lecture 39 - niches and interactions

Question 1

What is a niche?

Answer 1

The sum of the habitat requirements needed for
a species’ survival and reproduction.
The role that a species plays in a community.
A multidimensional habitat that allows a species
to practice its way of life.

Question 2

What determines a niche?

Answer 2

• Abiotic/physical parameters
– Climate
– Soil chemistry
– Environmental factors
• Biotic/biological factors
– Species characteristics such as	dispersal	or	
temperature	tolerance
– Interactions	with	other species;	competition and	
predation

Question 3

What is the fundamental niche?

Answer 3

The set of habitats occupied if there were no
limits (competitors, distance, predation, etc.)
• The full range of climate conditions and food resources that permit the individuals of a
species to survive.

Question 4

What are the Limits to species distributions? Why isn’t everything everywhere that is physically suitable?

Answer 4

• Competition
• Predation
• Dispersal

Question 5

What is the realised niche?

Answer 5

• The set of conditions which are actually
occupied.
• More restricted than the fundamental niche

Question 6

Anolis lizards:

Answer 6

Live in humid forests in Hispaniola
Can’t endure freezing temperatures
Commonly thrive if introduced to areas beyond their dispersal capabilities
Anolis lizards have evolved different adaptations for
feeding as a result of competition for space and
resources.

Question 7

What is resource partitioning?

Answer 7

• Species whose niches overlap may diverge to minimize the overlap. This is called resource
partitioning.

Question 8

Describe interactions in the niche

Answer 8

Populations and their physical surroundings: Adaptations as a result of natural selection

physical surroundings and biological surroundings:
Coexistence because species exploit
different combinations of resources

Question 9

Why are interactions important?

Answer 9

Very few populations live in isolation.
Interspecific interactions play a key role in the
distribution and abundance of populations.

Question 10

Describe different types of interaction

Answer 10

• Antagonism
– where at least one participant loses more than it
gains by the interaction
• Mutualism
– where interactions between species enhance the
reproduction and population growth of both
species

Question 11

What is competition between two individuals?

Answer 11

• An antagonistic interaction
• A lose-lose situation - both individuals expend
energy that they otherwise would have
conserved
• Results in slowed population growth as the
population approaches carrying capacity

Question 12

Describe how Competition promotes

niche divergence

Answer 12

• Antagonistic interactions mean that one species either becomes extinct or changes its
resource use (competitive exclusion).
• Ultimately, competitive exclusion leads to
resource partitioning and evolutionary
divergence, as with the Anolis lizards.
• Leads to the evolution of traits that aid
survival and reproduction by reducing
competition in some way

Question 13

Describe Gause’s principle of competitive

exclusion

Answer 13

When resources are limited, no two species

can occupy the same niche indefinitely.

Question 14

What are the types of competition?

Answer 14

• Direct
– a physical interaction between species
e.g. Sea anemones sting each other to deter other colonies from encroaching on their space
• Indirect
– species influence each other by competing for
shared resources
e.g. Diatom algae make silica skeletons thus depleting ocean surface waters.
Radiolaria (protozoa) have had to evolve silica-saving modifications in response to increasing sequestration of SiO2 by diatoms.

Question 15

Describe predator-prey interactions

Answer 15

• Another antagonistic interaction
• A win-lose situation - the predator benefits at
the expense of the prey
• Results in limited population sizes of prey
(when predators abundant) or predators
(when prey are scarce).
• Predators can limit prey populations to below
limits at which competitive exclusion occurs.
• Species can then have overlapping niches
without excluding one another.

Question 16

Describe parasite-host interactions

Answer 16

• Another antagonistic interaction
• A win-lose situation - the parasite benefits at
the expense of the host
• Limits the size of the host population.
e.g American chestnuts are attacked by chestnut blight. The sac fungus attacks the vascular system and kills off the trunk. Oak, beech and
other tree populations have increased as the
chestnut populations have declined.

Question 17

Describe Parasitoid-host interactions

Answer 17

Parasitoids typically kill their hosts.
May produce coupled cycles just like the
predator-prey antagonism.

Question 18

Give an example of social parasites

Answer 18

Large	blue	butterfly	caterpillars
secrete	chemicals	and	make	
queen ant	sounds	that	fool
ants	into	thinking	they	are	ant
grubs.	The	caterpillars	feed	on	
the	ants	and	are	sheltered	by
them.

Question 19

Give an example of brood parasites

Answer 19

Cuckoos lay their eggs in the nests of other
birds, so the young use food gathered by the
hosts.

Question 20

Weighing up interactions?

Answer 20

• Costs
– physical	resources,	leading	to	increased	mortality	
or	reduced	reproductive	ability
• Benefits
– increases	in	reproductive	rates
– reduction	in	mortality	rates
• What	is	the	cost-benefit	ratio?

Question 21

What is the cost-benefit ratio for antagonistic interactions?

Answer 21

• The benefit is greater than the cost for one participant
– the predator gets food
– the parasite gets food/space
– the winning competitor gets the food/space/mate
• The cost is greater than the benefit for the
other participant
– the prey gets eaten
– the host’s fitness is reduced
– the outcompeted individual loses out

Question 22

What is the cost-benefit ratio for mutualistic interactions?

Answer 22

• These are win-win situations – both participants have greater benefit than cost
• Benefit is measured in terms of increased
reproductive output and survival
• Benefits include access to nutrients, shelter,
reproductive facilitation
• Costs include energy-consumption, loss of food
resources, investment in protein/fat/carbohydrate structures and specialized tissue
e.g Midges benefit from cacao blossom food, at the (unwitting) cost of pollen transport
between flowers.
Chocolate trees obtain pollination from the midges at the cost of producing nectar
Urchins benefit from transport to new feeding grounds, with little cost.
Crabs benefit from reduced predation risk from fish, with the cost of carrying the urchin.

Question 23

Explain mutualistic interactions

Answer 23

• Important to remember that each
species/individual is acting in its own interests
• Associations that are beneficial to both
partners and are therefore more likely to be
represented in the next generation
• Mutualisms are subject to natural selection,
just like any other adaptation

Question 24

Give an example of evolving interdependence

Answer 24

Flowers evolved in response to pollinators…
who in turn adapted in response to flower
availability.

Question 25

Obligate or facultative?

Answer 25

• Obligate
– If one or both sides of a mutualism cannot survive
without the other.
– The cacao tree is only pollinated by the midge, so
the relationship is obligate.
• Facultative
– If one or both sides can survive without the other.
– The midge can find food elsewhere, so the
relationship is facultative.
• Many obligate relationships probably began as
facultative relationships that were reinforced
over time by natural selection.
e.g A mutation originally made cacao
flowers sprout from the trunk near the
ground, not on a high branch.
This made it more likely to be pollinated
by low flying midges and decreased the
chance of discovery and pollination by
bees.
If midges were reliable pollinators, the
mutation would be selected for.

Question 26

What is commensalism?

Answer 26

• An interaction in which one party benefits, with no cost or benefit to the other.
e.g Grey whales are often festooned with
barnacles, which benefit from a substrate
on which to grow, and transport to
areas rich with planktonic food.
Their presence does not appear to
affect the whales at all.
• Commensalisms can become mutualisms.
e.g Cattle egrets gain food (insects) by
following buffalo, with no benefit or cost
to the buffalo.
The egrets may begin to give warnings of
nearby predators, or eat insects carrying
buffalo diseases.

Question 27

What is cheating?

Answer 27

• Mutualisms can become antagonisms if one
partner begins to cheat.
e.g Tubular flowers mean that bees must go past the anthers or stigma, increasing
pollination but slowing down the bee.
If bees nip the flower base from the
outside, they can drink the nectar without
pollinating the flower, saving time and
allowing them to visit more flowers.

Question 28

What is Amensalism?

Answer 28

• An	interaction	in	which	one	party	is	harmed,	
with	no	cost	or	benefit	to	the	other.
• No	well	defended	examples	of	this	class	of interaction.