Trees Wanted—Dead or Alive! Host Selection and Population Dynamics in Tree-Killing Bark Beetles

Bark beetles (Coleoptera: Curculionidae, Scolytinae) feed and breed in dead or severely weakened host trees. When their population densities are high, some species aggregate on healthy host trees so that their defences may be exhausted and the inner bark successfully colonized, killing the tree in the process. Here we investigate under what conditions participating with unrelated conspecifics in risky mass attacks on living trees is an adaptive strategy, and what this can tell us about bark beetle outbreak dynamics. We find that the outcome of individual host selection may deviate from the ideal free distribution in a way that facilitates the emergence of tree-killing (aggressive) behavior, and that any heritability on traits governing aggressiveness seems likely to exist in a state of flux or cycles consistent with variability observed in natural populations. This may have implications for how economically and ecologically important species respond to environmental changes in climate and landscape (forest) structure. The population dynamics emerging from individual behavior are complex, capable of switching between “endemic” and “epidemic” regimes spontaneously or following changes in host availability or resistance. Model predictions are compared to empirical observations, and we identify some factors determining the occurrence and self-limitation of epidemics

Bryophyte colonisation and persistense in experimental microcosms-The role of nutrients, defoliation and vascular vegetation.

A three-year multi-factorial microcosms experiment simulating dry grassland was used to test five hypotheses concerning establishment and persistence of bryophytes in grassland vegetation. The experimental treatments included fertilisation, defoliation and species composition of vascular vegetation. ANOVA-modelling showed a significant response of bryophyte species richness to fertilisation (negative) and defoliation (positive). Species composition of vascular vegetation had no effect on bryophyte richness, but a significant negative relationship was found between vascular plant biomass and species richness of bryophytes. Vascular plant dry weight above 400 g m-2 appeared fatal to bryophytes. At high nutrient levels, bryophytes extinction seemed to be avoided by defoliation, but defoliation did not fully compensate for the negative effect of fertilisation on bryophyte richness. At the single species level, our experiment provided new autecological insight in the responses of Brachytecium rutabulum and Funaria hygrometrica. The relationship between bryophyte richness and bryophyte cover was shown to follow the theoretical species-area relationship, suggesting that bryophyte cover may be used as indicator of habitat quality for subordinate species such as bryophytes and lichens in grassland monitoring. The implications of the results for grassland conservation are discussed