Interactions among predators and plant specificity protect herbivores from top predators

Abstract

The worldwide loss of top predators from natural and agricultural systems has heightened the need to understand how important they are in controlling herbivore abundance. The effect of top predators on herbivore species is likely to depend on (1) the importance of the consumption of intermediate predators by top predators (intra-guild predation; IGP), but also on (2) plant specificity by herbivores, because specialists may defend themselves better (enemy-free space; EFS). Insectivorous birds, as top predators, are generally known to effectively control herbivorous insects, despite also consuming intermediate predators such as spiders, but how this effect varies among herbivore species in relation to the cascading effects of IGP and EFS is not known. To explore this, we excluded birds from natural fynbos vegetation in South Africa using large netted cages and recorded changes in abundance relative to control plots for 199 plant-dwelling intermediate predator and 341 herbivore morpho-species that varied in their estimated plant specificity. We found a strong negative effect of birds on the total abundance of all intermediate predators, with especially clear effects on spiders (strong IGP). In contrast with previous studies, which document a negative effect of birds on herbivores, we found an overall neutral effect of birds on herbivore abundance, but the effect varied among species: some species were negatively affected by birds, suggesting that they were mainly consumed by birds, whereas others, likely released from spiders by IGP, were positively affected. Some species were also effectively neutrally affected by birds. These tended to be more specialized to plants compared to the other species, which may imply that some plant specialists benefited from protection provided by EFS from both birds and spiders. These results suggest that the response of herbivore species to top predators may depend on cascading effects of interactions among predators and on their degree of plant specificity.

How competitive intransitivity and niche overlap affect spatial coexistence

“…Although neutral dynamics can explain a number of community assembly patterns (Chave et al 2002, Tilman 2004, Purves and Pacala 2005), its assumption of ecological equivalence between species might not be common in nature (Holt 2006, McGill et al 2006). Competition, and more generally the network of biotic interactions, plays an important role in explaining species distributions and community assembly patterns (Grime 1979, Schoener 1983, Gotelli and McCabe 2002, Bosc et al 2018, Latombe et al 2018, Hui and Richardson 2019, Steidinger et al 2019). As competitive intransitivity can allow species to coexist without neglecting the importance of competition itself in community assembly (Soliveres et al 2018), it is time to expand the coexistence theory in network and community ecology to incorporate competitive intransitivity (Laird and Schamp 2006).…”

Importance of biotic niches versus drift in a plant‐inhabiting arthropod community depends on rarity and trophic group

“…However, it must be noted that other non-neutral factors not considered in this study are also likely to influence those species. or top-down effects of predators (notably birds; Bosc et al 2018a) could also influence them. Also, it is possible that part of the very rare species in our study are transient species, i.e.…”

“…We sampled arthropods living in the entire plant communities, i.e. Additional information on the sampling method can be found in Bosc et al (2018a). In each quadrat, covers of all plant species were estimated in m 2 by visual inspection with the help of a 1-m 2 frame, and then arthropods found on plants were sampled with a vacuum sampler (modified leaf shredder vacuum/blower STIHL SH-86d; maximum airflow: 770 m 3 h −1 ) in a unique sampling session (between October and December 2014).…”

“…we sampled from all the plant species found in the quadrats. Data sets, species lists and photographs of all arthropod species are provided in Dryad Digital Repository: < https://doi.org/10.5061/dryad.6k939m4 > (Bosc et al 2018b). All plants and arthropods were separated by morpho-species and identified to the lowest taxonomic level possible (mostly species level for plants; family level for insects; genus level for spiders) with help of identification keys (Scholtz andHolm 1985, Manning 2007) and/or professional taxonomists.…”

“…As hemipterans are the dominant herbivorous insect group in our study, we expected a significant influence of hemipteran occurrence on spiders and wasps. Additional information on the sampling method can be found in Bosc et al (2018a). All morpho-species collected in this study are stored in 70% ethanol at the Stellenbosch Univ.…”