Community Ecology
BDC331
Mark J Gibbons, Room 4.102, BCB Department, UWC
Tel: 021 959 2475. Email: [email protected]
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
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