Transcript Living in communities - McGraw Hill Higher Education

Chapter 43: Living in communities
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Species coexist and interact in
communities
• 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
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
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Species diversity
• Simplest measure is the number of species
present—this is species richness
• Better to include a measure of relative abundance,
called alpha (α-) diversity
• Beta (β-) diversity measures the diversity
between communities separated in space
• When comparing diversity, the area sampled (or
time spent sampling) for all communities must be
standardised
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Slides prepared by Karen Burke da Silva, Flinders University
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Measuring diversity
• Simpson’s index (below) is one of several different
diversity indices
n 
D  1   i 
i 1  N 
s
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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Slides prepared by Karen Burke da Silva, Flinders University
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Structural diversity of plant
communities
• 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
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Fig. 43.4: Plant communities
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Interactions within communities:
symbiosis
• 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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Commensalism
• One species benefits, the other is unaffected
• Examples
– Moist forest: epiphytes on tree trunks
– Marine rocky shores: ‘decorator’ crabs and molluscs with
attached algae
• 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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Mutualism
• Both partners benefit
• Examples
– lichen (an alga and a fungus)
– mycorrhiza (a fungus and the root of a higher plant)
– symbiotic algae in tissues of coral animals and giant
clams
– sea-anemone and anemone fish
– ant colonies in domatia provided by plants
• Advantages may include food, protection, removal
of wastes, provision of living space
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Fig. 43.7: Anemone fish and its
symbiotic partner, the anemone
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Parasitism
• One partner benefits, the other is harmed but
not usually killed
• Parasites typically 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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Fig. 43.9: Parasitoid
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Question 1:
Which of the following statements is true?
a) Symbiosis refers to different organisms living
together.
b) Members of a symbiotic relationship cannot live
without each other.
c) Symbiosis refers to different organisms living
together and benefiting from each other.
d) A parasite is not in a symbiosis with its host.
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Predation
• One organism feeding on another
• Numbers of predators and prey may show
oscillations called predator–prey cycles, but these
are rarely simple because:
– unpredictable fluctuations in environment occur
– predator is likely to eat more than one prey species
– prey species is affected by the quality of its own food
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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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Fruit eaters aid seed dispersal
• Frugivores are herbivores that specialise as fruit
eaters
• In Queensland rainforests 84% 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
• Works best where the predator
– is monophagous (feeds on single prey type)
– drives prey to very low numbers
– survives in low numbers itself
• 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
• Natural selection has resulted in the evolution of
plant defence mechanisms against being eaten
–
–
–
–
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
• 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
• Presence or 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
• There are not many good examples of truly
keystone species
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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 competition
• Or between species: interspecific competition
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Types of competitive interaction
• If a resource is used up  exploitative competition
– nutrients exhausted; e.g. ants remove seeds from an
area
• Prevention of access to resources  interference
competition
– resource not consumed; e.g. tall trees shading
neighbours
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Ecological niche
• Defined by all the biotic and abiotic factors that
affect survival of a species
• 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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Niche overlap and character
displacement
• 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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Communities are not constant:
succession
• 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
• Stable sere is the climax community
– Larger, slower growing, best long-term competitors
– ‘K’ strategists
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Disturbance affects many
communities
• Disturbance may result from fire, grazing,
predation, pollution, trampling, flood, exotic pests
etc.
• 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 can act as novel
disturbances
• 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 the result of many
different interactions
• Competition, predation and disturbance all vary
and interact
• Community structure can be determined by
– bottom-up (nutrient) controls
– top-down (competition, predation) controls
– recruitment and immigration
• Manipulative ecological experiments are a
powerful tool to unravel these processes
• Rocky shore community structure results from a
combination of biotic processes modified by
physical environment and random disturbances
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Biomes: communities on a global
scale
• Biomes are ecological communities delineated by
climate
• 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. B43.4a: Major world biomes
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Gradients and ecotones
• 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
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Fig. 43.34a: Mangroves (Rhizophora
stylosa)
Copyright © marinethemes.com/Kelvin Aitken
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Fig. 43.34b and c: Distribution of
three mangrove species
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Summary
• Community structure can be summarised by
measures of diversity
• Species interact in many different ways. This can
include symbiosis, predation and competition
• Mutualism, commensalism and parasitism are all
forms of symbiosis
• Competition between different species for a limited
resource is interspecific competition
• Disturbance of a community may reduce the
frequency of competition and aid in maintaining
diversity
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
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