The Invertebrates of the Grass Layer
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INVERTEBRATES OF THE GRASS LAYER Among primary consumers, the short-horned grasshoppers (Acrididae) rank first in the two areas The most abundant invertebrates of the savanna studied; at the beginning of the dry season they layer are the arthropods. Among them, spi- represent 49.7% of the arthropod biomass at Fété s mites (Acari) has not yet been elucidated Argentinian chaco. Other primary consumers well vanna vegetatioa- Zaracas. Aciu Bor. viduals per square metre in some natural grass- feed upon grass. mode titiirteti/cvd By far, the most numerous invertebrate secon- tad de Ciencias. dary consumers in the two African sites studied are the spiders. At Lamto, they make up from 15 to reconstruction of 29% of the arthropod biomass according to the DOMINANT GROUPS OF SAVANNA Zev., 33. 376-396. season, and 4 to 12% at Fété Olé. In both areas, four families are particularly well represented in the s en lu GUUIUM The relative importance of the various taxo- spider community, but in different proportions: the nomic categories of arthropods is very different in Thomisidae, Salticidae, Drassidae and Lycosidae open savannas and in tropical forest habitats, (Blandin, 1974; D. Gillon and Gillon, 1974). although comparison is made difficult by the lack Together with the ants (see Ch. 24), their trophic of any reliable sampling method for the forest impact upon other invertebrates must be consider- canopy. Furthermore, some arthropod groups play able. In the savannas of the Argentinian chaco, the presented in West African savannas are the The relative numbers and biomasses of the major Mantodea among orthopteroid insects, the arthropod groups in the grass layer of two West Reduvidae among Hemiptera, the Diopsidae, African study sites are represented in Fig. 11.1 (see Chloropidae, Bombyliidae, Dolichopodidae and also Tables 11.1 and 11.2 for Lamto). Tachinidae among Diptera, the Meloidae among 290 Y.GILLON LAMTO FÉTÉ ,OLÉ .......I .. .. ... ..'. ...I... individuals per 100 sq.m. ,.... El dry weight per 100 sq.m. Fig. 11.1 Numbers and biomasses of the major groups of grass layer arthropods, in two contrasted West African savannas, at two different times of the year, the rainy season (left) and the dry season (right). After Gillon and Gillon (1974). Coleoptera, and the Sphecoidea among to 58% of the total arthropod biomass depending Hymenoptera. on the season; two species were even particularly Saprophages are numerous among savanna ar- abundant during the dry season' (D. Gillon and thropods. The most common are obviously the Gillon, 1974). Other groups well represented in termites (see Ch. 23), but other groups are also xeric African savannas are the Solifugae and SCOT- ' well represented, such as the Calliphoridae and pions among the Arachnida, and the Embiopten Drosophilidae among Diptera. The silverfishes among the insects. (Thysanura), cockroaches (Blattidae) and crickets The seed-eating insect community is also prob- (Gryllidae) are far less numerous in savannas than ably different in dry and humid tropics (D. Gillon in forest habitats. The same is true for Diplopoda and Gillon, 1974). The few studies carried out SO f3r I (Chilognatha and Polydesmoidea). on seed beetles (Bruchidae) all point out that thest Despite many common characteristics, there are seed predators are extremely host-specific (Cenla however some significant differences between the and Johnson, 1974; Janzen, 1977). arthropod communities of the driest and the more The savanna/forest boundary sometimes has humid savannas. Some of these differences are also a peculiar insect fauna. Pollet (197% for shown on Fig. 11.1 which compares the relative example, has found in the Ivory Coast tbc importance of the major taxonomic groups at Lygaeidae, Chrysomelidae, Languridae. * different seasons at Fété Olé and Lamto. The most punculidae, Tenthredinidae and Chalcidid3' obvious is the predominance of tenebrionid beetles were particularly numerous at the forest/savanno (Tenebrionidae) in the Sahelian savanna. Nineteen ecotone. Drosophilidae are also very ,Umerous species have been found there, representing from 19 there, more than 100 species having been NA INVERTEBRATES 29 1 of the various groups of Arthropoda in the grass layer of the Lamto savanna (Ivory Coast) during a yearly cycle 0.09 0.02 0.05 0.02 0.05 0.07 0.02 1.55 0.37 0.54 0.37 1.12 0.97 1.65 3.47 1.65 1.05 2.05 2.02 0.35 0.57 0.27 0.20 0.37 0.65 0.60 0.87 0.40 0.52 0.40 1.32 0.17 1.95 2.36 2.47 1.90 2.07 1.87 1.67 1.47 1.20 0.90 1.30 0.05 0.05 006 0.05 0.45 0.45 0.80 1.10 0.45 0.37 0.50 0.42 0.05 0.12 0.25 0.87 0.15 0.05 0.10 1.85 0.40 0.40 0.77 0.60 2.37 4.42 3.50 4.15 2.75 3.37 1.75 0.02 0.17 0.07 0.25 0.27 0.52 0.90 1.95 1.82 1.10 0.12 0.32 0.30 0.60 0.85 0.45 0.27 0.65 0.80 0.45 0.50 1.20 0.72 0.05 0.50 0.30 0.35 0.12 0.22 0.22 0.10 0.17 0.27 0.50 0.25 0.47 0.69 2.25 1.70 0.70 0.45 0.55 0.70 0.17 0.22 0.05 0.17 0.22 0.39 0.60 0.55 0.67 0.92 1.47 0.40 0.70 0.72 0.72 0.85 0.62 2.66 2.02 1.75 2.65 3.17 5.15 3.70 4.45 2.42 1.67 0.25 0.05 0.17 0.25 0.27 0.57 0.92 0.42 0.72 0.47 0.50 0.30 0.17 0.12 0.10 0.02 0.17.0.15 0.12 0.10 0.05 0.10 0.07 0.30 0.12 0.10 0.15 0.05 0.22 0.35 0.15 0.45 0.60 0.10 2.12 4.07 0.45 1.45 1.50 2.75 2.20 3.40 0.95 1.20 1.82 3.37 0.82 0.50 1.06 1.02 18.52 2.95 2.57 2.57 4.50 7.12 3.15 2.47 1.00 1.12 1.00 1.05 0.32 0.60 0.35 0.67 0.50 2.40 1.72 1.37 0.22 0.27 0.65 0.32 0.35 0.57 0.40 0.45 0.57 0.82 0.62 0.22 3.27 0.30 0.95 0.55 0.40 1.25 0.67 1.65 2.80 2.92 1.17 1.07 0.02 0.02 0.17 0.14 0.17 0.02 0.30 0.12 1.25 0.35 0.07 However, the comparison of the live weights of all the arthropods sampled in the same way, by the same observers, on two 25-m2 plots, in two very different West African savannas (Fété Olé and between individuals in the two samples. The live weight of an average arthropod was 6.7 mg at Fété Olé, as compared with 10.2 mg at Lamto (D. Gillon and Gillon, 1974). Two explanations come to mind Tropical insects are usually famous for their large to account for such a size difference. First, the 292 Y. GILLON TABLE 11.2 Biomasses (mg dry wt. m-*) of the various groups of Arthropoda in the grass layer of the Lamto savanna during a yearly cycle (D. Gillon and Gillon, 1974) - Jan. Febr. March April May June July Aug. Sept. Oct. NOV. Dec. - Spiders 100.37 45.12 42.50 76.25 39.85 59.60 59.10 79.58 39.48 72.98 84.17 78.90 Opiliones 0.47 0.40 0.61 0.37 0.60 0.60 Scorpions 1.37 0.35 0.27 0.91 0.60 1.42 0.25 Blattaria 10.25 0.62 5.22 2.37 4.58 13.17 11.01 23.25 16.36 10.15 34.83 30.50 , Mantodea 3.06 5.12 18.56 8.87 6.52 15.47 10.77 9.62 3.51 41.21 4.36 13.37 Acridids 11.25 51.37 86.25 155.12 72.93 48.80 53.86 38.63 54.75 61.73 72.05 161.00 Tetrigids 0.44 0.62 0.69 0.50 5.38 7.90 7.00 15.41 4.56 3.48 4.86 4.75 Tridactylids 0.09 0.15 0.51 1.40 0.42 0.06 0.18 Gryllidae 7.12 15.75 9.47 55.00 11.38 19.16 23.87 24.71 23.48 16.41 51.73 19.69 Tettigonids 1.25 10.56 2.81 9.71 13.08 5.96 2.76 9.72 80.35 129.36 2.77 Pentatomids 2.87 4.56 6.75 11.68 9.02 5.71 19.98 17.85 8.33 6.43 20.07 9.12 Coreids 0.87 5.25 5.62 3.07 2.10 2.70 1.80 0.38 1.30 2.51 3.87 Lygaeids 0.62 1.31 1.06 5.75 3.70 1.26 1.10 2.32 0.90 0.50 0.51 0.50 Reduvids 1.87 3.00 3.39 9.50 2.62 9.78 8.40 11.66 4.32 8.77 24.06 5.62 Homoptera 0.75 2.00 6.34 7.00 3.38 8.40 9.02 10.46 5.28 10.50 9.81 6.25 Carabid beetles 2.00 0.25 3.81 8.87 5.76 7.58 12.43 4.63 15.86 15.43 11.18 5.65 Coleoptera (others) 6.00 6.75 9.06 8.65 48.32 15.20 18.28 12.58 16.30 25.80 15.15 16.87 Lepidoptem 29.31 14.81 24.47 16.90 2.50 8.57 21.92 16.07 4.63 32.73 95.13 19.87 ' Hymenoptera 1.50 1.37 4.65 3.12 2.22 1.62 1.02 1.23 0.73 4.66 3.85 0.87 Diptera 7.62 0.87 3.19 1.62 1.75 3.70 1.86 2.68 6.36 7.77 5.37 4.31 Others 0.12 1.25 0.68 0.93 2.01 0.21 1.01 1.46 2.00 0.85 Toial 185.15 156.89 243.27 380.56 235.05 241.67 270.82 277.61 216.96 403.59 571.93 384.94 large size would not therFfore be adaptive for most 5.3 mg in September in northern Senegal and arthropods of the grass layer in the Sahel.