Overview Previously we considered fundamental groups… Autotrophs and autotrophy Heterotrophs and heterotrophy Consumers: primary, secondary, tertiary Now we want to consider interactions among species and trophic levels
General Observation Species interactions in aquatic systems are similar to those in terrestrial systems, with aquatic-specific characteristics.
Herbivory Most studies have focused on the effects of grazing on periphyton. Literature on consumption of higher plants is much more limited. Generalization: Grazers are attracted to the periphyton on plants and not the plant itself. Obvious exceptions: higher animals (e.g., geese, moose) that graze on higher plants
How does grazing affect the grazed? Grazing by snails limits biomass accumulation But…was primary production lower? Steinman et al. (1996) Epilithic algal biomass
Top-Down vs Bottom-Up Controls (Elimia, snail) Fig. 9.5 Allan and Castillo (2007), from Rosemond et al. (1993) This appears to be an N-limited system. However, grazing can overwhelm the nutrient enrichment effect.
Do grazers search for food randomly or “purposefully”? Fig. 9.1 Allan and Castillo (2007), from Kohler (1984) The grazing mayfly (Baetis) searches more thoroughly and moves slower if food quality is higher. Implication: Baetis lingers in food-rich patches
Do algivorous fish benefit from increased algal productivity? Fig. 9.2 Allan and Castillo (2007), from Power (1983) Yes and no… Greater total biomass Greater population density But basically same individual production Rainy season Dry season
Does grazing benefit the grazed organisms? Fig. 9.3 Allan and Castillo (2007), from Hill and Knight (1987) Top: Even light grazing reduces algal biovolume. Bottom: Ratio CHLa to biovolume increases with grazing density A grazing mayfly A = ambient density 0 = grazer removed # = experiment density Biovolume = volume of an average algal cell (biomass)
Hypothesized impacts of grazing Biomass (high confidence) Photosynthesis (some confidence) GPP (speculative) Fig. 9.4 Allan and Castillo (2007), from Lambertti and Moore (1984)
Predation (including Cannibalism) One consumer eats another Size and quality of the “food” (prey) matter as for grazers New variable: behavior – Of the prey (e.g. avoidance, defense) – Of the predator (e.g., foraging, feeding mode)
What does this tell you about the relationship between predator and prey? Caddisfly Alderfly # of predators / sample (N) Biomass of prey/sample (mg/m 2 ) Abundance of predators (caddisflies & alderflies) is directly related to the abundance of their prey (chironomids & stoneflies). More prey, more predators.
Are predators selective about their prey? Predators don’t appear to be too selective Predators consumed prey in approximate proportion to their in- stream abundance % Composition Benthic Samples Gut Contents: Alderfly Gut Contents: Caddisfly Macroinvertebrate Prey Species
Who eats what? Benthic macroinvertebrate predators Degree of overlap in what individual eat Difference in individual body size Individuals that are of similar size eat similar things; i.e., they have similar “dietary niches”. Largest Smallest Fig. 9.8 Allan and Castillo (2007), from Woodward and Hildrew (2002)
Who eats whom? Small predators eat small prey Large predators eat large prey
Size Matters Larger prey means greater benefit per prey item captured The “size refuge”: Too large to be eaten For invertebrates, size at different life stages may differ considerably, thus a prey at one life stage may be a predator at a later life stage, and vice versa.
Prey defenses Physical – Hard or spiny exoskeleton Chemical – Distasteful Behavioral – Nocturnal feeding Combinations – Live in a habitat the predator can’t access (e.g., shallows)
Do prey fish alter their behavior in the presence of predators? Fig. 9.9 Allan and Castillo (2007), from Dill and Fraser (1984) Distance fish will go to obtain food item “Risk” perceived by the predator Juvenile coho salmon feeding in presence or absence of model rainbow trout.
Do lower organisms behave differently in the presence of a predator? Night vs Day drift by Mayfly species Fig. 9.10 Allan and Castillo (2007), from Flecker (1992) Different rivers Predation intensity Different prey species
Predator impacts on the ecosystem may be complex - Competition - Trophic cascades Fig. 9.11 Allan and Castillo (2007), from Power (1990)
Competition Two species vie for a common resource One species disadvantages the other – Reduces the other species’ fitness – Reduces the other species abundance Modes of competition – Exploitation: Dominant species uses more of the critical resource to dominate the weaker species – Interference: Dominant species directly interacts to dominate the weaker species
Evidence for Competition Resource use – Nutrient use efficiency in algae – Assimilation efficiency in benthic macroinvertebrates Resource partitioning – In space: habitat use – In time: day-night, seasonal “Niche exploitation” But is this really competition? Or is it the “ghost of competition past”; i.e., an evolutionary response to efficient resource use?
What happens if you eliminate a key grazer? Natural collapse of grazing caddis fly Glossosoma Fig. 9.15 Allan and Castillo (2007), from Kohler and Wiley (1997)
Summary Species are inter-connected through food webs (herbivory, predation, competition) Bottom up effects: nutrient enrichment Top-down effects: trophic cascades Environmental stress may mask species interactions (e.g. extreme nutrient scarcity, frigid conditions)