What controls the productivity and abundance of plants

What controls the productivity and abundance of plants in this ecosystem? Nutrients Bottom-up Controls – refer to control of abundance or productivity of a species or functional group by supply of resources.

Global Ocean distribution of Chlorophyll and Benthic Faunal Biomass mg/m 3

Top-Down Controls – refer to control of abundance or productivity of prey species or functional group by the actions of its consumers Nutrients Bottom-up Controls – refer to control of abundance or productivity of a species or functional group by supply of resources.

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. ” Therefore the “usual condition is for populations of herbivores not to be limited by food supply…. ” and producers are limited by resources, not herbivores n But, plants may become depleted whenever herbivores become numerous enough (insect outbreaks, e. g. adelgid) n World is Green 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 Limited by nutrient availability, not herbivory Nutrients

Controlled manipulation of resources by ecosystem fertilization at Great Sippewissett Salt Marsh (Valiela, Teal, et al. 1970 -1990) False color aerial infra-red photo of fertilized plots at Great Sippewissett Salt Marsh • HF receives sewage sludge based fertilizer • UP receives equivalent does of urea/phosphate • C are control plots

Above ground biomass of salt marsh grasses in fertilized vs. control plots Despite naturally high porewater N levels, further N-additions increase productivity of marsh grasses

Annual Cycle of Benthic Chlorophyll in fertilized and control creeks at Great Sippewissett Salt Marsh • Lowest biomass of benthic microphytes in summer • Fertilization effect only occurs in winterspring period – why? Foreman 1989

Macrofaunal Density (1000’s per m-2) Seasonality of fish and macroinfaunal abundance Werme et al.

Caging Experiments in a Salt Marsh Exclude fish and crabs with cage

Caging Experiment Begin Caging Open one Cage -In cages, algae biomass remains high in cages as grazing pressure by fish and crabs is reduced and fertilization effect persists -Biomass converges on ambient (low) levels when cages are opened and grazers can gain access; diminishes fertilization effect -Suggests consumer control overwhelms resource control of benthic Foreman 1989

Effect of excluding fish and crabs using cages and fertilization on benthic productivity and respiration From Giblin, Foreman & Banta, 1995

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 al. , 1987. Ecology 68: 1863 -1876). ZOOPLANKTON (from Carpenter et 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 # Otters (% max count) 100 80 60 40 Amchitka I. N. Adak I. Kagalaska I. L. Kiska I. 20 400 0 300 gms 0. 25 m -2 Sea Urchin Biomass 200 060 50 40 30 20 10 10 0 8 6 4 2 1972 0 Grazing Intensity Total Kelp Density No. per 0. 25 m 2 % Loss 24 hr -1 100 1985 1989 1993 1997 Year From Estes et al. 1998 Science 282: 473

Are Trophic Cascades All Wet?

Comparative Strength of Trophic Cascades Across Ecosystems based on Manipulations of Predators in Six Types of Ecosystems (102 studies) predators for Plants Ratio B+predators/B- Plot loge (Abundance+predators/Abundance-predators) 20. 0 7. 4 2. 7 0. 7% 1. 0 1. 8% 5. 0% 13. 5% 36. 8% 100% N+predators/N-predators for Herbivores as % Ratio o Plant response greatest in marine benthos (biomass 4. 7 X > in systems with predators) vs. terrestrial (1. 1 X > with predators) o Across systems, as Herbivores ⇩ plant biomass response ⇧ (modified from Shurin et al. 2002, Ecology Letters 5: 785)

Reasons Why Trophic Cascades Might be Stronger in Aquatic Ecosystems than in Terrestrial Ecosystems ü Herbivores are bigger relative to plants in aquatic ecosystems (e. g. zooplankton vs. phytoplankton compared with insects vs. trees) ü Aquatic primary producers are more nutritious and have element composition more like their herbivores ü Terrestrial plants have lower P: B ratios than aquatic plants ü Aquatic herbivores consume about 3 X 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 3 X higher in Aquatic than Terrestrial Ecosystems (note: log scale) Aquatic Terrestrial

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

Are Top Down Controls and Trophic Cascades All Wet? Perhaps grazing in terrestrial ecosystems is inhibited by Antiherbivore Compounds/Lignin Derivitives

Feeding Experiments with Marsh Invertebrates on Agar Plates Mix Spartina detritus or grass with agar, spike with different concentrations of ferulic acid and allow invertebrates to feed. Count bite marks.

Relative Amount Eaten Effect of Tannins on palatability of grass to Geese Demonstrates that chemical composition of plants can affect feeding by herbivores From Buchsbaum et al

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;

Trophic Cascades and Feedbacks CX Nutrients H PP + + For simple food chains: But organisms feeding at multiple trophic levels can complicate picture C 1 + - C 2 + C 3 Links 1 (odd) 2 (even) - 3 (odd) 4 (even) 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 the abundance and productivity of popluations

Number of wolves in lower 48 (1, 000’s) WOLVES IN NORTH AMERICA The Study of ECOLOGY 250 200 150 100 1750 1850 1950 Wolves present Number of trees 50 30 2000 absent 15 ASPEN 0 150 75 0 1750 1850 1950 Ripple et al. 2005 Bio. Science 2000 COTTONWOOD

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

Behavior alters species roles in ecosystem Algae Inverts Mummichog Nutrients P P H - C 1

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 Fish -Fish Reference Fish -Fish Nutrient

Behavior alters species roles in ecosystem Removal of mummichogs allows shrimp to forage in more open areas and become more carnivorous. Shrimp d 15 N 12 10 8 6 David Johnson Kari Galvan Linda Deegan Fish -Fish Reference Fish -Fish Nutrient enrichment

THE IMPORTANCE OF INDIRECT EFFECTS IN ECOSYSTEMS 63% Number of studies 25 Non-Lethal Effects Aka Trait-mediated 15 5 40 0 51% Lethal Effects Aka: Density mediated 20 0 0. 5 1. 0 1. 5 2. 0 Ratio of effect size to total predator effect 0 Preisser et al. 2005. Scared to death? The effects of intimidation and consumption in predator prey interactions. Ecology 86: 501

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 • Trait mediated effects may be as important as consumption effects • Stronger in Aquatic or Terrestrial? No consensus just yet.

Top Down or Bottom-up? ? The End