Does snail grazing affect growth of the old forest lichen Lobaria pulmonaria?

Abstract

Grazing marks from snails are frequently observed in populations of the old forest epiphyte Lobaria pulmonaria. However, grazing marks are more numerous in thalli from deciduous broadleaved forests than in thalli from boreal Picea abies forests, due to higher populations of lichen-feeding molluscs in deciduous stands. Here we tested for deleterious effects of snails on the lichens by transplanting 600 more or less grazed L. pulmonaria thalli from deciduous forests to snail-free P. abies forests. Subsequent measurements showed that growth rates were as high in thalli with many grazing marks as those without, suggesting that growth of mature lobes of L. pulmonaria are not inhibited by the recorded grazing pressure imposed by lichen feeding snails.

Mollusc grazing limits growth and early development of the old forest lichen Lobaria pulmonaria in broadleaved deciduous forests

“…Various types of grazing marks were identiWed. In some thalli, the cortex and the photobiont layer was removed primarily on the ridges exposing the white medulla as illustrated by Gauslaa et al (2006a), whereas others had vague marks in between the ridges. Grazing from the underside, sometimes through the entire thallus, also occurred in some thalli.…”

“…No attempt was made to clarify how the diVerent kinds of marks aVected the lichen. Subsequent growth of mature L. pulmonaria in a snail-deWcient environment was not found to be aVected by previous grazing of molluscs (Gauslaa et al 2006a). However, the most grazed thalli investigated by Gauslaa et al (2006a) were not as severely grazed as some of the thalli in this study.…”

“…At a population level, grazing on juvenile thalli can be more destructive than on mature thalli. A given consumption of a small thallus leads more often to complete destruction, whereas large, mature thalli have a greater ability to regenerate (Fröberg et al 2006;Gauslaa et al 2006a). This implies that molluscs may have a great impact in a regenerating lichen community even though the biomass consumed is low.…”

“…Interactions between lichens and lichen-feeding molluscs have received some attention (e.g. Lawrey 1983;Fröberg et al 1993;Baur et al 1995;Lücking and Bernecker-Lücking 2000;Gauslaa et al 2006a), but knowledge about the impact of snails and slugs on lichens in forested habitats is generally scarce. Since a majority of threatened and declining lichen species in Norway and Sweden (Gärdenfors 2005;Timdal et al 2006) grow in forest ecosystems, there is a need for Weld experiments in forests with natural populations of lichen-feeding molluscs.…”

“…Invertebrate grazing may be one of these limiting factors. Grazing marks from lichen-feeding molluscs are common on L. pulmonaria thalli in calcareous broadleaved deciduous forests in southern Norway (Gauslaa et al 2006a). Juvenile lichen thalli are rarely found in such forests (personal observation), possibly because of a weaker herbivore defence in juvenile than in adult thalli due to their lower amounts of lichen substances .…”