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.
Highlights
The presence, abundance and biodiversity of predatory arthropods have significant impacts on the functioning of ecosystems [1,2,3,4]
While the importance of these trophic interactions in influencing and modifying ecosystem processes such as biomass production and nutrient cycling is increasingly recognized, and trophic complexity is increasingly being implemented in ecosystem functioning experiments [1,7,8], our basic knowledge on how the diversity and abundance of secondary consumers relates to plant diversity in natural ecosystems is still limited [9,10,11]
They attributed this negative relationship primarily to indirect effects of plant diversity on predator activity through diversity-dependent changes in microclimate and prey availability. This probably does not apply in the same way to our study, as characteristics of the plots and the immediate trap surroundings which are often considered to influence the activity of ground-dwelling arthropods, such as vegetation density or litter depth, had no effect on spider activity abundance
Summary
The presence, abundance and biodiversity of predatory arthropods have significant impacts on the functioning of ecosystems [1,2,3,4]. More information on the relationship between the biodiversity at different trophic levels is required to understand how natural ecosystems and their functioning are influenced by the potentially diversity-dependent effects of trophic interactions reported from experiments or theory [8,12]. This knowledge is of crucial importance in biodiversity conservation This knowledge is of crucial importance in biodiversity conservation (e.g. [13])
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