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ANIBARA

Association humanitaire et scientifique pour le Burkina Faso.

L’association s’est basée depuis sa création sur la construction et la mise en œuvre d’un petit campement au sud ouest du Burkina, près de Banfora : le Tilapia.
L’écotourisme et les expéditions scientifiques sont les ingrédients du succès de ce petit campement.
L’association cherche aussi à favoriser les échanges scientifiques entre les muséums de Ouagadougou et de Paris.
L’association vise également à promouvoir l’écotourisme et l’environnement, en diffusant des plaquettes dans les offices de tourisme et dans les associations naturalistes françaises.

Siège : Quartier Saint Pancrace, 3241 route de Très 06440 l’Escarène
anibara@hotmail.fr

Top-down and bottom-up effects on survival and migration of a neotropical herbivore.
Field experiments with nymphs of the stick insect Metriophasma diocles
jeudi 9 février 2006
par Arno
Juergen Berger & Rainer Wirth

Abteilung Allgemeine Botanik, Universität Kaiserslautern, Postfach 3049, 67653 Kaiserslautern, Germany, email : jrberger@rhrk.uni-kl.de

We studied top-down and bottom-up effects on first instar nymphs of the oligophagous stick insect, Metriophasma diocles. Prior investigations revealed that these herbivores are oligo-phagous specialists feeding on plants from the Piperaceae and Araceae, exclusively. In the tropical moist forest forest of Barro Colorado Island (Panama) they occur in low abundances. The purpose of this study was (1) to quantify top-down pressure on stick insect nymphs and (2) to directly link migratory activity of nymphs to predation avoidance or (3) to bottom-up factors, in particular to food availability.

We performed four 2-week long exclusion experiments following survival and migratory activity of a total of 238 nymphs in exclosures and controls during the rainy season. The exclusion of predators resulted in an almost threefold higher survival (69%) compared to nymphs on control plants (24%). As predation was significantly higher at night, birds can be excluded as relevant predators of these herbivores and motivate further studies to discriminate between nocturnal and diurnal predation rates.

After considering intrinsic mortality factors we estimated that 45% of the nymphs in controls died through predation. This is the first quantification of predation pressure on nymphs of hemimetabolous insects and within the range reported for holometabolous herbivores.

The rates of migration of nymphs off of plants could not be explained by the presence of predators and consequently does not reflect predation avoidance. Instead, we found evidence that emigration of nymphs was triggered by food availability.

In conclusion we state that top-down pressure in this system affects the abundance of M. diocles severely, whereas bottom-up forces may rather determine the herbivore’s distribution. to discriminate between nocturnal and diurnal predation rates. Survival did not differ between runs in the exclosures, but was highly and significantly different between runs in the controls.

Hence short term observations about predation pressure on insect la rva may only inadequately approach predation impact during larval development. Spatial effects of predation differed to a lower extent. Mortality in the controls varied between plots indicating locally increased predation pressure.

Patterns of predation pressure on a neotropical herbivore

Juergen Berger & Rainer Wirth

Abt. Allgem. Botanik, Universitaet Kaiserslautern, email : jrberger@rhrk.uni-kl.de

We studied top-down effects on first instar nymphs of the oligophagous stick insect, Metriophasma diocles. The purpose of this study was (1) to quantify top-down pressure on stick insect nymphs, and (2) to assess spatio-temporal patterns in predation rates.

We performed four 2-week long exclusion experiments in the forest of Barro Colorado Island, Panama. In total we followed survival of 238 nymphs in exclosures and controls during the rainy season.

The exclusion of predators resulted in an almost threefold higher survival (69%) compared to nymphs on control plants (24%). After considering intrinsic mortality factors we estimated that 45% of the nymphs in controls died due to predation. This is the first quantification of predation pressure on nymphs of hemimetabolous insects and is within the range reported for holometabolous herbivores.

Mortality was temporarily and spatially heterogeneous. The effects of predation were significantly increased at night. As a consequence, we concluded that birds as one prominent predator group were not responsible for predation on these herbivores and encourage further studies to discriminate between nocturnal and diurnal predation rates. Survival did not differ between runs in the exclosures, but was highly and significantly different between runs in the controls.

Hence short term observations about predation pressure on insect la rva may only inadequately approach predation impact during larval development. Spatial effects of predation differed to a lower extent. Mortality in the controls varied between plots indicating locally increased predation pressure.