Meelis Pärtel

Extinction debt: a challenge for biodiversity conservation

Local extinction of species can occur with a substantial delay following habitat loss or degradation. Accumulating evidence suggests that such extinction debts pose a significant but often unrecognized challenge for biodiversity conservation across a wide range of taxa and ecosystems. Species with long generation times and populations near their extinction threshold are most likely to have an extinction debt. However, as long as a species that is predicted to become extinct still persists, there is time for conservation measures such as habitat restoration and landscape management. Standardized long-term monitoring, more high-quality empirical studies on different taxa and ecosystems and further development of analytical methods will help to better quantify extinction debt and protect biodiversity.

Habitat fragmentation causes immediate and time‐delayed biodiversity loss at different trophic levels

Intensification or abandonment of agricultural land use has led to a severe decline of semi-natural habitats across Europe. This can cause immediate loss of species but also time-delayed extinctions, known as the extinction debt. In a pan-European study of 147 fragmented grassland remnants, we found differences in the extinction debt of species from different trophic levels. Present-day species richness of long-lived vascular plant specialists was better explained by past than current landscape patterns, indicating an extinction debt. In contrast, short-lived butterfly specialists showed no evidence for an extinction debt at a time scale of c. 40 years. Our results indicate that management strategies maintaining the status quo of fragmented habitats are insufficient, as time-delayed extinctions and associated co-extinctions will lead to further biodiversity loss in the future.

The species pool and its relation to species richness: Evidence from Estonian plant communities

Two types of species pool are distinguished. The regional species pool is defined as the set of species, occurring in a certain region (here: Estonia) which are capable of coexisting in a target community. The actual species pool is defined as the set of species present in a community. Field data from 14 different vegetation types in Estonia were used. The regional pool was compiled by including from the regional flora (1) all species for which the Ellenberg indicator values did not differ more than 1.5 relative units from the community mean and (2) all indifferent species. The actual pool was compiled by careful field observations. The aim of the paper is to test the validity of two null hypotheses about the species pool. HO 1 postulates that any size of the actual species pool is equally probable in the interval between zero and the size of the regional species pool. HO 2 postulates that any value of species richness per unit area (1 m 2 ) is equally probable in the interval between zero and the size of the actual species pool. To test the strengths of the relationships Monte Carlo modelling was used. It was shown that the relation between variables was stronger than proposed by the null models (P = 0.041 for HO 1 and P = 0.002 for HO 2 ). Consequently, the size of the actual species pool is largely determined by the regional species pool, and the species richness per 1 m 2 is largely determined by the actual pool. The results are discussed in the framework of coexistence theory. The size of the regional pool is determined by evolutionary (speciation) and historical (large-scale migration) processes. The size of the actual pool depends on local-scale migration, which can be a function of isolation, successional stage, local management history, etc.

Ecological assembly rules in plant communities-approaches, patterns and prospects

Understanding how communities of living organisms assemble has been a central question in ecology since the early days of the discipline. Disentangling the different processes involved in community assembly is not only interesting in itself but also crucial for an understanding of how communities will behave under future environmental scenarios. The traditional concept of assembly rules reflects the notion that species do not co‐occur randomly but are restricted in their co‐occurrence by interspecific competition. This concept can be redefined in a more general framework where the co‐occurrence of species is a product of chance, historical patterns of speciation and migration, dispersal, abiotic environmental factors, and biotic interactions, with none of these processes being mutually exclusive.

LOCAL PLANT DIVERSITY PATTERNS AND EVOLUTIONARY HISTORY AT THE REGIONAL SCALE

The effect of evolutionary history on local-scale diversity patterns has often been suggested, but not shown. I explored whether widely described local-scale relationships between plant species richness and soil pH are related to evolutionary history. I expected positive relationships to occur between richness and pH if the pool of species that is suited for high pH soil is larger than the pool of species that is suited for low pH soil. In contrast, I expected negative relationships to occur between richness and pH if the pool of species that is suited for low pH soil is larger than the pool of species that is suited for high pH soil. I call this the species pool concept, because the direction of the relationship between richness and pH depends on whether the species pool has evolutionary origin on soils of high or low pH. I used 85 published studies from all over the world and found that positive relationships between richness and pH were significantly more probable in floristic regions where evolutionary centers were on high pH soils, and negative relationships between richness and pH were more probable in regions where evolutionary centers were on low pH soils. Because soil pH increases with latitude, I also found that the relationship between richness and pH was positive at high latitudes and negative at low latitudes. Consequently, local relationships between plant diversity and soil pH are clearly related to evolutionary history.

Global assessment of arbuscular mycorrhizal fungus diversity reveals very low endemism

Cosmopolitan plant root symbionts The aboveground lives of plants are only sustainable because of the symbiotic soil fungi that encase their roots. These fungi swap nutrients with plants, defend them from attack, and help them withstand abrupt environmental changes. Out of necessity, fungal symbionts in the soil would appear to be restricted and local to certain plant species. Davison et al., however, discovered that some taxa are globally distributed. How these underground fungi have dispersed so widely remains a mystery; perhaps human farmers have had something to do with it. Science, this issue p. 970 The wide distribution of plant-root fungal symbionts seems to be driven by recent dispersal rather than ancient tectonics. The global biogeography of microorganisms remains largely unknown, in contrast to the well-studied diversity patterns of macroorganisms. We used arbuscular mycorrhizal (AM) fungus DNA from 1014 plant-root samples collected worldwide to determine the global distribution of these plant symbionts. We found that AM fungal communities reflected local environmental conditions and the spatial distance between sites. However, despite AM fungi apparently possessing limited dispersal ability, we found 93% of taxa on multiple continents and 34% on all six continents surveyed. This contrasts with the high spatial turnover of other fungal taxa and with the endemism displayed by plants at the global scale. We suggest that the biogeography of AM fungi is driven by unexpectedly efficient dispersal, probably via both abiotic and biotic vectors, including humans.

Integrative modelling reveals mechanisms linking productivity and plant species richness

How ecosystem productivity and species richness are interrelated is one of the most debated subjects in the history of ecology. Decades of intensive study have yet to discern the actual mechanisms behind observed global patterns. Here, by integrating the predictions from multiple theories into a single model and using data from 1,126 grassland plots spanning five continents, we detect the clear signals of numerous underlying mechanisms linking productivity and richness. We find that an integrative model has substantially higher explanatory power than traditional bivariate analyses. In addition, the specific results unveil several surprising findings that conflict with classical models. These include the isolation of a strong and consistent enhancement of productivity by richness, an effect in striking contrast with superficial data patterns. Also revealed is a consistent importance of competition across the full range of productivity values, in direct conflict with some (but not all) proposed models. The promotion of local richness by macroecological gradients in climatic favourability, generally seen as a competing hypothesis, is also found to be important in our analysis. The results demonstrate that an integrative modelling approach leads to a major advance in our ability to discern the underlying processes operating in ecological systems.

Functional species pool framework to test for biotic effects on community assembly

Functional trait differences among species are increasingly used to infer the effects of biotic and abiotic processes on species coexistence. Commonly, the trait diversity observed within communities is compared to patterns simulated in randomly generated communities based on sampling within a region. The resulting patterns of trait convergence and divergence are assumed to reveal abiotic and biotic processes, respectively. However, biotic processes such as competition can produce both trait divergence and convergence, through either excluding similar species (niche differences, divergence) or excluding dissimilar species (weaker competitor exclusion, convergence). Hence, separating biotic and abiotic processes that can produce identical patterns of trait diversity, or even patterns that neutralize each other, is not feasible with previous methods. We propose an operational framework in which the functional trait dissimilarity within communities (FDcomm) is compared to the corresponding trait dissimilarity expected from the species pool (i.e., functional species pool diversity, FDpool). FDpool includes the set of potential species for a site delimited by the operating environmental and dispersal limitation filters. By applying these filters, the resulting pattern of trait diversity is consistent with biotic processes, i.e., trait divergence (FDcomm > FDpool) indicates niche differentiation, while trait convergence (FDcomm < FDpool) indicates weaker competitor exclusion. To illustrate this framework, with its potential application and constraints, we analyzed both simulated and field data. The functional species pool framework more consistently detected the simulated trait diversity patterns than previous approaches. In the field, using data from plant communities of typical Northern European habitats in Estonia, we found that both niche-based and weaker competitor exclusion influenced community assembly, depending on the traits and community considered. In both simulated and field data, we demonstrated that only by estimating the species pool of a site is it possible to differentiate the patterns of trait dissimilarity produced by operating biotic processes. The framework, which can be applied with both functional and phylogenetic diversity, enables a reinterpretation of community assembly processes. Solving the challenge of defining an appropriate reference species pool for a site can provide a better understanding of community assembly.

Restoration of species-rich limestone grassland communities from overgrown land: the importance of propagule availability

A field experiment was established in the eastern coast of the Baltic Sea to test the relative roles of the availability of propagules and light competition in the restoration dynamics of a former calcareous grassland overgrown by pine (Pinus sylvestris L.). The treatments were: clear-cutting of trees with additional grazing and transplantation of sods from an open grassland with all possible sources of propagules. Species richness and composition were studied on a small-scale during 7 years in the transplanted patches of 20×20 cm and in their surroundings of 50×50 cm. In the cut and grazed site the species richness increased. Transplantation of sods from an open species-rich grassland did not result in higher richness even in their closest surroundings. In the forest, transplanted patches lost their high species richness by the second year. In the cut and grazed site, transplanted patches remained species-rich, but after 3 years, control patches reached the same level of species richness. In landscapes where former species-rich limestone grasslands are overgrown, but the local species pool has not yet changed, restoration of semi-natural grassland communities does not require the additional input of diaspores of grassland species. Transplantation of sods is potentially important method of community restoration in case of impoverished local species pools.

Agricultural policies exacerbate honeybee pollination service supply-demand mismatches across Europe

Declines in insect pollinators across Europe have raised concerns about the supply of pollination services to agriculture. Simultaneously, EU agricultural and biofuel policies have encouraged substantial growth in the cultivated area of insect pollinated crops across the continent. Using data from 41 European countries, this study demonstrates that the recommended number of honeybees required to provide crop pollination across Europe has risen 4.9 times as fast as honeybee stocks between 2005 and 2010. Consequently, honeybee stocks were insufficient to supply >90% of demands in 22 countries studied. These findings raise concerns about the capacity of many countries to cope with major losses of wild pollinators and highlight numerous critical gaps in current understanding of pollination service supplies and demands, pointing to a pressing need for further research into this issue.