A COMPARISON OF THE DIVERSITY AND COMPOSITION OF GROUND-ACTIVE SPIDERS IN MKOMAZI GAME RESERVE, TANZANIA AND ETOSHA NATIONAL PARK, NAMIBIA

6 Pages·2002·6.3 MB·English

by  A Russell-Smith

2002. The Journal of Arachnology 30:383-388
A COMPARISON OF THE DIVERSITY AND COMPOSITION OF
GROUND-ACTIVE SPIDERS IN MKOMAZI GAME RESERVE,
TANZANIA AND ETOSHA NATIONAL PARK, NAMIBIA
A. Russell-Smith: Natural Resources Institute, University of Greenwich, Central
Avenue, Chatham Maritime, Kent ME4 4TB, UK. E-mail: [email protected]
ABSTRACT. Pitfall traps were used to census ground-active spiders in 12 different habitat types in
protected savanna biomes in Tanzania and Namibia. With roughly equivalent trapping effort in the two
areas, a total of 229 spider species and 40 families were trapped in Mkomazi Game Reserve and 151
species and 34 families in Etosha National Park. The family composition of the fauna of the two areas
was similar, with Salticidae accounting for 17% (Mkomazi) and 14% (Etosha) of all species and Gna-
phosidae accounting for 16% (Mkomazi) and 14% (Etosha) of the total. Other families that accounted for
a significant proportion of species included Lycosidae (6-7%) and Zodariidae (6-7.5%). Despite the in-
tensive trapping effort, there was no indication from species accumulation curves that a complete estimate
of the spider species richness had been obtained from either area. The possible reasons for the differences
in spider species richness and family composition in the two areas are discussed.
Keywords: Spider biodiversity, family composition, Etosha National Park, Mkomazi Game Reserve.
Although much is known about invertebrate logistical reasons, most effort was focused on
diversity in temperate habitats, studies on spe- the ground layer fauna. Pitfall trapping in 16
cies diversity in tropical ecosystems are more habitats ranging from dry grasslands to wood-
A
recent. majority of studies have focused on land or dry forest in both areas was used to
tropical rainforest canopies because inverte- assess spider diversity over a three year peri-
brate diversity in them is so high. Very much od. Trapping effort was very similar in both
less attention has been paid to invertebrate di- areas and the results are thus directly com-
versity in savanna areas, despite the fact that, parable.
broadly defined, savannas occupy more than
METHODS
twice the total land area of rain forests. This
—
applies equally to research on spider diversity Study areas. Mkomazi Game Reserve in
in tropical areas where the majority of report- northern Tanzania lies at approximately 4° S
ed work has been on forests (reviewed by and between 38° and 39° E, adjacent to the
&
Russell-Smith Stork 1994). Kenya border and forming a southern exten-
In Africa, most inventories of savanna spi- sion of the much larger Tsavo (West) National
ders have either been undertaken for purposes Park in Kenya. The park covers an area of
other than biodiversity assessment (e.g. Blan- about 3600 km^ and varies in elevation from
&
din Celerier 1981; Russell-Smith 1981) or 630-1600 m. It is bounded to the South by
have been conducted over a limited time span the Usambara Mountains, to the West by the
or in a limited area (Russell-Smith et al. 1987; East Pare Mountains and to the East by the
Lotz et al, 1991). There have been few at- Indian Ocean. Apart from isolated hills rising
m
tempts to census spider diversity over the full to 1600 elevation, the land surface is a
range of savanna habitats within a given area. gently inclined peniplane falling gradually
m m
In this paper, assessments of spider biodi- from ca 1000 in the West to ca 700 in
mm
versity conducted as part of much larger sur- the East. Rainfall ranges from 800 in the
mm
veys of invertebrate diversity in Mkomazi West of the reserve to 200 in the East and
Game Reserve, Tanzania and Etosha National most of the rain falls in two periods, in March
Park, Namibia are described. Both surveys and April and October to December.
censused spiders of the ground layer, field lay- The vegetation of the reserve consists of a
er and tree canopies in the two sites but, for mosaic of woodland, bushland, wooded or
383
384 THE JOURNAL OF ARACHNOLOGY
bushed grassland and open grassland in which west of the reserve. Rainfall ranges from
mm
the boundaries between different vegetation about 450 in the East of the reserve to
mm
types are often gradual and indistinct. Many about 300 in the West. Nearly all rainfall
of the wooded or bushy habitats are dominate falls between November and April but is ex-
ed by species of Acacia and Commiphora, ei- tremely erratic, with some years with less than
mm
ther individually or as mixed stands and it is 30 total rainfall.
estimated that these may comprise some 70% The range of vegetation types within the re-
of the total area of the reserve. Forest habitats serve is, in many respects, similar to that in
are confined to isolated hilltops (principally in Mkomazi Game Reserve, with woodland,
the West of the reserve), where dry montane bush and grassland habitats which intergrade
forests are dominated by Spirostachys afri- gradually with one another. Differences in the
cana and Brachylaena huillensis and to a vegetation of the two reserves include the ab-
fringe of riverine forest along the Umba River sence of true forest habitats from Etosha and
in the East, dominated by leguminous trees of the much greater area of grassland or bushed
the genera Afzelia, Albizia, Newtonia and Ta- grassland habitats here than in Mkomazi. In
marindus. Pure grassland is almost confined general, closed woodland is confined to the
to low lying areas (locally known as “vleis"’) higher rainfall area of the east of Etosha and
on black cracking clay soils which are subject open grassland habitats become more frequent
to seasonal inundation. Grasslands vary in towards the West. Open grassland is however,
height from 25 cm or less in the dry season found throughout the reserve both on saline
to almost 200 cm in perennial grasslands in soils and on shallow soils on calcrete. Another
the wet seasons. difference is that Commiphora species are
Pitfall sampling was carried out in 12 dis- much less abundant in woodland and bushland
tinct vegetation types as follows: Seasonally in Etosha than in Mkomazi and Acacia spe-
inundated grassland with Acacia drepanolob- cies, although widespread and common, rarely
ium on black cracking clay (vlei grassland), dominate woodland and bush in Etosha. Fur-
near Ndea Hill; Seasonally inundated grass- ther details of the vegetation of Etosha can be
land with Acacia zanzibarica, near Ngurunga; found in Le Roux et ah, 1988.
Grassland derived from montane Spirostachys Pitfall trapping was conducted in the fol-
forest, summit of Ibaya hill; Unburnt mixed lowing 12 vegetation types: Terminalia/Com-
grassland on foot slope, near Ibaya Camp; Un- bretum woodland, Beisebvlakte and Oshivelo;
burnt Acacia/Commiphora woodland on lower TerminaliaiSpirostachys woodland, Leeu-
hill-slope, near Ibaya Camp; Open Acacia Se- drink; Mixed woodland, southeast corner of
negal woodland, near Ndea Hill; Mixed riv- reserve; Open Combretum/Kirkia woodland
erine scrub, margins of Umba River; Mixed on dolomite inselberg, Helio Hill; Dense
Combretum scrub, margins of Umba River; shrub mopane bushland on calcrete, Natukan-
Mixed Combretum bushland, near Dindira aoka and Otjivalunda; Colophospermumf
dam; Acacia senegal/A. nilotica woodland, Combretum bushland, Ombika and Ongava;
near Ubani; Dense Dichrostachys cinerea Open shrub mopane bushland on loam, Bit-
scrub, near Dindira dam; Montane Spirostach- terwater; Open Colophospermum!Combretum
ys forest, summit of Ibaya hill. These are list- bushland (“Kaokoveld”), Oliphantsrus;
ed in approximate order from habitats with the Mixed shrub/low tree savanna, near rain
least tree cover to those with the greatest. Fur- gauge 17; Mixed Acacia/Terminalia shrub
ther details of these habitats can be found in bushland, Duineveld; Eragrostis/Ennapogon
Coe et ah, 1998. grassland (“sweet grassveld”) on calcrete,
Etosha National Park is situated in the north Okaukuejo; Sporobolus/Odyssea grassland,
of Namibia and lies along latitude 19° South Andoni South and Andoni North. These are
between approximately 14° and 16° East lon- listed in approximate order from habitats with
gitudes. The park covers an area of 22,000 the greatest tree cover to those with the least.
km^ and is centred on Etosha pan, a very large Further details of these habitats can be found
area of seasonally flooded saltpan. The gen- in Le Roux et al. (198—8).
eral land surface is extremely flat with isolated Sampling Method. In Mkomazi, diversi-
inselbergs to the south of the pan but with ty of ground-living spiders was studied with
slightly larger hills in the extreme south and pitfall traps constructed from plastic coffee
RUSSELL-SMITH-DIVERSITY OF SPIDERS IN MKOMAZI AND ETOSHA 385
beakers each 7 cm in diameter and 10 cm fauna for the two study areas is shown in Ta-
deep. Traps were spaced at a minimum of 2 ble 1. In both areas, Salticidae and Gnaphos-
m
apart and filled to a quarter of their depth idae accounted for the largest proportion of
with water to which a trace of household de- spider species, each representing approximate-
tergent was added. To reduce disturbance re- ly 16%-17% of all species at Etosha and 14%
sulting from daily emptying, two plastic cups of all species at Mkomazi. Other families that
were placed one inside the other and only the were well represented in both areas included
inside cup was removed from the ground. Trap Lycosidae (~ 6%— 7% of all species) and Zo-
mm
contents were removed onto a 1 mesh dariidae (~ 6.%—7.5% of all species). Cor-
sieve and preserved in 70% ethanol for sub- innidae, Theridiidae and Thomisidae were
sequent sorting and identification. Normally, well represented in traps at Mkomazi but were
traps were placed in three rows of 10 traps rare in traps in Etosha. Prodidomidae repre-
m
spaced 5 apart, and left in the field for pe- sented 9% of all species at Etosha but less
riods of either three or six days before emp- than 2% in Mkomazi. The families Amauro-
tying. The contents of the 10 traps from each biidae, Clubionidae, Ctenidae, Hahniidae, Mi-
row were pooled at each site sampled. Sam- metidae, Miturgidae, Mysmenidae, Telemidae,
pling was most intensive at the two sites close Tetrablemmidae, Tetragnathidae, Theridioso-
to Ibaya camp where pitfall traps were oper- matidae and Uloboridae, 22 species altogether,
ated over six day periods in November 1994 were only recorded in traps at Mkomazi. Con-
and for six days in each month from April to versely, the families Ammoxenidae and Sica-
August 1995. Samples from the remaining site riidae, 7 species altogether, were only record-
were taken either over three days (vlei grass- ed from traps at Etosha. There were 78 more
land near Ndea, grassland derived from mon- spider species and 6 more families recorded
tane forest on Ibaya Hill and open Acacia Se- from Mkomazi Game Reserve than from Eto-
negal woodland near Ndea) or six days (all sha National Park (Table 1)—.
remaining sites other than the last one) in Efficiency of trapping. The conclusions
April 1995 or January 1996. Samples were drawn as to the differences in spider species
taken over three days in April 1995 and six richness between the two areas will depend to
days in January 1996 in the montane forest some extent on how complete the sampling
(site 12). Total trapping effort in Mkomazi effort was in each. The cumulative numbers
was 7,200 trap days.
of spiders trapped is plotted against the cu-
A
similar system was used in Etosha except mulative number of trap days for both sites in
that traps were 10 cm in diameter and 9 cm Fig. 1. While the rate of accumulation of new
A
deep. killing fluid of ethylene glycol diluted species trapped was always greater in Mkom-
to 50% with water was used. Here, traps were
azi than in Etosha, it is evident that in neither
placed in four rows of 10 and were emptied
area does the curve for species accumulation
after four days in the field. The catch from over sampling effort approach an asymptote.
each row of traps was pooled. Sampling was
However, the rate of species accumulation
carried out at the first 6 sites listed above in does appear noticeably lower in Etosha than
November 1996 and January, March, June and
in Mkomazi from 5,000 trap days to 7,600
November 1997. Sampling from the second
trap days.
six sites listed was in March 1998 only. The
total trapping effort at this site was 7,680 trap DISCUSSION
days. Voucher specimens of the Salticidae
Despite the relatively intensive sampling ef-
from Mkomazi Game Reserve are deposited
fort in both of the study areas, it is evident
in the Musee Royal de I’Afrique Centrale,
from Fig. 1 that the census of species was far
Tervuren, Belgium while those for all other
from complete. Although this may, in theory,
families remain in the author's collection.
mean that the difference between the two sites
Voucher specimens of all spiders from Etosha
in spider species richness is less than sug-
National Park are deposited in the National
Museum of Namibia, Windhoek. gested in this study, there is little reason to
suppose that species richness would actually
RESULTS
be higher at Etosha than at Mkomazi.
Family and species richness and diversi- The family composition of the ground-ac-
—
ty. The family composition of the spider tive spider faunas in both areas studied is fair-
386 THE JOURNAL OF ARACHNOLOGY
Table 1. The family and species composition of spiders from pitfall traps in Etosha National Park,
Namibia and Mkomazi Game Reserve, Tanzania.
Etosha National Park, Namibia Mkomazi Game Reserve, Tanzania
Family Species Family Species
Agelenidae 3 Agelenidae
1
Ammoxenidae 6 Amaurobiidae 3
Araneidae 0 Araneidae 6
Atypidae Atypidae
1 1
Barychelidae 1 Barychelidae 2
Caponiidae Caponiidae 2
1
Corinnidae Clubionidae
1 1
Cyrtaucheniidae 9 Corinnidae 10
Dictynidae Ctenidae 4
1
Dipluridae Cyrtaucheniidae 4
1
Gnaphosidae 24 Dictynidae
1
Hersiliidae Dipluridae
1 1
Heteropodidae Gnaphosidae 32
1
Idiopidae 4 Hahniidae
1
Linyphiidae 3 Hersiliidae 1
Lycosidae 11 Heteropodidae 3
Nemesiidae 2 Idiopidae 7
Ochyroceratidae Linyphiidae 8
.1
Oonopidae 4 Lycosidae 13
Oxyopidae 3 Mimetidae
1
Palpimanidae 5 Miturgidae 5
Philodromidae 5 Mysmenidae
1
Pholcidae 4 Nemesiidae
1
Prodidomidae 14 Ochyroceratidae
1
Salticidae 26 Oonopidae 8
Scytodidae 1 Oxyopidae 9
Segestriidae Palpimanidae 4
1
Selenopidae 1 Philodromidae 6
Sicariidae 1 Pholcidae 3
Theraphosidae Prodidomidae 4
1
Theridiidae 1 Salticidae 31
Thomisidae 3 Segestriidae 2
Zodariidae 9 Telemidae 2
Unidentified Tetrablemmidae
1 1
Tetragnathidae
1
Theridiidae 14
Theridiosomatidae
1
Thomisidae 15
Uloboridae
1
Zodariidae 17
Total species 151 Total species 229
Total families .34 Total families 40
ly typical of those found in earlier studies of cies rich in somewhat higher rainfall areas. In
semi-arid savannas elsewhere in Africa (Rus- the very humid savanna at Lamto, Cote
sell-Smith 1981, RusselLSmith et aL 1987, dHvoire (mean annual rainfall 1300 mm), saL
Lotz et aL 1991). In all of these areas, gna- ticids accounted for 19% of all species and
&
phosids and salticids are among the most di- gnaphosids for only 6.7% (Blandin Celerier
verse families, with the former apparently 1981). Other species rich families in previ-
dominating drier sites and the latter more spe^ ously studied sites include Zodariidae (4.8-
RUSSELL-SMITH-DIVERSITY OF SPIDERS IN MKOMAZI AND ETOSHA 387
though without supplying comparative data to
support the contention.
Although the range of annual rainfall in the
two areas studied overlaps (see site descrip-
tions above), there is little doubt that Etosha
National Park is, overall, considerably more
arid than Mkomazi Game Reserve. Much of
the former has annual rainfall of less than 400
mm
and complete failure of annual rains is a
— regular, if infrequent occurrence. By contrast
Figure 1 . Cumulative numbers of spider species much of Mkomazi, including the areas in
trapped in Mkomazi Game Reserve (Tanzania) and which most of the pitfall trapping was con-
Etosha National Park (Namibia).
ducted, has annual rainfall above 550 mm, and
here complete failure of both the short and
long rains is extremely rare. Some support for
12.8% of all species) Lycosidae (8.5-21.5%), the importance of rainfall in determining
Oxyopidae (2.1-5.7%) and Thomisidae (3.3- ground-active spider richness can be seen in
17.0%). However, the results differ from those
the relative proportion of gnaphosoid species
of Whitmore et al. (2002) who found that gna- (Ammoxenidae, Gnaphosidae and Prodidom-
phosids represented only a small proportion of idae) in the two areas, 29% of the total in
the total spider fauna of a savanna area adja- Etosha and 16% in Mkomazi. Evidence from
cent to Kruger National Park in South Africa.
studies of other savanna sites in Africa does
They suggest that this was probably a result
suggest that gnaphosids are characteristically
of the very high level of disturbance of pitfall
more diverse in more arid sites while the re-
traps by baboons (Whitmore, pers. comm.).
verse applies to salticids.
If it is assumed that the difference in spider A
final possibility, that there is a large dif-
species richness between Mkomazi and Eto-
ference in the size of the regional pools of
sha is real rather than an artifact of under-
spider species from which the fauna of the
sampling, what factors might account for the
two areas was drawn, is more difficult to es-
difference? There are at least three possible
tablish. That there are differences in the ocurr-
explanations, none of which are necessarily
ence of spider families represented in the two
mutually exclusive: a) a latitudinal gradient in
regions seems certain. Thus, Griffin (1998)
spider diversity exists, with highest diversity
does not include Tetrablemmidae, Telemidae,
at the equator and decreasing diversity with
Cyatholipidae and Symphytognathidae in her
increasing latitude; b) spider diversity is re-
lated to mean annual rainfall, with higher di- list of spider families recorded from Namibia
while all are recorded from Tanzania. Con-
versity in wetter areas; c) there are unique dif-
versely, Ammoxenidae are well represented in
ferences in the regional pools of available
spider species in the two regions which deter- Namibia but are known to be absent from East
Africa. However, none of these families con-
mines the diversity in each area. There is
some support for the concept of a latitudinal tributed many species to the total from either
gradient of diversity of spiders, at least be- area studied and it is probably the size of the
tween the equator and the northern hemi- regional pools of species in the more diverse
sphere. For example, Weselowska & Russell- families, such as Salticidae and Gnaphosidae,
Smith (2000) present empirical evidence for that is more pertinent here. Unfortunately, the
an area-specific decline in species richness of systematic data for any of the larger families
salticids between Mkomazi (4° S) and the UK of spiders in any region of Africa is currently
(~ 55° N) of approximately one order of mag- woefully inadequate. As an example, among
nitude, although there are insufficient data sets the 69 species of salticids recorded from
from Africa to place very great reliance on Mkomazi Game Reserve, nearly half were
&
this conclusion. However, Platnick (1991) has found to be new to science (Weselowska
suggested that there is little decline in diver- Russell-Smith 2000). Until the systematic un-
sity of spiders between the equator and south- derstanding of the larger spider families in Af-
ern latitudes of the neotropical region, al- rica is greatly improved, it is difficult to assess
THE JOURNAL OF ARACHNOLOGY
388
how far the size of regional species pools ef- fication of the vegetation of Etosha National
fects spider diversity of specific areas. Park. South African Journal of Botany 54:1-10.
&
Lotz, L.N., M.T. Seaman, D.J. Kok. 1991. Sur-
ACKNOWLEDGMENTS
face-active spiders (Araneae) of a site in semi-
arid central South Africa. Navorsinge van die
The studies reported here would have been Nasionale Museum Bloemfontein 7:529-540.
impossible without the assistance of Raphael Platnick, N.I. 1991. Patterns of biodiversity. Pp 15-
Abdallah, Elias Kihumo, George McGavin, 24. In Systematics, ecology and the biodiversity
Daniel Mafunde, Ramadan! Makusi, Paul crisis. (N. Eldredge ed.). Colombia University
New
Marenga, Mark Ritchie, Hamish Robertson Press, York.
and Simon van Noort in Mkomazi. Likewise Russell-Smith, A. 1981. Seasonal activity and di-
versity of ground-living spiders in two African
John Davies, Stuart Green and John Irish pro-
savanna habitats. Bulletin of the British Arach-
vided essential support in Etosha. The studies
nological Society 5:145-154.
UK
in both areas were supported by the Dar- Russell-Smith, A. & N.E. Stork. 1994. Abundance
win Initiative to which I make grateful ac- and diversity of spiders from the canopy of trop-
knowledgment. ical rainforests with particular reference to Su-
lawesi, Indonesia. Journal of Tropical Ecology
LITERATURE CITED 10:545-558.
&
& Russell-Smith A., J.M. Ritchie, N.M. Collins.
Blandin P. M.L. Celerier. 1981. Les araignees des
1987. The surface-active spider fauna of arid
savanes de Lamto. Organisation des peuple-
bushland in Kora Reserve, Kenya. Bulletin ofthe
ments, bilans energetiques, place dans
British Arachnological Society 7:171-174.
recosystemes. Publications du laboratoire de &
Weselowska, W. A. Russell-Smith. 2000. The
zoologie. No 21. Ease. 2. pp 507-&586. salticid spiders of Mkomazi Game Reserve, a
Coe M., K. Volleson, R. Abdallah E.I. Mboya.
semi-arid savanna in East Africa. Tropical Zo-
1998. Vegetation and habitats of Mkomazi. Pp.
ology 13:11-127.
17-144. In Mkomazi: The ecology, biodiversity &
1 Whitmore, C., R.H. Slotow, T.E. Crouch, A.S.
and conservation of a Tanzanian savanna. (M.
Dipenaar-Schoeman. 2002. Biodiversity of spi-
&
Coe, N. McWilliiam, G. Stone, M. Packer, XV
ders in a savanna ecosystem. International
eds.). Royal Geographical Society, London. Congress of Arachnology, Badplaas, South Af-
Griffin, E. 1998. Species richness and biogeography rica. 23-30 March 2001. Journal of Arachnology
of non-acarine arachnids in Namibia. Biodiver- 30:xx-yy.
&
sity Conservation 7:467-482.
Le Roux, C.J.G., J.O. Grunow, J.W. Monis, G.J. Manuscript received I July 2001, revised 10 April
& A
Bredenkamp, J.C. Scheepers. 1988. classi- 2002
.