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Background: Twelve distinct explanations have been proposed for the co-existence of species in ecological communities. Types of mechanism: The mechanisms can be divided into those that are stabilizing, i.e. with an increase-when-rare mechanism, and those that are equalizing, the latter on their own only delaying the exclusion of species. However, by evening out fitness, equalizing mechanisms can facilitate the operation of stabilizing mechanisms. Importance: It is suggested that circular interference networks, co-evolution of similar interference ability, cyclic succession, equal chance (neutrality) and initial patch composition are likely to be unimportant, or perhaps not even occur. Equal chance is an equalizing mechanism. Allogenic disturbance, alpha-niche differentiation, environmental fluctuation (relative non-linearity and/or the storage effect) and pest pressure are probably important. All four are stabilizing. More evidence is needed on aggregation, interference/dispersal trade-offs and the spatial mass effect. Aggregation and the spatial mass effect are equalizing. Suggestions are made of the evidence needed to make informed judgements on which contribute the most to co-existence in plant communities.

1. We sought evidence among the plant species of a New Zealand sand dune community that limiting similarity controls the ability of species to coexist. Sampling was at four spatial scales, from a single point up to a scale of 50 m2. Twenty-three functional characters were measured on each of the species, covering the morphology of the shoot and root systems and nutrient status, and intended to represent modes of resource acquisition. 2. Patterns of association between plant species at the four scales were examined for any tendency for plants with similar functional characters to coexist less often than expected at random (e.g. if a point has three species, do they have notably different characters?) The observed results were compared with the patterns expected under a null model using a range of test statistics. 3. A test over all characters found that the mean dissimilarity between nearest-neighbour species in functional space, and the minimum dissimilarity, were greater than expected under the null model at the 0.5 x 0.5 m scale. This supports the MacArthur & Levins model, although the actual community did not show an even spread of species over functional space. 4. Limiting similarity effects were seen even more consistently in separate characters when within-species variation was taken into account to calculate measures of overlap. The characters involved were mainly those related to rooting patterns and leaf water control, and thus perhaps reflecting the acquisition of nutrients and/or water. 5. Our results seem to be amongst the most convincing support for the theory of limiting similarity, and the only example involving vegetative processes in plant communities. The characters involved suggest that species can more readily coexist if they differ in their water-use pattern, reducing competition between them.

Question: Many broad-scale surveys are made, and local communities described, with time spent recording some measure of the abundance of each species. The results are always somewhat different from those obtained with presence/absence records, but which best represents the underlying structure of the community? Two partial answers to this question are suggested and tested here: which analysis correlates best with the habitat, and which gives more stable ordination scores under subsampling? Methods: Tests were made on ten field data sets, ranging widely in habitat type, spatial extent, spatial grain and measure of abundance. Correlation with the habitat was examined for the four larger-extent data sets with reasonably complete environmental information, using multiple regression of detrended correspondence analysis (DCA) ordination scores on environmental factors. Stability was tested for each data set using random subsets of the quadrats, and measuring stability as correlation between quadrat ordination scores in the subset and those using all quadrats. Results: Correlation with the habitat for the four data sets, where possible, was closer with presence/absence in most comparisons. Stability was greater with presence/absence in some cases and with abundance in others. Where abundance analyses were more stable, reduction to abundance categories, which are often used in field sampling, resulted in a loss of stability, although in two out of three data sets some advantage of abundance information over presence/absence was retained. Jittering to simulate subjective recording gave no further degradation. Conclusions: The data sets in which stability was higher in abundance analyses suggest that abundance is of value only in the rather homogeneous vegetation types that tend to occur over short distances, and with high-quality abundance data. From this, and environmental correlations being on the whole better with presence/absence analyses, I conclude that in broader-scale survey work, abundance information is unnecessary and may even be misleading. It seems that the primary assembly rule control on communities is on the presence of species, not their abundance.

1. We sought evidence for limiting similarity, a basic aspect of community structure, in three zones of a saltmarsh. Sampling was conducted at three spatial scales (grains), from a single point up to the scale of several square metres. Twenty‐three functional traits, related to the structure of the shoot and root systems and to nutrient status, were measured on each species present, separately in each community. 2. Patterns of association between plant species were compared with those expected under a null model, to assess whether plants with similar functional traits tended to coexist or to separate, i.e. whether there was environmental filtering or limiting similarity. A patch null model was used, a type that tends to be conservative but that avoids spurious evidence of limiting similarity caused by environmental pseudoreplication. One overall and four univariate test statistics were calculated, to capture possible patterns in trait space whilst minimising the problem of multiple testing. 3. In the Shrub community, overall evidence for even spacing of co‐occurring species in functional‐trait space, the pattern expected from the theory of limiting similarity, was seen at the area scale. In univariate tests in that community, there was evidence for even spacing in leaf lobation and leaf succulence, especially at small scales. 4. In the Rush community, there was significant evidence for limiting similarity in several traits, especially those related to canopy interactions, but also in some root characteristics. However, clustering in other traits, presumably owing to microenvironmental filtering, reduced overall tests for limiting similarity to ‘marginal significance’ (0.1 > P > 0.05). 5. In the species‐poor and salt‐stressed Salt turf, significant departures from the null model were sporadic and not consistent, although chlorophyll characters and leaf nitrogen concentration tended to be clustered. 6. Synthesis: There was evidence for niche limitation in two of the communities – Rush and Shrub – apparently based on canopy interactions in both cases and perhaps also root interactions in the latter community. Limiting similarity can be an important force in community assembly. However, in situations when it cannot be demonstrated, we do not know whether trait‐based competition is absent or whether its signal is overwhelmed by other processes.

Invasive plant eradication aims to restore an original state by eliminating the plant and its alteration of ecosystem structure and function. The method by which an invasive plant is eradicated is known to influence restoration success; however, success may be impeded by a legacy effect. We sought to examine how the soil nutrient pool and plant community assembly responded to the eradication of an invasive nitrogen-fixing legume, Lupinus arboreus , using three different techniques. We applied herbicide to six mature plants and with a handsaw removed the above ground biomass of 12 mature plants; six of these were left in situ to decompose and six were removed from site. Soil nutrient concentrations were measured, and plant community composition was monitored, in the treatment and reference plots over the following 13 months. Eradication resulted in a significant increase in extractable ammonium-N and potassium, in contrast nitrate-N declined. The plant community changed as a result of felling L. arboreus , and the removal of all biomass initially resulted in an increase in native plant cover. By the end of the experimental period, however, L. arboreus seedlings were increasing in abundance and native cover was declining. The application of herbicide resulted in the slow decay of the mature plant and as such delayed an increase in exotic species cover and inhibited L. arboreus germination. Nutrient concentrations in the eradication plots were not significantly different from the reference plots at the end of the experimental period. The impact of L. arboreus on the soil nutrient pool remains after eradication, a legacy effect which will hinder restoration of the original structure and function in the long term.

Questions: The co-existence of high numbers of species has always fascinated ecologists, but what and where are the communities with the world records for plant species richness? The species—area relationship is among the best-known patterns in community ecology, but does it give a consistent global pattern for the most saturated communities, the global maxima? Location: The world. Methods: We assembled the maximum values recorded for vascular plant species richness for contiguous areas from 1 mm2 up to 1 ha. We applied the power function to relate maximal richness to area and to make extrapolations to the whole Earth. Results: Only two community types contain global plant species maxima. The maxima at smaller spatial grain were from oligo- to meso-trophic, managed, semi-natural, temperate grasslands (e.g. 89 species on 1 m 2 ), those at larger grains were from tropical rain forests (e.g. 942 species on 1 ha). The maximum richness values closely followed a power function with z = 0.250: close to Preston's 'canonical' value of 0.262. There was no discernable difference between maxima using rooted presence (i.e. including only plants rooted in the plot) vs shoot presence (i.e. including any plant with physical cover over the plot). However, shoot presence values must logically be greater, with the curves flattening out at very small grain, and there is evidence of this from point quadrats. Extrapolating the curve to the terrestrial surface of the Earth gave a prediction of 219 204 vascular plant species, surprisingly close to a recent estimate of 275 000 actual species. Conclusions: Very high richness at any spatial grain is found only in two particular habitat/community types. Nevertheless, these high richness values form a very strong, consistent pattern, not greatly affected by the method of sampling, and this pattern extrapolates amazingly well. The records challenge ecologists to consider mechanisms of species co-existence, answers to the 'Paradox of the Plankton'.

1. The unpredictable component of community structure — chance/randomness/stochasticity — is increasingly discussed, recently in terms of Neutral Theory, but is never directly measured. We show that a direct estimate of chance can be made from spatial autocorrelation, and demonstrate this usage. 2. The dissimilarity between two patches of vegetation was plotted against the distance between the patches, and a curve fitted. We argue that the y-intercept of the curve, i.e. the 'nugget', represents the amount of chance variation in species composition, because when distance is zero all dispersal limitation and spatially correlated environmental differences are excluded, and only chance remains. 3. The method estimated that in 16 sites around the South Island of New Zealand, 0-71% of the variation in plant community composition was due to chance, with a mean of 34%. 4. Synthesis. The new analysis method provides information on the amount of randomness in community species composition and could be applied to the many dissimilarity/distance studies in the literature. The amount of randomness in the 16 study sites was not related to the type of plant community or to the species richness per quadrat. However, randomness was positively correlated with whole-community species richness, supporting previous suggestions of a relationship between chance, the size of the species pool and redundancy.

1. The mechanisms that structure biological communities hold the key to understanding ecosystem functioning and the maintenance of biodiversity. Patterns of species abundances have been proposed as a means of differentiation between niche-based and neutral processes, but abundance information alone cannot provide unequivocal discrimination. 2. We combined species niche information and species' relative abundances to test the effects of two opposing structuring mechanisms (environmental filtering and niche complementarity) on species' relative abundances in French lacustrine fish communities. The test involved a novel method comparing the abundance-weighted niche overlap within communities against that expected when relative abundances were randomized among species within the community. 3. Observed overlap was consistently significantly lower than expected at random for two (swimming ability and trophic status) of four primary niche axes across lakes of differing physical environments. Thus, for these niche axes, pairs of abundant species tended to have relatively low niche overlap, while rare species tended to have relatively high niche overlap with abundant species. 4. This suggests that niche complementarity may have acted to enhance ecosystem function and that it is important for species coexistence in these fish communities. The method used may be easily applied to any sort of biological community and thus may have considerable potential for determining the generality of niche complementarity effects on community structure.

Both local and regional filters can determine the invasion of alien species into native plant communities. However, their relative importance is essentially unknown. We used plot data from fragments of indigenous forests in southeastern New Zealand to infer which factors are important in explaining invasibility, measured as alien species richness. Twenty-eight predictor variables comprising both local factors (stand structure and soil) and regional ones (climate and land cover) were assessed. Reduction or increase in deviance in linear models was assessed, both individually and with a forward and backward stepwise variable selection procedure using the Akaike information criterion (AIC). We found that higher alien species richness was mainly associated with forest fragments of small area in warm and dry climates and where there were only small areas of surrounding indigenous forest. Local soil and stand structure variables had considerably smaller effects on alien species richness than the regional land cover and climate variables. Alien species richness showed no relationship with native species richness. We conclude that in the forest fragments investigated here, of the variables included in the analyses, regional land cover and climate variables are potentially important drivers for alien species richness at plot level. This has implications for projections of alien species spread in the future under different climate change and land use scenarios.

The recent Forum contribution by Grime (2006) contrasts the MacArthur/Diamond assembly-rule approach to studying plant communities with the study of environmental trait gradients. Both are valid and useful. In doing so, Grime declares that the assembly rules model, in which negative interactions between plants act with limiting similarity to cause local trait divergence, is “not supported by empirical study of plant communities”. This is, he says, the agony of community ecology. I show that there is now abundant evidence for assembly rules, and no agony.