J. Bastow Wilson

An index of functional diversity

Functional diversity has been seen as the key to predicting the stability, invasibility, resource capture, nutrient cycling and productivity of communities. However, it has been unclear how to estimate it. Ten criteria for an index of functional diversity are developed. These include that it should reflect the range of characters present and the abundance of the species with those characters in the community, and be unaf- fected by the measurement units used or by the number of species. An index that meets all ten criteria, FD var , is investigated. It is based on the variance in characters, weighted by the abundance of the species with those characters. Tested with artificial and randomly generated data, it showed reasonable use of the 0 - 1 range (mean 0.60, range 0.0009 - 0.975) and intuitive behaviour. Tested with field data from eight sites in New Zealand, it gave a good spread of values (mean 0.65, range across sites 0.34 - 0.84), showed good ability to distin- guish between the communities and its performance was eco- logically intuitive. Illustrative correlations are made with mean annual temperature and soil fertility, determined by a bio- assay. FD var is recommended for general use.

Properties of ecotones: Evidence from five ecotones objectively determined from a coastal vegetation gradient

Abstract Several properties have been suggested to be characteristic of ecotones, but their prevalence has rarely been tested. We sampled five ecotones to seek evidence on seven generalizations that are commonly made about ecotones: vegetational sharpness, physiognomic change, occurrence of a spatial community mosaic, many exotic species, ecotonal species, spatial mass effect, and species richness higher or lower than either side of the ecotone. The ecotones were in a sequence from scattered mangroves, through salt marsh, rush-marsh, scrub, woodland, to pasture. We developed a method to objectively define, by rapid vegetational change, the position and depth of an ecotone, identifying five ecotones. Their positions were consistent across three sampling schemes and two spatial grain sizes. One ecotone is a switch ecotone, produced by positive feedback between community and environment. Another is anthropogenic, due to clearing for agriculture. Two others are probably environmental in cause, and one may be largely a relict environmental ecotone. Sharp changes in species composition occurred. Three ecotones were associated with a change in plant physiognomy. In two, the ecotone was located just outside a woodland canopy, in the zone influenced by the canopy. Community mosaicity was evident at only one ecotone. There were few strictly ecotonal species; many species occurred more frequently within ecotones than in adjacent vegetation, but there were never significantly more ecotonal species than expected at random. There was little evidence for the spatial mass effect reducing ecotonal sharpness, or leading to higher species richness within ecotones. Species richness was higher than in the adjacent habitat in only one ecotone. It seems that supposedly characteristic ecotone features depend on the particular ecological situation, and the ecology of the species present, rather than being intrinsic properties of ecotones. Nomenclature: Connor & Edgar (1987) and references therein, and Stace (1997), except where indicated.

The effect of spatial scale on evenness

The effect of spatial scale on species evenness has not previously been investigated. As the area of each sample of vegetation (i.e. the spatial grain) increases, evenness could in theory increase, decrease, or stay the same, though the simplest model predicts an increase. We use biomass data from four dune slack sites and two semi-arid grasslands, sampled to allow calculation of evenness at a range of spatial grains. In all six sites, evenness decreases as grain size in- creases, almost monotonically. It is hypothesized that such a pattern is a result of a general feature of plant species abun- dance distributions and of vegetation response to environ- mental microheterogeneity.

Relative abundance distributions in plant communities: Effects of species richness and of spatial scale

. Relative abundance distributions (RADs) are an important feature of community structure, but little is known of the factors affecting which type of RAD is observed in a particular community. We examined the influences of species richness and of spatial scale on the RAD of plant communities. n n n nThe effect of species richness was examined by analysing simulated communities generated under the Broken stick model, the Sequential breakage model, and a randomized version of Niche pre-emption model. In all cases, when there were few species in the community the data only occasionally gave the best fit to the model that was used to generate it. With 40–65 species, a best fit was obtained for the correct model in about 75 % of cases, almost irrespective of the model. n n n nEffects of spatial scale were examined in data from four dune slacks and from two semi-arid grasslands, by analysing biomass values at a range of sample sizes. The model that best fitted the whole sample differed between the four slacks and between the slacks and the semi-arid grasslands. The change in which model of RAD fitted best, as sample size was reduced, varied between sites and between habitat types. At the smallest sample sizes, the Zipf-Mandelbrot model often fitted, and in the slack sites the Broken stick also, though neither fitted (in the vegetation examined) at larger spatial scales. n n n nIt is concluded the RAD is affected by species richness and by spatial scale, in ways that currently do not enable simple prediction. RADs can theoretically give information on the processes such as resource partitioning, immigration and competition that have structured the community, but they are a blunt tool for this purpose.

Which is the phyte in epiphyte

The term “Epiphyte” has been used for a plant growing on any object, and also for any organism growing on a plant. Evidence from analogy, dictionaries, use and precedence supports the two interpretations equally. We recommend that “epiphyte” be used only for an organism growing on a plant.

Do Plants Ever Compete for Space

The term “competition for space” occurs often in ecological literature, but there has never been a direct demonstration of this competition. In fact it has been shown that plant canopies are mainly empty of plant organs. F.E. Clements recognized this. He suggested that one of the few situations in which competition for space would occur was in an unthinned row of radishes. Even this has never been demonstrated. We sowed radishes (Raphanus sativa) in a dense row and let them grow to maximum size in favourable conditions. For comparison, the same number of radish seeds were sown in a double, offset row. Squeezing the radish plants into one row was largely compensated by sideways movement of their hypocotyl/taproots, but total plant mass was reduced by 7.5%. Even this cannot be definitely attributed to competition for space. We conclude that competition for space, if it exists at all, is only a small effect even in conditions very favourable for it. It can, as Clements said, be ignored for natural communities.

Do plant modules describe community structure better than biomass? A comparison of three abundance measures

Abstract Plants are composed of modules, such as shoots and leaves. However, modules have been overlooked as potential abundance measures in describing plant communities. We sampled 8 communities, to examine whether module counts gave different conclusions from biomass on community structure, and better discriminated between communities. Different abundance measures – number of leaves, number of shoots and photosynthetic biomass – gave different results for between-site comparisons of evenness and rank consistency. Since evenness is intended to represent a feature of the whole community, it should vary more between communities than within; module-based abundance achieved this better than biomass. It is speculated that the restrictions on species co-occurrences when plant communities assemble may sometimes be based on the number of modules rather than on biomass. Abbreviation: PSU = Photosynthetic unit.

Application and evaluation of Dale's non-parametric method for detecting community structure through zonation

Dales non-parametric method for analysing zonation was tested on three coastal ecosystems in New Zealand. The method, intended to investigate community structure, is based on a prediction that the upslope (i.e. top) boundary of one species will be followed upwards, more often than expected at random, by the downslope (i.e. bottom) boundary of another species. In the present study, the predicted pattern was not found: only one transect out of 12 had significantly more top/bottom boundary (TB) contiguities than would be expected on a random basis. Rather, the opposite trend was seen in two transects, of a deficit of TB contiguities. One of these deficits was significant even after Bonferroni correction for the number of tests made. Such TB deficits might be expected if there were community boundaries, and indeed TB deficits are predicted by several theoretical models of community structure. The results contrast with Dales own results, in which there were considerably more TB contiguities than would be expected on a random basis. This confirms suggestions that the pattern envisaged by Dale is likely to be uncommon.