COMPETITION AND NICHE SEPARATION BETWEEN 2 BARK BEETLES - EXISTENCE AND MECHANISMS

The existence and effects of competition as well as niche separation w ere investigated in a system of two bark beetle species which aggregat e on Norway spruce, Picea abies, to feed on and reproduce in the phloe m. In this and other bark beetle systems, there is evidence for compet ition and intra- and interspecific density effects at normally occurri ng densities, in contrast to the concept of ''density vagueness''. Und er natural conditions adults of the two species showed a spatial niche separation, where the smaller species, Ips duplicatus, occupied only the top of trees. Regression analysis of adult density patterns of eac h species showed effects of habitat variables and of density of the op posite species. When breeding at naturally occurring densities (repres enting both low, endemic, and high, epidemic, population phases), in t he same substratum in the laboratory, there was a strong, asymmetric l arval competition in favour of Ips typographus. The larger species had a higher reproductive rate in all eight density combinations where th e species were reared together. Thus, an adult individual of!. duplica tus would benefit most if it could avoid breeding closely together wit h I. typographus. Mean offspring body mass decreased for both species as inter- or intraspecific density increased. This resource partitioni ng between the individuals indicate that the larval competition was of the 'scramble' type. This conclusion is supported by a near complete consumption of the larval food (phloem) and similar egg-laying times ( adult re-emergence dates). When the two species were bred separately t he two reproductive rates were similar. There were small differences i n breeding performance in the laboratory on logs with a diameter betwe en 5 and 15 cm. When given a choice there was a preference by adults o f the larger species for larger logs. A clear separation in the aggreg ation pheromone signal is now known, which allows most colonizing indi viduals of the smaller species, except the pioneering males, to orient and land on areas outside the range of the larger, supreme competitor .