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Resource & Predation Controls on Ecosystem Production Bottom-up Controls – refer to control of abundance or productivity of a species or functional group by supply of resources. What controls production in an ecosystem? The “bottom up” view of Ecosystem production Nutrients Global Ocean distribution of Chlorophyll and Benthic Faunal Biomass mg/m3 “Trickle Up” effect Increases in primary production propagate through the food web What controls production in an ecosystem? The “bottom up” view of Ecosystem production Nutrients The “top down” view of Ecosystem production Top-Down Control & Inferences about Trophic Cascades Hairston, Slobodikin and Smith 1960 Basic Premise: “Any population which is not resource limited must, of course, be limited to a level below that set by its resources.” World is Green Therefore the “usual condition is for populations of herbivores not to be limited by food supply….” and producers are limited by resources, not herbivores But, plants may become depleted whenever herbivores become numerous enough (insect outbreaks, e.g. adelgid) The remaining general method of herbivore control is predation” “Predators and parasites in controlling herbivores … must be food limited.” “Green World Hypothesis” Limited by prey availability Limited by predation and parasitism Limited by nutrient availability, not herbivory Nutrients “Green World Hypothesis” Although the authors did not explicitly call this series of interactions a “trophic cascade” , that was the idea that was implicit in the Green World Hypothesis. Nutrients Systems with and without predators Brooks and Dodson (1965) examined 9 lakes: •Four lacked Alewifes •Four had natural Alewife populations •One had introduced Alewifes • sampled before and after the introduction Zooplankton Size Distribution in Lakes with: No Alewife Epischura Lepiodore 5 mm DOMINANT ZOOPLANKTERS LARGE Asplanchna Dioplomus Cyclops Daphnia Mesocyclops Ceriodaphnia Alewife Bosmina Tropocyclops Cyclops DOMINANT ZOOPLANKTERS SMALL Size-efficiency Hypothesis. Larger zooplankton moreout effectively than smaller Large zooplankton competegraze small phytoplankton zooplankton. Populations zooplankton. of small zooplankton increase in abundance when large Therefore if zooplanktivorous fish are large herbivorous zooplankton areabsent, depleted. zooplankton will increase and phytoplankton abundance will be reduced. A “Trophic Cascade” If piscivores added to lake, they will eat and deplete zooplanktivorous fish. Populations of larger herbivorous zooplankton will grow. Phytoplankton populations will be reduced. Food web manipulations in Peter, Paul and Tuesday Lakes - Bass Control + Bass Food web manipulation experiments of in 3 lakes (from Carpenter et al., 1987. Ecology 68:1863-1876). ZOOPLANKTON PHYTOPLANKTON Control +Bass -Bass Lakes with bass (piscivore added), zooplanktivorous fish depleted, large zooplankton increase, chl a is lower Lakes with piscivore removed have higher chl a TROPHIC CASCADE IN KELP ECOSYSTEMS Sea Otters and urchin grazers And Killer Whales Effect of Killer whale predation on Sea Otters Sea Otter Abundance 80 60 40 Sea Urchin Biomass 200 100 060 50 40 30 20 10 10 0 8 6 4 2 1972 0 % Loss 24 hr -1 -2 20 400 0 300 Amchitka I. N. Adak I. Kagalaska I. L. Kiska I. Grazing Intensity No. per 0.25 m2 gms 0.25 m # Otters (% max count) 100 Total Kelp Density 1985 1989 1993 1997 Year From Estes et al. 1998 Science 282:473 Trophic Cascades and Feedbacks CX Nutrients H PP + + But organisms feeding at multiple trophic levels can complicate picture C1 + - C2 + C3 Links 1 (odd) 2 (even) - 3 (odd) 4 (even) For simple food chains: An odd number of trophic links results in control of primary producers by grazing (top-down) An even number of trophic links results in control of primary producers by resources (bottom-up) And nutrients or other factors limiting producers can still increase producer biomass and have effects that propagate up the food web Both Top-down and Bottom-Up controls influence abundance and productivity of populations Are Trophic Cascades All Wet? Reasons Why Trophic Cascades Might be Stronger in Aquatic Ecosystems than in Terrestrial Ecosystems Herbivores are bigger relative to plants in aquatic ecosystems (e.g. zooplankton vs. phytoplankton compared with insects vs. trees) Aquatic primary producers are more nutritious and have element composition more like their herbivores Terrestrial plants have lower P:B ratios than aquatic plants (is this a cause or a consequence??) Aquatic herbivores consume about 3X more autotrophic production than terrestrial herbivores (but why??) Comparison of Herbivory in Aquatic and Terrestrial Ecosystems (Cyr and Pace, 1993 Nature 361:148) Median, 79% Median, 30% Median, 18% Frequency distributions showing the proportion of NPP removed by herbivores in ecosystems with different primary producers Regardless of amount of NPP, rates of Herbivory are on average about 3X higher in Aquatic than Terrestrial Ecosystems (note: log scale) Comparative Strength of Trophic Cascades Across Ecosystems based on Manipulations of Predators in Six Types of Ecosystems (102 studies) Ratio for Plants B+predators/B-predators Plot loge (Abundance+predators/Abundance-predators) o 20.0 7.4 2.7 0.7% 1.8% 5.0% 13.5% 36.8% 100% Herbivore response greatest in lentic & marine systems (e.g. density with predators ~6% to13% of that without predators) and least in terrestrial (density + predators ~71% of density - predators) o Plant response greatest in marine benthos (biomass 4.7X > in systems with predators) vs. terrestrial (1.1X > with predators) o Across systems, as Herbivores ⇩ plant biomass response ⇧ (modified from Shurin et al. 2002, Ecology Letters 5:785) Reasons why we might not ‘see’ top down cascades in land ecosystems Many terrestrial plants have complex structural tissue that is harder to digest and have evolved ‘anti-herbivore’ compounds Perhaps grazing in terrestrial ecosystems is inhibited by Antiherbivore Compounds/Lignin Derivitives Effect of Tannins on palatability of grass to Geese From Buchsbaum et al Reasons why we might not ‘see’ top down cascades in land ecosystems • Many terrestrial apex predators have been hunted to near or local extinction Willow Height (cm) Percent Browsing WOLVES AND WILLOWS 100 <2 m 50 2-3 m 3-4 m 4000 200 98 99 0 00 01 02 03 04 05 Wolves Re-introduced Ripple et al. 2006. For. Ecol & Mgt. 230:96 Reasons why we might not ‘see’ top down cascades in terrestrial ecosystems • Plants have complex tissues and anti-herbivore compounds • Terrestrial may have more complex and more detritus based food webs, less direct grazing. • Many terrestrial apex predators have been hunted to near or local extinction • Longevity of the plant community (decades to centuries for mature plants) makes it hard to measure the results • Terrestrial ecosystems are less experimentally tractable than their aquatic counterparts, in part because of extreme longevity of the plant community • Many of the more charismatic species now enjoy stringent legal protection, which hampers manipulation; "The ecology of fear" • Defensive strategies to avoid predation • Reduced energy intake • Alternate food sources • Increased susceptibility to other predators • Lower mating success • Emigration Mummichog Reduction No Trophic Cascade : Creek Infauna Abundance was lower Algae Inverts Mummichog Total Annelids (# / m 2 x 10 3) Expected 60 40 Observed 20 0 Fleeger et al. Accepted JEMBE Fish -Fish Reference Fish -Fish Nutrient Behavior alters species roles in ecosystem Algae Inverts (Shrimp) Mummichog Nutrients P P H - C1 Behavior alters species roles in ecosystem Removal of mummichogs allows shrimp to forage in more open areas and become more carnivorous. Shrimp d 15N 12 10 8 6 David Johnson Kari Galvan Linda Deegan Fish -Fish Reference Fish -Fish Nutrient enrichment SUMMARY • Top down controls can be important in ecosystem dynamics • Can affect more than just the level below (cascade), including effects on adjoining ecosystems • Interact with nutrient level • Indirect trait mediated effects may be as important as direct consumption effects • Stronger in Aquatic or Terrestrial? No consensus just yet.