Community Ecology

BDC331

Mark J Gibbons, Room 4.102, BCB Department, UWC

Tel: 021 959 2475. Email: mgibbons@uwc.ac.za

Image acknowledgements – http://www.google.com

Types of Predators

True predators – eat whole organisms; prey individuals

suffer immediately from the actions of an individual

predator; eat more than one prey item during their life

Herbivores – eat bits of organisms; prey individuals do

not usually suffer immediately from the actions of an

individual predator; eat more than one prey item during

their life

Parasites – eat bits of organisms; prey individuals do not

usually suffer immediately from the actions of an individual

predator; intimately associated with a single prey item

during their life

Parasitoids – “eat” whole organisms; prey individuals

suffer “immediately” from the actions of an individual

predator; intimately associated with a single prey item

during their life

The effects of predation on prey populations

Since the effects of predation on an individual prey item

are eventually deleterious, it might be supposed that

predators are bad news for prey populations

BUT……….

Predation may occur at a demographically unimportant

stage of the prey life

For example – if plant recruitment is not influenced by the

number of seeds produced then a seed-predator is unlikely

to have any effect on recruitment

Predators remove individuals from population that make, or

are likely to make, no contribution towards reproduction –

the old, the sick or the very young

Predation serves to reduce prey population numbers, thereby

reducing the effects of intra-specific competition

The impacts of predation may be limited by compensatory

reactions amongst survivors because of less competition

For example…………….

Massive shoot of wood pigeons in autumn has no impact

on numbers breeding the following year because numbers

determined by available food resources over winter

Link between predation and prey populations not easy

Murton et al (1966) J Applied Ecology 11: 61-81

Numerical responses of predator limited by generation time

If predator populations are unable to respond,

numerically, to the prey when they are abundant, they will

have a limited effect on the prey populations. Mismatch

Time (days)

Bio

mas

s

10

Generation Time = 30 days

Diatom Bloom in Southern Benguela

The effects of consumption on consumers

At the other extreme, growth and reproduction cannot

continue ad infinitum for an individual – it becomes satiated,

and consumption rate reaches a plateau whereafter an

increase in prey density is not reflected by any change to

consumption. This also means that the effect of the

predators on prey populations (per capita) is less at high

abundances than at low abundances.

An increase in the amount of food consumed leads to

increased growth and reproduction – subject to

intraspecific competition

All individuals require a certain amount of food to maintain

themselves – a threshold amount. Only if the threshold is

exceeded can the excess be diverted to growth and

reproduction, i.e. low consumption rates rather than leading

to small benefits to the consumer simply alter the rate at

which starvation occurs. This also means prey populations

have a refuge at very low abundances.

BUT….

TYPE I Functional Response

TYPE II Functional response – most common

Feeding Rate determined by Search and Handling Time

Constant volume of space per unit time

Plateau reached because food caught not eaten

Rigler (1961

) Cana

dian J Zoolog

y 39: 857-868

Thom

pson (1975) J A

nimal E

cology 44: 907-916

Berg

man

et al (2000) F

unctional Ecolog

y 14: 61-69

TYPE III Functional Response – S-shaped

Switching…….

Holling (195

9) Can

adian Entom

ologist 91: 385-398

Food quality also important

Eaten - Available Living - Dead

Animals eat most nutritious food available and those that

die during the dry season do so not only because of the

shortage of food but also the shortage of quality food – they

are in a poorer condition than those that survive

Sinclair (1975) J Animal Ecology 1974: 497-520

Diet widths and composition

Types

Monophagous – eat one prey type. E.g.?

Polyphagous – eat many prey types. E.g.?

Oligophagous – eat several prey types. E.g.?

Phylogenetic constraints to diet width….

Evolution of Diet Width…

GeneralistPredator

Prey Sp 3Prey Sp 2

Prey Sp 1 Camouflage

Speed

Fossorial habitMore Specialist

Predator

Prey Sp 2 Prey Sp 1

Over time, generalists increasingly become specialists due

to co-evolution

Problems with intra-specific

competition if too specialised:

also stochastic changes in

environment

Evolution of Diet Width…

Red Queen Hypothesis, ongoing adaptation may be the

evolutionary equivalent of this scene, requiring each

species to evolve continuously just to keep in the same

place among other evolving species (parasites,

competitors, predators, etc.).

Caroll (1872

) Throu

gh The

Looking G

lass and W

hat A

lice Fo

und Th

ere, MacM

illan

Alice finds herself hand-in-hand with the Red Queen,

running faster and faster but without getting anywhere.

 The Red Queen explains, "Now, here, you see, it takes all

the running you can do, to keep in the same place.  If you

want to get somewhere else, you must run at least twice as

fast as that."

Food Preferences

Oligo- and polyphagous predators are not indiscriminate –

they show preferences

Preferences based on maximizing energy returns

Two Types –

Ranked preferences – based on maximizing returns

Balanced preferences – based on ensuring a balanced intake

Davies (1977) J Animal Ecology 46: 37-57

Switching

Preferences maybe fixed

Preferences may vary with prey availability

Murdoch & Stewart-Oaten (1975) Advances in Ecological Research 9: 1-131

Murdoch et al (1975) Ecology 56: 1094-1105

Switching occurs in the following situations:

Different types of prey found in different microhabitats

Consumers develop a search image towards a common prey

Increased probability in pursuing a common prey

Increased probability of catching a common prey

Increased efficiency in handling a common prey

Often – observed population “preferences” are due to an

increase in the number of specialist individuals – not due to

an overall change in diet amongst all individuals

When tubificids and fruit-flies offered in equal numbers

Optimal Foraging Theory – Behavioural Ecology

Attempts to look at rules that

govern the behaviour of

individuals when foraging:

animals maximize energetic

returns

Ei ≥ E

(s + h)hi

E = Average energy content of current diet

s = Average search time of current diet

hi = handling time of the next most profitable prey item

Ei = Energy content of the next most profitable prey item

h = Average handling time of current diet

e.g.

Should a predator expand its

diet to include additional prey

items?

Predicts that:

Individuals with short handling times will be generalists

Individuals with long handling times will be specialists

Other things being equal, an individual in an

unproductive or very variable environment will have a

broader diet than one in a predictable and productive

environment

Predators should ignore insufficiently profitable prey,

irrespective of their abundance

Foraging in a patchy environment

Food is patchily distributed – and so as a consequence are

predators. Predator density in high density food patches is

greater than that in low density food patches

Two types of behaviour underlie aggregation by predators

1 - Location of patches

Change in rate of patch abandonment (profit margins)

2 - Responses of consumers in patches

Change in search pattern after encountering prey

Consumers aggregate in patches when the expected rate

of food intake exceeds background levels i.e. profitable

Immediately aggregated predators start feeding – the

profitability of the patch declines.

The rate at which the profitability declines will depend on

the number of consumers BUT…………

As predator density increases, the predators will also

spend an increasing amount of time interacting with each

other – and less time foraging – impacts on patch

profitability. Known as pseudo-interference

THE END

Image acknowledgements – http://www.google.com