Road Kills In Mikumi National Park, Tanzania - Drews 1995

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Afr. J. Eeo!. 1995, Volume 33, pages 89-100

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Road kills of animals by public traffic in Mikumi N atio Park, Tanzania, with notes 011 baboon mortality CARLOS DREWS* Department of Zoology, UniversitJ' of Cambridge. Cambridge CE23EJ, U.K

Summary

The Tanzania-Zambia highway crosses Mikumi National Park over a stretch of 50 km. The road was rehabilitated between May 1990-November 1991 resulting in higher average driving speeds. Data were collected on the incidence of road kills of animals within the park boundaries during this period. The list of 183 mammals, birds and reptiles killed by vehicles included at least 52 species, two of which are endangered: the African elephant (Loxodonta africana Blumenbach) and the African hunting dog (L,vcaon pictus Temminck). The road-kill rate increased and was estimated at a minimum of three road kills per day (21'8 road kills/km/yr) by the end of the study. Road kills were more frequent near water holes and traditionally used animal tracks. The demographic effect of road kills on troops of yellow baboons (Papio cynocephalus Linnaeus), which include the highway in their ranges, is in the order of 10% of the total yearly losses. High speed and the danger of collisions with wildlife are causing regular accidents in Mikumi National Park. Adequate signposts along the road and speed bumps are recommended in order to reduce the average driving speeds for the sake of humans and wildlife. Key words: conservation, demography, endangered species, wildlife management Resume

La grand route qui va de Tanzanie en Zambie traverse Ie Parc National de Mikumi sur une distance de 50 km. La route a ete refaite entre mai 1990 et novembre 1991, ce qui a entraine une augmentation de la vitesse moyenne des vehicules. On a recolte des donnees sur Ie nombre d'animaux tues sur la route dans les limites du parc pendant cette periode. La liste des 183 mammiferes, oiseaux et reptiles tues par des vehicules comprend au moins 53 especes dont 2 sont menacees: l'elephant africain (Loxodonta africana Blumenbach) et Ie lycaon (Lycaon pictus Temminck). Le taux d'animaux tues sur la route a augmente, et on l'estimait a environ trois morts par jour (21,8 animaux tues/kmlan) a la fin de l'etude. Les accidents mortels etaient plus frequents pres des points d'eau et des pistes traditionnellement utilisees par les animaux. L'effet demographique des accidents sur les groupes de babouins olive (Papio cynocephalus Linnaeus) dont l'aire de distribution couvrait la grand route est de l'ordre de 10% des pertes annuelles totales. La grande vitesse et Ie risque de collision avec des animaux *Present address: Programa de Vida Silvestre. Universidad Nacional. Apdo. 1350-3000. Heredia. Costa Rica.

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C. Drews

provoquent regulierement des accidents dans Ie Pare National de Mikumi. On recommande de placer des panneaux routiers ct des casse-vitesse pour reduire la vitesse moyenne, pour la securite des gens et des animaux. Introduction 'v1ikumi National Park in Tanzania is one of a number of African conservation areas which are traversed by a public road. The TANZAM (Tanzania-Zambia) highway traverses the reserve over a stretch of 50 km. Anecdotal reports of vehicles killing elephants (Loxodonta ajricana Blumenbach), giraffes (GiraOu camelopardalis Linnaeus) and hippopotamus (Hippopotamus amphibius Linnaeus), among others, indicated that the traffic was having an impact on the protected wildlife. Although public highways and railways pass through nature reserves in Africa (e.g. Kenya: Tsavo National Park, Marsabit National Park; Tanzania: Serengeti National Park, Selous Game Reserve; Uganda: Queen Elizabeth National Park; Zaire: Virunga National Park; Zambia: Kafue National Park), the range of species affected by vehicle traffic and the demo­ graphic implications of road kills has rarely been quantified in this continent (1. Lopez, pers. comm.; see review of road-kill literature by Lopez & Roviralta, 1993). This study complements the scarce information available about road kills in Africa (e.g. Broekhuysen, 1965; Lewis, 1989; Drews, 1991). The highway was in operation prior to the establishment of Mikumi National Park in 1964, 300 km south-west of Dar es Salaam. The park was expanded in 1975 to a present area of 3300 km 2 , which includes grassland, open woodland, miombo and montane rain forest (Norton et al., 1987; Lovett & Norton, 1989). The highway is of great economic importance in that it provides the main access to southern Tanzania and its bordering countries of Zambia and Malawi, which utilize the road for the transportation of goods to and from the port of Dar es Salaam. Drivers in transit through the park can contemplate African wildlife at no cost, an unusual privilege given the dwindling number of large mammals in the rural areas of Tanzania. The public road stretch inside the park was repaired between May 1990 and November 1991 as part of the rehabilitation scheme for the TANZAM highway. In early 1990 the poor condition of the road forced vehicles to cross the park at average speeds of 30 km h - I. By the end of 1991, however, buses, lorries and private vehicles passing at speeds above 90 km h ~~ 1 on the immaculate tarmac were common. During the same period, information on the occurrence of road kills within the national park boundaries was collected in order to assess (i) which taxa are affected by traffic in Mikumi; (ii) the road kill rate; (iii) the effect of an improvement in road condition on road kill rates; and (iv) to quantify the local effect of road-kill mortality on a subpopulation of yellow baboons (Papio cynocephalus Linnaeus). This report provides a basis for the design of manage­ ment measures targeted at improving road safety, the conservation of threatened species, and animal welfare in conservation areas intersected by public roads. Methods Road-kill data were collected during a study on the behaviour of yellow baboons in Mikumi National Park. The two-lane public highway is the only paved road

Road kills in a Tanzanian nalional park

91

in the park. Between 16 May 1990 and 16 August 1991 the species and location of each road kill encountered were recorded while driving at 40 km h - \ on the highway to and from the baboon study area (5-15 km north of the park headquarters). as well as during trips to Mikumi town along park territory. On average, a 20-km stretch (40'Yo of the highway within the park) was sampled regularly. The northern sector of the road was sampled much less often. Sampling occurred on 65% of the total number of days covered by the study period. In order to estimate the absolute rate of road kills along the 50-km road stretch within the park, the number of kills recorded was corrected for the proportion of highway covered during regular sampling as well as the proportion of days during which it was surveyed. This yields a correction factor of 3·75. Road kills generally disappeared from the road within one day as a result of the actions of scavengers. Occasional road-kill reports by other drivers are also included in this study. The recording effort remained more or less constant throughout the study. The records were limited to the more conspicuous animal classes: mammals, birds and reptiles. However. frogs and invertebrates, particularly ants, grasshoppers, and millipedes were also commonly found killed by vehicles. The speed of vehicles was measured by driving behind them and reading the speedometer. Unfortunately, the road toll office in Mikumi town was closed down during the study, and highly desirable information on traffic rates became unavailable. Quantitative information on road proximity and group size of baboons was used to assess the possible demographic effects of road kills on a gregarious mammal. A visit to the highway by a baboon troop was defined as any instance in which at least 10% of troop members was within 10 m of the road. Baboon troops were followed each day for seven hours, on average 0745-1545 h). When a troop visited the highway more than once during an observation day (9% of all cases, N= [44). the single durations of these visits were added to calculate the daily stay on the road (Table 3). The highway stretch considered for this analysis was 20 km long: from 15 km north of the park headquarters, to 5 km south of the headquarters. Non-parametric tests were used in all statistical analyses. Results Location of road kills

A total of 183 road kills were recorded within the national park over 15 months. The bulk (98°/,,) of the road kills occurred on the public highway. Kills were particularly common along road stretches near waterholes and where traditional, small-scale migration tracks cross the highway. Three birds (2%) were found dead on the unpaved tourist road system. Taxa

Taxa represented among road kill records are listed in Table I. At least 52 species of mammals (41°/<)), birds (42%) and reptiles (17%) were among the sample. Mammals accounted for 61 % of the total number of road kills, birds for 30% and reptiles for 9%. The most commonly killed mammal was the African civet (Viverra cil'effa Schreber), followed by spotted hyaena (Crocuta crocufa Linnaeus), yellow baboon and African buffalo (Syncerus caffer Sparrman). Other

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C. Drews

Table I. Number of road kills for each species recorded in ~likumi National Park. Tanzania (16 May 1990-16 August 1991)

Road kills in a Tanzanian national park

Species

Birds Birds of prey Gyps aFicanus Salvadori Gypoheirax angolensis Gmelin Circus ranivorus Daudin Aquila wahlbergi Sundevall Mil1'lIs migrons Boddaert Falco dickinsoni Sclater Aquila rapax Temminck Glaucidium capense Smith unidentified vultures unidentified raptors Others Coracias cal/data Linnaeus SlI'cplOpelia sp. Bonaparte Merops apiasler Linnaeus Melaenomis pallidus Muller Cenlropus superciliosus Hemprich & Ehrenberg Hirumlo sp. Linnaeus ,Numida meleagris Linnaeus GUllera pucherani Hartlaub Francolinus afer Statius Muller Cicon;a ciconia Linnaeus Macrodipleryx vexillaria Gould Macronv.\' sp. Swains Prinia subf/ava Gmelin Lanius sp. Linnaeus Lamprolomis chalibaeus Hemprich & Ehrenberg unidentified birds Total bird road kills

Number of species=23

Mammals Nocturnal Viverra civella Schreber Crocuta croc1I1a Linnaeus Genella sp. Oken Hystrix crislata Linnaeus Ietonyx striata Perry Triaenaps pers!c'a Dobson Diurnal Papio cynocephalus Linnaeus Lycaon pictus Temminck Plwcoc!lOerus aelhiopicus Pallas Other Syncerus caffer Sparrman Aepyceros melampus Lichtenstein

Number of kills

2 1 2 1 i 1 1 I

3 2

8 4 3 2

2

93

Table I. Conlinued Species

Mammals continued Other con I inued Lepus sp. Linnaeus Canis sp. Linnaeus Giraffa camelopardalis Linnaeus Helogale parvula Sundevall Loxodonta aFicana Blumenbach Connochaeles taurinus Burchell Panlhera pardus Linnaeus Procavia capensis Pallas Cephalophus sp. Hamilton Smith Mel!ivora capensis Schreber Panthera leo Linnaeus Total mammal road kills Number of species=22 Reptiles Lizards Varanus exanthematicus Schmidt Varanus sp. Merrem

Number of kills

6 5 2 2 o

2

111

2

Turtles Pelomedusidae 1 1 1 I 10 55

Snakes Python sebae Gmelin Rhamphiophis oxyrhynchus Reinhardt Bilis Gl'ielans Merrem Naja nigricol!is Linnaeus Psammophis phillipsii Hallowell Psammophis sp. Boie unidentified snakes Total reptile road kills Number of species=7

20

Total of road kills recorded

17

(III! taxa)

2

15 3 1

11 7

2 2

17

183

Total number of species identi{!ed= 52

mammals recorded more than twice include impala (Aepyceros melampus Lichtenstein), hare (Lepus sp. Linnaeus), jackals (Canis sp. Linnaeus), giraffe and African hunting dog (Lycaon pictus Temminck). The lilac-breasted roller (Coracias caudata Linnaeus) was the most common bird species among the road kills (Table I). The bird list includes two species of palaearctic migrants, the white stork (Ciconia deania Linnaeus) and the European bee-eater (Merops apiaster Linnaeus). Birds of prey accounted for 27% of all bird records. Most reptiles killed were snakes, among a list which included rock monitor lizard (Varanus exanthematicus Schmidt) and terrapin (Pelomedusidae).

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C. Drews

Road kills in a Tanzanian national park

50

95

Table 2. Baboon troops which include in their range part of a 20 km stretch of public highway in Mikumi National Park. Juvs. =juveniles and infants. The categorization of troops with respect to frequency of occurrence on the road is subjective (F. frequently; R, rarely). (-j no data

40

Troop

if)

On road

Size

Males

Females

Juvs.

F F F F F F F R R R R R R

28 32 21 60 16 20

4 3 I 6 I 3

8 II 8 17

8 8

16 18 12 37 7 9

21 66 7 20

3 II I 2

29 4 6

26 2 12

Dates of census

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30

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20 ./

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MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL 1m

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Fig. I. Monthly number of road kills recorded in Mikumi National Park, Tanzania, showing a significant increase with time (Spearman's 1'=0'878. n= 15. P
Road kill rate

Data from the last 90 days of the study (N= 73 road kills) yield a rate of 91 road kills per month, i.e. about three road kills per day within the park boundaries during that period. This figure is equivalent to 21·8 road kills/km/yr, consisting of 13·3 mammals/km/yr, 6·5 birds/km/yr and 2·0 reptiles/km/yr. These rates were calculated by taking into account the percentage of road distance sampled regularly as well as the proportion of days when it was sampled (see Methods). The rate calculated above is probably an underestimate given that an unknown proportion of animals were hit by vehicles and died in the nearby vegetation, without being recorded. In particular, this negative bias affects the bird counts and perhaps large mammals which were not killed outright and died later from injuries obtained on the road. Over the course of the study, the number of road kills increased significantly (Spearman's r=0'878, N= 15, P
Mgoda Viramba 2 Viramba I Yuma Hill Tented camp Kikoboga Highway Shy Mwanambogo Bwawadogo Lodge Kambini Troopx

26 March 16 August 20 August 27 April 22 February 26 October

1991 1991 1991 1991 1991 1991

19 March 1991 2 March 1991 27 March 1991 23 July 1991

Troop size: 0'=29'1 (SD=19'lj, Median=21. N=IO

traffic was diverted during road work. Temporary dirt tracks made up 45% of the total stretch considered. Thus, good road condition (i.e. higher driving speeds) is associated with significantly more road kills (X 2 = 5,69, df= I, P<0·05). It is noteworthy that doves (Streptopelia sp. Bonaparte) and impalas started to appear repeatedly in the records during the last months of the study when most of the highway had been newly paved. These animals are extremely agile and fast, which suggests that they are killed only by vehicles going at high speeds. On 3 November 1991 the road work inside the park was completed. Subsequently, vehicles going at more than 90 km h - I were regularly seen on the 50 km of excellent tarmac that traverses the park. Demographic effect of road kills in yellow baboons

The sample for this analysis is composed of, firstly, 15 baboons found killed within a 20 km road stretch over a period of 15 months, and secondly, of 13 troops that were seen more or less regularly within that 20 km road stretch (Table 2). The size of troops which were seen frequently on the road did not differ significantly from those troops which were only seen rarely on the highway (Mann-Whitney U-test, two-tailed, P=0·67). The estimated total of baboons killed along the 20 km stretch is 23, when the fact that the road was not sampled on 35% of the time is taken into account (i.e. 15 x 1'5). Three adult males and 10 juveniles, no infants and no adult females, were found among the dead baboons. The pooled troop compositions of nine troops (Table 2) indicate that the proportion of juveniles to adults in the subpopulation is 139 (51%) to 131. The proportion of juveniles among the road kills (77%) was higher than in the overall population. This difference just missed statistical significance (X 2 =3'309, df= I, P=0·0653). A larger sample size is required for a more powerful test of the susceptibility of certain age and sex classes to be killed by vehicles.

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C. Drews

Given a mean of 29 baboons per troop and 291 baboons among 10 troops of known size (Table 2), the estimated total of baboons among the 13 troops of the sample is approximately 378 (i.e. 291 +(29 x 3)). Thus, the estimated total of 23 baboons killed over the IS months of the study represented 6% of the baboon subpopulation that frequents the road. On average, 1·53 baboons were killed each month on the road or about 18·4 baboons per year (4'9% of subpopulation). Consequently, each troop lost, on average, 1·4 baboons per year to the traffic on the highway, i.e. 4·8% of the average troop size, when recruitment from births and immigrations is not accounted for. The estimated rate of baboons killed per troop and knowledge about the demography of baboon troops in Mikumi enables a crude assessment of which proportion of overall losses in a troop could be attributed to road kills. Demographic data over the same period of the road kill sample are available from two medium-sized troops eViramba I' and 'Viramba 2', Table 2). A 'loss' is defined here as any baboon which died or disappeared from the troop, except subadult males which left their natal troop upon attaining sexual maturity (four cases, all resighted later in other troops, pers. obs.). Adult males that transferred out of the troop are treated as 'losses'. 'Viramba I' lost four adult males, four adult females and eight juveniles (total of losses= 16, i.e. 12·8 per year). 'Viramba 2' lost eight adult males, three adult females and seven juveniles (total of losses= 18, i.e. 1404 per year). Thus, the estimated troop average of I A baboons killed by vehicles each year could represent 10·9% and 9'7% of the total yearly losses, respectively. None of the losses in the Viramba troops was confirmed to be attributed to road kills. Carcasses of baboons killed by vehicles in the study area could not be assigned to any particular troop unambiguously because (i) the killing was not witnessed; and (ii) damage to the baboon in question did not allow identification. Thus, the demographic data of Viramba troops were used to perform an indirect estimate of the potential extent of road kill mortality as a proportion of overall troop losses. Two factors should be considered for the interpretation of the average values calculated above. Firstly, the overall road-kill rate increased throughout the study (Fig. 1); towards the end of the study the road-kill risk was five times higher than at the beginning (Section 3). Thus, the mortality risk that baboons were experiencing on the highway at the end of the study is higher than the mean value calculated. Eight (53%) of all baboon kills recorded occurred during the last three months of the study. Only 33% of the IS kills occurred during the first half of the study. Second, the troops differed considerably in the frequency and duration of highway visits, i.e. some troops were at much higher risk than others (Table 2, Table 3). Troops which were seen regularly on the highway differed significantly in their mean daily stay on the road (Kruskal-Wallis one-way ANOVA, X2 =22'9, df=2, N=76, P
Road kills in a Tanzanian national park

97

Table 3. Frequency and duration of highway visits by three baboon troops in Mikumi National Park

Troop

Viramba 1 Viramba 2 Mgoda

Days observed

20 90 34

Percent on road

Min. per day on road

SD

Median

Maximum per visit

35,0"';, 50·0";',

36 58 17

16 52 23

30 46 9

60 220 94

76'5'~';'

Days observed=average duration of daily observation was seven hours (0745-1545 h); Percent on road=proportion of days during which the road was visited by the troop at least once; Min. per day on road=mean daily time spent on the road in minutes on days when the road was visited; SD=standard deviation; Maximum=maximum duration of a single visit in minutes

Road safety Any collision with an animal entails the risk of causing material damage and human casualties. Eighteen collisions between vehicles and large animals (elephant, buffalo, giraffe and wildebeest (Connochaetes taurinus Burchell» were recorded during this study. Two human fatalities were associated with these collisions. Several additional accidents occurred when the driver successfully avoided hitting an animal but left the road after losing control of the vehicle. On average, at least one large animal collision occurred each month on the highway stretch inside Mikumi National Park. Expensive freight losses and pollution due to wildlife-related accidents are further reasons for concern. In October 1991 a petrol tanker rolled over when the driver avoided a buffalo crossing the highway. Several tons of crude oil were spilled onto the grassland as a consequence. In another accident, a large load of DDT was spilled in Mikumi National Park in 1982 (G. Norton, pers. comm.). Deliberate killing of animals by drivers

On two occasions drivers were seen attempting to hit yellow baboons with their vehicles. It is conceivable that species considered as vermin (e.g. baboon), dirty (e.g. spotted hyaenas), or good to eat are killed deliberately by some drivers (c. Kidung'ho, pers. comm.). Impala was the only road kill observed to be, illegally, carried away by drivers for meat. In five of the seven impala kills recorded, only a few pieces of blood-stained fur enabled identification after the carcass had been taken by drivers. A similar situation applies to buffalo, which was occasionally found to have been butchered prior to the carcass being discovered. These observations suggest that there is a realistic potential for meat poaching on the highway by using vehicles as weapons. Discussion The different species observed killed on the highway are a representative sample of the mammalian diversity of Mikumi National Park. Interestingly, the bat Triaenops persica Dobson could be the first record of the species in the region (Kingdon, 1974; K. Howell, pers. comm.). Most mammals were probably killed at night when visibility is poor and the animals are blinded by the headlights of

98

C. Drews

the vehicles. The most commonly killed mammals were nocturnal species, namely African civet and spotted hyaena. Although not strictly nocturnal, buffalos too were generally killed at night. Interestingly there were no small rodents among the road kills in Mikumi National Park (cf. Lewis, 1989). This taxonomic group, together with domestic carnivores, represents the majority of mammal records in some road-kill studies in Europe (e.g. Frean et al., 1993; Lopez, 1993). The relatively high number of killed yellow baboons, a strictly diurnal species, may be partly explained by the acquired habit in some troops of expecting food from passing vehicles. The feeding of baboons, prohibited in all Tanzanian national parks, is still a common practice along the highway despite warning signs. In addition, drivers are often unaware of their accidental loss of food. On at least four occassions baboon troops stayed on or next to the road for up to 3·5 h when maize fell from passing lorries. Baboons utilized the highway in additional ways, including walking on the tarmac to cover long distances and foraging on the lush roadside vegetation. Reptiles seek the warm tarmac to absorb heat. This basking behaviour on the road makes them particularly vulnerable to passing vehicles. Overall. a large proportion (27%) of the killed birds were birds of prey. These exemplify the fact that road kills facilitate more road kills: dead animals on the road attract scavengers, which during the day are mainly raptors and vultures. These are, in turn, killed next to the carcass by passing vehicles. Some of the dead jackals (Canis sp.) and hyaenas were also in the proximity of fresh carcasses. The estimated rate of three road kills per day and its increasing trend indicates that the effect of public transport on the wildlife of Mikumi National Park is not negligible. The increase of the road-kill rate was associated with the rehabilitation of the highway which, in turn, led to higher average driving speeds. Also, the additional traffic resulting from the vehicles of the road workers could have contributed to the increase in road-kill rate. By the end of the study, the rate of large mammals killed on the road probably exceeded that of poached wildlife in the park (D. Njao, warden in charge of anti-poaching in Mikumi N.P., pers. comm.). Highway mortality is evidently one more demographic parameter in some of the populations which include the road in their range. For some of the baboon troops which frequently visit or cross the highway, road kills may have a measurable influence on survivorship, potentially accounting for about 10% of annual losses. The relatively high incidence of bone fractures among the adult male baboons of the Highway troop is probably associated with their high frequency of road visits (pers. obs). Although not fatal, impacts by vehicles are impairing these males in respect of their competitive ability and reproductive performance. The problems associated with road kills in African protected areas are similar to those identified in a recent, nationwide survey of road kills in Spain (Coordinadora de Organizaciones de Defensa Ambiental CODA, 1992; Asociacion Tecnica de Carreteras AIPCR, 1993) At least three reasons call for attempts to reduce the rate of road kills in Mikumi National Park. First, the risk of collisions with large animals within the park boundaries is increasing as traffic on the TANZAM highway increases in parallel with the development of southern Tanzania and its neighbouring countries. In addition to human casualties, the risk of pollution in the park and of considerable freight and

Road kills in a Tanzanian national park

99

vehicle losses in wildlife-related accidents should be reduced. Secondly, the list of road kills included at least two endangered species, namely African hunting dog (IUCN, 1990), and African elephant (Appendix I of CITES, 1991). Five elephants are known to have been killed on the highway inside the park between 1989-1992 (Mikumi National Park staff, pers. comm.). The killing of endangered animals is of immediate concern to conservation-oriented management plans. Thirdly and finally, from an ethical point of view, every individual animal deserves protection from unnecessary, human-induced suffering or death. Recommendations

Undoubtedly, the best way of reducing the rate of collisions between vehicles and animals is to reduce the driving speed. The existing speed limit of 80 km h - I on the TANZAM highway is too high within the park. A speed limit of 50 km h - I, which is the limit in all Tanzanian national parks, should be implemented on the public highway within the park boundaries. Adequate signposts at regular intervals should remind the drivers to reduce speed and of the presence of wild animals on the road. The most effective way of reducing average driving speed, however, is speed bumps. These should be placed close to areas identified as 'hot spots' for road kills, such as waterholes and traditional, frequently used animal tracks. A further step towards reducing additional road kills is to remove from the road any carcass found, so that its scavengers are not exposed to the traffic (also Barragan & Lopez, 1993). Acknowledgments

I am grateful to the Tanzanian Commission for Science and Technology, Tanzania National Parks, and the Serengeti Wildlife Research Institute for permission to work in Mikumi National Park. The national park authorities and rangers provided invaluable logistical support and background information about the road-kill problem. The Long Term Mikumi Baboon Project kindly provided unpublished demographic information on two of the baboon troops cited (Viramba 1 and 2) as well as accommodation in the park. Special thanks to Astrid Schmidl-Drews, Charles Kidung'ho, William Marwa and Ayoub Njalale who helped regularly in spotting, identifying and removing carcasses from the road. I thankfully acknowledge Michael O'Meara, who collected road-kill information during by temporary absence in October-November 1990. My sincere thanks to all those who reported road kills to me throughout the study, and to Keith Eltringham, Dawn Hawkins, Guy Norton, Sam Wasser, Javier Lopez and two anonymous referees for their valuable comments on a previous draft. The Leakey Foundation, SigrnaXi, Durham Fund, Bedford Fund and Kings College, Cambridge made this study possible by funding my stay in Mikumi National Park. References B.I. & LOPEZ. J. (1993) Soluciones a los atropellos de vertebrados en carreteras. In: II Simposio Naciollal sobre Carreteras y Medio Ambiel1le. Asociacion Tecnica de Carreteras (AIPCR), Madrid. BROEKHUYSEN, G. (1965) An analysis of bird casualties on the road in the southwestern Cape Province, South Africa. L·Oiseall. Rev. ji". Omit/wI. 35,35-51.

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CITES (1991) List of species of Appendices I, II. and III. In: Proceedings i!/ the 7th Meeting of the Conference of rhe Porties to CITES (Col1venrion on rhe Internalional Trade il1 Endangered Species 0/ Wild Fauna and Flora). LOllsanne. SlI'it:erlal1d. 9-20 October. 1989. Secretariat of the Convention. Lousanne. COORDINADORA DE ORGANIZACIONES DE DEFENSA AMBfENTAL CODA (Ed.) (1992) I Jornadas para el estudio v prevencion de la mortalidad de rerlebrados en carreteras, Vols. 1-3. CODA, Madrid. ASOCIACION TECNICA DE CARRETERAS AIPCR (1993) II Simposio Nacional sobre Carreteras y Media Ambienre. AIPCR. Madrid. DREWS. e. (1991) Road kills in Mikumi National Park. Miombo, Newsl. Wildl. Cons. Soc. Tan:ania 7, 6--7. FREAN. M .. VILLARINO. A .• ALVAREZ. I. & GONZALEZ. S. (1993) Mortalidad de vertebrados por atropello en las riberas del Millo en Orense. In: If Simposio Nacional sabre Carreteras y Media Ambiel11e. Asociaci6n Tecnica de Carreteras (AIPCR). Madrid. IUCN (1990) Red Ust of Threatened Animals. World Conservation Monitoring Centre. Cambridge. U.K. IUCN-World Conservation Union. Gland. Switzerland. KINGDON. I. (1974) East African Mammals. An Atlas oJ Evolution in Africa. II (A). Insecrivores and Bals. Academic Press. London. LEWIS. A.D. (1989) Road kills and other records of mainly smaller mammills from Kenya: data for a Kenyan mammal atlas. Eas/ Afi'. Nat. Hisr Soc. Bull. 19. 20-22. LOPEZ. I. (1993) Metodologia y resultados del proyecto de seguimiento de la mortalidad de vertebrados en carreteras (P.M.V.C.le.O.D.A.). In: II Simposio Nacional sohre Carreteras y Medio Ambiente. Asociaci6n Tecnica de Carreteras (AIPCR). Madrid. LOPEZ. J. & ROVIRALTA. F. (1993) Banco de datos y centro de documentaci6n acerca de la mortalidad de vertebrados en carreteras. In: II Sirnposio Nacional sobre Carre/eras F Media Amhien/e. Asociaci6n Tecnica de Carreteras (AIPCR). Madrid. LOVETT. I.e. & NORTON. G.W. (1989) Afromontane rainforest on Malundwe Hill in Mikumi National Park. Tanzania. BioI. Conser". 48. 13-19. NORTON. G.W .• RHINE. R.I., WYNN. G.W. & WYNN. R.D. (1987) Baboon diet: A five-year study of stability and variability in the plant feeding and habitat of yellow baboons Papio cynocephalus of Mikumi National Park. Tanzania. Folia primarol. 48, 78-120.

(Manuscript accepted 3 March 1994)

Population dynamics, pollination ecology and the significance of plant height in Aloe candelabrum C. J. STOKES and R. 1. YEATON Department of Botany and UN-FRD Research Unit for Plant Growth and Development. University of Natal, Pietermaritzburg 3200, South Africa

Summary Pollinator exclusion experiments showed that insect pollination of Aloe candelabrum Berger is negligible and that birds are the only effective pollinator guild. Bird activity is greatest where flowering aloes are most dense. Also fruit set is lower in those aloes that bloom later in the season after most of the population has already flowered. Thus the reproductive fitness of aloes is dependent on the density of flowering plants and there is selection for limited seed dispersaL As a result, spatial distributions show clumping in young populations. These clumps act as nuclei from which plants spread slowly over time with mature individuals forming the centres of the densest stands. As a further consequence of limited seed dispersal, intraspecific competition is intense, resulting in self-thinning of the population. The height of A. candelabrum does not enhance pollinator attraction. In closed environments, aloe height serves only to allow the persistence of those plants that establish before the development of a tree canopy. Under the constraint of limited seed dispersal capabilities, selected for by pollinator activity, the height of A. candelabrum serves as an adaptation to spread seeds away from the parent plant and from each other. Variability in seed morphology enhances the latter effect. Key words: Aloe, plant height, seed dispersal, competition

Resume Des experiences sur l'exclusion des pollinisateurs ont montre que la pollinisation par les insectes est negligeable chez Aloe candelabrum Berger et que les oiseaux sont les seuls pollinisateurs reellement efficaces. L'activite des oiseaux est maximale quand la floraison des aloes est la plus intense, et la fructification est plus faible chez les aloes qui fleurissent plus tard dans la saison, lorsque la plus grande partie de la population a deja fleuri. Done, l'aptitude reproductrice des aloes depend de la densite des plantes en fleurs et il y a une selection dans la dispersion limitee des semences. Par consequent, la distribution spatiale presente des agglomerats de jeunes populations. Ces groupes torment un noyau a partir duquelles plants se repandent lentement au cours du temps, avec des individus adultes qui forment Ie centre des peuplements les plus denses. Consequence ulrerieure d'une dispersion limitee des semences, la competition intraspecifique est intense et aboutit a un affaiblissement de la popuation. La hauteur de A. candelabrum n'augmente pas l'attraction sur les pollinisateurs. Dans des environnements fermes, la hauteur des aloes sert seulement a permettre