Predation, parasitism, herbivory

I. The initial motivation for these types of studies arose from concerns about managing populations of game species as well as controlling populations of agricultural pests.

II. Predators catch individuals, consume them and remove them from the prey population.

III. A parasite consumes a living host as well, but although it may increase the host's probability of dying or reduce fecundity, a parasite does not by itself remove an individual from the resource population.

IV. Herbivores eat whole plants or parts of plant. From the standpoint of consumer-resource relations, herbivore may function either as predators, consuming whole plants, or as parasites, consuming living tissue but not killing their victims.

V. We might expect to see a tight correlation between the prey population and the number of predators. Effects of predators and prey populatiions on each other are extremely varied and difficult to predict.

A. Effects of predators on prey

1. Detrimental effects. Two examples from field of biological control, a field of study which seeks to find long-term solutions to problems with agricultural pests by finding natural enemies of these pests and using predators or competitors to help keep the pest number down..

a. A ladybird beetle was released in CA to control cottony-cushion scale--an important pest on citrus--and the beetle virtually wiped out its prey.

b. Klamath weed is a European plant that was introduced in CA in the early 1900's. It thrived and by 1944 it covered about 2 million acres of rangeland. In 1944 a Chrysolina beetle that ate Klamath weed was released in Northern California and within 10 years it had reduced Klamath weed population by 99%. These two examples show how effective predators can be and are examples of an applied field called biological control. Biological control

2. Although predator effectiveness is not surprising, there are many counter examples. A species of weevil was released in New Zealand to control Gorse bushes. This weevil has become of the most abundant insects in NZ and eats about 95% of the seeds produced by the gorse bushes, but there has been no effect on the overall abundance or distribution of the plant.

3. Two examples also illustrate very different effects of predation on game populations.

a. Predators were removed from Cultus Lake in BC in an attempt to increase the production of Sockeye salmon and successfully increased the percent of young that returned as adults.



 

b. In contrast, in NY State, attempts were made to increase the number of Ruffed Grouse, a popular game bird, by removing predators. Removal of predators did increase nesting success but there was no increase in the number of adults available for hunting in the fall.

 



 

4. Very dramatic effects are observed when prey are exposed to new predators with which their species has no history of interacting. A single snake species the brown tree snake (Boiga irregularis), has been accidentally introduced to several Pacific Islands, including Guam where snakes did not naturally occur and has resulted in extinctions in less than a decade. In Guam, prior to its introduction in the 1940's, there were 18 abundant bird species. By 1980, there had been 7 extinctions and 4 are critically endangered.

B. The effect that prey populations will have on the populations of predators is also difficult to predict.

1. Ex. Moth larvae on a flowering ragwort plant, the number of moths is determined largely by the number of flowering plants in the previous year.



 

2. Wood mice and bank voles are prey for Tawny owls in the UK. Owls maintain a constancy of abundance despite fluctuations in the numbers of their small prey.



 

VI. Modelling simple predator-prey interactions where the predator densities affect prey densities and vice versa.

A. Prey population growth rate.

1. If a prey population is left with no predators, it will (at least until it approaches carrying capacity) increase exponentially. This is seen many times when a species is released into a new habitat where predators are absent. So start with the exponential growth rate.

dN/dt = rN

2. Introduce a predator that will slow the growth rate.

dN/dt = rN - (ones that get eaten)

3. Factors that determine how many get eaten?

a. How frequently the predator and prey encouter each other which is a function of the population size of predators (P) and the population size of prey (H).

b. Predator efficiency at capture, p.

4. Equation for growth rate of prey population moderated by predation is dH/dt = rH - pHP

B. Predator population growth rate.

1. When no prey around, we can describe the decline in the predator population as mP, where the population declines exponentially at a mortality rate of m.

2. When prey are present, the population of predators will change like:

dP/dt = number born - mp

3. Factors relating to predation that affect the number of predators born.

a. The population of predators already present, P.

b. The population of prey, H.

c. Predator efficiency at capture, p.

d. How many prey items it takes to make a new predator, or how nutritious the prey are, a.

4. So the predator population is expected to change as: dP/dt = apHP-mP

5. The Lotka Volterra predation model has two equations, one for prey populations and one for predator populations. To understand dynamics, find points when populations are in equilibrium (population growth rates of zero).

a. For prey, dH/dt = 0 when rH=pHP or P=r/p



 

b. For predators, dP/dt=0 when apHP=mp or H=m/ap



 

6. When these lines of equilibrium are combined. We see that the model predicts cycling between prey and predators.



 

VII. Do predator-prey population cycles exist?

A. Laboratory work by Gausse on the protists Paramecium (prey) and Didinium (predator) demonstrated that cycling occurred only when the experiment was restocked with predators at regular intervals.

 





B. Snowshoe hare and lynx are a classic example in which 8-11 year cycles have been observed for more than 80 years. But do predators determine abundances and cycling of hare populations or is it the food supply of the hares? Experiments demonstrate that both food and predators influence abundances. Experiments removing predators or augmenting food did not eliminate cycling.



 



 

VIII. Mutualisms:--a relationship between two species that benefits both.

A. Complementary ways of obtaining energy and nutrients.

B. Defense in exchange for food and shelter.

C. Pollination and seed dispersal types of relationships.

D. These relationships may be facultative, meaning that the partners can do with or without each other, or they may be obligate which means that they are dependent on each other

E. Two examples of mutualistic relationships.Ants and acacia. Yucca and yucca moth.

By the end of this lecture, you should understand

1) the variation in predator-prey interactions; 2) how simple predator-prey interactions are modelled (i.e., if given equations, you should be able to explain its terms and the dynamics it predicts); 3) how laboratory and field experiments support or do not support the models discussed in class; 4) what mutualism is; 5) the general types of mutualism.