Press perturbation experiments and the indeterminacy of ecological interactions: effects of taxonomic resolution and experimental duration

The outcomes of press perturbation experiments on community dynamics are difficult to predict because there is a high degree of indeterminacy in the strength and direction of ecological interactions. Ecologists need to quantify uncertainties in estimates of interaction strength, by determining all the possible values a given interaction strength could take and the relative likelihood of each value. In this study, we assess the degree to which fish effects on zooplankton and phytoplankton are indeterminate in direction using a combination of experimental data and Monte Carlo simulations. Based on probability distributions of interaction strength (i.e. effect magnitude), we estimated the probability of each fish interaction being negative, positive or undetermined in direction. We then investigated how interaction strength and its predictability might vary with experimental duration and the taxonomic resolution of food web data. Results show that most effects of fish on phyto- and zooplankton were indeed indeterminate, and that the effects of fish were more predictable in direction as the taxonomic resolution of food web data decreased and the experimental duration increased. Results also show that most distributions of interaction strength were not normal, suggesting that normal based statistical procedures for testing hypothesis about interaction strength may be misleading, as well as predictions of food web models assuming normal distributions of interaction strength. By considering the probability distributions and confidence intervals of interaction parameters, ecologists would better understand the outcomes of species interactions and make more realistic predictions about our perturbations in natural food webs

Interactions between Predation and Resources Shape Zooplankton Population Dynamics

Identifying the relative importance of predation and resources in population dynamics has a long tradition in ecology, while interactions between them have been studied less intensively. In order to disentangle the effects of predation by juvenile fish, algal resource availability and their interactive effects on zooplankton population dynamics, we conducted an enclosure experiment where zooplankton were exposed to a gradient of predation of roach (Rutilus rutilus) at different algal concentrations. We show that zooplankton populations collapse under high predation pressure irrespective of resource availability, confirming that juvenile fish are able to severely reduce zooplankton prey when occurring in high densities. At lower predation pressure, however, the effect of predation depended on algal resource availability since high algal resource supply buffered against predation. Hence, we suggest that interactions between mass-hatching of fish, and the strong fluctuations in algal resources in spring have the potential to regulate zooplankton population dynamics. In a broader perspective, increasing spring temperatures due to global warming will most likely affect the timing of these processes and have consequences for the spring and summer zooplankton dynamics

Fish-mediated nutrient recycling and the trophic cascade in lakes

The effects of planktivorous fish on phytoplankton through nutrient recycling and zooplankton herbivory were experimentally separated and their relative importance quantified in a eutrophic humic lake. Natural phytoplankton assemblages were incubated in nutrient-permeable chambers placed inside enclosures with or without fish. Outside these chambers, phytoplankton were exposed to zooplankton herbivory and to nutrient recycling by fish and zooplankton, whereas inside the chambers, phytoplankton were exposed only to nutrient recycling by these consumers. Our results show that fish had significant positive effects on cyanobacteria, cryptomonads, and chlorophytes inside the chambers, indicating that fish-mediated nutrient recycling had significant effects on these phytoplankton groups. However, our results also indicate that changes in zooplankton grazing induced by fish were an important mechanism by which fish affected all phytoplankton groups except cyanobacteria. Comparison of effect sizes revealed that the effects on cyanobacteria and chlorophytes through nutrient recycling were stronger than those through zooplankton grazing. Moreover, most of the fish-mediated nutrient recycling effects were due to increased nutrient recycling by zooplankton rather than direct nutrient excretion by fish. In conclusion, we provide experimental evidence supporting the hypothesis that fish-mediated nutrient recycling is an important mechanism affecting phytoplankton community structure and favoring cyanobacteria dominance in lakes

The relative importance of fish predation and excretion effects on planktonic communities

The effects of planktivorous fish on lower trophic levels through predation on zooplankton and nutrient excretion were experimentally separated and their relative importance quantified in a eutrophic humic lake. The experiment was performed in 12 enclosures (3 mi), which initially were identical with respect to all components except fish. At the start of the experiment, caged fish not able to feed on zooplankton were added to four of the enclosures (excretion treatment), and free swimming fish to four enclosures (excretion plus predation treatment). Four enclosures were left as controls. Samples for nutrients and all major groups of organisms from bacteria to zooplankton were taken after 14 and 28 d. The effect sizes of fish excretion and predation were calculated for each variable. Our results suggest that in eutrophic lakes fish predation on zooplankton may be more important than nutrient excretion by fish for the structure and dynamics of planktonic communities. Fish predation on zooplankton was the most important mechanism accounting for fish effects on nutrient concentrations in the water, on phytoplankton biovolume, on rotifers and total zooplankton biomass, as well as on protozoan densities. However our results suggest that nutrient excretion by fish may have important indirect effects on zooplankton. Hence, the effects of plankrivorous fish through both predation on zooplankton and nutrient excretion act in concert acid may be quantitatively important to shape the structure and dynamics of planktonic communities

Fish composition and species richness in eastern South American coastal lagoons: additional support for the freshwater ecoregions of the world

The relationships between fish composition, connectivity and morphometry of 103 lagoons in nine freshwater ecoregions (FEOW) between 2·83° S and 37·64° S were evaluated in order to detect possible congruence between the gradient of species richness and similarities of assemblage composition. Most lagoons included in the study were <2 km(2) , with a maximum of 3975 km(2) in surface area. Combined surface area of all lagoons included in the study was 5411 km(2) . Number of species varied locally from one to 76. A multiple regression revealed that latitude, attributes of morphometry and connectivity, and sampling effort explained a large amount of variability in species richness. Lagoon area was a good predictor of species richness except in low latitude ecoregions, where lagoons are typically small-sized and not affected by marine immigrants, and where non-native fish species accounted for a significant portion of species richness. Relationships between species and area in small-sized lagoons (<2 km(2) ) is highly similar to the expected number in each ecoregion, with systems located between 18·27° S and 30·15° S attaining higher levels of species richness. Similarities in species composition within the primary, secondary and peripheral or marine divisions revealed strong continental biogeographic patterns only for species less tolerant or intolerant to salinity. Further support for the FEOW scheme in the eastern border of South America is therefore provided, and now includes ecotonal systems inhabited simultaneously by freshwater and marine species of fishes.Fil: Petry, A. C.. Universidade Federal do Rio de Janeiro; BrasilFil: Guimarães, T. F. R.. Universidade Federal do Rio Grande do Sul; BrasilFil: Vasconcelos, F. M. Universidade Federal do Rio de Janeiro; BrasilFil: Hartz, S. M.. Universidade Federal do Rio Grande do Sul; BrasilFil: Becker, F. G.. Universidade Federal do Rio Grande do Sul; BrasilFil: Rosa, R. S.. Universidade Federal da Paraíba; BrasilFil: Goyenola, G.. Universidad de la República; UruguayFil: Caramaschi, E. P.. Universidade Federal do Rio de Janeiro; BrasilFil: Díaz de Astarloa, Juan Martín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; ArgentinaFil: Sarmento Soares, L. M.. Instituto Nacional da Mata Atlântica; BrasilFil: Vieira, J. P.. Universidade Federal do Rio Grande; BrasilFil: García, A. M.. Universidade Federal do Rio Grande; BrasilFil: Teixeira de Mello, Franco. Universidad de la República; UruguayFil: de Mello, F. T.. Universidade Federal Do Piaui.; BrasilFil: Meerhoff, M.. Universidad de la República; UruguayFil: Attayde, J. L.. Universidade Federal do Rio Grande do Norte; BrasilFil: Menezes, R. Fernandes de. Universidade Federal do Rio Grande do Norte; BrasilFil: Mazzeo, N.. Universidad de la República; UruguayFil: Di Dario F.. Universidade Federal do Rio de Janeiro; Brasi