Journal of Arid Environments (2001) 49: 541–554doi:10.1006/jare.2001.0805, available online at http://www.idealibrary.com on

Seed predation, germination and seedling establish-ment of Acacia erioloba in northern Botswana

Myra E. Barnes

Program in Ecology, Evolution and Conservation Biology, University ofNevada, Reno, U.S.A.

(Received 27 March 2000, accepted 21 February 2001, published electronically 3 September 2001)

The low rate of new Acacia erioloba seedling establishment in wildlife areas innorthern Botswana may be influenced by seed predation, insufficientrainfall for seed germination and seedling establishment or browsing. Mammaland insect predation on A. erioloba pods, seeds, and seedlings were monitoredin three wildlife areas to determine the effects on seed germination andseedling survival during 2 drought years followed by 2 years with averagerainfall. Although bruchid beetles infested half of the seeds, removal of unripepods by chacma baboons Papio cynocephalus and vervet monkeys Cercopithecusaethiops was the primary source of seed predation in areas near perennial watersources. The same number of seeds from pods collected under matureA. erioloba trees germinated as seeds from elephant dung when kept insaturated sand; however, seeds from dung began germinating in three days butthose from pods did not start germinating until the tenth day. During thedrought years, few seedlings emerged and none survived. New seedlings,identified by their cotyledons, emerged in December 1995 after '100 mmrain over several days, but by mid-January, more than half of the seedlings diedfrom dessication. Only 15% of the seedlings remained in June and nonesurvived through the dry season in 1996. Dispersal of more rapidly germinat-ing seeds in elephant dung would be beneficial in sandy semi-arid savannaswhere sufficient rainfall for seed germination and seedling establishmentappears to be rare. These results suggest that while seed predation reduces thenumber of seeds in the seed bank, inadequate rainfall limits A. erioloba seedlingestablishment, even during years with average annual rainfall.

( 2001 Academic Press

Keywords: Acacia erioloba; seed germination; seed predation; seedling estab-lishment; regeneration; Botswana; Africa

Introduction

Mature Acacia erioloba E. Mey. trees provide important forage and shade in wildlifeareas in southern Africa (Coe & Coe, 1987). Mortality of mature Acacia trees fromelephant browsing in northern Botswana has been well documented (Child, 1968;Sommerlatte, 1976; Melton, 1985; Wackernagel, 1992; Barnes, 1999); however, thelack of regeneration of new trees in wildlife areas has received little attention. Identifyingthe factors limiting the establishment of seedlings is as important as understanding thecauses of mortality, if the Acacia woodlands are to persist. Predators of Acacia pods,seeds, or seedlings include bruchid beetles and other insects (Coe & Coe, 1987;

0140}1963/01/110541#14 $35.00/0 ( 2001 Academic Press

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Hoffman et al., 1989; Ernst, 1992; Ernst et al., 1990; Miller, 1995; Mucunguzi,1995; Okello et al., 2001), chacma baboons Papio cynocephalus Kerr (Dunham, 1990),vervet monkeys Cercopithecus aethiops L. (Wrangham & Waterman, 1981; Isbell &Young, 1993; Isbell et al., 1998; Lee & Hauser, 1998), elephants Loxodonta africanaBlumenbach (Lamprey et al., 1974; Dunham, 1990), giraffe Giraffacamelopardalis L. (Pellew & Southgate, 1984; Miller, 1994a), smaller ungulates (Jarman,1976; Prins & Van Der Jeugd, 1993), and rodents (Miller, 1994c). The nutritiouspods and seeds are high in protein (Gwynne, 1969; Barnes et al., 1997) and are animportant component of browser diets during the dry season (Coe & Coe, 1987;Dunham, 1990).

Consumption of Acacia pods by elephants and other ungulates may facilitate seedgermination by scarifying the seed coat and increasing water uptake during passagethrough the digestive system (Lamprey, 1967; Lamprey et al., 1974; Pellew & South-gate, 1984; Hoffman et al., 1989; Miller, 1995). Hundreds of Acacia seeds havebeen found in individual elephant dung piles (Lamprey et al., 1974; Miller, 1995) withup to 50% of seeds consumed by captive elephants found intact in dung (Miller, 1995).Passage through the gut may also kill bruchid larvae within seeds (Jarman, 1976; Coe& Coe, 1987; Miller & Coe, 1993; Miller, 1994b). Some bruchid infested seeds remainviable if the embryo is not damaged. The bruchid exit hole increases the rate of wateruptake in the seed, resulting in more rapid germination (Lamprey et al., 1974). Bruchidbeetle infestation can vary widely among trees and years (Ernst et al., 1989) withinfestation rates of 54–96% in A. erioloba in Botswana (Ernst et al., 1990). While Acaciaseed germination may be enhanced by passage through an herbivore gut, usually (20%of seeds survive passage through smaller ruminants (Jarman, 1976; Miller, 1995).Herbivores may be important dispersers of mature seeds; however, giraffe (Pellewand Southgate, 1984), baboons (Dunham, 1990) and vervet monkeys (Isbell & Young,1993; Lee & Hauser, 1998) consume unripe Acacia pods before the seeds are fullydeveloped.

Studies of Acacia seedling establishment in savanna woodlands have reported anabsence of seedlings under canopy trees (Smith & Goodman, 1986; Young & Lindsay,1988) and episodic establishment related to rainfall (Wilson & Witkowski, 1998).Although Acacia seeds may germinate under the canopies of mature trees, there is a highrate of seedling mortality (Smith & Goodman, 1987). Seedlings grown at lower lightlevels under controlled conditions show a significant decrease in root/shoot ratio,resulting in less energy storage to facilitate survival during the dry season (Smith& Shackleton, 1988). However, O’Connor (1995) found that when rainfall was(500 mm, some seedlings did establish in shade while no seedlings survived in full sun.Frequent rainfall or occasional years of high rainfall appear to be essential for germina-tion of Acacia seeds and seedling establishment (Wilson & Witkowski, 1998). In theabsence of adequate rainfall, Acacia seeds may remain viable in the soil for up to 50 years(Tybirk et al., 1992). Ungulate browsing can also reduce seedling survival. Acaciaseedling establishment was highest in Tanzania during years when populations of impalaAepyceros melampus Lichtenstein were reduced by rinderpest epidemics (Prins & VanDer Jeugd, 1993).

This study examined A. erioloba seed predation, germination, and new seedlingsurvival in three wildlife areas in northern Botswana. Levels of seed predation bybruchid beetles, and mammals that consume unripe and mature pods or seeds, and theirinfluence on seed viability were measured to determine if seed predation enhanced orreduced germination and seedling establishment. Consumption by herbivores mayincrease seed coat permeability to water and disperse seeds to favorable sites away fromthe parent tree. However, removal of pods or seeds from trees before they are ripe, andchewing or digesting seeds, could reduce the number of viable seeds. Seedling emerg-ence and survival were monitored during two drought years and two average rainfallyears. The causes of seedling mortality and the relationship between seedling survival

ACACIA SEEDS AND SEEDLINGS 543

and location relative to the canopy of mature trees were recorded to determine theinfluence of browsers, rainfall, and sunlight on patterns of A. erioloba establishment inwildlife areas.

Methods

Study areas

Data were collected in Chobe National Park and Moremi Game Reserve in northernBotswana from 1994 to 1997. The Savuti study area is located in the western part of the11,000 km2 Chobe National Park. The flat topography is broken by seven rockyoutcrops or inselbergs and the Magwikhwe Sand Ridge which extends over 75 km alongthe west side of Savuti and the Mababe Depression (Thomas & Shaw, 1991). Alluvialand lacustrine deposits provide a diversity of soil types in the Kalahari sands, whichcover most of northern Botswana (Remmelzwaal et al., 1988). The Savuti River flowedfrequently until the 1880s and then was dry until 1958, allowing A. erioloba and otherwoody vegetation to establish in the flood plain. After 1958, the river flowed every yearexcept 1966, until it ceased flowing again in 1981 (Shaw, 1984). The Savuti River hasremained dry since 1982 leaving a broad dry channel and a dry grassland on the 2}4-kmby 15 km flood plain.

The Khwai study area is located along the terminal portion of the Khwai River, east ofKhwai village, in the 7000-km2 Moremi Game Reserve. The Khwai River is thenorthern distributary of the Okavango Delta, an 18,000-km2 alluvial fan and terminalbasin of an internal river system that arises in the central Angola highlands (Ellery et al.,1993a). The downstream movement of water is slow (Hughes, 1996), reaching Khwaiduring the dry season, 3–4 months after the rainy season. The catchment basin ispredominantly unconsolidated Kalahari sands of aeolian origin (Ellery et al., 1993b).Alluvial sand and sandy clay loam dominate the Khwai River flood plain and adjacentwoodland soils in the study area (Tinley, 1973). Additional observations were made ina Wildlife Management Area (WMA) 30 km south of Khwai, adjacent to Moremi GameReserve.

The November–March rainy season is highly variable and coincides with the period ofhighest evapotranspiration (Bhalotra, 1987; Bekker & DeWit, 1991). Dry periods ofseveral weeks are not uncommon during the rainy season (Vossen, 1988; Bhalotra,1989). Rainfall varies spatially and temporally with differences of '100 mmannually reported between rain gauges (1 km apart (Vossen et al., 1985). Savuti islocated on the 550 mm average annual rainfall isohyet and Moremi averages 500 mmwith a 35% coefficient of variation (Bhalotra, 1989). Since the Savuti River dried in1982, three artificial water points provide the only surface water available after theseasonal pans have dried. The flow of the Khwai River is seasonal and varies betweenyears. The extent of the flow decreased each year during the study from 1994–1997.The nearest water source to the study area in the WMA was '20 km away. Rainfall in1994 and 1995 was (50% of average while 1996 and 1997 had near average rainfall of550–600 mm each year.

Seed predation

Fifty pods were collected under mature A. erioloba trees in Savuti, Khwai, and the WMAsouth of Moremi Game Reserve in June 1995 and 1996. The pods were opened andeach seed was examined for bruchid beetle exit holes. In June 1995, over 4500 seedswere collected from 53 elephant dung piles in Savuti and examined for bruchid exitholes.

Table 1. Site characteristics at three study areas selected to compare the rate of podloss from A. erioloba canopies

Site Water source Baboon groups Monkey groups Elephants

Khwai Perennial river 5 3 '100Savuti Artificial water point 1 1 '100WMA River '20 km away Not resident Not resident Not resident

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Chacma baboons and vervet monkeys eat immature A. erioloba pods in trees duringMay and June and elephants shake trees from May to July and pick up and eat any podsthat fall (pers. obs.). A few mature pods may fall in late May and early June but mostseeds mature during the last half of June. The number of pods remaining in trees in May,June, and July was estimated from 50 trees in three study areas with differentnumbers of primates and elephants during 1995 and 1996. Estimates were recorded inthree abundance categories: (100, 100–500 or '500 pods/tree. The Khwai study areahas five large baboon groups and three vervet monkey groups along 5 km of river front.Hundreds of elephants drink and forage along this section of the Khwai River. Savuti hasone large baboon group and one large monkey group which were both found near oneartificial water point along the dry Savuti River channel. Up to 800 elephants werecounted at three artificial water points in Savuti in 1995. Resident baboons, monkeys,and elephants are not found in the study area within the WMA but elephants did movethrough the area throughout the year (Table 1).

Giraffe were observed browsing on immature and mature A. erioloba pods. InSavuti and Khwai giraffe browsing was limited by the high browse line maintainedby foraging elephants. Changes in pod abundance above and below the level reached bybrowsing giraffes were recorded in the WMA, where few elephants were observedduring the dry season.

Seed germination

Seeds for germination experiments were divided into five categories. Seeds collectedfrom elephant dung included (1) mature, undamaged seeds (1 cm; (2) mature un-damaged seeds '1 cm; (3) immature seeds '1 cm with light brown seed coats; and(4) mature seeds with bruchid beetle exit holes. In addition (5) mature, undamagedseeds were obtained from pods found under trees. All seeds used for germinationexperiments were collected in Savuti. Seeds were germinated in enamel basins filled toa depth of 50 mm with sand collected between the canopies of mature A. erioloba trees atthe study site. In each basin, 25 seeds were pressed into the sand which was keptcontinuously wet. There were eight replicate basins for each seed category, for a total of1000 seeds. The seeds were germinated in a well ventilated room with ambient temper-atures and natural indirect light during December 1995 when seeds were germinatingnaturally in Savuti. The number of seeds germinating each day was recorded. After 24days, all ungerminated seeds were cut open and examined for bruchid damage.

New seedlings

During the November–March rainy season in 1994, 1995, and 1997, 50 2]100-mtransects were sampled monthly in or adjacent to A. erioloba woodlands, searching fornew A. erioloba seedlings in Khwai and Savuti. New seedlings were identified by the

ACACIA SEEDS AND SEEDLINGS 545

presence of cotyledons to avoid confusion with seedlings which had resprouted after fireor browsing (Wilson & Witkowski, 1998). Less than 10 new seedlings were found ateither site and none survived into the dry season. In December 1995, after '100 mm ofrainfall, new seedlings emerged in Savuti. Ten new seedlings were marked with smallplastic tags in 1-m2 plots at four sites in Savuti. At each site, plots were located under thecanopy (n"3), at the edge of the canopy (n"4), and 10–15 m beyond the canopy(n"4) of a mature A. erioloba tree. The number and condition of seedlings wererecorded through June 1996. Rainfall was recorded daily and soil moisture wasmonitored with a Quickdraw 2000 soil moisture probe. Monthly searches for newlyemerged seedlings continued through April in Savuti. The Khwai area was not access-ible in 1996 because of flooded roads.

Statistical analysis

The differences in the number of pods in mature A. erioloba trees in May, June andJuly in Khwai (five baboon groups, three monkey groups, and '100 elephants), Savuti(one baboon group, one monkey group and '100 elephants), and WMA (no baboonsor monkeys and few elephants) were compared using chi-square analysis (SAS, 1992).Analysis of variance (SAS, 1992) was used to determine if there was a significantdifference in the time of germination and the number of seeds germinated for eachof the five seed categories. A mixed model ANOVA (SAS, 1992) with site as a randomeffect, time of observation as a repeated effect and the location in relation to thecanopy as a fixed effect was used to determine if distance from the canopyinfluenced seedling growth and survival. Arcsin transformations were used for allpercentage data.

Results

Seed production and predation

The number of seeds with bruchid beetle exit holes in pods collected under matureA. erioloba trees ranged from 47–52% at each of the sites (n"500–600 seeds per site) inJune 1995 and 1996 (Table 2). Only 2)9% of 4941 seeds found in elephant dung inSavuti in June 1995 had bruchid exit holes, suggesting that seeds with bruchid damageare more likely to be digested (Lamprey et al., 1974; Coe & Coe, 1987; Miller & Coe,1993; Miller, 1994b).

Chacma baboons and vervet monkeys at Khwai and Savuti began eating unripeA. erioloba pods in May. After a pod was removed from the tree by a baboon, the grayfuzzy pod coat was peeled off and a thin layer of the pulp was scraped with the teethfrom the pod and the inside of the pod coat. The seeds and most of the pulp were not

Table 2. Percentage of Acacia erioloba seeds with bruchid beetle exit holes collectedin June 1995 and 1996 from 50 pods at three sites in northern Botswana

Site 1995 1996% %

Savuti 49 47Khwai 51 52WMA 47 51

n"500–600 seeds per site.

Table 3. Percent of mature A. erioloba trees (n"50 at each site) with (100,100–500, '500 pods in three wildlife areas during May, June, and July 1995 and

1996

d pods/tree

Month Year Site (100 100–500 '500

May 1995 Khwai 0 10 90Savuti 0 16 84WMA 0 4 96

1996 Khwai 0 6 94Savuti 0 4 96WMA 0 2 98

June 1995 Khwai 76 24 0Savuti 0 26 74WMA 0 4 96

1996 Khwai 82 18 0Savuti 0 20 80WMA 0 6 94

July 1995 Khwai 100 0 0Savuti 88 12 0WMA 0 44 56

1996 Khwai 100 0 0Savuti 78 22 0WMA 0 34 66

546 M. E. BARNES

eaten. Vervet monkeys were more likely to eat some of the green seeds. Occasionally,hundreds of partially eaten pods with green seeds were found under A. erioloba trees butelephants, giraffe, kudu Tragelaphus strepsiceros Pallas, or impala were usuallywaiting under the trees while the primates foraged and immediately picked up thedropped pods.

Although baboons and monkeys foraged in A. erioloba trees in May, large numbers ofpods remained in trees at all sites. In May 1995 and 1996, '80% of the trees at all threesites had '500 pods (s2

"3)994 df."2 p"0)136) (Table 3). During June, more podsremained on trees in Savuti, with only one baboon and one monkey group, and in theWMA, without resident baboons and monkeys, than Khwai, where five baboon andthree monkey groups foraged daily on pods in the study area. At Khwai 79% of the treeshad (100 pods remaining while '75% of the trees at Savuti and the WMA retained'500 pods (s2

"258)744 df."4 p"0)001). In July, after seeds matured, primates nolonger foraged on pods but elephants shook trees daily and ate any pods that dropped.All of the trees at Khwai and 83% of the trees at Savuti had (100 pods in July andneither site had any trees with '500 pods while 61% of trees in the WMA retained'500 pods (s2

"256)833 df."4 p"0)001). Few pods remained on trees in July inKhwai and Savuti, where large numbers of elephants forage on pods, but over half of thetrees in the WMA, without resident elephants or primates, retained '500 pods throughSeptember.

Acacia erioloba canopies in Savuti and Khwai are usually kept above 6 m by browsingelephants, leaving few pods are within reach of giraffe. In the WMA, where treecanopies may be (3 m above the ground, an obvious browse line was observed in Maywith few pods remaining below the 5)75 m height reached by giraffe (Pellew, 1983).By June, no pods remained within the reach of browsing giraffe.

Table 4. Mean percent germination $S.E. in five categories of Acacia eriolobaseeds in Savuti, Chobe National Park, Botswana in January 1996

Seed category %Germinating BonGroup

Immature 0)0$0)0 A(1 cm 49)0$4)1 B'1 cm 20)0$2)8 CBruchid exit hole 4)5$1)6 APod 45)5$3)4 B

n"25 seeds]8 replicates for each category. Seeds in the first four categories were taken from elephantdung. Seeds in the fifth category were collected from pods found under mature trees. Bonferroni t-test meanswith the same letter are not significantly different (p(0)05).

ACACIA SEEDS AND SEEDLINGS 547

Seed germination

The mean number of A. erioloba seeds found in individual elephant dung piles was91)4$16)3 S.E. (range 9–793). Fifty-two percent of mature seeds without visibledamage were '1 cm and 48% were (1 cm. Only 2)9% had bruchid beetle exitholes and 7)8% had unidentified damage. The number of seeds that germinated variedamong seed categories (F"88)78 df."4)35 p"0)0001) (Table 4). None of theimmature seeds and only 5% of seeds with bruchid exit holes germinated. More thantwice as many seeds (1 cm from elephant dung germinated than seeds '1 cm fromelephant dung. Seeds from pods germinated at the same rate as seeds (1 cm fromelephant dung.

Seeds with bruchid exit holes and undamaged seeds from elephant dung germinatedfrom day 3–16 while seeds from pods collected under trees germinated on days10–20. Mean time to germination was twice as long in seeds from pods(12)6$0)2 days) as seeds (1 cm from elephant dung (6)3$0)2 days) and seeds'1 cm from dung (6)5$0)2 days) (F"259)36 df."2)21 p"0.0001). Ninety-onepercent of mature seeds that did not germinate had bruchid beetle damage to the embryoand cotyledons, but no exit hole in the seed coat. Nearly 60% of all seeds '1 cmfrom the elephant dung samples had bruchid damage without exit holes whileseeds from pods, immature seeds, and seeds (1 cm from elephant dung had45–47% bruchid damage in samples without exit holes (F"3)80 df."3)28 p"0)0215)(Table 5).

Table 5. Percent of seeds (mean $S.E.) with bruchid beetle damage but withoutbruchid exit holes in Savuti, Chobe National Park, Botswana in January 1996

Seed category % Bruchid damage BonGroup

Immature 46)5$2)9 A(1 cm 45)5$2)7 A'1 cm 59)5$4)3 BPod 47)0$2)8 A

n"25 seeds]8 replicates for each category. Seeds in the first three categories were taken from elephantdung. Seeds in the fourth category were collected from pods found under mature trees. Bonferroni t-testmeans with the same letter are not significantly different (p(0)05).

548 M. E. BARNES

New seedlings

After '100 mm of rain in early December 1995, new seedlings, identified by thepresence of cotyledons, emerged in Savuti. New seedlings were often clumped andassociated with elephant dung which was usually spread out by foraging baboons,monkeys, impala, birds, nocturnal rodents, and tree squirrels Paraxerus cepapi in Khwaiand Savuti. More new seedlings were found during the December 1995–March 1996rainy season than in previous years but they were not common. In 1994, 1995, and1997, (10 seedlings were located at Khwai or Savuti after searching 50 100-m transectsmonthly during the rainy season, and none survived after April.

Distance from mature trees did not influence A. erioloba seedling growth or survival.There was no difference in mean seedling height (F"2)94 df."2)46 p"0)0626)(Fig. 1) or survival (F"0)24 df."2)71 p"0)7842) (Fig. 2) among plots under thecanopy of mature A. erioloba, at the edge of the canopy or 10–15 m away from thecanopy. In December and January, the difference in mean seedling height inrelation to the canopy was (6 mm. After 70% of seedlings died from dessication inFebruary, the remaining seedlings under the canopy were taller but twice as manyseedlings survived outside of the canopy from April to June. Seedlings shaded by grassor herbs survived longer regardless of the proximity to the mature tree canopy. Only15% of the new seedlings survived in June and all leaflets were damaged by insects(Fig. 2). Two seedlings disappeared and may have been browsed but desiccation wasthe primary cause of mortality. None of the seedlings survived through the dry season.

Discussion

Seed predation

Mature and immature A. erioloba seeds and pods are consumed by insects, birds, andmammals. Although bruchid beetles may infest around half of the seeds, removal ofunripe A. erioloba pods from the canopy by foraging chacma baboons and vervet

Figure 1. Mean height$S.E. of Acacia erioloba seedling in plots under the canopy of matureA. erioloba trees (n"3; ), at the edge of the canopy (n"4; ), and in the open 10–15 maway from the canopy (n"4; ) in Savuti, Chobe National Park, Botswana from December1995 to 1996.

Figure 2. Mean percentage of Acacia erioloba seedling surviving in plots under the canopy ofmature A. erioloba trees (n"3; ), at the edge of the canopy (n"4; ), and in the open10–15 m away from the canopy (n"4; ) in Savuti, Chobe National Park, Botswana fromDecember 1995 to June 1996.

ACACIA SEEDS AND SEEDLINGS 549

monkeys may cause the highest seed predation in areas with large primate populations.Primates are able to reach pods throughout the canopy. More pods were removed fromthe canopy in Khwai, where five baboon and three monkey groups foraged daily onunripe pods during May and June than in Savuti where only one baboon and onemonkey group foraged primarily within 3 km of one AWP (Table 3). Most pods wereretained until maturity on A. erioloba trees away from the AWP in Savuti and in theWMA. Gwynne (1969) suggested that indehiscent Acacia pods were aromatic to attractseed dispersers. I could detect the aroma of A. erioloba pods from '200 m away whenprimates were foraging on unripe pods. Although primates, elephants and smallerungulates may be attracted by the aromatic pods (Coe & Coe, 1987), there is noapparent benefit for the tree until the seeds mature since immature seeds do notgerminate.

Ungulates browse unripe Acacia pods and may remove all within reach (Pellew& Southgate, 1984). Acacia erioloba trees flower and initiate pods from August toSeptember (Coates-Palgrave, 1983). By February, pods may reach full size but theseeds do not begin to swell until March. Delaying seed development, which requiresallocating high levels of protein to the developing ovules, until pods within reach ofbrowsers had been removed was suggested as a good evolutionary strategy by Coe& Coe (1987). However, browsing ungulates and primates in northern Botswana do notbegin foraging on A. erioloba pods until the green seeds have reached nearly full size inApril and May.

By the end of July, few pods remained on trees in Khwai and Savuti, where elephantsshake trees daily and eat any pods that fall. Elephant dung piles may contain a few tohundreds of undamaged seeds which are dispersed away from the parent tree. Sinceelephants can move long distances, seedlings and small trees are sometimes found10–20 km from the nearest seed-producing tree. In the WMA, without resident eleph-ants, 61% of trees retained '500 pods through September or longer. These seedsremain vulnerable to bruchid infestation, in the tree or on the ground, longer than seedsconsumed by herbivores.

550 M. E. BARNES

Seed germination

In germination trials, more seeds from pods and seeds (1 cm from elephant dunggerminated than seeds '1 cm from elephant dung (Table 4). The rate of bruchidinfestation was higher in seeds '1 cm than seeds (1 cm. The Caryedon sp. thatinfested A. erioloba seeds at Savuti is one of the larger bruchid beetles and it may selectlarger seeds to complete its life cycle. None of the immature seeds with light brown seedcoats found in elephant dung germinated. These seeds were more developed than seedsin pods removed by primates and giraffe in May, suggesting that the large numberof green pods and seeds removed from trees by browsers could not germinate even ifthey were dispersed in dung. Only 5% of seeds from elephant dung with bruchid exitholes germinated, demonstrating that some bruchid infested seeds remain viable butmost do not. Germination rates of 0)5–17% have been recorded in bruchid infested seedsof other Acacia species (Lamprey et al., 1974; Miller, 1994b; Mucunguzi, 1995).However, germination rates were higher in seeds without bruchid damage (Hoff-man et al., 1989; Okello et al., 2001). Ninety-one percent of seeds that did not germinatehad bruchid damage but no exit hole. Although seed germination may be limited duringmost years by insufficient rainfall, bruchid beetle damage appears to be the primaryfactor preventing germination in seeds with adequate water.

Similar rates of germination were observed in seeds from pods and smaller seeds fromelephant dung. This may be a result of the experimental conditions with seeds pressedinto continuously saturated sand that covered at least half of the seed surface, unlikemost studies in which Acacia seeds were germinated on filter paper in petri dishes(Hoffman et al., 1989; Miller, 1994b; Mucunguzi, 1995). Seeds from dung begangerminating after 3 days while the seeds from pods did not germinate until the tenth day.The sand in Savuti where Acacia woodlands are found drains very quickly. Within6 h after rainfall ceased, no available water was detected in the top 20 cm of the soil witha Quickdraw 2000 soil moisture probe. During the average rainfall year in 1996, rainover 3–4 days was recorded several times but there was never a period of rain lasting '5days. While the germination rates during the experiment were the same for seeds fromelephant dung as untreated seeds from pods, the seeds from dung germinated morequickly and would have a better chance of establishing under conditions found duringthe rainy season in northern Botswana.

New seedlings

The timing of rains during the wet season may influence seed germination and establish-ment. Over half of the seedlings in Savuti died from dessication by mid-Januaryfollowing a dry period with little rain for over 3 weeks. The highest mortality was at thecanopy edge with no difference between seedlings under the canopy and in theopen. Survival appeared to be influenced by microsite differences as individualseedlings shaded by grass or herbs survived longer, regardless of their location in relationto the mature tree canopy. Shading can increase seedling survival by reducing moistureloss (O’Connor, 1995). By mid-March (25% of seedlings survived. In June, 15% of theseedlings remained even though there was no additional rain after mid-March. None ofthe seedlings survived through the dry season. The root/shoot ratio in shaded Acaciaseedlings is lower than in seedlings grown in full sun (Smith & Shackleton, 1988). Whileshaded seedlings were more likely to survive dry periods during the first few weeks afterestablishment, they may not have produced sufficient root stores to survive the dryseason. In Savuti and Khwai, seedlings that remained after the rainy season werevulnerable to trampling along elephant paths and under mature trees where wildlife seekshade during the dry season. Although seedling establishment may not be expectedduring drought years, no seedlings in Savuti or Khwai survived the dry season, even after

ACACIA SEEDS AND SEEDLINGS 551

two average rainfall years. During the typical year in the Chobe area, there are two dryperiods '10 days and one '15 days during the wettest part of the rainy season fromDecember to February (Bhalotra, 1989). Above average rainfall or a rainfall pattern withfrequent rain and no dry periods longer than a few days may be necessary for establish-ment and survival of Acacia seedlings beyond a few months (Wilson & Witkowski,1998).

Sustained rainfall over several days occurred in Savuti during the first and secondweeks of February 1996, with '100 mm of precipitation during each storm. However,no new seedlings emerged. Coe & Coe (1987) observed germination at intervals in labstudies but were unable to determine what controlled episodes of germination. Meandaily maximum temperature varies (23C during the rainy season from December toApril (Bhalotra, 1987). Mean daytime temperatures from 30 to 333C are within therange identified by Hoffman et al. (1989) and Barnes et al. (1997). The absence ofgermination after the February rains may indicate a depleted viable seed source ora response to decreasing day length indicating the approaching end of the rainy season.Wilson & Witkowski (1998) found that frequent rainfall during the first 7 weeks wasessential for Acacia seedling establishment and survival. In Savuti, seedlings thatemerged in mid-February would have a reduced chance of survival because the rainsrarely extend beyond March.

Conclusion

While bruchid beetles may damage half of A. erioloba seeds produced each year, removalof unripe pods from trees by chacma baboons and vervet monkeys was the primarysource of seed predation in wildlife areas near permanent water sources where theseprimates are common. Giraffe also can remove hundreds of unripe pods from a treebut '75% of the pods in mature trees are beyond their reach. The effectiveness ofa seed disperser depends on the number and quality of seeds dispersed (Schupp, 1993).Smaller animals are more likely to chew and damage or digest seeds. When elephantsconsume mature pods, seeds are dispersed away from the parent tree and passagethrough the digestive tract may kill bruchid beetle larvae. Disruption to the seed coat thatincreases water permeability from digestive acids, enzymes or scarification (Lampreyet al., 1974; Pellew & Southgate, 1984; Miller, 1995), insect damage to the seed coatwithout damage to the embryo (Lamprey et al., 1974), or fire (Sabiiti & Wein, 1987)may allow seeds to germinate after fewer rainy days than intact seeds in pods droppedfrom trees. While seeds exposed to fire may have higher bruchid beetle larvae mortalityand increased permeability to water (Sabiiti & Wein, 1987), no relationship was detectedbetween seed germination or seedling establishment in burned and unburned areas inthis study. Dispersal of more rapidly germinating seeds would be beneficial in sandysemi-arid savannas where sufficient rainfall for seed germination and seedlingestablishment appears to be rare.

Predictions of species persistence in plants are often limited to demographic estimatesbased on size categories. Most Acacias, including A. erioloba, can produce new shootgrowth from roots after fires or browsing. In many studies, seedlings are classified bysize ((0)5 m or (1 m). On closer inspection, these plants have resprouted after fireor herbivory and are of unknown age (Wilson & Witkowski 1998). If seedlings,identified as plants (0)5 m, are recorded annually then it may be assumed incorrectlythat new seedlings established each year and that the regeneration potential is high.During the four years of this study, no new seedlings established although previouslyestablished seedlings (0)5 m of unknown age were found (Barnes, 1999). In additionto identifying animals that reduce or enhance seed and seedling survival, it is impor-tant to determine the interval between years with adequate rainfall for new seedlingestablishment.

552 M. E. BARNES

I am grateful to the Botswana Department of Wildlife and National Parks and the Office of thePresident for permission to conduct this research project. I received support from Nation ScienceFoundation dissertation improvement grant 9412510, Wildlife Conservation Society, and theProgram in Ecology, Evolution and Conservation Biology, University of Nevada, Reno. JoelBerger, Gary Haynes, Truman Young and Janet Rachlow provided valuable comments on anearlier draft. Anonymous reviewers provided helpful suggestions.

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