Hakea sericea: Development of a model of the impacts of biological control on population dynamics and rates of spread of an invasive species

This paper reports on the development and testing of a simulation model to assess the impacts of two seed-feeding biological control agents on the dispersal of an invasive perennial tree, Hakea sericea (Proteaceae), which was introduced into South Africa from Australia during the 1850s. The agents are known to reduce seed banks at the individual plant level but the population-level effects are not well understood. The aim of the study was to estimate the extent to which the biological control has reduced the population growth and rate of spread of this species. H. sericea is a serotinous species which releases its wind-dispersed seeds en masse when the parent plants are killed, usually by fire. Field data were used to develop functions describing the fecundity of the plants and the impacts of the biological control agents as well as seedling recruitment rates and density dependent mortality. A group of ‘experts’ provided estimates of the cumulative proportion of seeds that would disperse over distances from 50 to 1000 m following a fire. The estimates were used to fit various long-range dispersal functions. The Weibull distribution gave the overall best fit and was used to generate parameter sets from each expert's estimates of dispersal. Simulations were then run using a reasonable range of fire intervals and dispersal parameters for the two experts whose estimates represented the minimum and maximum dispersal distances. Biological control agents have reduced the seed loads on H. sericea plants by more than 95%. This, in turn, reduced population growth rates, maximum seed dispersal distances and the formation of new invasion foci. Population growth rates and spread rates were positively correlated because greater dispersal distances resulted in lower densities and, thus, lower levels of density dependent mortality. Numerous previous studies have found that biological control can limit population growth rates of invasive plants, but this is one of the few to have estimated the impacts on the invasion rates and to use an individual-based modelling approach to estimate population-level effects.