The influence of legacy effects and recovery from perturbations in a tritrophic salt marsh complex

Abstract

Abstract. 1. Much has been learned in recent years regarding the influence of environmental conditions on top-down and bottom-up effects acting on insect herbivores. Temporal variation in environmental conditions, however, has gone largely unstudied in spite of undoubtedly strong influences in most systems. 2. A 2-year study was conducted to examine the legacy effects of previous manipulations of host plant quality and parasitism pressure on the top-down and bottom-up effects influencing population densities of the salt marsh planthopper Pissonotus quadripustulatus. 3. For 10 months in 1998, a 2 × 2 factorial experiment was carried out, in which host plant quality was increased by the addition of nitrogen fertiliser, and parasitism pressure was decreased through the use of yellow sticky traps. This was followed by 2 months in the winter with no treatment applications. Treatments were then reversed in 1999 for a further 10 months. 4. In 1998, fertilisation treatments increased plant quality significantly, which resulted in strong effects on P. quadripustulatus density. Parasitism reduction treatments had weaker and time-dependent effects on the herbivore, increasing planthopper density in late summer and autumn. 5. After 2 months without treatments, previous fertilisation treatments were still influencing all response variables measured significantly. The legacy effects of fertilisation persisted for at least 7 months for the host plant, and at least 5 months for the herbivore and parasitoid. 6. Fertilisation treatments in 1999 increased P. quadripustulatus density by approximately the same percentage as in 1998, suggesting that previous reductions in parasitism had no influence on herbivore responses to increased nutrients. Parasitism reduction treatments in 1999, however, resulted in greater increases in herbivore density than in 1998, suggesting that previous increases in nutrients enabled greater responses to reductions in top-down pressure. 7. The results show that the top-down effects of parasitism attenuated more quickly than did the bottom-up effects of increased plant quality through greater nutrient availability. They also suggest that the recent history of nutrient status in an ecosystem may be important in determining the relative strengths of top-down and bottom-up forces.