Topic 14+15 (BIO-2220)

Optimal Forging Behaviour

Idea or theory → attempts to model how organisms feed as an optimizing process

• Natural selection is likely to favour individuals that are effective at aquring energy

• If energy supplies are limited, organisms cannot simultaneously maximize life’s functions

• attempts to predict what consumers will eat, and when, are where they feed

Functional Responses

The most common way to describe predators’ responses refers to how a predator varies it’s consumption of food. key in energy flow through ecosystems

Type I fuctional response

functional response of predators

• The rate of prey capture increases linearly with food density

• Assumption of this says time is required for a consumer to process food is minimal, or that eating does not disrupt the search for food

• rare response

Type II fuctional response

• Predators searching time decreases with increasing prey density 

• Predators handling time of prey remains constant 

• The proportion of prey eaten declines linearly, limited by handling time

Type III fuctional response

• The most frequent

• # of prey taken rises at a decreasing rate to a maximum level.

• may come from prey switching, more efficient for a predator to catch an adundant prey, rather than rare ones

• at high densities, prey consumption is limited by handling time and search efficiency.

Characteristics of type III fuctional response

Sigmodal Shape

• A decline phase in the consumption rate

• Predator handling time is determined by the physiology and ecology of the organisms and remains constant

• Both handling time and search efficiency determine the plateau

handling time in spiders

Spiders spend time capturing, subduing and consuming prey. Including building a web, venom, injection, and digesting prey externally before ingestion.

Satiation

even though spiders evenually become satiated, their feeding rate slows down as they approach it. Which is a type II Characteristic

Switching Behaviour in Predators

Associated with type III, it involves predators turning to a more abundant prey

• learning + behaviour → resulting in optimal foraging

3 types of behaviours are behind

1. Alternate prey offers a better choice

2. Predators ignore prey that are too rare

3. Concentrate the search on more rewarding areas

Type III at high prey densities

similar to type II (handling time, search efficiency) 

Type III at low prey densities

There is an accelerating phase where an increase in density leads to a more than linear increase in consumption rate

Role of competition in Anolis Lizards

over 150 anolis lizards inhabit the carribbean islands, making them a great model for studying the role of biotic interactions in shaping communities

Ecomorphs

local populations within a species that have similar body forms and behaviours determined by the ecological environment

Character Displacement

occurs when morphological differences among similar species are magnified in regions where they co-occur, but diminished or absent when their distributions do not overlap. Driven by interspecific competition

Connel said in 1980…

“the ghost of Compitition past

Resource Partitioning

• use of limiting resources in different ways

• partition along a certain gradiant (temporal or spatial)

• competing species can evolve to reduce niche overlap

Ways to reduce niche overlap

1. based on specific characteristics of a resource

2. based on the spatial distribution of the resource, or microhabitat resource partitioning

3. based on when the resource is used

4. based on abiotic conditions

Disturbance

defined as the toal or partial removal of vegetation

Predation

seen as a mode of disturbance since it removes individuals from the system

Herbivory

type of predation

Joseph Connel 1978

studied that communities tend to have low species diversity in the absence of disturbances

• A disturbance can increase species richness

• most communities tend to be dominated by the most competitive species

Came up with intermediate disturbance hypothesis

Intermediate Disturbance Hypothesis

Species richness is the highest at intermediate levels.

• Frequent or large disturbances prevent many species from establishing themselves and succeeding.

• Infrequent or small disturbances allow competitively superior species suppress the others

Intertidal Boulders

W. Sousa (1979) classified intertidal boulders of small, intermediate, or large depending on the force of a wave that would be required to move them. After monitoring for 2yrs he found that intermediate-sized ones had a more diverse algae community.

• Smaller Boulders are moved more frequently than large ones

Zeevalking and Fresco (1977) - test

examined the relationship between the intensity of grazing by rabbits and the species diversity of plants in coastal sand-dune vegetation in Europe.

• saw that diversity was highest in moderate grazing

Predator Mediated Coexistence

Specialist predators prefer the more competitive species. Therefore the predator mediates the abundance of the better species, allowing two species with the same niche to coexist

Keystone species

species that have disproportionately large effects on community structures. The removal of one of them leads to a change in community composition

• Ex. Otters