Testing the ecological communities of different areas for convergence, in the sense of remarkable similarity in the characteristics of the species present, has a long history in biology. Recently, numerical methods have been developed for comparing community-level convergence to an explicit null model. No valid method has been known for testing the significance of texture convergence when the species are weighted by their abundance. Six combinations of method variants are tested on random datasets. A valid P value (i. e., with P . 0. 05 in no more than 5% of the cases) is obtained so long as for each species the distribution of abundances across sites is retained, and only the assignment of character values is randomised. Further restriction is not necessary for obtaining a valid P value, and can lead to a test with considerably lower power to detect convergence. The power of the test with free matching of character values to species is only moderate with 10 sites, though improved with larger numbers of sites. Previous methods for detecting texture convergence have examined convergence only in the mean value for any character. It is possible that the external environment might be reflected in the community mean of a character, leaving the imprint of convergence on the shape of the distribution, rather than the mean. A method for comparing the shape is described, and it is shown that the null model is valid also for this test statistic.
Assembly rules are measures of community structure that link observed patterns with ecological processes, and as such may help to elucidate the mechanisms by which species coexist. We apply two approaches to a lawn community - limiting similarity and guild proportionality - hoping that agreement between them might give robust conclusions. We tested for agreement between these two assembly rules using functional characters that are related to two aspects of species function - light capture and response to defoliation. We combined point quadrat data and a null model approach to test for limiting similarity - a tendency for species differing in functional characters to co-occur more often than expected at random - in turves extracted from the lawn community. Examining the variance in the characters of the species co-occuring at each point, evidence for limiting similarity was found for mowing removal (the proportion of leaf area removed in mowing events). There was greater variation between the species co-occurring at a point than expected at random (i.e., under an appropriate null model). However, no such evidence was found for characters related uniquely to light capture, such as specific leaf area and pigment concentrations. In a previous study in the same community, “intrinsic” guilds had been determined from co-occurrences within the lawn community and from a competition experiment, as those effective in determining species assembly and co-existence. These intrinsic guilds are shown by t-test to differ in the proportion of their biomass removed in mowing (MRI), which is of course related to the height at which their leaf area is held. However, again no differences were seen in characters related uniquely to light capture. Thus, the two different approaches to assembly rules - guild proportionality and limiting similarity - agree that differences in response to mowing are responsible for species co-existence in the lawn community. The agreement between these two approaches, tested on independent datasets of quite different type from the same community, gives possibly the strongest evidence so far that niche differentiation may be responsible for local co-existence in plant communities. However, although MRI is related to this co-existence the lack of correlation with light-capture characters leads to speculation that the effects might be via the below-ground behaviour of the species.
Authors:R. Ohlemüller, P. Bannister, K.J.M. Dickinson, S. Walker, B.J. Anderson, and J.B. Wilson
Both local and regional factors determine local species richness. We investigated the relative role of local (13 soil and tree stand structure variables) and regional factors (19 climate, land cover and geographic location variables) in determining the richness of several vascular plant functional groups in indigenous forest fragments in southeastern New Zealand. The predictor variables explaining the largest fraction of the variation in species richness were identified using a backward and forward stepwise procedure, with adjustments for the number of variables and testing for multicollinearity. The total proportion of variation explained by local and regional factors was highest for tree species richness (54.1%) and lowest for herbaceous species richness (28.2%). We found differences between the functional groups in the extent to which species richness was explained by local vs. regional factors, but both showed some ability to explain the species richness of all functional groups. The abundance of the strongly competitive tree species Nothofagus menziesii (silver beech) had a strong negative effect on total, tree and herb species richness, but it had only minor effects on woody and fern species richness. Once the effect of this local variable was accounted for, the remaining variation in tree and woody species richness was mainly explained by regional variables. Herbaceous and fern species richness, on the other hand, was strongly correlated with local as well as regional variables. We emphasize the importance of using a fixed plot size when the relative effects of local and regional factors on patterns of species richness are to be compared and evaluated.
Authors:J.S. Khokhar, S. Sareen, B.S. Tyagi, L. Wilson, I.P. King, S.D. Young, and M.R. Broadley
Correlations between juvenile wheat root traits, and grain yield and yield component traits under optimal field conditions have previously been reported in some conditions. The aim of this study was to test the hypothesis that juvenile wheat root traits correlate with yield, yield components and grain mineral composition traits under a range of soil environments in India. A diverse panel of 36 Indian wheat genotypes were grown for ten days in ‘pouch and wick’ high-throughput phenotyping (HTP) system (20 replicates). Correlations between juvenile root architecture traits, including primary and lateral root length, and grain yield, yield components and grain mineral composition traits were determined, using field data from previously published experiments at six sites in India. Only a limited number of juvenile root traits correlated with grain yield (GYD), yield components, and grain mineral composition traits. A narrow root angle, potentially representing a ‘steep’ phenotype, was associated with increased GYD and harvest index (HI) averaged across sites and years. Length related root traits were not correlated with GYD or HI at most sites, however, the total length of lateral roots and lateral root number correlated with GYD at a sodic site of pH 9.5. The total length of lateral roots (TLLR) correlated with grain zinc (Zn) concentration at one site. A wider root angle, representing a shallow root system, correlated with grain iron (Fe) concentration at most sites. The total length of all roots (TLAR) and total length of primary roots (TLPR) correlated with grain S concentration at most sites. Narrow root angle in juvenile plants could be a useful proxy trait for screening germplasm for improved grain yield. Lateral root and shallow root traits could potentially be used to improve grain mineral concentrations. The use of juvenile root traits should be explored further in wheat breeding for diverse environments.