Week 6: Mutualisms and Food Web Stabiltiy

Mutualisms

both species have a dependency on each other→ net positive effect of partnership

When should we expect mutualisms

Under stressful conditions

1. Increasing physical stress, increasing the frequency of positive interactions → neighbourhood habitat amelioration

2. Increasing consumer pressure, decreasing the frequency of competitive interactions → associational defences

Neighborhood Habitat Amelioration

improving a local habitat

Associational Defenses

Protection gained by an organism from living in association with another species

Is mutualism obligate?

• Cannot grow without the other species

• Consequences

  • Below threshold: extinction

  • Above threshold: unlimited growth

  • Unrealistic (lots of mutualisms)→ scenarios not found in natural world

• Positive feedback (density dependence) is problematic in these very simple models

Do Lotka- Volterra models work for mutualism?

• tend to lead to silly solutions in which both populations undergo unbounded exponential growth, in an orgy of mutual benefaction

• non-linear response required

  • cannot benefit forever

  • saturation of benefit

Empirical examples of mutualisms: Pollination

• Pollen transfer

• Nectar

• Rapid diversification of flowers (angiosperms) 90-130 mya coincides with boom in insect species

Specialist pollinator

• Fig & fig wasp

• 700 spp of fig: 1-2 pollinator sp

Are figs full of dead wasps?

No. Ficin released by the plants decomposes the bodies

Fig wasp sex ratio

• sex ratio skewed to females

• male hatches→ blind + wingless → fertilises females → burrows a hole in fruit for females then die

Generalist pollinator

• Honey bees (Apis mellifera)

• 100s plants

Empirical examples of mutualisms: Coral- Algae

• can exist without each other but better together→ coral may have negative growth without algae + algae may take up minerals from water via diffusion → very slow process

• Facultative/Obligate symbiosis

  • Algae can leave when conditions not right (bleaching of coral)

  • Coral can feed by predation on plankton (but growth is slow or even negative)

The Role of Coral

• obtain photosynthate (carbon)

• sieve out minerals from huge volume of water that passes over via wave action

• coral reefs: clear water, shallow as wave action required

The role of algae (zooxanthellae)

• Factory for turning sunlight in C

• Obtain minerals extracted from sea by coral

• Free living individuals rely upon diffusion from nearby surrounding water (slow)

Empirical examples of mutualisms: Plants- Fungi

Fungi → photosynthates i.e carbon

Plant → soil nutrients

Xanthoria parietina

common pollution tolerant lichen found on trees across London

How old/ widespread is plants- fungi mutualism?

Fossil evidence → 400 Mya

Nutrient deficient soils

• Sand dunes

• Tropical rainforests(!)

Orchid mycorrhizas

• Some orchids non-photosynthetic

• Most chlorophyll only after seedling

• Obligate mutualist

• Seeds –size of dust particles

  • Very little nutrients

  • Produce large number

  • Good for colonisation

• Will not germinate without fungal infection

• Obtain carbon from fungus

• Parasitise other plants via fungal network (Also mutualism with pollinators)

mycoheterotrophic orchid

• no leaves

• can’t fix carbon

• e.g Erythrorchis altissima (Asia)

• decomposes wood on tree

• requires saprophytic fungi to germinate and grow

Arbuscular mycorrhizas (AM)

• majority of wild and crop plants

• Facultative

• Within plant cell → 20% of plant-fixed carbon transferred to fungus

• Oldest type (fossils)

AM Function: Root morphology modification and development of a complex, ramifying mycelial network in soil

Ecosystem service: increase plant/ soil adherence and soil stability

AM Function: Increasing mineral nutrient + water uptake by plants

Promote plant growth while reducing fertiliser requirement

AM Function: Buffering effect against abiotic stresses

Increased plant resistance to drought, salinity, heavy metals pollution and mineral nutrient depletion

AM Function: Secretion of ‘glomalin’ into the soil

Increased soil stability and water retention

AM Function: Protecting against root pathogens

Increased plant resistance against biotic stresses while reducing phytochemical input

AM Function: Modification of plant metabolism and physiology

Bioregulation of plant development and increase in plant quality for human health

Ectomyccorhiza

• less common than arbuscular mycorrhizas

• 10% plant families

• Basidiomycota, Ascomycota, and Zygomycota

• Hartig net

  • wraps round roots of plant → interface of plant and fungus

  • does not penetrate→ more generalist interaction

Experiment to show effect on competition

• 2 plants; 2 soils (high, low sand)

• 4 fungi 5 ratios of seedlings (8:0; 6:2; 4:4; 2:6; 0:8)

• plants will compete → seed ratios manipulated→ dominating plant

• trifolium good at fixing nitrogen from air

• good in nutrient poor soil

• pioneer species → dies → nitrogen in soil

• high sand = nutrient deficient

• low sand = high nutrients

• Trifolium – N-fixer - did better with mycorrhizae, high sand (low soil nutrient)

• Trifolium does bad without fungus

• Lolium did better with Trifolium. Better with no mycorrhizae→ mycorrhizae manipulates interaction

• Mixed communities produce higher yields. Mycorrhizal fungi improve community yield.

  Importance of “Higher order interactions” species A outcompetes species B, but not in presence of species C

Mutualism Summary

• Basic theory works with saturation of benefit

• Widespread

• Generalists –facultative

• Specialists –obligate

• Responsible for angiosperm diversification

• Interesting evolutionary dynamics (cheaters can destroy the mutualism)

• Mutualisms can mediate competition within and between species

• Higher order interactions (> 2 species)

• Diversity of plants dependent upon mutualists?

What do most food web studies not take into account?

parasites/ pathogens

Questions about diversity- stability

• Are more diverse ecosystems more stable?

• Does removing species risk ecosystem meltdown?→ anthropomorphic pressure

• How many species can we lose?

• Are there any particular species we need to keep?

Which is stabler: simple communities or richer ones?

• simple communities less stable

• more subjected to destructive oscillations in populations

• more vulnerable to invasions

What is stability?

• Species-level eg chaos, limit cycles

• Community eg stable community states

1. Multiple stable states

2. Fluctuations –intrinsic e.g oscillations predator-prey

3. Fluctuations –extrinsic→ environmental conditions→ climate change

4. Press perturbations→ e.g climate change/ global warming→ things slowly getting worse

5. (Removal) Extinction of species

6. Invasion of new species

Example of a system with multiple stable states

Coral reefs, Parrot fish, Diadema

Mumby et al. (2007)

Theoretical Results on Stability Using Lotka Volterra Models

• Using Lotka-Volterra models and random interactions (ie attack rates etc assigned random numbers)

• Mixed messages

• more diverse communities less stable

a(SC)^1/2 <1 for stability (Bob May 1976)

• a: average interaction strength→ strong interactions → harder for system to be stable

• S: species number→ many species = less stable

• C: connectance (interactions between species)→ more= less stability

Want S to be big → average interaction strength down and vice versa→ trade off

Why might stability with L-V be different to expected results?

Recent theory:

• Lots of weak interactions, omnivory -> stability at high diversity

• omnivores→ multiple trophic levels → important

Interactions assigned randomly → not replicate of natural world

Experiment on L-V Theory (Kokkoris et al.1999)

• Build communities using L-V competition models

• Assign interaction (competition) strengths from random pool of numbers.

• Build communities by adding in successive species→ looked at sequential arrival of species

• Keep track of average interaction strength

Result:

• Average interaction strength decreased as communities got bigger→ fits with Bob Mays results

• Few new species could invade = resilience?

Food Web Stability: Empirical Webs (Yodzis, 1981)

• 40 published food webs

• Assigned interaction strengths to L-V consumer-resource model

• Strong intraspecific interference stabilised dynamics

• Strong interspecific interference de-stabilize i.e weak interspecific interactions stabilise

St Martin’s Food web

Goldwasser and Roughgarden Ecology, Vol. 74, (Jun., 1993)

• Food web on Caribbean islands

  • Lizards; kestrels; spiders; hummingbirds; butterflies; snails; mites

  • Strength estimated by gut contents; hunting preference etc

Diversity- Stability Consequences

1. Weak interactions dominate

   • Lots of weak interactions stabilise

2. Generalist predators stabilise

   • Specialists lead to instability

3. Invader can destabilise community if interaction variability increased

   • Could be weak/strong interactions

4. Species removal increases mean interaction strength

   • Fewer links

   • Community becomes LESS stable

McCann (2000) Science

Empirical Evidence: Tropical Rain Forest

Asymmetric density dependence shapes species abundances in a tropical tree community

Comita et al. 2010 Science

• Barro Colorado Island, Panama

• 50Ha plot

• Every individual tree recorded every 5 years since 1981 300,000 individuals

• 300sp tree (<100 in North America)

• Seedling experiment Effect of conspecific & heterospecific seedling & adult neighbours on seedling survival over 5 years

• 31,000 seedlings; 180sp

Results

1. Conspecific effects much stronger than heterospecific effects

2. Heterospecific effects much less variable

Are species rich communities more stable?

Tilman, D. et al. 2006

Dave tilman → permanent experiment - over 20 years

• 9x9m plots

• Plots sown with 1, 2, 4, 8, or 16 species

• Spp drawn at random

• 30 replicates for each level

• Grasses (C3 and C4); legumes; woody species; forbs

• 10 years data

• Environmental variation→ outside experiment not in a greenhouse

• cf species v community stability

Results

1. Increase in community stability with increasing species richness

2. Decreasing species stability with increasing species richness

How were the results quantified? Tilman, D. et al. 2006

Community measure used = biomass production

Mechanism for community stability

portfolio effect

Portfolio Effect

In a diverse community- will always be a species that takes advantage of the environmental conditions so overall biomass stable → species producing biomass changes

Top Down Trophic Cascades

• A top-down cascade is a trophic cascade where the top consumer/predator controls the primary consumer population

• In turn, the primary producer population thrives

Bottom Up Trophic Cascades

change in nutrient supply leads to similar changes in equilibrium abundances at all trophic levels, at least until abundances are constrained by other factors

Example of Top Down Control: San Francisco Bay

• Over-fishing

  • Reduces ‘normal’ prey for killer whales

• Switch to sea otters

• Sea urchins released from predation

• Decimation of kelp forests

• Relatively ‘simple’ food web

• Fragile stability

• Mesopredator release→ middle predator not top

• Mangroves + kelp forests→ herbivore pressures → sea otters eat sea urchins

J. A. Estes et al. Science (1998)

Example of top predator re- introduction

• Wolves reintroduced to Yellowstone in 1995

• elk decline

• browsing decreasing

• aspen height increase

• Cottonwood trees increase

• Willow tree ring area increase

• Number of beaver colonies increase

• Bison increase

Examples of Bottom Up Control Baez et al. 2006

• Exclosure experiment

• Grass & shrub communities

  (i) with rodents

  (ii) without

• No difference between treatments = no top down control

• Rodent numbers dependent upon (current and previous year) precipitation