Trophic networks

Bacterial producers

Small herbivores

Small carnivores

Larger carnivores

Large carnivores

Medium omnivores

Large omnivores

Large herbivores

Parasitoids

Hyperpara-sitoids

Parasites

Smaller decomposer

Larger decomposer

Plant producers

Bacterio-phages

Typical terrestrial food with six trophic levels and 15 functional groups. Each functional group (guild) may contain many species.

Plant producers

Herbivores

Small predators

Large predators

Parasites

Food chainFood web

Web size The total number of elements S (species) in the web

Connectance The proportion of realized links L in the web

Linkage density The proportion of realized connections per species

Food chain length The average length of single food chains

𝐶=𝐿

𝑆(𝑆−1)/2≅𝐿𝑆2

𝐷=𝐿𝑆 ≅ 𝐶𝑆

Proportions of top, intermediate, and basal species. The proportion of omnivores. Omnivores are species that feed on more than one basic source of food (more than one trophic level)

Plant

Herbivore

Predator

Herbivore

Predator

Web size: 5Links: 4Connectance: 4/(5*4/2) = 0.4Linkage density: 4/5 = 0.8Food chain length: 2Omnivores: 0Basal species: 1Intermdeiate species: 2Top species: 2

Loop, intraspecific feeding

Loops do not count

1

2 3

4 5 6 7 8 9 10 11

12 13 14 15 16

17 18 19

S = 19Lmax = 19x18/2=171L = 35C = 35 / 171 = 0.2Ch = 100Li = 40ChL = 100 / 40 = 2.5L / S = 35 / 19 = 1.8

Photo Nepenthes

Pitcher plant food (Nepenthes albomarginata) web

Loop,Cannibalism

100

80

2

Herbivores

Small predators

Large predators

100

15

2

20Plants 120

10

1

100

0.1

Abundance Biomass Energy

Trophical cascades

Trophical cascades vary from habitat to habitat.They are habitat specific

In terrestrial habitats about 10% of energy is passed from each level to the next higher level (rule of Lawton).

In marine food webs the biomass and abundance pyramids are sometime inverted.

The paradox of the plankton

Why is the world green?

With predators of herbivores

Without predators of herbivores

Terborgh et al. 2006, J. Ecol.

Venezuela

Comparing the defoliation by herbivores on small (without predators), intermediate (some arthopod predators ) and large islands/mainlands (all types of predators) Terborgh et al. (2006) corroborated the hypothesis of Hairston, Smith, and Slobodkin (HSS) that herbivore predators control defoliation

and keep the world green.

Mortality Recruitment

Plant defense did not play a major role

Predaceous birds and snakes Parasitoid spider wasps Songbirds

Lizards

Predaceous insects

Scorpions

Spiders

Herbivorous insects

Land plands, seed detritus

Rodensts

Scavenging insects

Detritovorous insects

Seabird ectoparasites

Algal detritusSeabird guanoFish and bird carcasses

Seabirds

Marince planctonic food web Marine macroalgae

An example how complex food webs might be. Each trophic level may contain several up to several hundreds of species. Islands in the Gulf of California.

Polis 1998, Nature395:

744-745

Terrestrial arthropod dominated food chains are often shorter than marine food chains

Schoenly et al. 1991, Am. Nat 137: 597-638

Terrestrial food chains have rarely more than five levels.

Mikiola fagiTorymus auratus Platygaster spec. galls

Food chain involing insect parasitoids have often more

than five levels.

Do terrestrial and marine food webs differ in structure ?

Haven, 1997, Oikos 78: 75-80

Schoenly et al. 1991, Am. Nat 137: 597-638 Terrestrial webs

Marine webs

Numbers of food chain in a web increase to the power of species richness.

The upper boundary marks the limit of stability.

Predator numbers increase linearly with the number of asvailable prey

species

The total richness of predators is often higher than the number of prey

species

Schoenly et al. 1991, Am. Nat 137: 597-638

Betula pendula

Quercus robur

Thelaxes dryophila

Tuberculoides annulatus

Myzocallis castanicola

Stomaphis quercus

Euceraphis betulae

Betulaphis quadrituberculata

Calaphis betulicola

Betulaphis brevipilosa

Mamamelistes betulinus

Symydobius oblongus

Lysiphlebus thelaxis

Aphelinus chaonia

Trioxys betulae

Protaphidius wissmannii

Trioxys pallidus

Trioxys curvicaudus

Trioxys tenuicaudus

Aphidencyrtus aphidivorus

Praon flavinode

Aphidencyrtus quercicola

Aphidius aquilus

Callaphidius elegans

Trioxys compressicornis

Rajmanek and Stary 1979, Nature 280:

311-313

Parasitoid – aphid relationship on oaks

Rajmanek and Stary 1979, Nature 280: 311-313

𝐷√𝑆𝐶<1

Food web connection and stability

The May equation predicts low linkage density at higher connection rate

D: Linkage densityS: species numberC: connectivity

The May equation predicts an upper limit of connectance for a stable food web.

Schoenly et al. 1991, Am. Nat 137: 597-638

𝐷√𝑆𝐶<1

Food web complexity is limited by species richness

Aquatic food webs

Schoenly et al. 1991, Am. Nat 137: 597-638

Mechanisms that stabilize food webs:

• Weak and variable links• Low connectance• Dietary switches• Omnivory

SC: measure of food web complexity

The May eqaution is based on simplified random food webs with density dependent regulation.

Omnivory stabilizes food webs

Undisturbed Disturbed

High proportion of specialist species

Intermediate proportion of specialist species

High proportion of omnivorous species

Tem

pora

l var

iabi

lity

amon

g sp

ecie

s

Fagan 1997, Am. Nat. 150: 554-567

Mount Saint Helens blowdown zone

Mount St. Helen’s recovery is a natural experiment on succession and community ecology.

The temporal stability of food webs

Food chain length and habitat properties

Post et al. 2000,

Nature 405: 1047-1049

Fresh water food chain length of North American lakes increase with lake size but

not with productivity

Average food chain length asymptotically reaches a plateau independent of species

richness.

Hall and Raffaelli 1991, J. Anim. Ecol. 60: 823-841.

Compilation of well resolved food chains

Schneider 1997, Oecologia 110: 567-575

Food web complexity and ecosystem

variability in ponds

Linkage density of fresh water insect dominated small pond food webs increased with• Species richness• Habitat duration and decreases with• Pond environmental

variability

Connectance was lowest at average species richness, variability, and pond duration.

Empirical interaction matrices

Pollination networks

Plants

BeesKratochwil et al. 2009, Apidologia 40: 634-650

Plant Asclepias AsclepiasAspidonepsisMiraglossumMiraglossumPachycarpusSisyranthusXysmalobiumXysmalobiumPollinators cucullata woodii diploglossa verticillare pilosum natalensistrichostomus gerrardii involucratumHemipepsis 0 0 0 18 9 20 2 41 1Pompilidae sp. 2 0 0 0 0 0 0 0 1 0Tiphia 0 1 0 0 0 0 0 0 0Arge 0 0 0 0 0 0 1 0 0Apis 0 0 1 0 0 0 1 3 0Halictidae sp. 1 0 0 2 0 0 0 0 0 0Halictidae sp. 2 1 0 0 0 0 0 0 0 0Other wasps 0 1 1 0 0 0 0 1 3Other bees 0 0 0 0 0 0 0 1 1Other solitary bees 0 1 2 0 0 9 0 0 0Atrichelaphinis 0 15 0 1 0 0 35 15 6Cyrtothyrea 0 8 0 1 0 0 42 6 0Lycidae sp. 0 0 0 0 0 0 0 2 0Cantharidae sp. 0 0 0 0 0 0 0 2 0Elateridae sp. 0 0 0 0 0 0 0 0 4Chrysomelidae sp. 1 0 0 0 0 0 1 0 0 1Chrysomelidae sp. 2 0 0 0 0 0 0 1 1 1Scarabaeinae sp. 1 0 0 0 0 0 0 0 3 0Scarabaeinae sp. 2 0 0 0 0 0 0 0 3 1Scarabaeinae sp. 3 0 0 0 0 0 0 0 1 0Curculionidae sp. 1 0 0 0 0 0 0 10 4 1Curculionidae sp. 2 0 2 0 0 0 0 0 0 0Coleoptera sp. 3 0 0 0 0 0 0 0 2 0Coleoptera sp. 8 0 0 0 0 0 0 1 0 0Other Coleoptera 0 0 0 0 0 0 0 4 4Aspilocoryphus 1 0 0 1 0 4 1 139 1Lygaeidae sp. 2 0 0 0 1 0 1 0 8 2Coreidae sp. 0 0 0 0 0 0 0 1 0Spilostethus 0 0 0 0 0 1 0 0 0Homoecerus 0 0 0 0 0 1 0 0 0Pentatomoidea sp. 0 0 0 0 0 0 0 1 0Other Heteroptera 0 0 0 0 0 0 0 1 0Calliphoridae genus 1 0 0 0 0 0 0 0 1 0Calliphoridae genus 2 0 0 0 0 0 0 2 6 0Calliphoridae genus 3 0 0 0 0 0 0 0 1 0Sarcophaga sp. 0 1 0 6 0 11 0 53 1Musca 0 0 2 0 0 0 0 3 0Muscidae genus 2 0 0 1 0 0 0 0 0 0Empididae sp. 1 2 0 0 0 0 1 0 0 0Empididae sp. 2 0 0 0 0 0 0 0 1 0Chloropidae 0 0 1 0 0 0 0 1 0Microphthalma 0 0 0 0 0 1 0 0 0Microphthalma 0 0 0 0 0 0 0 1 0Tachinidae subfamily Goniinae 0 0 0 0 0 0 0 1 0Tachinidae genus 2 0 0 0 0 0 0 0 1 0Actea 0 0 0 0 0 0 0 1 0Sepsidae sp. 1 0 0 0 0 0 0 0 3 1Sepsidae sp. 2 0 0 0 0 0 0 0 0 1Sepsidae sp. 3 0 0 0 1 0 0 0 0 0Dacus 0 0 0 0 0 1 0 0 0Bibionidae 0 0 0 0 0 0 0 1 0Diptera sp. 3 0 0 0 0 0 0 1 0 0Diptera sp. 22 0 0 0 0 0 1 0 0 0Other Diptera 0 1 0 1 0 1 0 15 0Unidentified butterfly 0 0 0 0 0 0 1 0 0Unidentified micromoth 2 0 0 0 0 0 0 0 0

From Ollerton et al. 2003, Ann. Botany

92: 807-834

The matrix approach to mutualistic and food webs

What are mutualistic webs:

• Plant – pollinator webs• Plant seed disperser webs• Plant herbivore webs• Predator prey webs• Host parasite webs• Competition webs

Pollinators

Plan

ts

Nestedness is defined as the ordered loss of links in a mutualistic matrix where rows and coloumns are sorted according to species richness.

Unexpected link

Linkages: number of filled cells in the matrixLinkage density: L/S1

Connectance: Matrix fill, L/(S1S2)

Generalists

Specialists

Generalists Specialists

• Generalist pollinator visit most plant species

• Specialist pollinator visit the most popular plant species

• Mutualistic networks contain forbidden links

Foods webs

Pollination networks

Seed disperser

Bascompte 2003, PNAS 100: 9383-9387

Bastolla et al. 2009, Nature 458: 1018-1021

• Mutualistc networks are often nested.• The nested architecture promotes diversity

and stability

The architecture of mutualistic networks

Jordi Bascompte1967-

Weak Anthropic Principle (Carter 1973): We must be prepared to take account of the fact that our location in the universe is necessarily privileged to the extent of being compatible with our existence as observers.

In ecology this means:Ecological systems do not have a random strucure. They have that non-random structure that enabled them to survive during evolution.

Nestedness as an emergent property of ecological systems

Nestedness tends to stabilize mutualistic networks.

1 2 3 4 5 6 7 8 SA 1 1 1 1 0 0 0 0 4B 1 1 1 1 0 0 0 0 4C 1 1 1 1 0 0 0 0 4D 1 1 1 1 0 0 0 0 4E 0 0 0 0 1 1 1 1 4F 0 0 0 0 1 1 1 1 4G 0 0 0 0 1 1 1 1 4

4 4 4 4 3 3 3 3 28

1 2 3 4 5 6 7 8 SA 1 1 1 1 1 1 1 1 8B 1 0 1 1 1 1 0 0 5C 1 1 1 1 1 0 0 0 5D 1 1 1 1 0 0 0 0 4E 1 1 0 0 0 1 0 0 3F 1 0 0 0 0 0 0 0 1G 1 0 0 0 0 0 0 0 1

S 7 4 4 4 3 3 1 1 27

Food webs are often compartmented

Foods webs have a modular structure.

Modularity tends to stabilize food webs.

Modules itself have a nested structure.

Mutualistic webs (comparirson of two trophic levels) are most often nested.

Stability, resilience and tipping points

Instable equilibrium Local stable

state Global stable state

Ecologial systems (particularly networks) can be in various states:• Instable equilibria are at tipping points

and can move towards different directions.

• Local stable equilibria can easily be forced to achieve other stable states.

• Global equilibria need much energy to leave their state.

• Inequilibria can easily move between different states.

• Resilience refers to the speed of a systen to return to a stable state.

• Resistence is the ability of a system to avoid displacement.

• Robustness is the ability of a system to exist witin a wide range of conditions.

• Stability refers to the amplitude of variability

• Sustainability i the capacity to endure

Low stability

Tipping point

Low resistence

Local stabilityGlobal instanility

Instable equilibrium Local stable

state Global stable state

Multiple states

State

Prob

abili

ty

Tipping points define states where a system irreversably changes the probability distribution of states.

Food webs and tipping points

Indicators of critical tipping points:• Resilience slows down• Dominant eigenvectors of the food web

matrices shorten• Increased variance• Variance / mean relationships increased• Multimodality of states• Increasing connectivity and decreasing

diversity

A state reaches its tipping point

Robustness

Dunne 2002, Ecol. Lett 5: 558

In empirical foods webs robustness increases with connectance.

In random foods webs robustness decreases with connectance.

Therefore, empirical food webs have a special non-random structure that promotes stability.

The importance of wild bees for pollination stability

Meta-analysis of empirical food ebs

Wild insects increase fruit production more effectively than honey bees alone.

Species richness increases ecological functioning

Garibaldi et al. 2013. Science 339: 1608