Lecture 16

Slide1: 

Grazing and Top Down vs. Bottom Up Regulation

Slide2: 

Grazing: a functional definition

Slide3: 

Most grazers are herbivores Leucania unipunctata (Army worm) Bison bison

Slide4: 

Although there are exceptions! Chrysops species Deer fly

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Grazers Generally herbivores Remove tissue from a large number of ‘prey’ individuals Are rarely lethal What limits grazer population density?

Slide6: 

Top down vs. bottom up regulation Top down Bottom up

Slide7: 

We have already seen that predators can control prey densities Prey Predator Time Population density

Slide8: 

But can plant abundance also control grazer densities? How can we answer this question?

Slide9: 

We could apply the Lotka-Volterra model… Prey (Plants) Predator (Grazer)  is the per capita impact of the predator on the prey  is the per capita impact of the prey on the predator q is the predator death rate But this implies that grazers kill ‘prey’ individuals outright

Slide10: 

But by definition, grazers do kill ‘prey’ individuals Plant parts differ in nutritional quality, so only some parts are eaten Plant parts differ in levels of chemical defense, so only some parts are eaten

Slide11: 

As a result, graze biomass changes, but population density does not Before grazing After grazing

Slide12: 

The re-growth of graze biomass should not be logistic Biomass Time Logistic Regrowth re-growth should be more rapid

Slide13: 

A reasonable model of plant-grazer interactions A Lotka-Volterra model with the following changes: 1. Prey (plant) biomass changes in response to grazing, but prey (plant) population density does not. 2. Prey (plant) biomass increases in a ‘re-growth’ rather than logistic fashion. 3. A Type II functional response

Slide14: 

What does the model tell us? Interactions between grazers and plants limit plant biomass Interactions between grazers and plants limit grazer population densities Interactions between grazers and plants lead to stable equilibria, not permanent cycles Graze biomass Plant biomass Grazer population density No Grazers Grazers

Slide15: 

A comparison of interactions Predation Grazing Predators can control prey population density Prey density can control predator density Frequently causes permanent cycles in population density Grazers can control plant biomass Plant biomass can control grazer population density Generally does not lead to cycles in population density

Slide16: 

Top down vs. bottom up regulation Top down Bottom up Mathematically, both can work… But what about real data?

Slide17: 

Another look at snowshoe hare cycles Year The strong cyclical nature of this data would seem to be more compatible with top down regulation. However the simple re-growth model considers only graze quantity and ignores graze quality

Slide18: 

An alternative hypothesis Hare population density is regulated from the bottom up This bottom up regulation is due to both graze biomass and graze quality Lynx density simply tracks hare density

Slide19: 

Interactions between the hare and its food plants

Slide20: 

Evidence for importance of vegetation (Quantity) Pease et. al. 1979 Studied a population of hares in Alberta from the peak of the cycle to its trough (1970-1975) Measured food availability to hares during these years Results showed that in the peak years of 1970 and 1971 food plant biomass was too low to support observed hare population densities

Slide21: 

Evidence for importance of vegetation (Quality) Bryant et. al. 1979 Studied the chemical composition of plants used by hares as food Found that secondary shoots (produced after intense hare grazing) had significantly greater concentrations of toxic chemicals that deter feeding by snowshoe hares These results suggested that hare population cycles might be driven by fluctuations in the level of plant defenses

Slide22: 

This led to a new hypothesis 1. Hare population density increases causing increased removal of plant tissues 2. As a result, plant biomass decreases, plant quality decreases, and plants become increasingly well defended with toxic chemicals 3. Consequently, hares population begins to decline due to a shortage of food 4. As hare population density decreases, plant biomass increases and the concentration of toxic chemicals is reduced 5. Lynx do nothing but track the density of the hare population The ‘bottom up’ or ‘food shortage’ hypothesis

Slide23: 

Comparison of the two hypotheses Which is correct?

Slide24: 

Kluane studies (Krebs et. al.) Studied an entire lynx-hare cycle from 1986-1994 in the Canadian Yukon Experimentally manipulated both predation and food supply Followed lynx and hare densities within 1km square enclosures Attempted to ‘stop the cycle’

Slide25: 

Design of the Kluane study Control Control Control Food added (by plane!) Food added Predators excluded Food added and Predators excluded Fertilizer added (by plane!) Fertilizer added 1km

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Results of the Kluane study Food added Hare density was tripled during peak years Predators excluded Hare density was doubled during peak years Predators excluded & Food added Hare density was increased eleven fold during peak years Both food supply and predators play a role in regulating hare population density

Slide27: 

But the cycles didn’t stop Avian predation was not excluded Enclosures allowed hares to move into and out of treatments - Hares tended to move into food addition enclosures - Hares tended to move into predator exclusion enclosures So we still don’t know what is causing the cycles! Possible explanations

Slide28: 

A mathematical approach to the experiment f (Plant biomass, Hare density) f (Plant biomass, Hare density, Lynx density) f (Hare density, Lynx density) Using this model, one can ‘experimentally’ remove any one species and determine the outcome (King and Schaffer)

Slide29: 

Suggests that predators and not prey are responsible for the cycles Vegetation, hare, lynx Hare, lynx Vegetation and hare Model results Model results Cycles are qualitatively indistinguishable No cycles

Slide30: 

Does this contradict the results of the Kluane experiment? Real data Model simulations Controls (Solid line) Food addition (Dotted line) Predator exclusion (Dash-dot line) Predator exclusion & Food addition (Dashed line) NO, both model and data predict that hare density is regulated by both predation and graze The model shows, however, that the cycles are likely due primarily to interactions with predators

Slide31: 

What about other systems? (An example from the diverse mammal community of the Serengeti) Golden Jackal Serval Leopard Cheetah Hyenah Lion Oribi Impala Wildabeest Zebra Black Rhino Hippo Elephant

Slide32: 

Predator species differ in the size of prey they consume (Sinclair et. al. 2003. Nature 425:288-290)

Slide33: 

Therefore, prey species differ in their # of predators (Sinclair et. al. 2003. Nature 425:288-290)

Slide34: 

As a result, some prey species experience more predation (Sinclair et. al. 2003. Nature 425:288-290) Oribi Elephant Predation limited Food limited

Slide35: 

Summary: Grazing and Top Down vs. Bottom Up Regulation Interactions between grazers and plants can control both the density of grazers and plants Plant-grazer interactions are less likely to cycle than are predator-prey interactions Mathematical models show that both bottom up and top down population regulation are possible and not mutually exclusive Empirical studies show that prey density is regulated by both predators and food supply, and that which is more important may be species and system specific