Grasslands In Sweden Research Articles

Sustainable Grazing by Cattle and Sheep for Semi-Natural Grasslands in Sweden

Despite their importance for biodiversity and other ecosystem services, many semi-natural grasslands deteriorate or have even disappeared due to insufficient grazing and neglect. Preservation of grassland habitats depends on a good understanding of sustainable grazing management as well as effective agricultural policy measures that ensure long-term economic sustainability for the farmer. Through meta-evaluation and synthesis of previous investigations and discussion of scientific literature, we aimed to evaluate factors that determine the extent to which cattle and sheep in Sweden graze semi-natural grasslands instead of more productive land and what this means for biodiversity and sustainability. We also aimed to propose which practises and policy measures may be the most cost-effective to promote habitat quality and the sustainable use of grasslands. Results from a nationwide survey of Swedish farmers’ attitudes towards agri-environmental payment schemes are discussed in relation to farm characteristics and other factors influencing the use of cattle and sheep for sustainable grazing. This study supports recommendations by environmental economists that payments should be targeted more strongly at the most valuable grasslands, emphasising the need for a more detailed and nuanced framework for classifying grasslands in Europe. A comparison with independent estimates of the area of agricultural land from nation-wide, sample-based monitoring shows that the data from official statistics normally used for nationwide evaluations are partly biased and of insufficient quality, underscoring the need for more sophisticated and precise methods for monitoring both overall trends and detailed environmental effects related to the preservation of semi-natural grasslands.

A cross‐disciplinary forum promoting efficient biodiversity conservation in “new” grasslands in Sweden

AbstractDue mainly to changes in land use, high nature value grasslands are becoming increasingly rare in agricultural areas. As a consequence, biodiversity is decreasing and many species are facing the risk of extinction. At the same time, the present‐day landscape contains many “new” grassland habitats, for example golf courses, roadside verges, power line corridors, and railway banks. These habitats are well distributed over the landscape with a potential to connect remnant patches of valuable grasslands. As all kinds of grasslands, they need appropriate management to preserve biodiversity. To achieve this, innovative solutions are needed. In order to exchange knowledge and experiences in this field, we established a cross‐disciplinary working group, with members from a large number of National authorities, Universities, power grid companies, and various other stakeholders such as the Swedish Golf Federation. This group has become an invaluable forum for project cooperation and discussions about practical management issues, monitoring, and research. After 10 years we are now approaching a common understanding on how infrastructure habitats can play an important role in nature conservation. At present, this has led to adaptive management of a large number of high value grasslands, for example in power line corridors, along roadside verges, and on golf courses. In addition, three landscape‐scale projects have been initiated with the long‐term aim to establish a green infrastructure.

Butterflies in Swedish grasslands benefit from forest and respond to landscape composition at different spatial scales

ContextLoss and fragmentation of semi-natural grasslands has critically affected many butterfly species in Europe. Habitat area and isolation can have strong effects on the local biodiversity but species may also be strongly affected by the surrounding matrix.ObjectivesWe explored how different land cover types in the landscape explained the occurrence of butterfly species in semi-natural grasslands.MethodsUsing data from 476 semi-natural grasslands in Sweden, we analysed the effect of matrix composition on species richness and occurrence. Additionally, we analysed at which spatial scales butterflies responded to matrix types (forests, semi-natural grasslands, arable land and water).ResultsForest cover showed the strongest positive effect on species richness, followed by semi-natural grasslands. Forest also had a positive effect on red-listed species at local scales. Responses to matrix composition were highly species-specific. The majority of the 30 most common species showed strong positive responses to the amount of forest cover within 200–500 m. There was a smaller group of species showing a positive response to arable land cover within 500–2000 m. Thirteen species showed positive responses to the amount of semi-natural grasslands, generally at larger scales (10–30 km).ConclusionsOur study showed that surrounding forest is beneficial for many grassland butterfly species and that forests might mitigate the negative effects of habitat loss caused by agricultural intensification. Also, semi-natural grasslands were an important factor for species richness at larger spatial scales, indicating that a landscape consisting mainly of supporting habitats (i.e. forests) are insufficient to sustain a rich butterfly fauna.

The conservation benefit of mowing vs grazing for management of species‐rich grasslands: a multi‐site, multi‐year field experiment

Species‐rich semi‐natural grasslands in Europe are becoming more fragmented and many species that depend on this habitat type are rare and threatened today. Management methods like mowing and grazing are needed to preserve remaining grasslands. Because management is costly it is important to use the most cost‐effective as well as the most beneficial management method, but few studies have compared mowing and grazing. We investigated the effect of mowing and grazing on grassland vegetation using data from 11 long‐term field trials situated in southern Sweden. We calculated the change in the odds of finding species belonging to three different groups of indicators at the start of the treatment and after 8 and 14 years. The used indicator groups were indicators of good management, excess nitrogen and poor management. The results revealed an increase in the odds of finding indicators of good management in mowed plots and an increase in finding indicators of excess nitrogen in grazed plots. The odds of finding indicators of poor management remained unchanged. Results from sub‐analysis of the grazing intensity showed a more negative effect from grazing with low grazing intensity than normal/high grazing intensity. Therefore, mowing is the best long‐term management method for semi‐natural grasslands in Sweden and grazing using a low grazing intensity should be avoided.

Dark diversity in dry calcareous grasslands is determined by dispersal ability and stress‐tolerance

Temperate calcareous grasslands are characterized by high levels of species richness at small spatial scales. Nevertheless, many species from a habitat‐specific regional species pool may be absent from local communities and represent the ‘dark diversity’ of these sites. Here we investigate dry calcareous grasslands in northern Europe to determine what proportion of the habitat‐specific species pool is realized at small scales (i.e. how the community completeness varies) and which mechanisms may be contributing to the relative sizes of the observed and dark diversity. We test whether the absence of particular species in potentially suitable grassland sites is a consequence of dispersal limitation and/or a low ability to tolerate stress (e.g. drought and grazing).We analysed a total of 1223 vegetation plots (1 × 1 m) from dry calcareous grasslands in Sweden, Estonia and western Russia. The species co‐occurrence approach was used to estimate the dark diversity for each plot. We calculated the maximum dispersal distance for each of the 291 species in our dataset by using simple plant traits (dispersal syndrome, growth form and seed characteristics). Large seed size was used as proxy for small seed number; tall plant height and low S‐strategy type scores were used to characterise low stress‐tolerance.Levels of small‐scale community completeness were relatively low (more species were absent than present) and varied between the grasslands in different geographic areas. Species in the dark diversity were generally characterized by shorter dispersal distances and greater seed weight (fewer seeds) than species in the observed diversity. Species within the dark diversity were generally taller and had a lower tolerance of stressful conditions.We conclude that, even if temperate grasslands have high levels of small‐scale plant diversity, the majority of potentially suitable species in the regional species pool may be absent as a result of dispersal limitation and low stress‐tolerance.

Historical Landscape Perspectives on Grasslands in Sweden and the Baltic Region

A landscape perspective is generally recognized as essential for conservation biology. The main underlying reason is that species respond to features of the landscape at various spatial scales, for example habitat area, connectivity, and matrix habitats. However, there is also an “historical” component of a landscape perspective, which has not received similar attention. The underlying reasons for historical effects are that humans have influenced landscapes during several millennia and that species and communities may respond slowly to land use change. An historical perspective on landscapes also relates to how we perceive “natural” vs. “cultural” landscapes, and thus how conservation actions are motivated and valuated. We review studies from Sweden and the Baltic region in the context of an historical landscape perspective, focusing on semi-natural grasslands, i.e., grasslands formed by long-term human management for grazing and hay-making. Semi-natural grasslands are today a high concern for conservation. Historical effects are ubiquitous on species distributions and patterns of species richness, and have important implications for developing informed conservation programs in semi-natural grasslands, particularly with regard to assumptions of historical baselines, the choice of conservation targets, and insights on time-lags in the response of species to current landscape change.

Are restricted species checklists or ant communities useful for assessing plant community composition and biodiversity in grazed pastures?

Semi-natural grasslands in Sweden are species-rich, and their natural values are strongly dependent on continuous management, mainly by grazing. However, the large heterogeneity in vegetation within and between grassland sites must be taken into account when designing management and preservation schemes, calling for precise field monitoring and assessment of habitat type and land use history. We have evaluated different surrogate measures to assess community composition and biodiversity of the most common vegetation types in grazed semi-natural pastures. We compared the complete plant community, two reduced checklists intended for quick surveys of the plant community, and the ant community. The results suggest that the taxonomic resolution in a plant inventory is important for both biodiversity assessment and recognition of vegetation types. The extent of a reduced species checklist was of greater importance than its quality for describing the plant community. Reduced checklists should only be used if they comprise species with known affinity to the studied vegetation types. We also found that plants and ants experience grazed semi-natural grasslands in different ways. Ant communities did not resemble the communities deduced from plant inventories, or vegetation types recognised by field staff.

Ant and plant species richness in relation to grazing, fertilisation and topography

Summary The preservation of biodiversity within agricultural landscapes in Europe is of great importance. Conservation priorities in semi-natural grasslands in Sweden have mainly focused on preventing accumulation of litter by management with grazing or haymaking. The effects of specific management regimes on species diversity of different taxa or taxonomic groups can differ considerably. In this study we investigated how plant and ant diversity and community composition were affected by grazing, fertilisation and topography. The results show that plant species richness was higher in grazed than abandoned pastures but ant species richness was not affected by grazing. Both plant and ant species richness was negatively affected by fertilisation. Neither ant nor plant species richness was affected by topography. No significant relation was found between the total numbers of ant and plant species. Both plant species composition and ant species composition were affected by fertilisation and grazing. When considering management or restoration tools for grasslands it is important to focus on more than one taxonomic group.

Validation of plant diversity indicators in semi-natural grasslands

The use of plant biodiversity indicators in a nationwide survey of semi-natural grasslands in Sweden was examined by comparing results from the survey with additional in-depth studies of plant diversity at the same 30 sites in southern Sweden. Additional grasslands, that were not subjected to the nationwide survey (i.e. rejected after an initial site selection), were also investigated in order to examine the quality of the initial site selection and to assess to what extent they harboured species rich plant communities. Results showed that grasslands that were not included in the nationwide survey were generally smaller in size and that they contained significantly lower plant diversity than sites that were included by the survey. However, some indicator species, including indicators for poor management, were abundant in both types of sites. Biodiversity indicators correctly indicated both plant species richness and plant species density when using data from the in-depth study. However, by comparing indicator species found by the nationwide survey and by the in-depth study, it was shown that the survey overlooked 42% of all indicator species occurrences, which removed the significant relationship between indicator richness and total plant species richness. Furthermore, a null model showed that the chosen indicator species did not perform significantly better than species chosen at random from the available species pool. The conclusion was that validation of indicators is crucial, because even though real correlations exist between taxa or between a subset of species and overall diversity, poor precision of surveys might make these indicators useless. This also suggests that the effort put into searching for the indicator species may have to be so high, that it may be more efficient to go directly into assessing the biological values they are supposed to indicate.

Importance of boreal grasslands in Sweden for butterfly diversity and effects of local and landscape habitat factors

A widespread decline in biodiversity in agro-ecosystems has been reported for several groups of organisms in Western Europe. The butterfly fauna was studied in 60 selected semi-natural grasslands in a coniferous-dominated boreal landscape in south-eastern Sweden. The aim was to investigate how butterfly assemblages were affected by the amount of semi-natural grasslands in the surrounding landscape. Furthermore, we wanted to determine if semi-natural grasslands in boreal landscapes harboured species otherwise declining in other parts of Europe. For each study site, the amounts of semi-natural grasslands in the landscape within radii of 500, 2,000 and 5,000 m were studied. Nine local habitat factors were also recorded. Only the amount of semi-natural grasslands within a 5,000 m radius could explain a significant part of the variation in butterfly composition, but there was no clear relationship between the amount of semi-natural grassland and butterfly diversity. Instead, this study showed that local habitat quality was very important for butterfly diversity at individual sites. Flower abundance, sward height and herb composition were the most important local factors. Patches surrounded by a small amount of semi-natural grasslands had high butterfly diversity, contrary to expectations. This may be explained by the fact that forest habitat provides a matrix with several features suitable for butterflies. The butterfly fauna was rich in species representative of low-productivity grasslands, species that are declining in other countries in Western Europe.

Local seed rain and seed bank in a species-rich grassland: effects of plant abundance and seed size

In this study, we examined the relationship between seed size, seed rain, and seed bank in a species-rich perennial grassland in Sweden. The seed rain was monitored by 100 seed traps placed in a 10 m × 10 m area for 1 year. The seed bank was sampled by taking 100 soil samples, each in close vicinity to a seed trap. Abundance of reproductive ramets in the area was estimated, since this is likely to affect the proportion of hit seed traps and seed bank samples. When abundance of reproductive ramets was accounted for, we found a negative relationship between seed size and proportion of hit seed bank samples, but we found no relationship between seed size and proportion of hit seed traps. We found strong positive relationships between the abundance of reproductive ramets and proportion of hit seed traps and seed bank samples. We also found strong positive relationships between abundance of reproductive ramets and abundance of seeds in the seed rain and the seed bank, but no relationship between seed size and abundance of seeds in the seed rain or the seed bank. We discuss these results in the context of theory suggesting that large-seeded and small-seeded species may coexist because of a trade-off between colonization and competitive abilities, where smaller-seeded species are able to reach more sites than seeds of larger-seeded species, because they are more numerous and (or) better dispersed.

Sex ratio mediated pollen limitation in the dioecious herb Antennaria dioica

We examined how population structure affects seed set and recruitment in the dioecious plant Antennaria dioica, which is a declining species inhabiting semi-natural grasslands in Sweden. Flowering and sex ratio in A. dioica were studied over 4 y at a local (1.2 km2) scale and over 1 y at a regional (742 km2) scale in southern Sweden. Antennaria dioica was also hand-pollinated during 2 y to examine whether sex ratios, male abundance, and distance to nearest male influence the degree of pollen limitation. Seed-sowing experiments were conducted to assess whether recruitment is limited by seed or microsite availability. There was a considerable spatio-temporal variation in both flowering and sex ratios. The regional scale survey showed that patch size and number of patches per site were positively correlated, and small patches of A. dioica tended to have biased sex ratios. Experimental hand-pollinations showed that the degree of pollen limitation increased with increasingly female-biased sex ratios in the closest vicinity of the experimental plants. Thus, even though A. dioica is pollinated by many different insects, a fragmented population structure has a large influence on reproductive performance of A. dioica. The seed-sowing experiments showed that recruitment is limited by a combination of seed and microsite availability. It is therefore plausible that reduced seed production due to pollen limitation translates into reduced recruitment. The results from the local and the regional scale indicate that a large fraction of local patches and populations of A. dioica have decreased sexual reproduction. The conclusion is that A. dioica is likely to be particularly sensitive to habitat fragmentation.

Invasibility: the local mechanism driving community assembly and species diversity

Recent theoretical developments involving community assembly on the one hand, and invasion biology on the other, suggest a gradual convergence in thought between what have been two largely separate theoretical initiatives. The term ‘‘invasibility’’ emerged in the field of invasion ecology to describe the susceptibility of environments to invasion by species from other regions of the world. Although Elton did not use the term ‘‘invasibility’’ in his pioneering book (1958), he did employ the concept, referring to an ecosystem’s ‘‘vulnerability to invasion’’. Given its original definition, the concept of invasibility has been limited in its scope and use, with rather little application to the larger field of community ecology. However, our assessment and usage of the concept (Davis et al. 2000, 2001) has prompted us to consider invasibility as a more general condition of all environments. This expanded perspective of invasibility has caused us to reconsider some fundamental questions and issues regarding community assembly and species diversity as well as recent discussions involving the notion of metacommunities (Leibold and Miller 2004, Leibold et al. 2004). By metacommunity, we mean a set of local communities that are linked by dispersal of multiple, and potentially interacting, species (Leibold et al. 2004). Recent theoretical efforts to characterize community assembly processes have reemphasized understanding the importance of interactions between local and regional processes (Levine 2000, Hubbell 2001, Tilman 2004, Foster and Dickson 2004, Jiang and Morin 2004, Steiner and Leibold 2004, Leibold et al. 2004). There is general agreement that the diversity of the regional species pool and the extent of dispersal of the species from this pool throughout the region are the principal regional processes involved. However, investigators have emphasized the importance of different local conditions and processes, including productivity (Jiang and Morin 2004, Steiner and Leibold 2004), demographic stochasticity (Tilman 2004), ecosystem size (Fukami 2004), biotic limitation of diversity (Tilman 2004), and even extent of tree lean, the latter which affects colonization success of epiphyes (Snall et al. 2005). We propose that the notion of invasibility can serve as a unifying concept in these discussions and thereby can facilitate current efforts to develop a more comprehensive and realistic theory of community assembly and metacommunity dynamics.

Effects of Restoration on Plant Species Richness and Composition in Scandinavian Semi‐Natural Grasslands

Abstract Plant species richness in rural landscapes of northern Europe has been positively influenced by traditional management for millennia. Owing to abandonment of these practices, the number of species‐rich semi‐natural grasslands has decreased, and remaining habitats suffer from deterioration, fragmentation, and plant species decline. To prevent further extinctions, restoration efforts have increased during the last decades, by reintroducing grazing in former semi‐natural grasslands. To assess the ecological factors that might influence the outcome of such restorations, we made a survey of semi‐natural grasslands in Sweden that have been restored during the last decade. We investigated how plant species richness, species density, species composition, and abundance of 10 species that are indicators of grazing are affected by (1) the size of the restored site, (2) the time between abandonment of grazing and restoration, (3) the time elapsed since restoration, and (4) the abundance of trees and shrubs at the restored site. Only two factors, abundance of trees and shrubs and time since restoration, were positively associated with total species richness and species density per meter square at restored sites. Variation in species composition among restored sites was not related to any of the investigated factors. Species composition was relatively similar among sites, except in mesic/wet grasslands. The investigated factors had small effects on the abundance of the grazing‐indicator species. Only Campanula rotundifolia responded to restoration with increasing abundance and may thus be a suitable indicator of improved habitat quality. In conclusion, positive effects on species richness may appear relatively soon after restoration, but rare, short‐lived species are still absent. Therefore, remnant populations in surrounding areas may be important in fully recreating former species richness and composition.

Modelling the effects of landscape pattern and grazing regimes on the persistence of plant species with high conservation value in grasslands in south-eastern Sweden

Semi-natural grasslands in Sweden are threatened by land-use change and lack of management with attendant risk to their biodiversity. We present a model to explore the effects of grazing frequency ...

Functional response, seasonal decline and landscape differences in nest predation risk.

Previous studies investigating density-dependent nest predation risk have suffered from unnaturally high nest densities (artificial nests) or lack of controls for nest predator densities (observational data on real nests). I designed an experiment with artificial nests within the natural range of nest densities using a Latin square design (to control for differences in densities of nest predators in time and space) to avoid the shortcomings of previous studies. I incorporated confounding factors, such as time in the breeding season and landscape type, in order to study whether they affected nest predation risk. The experiment was performed in dry semi-natural shrub-rich grasslands in Sweden, using artificial shrub-nests containing two quail (Coturnix coturnix) eggs and a plasticine egg. Finally, I validated my results obtained from the artificial nest experiment by comparing relative nest predation risk on artificial nests with nest predation risk on real nests of red-backed shrikes (Lanius collurio), a shrub-nesting passerine. Corvids were the major nest predators on artificial shrub nests as revealed by marks in plasticine eggs. Within the natural variation in densities of simultaneously active shrub nests, corvids increased their rates of predation with increasing densities of artificial nests, indicating a functional response. Nest predation risk decreased with time in the season and differed between grassland plots in farmland-dominated (high risk), farmland-forest mosaic (low risk), and forest-dominated (low risk) landscape surroundings. Furthermore, predation risk on artificial nests increased with decreasing distance to nests of at least one corvid species. Breeding red-backed shrikes selected grasslands with a low nest predation risk on artificial nests and reproductive success of shrikes was positively related to success of artificial nests. Moreover, the probability of success for both artificial and real red-backed shrike nests increased with increasing distance from the nearest corvid nest. Thus, results from the artificial nest experiment were validated by the results from the red-backed shrike population study. My results therefore suggest that predation risk measured on artificial nests can be used as a relative index of spatial and temporal variation in nest predation risk. My results also indicate that grasslands in forested landscapes should be given extra attention for conservation aspects, since nest predation risk is lower there compared to grasslands in more open habitats.

Floristic composition of a Swedish semi‐natural grassland during six years of elevated atmospheric CO2

Abstract. A semi‐natural grassland in Sweden was exposed to an elevated CO2 concentration during a six‐year open‐top chamber experiment. Vegetation composition was assessed twice a year using the point‐intercept method. The field had been grazed previously, but when the experiment started this was replaced with a cutting regime with one cut (down to ground level) each year in early August. From the third to the sixth year of the study the harvested material was divided into legumes, non‐leguminous forbs and grasses, dried and weighed.Elevated CO2 had an effect on species composition (as analysed by Principal Component Analysis) that increased over time. It also tended to increase diversity (Shannon index) in summer, but reduce it in spring. However, the effects of the weather and/or time on species composition and diversity were much more prominent than CO2 effects. Since the weather was largely directional over time (from dry to wet), with the exception of the fifth year, it was difficult to distinguish between weather effects and changes caused by a changed management regime.In all treatments, grasses increased over time in both mass and point‐intercept measurements, whereas non‐leguminous forbs decreased in mass, but not in point‐intercept measurements. Legumes increased in the point‐intercept measurements, but not in biomass, at elevated CO2, but not in the other treatments.Overall, we found that elevated CO2 affected species composition; however, it was only one of many factors and a rather weak one.

Size‐related deterioration of semi‐natural grassland fragments in Sweden

Abstract.One of the most dramatic landscape changes during the 20th century in Sweden, like in most of Europe, has been the reduction and fragmentation of semi‐natural grasslands. Using a set of remnant semi‐natural grasslands, chosen to be as similar as possible, but differing in size, we have examined whether size of remnant fragments of traditionally managed semi‐natural grasslands in Sweden is related to patterns of species richness and composition. We focused on edge‐to‐interior relationships, since we expected that a possible impact from invasive habitat generalists would be manifested in a gradient from the edge of fragments to their interior. We found no relationship between size of grassland fragments and (a) overall species richness, (b) species richness at different spatial scales, and (c) abundance of some typical invader species or species characteristic of semi‐natural grasslands. However, the results indicated that larger grasslands have a comparatively larger number of species in the edges, whereas the opposite pattern was found in smaller grasslands. The similarity in species composition between the edge and the interior of the pastures also increased with grassland size. Thus, even though the overall species richness is still unaffected by reduction in grassland fragment size, the edges of smaller grasslands show signs of degradation, i.e. reduction in species richness and a decreased similarity to the grassland interior. We suggest that these kinds of effects may be early signs of fragmentation effects that in the future will result in species loss even if the present distribution of semi‐natural grasslands is maintained.

Floristic composition of a Swedish semi-natural grassland during six years of elevated atmospheric CO2

Abstract A semi-natural grassland in Sweden was exposed to an elevated CO2 concentration during a six-year open-top chamber experiment. Vegetation composition was assessed twice a year using the point-intercept method. The field had been grazed previously, but when the experiment started this was replaced with a cutting regime with one cut (down to ground level) each year in early August. From the third to the sixth year of the study the harvested material was divided into legumes, non-leguminous forbs and grasses, dried and weighed. Elevated CO2 had an effect on species composition (as analysed by Principal Component Analysis) that increased over time. It also tended to increase diversity (Shannon index) in summer, but reduce it in spring. However, the effects of the weather and/or time on species composition and diversity were much more prominent than CO2 effects. Since the weather was largely directional over time (from dry to wet), with the exception of the fifth year, it was difficult to distinguish ...

High species mobility in species-rich plant communities: An intercontinental comparison

Using results from a long-term study of fine-scale dynamics in grasslands in four widely separated study areas from two continents, we provide further evidence to support the idea of the carousel model as an aid to describe the high fine-scale temporal and spatial species mobility found in grassland communities. Cumulative species numbers on small subplots in plots situated in stable plant communities, determined as the sum of species appearing in these subplots in one or more years over a period of time, are very high. In floristically different species-rich grasslands, varying from moist pine savannas in North Carolina and Mississippi, to humid chalk grassland in the Netherlands and seasonally dry limestone grassland in Sweden, average species numbers on subplots of 0.01 m2 in plots of 2.5 m2 over the period 1985–1989 were similar, most plots falling in the range 10.8–13.2. The total cumulative species numbers were similar as well, most plots falling in the range 17.4 and 20.9. Yearly average species numbers remained relatively constant. Considerable species turnover is occurring in all these communities; on average three species appear and three disappear each year in each 0.01 m2 subplot. Total species accumulation on 0.01 m2 subplots over the period 1985–1989 varied considerably, from 4.1 to 11.6, and is correlated with the cumulative species total on the plot, the latter figure being considered as correlated with the size of the species pool.