over the three seasons, recorded during the more intensive study in the southern granites (a) and basalts (b).

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Fig. 4 – The average number of faecal deposits per 100 m2 of the common herbivores in the KNP on crests and sodic sites of the southern granites (left) and basalts (right).

Fig. 5 – Foliar N concentrations (top row) in the granites (a) and basalts (b) and P concentrations (bottom row) in the granites (c) and basalts (d) in tufts collected from study sites in the northern and southern KNP for each of the three foraging patches in the three seasons sampled. The solid line in the graphs indicates maintenance requirements, the broken lines in the N graphs indicates requirements for growth and reproduction.

differ significantly between the sample sites on a particular geology, but concentrations were significantly higher on the basalts than the granites (F = 38.25, n = 110, p < 0.001) (Fig. 5c and d). Sodium concentrations were significantly higher in the sodic patches of the northern granites (0.86 ± 0.12%) and

southern basalts (0.75 ± 0.09%) when the four land systems were compared (F = 5.06, n = 110, p < 0.001). Nutrient concentrations differed significantly among forage patches in the more intensive southern study (Table 2). The highest foliar N concentrations were found in samples

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Table 2 – The results of a multiple ANOVA examining differences in foliar nutrient concentration in utilized and unutilized tufts on the three different forage patches (crest, termite mound, sodic) in the southern granites and basalts in three seasons (growth, end of the growth and end of the dry season) (also see Fig. 6) P

Season Forage patch Utilization Season · forage patch Season · utilization Forage patch · utilization

Na

N

p

F

p

F

p

F

<0.0001 <0.0001 0.08 <0.0001 NS NS

22.9 8.0

<0.0001 <0.0001 <0.0001 <0.0001 0.0001 NS

184.3 48.5 26.0 19.1 9.4

<0.0001 <0.0001 <0.002 <0.0001 0.001 0.002

619.2 27.5 3.84 9.5 7.1 3.8

19.7

that were collected during the growth season (January), with higher N concentrations in samples from sodic sites and or termite mounds (Fig. 6a and b). These concentrations were much higher in January than in March and September. Prins and Beekman (1987) suggest that herbivores need about 1.8% N in forage for reproduction. On the basalts, only sodic sites and termite mounds had foliar N concentrations above the level required for maintenance after the growth season (Fig. 6b). However on the granites, both utilized and unutilized tufts on all the forage sites still had foliar N concentrations above the maintenance requirements of 1% N (Prins and

Beekman, 1987) after the growth season (Table 3). Phosphorus concentrations were sufficient for maintenance in utilized and unutilized tufts in all the forage patches of the basalts in January, but on the granites, only grasses growing on sodic sites had P concentrations above maintenance levels (Fig. 6c). By March, only the termite mounds on the basalts still had P concentrations above the maintenance requirement of 0.24% P (Fig. 6d). During the dry year of the more intensive study, none of the forage patches on the granites had P concentrations above maintenance levels after the growth season (Fig. 6c); however, during the average rainfall year of the

Fig. 6 – Foliar nutrient concentrations in utilized tufts in the intensive study in southern KNP; N concentrations (top row) in granites (a) and basalts (b) and P concentrations (bottom row) in granites (c) and basalts (d) in each of the three foraging patches in the growth season (January), the end of the growth season (March) and the end of the dry season (September). The solid line on the graphs indicates maintenance requirements; the broken line indicates the N requirements for growth and reproduction.

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Table 3 – Foliar P, Na and N concentrations expressed as an percentage, in utilized and unutilised tufts in three seasons on the three forage patches of the southern basalts and granites Season

Forage patch

Utilization

n

%P

sd

% Na

sd

%N

sd

Basalt Jan March Sept Jan March Sept Jan March Sept

Crest Crest Crest Sodic Sodic Sodic Termite Termite Termite

Utilized Utilized Utilized Utilized Utilized Utilized Utilized Utilized Utilized

8 8 8 8 8 10 8 8 8

0.24 0.17 0.09 0.28 0.18 0.19 0.27 0.35 0.11

0.04 0.03 0.02 0.04 0.05 0.08 0.04 0.07 0.01

0.42 0.19 0.06 0.79 0.37 0.12 0.55 0.42 0.09

0.07 0.13 0.02 0.13 0.25 0.07 0.12 0.15 0.01

2.03 0.73 0.78 2.39 0.65 1.01 2.89 1.27 0.96

0.25 0.28 0.11 0.51 0.12 0.28 0.13 0.21 0.17

Granites Jan March Sept Jan March Sept Jan Sept

Crest Crest Crest Sodic Sodic Sodic Termite Termite

Utilized Utilized Utilized Utilized Utilized Utilized Utilized Utilized

6 10 10 8 10 8 8 9

0.12 0.15 0.35 0.26 0.16 0.11 0.19 0.10

0.02 0.16 0.10 0.02 0.04 0.02 0.05 0.01

0.15 0.31 0.04 0.67 0.56 0.20 0.29 0.11

0.06 0.16 0.01 0.05 0.17 0.03 0.08 0.04

2.25 1.11 1.04 2.57 1.54 1.27 2.27 1.33

0.30 0.31 0.09 0.20 0.29 0.14 0.29 0.14

Basalts Jan March Sept Jan March Jan March Sept

Crest Crest Crest Sodic Sodic Termite Termite Termite

Unutilized Unutilized Unutilized Unutilized Unutilized Unutilized Unutilized Unutilized

8 7 8 8 7 8 8 1

0.26 0.13 0.07 0.28 0.16 0.27 0.34 0.12

0.06 0.02 0.02 0.05 0.08 0.05 0.11

0.36 0.08 0.08 0.75 0.28 0.56 0.24 0.11

0.09 0.01 0.02 0.17 0.16 0.11 0.09

1.72 0.57 0.78 2.30 0.68 2.74 1.05 0.98

0.26 0.11 0.13 0.41 0.10 0.25 0.12

Granites Jan March Sept Jan March Sept Jan Sept

Crest Crest Crest Sodic Sodic Sodic Termite Termite

Unutilized Unutilized Unutilized Unutilized Unutilized Unutilized Unutilized Unutilized

8 9 10 8 10 3 8 7

0.12 0.16 0.29 0.26 0.08 0.09 0.17 0.09

0.04 0.25 0.09 0.04 0.04 0.03 0.04 0.02

0.10 0.22 0.04 0.66 0.30 0.13 0.12 0.08

0.06 0.16 0.02 0.05 0.16 0.03 0.08 0.03

1.81 1.00 1.08 2.30 0.78 1.13 1.96 1.30

0.45 0.22 0.17 0.15 0.28 0.07 0.19 0.14

Jan, active growth season; March, end of the growth season; Sept, end of the dry season; n, number of samples analysed; sd, standard deviation.

broader study, foliage from the sodic sites on the southern granites still had foliar P concentrations above maintenance levels in March (Fig. 5c). In the smaller scale study, all of the forage patches in the basalts had foliar P concentrations above the maintenance level at the end of the growth season (March) and termite mounds still had foliar P concentrations above maintenance levels at the end of the dry season (September) (Fig. 5d). The high P concentration in the granite crests in September during the intensive study may be an anomaly and needs to be further investigated. Sodium concentrations were highest in utilized tufts on termite mounds and sodic sites in March and January and much lower in September in all patches (Table 3). Sodium concentrations were sufficient in January and March, but were below the required 0.08–0.1% in the crest at the end of the dry season. Utilized tufts had significantly higher Na and N concentrations than unutilized tufts (Table 3). However, within a spe-

cific forage patch at one time, only N concentrations were significantly higher in utilized tufts than unutilized tufts (Tables 2 and 3). The P:N ratio of the foliage differed significantly between habitats with the highest ratio in foliage from termite mounds on granite (12.2 ± 0.62) (F = 27.8, n = 262, p < 0.0001). Ratios were significantly lower on the basalts where ratios varied between 4 and 10, than the granites where ratios varied between 6 and 14, reflecting the higher N and lower P concentrations in the granites. The second hypothesis that forage patches, and the fourth hypothesis, that utilized tufts and patches have higher foliar nutrient concentrations, is thus supported.

4.5.

Soil nutrient concentrations in forage patches

Nitrogen mineralization differed significantly between seasons, with net mineralization being recorded only after the

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a

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435

b

Fig. 7 – N mineralization rates in soil from the different forage patches in the KNP just after the first rains (November) on the granites (a) and basalts (b) in the south (left hand side of each graph) and north (right hand side of each graph). first good rains and immobilization being the most important process in the other seasons (F = 26.7, n = 201, p < 0.0001). Nitrogen mineralization rates also differed significantly between the forage patches examined (F = 2.73, n = 201, p = 0027), but were not significantly higher in the favoured patches (Fig. 7). Available P concentrations were significantly higher just after the first rain than at the end of the growth season (F = 5.4, n = 151, p = 0.02). P concentrations differed significantly between forage patches (F = 5.94, n = 151, p < 0.0001), with the highest P concentrations found in the soil from the crests and termite mounds on the basalts after the first rains in November. Concentrations of P were much lower on the granites. Sodium was significantly higher in the sodic patches than the other forage patches of the granites and basalts (F = 25.52, n = 35, p < 0.0001). No simple relationship between forage and soil nutrients could be illustrated. Nitrogen in forage was not significantly correlated with N mineralization rate (r = 0.19; n = 44; R2 = 2.0%, p = 0.22). Resin available P showed a significant (p = 0.016) but weak correlation (r = 0.29; R2 = 8.3%, n = 68) with total forage P. There was also no correlation between soil Na and plant Na concentrations (r = 0.22, R2 = 4.78%, n = 34, p = 0.21). The highest soil nutrient concentrations were thus not always recorded in the favoured forage patches, and the third hypothesis that higher foliar nutrient concentrations are associated with higher soil nutrient concentrations cannot be supported by this study.

5.

Discussion

Tuft utilization and number of faecal deposits support the first hypothesis that sodic sites and termite mounds are more favoured as forage patches than crests. From the recorded faecal deposits, concentrate grazers (blue wildebeest and impala), which are more dependent on quality forage (Collinson and Goodman, 1982), were responsible for most of the utilization of the favoured forage patches, specifically the sodic patches on granites. However, evidence of the presence of all three groups of herbivores was found on crests and sodic sites. In the broad scale study, the highest number of faecal deposits and utilized tufts was recorded at the end of the growth season (March) in the southern basalts. This corre-

sponded with the highest density of herbivores and forage biomass according to aerial and vegetation surveys (Naiman et al., 2003). At the larger, KNP-wide scale it can thus be concluded that more animals are attracted to areas with higher quantity forage, as was proposed by Turner et al. (1997). At the finer scale, patches of higher utilization tended to be higher in quality as exhibited by higher foliar N, P and Na concentrations thus supporting the second hypothesis. At the finest scale, tuft level quality also determined utilization as utilized tufts had higher N concentrations than unutilized tufts, supporting the fourth hypothesis. The third hypothesis suggesting a link between soil and plant nutrients could not be supported in contrast to the close link between soil nutrients and foliage nutrients recorded in the Serengeti. An explanation may be that habitat specific grass species that are preferred and adapted to herbivory are inherently higher in nutrients. On the other hand intense grazing may increase the forage production and quality as described by McNaughton (1984). Such a cycle of higher production and quality could have been initiated by the patch being selected for Na content. These patches also provide areas of good visibility which are safer from predators. The longer period of herbivore presence in such suitable patches could also start the cycle of nutrient enrichment. To be able to unravel the complex nutrient – foliage – herbivore links in the KNP, more detailed studies examining nutrient cycling in these favoured forage sites in the absence of herbivores are required. The newly developed exclosures along two major rivers render such an opportunity. Favoured forage patches and selected tufts play an essential role in providing the nutrients required by large herbivores for production and maintenance. In the growth season, in January, both P and N concentrations were sufficient to support lactation and growth in all the forage patches of the basalts. However, on the granites, only grasses growing on sodic sites had sufficient P concentrations to support production in spite of N concentrations being high enough to support production in all the forage patches. By March, all the basaltic forage patches, still had sufficient P and N concentrations to support production in the average rainfall year, but only termite mounds had sufficient nutrient concentrations in the year of below average rainfall. On the granites, only the favoured forage patches could supply sufficient P and N for maintenance in March of the average rainfall year. Sodium concentrations were sufficient in January and March

436

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apart from on the crest at the end of the dry season where the levels were below the required 0.08–0.1% . The favoured forage patches are therefore nutrient hot-spots in a relatively nutrient-poor environment, supplying sufficient P, Na and N for production, especially when the forage is in an actively growing stage, and throughout the growth season in average rainfall years. These nutrient hot-spots are the only areas where the concentrate grazers can build up sufficient body reserves to carry them through the lean time of the dry period, when forage nutrients are below maintenance levels. This finding supports the fifth hypothesis that patches that provide quality forage are essential in maintaining herbivore populations. McNaughton (1990) recorded concentrations of 2.33% N in the foliage in the wet season of the Serengeti. In the KNP, levels on the favoured forage sites are comparable in the wet season with 2.03–2.89% N in the basalts and 2.25– 2.57% N on the granites at the beginning of the growing season. Towards the end of the growth season, however (i.e., from March onwards), the N concentrations in all the forage patches were below the 1.56% recorded for the dry season in the Serengeti. This may explain why the medium sized (61– 450 kg) herbivore biomass is more than four times higher in the Serengeti than in the Kruger Park, while the biomass of small (15–60 kg) herbivores, which require less forge biomass, is twice as high in the Serengeti (East, 1984). Grass productivity data from the grazing lawns of East Africa suggest that these key areas are sufficient to maintain the biomass of herbivores in the Serengeti (McNaughton, 1985). If this holds for the KNP, then the number of animals that can be supported may be determined by the area covered by sodic patches and termite mounds and other similar nutrient hot-spot areas such as the clay-rich, gabbro patches on the granites. These hot-spots need to provide sufficient quality biomass to support the majority of concentrate grazers such as impala and blue wildebeest which form 57% of the herbivores in the KNP. The higher quantity, poorer quality grasses, growing on the crests, support the bulk grazers, such as zebra, buffalo and white rhino, which are only 17% of the large herbivores. Forage on the crest sites could supply sufficient forage biomass in most years to allow bulk grazers to utilize the quantities of forage they need to supply the required nutrients for maintenance. Herbivores that are able to consume larger volumes of low quality forage are thus more likely to maintain their body condition over the dry period and to survive in below average rainfall years when nutrient levels tend to drop earlier in the season. We conclude that rainfall models, such as proposed by Coe et al. (1976) to estimate of the number of herbivores that could be supported by a specific area should be improved by including the area of high nutrient patches as a variable. In a semi-arid savanna (and in the KNP in particular), where herbivore numbers are relatively low, most grazers can afford to select patches with relatively high forage quality, even on nutrient-rich soils such as the basalts. This may not be the case in smaller and more densely populated reserves, bordering the KNP. The lack of quality forage in such areas may lead to poor production by herbivores dependent on quality forage. In such areas, managers should probably tend towards managing for higher densities of bulk feeders and browsers.

1 3 0 ( 2 0 0 6 ) 4 2 6 –4 3 7

6.

Conclusions

This study contributed to the broader understanding of functioning of the ecosystem in a large conservation area by identifying some key resource areas in the form of nutrient hot-spots. These patches supply the nutrients needed for reproduction in this nutrient-poor environment. Although forage quantity probably determined the large-scale herbivore distribution patterns and population size in the KNP (Naiman et al., 2003), as seen in the larger number of herbivores in the higher rainfall nutrient rich southern basalts, forage quality determined the lower scale patch and tuft utilization patterns in this semi-arid system. Forage on the less favoured sites however; supply enough biomass to allow animals to utilize high forage quantities in the dry season of average rainfall years, in an effort to obtain sufficient nutrients for maintenance. Models predicting herbivore densities, impacts or carrying capacity in small conservation areas can thus be improved by taking the forage types provided by the different forage sites in the different seasons into account. In a large conservation area, these high nutrient areas seem to form a chain of high quality forage patches that enables herbivores to fulfil their nutritional demands despite large-scale shifts in climatic patterns (e.g., drought). This has significant implications for conservation and monitoring design, and indicates focus areas for monitoring programmes. Because of the expanses of conservation areas such as the KNP, monitoring is very demanding and identifying these key resource areas for herbivores can help to select monitoring sites that will give an accurate representation of resources available to herbivores. This could also be a step towards the management of ecosystems with emphasis on single species (or group of similar species) as suggested by Simberloff (1998). Further studies, such as the forage production and extent of sodic areas and the use of faecal deposition as a surrogate for species distribution are currently under way. Such studies are critical to help elucidate the intricate relationship between herbivore body size and forage, which in turn influences the functioning and conservation of the ecosystem as a whole.

Acknowledgements We thank SANParks for their support and the University of the Witwatersrand and the National Research Foundation for funding the project. All the data custodians with scientific services are thanked for providing data. The valuable assistance with the sample collection by Glynn Alard, Abri de Buys and other field assistants, game-guards and helpers are gratefully acknowledged. Harry Biggs, Jessica Redfern, Gay Bradshaw, and Oonsie Biggs are thanked for their help with the preparation of the manuscript.

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