Do distances among host patches and host density affect the distribution of a specialist parasitoid?

Abstract

The effect of spatial habitat structure and patchiness may differ among species within a multi-trophic system. Theoretical models predict that species at higher trophic levels are more negatively affected by fragmentation than are their hosts or preys. The absence or presence of the higher trophic level, in turn, can affect the population dynamics of lower levels and even the stability of the trophic system as a whole. The present study examines different effects of spatial habitat structure with two field experiments, using as model system the parasitoid Cotesia popularis which is a specialist larval parasitoid of the herbivore Tyria jacobaeae. One experiment examines the colonisation rate of the parasitoid and the percentage parasitism at distances occurring on a natural scale; the other experiment examines the dispersal rate and the percentage parasitism in relation to the density of the herbivore and its host plant. C. popularis was able to reach artificial host populations at distances up to the largest distance created (at least 80 m from the nearest source population). Also, the percentage parasitism did not differ among the distances. The density experiment showed that the total number of herbivores parasitised was higher in patches with a high density of hosts, regardless of the density of the host plant. The percentage parasitism, however, was not related to the density of the host. The density of the host plant did have a (marginally) significant effect on the percentage parasitism, probably indicating that the parasitoid uses the host plant of the herbivore as a cue to find the herbivore itself. In conclusion, the parasitoid was not affected by the spatial habitat structure on spatial scales that are typical of local patches.

Metacommunity Dynamics: Decline of Functional Relationship along a Habitat Fragmentation Gradient

“…Modest degrees of isolation of suitable patches within a metapopulation can contribute to the stability of the system because this offers to herbivores the opportunity to escape temporally from parasitoids by continuously colonizing new habitat patches. However, large scale fragmentation and the resulting isolation of the patches, can lead to the destabilization of multi-trophic systems, and hence of metacommunity functioning because parasitoids will be absent in many isolated patches [32]. Yet, few studies have mentioned so far that the relaxation of the parasitism rate with increasing habitat fragmentation that we document here.…”

“…Indeed, abundance and diversity of parasitoids were often more strongly affected by habitat fragmentation than the abundance and diversity of herbivorous hosts, even at the scale of few hundred meters [29]. More generally, parasitoids were more sensitive to urbanization than their hosts [12], [30], [31] and the absence of higher trophic levels can affect the population dynamics of lower levels and even the stability of the trophic system as a whole [32].…”

Parasitoid dispersal and colonization lag in disturbed habitats: biological control of cereal leaf beetle metapopulations

“…Additional species of parasitoids have been documented to disperse as well or better than their hosts across hundreds of metres between habitat patches in fragmented landscapes, such that parasitism rates vary little among sites (Van Nouhuys and Hanski ; Esch et al. ; Van Nouhuys ; Elzinga et al. ).…”

Organic farming and host density affect parasitism rates of tortricid moths in vineyards

“…Several traits of the parasitoid, such as searching behavior or dispersal abilities, could lead to density dependent parasitism rates. Parasitoids use two main categories of information to localize and parasitize their host: those related to the resource of their host (plant kairomones or plant habitats characteristics such as plant abundance or shape), and those related to the host itself, such as host kairomones (Esch et al, 2005;Finch and Collier, 2000). Host density is probably the most documented driving factor of parasitoid attraction (Walde and Murdoch, 1988).…”