Herbivore vs. Nutrient Control of Marine Primary Producers: Context-Dependent Effects

Abstract

Pervasive overharvesting of consumers and anthropogenic nutrient loading are changing the strengths of top-down and bottom-up forces in ecosystems worldwide. Thus, identifying the relative and synergistic roles of these forces and how they differ across habitats, ecosystems, or primary-producer types is increasingly important for understanding how communities are structured. We used factorial meta-analysis of 54 field experiments that orthogonally manipulated herbivore pressure and nutrient loading to quantify consumer and nutrient effects on primary producers in benthic marine habitats. Across all experiments and producer types, herbivory and nutrient enrichment both significantly affected primary-producer abundance. They also interacted to create greater nutrient enrichment effects in the absence of herbivores, suggesting that loss of herbivores produces more dramatic effects of nutrient loading. Herbivores consistently had stronger effects than did nutrient enrichment for both tropical macroalgae and seagrasses. The strong effects of herbivory but limited effects of nutrient enrichment on tropical macroalgae suggest that suppression of herbivore populations has played a larger role than eutrophication in driving the phase shift from coral- to macroalgal-dominated reefs in many areas, especially the Caribbean. For temperate macroalgae and benthic microalgae, the effects of top-down and bottom-up forces varied as a function of the inherent productivity of the ecosystem. For these algal groups, nutrient enrichment appeared to have stronger effects in high- vs. low-productivity systems, while herbivores exerted a stronger top-down effect in low-productivity systems. Effects of herbivores vs. nutrients also varied among algal functional groups (crustose algae, upright macroalgae, and filamentous algae), within a functional group between temperate and tropical systems, and according to the metric used to measure producer abundance. These analyses suggest that human alteration of food webs and nutrient availability have significant effects on primary producers but that the effects vary among latitudes and primary producers, and with the inherent productivity of ecosystems.

Ecological rules governing helminth–microparasite coinfection

“…In some senses, this is unsurprising; microparasites are organisms, after all, and there is no reason to suspect that they are exempt from ecological processes that shape the abundance of organisms in macroecosystems (e.g., refs. [17][18][19][20]. Furthermore, IFN-␥ is central to the clearance of virtually all microparasites, despite differences in the details of which cell types or anatomical location might be involved (27,30).…”

“…However, the complexity of within-host interactions (12)(13)(14)(15)(16) can make it difficult to predict how one infection will affect the course of others. Disentangling such complexity is a core activity of community ecologists, who in recent years have successfully identified trophic rules that determine, for example, the abundance of organisms in marine (17) as well as tropical (18,19) and temperate (20) terrestrial ecosystems. These trophic rules include ''bottom-up'' control of population size via resource limitation and ''top-down'' control via predation (21).…”

“…These trophic rules include ''bottom-up'' control of population size via resource limitation and ''top-down'' control via predation (21). Only by considering regulation by both resources and consumers have ecologists been able to explain the abundance of organisms across such diverse communities (17)(18)(19)(20). One major finding is that the relative importance of top-down and bottom-up control mechanisms varies among species and ecosystems; a charismatic example of this is provided by African savannah systems, in which zebra populations are primarily controlled by predators, whereas wildebeest are more strongly regulated by the quality and quantity of plant matter available for them to eat (18).…”

Interactions among ecosystem stressors and their importance in conservation

“…The conceptual Burkepile & Hay [34] Ban et al [23] synergy interaction types responses physiological individual/ population community/ ecosystem e.g. respiration, photosynthesis, calcification e.g.…”

Chemically rich seaweeds poison corals when not controlled by herbivores

“…In the Caribbean, average cover of hard corals has declined by ∼80% in the last 3 decades (5) and more than 30% of the world's coral species face elevated risk of extinction (6). Monitoring (7), field experiments (8)(9)(10), and a meta-analysis (11) all indicate that herbivory is critical in preventing seaweed replacement of corals. However, the extent to which seaweeds drive these shifts by outcompeting adult corals in the absence of herbivory, or proliferate only after coral mortality is triggered by other causes (such as disease or bleaching) is debated (12)(13)(14)(15).…”