43-1 copyright 2005 mcgraw-hill australia pty ltd ppts t/a biology: an australian focus 3e by knox,...

43-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Chapter 43: Living in communities


Species coexist 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


Species diversity

• 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


Measuring diversity

• Simpson’s index (below) is one of several different diversity indices

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

2

1

1

s

i

i

N

nD


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

• See Fig. 43.4


Fig. 43.4: Plant communities


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


Symbiosis: 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

• Note: over time the relationship may change, e.g. a strangler fig is a harmless epiphyte when small, but grows into an aggressive competitor


Symbiosis: mutualism• 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

• Advantages may include food, protection, removal of wastes, provision of living space


• 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

Symbiosis: parasitism


Predation

• 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


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


Frugivores

• 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)


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


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)


Animal defences against predators• 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)


Keystone predators

• 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

• Note: there are not many good examples of truly keystone species


Competition

• 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


Types of competitive interaction

• If a resource is used up Exploitative competition– nutrients exhausted; ants remove seeds

• Prevention of access to resources Interference competition

– resource not consumed, e.g. tall trees shading neighbours


Ecological niche

• 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


Fig. 43.22: Idealised niche space


If niches overlap

• 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


Community 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


Ecological disturbance• 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


Humans causes of disturbance

• 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


Communities are affected by many different interactions• 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

• 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


Biomes: communities on a global scale• 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


Fig. B43.3a: Major world biomes


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 (see Fig. 43.33)


Fig. 43.33b and c: Distribution of three mangrove species

43-1 copyright 2005 mcgraw-hill australia pty ltd ppts t/a biology: an australian focus 3e by knox,...

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