Landscape patterns in top-down control of decomposition: omnivory disrupts a tropical detrital-based trophic cascade.

Detrital-based trophic cascades are often considered weak or absent in tropical stream ecosystems because of the prevalence of omnivorous macroconsumers and the dearth of leaf-shredding insects. In this study, we isolate top-down effects of three macroconsumer species on detrital processing in headwater streams draining Trinidad's northern mountains. We separated effects of different macroconsumers by experimentally manipulating their temporal access to isolated benthic habitat over the diel cycle. We found no evidence that omnivorous macroconsumers, including a freshwater crab (Eudaniela garmani) and guppy (Poecilia reticulata), increased leaf decomposition via consumption. By contrast, above a waterfall excluding guppies, the insectivorous killifish, Anablepsoides hartii, reduced the biomass of the leaf-shredding insect Phylloicus hansoni 4-fold, which consequently reduced leaf decomposition rates 1.6-fold. This detrital cascade did not occur below the barrier waterfall, where omnivorous guppies join the assemblage and reduce killifish densities; here killifish had no significant effects on Phylloicus or decomposition rates. These patterns of detrital processing were also observed in upstream-downstream comparisons in a landscape study across paired reaches of six streams. Above waterfalls, where killifish were present, but guppies absent, leaf decomposition rates and Phylloicus biomass were 2.5- and ~35-fold lower, respectively, compared to measurements below waterfalls. Moreover, the strength of top-down control by killifish is reflected by the 20- and 5-fold reductions in variability (±SE) surrounding mean Phylloicus biomass and leaf decomposition rates in upstream relative to downstream reaches where no top-down control was detected. Findings show a clear, detrital-based trophic cascade among killifish, a leaf-shredding insect, and leaf decomposition rates. Results also show how omnivorous guppies disrupt this cascade by depressing killifish densities, thereby releasing invertebrate shredders from predation, and significantly increasing decomposition rates. Moreover, this combination of direct and indirect trophic interactions drives patterns in decomposition rates in stream networks at a landscape scale, resulting in significantly lower rates of decomposition above vs. below barrier waterfalls. Our findings reveal that omnivory can result in significant indirect effects on a key ecosystem process, illustrating the importance of these hidden trophic pathways in detrital-based systems and suggesting that resource control in tropical systems may be even more complex than previously envisioned.

Measuring fertilization success of broadcast-spawning marine invertebrates within seagrass meadows.

Increasing current velocity has been negatively correlated with the fertilization success of marine broadcast-spawning invertebrates. Seagrass has been shown to affect seawater hydrodynamics by slowing the movement of water. In this study we aimed to tease apart the relationship between fertilization success in sea urchins inside and outside of seagrass beds in St. Joseph Bay, Florida. Fluorescein dye diffusion, as a proxy for gamete diffusion, indicated higher rates of diffusion in sand habitats outside of seagrass beds. We quantified the proportion of eggs that remained on a female compared to being advected off a female over a 2-min interval in and out of grass beds. More eggs were collected inside of seagrass beds than over sand habitats, suggesting increased residence time of gametes within the beds. We induced sea urchins to spawn in experimental arrays in and out of grass beds and measured the fertilization success of eggs released from females and captured in the water column with a plankton pump. The fertilization success of eggs was significantly higher in grass beds. We concluded that seagrasses have the potential to mitigate gamete diffusion and increase the reproductive success of broadcast-spawning species that spawn in them.