Summary1. An unresolved question in plant ecology is how the balance between positive and negative interactions changes with environmental conditions. Recently, the debate has been expanded by Brooker & Kikvidze (2008), reintroducing the importance, rather than the intensity, of interactions as the appropriate concept for empirical studies. Importance is the difference in performance with interactions relative to all affecting factors. 2. Positive interactions among plants (facilitation) are common in nature. However, the advocated importance indices fail to address facilitation and can thus not be applied to studying shifts in the balance between positive and negative interactions. The deficiencies of the current importance indices are both conceptual as well as mathematical. In particular, their maximum performance estimator is based on the assumption that it can only be attained in the absence of neighbours. 3. We suggest two major improvements. First, we use the target's overall maximum performance as a surrogate for the plant optimum, i.e. we allow the possibility of higher performance with neighbours (i.e. facilitation). Second, we propose an alternative approach to treat the factors affecting measured performance. The new index is limited in its range [)1, 1], is symmetrical for competition and facilitation, and it preserves the intuitive nature of the original index by Brooker & Kikvidze (2008). 4. Synthesis. The concept of interaction importance is useful for studying interactions along environmental gradients. In contrast to previous indices, our index accommodates facilitative interactions and thus offers a significant conceptual and methodological advancement. We advocate its use in future empirical studies.

Summary1. Abiotic stress and interactions with neighbours are major selective forces but their relative importance for local adaptation has rarely been separated. Plant community theory predicts increasing importance of competition with decreasing stress. We experimentally separated the role of neighbours and drought stress for local adaptation of two annual plant species. We predicted that neighbours amplify patterns of local adaptation with adaptation to competition prevailing in benign habitats. 2. During one growing season, we combined reciprocal transplants with neighbour removals along a gradient of more than an eightfold increase in annual rainfall using populations from sites representative of Arid, Semi-arid, Mediterranean and Mesic Mediterranean ecosystems. We evaluated statistical interactions between origin and planting site with and without neighbours and quantified plant-plant interaction importance.3. An extreme drought (<63% of the average annual rainfall at the arid sites) reduced the probability of detecting neighbour effects. There was weak but non-significant evidence for local adaptation and for the amplification of adaptation patterns with neighbours for populations from wetter sites of the two species. Our results together with previous findings in similar gradients suggest that detecting local adaptation to neighbours in unpredictable environments depends on the climatic conditions of the study season. 4. We coupled a central concept from plant community ecology with an evolutionary approach to separate the role of abiotic vs. biotic factors for local adaptation. This permits quantifying the importance of interactions with neighbours and we advocate its use in future studies of local adaptation, which should be conducted over several years.

The effects of postmaturation ambient temperatures, light, as well as chemical and structural properties of the substrate on germination and patch distribution of Stipa capensis in a shrubland landscape were studied. This species is a dominant annual grass in exposed intershrub areas covered with biological loess soil crusts in the northern Negev Desert. Freshly matured caryopses do not germinate. After 7-8 months of dry storage at high temperatures, there was a significant reduction in primary dormancy and an increase in the rate and percentage of germination of caryopses stored at high temperatures. The speed and percentages of germination were lower on various sterile substrates, such as organic matter and loose loess soil, in comparison with similar live substrates or the control on filter paper. In 3 of 4 years of field observations in semi-arid shrubland, the density of Stipa capensis plants on soil crust in the exposed intershrub areas was significantly higher than below shrubs. The positive effects of higher temperatures on the dry soil during summer, before the season with rains, and of light during germination, can favor germination in exposed patches. Although germination responses to storage temperature and light regime differentiate between patch types before and during germination, other processes may be critical for the pattern of distribution of Stipa capensis. These include dispersal of caryopses, arrival, soil penetration and density of local seed banks, as well as substrate properties affecting germination and plant density, in different landscape patches.

We studied the composition, species richness, and relative abundance of bat assemblages in the Colombian dry forests of Chicamocha and Patía. In Chicamocha, 11 bats of the family Phyllostomidae were captured with mist-nets, corresponding to 85-100% of the potential phyllostomids species in the area. Two bats of the family Vespertilionidae were also captured in Chicamocha. In Patía, 12 species were captured with mist-nets, all Phyllostomidae, representing 72-100% of the estimated total number of species in the zone. Minor differences in number of species and composition were detected among sampling periods in Chicamocha. The most common species in this dry forest were Glossophaga longirostris and Sturnira lilium. In Patía, notable differences in the number of species and composition were observed among sampling periods, and the most common species were Artibeus jamaicensis, Carollia perspicillata and Phyllostomus discolor. Arid-zone dwelling bats were absent in Patía and we suggest that this absence may be associated with the isolation of Patía from other northern dry zones of Colombia since Quaternary times. There was also low abundance of bats in Patía, which appears to be related to human disturbance. The most abundant phyllostomid bat species in the two dry forests studied are those that include fruit and/or nectar-pollen from columnar cacti as an important proportion of their diets.

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Abstract. Despite efforts to discern the role of plant size in resource competition, the circumstances under which size-dependent plant-plant interactions occur are still unclear. The traditional assumption is that competition intensifies with increasing neighbour size. However, recent studies suggest that the size (biomass) dependence of competitive interactions is strongest at very low biomass levels and becomes negligible after a certain threshold neighbour biomass has been reached. We searched for the generality of such patterns for three common annual plant species in Israel. We monitored target and neighbour biomass along their entire lifecycle using an even-aged, intraspecific and intrapopulation competition screenhouse experiment under water-limited conditions. For all focal species, neighbour presence had a net negative effect on vegetative biomass at harvest. However, this was not explained by increasing neighbour biomass over time, as a consistent pattern of sizedependent facilitative, rather than competitive, interactions was observed at all life stages. We explain these observations in terms of co-occurring aboveground facilitation and dominant belowground competition for water. Since our findings are the first of their kind and contradict theoretical predictions of biomass dependence of net negative interactions, we advocate further experiments addressing size dependence in interactions among plants. In particular, theoretical models addressing size dependence of positive interactions must be developed.

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