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Keywords:

  • diaspores;
  • dispersal;
  • grassland management;
  • Iron Age;
  • landscape;
  • land-use history;
  • prehistoric settlements;
  • species density;
  • species packing;
  • species pool

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • 1
    Species-rich semi-natural grasslands in Europe developed during prehistoric times and have endured due to human activity. At the same time, intensive grassland management or changes in land use may result in species extinction. As a consequence, plant diversity in semi-natural calcareous grasslands may be related to both historical and current human population density.
  • 2
    We hypothesize that current vascular plant diversity in semi-natural calcareous grasslands is positively correlated with the Late Iron Age (c. 800–1000 years ago) density of human settlements (indicated by Late Iron Age fortresses and villages) due to enhancement of grassland extent and species dispersal, and negatively correlated with current human population density due to habitat loss and deterioration.
  • 3
    We described the size of the community vascular plant species pool, species richness per 1 m2 and the relative richness (richness divided by the size of the species pool) in 45 thin soil, calcareous (alvar) grasslands in Estonia. In addition to historical and current human population density we considered simultaneously the effects of grassland area, connectivity to other alvar grasslands, elevation above sea level (indicating grassland age), soil pH, soil N, soil P, soil depth, soil depth heterogeneity, geographical east–west gradient, precipitation and spatial autocorrelation.
  • 4
    Both the size of the community species pool and the species richness are significantly correlated with the Late Iron Age human population density. In addition, species richness was unimodally related to the current human population density. The relative richness (species ‘packing density’) was highest in the intermediate current human population densities, indicative of moderate land-use intensity.
  • 5
    Community species pool size decreased non-linearly with increasing soil N, and was highest at intermediate elevation. Small-scale richness was greater when sites were well connected and when the elevation was intermediate. Spatial autocorrelation was also significant for both species pool size and small-scale richness.
  • 6
    In summary, human land-use legacy from prehistoric times is an important aspect in plant ecology, which could be an important contributor to the current variation in biodiversity.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The number of species in a local community depends on both the arrival of new species and local extinctions. The quantity of potential arriving species depends on the size of the species pool (Pärtel et al. 1996; Zobel 1997; Foster et al. 2004), which is a function of large-scale processes, such as evolution and historical migration of species (Ricklefs 1987; Taylor et al. 1990; Pärtel 2002). Arrival of plant species into a particular community depends on the proximity of potential seed sources (Bruun 2000; Matlack 2005), the presence of dispersal vectors (Ozinga et al. 2004) and the permeability of the matrix landscape (Ricketts 2001; Zobel et al. 2006). While loss of species in a particular community is a local event (Fischer & Stöcklin 1997; Williams et al. 2005), the loss of a single species in all local communities results in a regional or even global extinction (Dirzo & Raven 2003).

Many contemporary plant communities have developed in close association with human settlements (Hayashida 2005). Although this is certainly true for European forests and semi-natural grasslands (Behre 1988), forests and grasslands in North America (Foster 1992; Foster & Motzkin 1998), and even tropical rainforests (Heckenberger et al. 2003), also exhibit human land-use legacies. It is possible that these legacies manifest themselves in a relationship between current biodiversity and past human population density. Although some studies demonstrate biodiversity loss due to increasing human population densities (Thompson & Jones 1999; Duncan & Young 2000), other authors have reported positive correlations between human population density and plant or animal diversity on large scales (Balmford et al. 2001; Araujo 2003; Luck et al. 2004). These positive relationships may be due simply to spatial artefacts of densely populated areas with natural biodiversity hotspots or, alternatively, due to increased biodiversity caused by human activities (Cincotta et al. 2000; Kühn et al. 2004).

Semi-natural grasslands are among the most species-rich vegetation types in Europe and hold great conservational value (Eriksson et al. 2002; Poschlod & WallisDeVries 2002; Sutherland 2002; WallisDeVries et al. 2002). Although such grasslands represent ecosystems that have developed and endured due to historical and current human impact (animal husbandry, mowing, collection of firewood), the flora of European semi-natural grasslands is spontaneous (Svenning 2002; Mitchell 2005). Development of calcareous grasslands in Europe began in the Roman Era due to different types of land use (Poschlod & WallisDeVries 2002), and these grasslands have become especially widespread since the Middle Ages.

Several attempts have been made to disentangle the mechanisms behind plant diversity patterns in semi-natural grasslands. At the landscape level, current high plant species richness is related to both grassland area and connectivity 50–100 years ago (Lindborg & Eriksson 2004; Helm et al. 2006). However, the relationship with the current landscape may be less apparent as these grasslands have suffered dramatic habitat loss and fragmentation during the last century. Age of grassland ecosystems may be related to increasing diversity given that soil formation is gradual and that species accumulate relatively slowly (Pärtel & Zobel 1999). Furthermore, soil pH is often positively correlated with plant diversity within the temperate and boreal regions (Pärtel 2002). European grasslands often exhibit a unimodal relationship between plant diversity and habitat productivity (Grime 1979; Pärtel et al. 2007), as well as with other parameters related to productivity, such as thickness of the humus layer on top of the parent limestone material (Zobel 1985; Belcher et al. 1995). Diversity may also be limited by excessive soil nitrogen (e.g. McCrea et al. 2004; Crawley et al. 2005) or phosphorus availability (Janssens et al. 1998). In addition, soil resource heterogeneity is often cited as a factor increasing local species richness (Tilman 1988; Wilson 2000), which has been supported by field observations (e.g. Lundholm & Larson 2003). Furthermore, geographical and climatic gradients may be important determinants of regional species pools. Finally, spatial autocorrelation may strongly influence the patterns and analyses of causal relationships in observational data (Koenig 1999; Wagner & Fortin 2005).

Although the aforementioned factors have been shown to account for some of the variation in the plant diversity in grasslands, part of the variation still remains unresolved. In particular, historical factors are usually disregarded due to a lack of data (Foster et al. 2003). Although historical vegetation maps may exist for the last 300 years (Pärtel et al. 1999b), the effects of land-use legacy on current vegetation may extend up to 1000 years, as demonstrated in studies from North America (Foster et al. 2003; Briggs et al. 2006) and Denmark (Bruun et al. 2001).

We examine the interrelationships between plant diversity in semi-natural calcareous grasslands and both past and current human population density. We hypothesize that semi-natural grasslands occurred only in places with sufficient inhabitants to maintain many grazing animals, and that human activity enhanced the dispersal of many grassland species. During the Viking Period (800–1100 ad), as the human population increased greatly in northern Europe, the area covered by grasslands increased considerably (Berglund 1969). For example, the present extensive calcareous alvar grasslands on the Baltic island of Öland were actually formed during the Iron Age, when this region was wealthy and densely populated (Enckell et al. 1979). Because the expansion of northern European grassland communities resulted from human activities, we predict a significant relationship between the abundance of historical human settlements and the distribution of grassland species.

Human activity during the Late Iron Age (c. 1050–1250 ad) may have built the foundation for grassland species diversity that is still evident today. Current human settlements, however, do not necessarily support high plant diversity because of increasing habitat loss caused by urbanization and intensive agriculture (Stoate et al. 2001). In contrast to past land use, current agricultural practices do not support species dispersal (Poschlod et al. 1998). Indeed, negative relationships have frequently been found between current human population presence and plant biodiversity at smaller scales (Pautasso 2007).

We hypothesize that current vascular plant diversity in semi-natural calcareous grasslands is positively correlated with the Late Iron Age density of human settlements. We also hypothesize that plant diversity decreases with current human population density due to habitat loss and deterioration of the quality of the remaining habitats. We test these hypotheses while also considering the effect of other possible drivers of grassland diversity.

Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We conducted our study in Estonia, a northern European country on the eastern shore of the Baltic Sea (58–60°N, 22–28°E). The vegetation of Estonia belongs to the boreo-nemoral zone (Sjörs 1965). Calcareous grasslands are found mainly in the western and northern parts of the country, where the climate is relatively maritime; the annual precipitation is 500–700 mm and the mean temperature is 17 °C for July and −5 °C for January (Raukas 1995). Calcareous grasslands occur in areas where Ordovician or Silurian limestone bedrock is covered by only a thin layer of weathered material. The soil type is rendzic leptosol and the humus layer is 2–25 cm thick (Pärtel et al. 1999a).

Forty-five calcareous grasslands were included in the study, all representing Avenetum-type alvar grasslands (Albertson 1950). Detailed descriptions of these study sites are given in Pärtel et al. (1999a) and Znamenskiy et al. (2006). All these grasslands still exist, although management intensity has decreased considerably in recent decades.

In each of the 45 study sites, vascular plant diversity was recorded in 15 1 × 1 m plots located randomly in the grassland vegetation (675 plots in total). Large-scale plant diversity was denoted by the size of the community species pool, i.e. the total number of characteristic grassland species in a particular study site. The list of characteristic grassland species was defined by a general numerical classification of alvar grassland vegetation. We were able to recognize 68 species characteristic for this grassland type. Ruderal plant species associated with disturbed habitats, such as road verges and field margins, forest species found under juniper shrubs, or species from more rocky or moist habitats, were excluded (for the list of species, see Pärtel et al. 1999a). The mean number of plant species in the 15 plots was used to quantify the small-scale plant diversity. No species were excluded from these data because plots were only described in terms of grassland vegetation.

There is a strong positive correlation in plant species richness between the small scale (1 × 1 m plots) and the large scale (the size of the community species pool) (Fig. 1). Consequently, similar patterns are expected for small-scale richness and community species pool size. Therefore, we also calculated the relative richness, i.e. the average richness per 1 m2 divided by the size of the community species pool (Zobel & Liira 1997; Liira & Zobel 2000; Ingerpuu et al. 2001). This parameter describes how densely the pool of available species is ‘packed’ within a limited space.

image

Figure 1. Scatterplot between the average small-scale vascular plant species richness (number of species m−2) and the size of the community species pool (the number of grassland species per site) in shallow soil, calcareous grasslands.

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Extensive archaeological information on the historical distribution of human settlements (fortresses and villages) during the Late Iron Age is available for the region (Jaanits et al. 1982; Poska & Saarse 2002b; Veski et al. 2005). We quantified relative human population densities during the Late Iron Age for each study site dependent upon their distance from the Late Iron Age fortresses and villages (Fig. 2a; Kriiska & Tvauri 2002), using the formula of Moilanen & Nieminen (2002):

image

Figure 2. Map of the study area. (a) Iron Age human population distribution (larger dots represent fortresses, smaller dots villages) and elevation above sea level (the darker the higher in the scale of 0–60 m). (b) Current human population density (darker shading represents higher density). Study sites are indicted by squares on both maps. Some sites have been slightly shifted for clarity.

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  • Human population density =Σ exp(−αdi)Ai.

The summation included all settlement points; di is the distance in kilometres between a grassland and settlement point i with relative size Ai (we set 1 for a fortress and 0.01 for a minor settlement). This measure uses a negative exponential influence, where α describes the average distance of influence (average influence radius = 1/α). Here we set α to 0.1 (analogous to 10 km). In this way we considered much more data than methods based on the nearest neighbour or count of settlement points within a radius. This parameter is relative, showing the variation of historical human population density, not the absolute number of humans per area. Current population densities (Fig. 2b) were obtained from the official population census in 1999 from Statistics Estonia (http://www.stat.ee). Iron Age population density is the oldest one that can be reliably quantified, and the 1999 data are the most recent. In this instance these two population densities were not significantly correlated (r = 0.3, P > 0.05).

In addition to human population density, the following variables that may affect plant diversity were also incorporated in the analysis. (i) Grassland area in the 1930s, as previous studies have shown no relationship with current grassland area but with the area 70 years ago (Helm et al. 2006). (ii) Connectivity to other similar grasslands, derived from the same map from the 1930s (for details, see Helm et al. 2006). (iii) Elevation above sea level, which serves as a proxy for grassland age, a reflection of a continuous isostatic land uplift of approximately 2.5 mm yr−1 (Pärtel & Zobel 1999). The elevation was measured from a 1 : 50 000 digital map (Fig. 2a). Twenty soil samples from the top 10 cm were collected from random locations in each grassland, amalgamated into a single sample, kept air-dry and analysed for (iv) pH (KCl solution), (v) total N content (Kjeldahl method) and (vi) P content (extracted in ammonium lactate solution). (vii) Soil depth was measured with a metal stick in 20 locations within each grassland and the mean was calculated. In thin alvar soils on calcareous parent material, the depth of the humus layer is a proxy of soil fertility (Zobel 1985; Belcher et al. 1995). (viii) Soil heterogeneity was calculated as the coefficient of variation of the soil depth (standard deviation divided by the mean). We included also (ix) the geographical east–west gradient because western Estonia is much more species-rich than eastern Estonia (Kull et al. 2002), and (x) local precipitation variability (long-term mean) because this may affect the distribution of many grassland species in some regions (Ejrnæs & Bruun 2000).

All variables were tested for normality (Kolmogorov–Smirnov test) and log10-transformed when necessary. A general linear mixed model (SAS proc mixed procedure; Littell et al. 1996) was used to generate models in which different plant diversity parameters (the size of the community species pool, small-scale species richness and the relative richness) were related to all possible combinations of dependent parameters (testing both linear and quadratic relationships). We used a forward stepwise selection procedure to combine significant factors. In the next step we tried all other factors that were correlated to any of the factors in the model of stepwise selection. From all these models we chose the optimal according to the Akaike information criterion (AIC; Akaike 1973). In order to consider the effect of spatial autocorrelation, we used a spatial correlation setting that included a distance-dependent covariance matrix into the model and adjusted the test statistics accordingly (see Littell et al. 1996; Evans et al. 2006). We used the power function for the spatial covariance structure as it gave the most stable results and the lowest AIC values. Note that it is not possible to calculate R2 values for mixed models with spatial autocorrelation settings, but the approximate estimate of R2 can be obtained from the likelihood ratio test statistic of a model (Magee 1990). To test for possible outlier effects on models we performed the leave-one-out cross-validation (jackknife) of model parameter estimates. There were, however, no notable outliers found in any model.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The size of the community species pool in calcareous grasslands showed a positive linear correlation with the Late Iron Age human population density (Fig. 3, Table 1). Two of the additional factors had a significant non-linear effect on the size of the community species pool: elevation, indicating the age of the land plant community, and soil N content. In addition, spatial autocorrelation was highly significant in the model.

image

Figure 3. The size of the community species pool related to the Iron Age human population density (relative units), elevation above sea level and soil nitrogen content. For statistics, see Table 1.

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Table 1.  Results of the general linear mixed model analysis, addressing variation of species diversity in shallow soil, calcareous grasslands. The approximate estimates of R2 were obtained from the likelihood ratio test statistics
Dependent variableEffectsStatisticsP
Community species poolIron Age human density d.f. = 1, 39; F = 13.20.0008
R2 = 0.31Soil N content d.f. = 1, 39; F = 7.90.0078
Soil N content2 d.f. = 1, 39; F = 10.20.0027
Log elevation d.f. = 1, 39; F = 11.40.0017
Log elevation2 d.f. = 1, 39; F = 14.80.0004
Spatial autocorrelationZ = −9.0< 0.0001
Species richness per 1 m2Iron Age human density d.f. = 1, 38; F = 4.20.0467
R2 = 0.66Current human density d.f. = 1, 38; F = 12.90.0009
Current human density2 d.f. = 1, 38; F = 5.80.0210
Connectivity d.f. = 1, 38; F = 6.70.0137
Log elevation d.f. = 1, 38; F = 5.30.0274
Log elevation2 d.f. = 1, 38; F = 6.00.0137
Spatial autocorrelationZ = 17.1< 0.0001
Relative richnessCurrent human density d.f. = 1, 42; F = 21.4< 0.0001
R2 = 0.34Current human density2 d.f. = 1, 42; F = 13.80.0006

Small-scale species richness showed a positive linear correlation with the Late Iron Age human population density and a significant unimodal relationship with the current human population density (Fig. 4, Table 1). In addition, small-scale species richness showed a positive linear correlation with the historical connectivity to other similar grasslands, and a unimodal relationship with elevation. Spatial autocorrelation was also significant.

image

Figure 4. Small-scale plant species richness (average of 15 1-m2 plots) related to the Iron Age human population density (relative units), current human population density (persons km−2), connectivity to other grasslands and elevation above sea level. For statistics, see Table 1.

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Relative richness had a unimodal relationship with the current human population density (Fig. 5, Table 1). Unlike the models with other diversity parameters, no significant spatial autocorrelation was detected.

image

Figure 5. Relative richness (average small-scale richness divided by the size of the community species pool) related to the current human population density (persons km−2). For statistics, see Table 1.

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We hypothesized that species diversity in calcareous grasslands is positively dependent on human population density 1000 years ago. Indeed, we found that both the size of the community species pool and the small-scale richness are significantly positively correlated with the Late Iron Age human settlement density (Figs 3 & 4). We show for the first time that high prehistorical human population density coincides with the high current plant diversity in semi-natural grasslands.

The possible effect of historical human influence on contemporary plant communities has been overlooked by plant ecologists (Briggs et al. 2006). European semi-natural grasslands were kept open by grazing, mowing and collecting firewood. Palynological records combined with archaeological data from northern Estonia reveal a high diversity of grassland species and increasing and constant animal husbandry since 1500 bc (Poska & Saarse 2002a). During the Late Iron Age (c. 1000 years ago), the entire Baltic Sea region was densely populated, grasslands were common and palynological records indicate high diversity of grassland species (Enckell et al. 1979; Lindbladh 1999; Poska & Saarse 2002b). In general, the prehistorical pattern of grasslands has endured throughout the following centuries, as indicated by studies from the Baltic island of Gotland (Carlsson 1979) and the British Isles (Spratt 1991). The most dramatic changes in Estonian calcareous grasslands have probably occurred since the middle of the last century (Pärtel et al. 1999b). In addition, plant diaspores may have historically been transported unintentionally together with animals and hay. During times of war, people and animals were assembled in fortresses, which also served as administrative and market centres (Enckell et al. 1979). The use of hay as the packing material for the transport of fragile objects may have led to considerable diaspore dispersal, even between different regions in Europe (Fries 1969). Thus, relatively continuous land use and enhancement of plant dispersal have most likely contributed to the diversity patterns that we can observe even today.

Small-scale plant species richness was related to the size of the community species pool, and both were related to the Late Iron Age human population density. Community species pool size did not show a significant relationship with the current human population density. By contrast, small-scale richness was unimodally related to the current density of the human population (Fig. 4). Proper grassland management prevents local species extinctions, but dispersal between grassland fragments is very limited today (Fischer & Stöcklin 1997; Poschlod et al. 1998). The tremendous decrease in human population density in many rural areas in northern Europe has led to a reduction in land-use intensity (Eriksson et al. 2002; Poschlod & WallisDeVries 2002). This reduction may result in extensive overgrowth of former semi-natural grasslands by shrubs and trees, accompanied by a decrease in species richness (Mitlacher et al. 2002). By contrast, areas more suitable for modern agriculture are distinguished by a higher density of rural population and more intensive land use, resulting in decreasing area and habitat quality of semi-natural grasslands (Stoate et al. 2001). High current human population density may also reflect urbanized areas close to larger towns where (semi)natural communities have been transformed into urbanized or industrial areas. Consequently, the unimodal relationship between small-scale richness and current human population density may indicate the loss of grassland species to land abandonment in areas of low population density, and to urban or industrial development at high population density.

In order to study the effect of local-scale processes only, we examined the variation of relative richness (small-scale richness divided by the size of the species pool). Relative richness was unimodally related to current human population density (Fig. 5). Within a particular species pool, the size of which is determined primarily by historical human activity, species may assemble at different densities within a limited space. The species ‘packing density’ was highest at intermediate human influence. A study conducted on a smaller scale in a calcareous wooded meadow concluded that the level of small-scale species richness within the same species pool depends on the continuity of the traditional management regime (Kull & Zobel 1991). The present study, conducted on a larger scale, confirms that relative richness may be a suitable indicator of the recent management conditions in semi-natural grassland communities. Similarly, Bruun et al. (2001) found that the beta diversity (inversely related to the relative richness) in Danish grasslands was dependent on contemporary environment, and not on variables describing land-use history.

We also considered several other parameters that have been related to plant species richness in semi-natural grasslands. Whereas elevation above sea level, connectivity to other similar grasslands, soil N content and spatial autocorrelation were included in the models, historical and current human population density both exhibited significant correlation with current plant species diversity.

The size of the community species pool was greatest in plant communities of intermediate age (c. 2000–3000 years old, corresponding to an elevation of approximately 10 m above sea level). In younger communities, both the lack of propagules and ongoing soil formation may limit the number of species (Pärtel & Zobel 1999). In older areas, the dominance of different land uses (e.g. forests or arable fields) on mature soils may isolate grassland stands and hinder dispersal of grassland species. Historical connectivity has been shown to be an important determinant of species richness in grasslands (Lindborg & Eriksson 2004; Helm et al. 2006), although we found a significant relationship only with the small-scale richness. Thus, the impact of both age and historical connectivity is evidently related to the effect of limited seed dispersal, in turn constraining the diversity of semi-natural grasslands. Community species pool size also decreased with soil N content. A decline of diversity with habitat productivity is often found in temperate regions (Pärtel et al. 2007), and this can be caused by processes such as interspecific competition (Al Mufti et al. 1977; Grime 1979), dispersal limitation (Pärtel & Zobel 2007) and evolutionary constraints (Bruun & Ejrnæs 2006).

Our statistical models also included spatial autocorrelation settings for diversity variables (response variables). The spatial autocorrelation effect was significant for the size of the community species pool and for the small-scale species richness (Table 1). This phenomenon is understandable on the larger spatial scales as species dispersal between sites is spatially constrained, and the historical and environmental conditions of nearby grasslands tend to be more similar than distant ones. At the same time, spatial autocorrelation was not significant for relative richness. As discussed above, the relative richness depends more on recent local management, which is not necessarily similar in adjoining sites.

To summarize, we were able to demonstrate for the first time that the size of the community species pool and the small-scale species richness are positively correlated with the human settlement density as far back as 1000 years ago. Current human influence of intermediate intensity is needed for dense ‘packing’ of grassland plant species within a limited space. Therefore, human land-use legacy is an important aspect in plant ecology that may contribute substantially to the current variation in biodiversity.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We thank Dr Hans Cornelissen, Dr Robert Szava-Kovats and anonymous reviewers for valuable comments, and the Estonian Science Foundation (Grants 6614 and 6619), EU 6FP projects ALARM (GOCECT-2003–506675) and COCONUT, and the University of Tartu for their support.

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  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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