An intense debate is underway on the different approaches to measuring the importance of neighbour interaction. Both the ecological meaning and the statistical suitability of one of the most popular indices have been seriously questioned, but no simpler and practical alternative tools have been proposed up to now. This paper proposes a novel approach based on the use of new normalized indices which scale the effects of neighbours and environment to the maximum target-plant potential. Two indices related to environmental suitability and size-asymmetry are suggested as tools to stratify data in homogeneous subsets before analysis, and an index of normalized neighbour effect (Nn) is proposed to integrate the measuring of neighbour importance and intensity. When tested on literature data, Nn index proves to be very highly correlated to the most currently used importance index. At the same time, it is moderately but significantly correlated to the intensity index. Yet, an accurate reanalysis of three published datasets proves that several detected trends are predictable on the basis of the inherent properties of the used indices. This is inextricably linked to the use of the same phytometers at different productivity levels. Thus, a glimpse is proposed towards the opportunity to use groups of equivalent competitors, each one working at a different point of the gradient, but all in a comparable range of environmental suitability and potential size-asymmetry relative to neighbours. Once defined these equivalence conditions, the normalized Nn metric is suited to measure how the relative weight of neighbour impact changes along the productivity gradient.
Authors:E. Ruprecht, S. Bartha, Z. Botta-Dukát, and A. Szabó
Studies addressing the question of how communities develop reported contrasting temporal patterns of species associations during succession. Several hypotheses were formulated about succession, but a general explanation of community assembly is missing. We analysed trends of species associations during old-field succession in two contrasting habitats: the first with chernozemic brown forest soil and temperate climate, and the second with sand soil and dryer climate. Significant pair-wise associations were calculated across a range of spatial scales. Comparing the two succession seres, one under harsh and the other under favourable environment, we attempted to make generalisations about species relation patterns. We found no trend but fluctuation in the level of community organization during succession. None of the existing succession models explained our results about changes in spatial structure of grassland communities during succession. Fluctuation in the number of significant associations was more intense and took longer under less favourable environmental conditions. Our results suggest that the stressed habitat type posed stronger constraints on species coexistence during succession than the favourable habitat did, but validating this hypothesis needs further investigations.
Authors:R. González-Vaquero, A.-I. Gravel, and M. Devoto
Numerous studies analyze the interactions between plants and their pollinators in ecological communities using a network approach. However, field studies rarely record all the interactions occurring in the field. In this sense Natural History Collections (NHCs) can provide information on interactions that may have been missed by field sampling. In this study we compare a network based on field sampling with a network based on data retrieved from specimens at NHCs, and we assess the degree to which these two sources of data are complementary. For this we used data available from a bee biodiversity study conducted in Southern Argentina for the South American bee genus Corynura (Halictidae: Augochlorini). Data on the floral associations of the specimens at NHCs were retrieved from the specimens’ labels, as the name of the plant species on which a given bee was captured is often recorded for many specimens at NHCs. Although field sampling recorded an unusually high number of insect-plant interactions, it misses some unique interactions present in the NHCs networks. Some structural properties of these networks are briefly analyzed, and usefulness and limitations of using NHCs data are discussed. We conclude that the information about insect-plant interactions extracted from NHCs could complement field-based data, especially in poorly sampled communities.
Authors:N. Naz, M. Hameed, M. Sajid Aqeel Ahmad, M. Ashraf, and M. Arshad
Five distinct habitats along salinity gradient were explored for plant ecological attributes including soil plant interaction, vegetation composition and species distribution in the Cholistan desert. Higher saline sites supported
Aeluropus lagopoides, Cymbopogonjwarancusa, Ochthochloa compressa, Haloxylon recurvum
, whereas moderately saline habitats supported predominantly
Fagonia indica, C. jwarancusa
. The community structure and composition of each habitat type were very specific, the most dominant component being
. Each species has very specific relation to different environmental variables, and this reflects the habitat status, ecological adaptations and stress tolerance degree of the individual species. On the whole, it can be concluded that salinity alone was not responsible for the distribution of species at salt affected habitats.
The Plantaginaceae family has a wide distribution in Jordan during spring mainly in the northern and southern highlands and even in Jordan valley regions parallel to Jordan River. Plantago is one of the most important genera of the family with approx. 25 species. The objective of this work was to study the pollen morphology as well as to investigate the pollen visitors of Plantago arabica Boiss. Preliminary data indicate the presence of one species of stinging bee and three species of stingless bee as pollen visitors. The pollen of Plantago arabica Boiss. is pantoporate without annulus and operculum, circular, verrucate with few or no puncta, sexine scabrae, nexine surface is almost not well-defined as pollen ornamentation. In a word, Plantago arabica Boiss. as well as other Plantago species in Jordan produce a large amount of whitish polliniferous dust and constitutes a potential source of pollen for different species of bees, providing an interesting field for germination studies, insect-plant interactions and pollen morphology that are already under way.
Small-scale vegetation patterns are frequently the results of plant-plant interactions such as facilitation and competition. Facilitation should be particularly pronounced when both abiotic and biotic stresses are high, but few studies were conducted in such habitats. In heavily-grazed pastures on the eastern Tibetan Plateau, an area with both high abiotic stress and strong biotic disturbance, we made relevés of herb species both beneath and outside canopies of three shrub species (Spiraea alpina, Sibiraea angustata and Potentilla fruticosa) differing in palatability and canopy structure. Herb species richness (S), pooled cover (PC) of all species, number of flowering species (FS) and number of inflorescences of all species (IN) were greater outside than beneath the shrub canopies. Evenness (J), in contrast, was smaller outside, while Shannon’s diversity index (H) was the same. Differences in S and J between plots beneath and outside the shrub canopies were greater in the case of P. fruticosa than in the cases of S. angustata and S. alpina, but differences in PC, FS or IN did not depend on the shrub species. Among the common species (frequency ≥6), 47–85% were equally frequent beneath and outside the shrubs, 13–39% were more frequent outside and 3–13% were more frequent beneath the shrubs. For the rarest species (frequency < 6), however, more species occurred beneath than outside the shrubs. The ordination diagram showed a clear separation between the relevés outside and beneath the shrubs and a gradient from P. fruticosa via S. alpina to S. angustata, accompanied by a distinct decrease in the extent of the difference between the vegetation beneath and outside the shrub canopies. In conclusion, the three shrub species facilitated some species in the herb layer and each shrub species had a specific impact, related to its canopy structure and palatability but also to the grazing pressure, which was greater around the P. fruticosa shrubs than around S. alpina and S. angustata.