The dynamics of a tephritid seed predator on Tripolium vulgare in a stochastic and heterogeneous environmentRiber Albrectsen, Benedicte
In this thesis the factors regulating the interaction between the seed predatory fly Paroxyna plantaginis (Diptera: Tephritidae) and its host plant Tripolium vulgare, (former Aster tripolium, Asteraceae) are determined and evaluated. The host is patchily distributed on islands in the study area (the archipelago of Skeppsvik 63°44-48' N, 20°34-40'E) in northern Sweden. Possible factors that may cause the persistence of this system include direct density-dependent feedback processes, temporal and spatial heterogeneity, and the effect of stochasticity (chance). The impact of these forces is analysed through both pattern and process approaches, concentrating on the spatial and temporal distribution of the resource and the timing and dispersal of the fly, as well as the influence of stochasticity.
Relating attack frequencies to host density showed no strong indication of tight regulation between the specialist seed predator and its host plant (Paper I). Smaller plants and subpopulations were subject to the highest variation in attack frequency, suggesting that there is a strong element of chance influencing the risk of attack. There were, however, indications of a spatial effect at low fly densities, which seemed to break down at higher densities, implying that dispersal behaviour and strength may be related to adult fly density. The fragmented resource is more evenly used at higher attack frequencies (Paper II) : an effect which may be coupled directely to fly densities (Paper III). A mismatch between the phenology of the fly and the sea aster flowering phenology seems to benefit the first flowering flower heads, which are less often attacked in a normal year (Paper III) . However, it was experimentally shown that the flies compensate for the temporal and spatial unpredictability by a female-biased density-dependent dispersal (Paper IV). Density-independent water-borne dispersal during the puparia stage may also affect the dynamics of the tephritid (Paper V).
This system is an example of a highly persistent interaction, for which stochastic and spatial effects are of major importance. The density-dependent dispersal of females may act within generations in the spatially heterogeneous environment and this may be transformed from a local response to a mode of regulation that works between generations.
Keywordsplant-animal interactions; population dynamics; regulation; densitydependence; stochasticity; temporal and spatial refuges; plant defence; dispersal; optimal patch use
Published inActa Universitatis Agriculturae Sueciae. Silvestria
2000, number: 156
Publisher: Department of Animal Ecology, Swedish University of Agricultural Sciences
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