Kevin McConway

Phylogeny and the hierarchical organization of plant diversity

R. H. Whittakers idea that plant diversity can be divided into a hierarchy of spatial components from alpha at the within-habitat scale through beta for the turnover of species between habitats to gamma along regional gradients implies the underlying existence of alpha, beta, and gamma niches. We explore the hypothesis that the evolution of alpha, beta, and gamma niches is also hierarchical, with traits that define the alpha niche being labile, while those defining beta and gamma niches are conservative. At the alpha level we find support for the hypothesis in the lack of close significant phylogenetic relationship between meadow species that have similar alpha niches. In a second test, alpha niche overlap based on a variety of traits is compared between congeners and noncongeners in several communities; here, too, there is no evidence of a correlation between alpha niche and phylogeny. To test whether beta and gamma niches evolve conservatively, we reconstructed the evolution of relevant traits on evolutionary trees for 14 different clades. Tests against null models revealed a number of instances, including some in island radiations, in which habitat (beta niche) and elevational maximum (an aspect of the gamma niche) showed evolutionary conservatism.

RAINFALL, BIOMASS VARIATION, AND COMMUNITY COMPOSITION IN THE PARK GRASS EXPERIMENT'

We used data on grassland plant community composition over a 90-yr period in the history of the Park Grass Experiment, England to look for relationships between variation in composition and annual variation in rainfall and biomass. This was investigated by regressions of biomass and rainfall on each other, and of these variables separately on each of three different measures of variation in plant community composition. Two of these measures, principal components analysis scores based on variation in species abun- dance and the ratio by mass of nongrass/grass species, showed significant relationships with biomass variation on many experimental plots, although relationships with rainfall were relatively slight or nonexistent. The third measure employed similarity indices to detect changes in species composition in response to variation in biomass, but failed to find any. Biomass was significantly increased by rainfall on all plots. We propose that variation in community composition was more closely related to biomass variation than to rainfall because rainfall selectively favored the grasses in the community, which we believe com- peted asymmetrically (for light) with the other species when rainfall was high. The severity of this competition would depend upon biomass more directly than upon rainfall, although it is rainfall that enhanced grass growth. In effect, asymmetric competition magnified the effect of rainfall on community composition.

Marginalization and Linear Opinion Pools

Abstract Suppose a decision maker has asked a group of experts to assess subjective probability distributions over some space, and he wishes to find a consensus probability distribution as a function of these. The assumption that finding the consensus distribution commutes with marginalization of the distributions implies that the consensus must be found using a linear opinion pool (weighted average), provided the space being considered contains three or more points. Some of the consequences of this result are discussed.

Absence of phylogenetic signal in the niche structure of meadow plant communities

A significant proportion of the global diversity of flowering plants has evolved in recent geological time, probably through adaptive radiation into new niches. However, rapid evolution is at odds with recent research which has suggested that plant ecological traits, including the β- (or habitat) niche, evolve only slowly. We have quantified traits that determine within-habitat α diversity (α niches) in two communities in which species segregate on hydrological gradients. Molecular phylogenetic analysis of these data shows practically no evidence of a correlation between the ecological and evolutionary distances separating species, indicating that hydrological α niches are evolutionarily labile. We propose that contrasting patterns of evolutionary conservatism for α- and β-niches is a general phenomenon necessitated by the hierarchical filtering of species during community assembly. This determines that species must have similar β niches in order to occupy the same habitat, but different α niches in order to coexist.

A fundamental, eco‐hydrological basis for niche segregation in plant communities

• Ecologists still puzzle over how plant species manage to coexist with one another while competing for the same essential resources. The classic answer for animal communities is that species occupy different niches, but how plants do this is more difficult to determine. We previously found niche segregation along fine-scale hydrological gradients in European wet meadows and proposed that the mechanism might be a general one, especially in communities that experience seasonal saturation. • We quantified the hydrological niches of 96 species from eight fynbos communities in the biodiversity hotspot of the Cape Floristic Region, South Africa and 99 species from 18 lowland wet meadow communities in the UK. Niche overlap was computed for all combinations of species. • Despite the extreme functional and phylogenetic differences between the fynbos and wet meadow communities, an identical trade-off (i.e. specialization of species towards tolerance of aeration and/or drying stress) was found to cause segregation along fine-scale hydrological gradients. • This study not only confirms the predicted generality of hydrological niche segregation, but also emphasizes its importance for structuring plant communities. Eco-hydrological niche segregation will have implications for conservation in habitats that face changing hydrology caused by water abstraction and climate change.

community stability - a 60-year record of trends and outbreaks in the occurrence of species in the park grass experiment

1 A 60-year time-series of species recorded in visual surveys of the plant communities of the Park Grass Experiment is analysed to detect changes through time in the frequency of species on seven plots with acidified soil and on 35 non-acidified plots. 2 Of 14 species recorded on the acidified plots, eight decreased with time, one ( Agrostis capillaries ) increased, four showed no trend and one ( Chamerion angustifolium ) showed an outbreak with a peak in 1946. Of the 43 species recorded on the nonacidified plots, six increased, five decreased, 10 showed outbreaks and 22 showed no trend. 3 We used discriminant analysis to try to identify combinations of seven life-history and two habitat variables that would correctly classify species according to how their frequency changed on the non-acidified plots. Habitat variables (mean pH and mean hay yield of plots) were poor discriminators, but some life history variables (notably ruderalness, mating system and flowering time) were more successful. 4 Species which increased were more outcrossing whereas outbreak species were more selfing than the average for all species, both were more ruderal than average. We speculate that the ruderal species were all well equipped to spread across plots, but that only outcrossing species possessed sufficient genetic variation to be able to sustain a broad distribution in the heterogeneous environment represented by the PGE. 5 In view of the otherwise stable nature of the Park Grass communities we suggest that the existence of outbreaks in a significant number of species calls for a reevaluation of the concept of the stable plant community.

Citizen Science Reveals Unexpected Continental-Scale Evolutionary Change in a Model Organism

Organisms provide some of the most sensitive indicators of climate change and evolutionary responses are becoming apparent in species with short generation times. Large datasets on genetic polymorphism that can provide an historical benchmark against which to test for recent evolutionary responses are very rare, but an exception is found in the brown-lipped banded snail (Cepaea nemoralis). This species is sensitive to its thermal environment and exhibits several polymorphisms of shell colour and banding pattern affecting shell albedo in the majority of populations within its native range in Europe. We tested for evolutionary changes in shell albedo that might have been driven by the warming of the climate in Europe over the last half century by compiling an historical dataset for 6,515 native populations of C. nemoralis and comparing this with new data on nearly 3,000 populations. The new data were sampled mainly in 2009 through the Evolution MegaLab, a citizen science project that engaged thousands of volunteers in 15 countries throughout Europe in the biggest such exercise ever undertaken. A known geographic cline in the frequency of the colour phenotype with the highest albedo (yellow) was shown to have persisted and a difference in colour frequency between woodland and more open habitats was confirmed, but there was no general increase in the frequency of yellow shells. This may have been because snails adapted to a warming climate through behavioural thermoregulation. By contrast, we detected an unexpected decrease in the frequency of Unbanded shells and an increase in the Mid-banded morph. Neither of these evolutionary changes appears to be a direct response to climate change, indicating that the influence of other selective agents, possibly related to changing predation pressure and habitat change with effects on micro-climate.

A Demographic Interpretation of Grime's Triangle

1.The CSR theory of life-history strategies of Grime, and demographically based theories of life history represent strongly contrasting approaches that have yet to be reconciled. 2. It is argued that there are a priori grounds for analogy between Grimes three primary strategies of the established phase in plants and the demographic processes of growth (≈C), survival (≈S), and fecundity (≈R). 3. The contribution of growth, fecundity and survival to the finite rate of population increase λ was calculated for populations of 18 plant species that have also been classified according to Grimes CSR scheme. The match between the demographic classification of species based on these data and their CSR status was determined using a randomization test, and no significant match between the two was found. The reasons for this result are discussed, and it is concluded that it would be premature to abandon attempts to reconcile these two important approaches to plant life history.

Crowdsourcing the identification of organisms: A case-study of iSpot

Abstract Accurate species identification is fundamental to biodiversity science, but the natural history skills required for this are neglected in formal education at all levels. In this paper we describe how the web application ispotnature.org and its sister site ispot.org.za (collectively, “iSpot”) are helping to solve this problem by combining learning technology with crowdsourcing to connect beginners with experts. Over 94% of observations submitted to iSpot receive a determination. External checking of a sample of 3,287 iSpot records verified > 92% of them. To mid 2014, iSpot crowdsourced the identification of 30,000 taxa (>80% at species level) in > 390,000 observations with a global community numbering > 42,000 registered participants. More than half the observations on ispotnature.org were named within an hour of submission. iSpot uses a unique, 9-dimensional reputation system to motivate and reward participants and to verify determinations. Taxon-specific reputation points are earned when a participant proposes an identification that achieves agreement from other participants, weighted by the agreers’ own reputation scores for the taxon. This system is able to discriminate effectively between competing determinations when two or more are proposed for the same observation. In 57% of such cases the reputation system improved the accuracy of the determination, while in the remainder it either improved precision (e.g. by adding a species name to a genus) or revealed false precision, for example where a determination to species level was not supported by the available evidence. We propose that the success of iSpot arises from the structure of its social network that efficiently connects beginners and experts, overcoming the social as well as geographic barriers that normally separate the two.

NONSTOCHASTIC VARIATION OF SPECIES-LEVEL DIVERSIFICATION RATES WITHIN ANGIOSPERMS

Abstract Variations in the origination and extinction rates of species over geological time often are linked with a range of factors, including the evolution of key innovations, changes in ecosystem structure, and environmental factors such as shifts in climate and physical geography. Before hypothesizing causality of a single factor, it is critical to demonstrate that the observed variation in diversification is significantly greater than one would expect due to natural stochasticity in the evolutionary branching process. Here, we use a likelihood‐ratio test to compare taxonomic rate heterogeneity to a neutral birth‐death model, using data on well‐supported sister pairs of taxa and their species richness. We test the likelihood that the distribution of extant species among angiosperm genera and families could be the result of constant diversification rates. Results strongly support the conclusion that there is significantly more heterogeneity in diversity at the species level within angiosperms than would be expected due to stochastic processes. This result is consistent in datasets of genus pairs and family pairs and is not affected significantly by degrading pairs to simulate inaccuracy in the assumption of simultaneous origin of sister taxa. When we parse taxon pairs among higher groups of angiosperms, results indicate that a constant rates model is not rejected by rosid and basal eudicot pairs but is rejected by asterid and eumagnoliid pairs. These results provide strong support for the hypothesis that species‐level rates of origination and/or extinction have varied nonrandomly within angiosperms and that the magnitude of heterogeneity varies among major groups within angiosperms.