Predator Diversity and Abundance Provide Little Support for the Enemies Hypothesis in Forests of High Tree Diversity

Abstract

Predatory arthropods can exert strong top-down control on ecosystem functions. However, despite extensive theory and experimental manipulations of predator diversity, our knowledge about relationships between plant and predator diversity—and thus information on the relevance of experimental findings—for species-rich, natural ecosystems is limited. We studied activity abundance and species richness of epigeic spiders in a highly diverse forest ecosystem in subtropical China across 27 forest stands which formed a gradient in tree diversity of 25–69 species per plot. The enemies hypothesis predicts higher predator abundance and diversity, and concomitantly more effective top-down control of food webs, with increasing plant diversity. However, in our study, activity abundance and observed species richness of spiders decreased with increasing tree species richness. There was only a weak, non-significant relationship with tree richness when spider richness was rarefied, i.e. corrected for different total abundances of spiders. Only foraging guild richness (i.e. the diversity of hunting modes) of spiders was positively related to tree species richness. Plant species richness in the herb layer had no significant effects on spiders. Our results thus provide little support for the enemies hypothesis—derived from studies in less diverse ecosystems—of a positive relationship between predator and plant diversity. Our findings for an important group of generalist predators question whether stronger top-down control of food webs can be expected in the more plant diverse stands of our forest ecosystem. Biotic interactions could play important roles in mediating the observed relationships between spider and plant diversity, but further testing is required for a more detailed mechanistic understanding. Our findings have implications for evaluating the way in which theoretical predictions and experimental findings of functional predator effects apply to species-rich forest ecosystems, in which trophic interactions are often considered to be of crucial importance for the maintenance of high plant diversity.

Do birds see the forest for the trees? Scale-dependent effects of tree diversity on avian predation of artificial larvae

“…The enemies hypothesis has received much experimental scrutiny and support from studies in agricultural ecosystems and grasslands (Tonhasca 1993;Siemann et al 1998;Sobek et al 2009;Letourneau et al 2011;Straub et al 2014); however, fewer tests of this hypothesis have been conducted in forest ecosystems. These studies have produced mixed results with some reporting negative effects of tree diversity on predator effectiveness (Schuldt et al 2011;Zou et al 2013) and others showing stronger effects of tree species composition (Riihimäki et al 2005;Kaitaniemi et al 2007;Vehviläinen et al 2008), density (Sperber et al 2004;Schuldt et al 2008) or tree species identity (Sobek et al 2009), rather than tree species richness per se (Zhang and Adams 2011). Thus, more studies are required to better understand relationships between diversity and top-down control of insect pests in forest ecosystems.…”

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).…”

Plant diversity in a nutshell: testing for small‐scale effects on trap nesting wild bees and wasps

“…Thus declining plant species richness can have cascading effects, releasing herbivores from this control and leading to reduced ecosystem functioning (Knops et al 1999, Balvanera et al 2006. The relationships between abundance and diversity of different trophic levels is therefore difficult to predict, and it is not surprising that recent studies showed varying effects of plant diversity on the abundance or diversity of higher trophic levels, yielding positive (Albrecht et al 2007, Dinnage 2013, neutral (Ebeling et al 2012) or negative (Schuldt et al 2011) relationships. However, positive effects dominate, with a reported decrease in the strength of the relationship for higher trophic levels (Scherber et al 2010a).…”