Host-parasitoid extinction and colonization in a fragmented prairie landscape

Abstract

Few field studies of natural populations have examined the factors influencing local extinctions and colonization of empty habitat patches for a prey species and its predator. In this study, I carried out a census of planthopper (Prokelisia crocea; Hemiptera: Delphacidae) and egg parasitoid (Anagrus columbi; Hymenoptera: Mymaridae) incidence and densities in 147 host-plant patches (Spartina pectinata; Poaceae) over seven planthopper generations in a tall-grass prairie landscape. For both species, the likelihood of going extinct in a patch was related to a number of patch-specific variables: density, temporal variability in density, proportion of hosts parasitized (planthopper only), host-plant density, patch size, patch isolation, and composition of the surrounding matrix. Colonization likelihood was related only to the physical attributes of the patch. There was high patch turnover in this prairie landscape. On average, planthoppers went extinct in 23% of the patches and A. columbi went extinct in 51% of the patches in each generation. For the planthopper, extinction likelihood increased with a decrease in patch size and the proportion of the matrix composed of mudflat. Parasitism of eggs had no effect on the extinction likelihood of local P. crocea populations, suggesting that A. columbi may not play a major role in the patch dynamics of its host. The likelihood of extinction for A. columbi was dependent on factors that spanned three trophic levels. An increase in plant density, decrease in host density and decrease in parasitoid density all increased the likelihood of A. columbi extinction within a patch. The dependency on multiple trophic levels may explain the higher extinction risk for the parasitoid than its host. A. columbi extinction was also affected by the matrix habitat surrounding the patch--the effect was the opposite of that for P. crocea. Finally, vacant patches were colonized at rates of 53% and 34% per generation for the planthopper and parasitoid, respectively. For both species, colonization probabilities decreased with an increase in patch isolation. High host densities in a patch also favored high rates of colonization by A. columbi. I discuss how anthropogenic changes to the prairie landscape can affect the metapopulation dynamics and persistence time of this host-parasitoid interaction.

Conservation Biocontrol in Fragmented Landscapes: Persistence and Parasitation in a Host-Parasitoid Model

“…3). This increase in parasitoid persistence with patch size was also found in the field (Cronin 2004) and can be explained by the spatial structure that the oscillating host-parasitoid populations develop in a single patch. In this case, a threshold area is needed to sustain the typical spatial wave-like or chaotic pattern.…”

Biodiversity, Species Interactions and Ecological Networks in a Fragmented World

“…Besides food webs, several examples from other multitrophic systems give an indication of how antagonistic host-parasitoid networks may be affected by fragmentation (Cronin, 2004;Kruess, 2003;Thies et al, 2005 Figure 18 Impacts of habitat fragmentation caused by drought in experimental stream food webs: results from a long-term field experiment in artificial streams Ledger et al, 2008Ledger et al, , 2009Ledger et al, , 2011. Drought can have patchy effects in river networks and individual stream channels can be viewed as fragmented patches in the wider riverscape.…”

Conservation of Grassland Leafhoppers: A Brief Review

“…Most grassland habitats have been fragmented to some extent. Recent studies have shown that leafhopper species are affected by the effects of fragmentation, especially reduced habitat area, small habitat patches being associated with an increased risk of local extinction for many species (Biedermann 2000;Cronin 2004). In a review of existing studies on leafhopper populations in fragmented landscapes, Biedermann (2002) found that a high proportion of species were area-sensitive.…”

“…However, species differ widely in their response to habitat fragmentation (Biedermann 2004), some species being only marginally affected, whilst others require large areas for persistence. Furthermore, the import effect of habitat connectivity on the occurrence and persistence of grassland leafhopper species has been highlighted (Biedermann 2002;Cronin 2004). …”