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THE EFFECTS OF SALINITY AND NUTRIENTS ON A TRITROPHIC SALT‐MARSH SYSTEM
Author(s) -
Moon Daniel C.,
Stiling Peter
Publication year - 2002
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.1890/0012-9658(2002)083[2465:teosan]2.0.co;2
Subject(s) - gall , biology , parasitism , parasitoid , abiotic component , nutrient , salinity , gall wasp , herbivore , agronomy , ecology , biological pest control , host (biology)
In recent years, ecologists have begun to develop a more fine‐grained and integrative approach to examining the importance of top‐down and bottom‐up effects on herbivore populations by investigating how changes in abiotic heterogeneity affect the relative roles of these forces. We performed two factorial field experiments to determine how increasing salinity and nutrient supply affected the relative strengths of top‐down and bottom‐up forces among the gall‐making midge, Asphondylia borrichiae , its host plant Borrichia frutescens , and a suite of parasitoids. Salinity was increased by the addition of salt pellets, and nutrient supply was increased by the addition of fertilizer. In both experiments, parasitism pressure was decreased by trapping hymenopteran parasitoids with yellow sticky traps. In both experiments, bottom‐up manipulations had significant effects on gall density. Elevated salinity levels decreased the number of galls per 200 Borrichia stems, and fertilization increased the number of galls. In the salt‐stress experiment, increased salinity decreased gall parasitism, and in the fertilization experiment, fertilizer increased parasitism of galls, but the direct effects of these treatments on gall density outweighed the indirect effects mediated by parasitoids. Parasitoid‐removal treatments resulted in only small (and not statistically significant) changes in gall parasitism in the salt‐stress experiment, and there was no significant effect on gall density. In the fertilization experiment, parasitoid‐removal treatments reduced parasitism of galls, and this resulted in an increase in gall density over time. Thus, bottom‐up effects were strong and consistent in both experiments, but top‐down effects appeared weaker and inconsistent. Stem morphology, which was altered by salt and fertilizer, also affected gall density and parasitism, and there was some evidence that the strength of top‐down forces on gall density varied with stem type. The results of this study show that, in this system, top‐down effects of parasitism are dependent upon the bottom‐up effects of host plant quality and morphology resulting from abiotic heterogeneity. As such, the relative importance of top‐down effects can change depending upon the relative nutrient supply or salinity of the environment.