Jari Oksanen

Continuum theory revisited: what shape are species responses along ecological gradients?

Abstract The shape of species’ responses along ecological gradients has important implications for both continuum theory and community analysis. Most current theories and analytical models in community ecology assume that responses are unimodal and symmetric. However, interactions between species and extreme environmental stress may cause skewed or non-unimodal responses. To date, statistical tools for evaluating response shapes have been either inappropriate, inefficient or biased. Using a data set on vascular plant distributions along an elevation gradient, we show that Huisman–Olff–Fresco (HOF) models are an effective method for this purpose, allowing models of various forms (skewed, symmetric, plateau, monotonic) to be tested for adequacy. HOF modeling was compared with alternative methods for response fitting, including Gaussian responses as Generalized linear model (GLMs), Generalized Additive Models (GAM) and Beta Functions with fixed or estimated endpoints. In our data set, skewed and plateau responses are less common than symmetric ones. Less than half of the species have skewed or plateau responses that can not be adequately modeled by Gaussian models. We show that Beta function models with fixed endpoints are biased, confounding skewness and the location of the mode and should not be used to test response shapes. Beta models with estimated endpoints are fairly consistent with other models. GAMs cannot provide clear tests of skewness or kurtosis of response curves, though we show that GAMs, in general, confirmed the shapes chosen by HOF modeling. We provide free software for fitting HOF models and encourage further applications to community data collected along different types of ecological gradients.

Instability of ordination results under changes in input data order: explanations and remedies

. Correspondence analysis (CA) and its Detrended form (DCA) produced by the program CANOCO are unstable under reordering of the species and sites in the input data matrix. In CA, the main cause of the instability is the use of insufficiently stringent convergence criteria in the power algorithm used to estimate the eigenvalues. The use of stricter criteria gives results that are acceptably stable. The divisive classification program TWINSPAN uses CA based on a similar algorithm, but with extremely lax convergence criteria, and is thus susceptible to extreme instability. We detected an order-dependent programming error in the non-linear rescaling procedure that forms part of DCA. When this bug is corrected, much of the instability in DCA disappears. The stability of DCA solutions is further enhanced by the use of strict convergence criteria. In our trials, much of the instability occurred on axes 3 and 4, but one should not assume that published two-dimensional ordinations are sufficiently accurate. Data sets which have pairs of almost equal eigenvalues among the first three axes could suffer from marked instability in the first two dimensions. We recommend that a debugged, strict version of CANOCO be released. Meanwhile, users can check the stability of their CA and DCA ordinations using the software that we have made available on the World Wide Web (http://www.helsinki.fi/jhoksane/). An accurate program for CA, a debugged, strict version of DECORANA (for DCA) and a strict version of TWINSPAN are also available at our site.

Community ecology in the age of multivariate multiscale spatial analysis

Species spatial distributions are the result of population demography, behavioral traits, and species interactions in spatially heterogeneous environmental conditions. Hence the composition of species assemblages is an integrative response variable, and its variability can be explained by the complex interplay among several structuring factors. The thorough analysis of spatial variation in species assemblages may help infer processes shaping ecological communities. We suggest that ecological studies would benefit from the combined use of the classical statistical models of community composition data, such as constrained or unconstrained multivariate analyses of site-by-species abundance tables, with rapidly emerging and diversifying methods of spatial pattern analysis. Doing so allows one to deal with spatially explicit ecological models of beta diversity in a biogeographic context through the multiscale analysis of spatial patterns in original species data tables, including spatial characterization of fitted or residual variation from environmental models. We summarize here the recent progress for specifying spatial features through spatial weighting matrices and spatial eigenfunctions in order to define spatially constrained or scale-explicit multivariate analyses. Through a worked example on tropical tree communities, we also show the potential of the overall approach to identify significant residual spatial patterns that could arise from the omission of important unmeasured explanatory variables or processes.

IS THE HUMPED RELATIONSHIP BETWEEN SPECIES RICHNESS AND BIOMASS AN ARTEFACT DUE TO PLOT SIZE

If plants vary in size, then the use of small quadrats of fixed size will show maximum species richness at intermediate biomass. Fewer plants are found in plots of fixed size both if plants increase in size (and biomass therefore increases), or if vegetation becomes sparse (biomass decreases).The supposed response of species richness to biomass is therefore produced as an artefact of varying plant number. Grime derived a humped relationship between species richness and biomass from studies using relatively small quadrats of 50 cm x 50 cm, or 1/4 m2 (AlMufti et al. 1977; Grime 1979), and similar patterns have been found in many later studies on terrestrial vegetation (e.g. Moore & Keddy 1989; Wisheu & Keddy 1989; Wheeler & Shaw 1991; Garcia et al. 1993; Rosenzweig & Abramsky 1993; Abrams 1995). The ecological reason usually proposed to explain the humped relationship is that stress reduces diversity at both extremes (harsh environments at low biomass and intensive competition at high biomass, Grime 1979; Abrams 1995), although other explanations have been proposed (Rosenzweig & Abramsky 1993). In this paper I show how a humped response can be produced without assuming any biological interactions.

Effects of reindeer grazing on understorey vegetation in dry Pinus sylvestris forests

Data on floristic composition and environmental variables were collected in floristically homogeneous oligo- trophic pine (Pinus sylvestris) forests with heath-like under- storey vegetation in eastern Fennoscandia, and ordinated by non-linear multidimensional scaling (NMDS) in order to study the effect of lichen grazing by reindeer on the understorey vegetation. The study sites included areas with varying graz- ing pressure, as well as 50-yr old grazing exclosures. Sites rich in respectively bryophytes and lichens were placed at opposite ends of the ordination axes, and heavily grazed sites were placed in between them. Reindeer grazing increased the abundance of bryophytes, especially Dicranum spp. and Pleurozium schreberi. Grazing changed the vegeta- tion to the extent that it resembled more mesotrophic sites, but this did not show any relationship with tree volume or other site productivity indicators. This was observed both in the ordination and, in a more compelling way, when exclosures with adjacent grazed areas were compared. No such signs were evident at ungrazed sites, where especially Cladina spp. spatially replace Cladonia spp. and tiny bryophytes like Barbilophozia spp., Polytrichum spp. and Pohlia nutans dur- ing succession. Cladina stellaris had almost disappeared from the most intensively grazed sites. The soil at ungrazed sites was characterized by high Al and Fe concentrations and bryophyte-rich sites by high Mn concentrations. Shannons diversity index, depth of humus layer and proportion of bare ground also increased in sites getting richer in bryophytes.

Palm distribution patterns in Amazonian rainforests: What is the role of topographic variation?

Abstract Question: Are soil properties and topographic variation related with palm (Arecaceae) species composition and distribution patterns? If so, are species distribution patterns consistent across sites? Location: 100–200 m a.s.l, non-inundated Amazonian rainforest, NE Peru. Methods: One 0.65-ha line transect divided into 5 m by 5 m subunits was inventoried for all palm individuals at each site. Soil samples were collected, and topography was measured. Results: A total of 56 palm taxa were recorded. Floristic similarity among transects clearly corresponded with similarity in soil cation content when species abundances were taken into account, but less so when only presence-absence data were used. Taxon-wise distribution analyses were done for the 37 most abundant palm taxa. Quite a few of these taxa proved not to be randomly distributed along the transects, but instead were clearly more abundant in some topographic positions than in others. However, the consistency of the distribution patterns across study sites proved to be rather low, and many of the palm taxa showed different distribution patterns at different sites. Conclusions: The ambiguity in distribution patterns across study sites may partly be due to the complexity of topography as a measure of ecological conditions, and the probability that it is related to the variation in different environmental variables at different sites. Nomenclature: Henderson (1995, 2000); see also Material and Methods section.

Reindeer reduce biomass of preferred plant species

. Reduced weights in reindeer that graze in pastures with high reindeer densities have raised the question if coastal summer pastures are modified by grazing. To evaluate this, the impact of reindeer grazing on standing crop was measured by the plant intercept method inside and outside grazing exclosures in the understorey of a coastal mountain birch forest in northern Norway. The understories of coastal birch forests are dominated by vascular plants and are important summer pastures to reindeer. Based on the literature, we made a priori categorization of the vascular plant species into functional groups of preferred forage, less preferred forage and forage of unknown value to reindeer. Intercept frequency was measured within the same plots on three occasions in the summer of 1996. At the end of the grazing season, total standing crop was 33% lower in open plots compared to plots protected by exclosures. However, the reduction varied between the functional groups, with only preferred forage plants being significantly reduced in standing crop (by 49%). Results suggest that reindeer have a strong annual impact on most of the preferred forage species. However, some of the preferred graminoids are tolerant of grazing and dominate the understorey despite decades of high grazing pressure. We suggest that current grazing pressure is favouring the establishment of a few grazing tolerant graminoids, and that this reduces the forage plant variability. The results are discussed in relation to the grazing optimization hypothesis and the potential importance of plant variability for pasture quality.

Epigeic lichen communities of taiga and tundra regions

The major physiognomic and ecological categories of the lichen-rich, epigeic communities in the boreal (taiga) and arctic (tundra) zones are defined and their syntaxonomy and ecology in Europe, Asia and North America is reviewed. In the boreal and hemiarctic areas open, dry, acidophytic lichen woodlands are widespread, especially on sandy habitats. Their epigeic lichen synusiae are usually dominated by four fruticoseCladina species, being extremely homogeneous in species composition and structure throughout the boreal zone, while the dominant trees and the other vascular plant flora of the woodlands are geographically more variable. No phytosociological classification system exists that would cover most of these communities over the circumpolar regions. Very similar communities, though much more poorly known, are found on thin soils over Precambrian rock outcrops. Other sites to produce epigeic lichen communities include open sand dunes, treeless heathlands, drier bogs and many seral stages, like those on road banks. Boreal lichen-rich communities on eutrophic soils may be developed in semiarid regions, in particular. In the Arctic, lichens are common in most communities, and the driest ones are regularly lichen-dominated, whether acidophytic or eutrophytic, chionophytic or achionophytic. Detailed syntaxonomic systems for their classification have been developed, especially in Greenland and Scandinavian mountains (in oroarctic zones in the latter regions). The richest fruticose lichen areas are in continental, hemiarctic timberline regions in northern Siberia and Canada. The southern and middle arctic subzones are also characterized by many macrolichens, such asCetraria cucullata, C. nivalis, Alectoria ochroleuca, andThamnolia vermicularis, but everywhere also small, crustose lichens are common on soil, such asRinodina turfacea andLopadium pezizoideum, which are often overlooked in vegetation analyses. The presence of microlichens and the formation of mosaic micropatterns of soil lichen communities is particularly typical of the northern arctic subzone. The conservation problems of the boreal and arctic lichen communities include overgrazing by reindeer or caribou, which has caused delichenization in some regions, extensive forest and tundra fires, use of heavy transport vehicles in forestry and tundra operations, and, locally, heavy industrial air pollution.

Niche characteristics of Danish woody species as derived from coenoclines

Abstract A main floristic coenocline is extracted from a Danish forest vegetation data set with Detrended Correspondence Analysis. Huisman-Olff-Fresco (HOF) modelling of 262 species occurrences relative to site scores along the coenocline is compared with Gaussian and Generalized Linear Modelling. HOF models allow for responses of variously skewed forms, which may result from interspecific interactions. Most species have symmetric (60%) or skewed (29%), and only few species have flat or monotone response curves. This provides evidence for the generality of unimodal plant species response curves, provided the gradient in question comprises a sufficient amount of compositional turnover. The Gaussian niche width is correlated with density of optima and probability of occurrence of the species. Detailed analysis of HOF estimates for 28 woody species indicates that early succession species such as Pinus sylvestris and Populus tremula have wider niches than middle to late successional canopy species such as Alnus glutinosa, Carpinus betulus, Fagus sylvatica, Fraxinus excelsior, Quercus robur and Tilia cordata. A marked overlap in niche range along the coenocline is interpreted in terms of species competition. Rubus idaeus intrudes into the niches of all other shrub species. Several canopy tree species, such as Fagus sylvatica, Fraxinus excelsior and Quercus robur overlap more than 60% of the ranges of all other species, while their internal overlaps are fairly equal, presenting evidence for their likely co-existence given more natural forest conditions. The combined analysis of coenoclines with HOF modelling of species behaviour is the most robust among the compared methods for estimating model parameters and niche characteristics of plant species. It is recommended for future ecological studies. Nomenclature: Lawesson et al. (1997).

Membrane permeability response of lichen Bryoria fuscescens to wet deposited heavy metals and acid rain

Abstract Lichen Bryoria fuscescens were treated in the field experiment with heavy metal solutions, containing a mixture of Cu 2+ and Ni 2+ ions alone or in combination with acidity (H 2 SO 4 , pH 3), to study the effects of simulated acid rain and heavy metal deposition on the membrane integrity of lichen. Membrane responses to pollutant treatments were assessed by measurements of the leachate conductivity, the K + leakage, and the ergosterol concentration. Both Cu and Ni concentrations of lichen were significantly increased by metal treatments. In addition a significant reduction of Ni with acid solution was found. The conductivity of leachate was significantly increased by the acidity but not affected by the metal treatments alone. Increased thallus metal concentrations (>400 μg g −1 Cu, >100 μg g −1 Ni) significantly increased K + leakage and reduced ergosterol concentration. The results suggest that the differences in the membrane responses are associated with the different sensitivity of algal and fungal partners to the acidity of heavy metal solutions.