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

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Presentation on theme: "Trophic networks Typical terrestrial food with six trophic levels and 15 functional groups. Each functional group (guild) may contain many species. Plant."— Presentation transcript:

1 Trophic networks 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 chain Food web

2 Web sizeThe total number of elements S (species) in the web ConnectanceThe proportion of realized links L in the web Linkage densityThe proportion of realized connections per species Food chain lengthThe average length of single food chains 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: 5 Links: 4 Connectance: 4/(5*4/2) = 0.4 Linkage density: 4/5 = 0.8 Food chain length: 2 Omnivores: 0 Basal species: 1 Intermdeiate species: 2 Top species: 2 Loop, intraspecific feeding Loops do not count

3 Photo Nepenthes Pitcher plant food (Nepenthes albomarginata) web Loop, Cannibalism

4 Herbivores Small predators Large predators Plants AbundanceBiomass 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

5 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. MortalityRecruitment Plant defense did not play a major role

6 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:

7 Terrestrial arthropod dominated food chains are often shorter than marine food chains Schoenly et al. 1991, Am. Nat 137: Terrestrial food chains have rarely more than five levels. Mikiola fagi Torymus auratus Platygaster spec. galls Food chain involing insect parasitoids have often more than five levels.

8 Do terrestrial and marine food webs differ in structure ? Haven, 1997, Oikos 78: Schoenly et al. 1991, Am. Nat 137: Terrestrial webs Marine webs

9 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:

10 Rajmanek and Stary 1979, Nature 280: Parasitoid – aphid relationship on oaks

11 Rajmanek and Stary 1979, Nature 280: Food web connection and stability The May equation predicts low linkage density at higher connection rate D: Linkage density S: species number C: connectivity The May equation predicts an upper limit of connectance for a stable food web. Schoenly et al. 1991, Am. Nat 137:

12 Food web complexity is limited by species richness Aquatic food webs Schoenly et al. 1991, Am. Nat 137: 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.

13 Omnivory stabilizes food webs Undisturbed Disturbed High proportion of specialist species Intermediate proportion of specialist species High proportion of omnivorous species Temporal variability among species Fagan 1997, Am. Nat. 150: 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

14 Food chain length and habitat properties Post et al. 2000, Nature 405: 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: Compilation of well resolved food chains

15 Schneider 1997, Oecologia 110: 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.

16 Empirical interaction matrices Pollination networks Plants Bees Kratochwil et al. 2009, Apidologia 40: From Ollerton et al. 2003, Ann. Botany 92:

17 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 Plants 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 matrix Linkage density: L/S 1 Connectance: Matrix fill, L/(S 1 S 2 ) Generalists Specialists Generalists Specialists Generalist pollinator visit most plant species Specialist pollinator visit the most popular plant species Mutualistic networks contain forbidden links

18 Foods webs Pollination networks Seed disperser Bascompte 2003, PNAS 100: Bastolla et al. 2009, Nature 458: Mutualistc networks are often nested. The nested architecture promotes diversity and stability The architecture of mutualistic networks Jordi Bascompte 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.

19 Nestedness as an emergent property of ecological systems Nestedness tends to stabilize mutualistic networks. 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.

20 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 stability Global instanility

21 Instable equilibrium Local stable state Global stable state Multiple states State Probability 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

22 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 Science 339: 1608


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