The effect of spatial scale on interactions between two weevils and their parasitoid

Abstract

Summary 1. The effect of spatial scale on the interactions between three hymenopteran parasitoids and their weevil hosts was investigated. The parasitoid Mesopolobus incultus (Walker) parasitised Gymnetron pascuorum Gyll.; the parasitoids Entodon sparetus (Walker) and Bracon sp. parasitised Mecinus pyraster Herbst. Both of these weevils develop inside the seedhead of Plantago lanceolata L. but occupy different niches. Seedheads were sampled annually from 162 plants at each of two experimental sites consisting of a series of habitat patches of two distinct sizes. Data were analysed from three site‐years. 2. Parasitoid densities at each site‐year were closely related to the abundance of their respective weevil hosts. The overall proportion of hosts parasitised was more variable for M. incultus than for E. sparetus and Bracon sp. 3. Changes in spatial scale affected the variability of parasitoid densities. For M. incultus, there was generally a greater degree of additional heterogeneity for all increases of scale; for E. sparetus, this was true only at the largest scales; for Bracon sp., all components of variance were negative. 4. The rate of parasitism was related to host density in different ways at different spatial scales. Mesopolobus incultus exhibited inverse density dependence at the finest (seedhead) scale, direct density dependence at the intermediate (plant) scale, and density independence at the large (habitat area 729 m2) scale. Entodon sparetus showed no response to variation in host density at any spatial scale. Bracon sp. showed direct density dependence only at the intermediate and largest scales. 5. Parasitoids E. sparetus and Bracon sp. seemed able to detect more than one M. pyraster individual in seedheads with multiple host occupancy; a greater incidence of conspecific parasitoids than expected emerged from such seedheads.

An experimental investigation of patterns of parasitism at three spatial scales in an aphid-parasitoid system (Hymenoptera: Aphidiidae)

“…To date, there has been only one published case of switching from inverse to direct density dependence, concerning the solitary pteromalid parasitoid Mesopolobus incultus (Walker) in a field manipulative experiment (Norowi et al, 2000). This parasitoid, quite consistently with our results, exhibited inverse density dependence of parasitism rate at the small scale of the seedhead of the plant Plantago lanceolata L., but direct density dependence at the intermediate scale of the whole plant.…”

Scale-dependent variation in composition of fish fauna among sandy tropical estuarine embayments

“…No understanding of the forces that structure tropical estuarine faunas is possible until we develop a full appreciation of the patterns of faunal distribution and the scales at which variability operates. This is because major biological and physical processes are focussed at characteristic spatial scales (Stommel 1963, Schneider 1994a, Mohd Norowi et al 1999) and produce characteristic patterns of faunal distribution (Azovsky 2000). Consequently, comparison of variability at different scales can be used to assess both the relative importance of processes operating within an ecosystem (Kratz et al 1987, Weins 1989 and the scales at which important processes operate.…”

Ecological drivers of spatial variability among fish fauna of 21 tropical Australian estuaries

“…Estuary-level ecological variables had greater explanatory power than site-level physical variables and, importantly, are focussed at the scale that accounted for the greatest proportion of systematic variability. This scale is ecologically significant because it is the scale at which the major ecological structuring processes are focussed (Mohd Norowi et al 1999) and is thus the scale at which characteristic faunal patterns are determined (Azovsky 2000). Consequently, variables that have strong explanatory power at this focal scale are likely to be important in determining the major features of the spatial pattern.…”