Multi-trophic effects of ungulate intraguild predation on acorn weevils

Abstract

Predators and parasitoids may contribute to controlling the population sizes of phytophagous insects, and this has been shown to benefit plants. Phytophagous insects may also be killed by other herbivores (intraguild predation), usually larger-sized vertebrates that ingest insects accidentally while feeding on common food sources. We studied the intraguild predation on acorn weevils by ungulates and assessed the consequences for weevil populations. Infested acorns are prematurely abscised and the weevil larvae finish their development inside the acorns after being dropped. Our results show that weevil larvae were killed by ungulates eating the infested acorns on the ground. Ungulates did not discriminate between infested and sound acorns, and the probability of a larva being incidentally eaten was inversely related to acorn availability. Thus, predation risk was enhanced by the premature drop of infested acorns when acorn availability on the ground was low. Predation rates on infested acorns were much higher where ungulates were present, and acorn infestation rates were significantly lower. However, ungulates did not provide the oaks any net benefit, since the reduction of infestation rates was not enough to compensate for the large amounts of sound acorns eaten by ungulates. Seed predation is usually studied as a progressive loss of seeds by pre- and post-dispersal predators, but the interactions between them are usually not considered. We show that intraguild predation on insects by large ungulates had an effect on the structure of the foraging guild, as the proportion of acorns predated by insects decreased; however, replicating the same experimental design in different ecological scenarios would increase the strength of these results. In conclusion, the present study shows the importance of considering the multi-trophic interactions between seed predators in order to have a complete picture of granivory.

Effects of vertebrate herbivores and shrub characteristics on arthropod assemblages in a northern Arizona forest ecosystem

“…It is well known that arthropods living in fruits, flowers, leaves and seeds can be consumed as larger vertebrates forage (Polis et al, 1989;Gomez and Gonzalez-Megias, 2002;Bonal and Munoz, 2007). Our results support those of other studies which have documented such direct effects to be quite large (60-80% of the population in one study) and detrimental to the arthropod community (Gomez and Gonzalez-Megias, 2002).…”

Impact of grazing management on hibernating caterpillars of the butterfly Melitaea cinxia in calcareous grasslands

“…van Noordwijk, unpublished data). It thus appears that the effect of grazing on nest survival may vary substantially between years, as was previously reported for incidental omnivory on phytophagous beetles (Bonal and Munoz 2007;Gómez and Gómez-Megías 2002). This variation may be due to the fact that the proportion of removed vegetation does not only depend on grazer density, but also on the amount and quality of food available to the grazers Roguet et al 1998).…”

“…In contrast, at undamaged nest sites the vegetation height after grazing was on average still higher than the nest height prior to grazing. Such ingestion of insects by grazers is known as incidental omnivory (Bonal and Munoz 2007;Gómez and Gómez-Megías 2002;Polis et al 1989). For butterflies our study seems to be the first direct proof of such incidental omnivory, although it has previously been suggested to be a major cause of butterfly declines at grazed sites (Baines et al 1994).…”

Pre‐dispersal acorn predation in mixed oak forests: interspecific differences are driven by the interplay among seed phenology, seed size and predator size

“…(i.e. having the typical mark caused by the oviposition of Curculio spp., see Bonal & Muñoz 2007) and (iii) sound, mature acorns (i.e. naturally fallen sound acorns, present only at the end of the season from 30 September to 30 October).…”