Testing the enemies hypothesis in forest stands: the important role of tree species composition

Abstract

Numerous studies conducted in agro-ecosystems support the enemies hypothesis, which states that predators and parasites are more efficient in controlling pest densities in polycultures than in monocultures. Few similar studies, however, have been conducted in forest ecosystems, and we do not yet have evidence as to whether the enemies hypothesis holds true in forests. In a 2-year study, we investigated whether the survival of autumnal moth ( Epirrita autumnata) larvae and pupae differs between silver birch monocultures and two-species mixtures of birch with black alder, Norway spruce and Scots pine. We placed young larvae on birch saplings and monitored their survival until the end of the larval period, when we checked whether they had been parasitized. After the larvae had pupated, pupal survival was tested in a field trial. In 2002, the larvae disappeared earlier and their overall survival was lower in birch-pine mixtures than in other stand types. In 2003, survival probability was lowest in birch-pine stands only during the first week and there were no differences between stands in overall survival. Larval parasitism was not affected by tree species composition. Pupal weight and pupal survival were likewise not affected by stand type. Among the predators, wood ants were more abundant on birches growing in birch-pine mixtures than in other stand types probably because colonies of myrmecophilic aphids were common on pines. In contrast, spider numbers did not differ between stand types. Ant exclusion by means of a glue ring around the birch trunk increased larval survival, indicating that ants are important predators of the autumnal moth larvae; differences in larval survival between stands are probably due to differential ant predation. Our results provide only partial support for the enemies hypothesis, and suggest that it is both tree species composition and species diversity which affect herbivore survival and predation.

Experimental evidence for associational resistance against the European pine sawfly in mixed tree stands

“…A similar effect at fine spatial scale was observed in another study where survival of the lepidopteran Epirrita autumnata larvae was lowest in pine-birch mixtures compared with three other stand types with the same plot size (Riihimäki et al 2005).…”

“…However, what is associational resistance against one species may not be against another. At the same time as the number of sawflies decreased in the pines growing in mixed stands with birch, the number of ants and anttended aphid colonies increased in the same trees (Riihimäki et al 2005). The predator community which appeared to be underlying resistance also differed between the study sites.…”

“…At least there was no detectable effect on pine diameter yet. However, as a whole, the stands with pine and birch mixed may benefit from the associational resistance as the birches within the same mixed plots also experienced associational resistance (Riihimäki et al 2005). Thus, following the original formulations (Root 1973), it may be practical to use the concept of associational resistance as a trait characterizing the resistance of whole communities against damage by herbivores in general, instead of concentrating only on some specific interactions.…”

Plant neighbour identity and invasive pathogen infection affect associational resistance to an invasive gall wasp

“…The ''enemies hypothesis'' (Elton 1958;Root 1973) posits that plant communities with higher species richness provide more resources and habitats and thus can shelter more diverse predator or parasitoid communities (Wilby and Thomas 2002;Schuldt et al 2011;Castagneyrol and Jactel 2012), which could in turn provide a better control of herbivore populations (Riihimäki et al 2005;Leles et al 2017). A spill-over of natural enemies from associated to target trees is expected if associated and target trees share common or alternative prey or hosts (Cappuccino et al 1998).…”

The Sabah Biodiversity Experiment: a long-term test of the role of tree diversity in restoring tropical forest structure and functioning

“…Most of the evidence for the relationship between diversity and ecosystem functioning comes from observational surveys in temperate and boreal forests [24][25][26], where it is difficult to separate the effects of diversity from other confounding environmental variables. For example, these natural gradients of diversity often include one species that is present along the entire gradient, confounding species diversity with species identity [24].…”