(The Hebrew University)
Temperate grassland communities harbor a great fraction of the Central European plant species diversity and responses of these communities to anthropogenic disturbances have been investigated by numerous studies during the last decades. Our research focuses on one aspect that has been overlooked in previous studies: the role of micro-scale spatial heterogeneity in habitat conditions. This question is planned to be investigated in two phases. In the present phase we use a series of microcosm experiments and related models to uncover the fundamental mechanisms by which spatial heterogeneity in habitat conditions (habitat heterogeneity) affects the diversity of grassland communities. Our models and experiments are based on a novel concept termed 'the area-heterogeneity trade-off'. The results obtained from this phase will be used as a baseline for planning a second phase of experiments that will be designed to test predictions obtained from our models and microcosm experiments under natural field conditions. Importantly, although our research focuses on grassland communities, we expect that our theoretical and empirical findings would provide general insights that will be applicable to studies focusing on larger scales and other kinds of organisms.
The 'area-heterogeneity trade-off' (Allouche et al. 2012) states that any increase in habitat heterogeneity must be associated with a corresponding decrease in the average amount of suitable area available per species (Fig.1). This geometric trade-off leads to a number of testable hypotheses:
1. Increasing habitat heterogeneity should increase the potential number of species in a community by providing suitable conditions for a larger number of species with different ecological requirements (this is the traditional prediction of niche theory).
2. At the same time, increasing heterogeneity reduces average population size (due to the proposed area-heterogeneity trade-off).
3. The reduction in population size should increase the likelihood of stochastic extinction.
4. The increase in extinction rates should reduce the number of species.
5. These contrasting positive and negative effects should lead to unimodal heterogeneity-diversity relationship, with habitat availability limiting richness at low levels of heterogeneity and area limiting richness at high levels of heterogeneity.
6. Specialist species should be more sensitive to the area-heterogeneity trade-off than generalist species and should therefore show stronger responses (both positive and negative) to variation in habitat heterogeneity.
7. The position of the inflection point (i.e., the level of heterogeneity that maximizes species diversity) should depend on properties of the relevant system. In general, any factor that increases the likelihood of stochastic extinction (habitat fragmentation, environmental stress, disturbance) should shift the inflection point to lower levels of heterogeneity and vice-versa.
The latter hypothesis implies that natural grassland communities can be expected to show various responses (positive, negative, unimodal, or flat) to increased habitat heterogeneity, depending on properties of the relevant system.
The main objective of the theoretical work in Phase 1 is to examine the validity of the AHTO under a wider range of scenarios and conditions than those examined in the original model. Therefore, our modelling strategy was designed to maximize generality by adopting a general approach that takes into account those mechanisms and interactions which are common to any kind of organisms. This approach, however, has the limitation that it ignores processes and interactions that are important in structuring grassland communities, our target in this project. Thus, a major need at this stage is to shift from our current, highly abstract and general modeling approach into a more mechanistic approach that takes into account those processes and interactions that are most important in determining the diversity of real grasslands. Such an extension will allow us to directly and tightly link our theoretical, experimental, and observational findings.
Our main objective in the experimental component of Phase 1 was to establish a long-term microcosm experiment that will allow us to test hypotheses generated by our models under highly controlled conditions. A long-term monitoring of this extensive experiment is crucial because community level responses to manipulations of the environment are driven by demographic processes (birth, death and dispersal) which operate at relatively long time scales, especially in perennials and plants with clonal propagation. Thus, a major need of our experimental component is to continue monitoring the responses of the artificial communities to the various manipulation treatments. Another need resulting from the shift in our modelling approach into a more mechanistic approach is to obtain appropriate empirical data for parameterizing our new models. To this end we plan to establish a new experiment in which we will measure growth responses of individual plants to experimental manipulations of relevant environmental variables.
The most important need of our current work is to link it more directly to the general framework of the BEs. This extension is strongly supported by recent syntheses from the BEs, all of which highlighting habitat heterogeneity as a main potential - but not confirmed - explanation for various patterns and processes found across the land use intensity gradient of the Exploratories (e.g. Blüthgen et al. 2016, Solivieres et al. 2015, Manning et al. 2015). Thus, in the second phase we plan to add an observational component in which all aspects investigated in the theoretical and experimental subprojects will be studied also in a wide range of grassland BEs that will be selected to represent the main land-use gradients.