Trophic trickles rather than cascades: Conditional top-down and bottom-up dynamics in an Australian chenopod shrubland

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Abstract Despite continued interest in the relative roles of top-down (predation) and bottom-up (productivity) factors in structuring ecological communities, there have been few studies of diverse terrestrial arthropod systems. Top-down theory predicts that decreased predator populations will result in increased herbivore populations and reduced plant populations. Bottom-up theory predicts that a positive producer level response to nutrients will support greater herbivore and predator populations. Few studies simultaneously examine both theories. In the present study, the roles of predators and nutrients in structuring an Australian chenopod shrubland community were investigated, by using a complete 2 × 2 factorial design with three replicated plots with and without nutrients (fertilizer) and predators (two wolf spiders and two ants). Producer and major arthropod taxa responses, predator densities and soil nutrient levels were monitored from April 1996 to February 1998. In agreement with bottom-up theory, the abundances of some plants and herbivores increased with the addition of nutrients, but most effects varied temporally and did not increase the abundances of any predator taxa. The effects of predator removal propagated down the food web to influence some herbivore and producer taxa but, in contrast to top-down theory, effects were temporally dependent and varied in the direction of response within each trophic level. Predators and nutrients only interacted significantly to determine the abundance of one taxon. The findings suggest that true trophic cascades are not a primary determinant of chenopod community structure, with ‘conditional trophic trickles’ a better descriptor for the effects of predators and nutrients on this community. It is suggested that the expression of top-down and bottom-up effects is conditional on appropriate timing and extent of rainfall. The results provide further support for the theory that food webs are more complex in terrestrial than in aquatic systems, with abiotic and biotic heterogeneity, intraguild interactions and non-consumptive interactions between adjacent trophic levels all confounding the classic consumptive interactions that are required for the expression of top-down and bottom-up forces.