Association Patterns in Saproxylic Insect Networks in Three Iberian Mediterranean Woodlands and Their Resistance to Microhabitat Loss

The assessment of the relationship between species diversity, species interactions and environmental characteristics is indispensable for understanding network architecture and ecological distribution in complex networks. Saproxylic insect communities inhabiting tree hollow microhabitats within Mediterranean woodlands are highly dependent on woodland configuration and on microhabitat supply they harbor, so can be studied under the network analysis perspective. We assessed the differences in interacting patterns according to woodland site, and analysed the importance of functional species in modelling network architecture. We then evaluated their implications for saproxylic assemblages’ persistence, through simulations of three possible scenarios of loss of tree hollow microhabitat. Tree hollow-saproxylic insect networks per woodland site presented a significant nested pattern. Those woodlands with higher complexity of tree individuals and tree hollow microhabitats also housed higher species/interactions diversity and complexity of saproxylic networks, and exhibited a higher degree of nestedness, suggesting that a higher woodland complexity positively influences saproxylic diversity and interaction complexity, thus determining higher degree of nestedness. Moreover, the number of insects acting as key interconnectors (nodes falling into the core region, using core/periphery tests) was similar among woodland sites, but the species identity varied on each. Such differences in insect core composition among woodland sites suggest the functional role they depict at woodland scale. Tree hollows acting as core corresponded with large tree hollows near the ground and simultaneously housing various breeding microsites, whereas core insects were species mediating relevant ecological interactions within saproxylic communities, e.g. predation, competitive or facilitation interactions. Differences in network patterns and tree hollow characteristics among woodland sites clearly defined different sensitivity to microhabitat loss, and higher saproxylic diversity and woodland complexity showed positive relation with robustness. These results highlight that woodland complexity goes hand in hand with biotic and ecological complexity of saproxylic networks, and together exhibited positive effects on network robustness.The research Projects I+D CGL2011-23658 y CGL2012-31669 of the Spanish Minister of Science provided economic support

Oxygen glucose deprivation-induced astrocyte dysfunction provokes neuronal death through oxidative stress

International audienceUnderstanding the role of astrocytes in stroke is assuming increasing prominence, not only as an important component on its own within the neurovascular unit, but also because astrocytes can influence neuronal outcome. Ischemia may induce astrogliosis and other phenotypic changes, but these remain poorly understood, in part due to limitations in reproducing these changes in vitro. Dibutyryl cyclic AMP-differentiated cultured astrocytes are more representative of the in vivo astroglial cell phenotype, and were much more susceptible than undifferentiated astrocytes to an ischemic-like stress, oxygen-glucose deprivation (OGD). OGD altered the expression/distribution and activity of glial glutamate transporters, impaired cellular glutamate uptake and decreased intracellular levels of glutathione preferentially in differentiated astrocytes. Resistance to OGD was conferred by inhibiting caspase-3 with DEVD-CHO and oxidative stress by the antioxidant N-acetylcysteine (NAC). The resistance of undifferentiated astrocytes to OGD may result from a transient but selective morphological transformation into Alzheimer type II astrocytes, an intermediary stage prior to transforming into reactive astrocytes. Co-culture of neurons with OGD-exposed astrocytes resulted in neurotoxicity, but at surprisingly lower levels with dying differentiated astrocytes. The antioxidant NAC or the 5-LOX inhibitor AA861 added upon co-culture delayed (day 1) but did not prevent neurotoxicity (day 3). Astrocytes undergoing apoptosis as a result of ischemia may represent a transient neuroprotective mechanism via ischemia-induced release of glutathione, but oxidative stress was responsible for neuronal demise when ischemia compromised astrocyte supportive functions