Bio209- Chp 13- Predation and Herbivores

Thompson’s Definitions of Consumers

1. A true predator kills many prey individuals, and consumes most or all of it

2. A grazer feeds on many individuals but does not kill them

3. A parasite consumes part of a host without killing it, and feeds on a single host

4. A parasitoid feed on a single host, does not cause immediate death death, but eventually kills it

• Although Thompsons definitions are useful in terms of ecological consequences of interactions, common uses of these terms may sometimes differ; for example zooplankton that feed on phytoplankton are usually called grazers even though the consequences to single-celled algae is like Thompson's definition of predation

Measuring Effects of Predators on Prey Populations

1.Time-series analysis of predator and prey populations, e.g. lynx-
hare cycles
2. Before-after comparisons of predator invasions, e.g. the brown
tree snake on Guam
3. Comparisons of predator presence-absence among localities,
mayflies in fish vs fishless lakes
4. Predator removal experiments, e.g. lizards and spiders
5. Observations of predator feeding rates and population responses
to prey density, e.g. wolf predation on moose
6. Models of predator-prey dynamics (not covered in this class)

Caudill's (2005)

example where he measured mayfly abundance and adult emergence in alpine lakes where brook trout were absent or introduced; there was a strong inverse relationship between trout abundance and mayfly adult emergence from lakes

Spiller and Schoener (1988)

conducted experiments where Anolis lizards were maintained in fenced enclosures, or fenced enclosures where lizards were removed, and unfenced controls; the abundance of web-building spiders was 2-3 times higher in enclosures where lizards were removed than when lizards were present or in unfenced controls

Functional, numerical and total responses of predators to prey populations

The functional response of an individual predator is the number of prey eaten per predator per unit time as it varies with changes in prey density (sometimes called the kill rate); there are three types of functional responses that mainly differ in kill rates at low prey densities:

Types of functional responses

• Type I - kill rate increases linearly with prey density then flattens when the gut capacity is reach - applied to zooplankton grazers where intake rate = passage rate at higher prey densities

• Type II - this is the most commonly used functional response, and is a saturation curve - at low prey densities the predator spends most of its time searching and encountering prey so kill rate increases rapidly with prey density; at higher prey densities the kill rate saturates to a maximum because the predator reaches satiation and/or spends more time handling prey than search for prey

• Type III - less commonly used but applies most to generalist predators that ignore prey items at low densities but increase predation rate as prey density increases then saturates as with type II