Body size determines the outcome of competition for webs among alien and native sheetweb spiders (Araneae: Linyphiidae)

Abstract 1. Biotic invasions are one of the most important reasons for changes in biodiversity. The alien sheetweb spider Mermessus trilobatus (Araneae: Linyphiidae) has become abundant in large parts of Central Europe within the past three decades. Its invasion might negatively influence native spiders, for instance via competition for webs. 2. Laboratory experiments were developed to test if M. trilobatus is competitively superior to native spiders. The alien M. trilobatus and five native sheetweb spiders (Erigone dentipalpis, E. atra, Gnathonarium dentatum, Dicymbium nigrum and Micrargus herbigradus) were compared with respect to their success to take over occupied webs from E. dentipalpis. 3. The rate of web takeover or defence was determined by body size, whereby individuals with a wider thorax invaded webs more successfully. After taking body size into account, the frequency of defence or web takeover did not differ between species. In 13% of all confrontations, predation against generally smaller opponents was recorded. Contrary to the predictions, raising the web value with food resources did not raise the effort expended on web defence but reduced predation by the web owner. 4. The current study does not indicate that the invasion of the relatively small‐sized M. trilobatus is facilitated by strong competitiveness. Nevertheless, M. trilobatus may displace smaller‐sized immature specimens and thereby threaten native spiders. Still, other reasons are likely to underlie the success of M. trilobatus in Europe, such as rapid reproduction or release from natural enemies.     

Vieillissement de l'appareil photosynth?tique I. Effet synergique de la lumi?re et du vieillissement in vitro sur les changements de volume de chloroplastes isol?s d'?pinard

In view of the difficulty of studying all of the aspects ofageing of the photosynthetic apparatus in vivo and of the actionof light upon this phenomenon, the consequences of chloroplastageing were investigated in vitro, especially on the volumeof these organelles. It was found that at 20?C in a Tris-NaCl medium, chloroplastswelling occurred slowly in the dark for the 6 hr experimentalperiod, while it was activated in the light, with an optimumafter 90 min. The most characteristic feature of these 2 curvesis that they cross each other after 5 hr, indicating that, independentof the previous treatments (dark or light), the chloroplastsin vitro reach the same volume after a few hours. Special attentionwas devoted to the pH effect. In dark and light conditions,swelling was found to be maximum at pH 7.5 to 8.5 and was independentof the buffer used. There was no major difference between swellingin Tris and Tricine. A marked depression in the chloroplastvolume was observed at pH 4.5 in the dark and at pH 5.5 in thelight. Simultaneous with the swelling phenomenon, the capacity of chloroplaststo carry out light-dependent shrinkage diminished. There wasa striking parallelism between swelling rate and loss of chloroplastshrinkage capacity. The action of light is synergetic on boththese processes in that it accelerated to the same extent boththe dark-swelling and the inhibition of light-dependent shrinkingin chloroplasts incubated in the dark. (Received May 24, 1969; )     

Vieillissement de l'appareil photosynth?tique II. Effet synergique de la lumi?re et du vieillissement in vitro sur les activit?es photochimiques de chloroplastes isol?s d'?pinard

The consequences of chloroplast ageing in vitro were furtherinvestigated, especially on the photochemical activities ofthese organelles. Ageing of chloroplasts in dark was accompanied by decreasesin activities for photohydrolysis and cyclic and non-cyclicsyntheses of ATP, photoreduction of NADP⁺ and O₂ evolution;but there was no decrease in ferricyanide photoreduction. Therates of decrease in these activities were comparable to therate of increase in chloroplast volume. Complete inhibitionswere reached when maximum chloroplast swelling had occurred,i.e. after 5 to 6 hr of incubation at 20?C in a Tris-NaCl (pH8) medium. Ageing in the light resulted in much accelerateddecreases in activities tested; the loss of capacity for light-inducedshrinkage was also accelerated by the light during ageing. Thus,light acts synergetically towards the ageing process. Moreover,light and, to a less extent, dark ageing, resulted in an uncouplingof chloroplast photophosphorylation and associated electronflow measured by ferricyanide photoreduction. The part of theelectron flow which is induced by coupling (+ ADP, Pi, MgCl₂,pH 8) or by uncoupling (+ NH₄C1, pH 7) was found to be verysensitive to light ageing. The NADP⁺ photoreduction loss wasrestored by addition of the ascorbate-DCPIP electron donor system,suggesting that ageing interferes with the integrity of photosystemII. In many respects, these effects of ageing are comparable withthe action of detergents and fatty acids on the structure andphotochemical activities of chloroplasts, especially in thatthey attack the energy transducing mechanism in chloroplasts. (Received May 24, 1969; )