C4 Maintenance of biodiversity through mutualistic networks between arbuscular mycorrhizal fungi and plants: robustness against human disturbance and climate change

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Arbuscular mycorrhizal fungi (Glomeromycota) are the most prominent mycobionts in the tropics and crucial for providing minerals to the vast majority of plants including nearly all tree species in the nutrient poor environments. Thus, knowledge of the mutualistic plant-fungus associations is indispensable for understanding the biology and ecology of tropical ecosystems and give guidelines for monitoring and conservation efforts under global change expectations. In contrast to surveys on birds, mammals, soil arthropods or aquatic animals, arbuscular mycorrhizal fungal communities have been rarely studied in the tropics, and nothing is known from the high elevation Ecuadorian Páramos. The main reason for the widespread underestimation of the importance of mycobionts in ecological studies and the lack of a monitoring system is the technically difficult approach to the organisms. During recent years molecular methods were increasingly applied to identify the arbuscular mycorrhizal fungi (AMF) because morphological features proved as insufficient even with detailed description. Meanwhile, nested PCR, cloning and Sanger sequencing are well-established methods for identification of the fungi and a comprehensive data set of AMF sequences has been provided for the Andean tropical mountain forest based on these methods during the previous projects. Application of ecological network theory revealed that all studied AMF-assemblages were significantly nested. According to network simulations, significant nestedness, the observation that rarely linked species preferentially link to subsets of highly linked species, indicates that richness of fungi and plants are promoted by the mutualistic interaction. AMF, therefore, not only improve nutrition of individual plants but also integrate and thus preserve locally rare plant species and fungi in a common network. Thus, monitoring the changes of AMF communities among plots along a large elevation gradient as present in the Ecuadorian Andeans will supply data that can be interpreted on a functional level with respect to climate changes. Similar analyses can be carried out for destroyed sites in comparison to undisturbed sites. The sensitivity of AMF to site elevation and disturbance needs to be assessed. It is necessary to know if AMF can shift among the modules related to elevation and disturbance to evaluate in as much the mycobionts are resilient or sensitive to climate change or human affection with consequences for plant establishment and growth. Thus, to install a monitoring system for AMF, further investigation together with the Ecuadorian partners including low and high elevation is necessary to estimate the sensitivity of AMF as indicator organisms for climate change and human destructions.



Work package 1: Sampling and identification of arbuscular mycorrhizal fungi to complete the AMF data set
In order to complete the current data set on AMF obtained from an insufficient longitudinal gradient, further root sampling will be carried out in the mountain rain forest at RBSF (2000 m), and sampling will be expanded to the premontane forest in Bombuscaro (bordering San Francisco river at 1000 m), the high elevation Cajanuma (2800 - 3000 m) and Cajas Páramo (3200 – 4000 m). The large altitudinal gradient is serving as kind of “climate change experiment”. The mycorrhization will be checked with light microscopy and arbuscular mycorrhizal fungi will be identified with molecular methods.


Work package 2: Analysis of network structure for evaluation of degree of stability (nestedness) and compartmentalisation (modularity)
Networks as defined here are a set of nodes (species) connected through links (interactions). Mutualistic networks involving arbuscular mycorrhizas are two-node networks since there are two well-defined types of nodes, the arbuscular mycorrhizal fungi belonging to the Glomeromycota and the plant species, and mutualistic interactions occur between but not within these node types. We will test for potential nestedness of plant-fungal networks at our study sites and in the whole network. We will test for stability of the overall network by simulation loss of species. By stepwise eliminating highly linked or rarely linked species and recalculation NODF as measure of stability we will obtain insight on importance of diversity and of frequent or rare species on stability.

Work package 3: Analysis of network modularity in respect to altitudinal gradient and habitat destruction
We will test for modularity in the network structure. Levels of specialisation between plants and AMF will be assessed.

Work package 4: Define target AMF for the monitoring system
The network matrix will be used to calculate the degree of links of each AMF OTU by the marginal totals in the overall network and the site specific networks. AMF-OTU frequencies and occurrences will be evaluated taking into account also the results from workpackage 2 to select the keystone AMF, indicative of a stable network, for future long-term monitoring at different altitudes and land use gradients. 

Work package 5: Define target plant species and plots at the different sites for monitoring of the target AMF species.

Similar to definition of target AMF, plants with high linkage degree will be visible from the marginal totals of the network matrices. It needs to be checked if the defined keystone AMF are frequently linked to the frequently occurring and easily accessible plant species for root excavation. Three to five plant species will be selected at each plot for future monitoring.

Work package 6: Development of long-term monitoring
The method of choice for monitoring of mycorrhizal fungi will be developed together with the Ecuadorian partners.

Work package 7: Transfer of basic and technical knowledge by integrating local people into AMF research and further monitoring; supplying written advice for AMF monitoring

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Radar Network Ecuador - Peru