Transcript community structure

Chapter 43: Living in communities
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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Species coexist in communities
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Ecological views of coexisting species range from
coevolved, highly interactive communities to loose
assemblages of species
Community ecology focuses on:
– community structure i.e. the relative abundance of
component species
– species dynamics (changing abundance) in space and
time
– species interactions
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Species diversity
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Simplest measure is the number of species
present—this is species richness
Better to include measure of relative abundance,
then this is called alpha diversity
Beta diversity measures the diversity between
communities separated in space
Note: When comparing diversity, the area sampled
(or time spent sampling) for all communities must
be standardised
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Measuring diversity
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Simpson’s index (below) is one of several different
diversity indices
 ni 
D  1   
i 1  N 
s
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2
where ni is number of individuals in species i, N is
total no. of individuals in the community and S is
the no. of species present.
D ranges from 0 to 1, and is affected by the total
number of species as well as their relative
abundance
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Structural diversity of plant
communities
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Describes variations in size and shape irrespective
of species
Provides information on biomass and productivity
Uses two components
– projective foliage cover = % of ground above which there
is foliage
– height of foliage
R. L. Specht’s structural classification has been
widely used in mapping Australian vegetation
• See Fig. 43.4
•
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Fig. 43.4: Plant communities
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Interactions within communities:
symbiosis
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Symbiosis: occurs when two organisms live closely
together and at least one benefits e.g. coral polyps
and symbiotic algae
Partners may have coevolved e.g. orchid flower
mimics female wasp
Symbiosis is common in complex communities
There are three kinds of symbiotic interactions
– commensalism
– mutualism
– parasitism
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Symbiosis: commensalism
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One species benefits, the other is unaffected
• Examples
– Moist forest: epiphytes on tree trunks
– Marine rocky shores: ‘decorator’ crabs and molluscs with
attached algae
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Note: over time the relationship may change, e.g.
a strangler fig is a harmless epiphyte when small,
but grows into an aggressive competitor
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Symbiosis: mutualism
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Both partners benefit
Examples
– lichen (an alga and a fungus)
– mycorrhiza (a fungus and the root of higher plant)
– symbiotic algae in tissues of coral animals and giant
clams
– sea-anemone and anemone fish
– ant colonies in domatia provided by plants
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Advantages may include food, protection, removal
of wastes, provision of living space
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Symbiosis: parasitism
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One partner benefits, the other is harmed but
not often killed
• Parasites usually smaller than host
• Ectoparasites live on surface of host e.g.
lampreys
• Endoparasites live internally e.g. tapeworms
• Parasitoids are insects that are parasitic only in
their larval stage
• Parasitism is a special kind of predation
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Predation
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One organism feeding on another
• Numbers of predators and prey may show
oscillations called predator-prey cycles, but these
are rarely simple because of
– unpredictable fluctuations in environment
– predator is likely to eat more than one prey species
– prey species is affected by the quality of its own food
•
See Fig. 43.12
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Fig. 43.12: Seasonal changes in the number of insects on the
grass Holcus mollis in relation to changes in food quality,
measured as nitrogen in leaves and stems
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Frugivores
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These are herbivores that specialise as fruit eaters
• In Qld rain forests 84 per cent of plants were found
to produce fleshy fruits (CSIRO data)
• Frugivorous birds digest only the fruit
• Fruit pigeons and cassowaries have thin-walled
gizzards that do not grind up the seed
• Seeds are dispersed and voided to forest floor
(with a small dose of fertiliser!)
• This is an example of an ecosystem service (see
Ch 44)
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Predation and biological control
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Works best where the predator
– is monophagous (feeds on single prey type)
– drives prey to very low numbers
– survives in low numbers itself
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Successful  Opuntia (prickly pear cactus) has
been controlled by Cactoblastis moth larvae
Unsuccessful  Cane beetle Dermolepida was
unaffected by the cane toad Bufo marinus, which
has now become a major pest species
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Plant defences
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Natural selection has resulted in the evolution of
plant defence mechanisms against being eaten
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tough leaves
spines
hairs
chemical deterrents (e.g. tannins and toxic alkaloids)
Many animals have evolved ways to cope with
plant toxins
– eat a small amount of a wide variety of plants
– contain detoxifying enzymes (e.g. Colobine monkeys)
– tolerate poisons (e.g. kangaroo and fluoroacetate in pea
family)
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Animal defences against
predators
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These include speed, agility, size, camouflage,
toxic chemicals, physical barriers and behavioural
adaptations
• Animal may accumulate toxic compounds from its
food to make it unpalatable e.g. Monarch butterfly
• Harmless insects may mimic toxic models. This is
Batesian mimicry (successful provided there are
more models than mimics)
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Keystone predators
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Presence/absence of keystone predator has a
major effect on community structure
Sea star Pisaster ochraceus controls diversity of
rocky shore community by controlling mussel
abundance
– removal of Pisaster  mussels out-compete other
species for space
– in presence of Pisaster  weaker competitors survive
because mussels are controlled
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Note: there are not many good examples of truly
keystone species
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Competition
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Competition only occurs when a resource is in
short supply e.g. food, light, shade, space,
moisture
Can be within a species: intraspecific
Or between species: interspecific
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Types of competitive interaction
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If a resource is used up  Exploitative competition
– nutrients exhausted; ants remove seeds
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Prevention of access to resources  Interference
competition
– resource not consumed, e.g. tall trees shading
neighbours
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Ecological niche
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Defined by all the biotic and abiotic factors that
affect survival of a species, see Fig. 43.22
• A species uses only the realised niche (a part of
the multidimensional and bigger fundamental
niche)
• Niche breadth is the range of a resource that the
species uses
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Fig. 43.22: Idealised niche space
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If niches overlap
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Organisms compete if their realised niches overlap
in time or space
• Resource partitioning is a mechanism that
minimises niche sharing e.g. forest birds feeding at
different heights above ground
• Morphological changes that assist in resource
partitioning are promoted by natural selection e.g.
bill size in seed-eating birds
• Natural selection acts on individuals, and evolution
acts on the population, to promote character
displacement in sympatric populations
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Community succession
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Succession is a theory about how communities
change
Each stage is called a sere
Earliest sere consists of disturbance opportunists
– Small, multiply quickly, short-lived
– ‘r’ strategists
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Stable sere is the climax community
– Larger, slower growing, best long-term competitors
– ‘K’ strategists
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Ecological disturbance
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Disturbance may result from fire, grazing,
predation, pollution, trampling, flood, exotic pests
etc.
Effects of fire in Australia are well studied. Fig.
43.27 shows use of post-fire vegetation by birds
Fire and other disturbances can promote diversity
Intermediate disturbance hypothesis (Connell,
1977 and others): frequency of disturbance is
important
Mosaic of patches of different ages enhances
diversity
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Humans causes of disturbance
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Vegetation clearing, uncontrolled burning, dredging
seabed, trampling rocky shores
• Temporal and spatial scale of human activities may
be unlike natural processes e.g. fire as tool for
hunting
• Combined effects of natural and man-made
disturbance exert new selective pressures and
competitive interactions
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Communities are affected by
many different interactions
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Competition, predation, disturbance all vary and
interact
Community structure can be determined by
– bottom-up (nutrients) controls
– top-down (competition, predation) controls
– recruitment and immigration
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Manipulative ecological experiments are a
powerful tool to unravel these processes
Rocky shore community structure results from
combination of biotic processes modified by
physical environment and random disturbances
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Biomes: communities on a
global scale
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Biomes are ecological communities delineated by
climate (see Fig. B43.3 (a))
Biomes result from evolutionary convergence
A biome may comprise several plant communities,
e.g. alpine biome includes herb-fields, heaths, fens
and bogs
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Fig. B43.3a: Major world biomes
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Ecotones
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Community boundaries are called ecotones
• Ecotones may be species rich because they
include individuals from adjacent community types
• Species are replaced along gradients, resulting in
patterns of zonation e.g. mangrove trees (see Fig.
43.33)
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Fig. 43.33: Mangroves (Rhizophora
stylosa)
Copyright © marinethemes.com/Kelvin Aitken
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PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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Fig. 43.33b and c: Distribution of three
mangrove species
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