Lecture 11: Grazers & Mutualisms

Lecture 11: Grazers & Mutualisms EEES 3050

The Green World Hypothesis • The world is green • because herbivores are held in check by their predators, parasites, and diseases such that they cannot consume all the plant biomass. • Briefly discussed on pg. 551 of text. • Proposed by Hairston, Smith and Slobodkin in 1960.

The Green World Hypothesis • Not everyone was convinced that predators can limit herbivore numbers. • Why else could cause the world to be “green”? • Plant defenses • Nutrients limit herbivores, not energy. • Abiotic factors limit herbivores • Spatial and temporal heterogeneity reduce the availability of plants.

Plant defenses • What types of defenses do plants have? • Structural adaptations, i.e. thorns. • Secondary plant substances • Acetogenin, juglone • remember the apple tree/grass relationship. • Phenylpropanes • E.g. cinnamon and cloves • Terpenoids • Peppermint oil • Alkaloids • Nicotine, morphine, caffeine.

Secondary defense example • Oak trees • Produce tannins • Toughness

Secondary defense example • Oak trees • Produce tannins • Toughness • Herbivores have compensated by concentrating feeding in the early spring on young leaves or altering their life cycle. • Some moths overwinter as larvae.

Herbivores on the Serengeti Plains • Several herbivores • Zebra, wildebeest, Thompson’s gazelle

Temporal differences in foraging.

Differential eating of plant parts.

Grazing facilitation: Does the grazing of one species increase the availability of food for following species? • Observation: Gazelles concentrated their feeding on areas that wildebeest ate. • Hypothesis: Gazelles will increase in # if wildebeest increase in #. • Test/Experiment: Examine time periods when wildebeest #’s go up.

Results No evidence of facilitation

Herbivores on the Serengeti Plains • We have looked at only 3 of the herbivores on the Serengeti Plains • 38 species of grasshoppers • In some communities, grasshoppers consumed almost half as much grass as the ungulates. • 36 species of rodents

Can grazing benefit plants? • Observations: Overgrazing is clearly detrimental, but some have observed mutualistic relationship between grasses and grazers. • Hypothesis: Biomass of grass will increase with moderate levels of grazing.

Experiment: Measure biomass in grazed vs. ungrazed areas.

Results

Herbivores can be selective (snowshoe hares) • Observation: Some herbivores select certain plants over others. • How does this relate to the “Green World”? • Krebs suggests: “selectivity is a major reason why the world is not completely green for a herbivore” • Is this a valid statement?

Herbivores can be selective (snowshoe hares) • I would suggest… • A herbivore being selective is only relevant if food is a limiting factor. • With the snowshoe hares, they still eat the other plants, suggesting that if Dwarf birch disappears the hares can still survive on spruce and willow. • What could be limiting the population of snowshoe hares? • Lynx, harsh winters, …

Testing the ‘green world’ hypothesis. • Vegetation dynamics of predator-free land-bridge islands • By Terborgh et al. Journal of Ecology 2006. • Green world hypothesis = • Predators keep herbivores in check allowing plants to grow. • How to test this?

Testing the ‘green world’ hypothesis. – Terborgh et al. • Studied 14 sites in an area where a new reservoir was constructed in the 1986’s. • Small islands – no predators, herbivores = leaf-cutter ants, howler monkeys, iguanas • Medium islands – no vertebrate predators, but armadillos are present and are a predator of leaf-cutter ants. • Large islands – had vertebrate predators, including snakes, raptors, and ocelot. • Hypothesis: Islands without predators will have a decline in plant production:

Fig. 2 Per-capita mortality plus growth out of a sapling class (left bars) vs. per-capita recruitment into the class (right bars) of small (top) and large (bottom) saplings at small, medium and large landmasses at Lago Guri, Venezuela. Values shown are the means of the proportions dying and recruiting at each site. Black portions of bars = mortality; white = growth out of the size class into the next larger; grey = recruitment into the size class from the next smaller. Post hoc analyses were performed on t-tests from model contrasts and Bonferroni corrected for multiple comparisons.

Further comments: • Strong herbivore effects have been found many times when herbivores were introduced to a previously herbivore free location. • What did this study account for that others may not have? • All species were native. • On herbivore free islands there is a tendency for plants to lose their secondary chemical defenses.

Interspecific Interactions • Herbivory: one species benefits at the other’s expense • Interactive herbivore systems • Mutualisms: both species benefit • Commensalism: one benefits, one unaffected • Noninteractive herbivore systems

Spruce budworm irruptions (outbreaks) • Spruce budworms have a 35-40 year cycle. • During an irruption they kill many trees in a stand.

Emerald ash borer: an invasive species

How did EAB arrive in North America? • Arrived in solid wood packing material from Asia 10 -12 years ago. • First detected in Detroit/Windsor area in July 2002.

Impacts

Are there any ash trees in Asia? • Mongolian ash trees have different secondary chemicals. • Ash borers in North America could have escaped a disease.

Evaluating the economic costs and benefits of slowing the spread of the emerald ash borer in Ohio and Michigan • Objectives: • 1) to provide estimates of the regional economic impact emerald ash borer will potentially inflict upon the ash forestry in Ohio and Michigan; • 2) to provide policy-makers with quantitative guidance for cost-effective alternative strategies to control, prevent, or slow the spread of emerald ash borer.

Objective 1: estimate regional economic impact emerald ash borer. • Estimate the current distribution of ash trees and emerald ash borer • Predict the spread of emerald ash borer • Estimate value of ash in spatially explicit manner • Determine the regional economic consequences of emerald ash borer spread through the development of a CGE model

Objective 1: to provide estimates of the regional economic impact of an invasive species. A • Estimate the potential habitat - A • Predict the spread - B • Estimate economic impact in a spatially explicit manner - C • Determine the regional economic consequences of spread through the economy - D B C D

Estimate the current distribution of ash trees and emerald ash borer From Dr. Louis Iverson and Anatha Prasad - USFS

Predict the spread of emerald ash borer • Natural Dispersal • Human-mediated dispersal

Wood products Roads Campgrounds PopulationDensity Ash Abundance Multiplier Spread Model Insect Ride Model (GIS) Ash Abundance (Basal Area) Weighted Random Seeding Gravity Model Insect Flight Model (EAB SHIFT) Outlier Generator Maturity of Infection (EAB Abundance) Random Seeding Spatially Explicit Probability of Colonization

Predict the spread of emerald ash borer: Natural Dispersal 270-m cells • SHIFT model for EAB spread(spatially explicit cell-based model) • Calculates the probability of colonization of currently unoccupied cells based on abundance of EAB, habitat availability of ash, and distance between all cells. EAB occupied zone Unoccupied zone From L. Iverson

Predict the spread of emerald ash borer:Human-mediated dispersal Campers with Firewood

Model Needs • Number of campers • Attractiveness of campgrounds • Distance • Distance coefficient • Data for parameterization and validation. • Primary Question – • How many campers from areas with emerald ash borer are traveling to Ohio and Michigan campgrounds?

Number of campers traveling to each campground from EAB region.

Why does long distance dispersal matter? From. Spatial Ecology 1997

Lecture 11: Grazers & Mutualisms