Communities of ground-living spiders in deciduous forests: Does tree species diversity matter?

The relationships between species diversity and ecosystem functions are in the focus of recent ecological research. However, until now the influence of species diversity on ecosystem processes such as decomposition or mineral cycling is not well understood. In deciduous forests, spiders are an integral part of the forest floor food web. In the present study, patterns of spider diversity and community structure are related to diversity of deciduous forest stands in the Hainich National Park (Thuringia). In 2005, pitfall trapping and quantitative forest floor sampling were conducted in nine plots of forest stands with one (Diversity Level 1), three (DL 2) and five (DL 3) major deciduous tree species. Species richness, measured with both methods, as well as spider abundance in forest floor samples were highest in stands with medium diversity (DL 2) and lowest in pure beech stands (DL 1). The Shannon-Wiener index and spider numbers in pitfall traps decreased from DL 1 to DL 3, while the Shannon-Wiener index in forest floor samples increased in the opposite direction. Spider community composition differed more strongly between single plots than between diversity levels. Altogether, no general relationship between increasing tree species diversity and patterns of diversity and abundance in spider communities was found. It appears that there is a strong influence of single tree species dominating a forest stand and modifying structural habitat characteristics such as litter depth and herb cover which are important for ground-living spiders.     

Is there a trade-off between species diversity and genetic diversity in forest tree communities?

The two most important components of biodiversity, species diversity and genetic diversity, have generally been treated as separate topics, although a coordination between both components is believed to be critical for ecosystem stability and resilience. Based on a new trait concept that allows for the assessment of genetic diversity across species, the relationship between species diversity and genetic diversity was examined in eight forest tree communities composed of different tree genera including both climax and pioneer species. It was intended to check whether a trade-off exists between the two diversity components as was found in a few studies on animal species.Using several isozyme-gene systems as genetic markers, the genetic diversity across species within each of the tree communities was determined by two measures, the commonly used intraspecific genetic diversity averaged over species and the recently developed transspecific genetic diversity per species. Both data sets were compared with the corresponding community-specific species diversity resulting in a positive relationship between the two diversity components. A statistically significant positive correlation was established between the transspecific genetic diversity per species and the species diversity for three isozyme-gene systems. Beyond that, consistent results were obtained using different parameters of the diversity measure which characterize the total, the effective and the number of prevalent variants. The number of prevalent variants reflected most significantly the non-randomness of the observed diversity patterns.These findings can be explained by the observation that the pioneer tree species reveal a by far higher genetic diversity than the climax tree species, which means that an increase in species diversity, due to the addition of several pioneer species at the expense of one or two climax species, goes along with an increase in the level of genetic diversity. Forest tree communities with the highest degree of species diversity exhibit therefore the highest transspecific genetic diversity per species. This result was discussed with regard to the particular composition and stability of forest tree communities.     

Does tree diversity increase wood production in pine forests?

Recent experimental advances on the positive effect of species richness on ecosystem productivity highlight the need to explore this relationship in communities other than grasslands and using non-synthetic experiments. We investigated whether wood production in forests dominated by Aleppo pine (Pinus halepensis) and Pyrenean Scots pine (Pinus sylvestris) differed between monospecific and mixed forests (2-5 species) using the Ecological and Forest Inventory of Catalonia (IEFC) database which contains biotic and environmental characteristics for 10,644 field plots distributed within a 31,944 km(2) area in Catalonia (NE Spain). We found that in Pyrenean Scots pine forests wood production was not significantly different between monospecific and mixed plots. In contrast, in Aleppo pine forests wood production was greater in mixed plots than in monospecific plots. However, when climate, bedrock types, radiation and successional stage per plot were included in the analysis, species richness was no longer a significant factor. Aleppo pine forests had the highest productivity in plots located in humid climates and on marls and sandstone bedrocks. Climate did not influence wood production in Pyrenean Scots pine forests, but it was highest on sandstone and consolidated alluvial materials. For both pine forests wood production was negatively correlated with successional stage. Radiation did not influence wood production. Our analysis emphasizes the influence of macroenvironmental factors and temporal variation on tree productivity at the regional scale. Well-conducted forest surveys are an excellent source of data to test for the association between diversity and productivity driven by large-scale environmental factors.     

Fouling reefal communities on artificial reefs: does age matter?

Man-made submerged structures, including shipwrecks, offering substrata for fouling organisms and fish, have been classified secondarily as artificial reefs (ARs). The current approach in AR design is that of low-profile structures placed on the seabed and attempting to mimic natural reef (NR) communities with the aim of mitigating degraded marine ecosystems. To examine the validity of this concept, a long-term comparison of the developing AR fouling communities to those of nearby NRs is required. A survey of the fouling reefal organisms was conducted on seven shipwrecks (Red Sea, Egypt), comprising three young (ca 20 years old) and four old (>100 years old) unplanned ARs, in comparison to nearby NR communities. The hypothesis tested was that the age of the ARs shapes the structure of their fouling coral communities. The results demonstrated distinct differences between ARs and NRs and between young and old ARs. While the species composition on ARs may resemble that of NRs after approximately 20 years, obtaining a similar extent of coral cover may require a full century. Moreover, differences in structural features between ARs and NRs may lead to differences in species composition that persist even after 100 years.     

Fungal diversity on decaying beech logs – implications for sustainable forestry

An analysis of factors influencing the diversity of macrofungi fruiting on decaying beech logs at site level is presented. Variables related to log size and shape and decay stage were found to explain up to 56% of the variation in total species richness and 42% of the variation in the richness of threatened (red-listed) species. Inclusion of variables relating to the vernal flora and the degree of soil contact further increased the explained variation in total species richness to 71%, but these variables were non-significant with respect to red-listed species. However, inclusion of the variable log type, distinguishing uprooted logs, logs broken at root neck and logs broken 1–7 and 8–15 m above ground, increased the amount of explained variation in richness of red-listed species to 50%. Among the log size and shape variables, the number of bole forks was superior in describing the variation in both total and red-listed species richness. Accordingly, forked trees should preferably be selected for decay in order to improve biodiversity, since they support comparably higher species diversity than unforked logs and have limited economical value. The importance of log type for the richness of threatened species appears to be connected with the occurrence of certain non-dominant primary decayers, causing heart rot, subsequently allowing the establishment of red-listed species. Accordingly, it is suggested that a high diversity of primary decayers may be a key to the conservation of wood decaying fungi. Microclimatic variables were found to have a limited effect on fruit body diversity on the studied logs; however, the microclimatic regime is discussed as an important factor in relation to management of dead wood for fungal biodiversity.     

Temporal shifts on tree species niches: how do they affect species dynamics and community diversity?

Plant Ecology - Species–habitat associations can be used as a proxy for species niches. Previous research has shown that niche plasticity may increase diversity in plant communities, and that...     

Does species diversity limit productivity in natural grassland communities?

Theoretical analyses and experimental studies of synthesized assemblages indicate that under particular circumstances species diversity can enhance community productivity through niche complementarity. It remains unclear whether this process has important effects in mature natural ecosystems where competitive feedbacks and complex environmental influences affect diversity–productivity relationships. In this study, we evaluated diversity–productivity relationships while statistically controlling for environmental influences in 12 natural grassland ecosystems. Because diversity–productivity relationships are conspicuously nonlinear, we developed a nonlinear structural equation modeling (SEM) methodology to separate the effects of diversity on productivity from the effects of productivity on diversity. Meta-analysis was used to summarize the SEM findings across studies. While competitive effects were readily detected, enhancement of production by diversity was not. These results suggest that the influence of small-scale diversity on productivity in mature natural systems is a weak force, both in absolute terms and relative to the effects of other controls on productivity.     

Invasion of temperate deciduous broadleaf forests by N-fixing tree species – consequences for stream ecosystems

Biological invasions are a major threat to biodiversity and ecosystem functioning. Forest invasion by alien woody species can have cross-ecosystem effects. This is especially relevant in the case of stream–riparian forest meta-ecosystems as forest streams depend strongly on riparian vegetation for carbon, nutrients and energy. Forest invasion by woody species with dissimilar characteristics from native species may be particularly troublesome. The invasion of temperate deciduous broadleaf forests with low representation of nitrogen (N)-fixing species by N-fixers has the potential to induce ecosystem changes at the stream level. Although effects of tree invasion on stream ecosystems have been under assessed, knowledge of native and invasive tree characteristics allows prediction of invasion effects on streams. Here we present a conceptual model to predict the effects of forest invasion by alien N-fixing species on streams, using as a background the invasion of temperate deciduous broadleaf forests by leguminous Acacia species, which are among the most aggressive invaders worldwide. Effects are discussed using a trait-based approach to allow the model to be applied to other pairs of invaded ecosystem–invasive species, taking into account differences in species traits and environmental conditions. Anticipated effects of N-fixing species invasions include changes in water quality (increase in N concentration) and quantity (decrease in flow) and changes in litter input characteristics (altered diversity, seasonality, typology, quantity and quality). The magnitude of these changes will depend on the magnitude of differences in species traits, the extent and duration of the invasion and stream characteristics (e.g. basal nutrient concentration). The extensive literature on effects of nutrient enrichment of stream water, water scarcity and changes in litter input characteristics on aquatic communities and processes allows prediction of invasion effects on stream structure and function. The magnitude of invasion effects on aquatic communities and processes may, however, depend on interactions among different pathways (e.g. effects mediated by increases in stream nutrient concentration may contrast with those mediated by decreases in water availability or by decreases in litter nutritional quality). A review of the literature addressing effects of increasing cover of N-fixing species on streams suggests a wide application of the model, while it highlights the need to consider differences in the type of system and species when making generalizations. Changes induced by N-fixing species invasion on streams can jeopardize multiple ecosystem services (e.g. good quality water, hydroelectricity, leisure activities), with relevant social and economic consequences.     

Do foliar fungal communities of Norway spruce shift along a tree species diversity gradient in mature European forests?

Foliar fungal species are diverse and colonize all plants, though whether forest tree species composition influences the distribution of these fungal communities remains unclear. Fungal communities include quiescent taxa and the functionally important and metabolically active taxa that respond to changes in the environment. To determine fungal community shifts along a tree species diversity gradient, needles of Norway spruce were sampled from trees from four mature European forests. We hypothesized that the fungal communities and specific fungal taxa would correlate with tree species diversity. Furthermore, the active fungal community, and not the total community, would shift along the tree diversity gradient. High-throughput sequencing showed significant differences in the fungal communities in the different forests, and in one forest, tree diversity effects were observed, though this was not a general phenomenon. Our study also suggests that studying the metabolically active community may not provide additional information about community composition or diversity.     

The pristine rain forest? Remnants of historical human impacts on current tree species composition and diversity

Abstract Aim Tropical rain forests are often regarded as pristine and undisturbed by humans. In Central Africa, community‐wide disturbances by natural causes are rare and therefore current theory predicts that natural gap phase dynamics structure tree species composition and diversity. However, the dominant tree species in many African forests recruit poorly, despite the presence of gaps. To explain this, we studied the disturbance history of a species‐rich and structurally complex rain forest. Location Lowland rain forest in Southern Cameroon. Methods We identified the recruitment conditions of trees in different diameter classes in 16 ha of species‐rich and structurally complex ‘old growth’ rain forest. For the identification of recruitment preference we used independent data on the species composition along a disturbance gradient, ranging from shifting cultivation fields (representing large‐scale disturbance), to canopy gaps and old growth forest. Results In nine of sixteen 1‐ha forest plots the older trees preferred shifting cultivation fields for recruitment while younger trees preferred gaps and closed forest conditions. This indicates that these nine sites once experienced large‐scale disturbances. Three lines of evidence suggest that historical agricultural use is the most likely disturbance factor: (1) size of disturbed and undisturbed patches, (2) distribution of charcoal and (3) historical accounts of human population densities. Main conclusions Present‐day tree species composition of a structurally complex and species‐rich Central African rain forest still echoes historical disturbances, most probably caused by human land use between three to four centuries ago. Human impact on African rain forest is therefore, contrary to common belief, an issue not of the last decades only. Insights in historical use will help to get a more balanced view of the ‘pristine rain forest’, acknowledging that the dualism between ‘old growth’ and ‘secondary’ forest may be less clear than previously thought.     

How much does climate change threaten European forest tree species distributions?

Although numerous species distribution models have been developed, most were based on insufficient distribution data or used older climate change scenarios. We aimed to quantify changes in projected ranges and threat level by the years 2061–2080, for 12 European forest tree species under three climate change scenarios. We combined tree distribution data from the Global Biodiversity Information Facility, EUFORGEN, and forest inventories, and we developed species distribution models using MaxEnt and 19 bioclimatic variables. Models were developed for three climate change scenarios—optimistic (RCP2.6), moderate (RCP4.5), and pessimistic (RPC8.5)—using three General Circulation Models, for the period 2061–2080. Our study revealed different responses of tree species to projected climate change. The species may be divided into three groups: “winners”—mostly late‐successional species: Abies alba, Fagus sylvatica, Fraxinus excelsior, Quercus robur, and Quercus petraea; “losers”—mostly pioneer species: Betula pendula, Larix decidua, Picea abies, and Pinus sylvestris; and alien species—Pseudotsuga menziesii, Quercus rubra, and Robinia pseudoacacia, which may be also considered as “winners.” Assuming limited migration, most of the species studied would face a significant decrease in suitable habitat area. The threat level was highest for species that currently have the northernmost distribution centers. Ecological consequences of the projected range contractions would be serious for both forest management and nature conservation.     

Does insect herbivory on oak depend on the diversity of tree stands?

Studies on the effects of plant diversity on insect herbivory have produced conflicting results. Plant diversity has been reported to cause positive and negative responses of herbivores. Explanations for these conflicting responses include not only various population-level processes but also changes in plant quality that lead to changes in herbivore performance. In a tree diversity experiment, we investigated the effects of tree diversity on insect herbivory on oak in general and whether the effects of tree diversity on herbivore damage are reflected by the performance (leaf consumption, growth) of the generalist herbivore Lymantria dispar. Our study showed that the feeding damage caused by naturally occurring herbivores on oak trees decreased with increasing diversity of tree stands. The performance of L. dispar on oak leaves was not affected by tree diversity, neither in field nor laboratory experiments. Our results can be explained by the various processes behind the hypothesis of associational resistance.     

Regional and local impact on species diversity – from pattern to processes

The impact of regional factors (such as speciation or dispersal) on the species richness in local communities (S_L) has received increasing attention. A prominent method to infer the impact of regional factors is the comparison of species richness in local assemblages (S_L) with the total number of species in the region (S_R). Linear relations between S_R and S_L have been interpreted as an indication of strong regional influence and weak influence of interactions within local communities. We propose that two aspects bias the outcome of such comparisons: (1) the spatial scale of local and regional sampling, and (2) the body size of the organisms. The impact of the local area reflects the scales of ecological interactions, whereas the ratio between local and regional area reflects the inherent moment of autocorrelation. A proposed impact of body size on the relation is based on the high dispersal and high abundance of small organisms. We predict strongest linearity between S_R and S_L for large organisms, for large local areas (less important ecological interactions) and for sampling designs where the local habitat area covers a high proportion of the regional area (more important autocorrelation). We conducted a meta-analysis on 63 relations obtained from the literature. As predicted, the linearity of the relationship between S_L and S_R increased with the proportion of local to regional sampling area. In contrast, neither the body size of the organisms nor the local area itself was significantly related to the relation between S_L and S_R. This indicated that ecological interactions played a minor role in the shape of local to regional richness plots, which instead was mainly influenced by the sampling design. We found that the studies published so far were highly biased towards larger organisms and towards high similarity between the local and regional area. The proposed prevalence of linear relationships may thus be an artefact and plots of S_L to S_R are not a suitable tool with which to infer the strength of local interactions.     

Does predation contribute to tree diversity?

Seed and seedling predation may differentially affect competitively superior tree species to increase the relative recruitment success of poor competitors and contribute to the coexistence of tree species. We examined the effect of seed and seedling predation on the seedling recruitment of three tree species, Acer rubrum (red maple), Liriodendron tulipifera (yellow poplar), and Quercus rubra (northern red oak), over three years by manipulating seed and seedling exposure to predators under contrasting microsite conditions of shrub cover, leaf litter, and overstory canopy. Species rankings of seedling emergence were constant across microsites, regardless of exposure to seed predators, but varied across years. A. rubrum had the highest emergence probabilities across microsites in 1997, but Q. rubra had the highest emergence probabilities in 1999. Predators decreased seedling survival uniformly across species, but did not affect relative growth rates (RGRs). Q. rubra had the highest seedling survivorship across microsites, while L. tulipifera had the highest RGRs. Our results suggest that annual variability in recruitment success contributes more to seedling diversity than differential predation across microsites. We synthesized our results from separate seedling emergence and survival experiments to project seedling bank composition. With equal fecundity assumed across species, Q. rubra dominated the seedling bank, capturing 90% of the regeneration sites on average, followed by A. rubrum (8% of sites) and L. tulipifera (2% of sites). When seed abundance was weighted by species-specific fecundity, seedling bank composition was more diverse; L. tulipifera captured 62% of the regeneration sites, followed by A. rubrum (21% of sites) and Q. rubra (17% of sites). Tradeoffs between seedling performance and fecundity may promote the diversity of seedling regeneration by increasing the probability of inferior competitors capturing regeneration sites.     

How many species of host-specific insects feed on a species of tropical tree?

The assumptions on the host specificity of beetles that led Terry Erwin to suggest that there may be over 30 million arthropod species were tested for 10 species of trees and their insect associates at a rainforest site in Papua New Guinea. The data included 391 species and 4696 individuals of herbivorous beetles collected during a one year period using hand collecting, beating, branch clipping, intercept flight traps and pyrethrum knockdown. Insect host specificity was assessed by feeding trials in captivity. The data suggest that between 23 and 37 monophagous leaf-feeding species are most likely to be present in this system, whereas Erwin's method yields an estimate of 138 monophagous species. The major factors responsible for the discrepancy between our observations and Erwin's assumptions appears to be (a) the importance of transient species; (b) the insect fauna that is shared among tree species; (c) some generalist wood-eating species may inflate the apparent species richness of leaf-feeding beetles; and (d) the proportion of specialist species varies significantly among tree species. We conclude that studies reporting the proportion of specialist insect herbivores associated with particular tropical tree species will yield only a portion of the information needed to estimate global arthropod species richness, but may be useful for elucidating certain aspects of food-web ecology in tropical rain forests.     

A consistent terminology for quantifying species diversity? Yes, it does exist

The prevailing terminological confusion around the concept ‘diversity’ has hampered accurate communication and caused diversity issues to appear unnecessarily complicated. In fact, a consistent terminology for phenomena related to (species) diversity is already available. When this terminology is adhered to, diversity emerges as an easily understood concept. It is important to differentiate between diversity itself and a diversity index: an index of something is just a surrogate for the thing itself. The conceptual problem of defining diversity also has to be separated from the practical problem of deciding how to adequately quantify diversity for a community of interest. In practice, diversity can be quantified for any dataset where units of observation (such as individuals) have been classified into types (such as species). All that needs to be known is what proportion of the observed units belong to a type of mean abundance. Diversity equals the inverse of this mean, and it quantifies the effective number of the types of interest. In ecology, interest often (but not always) focuses on species diversity. If the dataset consists of (or gets divided into) subunits, then the total effective number of species (gamma diversity) can be partitioned into the effective number of compositionally distinct subunits (beta diversity) and the mean effective number of species per such subunit (alpha diversity). Species richness is related to species diversity, but they are not the same thing; richness does not take the proportional abundances into account and is therefore the actual—rather than the effective—number of types. Most of the phenomena that have been called ‘beta diversity’ in the past do not quantify an effective number of types, so they should be referred to by names other than ‘diversity’ (for example, species turnover or differentiation).     

Asymmetry patterns across the distribution range: does the species matter?

An important question in evolutionary ecology is whether different populations across a species range, from core to periphery, experience different levels of stress. The estimation of developmental instability has been proposed as a useful tool for quantifying the degree of environmental and genetic stress that individuals experience during their development. Fluctuating asymmetry, the unsigned difference between the two sides of a bilaterally symmetrical trait, has been suggested to reflect the levels of developmental instability in a population. As such, it has been proposed as a useful tool for estimating changes in developmental instability and in stress response in populations across a range of environmental conditions. Recent studies focusing mostly on birds have detected increasing fluctuating asymmetry from core to periphery across the distribution range, suggesting that peripheral populations may experience higher levels of environmental and/or genetic stress. Most of these comparisons were done for single taxa across a single gradient. However, different species are predicted to respond differently to environmental shifts across the range. We compared asymmetry patterns in wing morphology in populations of two Euchloe butterfly species across their opposing ranges in Israel. Contrary to the patterns observed in birds across the same gradient, bilateral asymmetry did not increase or shift towards the periphery in either of the butterfly species. If fluctuating asymmetry in these traits reflects levels of stress, these results may partly reflect the fact that the range of these two butterfly species is limited by the distribution of their host plant, rather than by abiotic environmental variables. In addition, developing pierids can diapause during harsh seasons and can persist in resource‐rich patches, thus minimizing the environmental stress perceived by developing individuals. We conclude that accounting for differences in species’ life histories and range‐limiting factors is necessary in order to better predict patterns of developmental instability across spatial and environmental gradients. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 81 , 313–324.     

Measuring diversity of endophytic fungi in leaf fragments: does size matter?

Endophytic fungi inhabit living plant tissues without causing disease symptoms. Although abundant, the extent of their contribution to fungal biodiversity remains unclear. Since endophytic fungi are poorly known, especially in the tropics, current estimates of fungal species are probably conservative. Here we tested strategies for sampling endophytic fungi in tropical plants. We compared the number of fungi isolated from 400 mm2 leaf pieces that were divided into increasingly small fragments. Leaf pieces were surface-sterilized, cut into fragments and plated on culture media. For a given area, cutting leaf pieces into smaller fragments significantly increased the number of fungal morphospecies recovered. There was a strong linear relationship between size of fragments and number of fungi isolated. By extrapolation, an estimated 16 +/- 3 fungi could be recovered from a 2 x 2 cm leaf piece, using infinitely small fragments. This represents a large part of the fungal diversity estimated to exist in leaf endophytes in a population. We conclude that reducing the size and increasing the number of leaf fragments will increase the number of fungal species isolated. This strategy will help to estimate real values of endophytic fungal diversity.