bios 3010: ecology lecture 21: food webs - homepages at wmu

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Dr. S. Malcolm BIOS 3010: Ecology Lecture 21: slide 1

BIOS 3010: Ecology Lecture 21: Food Webs:

•  Lecture summary: –  Indirect effects:

•  Processes. •  Keystone species.

–  Top-down & bottom- up effects.

–  Community stability: •  Classical view. •  Modern view.

–  Properties of food webs.

Fig 6-27, Miller (2002) Essentials of Ecology


2. Indirect effects in food webs:

•  In addition to direct effects such as competition, predation, herbivory and parasitism, indirect effects are also important in structuring communities.

–  Such as exploitation (scramble) competition mediated via the resource, and consumer-mediated coexistence where predation alters the competitive status of species in the next trophic level down.

•  For example, experimental removal of a species from a food web can reveal its role in structuring a community.

•  But unexpected, indirect effects appear to occur in a third of 100 experimental studies of predation.


3. Indirect effects in food webs:

•  Keystone species: –  A species whose removal would produce a significant

effect (extinction or a large change in density) in at least one other species and significant changes that spread throughout a food web.

–  The keystone locks other components together and its removal leads to collapse of the structure producing a community with a different species composition and different physical appearance: •  “keystone” vs “foundationstone”? at any trophic level.

–  Exclusion of bird predators in Fig. 20.3 showed their complex direct and indirect effects on community structure of a rocky shore.

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4. Top-down or bottom-up control of food webs?:

•  Top-down control: –  Invokes the control of prey numbers by predators and consequent

benefits to plants. •  Hairston, Smith & Slobodkin, 1960 - “the world is green” so

predators must control herbivores.

•  Bottom-up control: –  Invokes energy and nutrient flux from lower to higher trophic levels.

•  Resource availability is the key process and so competition is more important than predation.

•  Murdoch's view that “the world is prickly and tastes bad.” •  In simpler communities bottom-up control may be most likely, but

both bottom-up and top-down control appear as trophic levels increase (Figs. 22.2 & 20.2).


5. Community stability and food- web structure:

•  Resilience: –  Speed with which a community returns to its former state after

perturbation and displacement from that state. •  Resistance:

–  Ability to avoid displacement in the first place (Fig. 20.7). •  Stability:

•  Local stability - tendency to return to original state after a small perturbation.

•  Global stability - tendency to return to original state after a large perturbation.

•  Fragility: •  Dynamically fragile - stable over a narrow range of environmental

conditions. •  Dynamically robust - stable over a wide range of environmental

conditions.


6. Classical versus modern ideas of community stability:

– Classical idea: •  Believed up to 1970 that increased complexity within a

community leads to increased stability: –  Elton, 1958 see Table 22.1; MacArthur, 1955.

– Modern idea: •  Since 1970 theoretical work of May and Pimm indicated

that increase in number of species, increase in connectance (fraction of all possible pairs of species that interacted directly), and increase in interaction strength all tend to decrease stability.

•  All measures of increased complexity that lead to instability (reverse of classical idea).

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7. Classical versus modern ideas of community stability:

•  The modern idea is corroborated in Fig. 22.6 (stability decreases with complexity) for all species under top-down control (top- predators removed).

•  But the classical idea is upheld (stability increases with complexity) under bottom- up control (basal species removed).

– Much as envisaged originally by MacArthur!


8. May’s criterion for community stability:

•  May (1972) established stability criterion of: β(SC)1/2 < 1

•  where, β = average strength of interaction between species; S = the number of species; C = connectance of the web (fraction of all possible pairs of species that interacted directly)

– Thus increasing S leads to decreased stability unless there is a decrease in C or β •  Thus S x C should be approximately constant to retain

stability as in real communities Fig. 20.9a. •  But see exceptions in 20.9b-d.


9. May’s criterion for community stability:

•  Overall it does now appear that complex, fragile communities in relatively constant environments (e.g. tropical forests: more K-selected + high resistance, but low resilience) are more susceptible to outside disturbance than the simpler, more robust communities of areas with more frequent disturbance (e.g. temperate regions: more r-selected + low resistance, but high resilience).

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10. Empirical properties of food webs:

•  Why are food chains generally short at between 2 & 5 trophic levels? (Fig. 20.14)

– Energy flow hypothesis: •  Energy transfer inefficiency limits trophic levels (1-30%)

– Dynamic fragility: •  Extinction of top predators more likely in long food

chains and return times longer (Fig. 22.11). – Constraints on predator design & behavior:

•  Limit to predator size (is this why Tyrannosaurus rex or Carcharodontosaurus went extinct?).


11. Empirical properties of food webs:

•  Omnivory was at first thought to be destabilizing.

•  But its prevalence is high and it may stabilize

– (Fig. 20.17). •  Predator:prey ratios also appear to

be constant in food webs – (Fig. 22.13).


Figure 20.3:

Direct and indirect influences of predatory seabirds on intertidal community structure

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Figure 22.2 (3rd ed.): Bottom-up (B) or top-down (T) influences in communities with 1, 2, 3, or 4 trophic levels


Figure 20.2:

Effect of salinity on Great Salt Lake ecosystem structure


Figure 20.7:

Aspects of community stability

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Table 22.1 (3rd ed.):


Figure 22.6 (3rd ed.) : Species-deletion stability against connectance for 6-species models.

plants

herbivores

predators


Figure 20.9: Connectance (C) against species richness (S) for (a) 40 mixed webs, (b) 95 insect-dominated webs, (c) seasonal pond webs, (d) food webs in swamps and streams in Costa Rica & Venezuela

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Figure 20.14: Food chain lengths in a community matrix for an exposed intertidal rocky shore in Washington


Figure 22.11 (3rd ed.): Frequency (F) of return times (RT) for 2-, 3-, & 4-trophic level models.


Figure 20.17: Prevalence of omnivory in glacial lakes greater than in Briand webs.

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Figure 22.13 (3rd ed.): Numbers of prey species against numbers of predator species in (a) freshwater invertebrate communities, (b) model community with species subject to Lotka-Volterra dynamics.

bios 3010: ecology lecture 21: food webs - homepages at wmu

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