SEASONALITY AND STRUCTURE OF THE ARTHROPOD COMMUNITY IN A FORESTED VALLEY IN THE ULUGURU MOUNTAINS, EASTERN TANZANIAM. Nummelin⟨sup⟩1⟨/sup⟩FORST ProjectP.O. Box 292, Morogoro, TanzaniaL. NshubemukiTanzania Forestry Research InstituteP.O. Box 1854, Morogoro, TanzaniaE-mail: tafori@twiga.comABSTRACTThe structure of an arthropod community in the forest floor vegetation was studied in a low altitude (about 700 m a.s.l.) forest valley in the Uluguru Mountains near Morogoro, Tanzania, by monthly sweep net sampling during one year (December 1996-November 1997). The community structure of arthropods changed relatively little during the study period. Eight groups (Araneae, Hymenoptera, Heteroptera, Homoptera, Diptera, Coleoptera, Orthoptera and Lepidoptera) made up over 95 % of all the arthropod individuals caught. There were seasonal peaks in the abundances. The highest numbers of arthropods per sample were found during the late rainy season and early dry season (May-August). Arthropod groups generally follow the rainfall pattern; in particular the abundance of Diptera, Hymenoptera and Hemiptera increases with rains.INTRODUCTIONData documenting arthropod diversity in the tropics are scarce (Hammond, 1990; Stork & Brendell, 1993; Nummelin, 1996) and papers on insect diversity in African non-managed rain forests are virtually non-existent. The main focus of insect studies in African forests has been on forest plantations and pest species (see e.g. Madoffe & Bakke, 1995 and citations therein). The arthropods in clear-cuts (Cambefort, 1984; Howden & Nealis, 1975) as well as in selectively logged sites have been studied to some degree (Holloway, 1987; Nummelin, 1989, 1992, 1996; Nummelin & Borowiec, 1991; Nummelin & Fursch, 1992; Nummelin & Hanski, 1989; Nummelin & Roininien, 1993; Wolda, 1987; Wolda et al., 1983, see also Wolda, 1983a, b; Wolda & Fisk, 1981). However, little information is available on seasonality of arthropod communities in African rain forests (Nummelin, 1989, 1996; see also Struhsaker, 1996) and the seasonality of arthropods in the forests of the Eastern Arc Mountains remains virtually undescribed (see Scharff, 1993; Jong & Congdon, 1993).⟨sup⟩1⟨/sup⟩ Present address: Division of Environmental Biology, Department of Ecology and Systematics, P.O. Box 7, FIN-00014 University of Helsinki, Finland. E-mail: Matti.Nummelin@Formin.FIThis paper gives the first description on the seasonality of an arthropod community in a non- managed forest in the East Africa and is the first one performed in the Eastern Arc Mountains. Lovett & Wasser in their book (1993) give a synopsis of the studies on the natural history of the Eastern Arc Mountains.MATERIAL AND METHODSStudy SiteThe Uluguru Mountains (7°S 38°E, max. height 2,646 m) are a part of the Eastern Arc Mountains. These are Precambrian formations uplifted during the Tertiary but assumed to have been forested since Gondwana times (see e.g. Lovett & Wasser, 1993). The study site is situated on the eastern slopes of Uluguru, near Morogoro Teachers College at about 700 m a.s.l. The sampled valley has a closed forest contiguous with the Uluguru Forest Reserve, which covers the higher altitudes of the Mountains. This is the only site in the Ulugurus where a continuous forest strip still exists down to this low altitude (although the strip is narrow, secondary and severely disturbed in nature). Rainfall usually has two annual peaks in this area (March-May and October-November, figure 1).Figure 1. Rainfall in 1996-1997 on the Uluguru slopes in Morogoro (data from Sokoine University Meteorological Station, about 5 km east of the study site). (Note exceptional lack of short rains in late 1996).The vegetation is submontane, semi-evergreen rain forest. Illegal, manual selective felling by local residents has reduced the number of canopy trees, mainly Albizia spp, Newtonia buchananii and Milicia excelsa. However, the canopy cover has remained and there are no large gaps. The herb layer covers about 45 % of the ground and is 40-120 cm tall. It is not characterised by any particularly dominant species (Pócs, 1974). Broad-leaved members of Graminae, sedges, and many other dicotyledonous plants (notably members of Leguminosae) form a very variable vegetation carpet. Dioscorea spp are the major climbers. The shrub layer is mainly 1-4 m tall. Lantana spp is dominant in open spaces.METHODSArthropod samples were collected monthly by sweep-netting. Each sample consisted of 800 sweeps. The sweep-net was a standard 38 cm diameter muslin insect net with a 61 cm handle (see e.g. Janzen, 1973a). Sweeps were made only in places where the net could hit the forest floor vegetation. Thus, open gaps in forest floor vegetation were avoided. Depending on the height of the vegetation, the minimum sweeping height varied from 20 to 80 cm. Sweeping took place on dry early afternoons when the vegetation was also dry. After 50 sweeps the net was emptied to a big plastic bag and sprayed with insecticide. Sorting and counting was done during the same and the following day in a laboratory. For more details on the sweeping method and equipment used see Janzen's seminal paper (1973a).Correlation was calculated between cumulative rainfall and arthropod numbers (statistical software: Microsoft Excel 5.0) and statistical significance obtained from tables in Ranta et al. (1989). Relative similarity of arthropod groups and sampling months were described by factor analysis (statistical software: Statistical Graphics Corporation: Statgraphics) and percentile similarity index (Renkonen index, Wolda, 1981).RESULTSThe number of arthropod individuals in the sweep-net samples varied greatly depending on the season (figures 1 and 2). The most common groups were: Araneae (Opiliones excluded since they occurred only occasionally), Diptera, Hymenoptera, Heteroptera, Homoptera, Coleoptera, Orthoptera, Lepidoptera, and caterpillars (almost solely Lepidoptera larvae). They made up over 95 % of all the arthropods caught by sweep net from the forest floor vegetation in the Uluguru valley. The group remaining 'others' consisted mostly of Phasmidae, Mantidae and Blattidae. Occasional Thysanoptera, Psocoptera, Collembola and Acarina were found in the samples.There are significant correlations between total numbers of arthropods caught and rainfall. The arthropod abundance correlated with the 30-days cumulative rainfall. However, there was a time lag in the response. The most usual time lag was 90 days i.e. cumulative rainfall 120-90 days prior the sampling time. The best fit for the correlation between the number of arthropods in the sample was searched with 0-, 14-, 30-, 60- and 90-day time lags (see Nummelin, 1996). The statistically significant correlations (p ⟨0.05, df=10) were as follows: total number of arthropods in the samples r=0.85, Diptera r=0.76, wasps r=0.79, Araneae r=0.71, Hemiptera r=0.94, Acrididae r=0.87, Formicidae r=0.63, Hymenoptera r=0.70, caterpillars r=0.64 (60), Lepidoptera r=0.84. The best fit in all significant correlations was with 90 days time lag except in caterpillars, which had best fit with 60-day time lag. The numbers of Coleoptera, Heteroptera and Tettigonidae had no significant correlations with the rainfall. In other groups the data were too small to make sensible analysis.In factor analysis over the seasons, the relative distances of Acrididae, Araneae, Lepidoptera and Formicidae are closer to each other than mean distances in two first axes factor loadings (figure 3). This indicates that there is a closer correspondence in the variation of the numbers of Acrididae, Araneae, Lepidoptera and Formicidae individuals over seasons than that of the other arthropod groups. Strong reaction to the rains pushes Diptera, Hemiptera and Heteroptera apart from the other groups.Figure 2. Seasonal occurrence of arthropods in sweep-net samples on the slopes of the Uluguru Mountains, near Morogoro, Tanzania.When the months are grouped with factor analysis, February, March and December are closer to each other than the other months (figure 4). When considering month to month change, there is little change in February-March-April and June-July-August, but a very marked difference between April and May obviously as the result of a reaction to the rains starting a montii earlier. The percentage similarity index (Renkonen Index: Wolda, 1981) of the arthropod community was in its minimum between April and July (47 %) as well as between January and August (52 %) indicating a difference in the arthropod community during late wet and dry seasons.DISCUSSIONThe total number of arthropod individuals caught in 800 sweep samples was of the same order magnitude when compared with similar samples in virgin forest sites in the Kibale Forest, Uganda (Nummelin, 1989, 1996). Kibale Forest resembles physionomically the Uluguru sampling site, but is twice as high from sea level. There is also a similarity in arthropod numbers in the secondary vegetation in Osa, Taboga and San Vito in Costa Rica, Central America (Janzen & Pond, 1975). However, the numbers are less than those recorded in mid summer in secondary vegetation in Michigan and England in the temperate zone (Janzen & Pond, 1975). This suggests that arthropods are rarer in tropical forest floor vegetation than in a temperate meadow. However, the arthropod activity is concentrated to a much shorter time period in the temperate regions, which increases their numbers within a shorter active period.Figure 3. Results of the factor analysis on sweep-net samples of Uluguru arthropods. Sampled arthropod groups are arranged on the basis of monthly samples (two first axes explain 84 % of the variation).Figure 4. Results of the factor analysis on sweep-net samples of Uluguru arthropods. Sampling months are arranged on the basis of arthropods community (two first axes explain 87 % of the variation).The community structure of arthropods in Uluguru forest floor does not differ drastically from that in Uganda rainforest (Nummelin, 1996) and Neotropical rainforests (Boinski & Fowler, 1989; Elton, 1973; Janzen, 1973a, b; Janzen & Schoener, 1968; Janzen et al., 1976; Nentwig, 1989; Tanaka & Tanaka, 1982).Compared with the arthropod samples collected with vacuum sampler from the herb-layer vegetation from a rain forest in South East Asia (Stork & Brendell, 1993), Uluguru seems to harbour less Thysanoptera, Psocoptera, Collembola and Acarina. However, it is known that the sweep method used in Uluguru is much more destructive to small specimens than vacuum sampling (during the study period no vacuum sampler was available in Tanzania). All the above- mentioned arthropods are small and mostly soft-bodied, therefore they are often destroyed by the sweep method, which is notoriously selective in favour of larger insects (Hespenheide, 1979). For further discussion of restrictions in the sweep method, see e.g. DeLong (1932), Beall (1935) and Janzen (1973a).Tanaka & Tanaka (1982) reported an approximate three-week time lag relative to rainfall in the correlation of abundance of various insect groups on the West Indian island Grenada. Thus, the response in insect numbers to the rainfall in Grenada appears to be quicker than that in Uluguru and Kibale.ACKNOWLEDGEMENTSFinnish support to forestry research in Tanzania FORST Project made this study possible. Morogoro Teachers Training College allowed access to the forested area where the study was conducted. We are also grateful to the comments we have received from reviewers.REFERENCESBeall, G. (1935). Study of arthropod populations by method of sweeping. 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