Relative Importance Of Top-down Effects Research Articles

Biological Control ofSolenopsisFire Ants byPseudacteonParasitoids: Theory and Practice

Pseudacteonparasitoids are potential biocontrol agents of invasiveSolenopsisfire ants.Pseudacteonspecies that parasitize the invasiveS. invictaBuren andS. richteriForel have been introduced to, and naturally dispersed across, the southeastern USA, although there is no evidence yet thatSolenopsishost ant populations have decreased. The ability of introducedPseudacteonspecies to regulateSolenopsispopulations will depend upon the relative importance of top-down effects in the recipient communities. In this paper, I examine the characteristics of thePseudacteon/Solenopsisparasitoid/host system and evaluate the extent to which research findings are consistent with top-down control. Laboratory and field experiments evaluatingSolenopsispopulation regulation have been equivocal, and overall the available evidence provides little support for strong top-down effects in this system. Competitive exclusion may occur among introducedPseudacteonspecies, and future efforts at biological control are likely to be more efficacious if they focus on other types of natural enemies.

Shifting species interactions in terrestrial dryland ecosystems under altered water availability and climate change

Species interactions play key roles in linking the responses of populations, communities, and ecosystems to environmental change. For instance, species interactions are an important determinant of the complexity of changes in trophic biomass with variation in resources. Water resources are a major driver of terrestrial ecology and climate change is expected to greatly alter the distribution of this critical resource. While previous studies have documented strong effects of global environmental change on species interactions in general, responses can vary from region to region. Dryland ecosystems occupy more than one-third of the Earth's land mass, are greatly affected by changes in water availability, and are predicted to be hotspots of climate change. Thus, it is imperative to understand the effects of environmental change on these globally significant ecosystems. Here, we review studies of the responses of population-level plant-plant, plant-herbivore, and predator-prey interactions to changes in water availability in dryland environments in order to develop new hypotheses and predictions to guide future research. To help explain patterns of interaction outcomes, we developed a conceptual model that views interaction outcomes as shifting between (1) competition and facilitation (plant-plant), (2) herbivory, neutralism, or mutualism (plant-herbivore), or (3) neutralism and predation (predator-prey), as water availability crosses physiological, behavioural, or population-density thresholds. We link our conceptual model to hypothetical scenarios of current and future water availability to make testable predictions about the influence of changes in water availability on species interactions. We also examine potential implications of our conceptual model for the relative importance of top-down effects and the linearity of patterns of change in trophic biomass with changes in water availability. Finally, we highlight key research needs and some possible broader impacts of our findings. Overall, we hope to stimulate and guide future research that links changes in water availability to patterns of species interactions and the dynamics of populations and communities in dryland ecosystems.

Underestimation of Spartina productivity in western Atlantic marshes: marsh invertebrates eat more than just detritus

Quantifying primary productivity and understanding the factors that control plant growth are primary goals of ecosystem ecology. Although some methods have been very successful in providing accurate measurements of plant growth and elucidating the importance of both physical and biological factors in regulating primary production (e.g. terrestrial systems: Smith 1996; marine communities: Bertness et al. 2001), many techniques are still hampered by methodological biases that greatly affect productivity estimates and overall experimental results. For example, a recent review of tropical seagrass systems suggests that past experiments investigating seagrass productivity may have been compromised because they did not control for the confounding effects of grazing (i.e. monitoring and/or excluding fish and invertebrate grazers – Valentine and Heck 1999). The authors argue that the probable consequences of not accounting for herbivore effects are: (1) inaccurate estimates of both net and gross primary production and (2) an intellectual bias concerning the relative roles of bottom-up (i.e. nutrients and nutrient regulating factors) and top-down (e.g. herbivory) forces in controlling seagrass growth. In this paper, we argue that these conclusions also apply broadly to salt marshes along the East Coast of both North and South America. Specifically, we suggest that past studies of plant productivity in Western Atlantic marshes, by not accounting for the confounding effects of grazing (i.e. excluding herbivorous crabs and snails), are likely to have significantly underestimated plant growth and overestimated the relative importance of bottom-up factors in regulating marsh primary production. We contend that current methods of measuring marsh grass growth must be amended to include grazer exclusions in carefully controlled field experiments. Only then will we be able to obtain accurate measurements of marsh plant productivity and assess the relative importance of top-down effects in controlling marsh grass growth.

Symbiosis and the Regulation of Communities1

Ecologists have long been interested in factors that control the structure of communities and the relative importance of top-down effects of predators versus bottom-up effects of resources. There is a growing body of evidence that microbial symbioses are important determinants of plant community structure and indirectly affect herbivore and predator trophic levels. Studies with mycorrhizal fungi, N-fixing bacteria and endophytes of grasses have demonstrated that they can affect competition, coexistence, soil nutrient dynamics and plant-herbivore interactions. Long-term field experiments with one grass/endophyte interaction suggest that grassland community structure is determined by the fungus. While total plant productivity of experimental plots was similar, the composition of the vegetation was altered by endophyte symbiosis. The host grass tall fescue (Festuca arundinacea) dominated plots when infected while other grasses greatly increased in uninfected plots. Indirect evidence suggests that changes in prairie vole (Microtus ochrogaster) grazing patterns and reproductive physiology may be in part responsible for vegetational changes. These results provide evidence that, in addition to bottom-up and top-down forces, microbial symbionts of plants are important determinants of community structure.

Symbiosis and the Regulation of Communities

Ecologists have long been interested in factors that control the structure of communities and the relative importance of top-down effects of predators versus bottom-up effects of resources. There is a growing body of evidence that microbial symbioses are important determinants of plant community structure and indirectly affect herbivore and predator trophic levels. Studies with mycorrhizal fungi, N-fixing bacteria and endophytes of grasses have demonstrated that they can affect competition, coexistence, soil nutrient dynamics and plant-herbivore interactions. Long-term field experiments with one grass/endophyte interaction suggest that grassland community structure is determined by the fungus. While total plant productivity of experimental plots was similar, the composition of the vegetation was altered by endophyte symbiosis. The host grass tall fescue (Festuca arundinacea) dominated plots when infected while other grasses greatly increased in uninfected plots. Indirect evidence suggests that changes in prairie vole (Microtus ochrogaster) grazing patterns and reproductive physiology may be in part responsible for vegetational changes. These results provide evidence that, in addition to bottom-up and top-down forces, microbial symbionts of plants are important determinants of community structure.

Top-down versus bottom-up and the Ruritanian bean bug.

In a recent article, Hunter uses the late George Varley and George Gradwell's long-term data on the winter moth (Operophtera brumata) and green tortrix (Tortrix viridana) populations to propose a method of quantifying the relative importance of top-down effects (because of natural enemies) and bottom-up effects (because of resource competition) in influencing population dynamics. We believe this approach is deeply flawed. Using Varley and Gradwell's winter moth study, we show that the problems with Hunter's analysis lie in his misinterpretation of the population dynamics and his inappropriate use of statistical techniques. We also emphasize the importance of distinguishing clearly between two quite different things: firstly, top-down and bottom-up regulation of populations and secondly, the much simpler task of categorizing factors affecting changes in population density as either top-down or bottom-up processes.