Guild structures and biomass of spiders in forests and agroforestry systems in central Amazonia, Brazil FLORIAN RAUB & HUBERT HÖFER State Museum of Natural History Karlsruhe contact: fl
[email protected];
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Financial support was granted by Federal Ministry of Education and Research, Germany (BMBF)
Different polyculture systems in a matrix of primary and secondary forest.
Introduction An analysis of the size distribution of adult spiders showed a distinct pattern in the primary forest with small spiders being most frequent, whereas in poly- and monocultures more medium sized spiders were captured (Tab.2, Fig.2).
Data were collected during the German-Brazilian research program SHIFT (Studies on Human Impact on Forests and Floodplains in the Tropics). The studied sites are situated in Central Amazonia, 30 km outside the city of Manaus, within the experimental area of the Brazilian research institute Embrapa - Amazonia Ocidental (02°53’S, 59°59’W).
Tab. 1. Size distribution of all spiders of Berlese sampling pooled together per system. (FLO - primary forest, SEC - secondary forest, POA - polyculture A, POC - polyculture C)
FLO SEC POA POC
Number Ind
< 1 mm
1-5 mm
> 5 mm
1204 935 498 436
133 124 48 137
1065 805 450 298
6 6 0 1
% < 1 mm % 1-5 mm % > 5 mm 11.0 13.3 9.64 31.4
88.5 86.1 90.4 68.4
0.498 0.642 0 0.229
Tab. 2. Size distribution of all adult spiders of the pitfall sampling pooled together per system. Fig. 1. Size distribution of spiders (Berlese).
dates pooled together showed a significant influence of the forest type on the size distribution (Tab.1, Fig.1). This difference is only found significant for spiders of the “soil fraction” but not the “litter fraction” of the soil cores. Polyculture with two crop species and a legume cover crop. (Foto: P. Schmidt)
Material and methods Soil/litter fauna has first been collected at two sites of a polyculture agroforestry system, one site of secondary forest, and one site of primary forest by eightfold repeated sampling between July 1997 and March 1999 with 20 soil cores (diameter 21 cm) from each system, separated into litter and soil fractions and extracted by a Berlese apparatus.
Soil core sampling
The highest spider diversity at family level was observed in the primary forest, and the lowest in the secondary forest. Mygalomorph spiders were nearly exclusively found in the undisturbed forest, only single individuals were found in the anthropogenic influenced areas. Overall 21 araneomorph families were identified, 18 in the primary forest, 14 in the secondary forest and 16 at the polyculture sites. The Oonopidae dominated, followed by Symphytognathidae, Ochyroceratidae and Salticidae. Web builders were less abundant than stalking, ambushing and running ground spiders. Overall these “active” feeding/hunting strategies seem to dominate “passive” web-building at all sites. Ochyroceratids and symphytognathids were the only abundant web-building spiders. Sheet web builders (e.g. mygalomorphs) had practically disappeared in the disturbed sites.
Results and discussion 1. Soil core sampling In total 3,073 spiders were collected from the soil cores, 1,204 in the primary forest, 935 in the secondary forest and 934 in the two polyculture sites together. In the primary forest (FLO) we sampled a mean of 221 individuals m-2, in the secondary forest (SEC) 178.6 and in the timber cultures 184.4 (POA) and 158.8 (POC) individuals m-2. The difference in spider biomass in Berlese samples between the primary forest and disturbed areas (secondary and polycultures) was distinct: 75.1 mg m-² in primary forest (set 100%), 46.2 mg m-² in the secondary forest (62%) 23.1 mg m-² in the polycultures (31%). This difference is only partly caused by the higher abundance, but also by different body size distributions. The analysis of the body length of all spiders of all sampling
< 1 mm
1-5 mm
> 5 mm
156
110
13
55.9
39.4
4.7
65
152
19
27.5
64.4
8.1
26
79
10
22.6
68.7
8.7
Primary 279 Forest Rubber 236 Polycult. Rubber 115 Monocult.
% < 1 mm % 1-5 mm % > 5 mm
Fig. 2. Size distribution of spiders (Pitfall).
Comparing pitfall and Berlese sampling similar size distribution patterns were recognized. Most sampled individuals were between 1 and 5 mm in both methods. Remarkable is the high number of spiders (i.e. ochyroceratids) less than 1 mm in the primary forest captured in pitfall traps.
Pitfall trap
From June to August 2001 spiders were sampled with pitfall traps during 8 weeks in 16 replicate plots of another polyculture system, another natural forest and a rubber tree monoculture. We analysed the data for expected differences between natural and anthropogenic sites in abundance, body size distribution, biomass and guild structure of spiders.
Number Ind
Theraphosa blondii (Theraphosidae)
Ctenus amphora (Ctenidae)
2. Pitfall sampling From the activity based pitfall samples we identified 1,001 spiders (686 adults) of 27 families. We captured a mean of 3.3 adult spiders per trap: 4.4 in the primary forest, 3.8 in the poly- and 1.8 in the monoculture. In the pitfall traps sheet web building mygalomorphs were abundant only in the primary forest and litter stalking oonopids were most abundant in the polyculture. In the primary forest the small and fragile nocturnal ground weaving ochyroceratid spiders were the dominant family, followed by oonopids and the sedentary web-weaving pholcids, lycosids were not captured at all. To our surprise in the pitfall samples slightly more web builders (513) were captured than hunters or stalkers (488). Family diversity was highest in the polyculture and lowest in the monoculture. The biomass of adult spiders in pitfalls was highest in the primary forest (set 100%) and distinctly lower in the polyculture (32%) and in the monoculture (20%), but this difference was not statistically significant at the 5% level.
Conclusion Differences in abundance, biomass and guild structure of spider assemblages between primary forest and disturbed (anthropogenic) sites in central Amazonia were shown by soil core sampling and pitfall trapping. Ochyroceratids are frequently sampled in pitfall traps in the primary forest, these tiny web builders and small mygalomorphs (Microstigmatidae, Idiopidae, Dipluridae) do not nearly appear as frequent in plantations and secondary forest, where in contrast lycosids and oonopids are abundant.
Nops sp. (Caponiidae)
