Too good to be true: pitfalls of usingmean Ellenberg indicator values in vegetation analyses

Question: Mean Ellenberg indicator values (EIVs) inherit information about compositional similarity, because during their calculation species abundances (or presence–absences) are used as weights. Can this similarity issue actually be demonstrated, does it bias results of vegetation analyses correlating mean EIVs with other aspects of species composition and how often are biased studies published? Methods: In order to separate information on compositional similarity possibly present in mean EIVs, a new variable was introduced, calculated as a weighted average of randomized species EIVs. The performance of these mean randomized EIVs was compared with that of the mean real EIVs on the one hand and random values (randomized mean EIVs) on the other. To demonstrate the similarity issue, differences between samples were correlated with dissimilarity matrices based on various indices. Next, the three mean EIV variables were tested in canonical correspondence analysis (CCA), detrended correspondence analysis (DCA), analysis of variance (ANOVA) between vegetation clusters, and in regression on species richness. Subsequently, a modified permutation test of significance was proposed, taking the similarity issue into account. In addition, an inventory was made of studies published in the Journal of Vegetation Science and Applied Vegetation Science between 2000 and 2010 likely reporting biased results due to the similarity issue. Results: Using mean randomized EIVs, it is shown that compositional similarity is inherited into mean EIVs and most resembles the inter-sample distances in correspondence analysis, which itself is based on iterative weighted averaging. The use of mean EIVs produced biased results in all four analysis types examined: unrealistic (too high) explained variances in CCA, too many significant correlations with ordination axes in DCA, too many significant differences between cluster analysis groups and too high coefficients of determination in regressions on species richness. Modified permutation tests provided ecologically better interpretable results. From 95 studies using Ellenberg indicator values, 36 reported potentially biased results. Conclusions: No statistical inferences should bemade in analyses relatingmean EIVs with other variables derived from the species composition as this can produce highly biased results, leading to misinterpretation. Alternatively, a modified permutation test using mean randomized EIVs can sometimes be used

Effectafstand van stikstof uit verkeersemissies op de vegetatie - een inventarisatie van de literatuur

Stikstof afkomstig van het wegverkeer kan in natuurgebieden leiden tot een afname van de natuurkwaliteit en kan het behalen van de natuurdoelen in Natura 2000-gebieden in de weg staan. Het is echter niet eenvoudig om op één bepaalde plaats vast te stellen welk deel van de stikstofbelasting aan het wegverkeer op een nabij gelegen weg toe te schrijven is en welk deel aan andere bronnen. Als het gaat om daadwerkelijk aangetoonde effecten is echter maar weinig kennis aanwezig. Daarom is vanuit Rijkswaterstaat de volgende vraag gedefinieerd: Welke onderzoeksgegevens bestaan er m.b.t. dosis - effect relaties van stikstof (NH3 en NOx) geproduceerd door het verkeer, op langs de weg gelegen natuurgebieden. Daarbij staat vooral de vraag centraal tot op welke afstand deze effecten zich nog doen gelden

Co-correspondence analysis: a new ordination method to relate two community compositions

A new ordination method, called co-correspondence analysis, is developed to relate two types of communities (e.g., a plant community and an animal community) sampled at a common set of sites in a direct way. The method improves the simple, indirect approach of applying correspondence analysis (reciprocal averaging) to the separate species data sets and correlating the resulting ordination axes. Co-correspondence analysis maximizes the weighted covariance between weighted averaged species scores of one community and weighted averaged species scores of the other community. It thus attempts to identify the patterns that are common to both communities. Both a symmetric descriptive and an asymmetric predictive form are developed. The symmetric form relates to co-inertia analysis and the asymmetric, predictive form to partial least-squares regression. In two examples the predictive power of co-correspondence analysis is compared with that of canonical correspondence analyses on syntaxonomic and environmental data. In the first example, carabid beetles in roadside verges are shown to be more closely related to plant species composition than to vegetation structure (biomass, height, roughness, among others), and, in the second example, bryophytes in spring meadows are shown to be more closely related to the species composition of the vascular plants than to the measured water chemistry

Ecology of roadside plant communities

Roadside habitats nowadays contribute considerably to the amount of natural areas in the Netherlands. The ecological role of roadsides is recognized by the Dutch government and road authorities, and the interest in ecological management is growing. The study considers a cross-section of the more valuable plant communities occurring on Dutch road verges and investigates a large number of environmental factors. Aiming to provide an ecological framework that can be used as a reference for ecological management, the study presents an accurate synecological description of the studied syntaxa. An additional field experiment shows that large amounts of nutrients are lost from cuttings, already in the first few weeks after mowing. The effect of various hay removal delay times on the nutrient balance of the different communities is modelled. In most cases, cuttings should be removed within one week (or two at most) if the amounts of nutrients removed are to exceed the inputs through atmospheric deposition. Exploiting the unfertilized, semi-natural character of the sites and the wide range of conditions involved, the data are also used to detect and test general relationships. Investigating soil versus biomass relations it is shown that, over a wide environmental gradient, no simple relationship exists between the vegetation tissue concentration and soil availability of a nutrient (with the possible exception of K).Focussing on diversity, unimodal species richness relationships are identified for both maximum standing biomass and productivity, but these only explain a small part of the variation and are apparent only if soil factors are not considered. The "hump-shape" is more pronounced for standing biomass than for productivity, suggesting that competition for light is the key-factor. Particularly the numbers of rare and endangered species are curtailed strongly by high biomass values. Management affects species richness positively, but also this effect is apparent only if soil factors are not considered. To investigate the nature of Ellenberg indicator values, these are correlated to the measured soil and vegetation parameters. Soil pH is shown not to be adequately indicated by the so-called Ellenberg reaction values, but instead they properly reflect soil total calcium ( i.e. both exchangeable calcium and calcium in the form of carbonates). Ellenberg nitrogen values best indicate productivity. Ellenberg moisture values best indicate the average lowest soil moisture content, but they also provide an appropriate indication of the average annual groundwater level or alternatively the average spring groundwater level. Over the wide range of unfertilized conditions studied, in situ average annual N mineralization can be predicted well from a few directly measured soil parameters. The pool of mineral N just before the growing season is the best correlate. Together with moisture content and pH, this variable can explain 83.5% of the variation in annual N mineralization.Key-words:diversity, ecological management, indicator values, infrastructure, nitrogen mineralization, nutrient availability, productivity, standing biomass, vegetation.</p

Wegberm biedt hulp tegen bestuivingscrisis

De achteruitgang van bloemzoekende insecten is een bedrieging voor een scala aan ecologische processen en diensten die deze dieren verzorgen. Wegbermen zijn vaak rijk aan bloeiende kruiden en kunnen daardoor van groot belang zijn voor deze dieren. Maar hoe kunnen deze bermen het beste beheerd worden? Wageningen Universiteit deed een experiment in een grazige berm, waarbij bloembezoek bekeken werd in relatie to vijf maairegime

De droge stroomdalgraslanden van het Sedo-Cerastion in Nederland : verspreiding en soortensamenstelling van het Medicagini-Avenetum en het Sedo-Thymetum vóór 1960 en daarna

Dit rapport geeft de vegetatiekundige geschiedenis weer van de stroomdalgraslanden in Nederland. Voor het eerst zijn gegevens uit de jaren ’50 en ’60 gedigitaliseerd en gebruikt om een referentiebeeld te bepalen van het voorkomen, de standplaatsen en de soortensamenstelling van droge stroomdalgraslanden. Vergelijking met recentere vegetatieopnames bevestigt de achteruitgang in de verspreiding, maar ook in de kwaliteit van deze vegetaties. Sommige associaties zijn helemaal verdwenen, andere zijn sterk achteruitgegaan in verspreiding of hebben meer soorten van voedselrijkere bodems als kenmerkende soorten gekregen

Successful overwintering of arthropods in roadside verges

In densely populated areas, roadside verges often provide the last semi-natural habitats available. Their ecological value is often stressed by bio survey results. Yet insect summer surveys potentially misjudge the value of a biotope (roadside or otherwise) since the occurrences of species may only be seasonal, or even transient. To effectively ascertain the importance of a site for insects, species must be shown to complete their life cycle there or at least be shown to successfully accomplish critical life stages, such as reproduction or overwintering. To confirm overwintering of arthropods in roadsides, sods were cut in late winter in a verge where several years of summer survey data were available. The sods were placed in transparent semi-permeable cages and kept alive during subsequent spring, summer and autumn. All emerging arthropods were caught inside the cages using pitfall and funnel traps and identified to order or family level. Most of the terrestrial arthropod phyla and orders occurring in NW-Europe appeared to be represented. Several groups were further identified to the species level: Carabidae (Coleoptera), Curculionidae (Coleoptera), Araneae, Orthoptera, Apidae (Hymenoptera), Syrphidae (Diptera) and Dephacidae/Cercopidae (Hemiptera). Particularly for the Carabidae, Araneae and Curculionidae, many species recorded in summer were also found to overwinter; species overlap amounted to approximately 67%. Rarefaction of the summer sample or excluding possible summer vagrants, raised this overlap to as much as 88% for the Carabidae. Many of the species successfully overwintering in the roadside verge were generalists, but less common, more stenotopic species were present as well. Not only species hibernating as adults were involved, but also species overwintering in immature stages, indicating reproduction also takes place in the roadside verge. Apparently the roadside occurrence of many arthropod species, including stenotopic and declining ones, is not merely seasonal or incidental, and roadside verges do not necessarily act as a sink only. The ecological importance often attributed to roadside verges should clearly be taken seriously

Shifts in functional plant groups in ditch banks under agri-environment schemes and in nature reserves

Management of ditch banks of agricultural fields is considered to be a promising and multifunctional application of agri-environment schemes (AES) on farmland. Our previous research has shown that in the Netherlands, there is a small increase in the number of target plant species of AES in ditch banks. However, the productivity and Ellenberg indicator value for nitrogen also increased. This suggests a change in species composition towards more competitive species. This is important, because management mainly focuses on restoring disturbance tolerant species that used to be common in meadows, rather than competitive dominants. In this study we use a large scale dataset of target species composition in ditch banks of nature reserves and ditch banks with and without AES over 10 years to monitor results of functional plant species groups under these different management regimes. Our analyses show that plant functional type composition in ditch banks of agricultural fields indeed shifted towards more competitive species over the last 10 years, independent of AES. In nature reserves, a similar increase in competitive species was observed. The shift towards more competitive species was reflected in the increase of the average height of the vegetation and the increase in species with a leafy canopy structure, whereas species with a semi-basal canopy structure were decreasing. We conclude that current AES does not increase the number of targeted disturbance tolerant species and that more disturbance such as more frequent mowing is required to obtain these species

Kansen voor geleedpotigen in bermen : acht jaar onderzoek langs de weg

Geleedpotigen leven onder steeds moeilijker omstandigheden. De gebieden waar ze leven raken steeds verder versnipperd. Om die reden is het belangrijk om de overgebleven soorten beter te beschermen. Zo ontstond er ook hernieuwde belangstelling voor het leven van geleedpotigen in bermen. Bermen blijken een belangrijke ecologische waarde te hebben. Dit vraagt een doordachte aanpak van het maaien van berme