Population Dynamics
at Rhyd y creuau
Population Dynamics of the Holly Leaf Miner (Phytomyza ilicis)
Aims
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To describe the mortality within a generation of the holly leaf miner insect
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To determine factors that could regulate, or cause changes in, population size
Objectives:
Tick when achieved
This study will allow us to:
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Use microscopes and dissection tools for the collection of ecological data
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Cover some of the ecological terminology and concepts needed for the study
of population dynamics
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Your Objective:
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Concept’s we’ll be covering:
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Before
After
Population growth (lag time and exponential growth)
Density dependent limiting factors
Density independent limiting factors
Carrying capacity
Predator-prey relationships
Pest control (chemical, biological and integrated pest management)
Introduction:
The holly leaf miner (Phytomyza ilicis) is a species of insect whose life cycle revolves around holly (Ilex
aquifolium). Adult holly leaf miners, which resemble small house flies, lay their eggs on holly leaves.
When the eggs hatch, the maggot-like larvae feeds on the leaf’s tissue, creating a distinctive blotch
mine. As most of the insect’s life cycle takes place within holly leaves, mortality factors can be identified
and quantified.
Population dynamics:
Starting from a population of two holly leaf miners, if all mortality factors were removed, how many
would you have in 3 years?
What percentage mortality is required to keep the population at a constant level?
Population after 3
years:
Required %
mortality:
Life cycle of the Holly Leaf Miner (Phytomyza ilicis):
1. During late May to June
the adult fly emerges from
a pupa within the mine
through the leaf, creating
a small exit hole
Spring:
Parasitic wasps may attack the
larva or pupa of the miner. The
parasite larva is yellow and the
parasite pupa or pupal skin is
shining black. The adult parasite
emerges from within the mine
creating a minute exit hole
3. The holly leaf miner
pupates inside the leaf in
March, after preparing a
small patch in the surface
of the mine. The pupa is
light brown
2. The adult fly, which
resembles a tiny house
fly, mates. A single egg is
laid on each holly leaf,
into the base of the
underside of the midrib
Life Cycle
of the
Holly Leaf
Miner
Autumn:
Disease may kill young
larvae, in which case the
mine is small and contains
black and white grot
4. After hatching, the larvae
eat through the midrib
and enter the outer
parenchyma of the leaf
during the autumn
Winter:
Predators (blue tits) may tear open the
mine and feed on larva, leaving a
characteristic v-shaped tear
The larva continues to
feed throughout the
winter months, creating
distinctive blotch mines
Data Collation –Population Dynamics of the Holly Leaf Miner
Eggs don’t hatch
(D1)
Key:
No. of mines
(N0)
eggs (N0)
No. of mines (N1)
Larval disease
(D2)
N = Number of surviving
individuals
NO ESTIMATE
Death of adults
(D7)
D = Mortality factor
No. of mines (N2)
No. of mines (N7)
NO ESTIMATE
Larval parasite
(D3)
No. of mines (N3)
No. of mines (N6)
Calculations:
No. of mines (N5)
Pupal failure
(D6)
No. of mines (N4)
Overall percentage mortality:
Predation by birds
(D4)
N1 – N6
= -------------N1
x
Pupal parasites
(D5)
= ...................%
100
YES
Who Killed the Holly Leaf Miner?
Emergence hole c0.5mm. Dissection
reveals black pupal skin of parasite) but
no puparium
NO
Emergence hole c0.5mm. Mine contains
brown puparium (of leaf miner) with
black pupal skin (of parasite) inside
NO
Large emergence hole (>1mm).
Dissection reveals shiny light brown
puparium left by emerging leaf miner
NO
YES
NO
Tear or hole present in the surface of
mine (check lower and upper epidermis
No hole or tear in surface of mine.
Dissection reveals shiny light brown
puparium of leaf miner
YES
d5 =
YES
Puparium of leaf miner contains black
pupa of parasite
YES
d5 =
Other death of
pupa
d6 =
YES
Healthy larva
of leaf miner
Dissection reveals dead larva of leaf
miner with black pupa of parasite lying
near to remains
YES
Dissection reveals dead larva of leaf
miner. Remains appear flaccid and dirty
yellow in colour
YES
Dissection reveals no conspicuous
remains of larva at all
Parasitic wasp
killed pupa
YES
Puparium of leaf miner does not
contain black pupa of parasite
NO
No hole or tear in surface of mine.
Dissection reveals living larva – shiny
and lemon yellow in colour
d4 =
Healthy pupa
of leaf miner
No hole or tear in surface of mine.
Dissection reveals dull and dark brown
puparium
NO
Parasitic wasp
killed pupa
Predation by
blue tit
NO
NO
d3 =
Successful leaf
miner
YES
V shaped tear indicating peck mark
present. No remains of leaf miner
present inside mine
START HERE
YES
Parasitic wasp
killed larva
NO
Parasitic wasp
killed larva
d3 =
Other death of
larva
d2 =
YES
Dissection reveals dead larva of leaf
miner. Remains appear shrivelled and
greenish in colour
Population Regulation and Dynamics Discussion
Although population explosions or extinctions may grab the news, in reality most populations of
organisms in stable ecosystems remain remarkably constant over time. What causes this regulation of
populations has vexed ecologists for almost a century and much on this subject remains to be learnt.
Mortality factors:
Mortality factors are factors which prevent populations from increasing rapidly in number:
Density _______________________ Factors
Density _______________________ Factors
Disease
Predation
Fire
Frosts
These two types of mortality factors act on populations in different ways. Very generally, these factors
combine to give the following growth curve:
What type of growth
curve is this?
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Pop.
Size
K=
Time
Stage
1
2
3
4
Name
Description
A small population colonizes an area where space and food are
plentiful. The population slowly establishes itself.
Exponential growth occurs.
Growth slows due to the constraints of the environment (these
constraints are called limiting factors).
The population size stabilises around the environment’s carrying
capacity (k) for that population. The carrying capacity is the
average number of a population the area can hold. Oscillations
around the carrying capacity are generally caused by seasonal or
climatic factors.
Think: Why is it important to study population dynamics?
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