Community Ecology

BCB331

Mark J Gibbons, Room Z108, BCB Department, UWC

Tel: 021 959 2475. Email: mgibbons@uwc.ac.za

Individual species can influence communities in a variety of ways

Provide habitats

Provide food resources

Quercus

Alter conditions

The role of inter-specific competition

IMPORTANT – YES

BUT……… HOW IMPORTANT

In a stable, homogeneous environment, where species

compete with each other on an ongoing basis – competitive

interactions will reach equilibrium: IMPORTANT

But if other factors prevent equilibrium being reached,

competition may not be so important

E.g. phytoplankton communities: diverse despite limited

scope for resource partitioning

Hutchinson (1961: American Naturalist 95: 137-145) suggested this

reflected short term fluctuations in conditions and

resources that prevented competitive exclusion

Flo

de

r et a

l. (20

02

) Oe

co

log

ia 1

33

: 39

5-4

01

Diversity higher in unstable environments – competitive

exclusion prevented

Schoener (1983) American Naturalist 122: 240-285

In a literature survey of inter-specific competition, Schoener

(1983) noted that approx equal numbers of studies had been

published on marine organisms, terrestrial plants and

terrestrial animals; paucity in freshwater

Amongst terrestrial studies, most conducted in temperate

areas – few dealt with phytophagous insects

Conclusions about generalities restricted to environments

and taxa studied – BUT…………..

89% - Terrestrial

91% - Freshwater

94% - Marine

Competition demonstrated

BEWARE – people don’t publish negative results, people

tend to study species that hint at competition and journals

don’t publish all papers submitted to them!

In a similar study, Connell (1983) noted that in > 90% of

studies of one species pairs, competition demonstrated but

that this dropped to < 50% in studies of more species

Connell (1983) American Naturalist 122: 661-696

The case of phytophagous insectsL

aw

ton

(19

84

) In: E

co

log

ica

l Co

mm

un

ities

, Stro

ng

et a

l. (Ed

s), P

rinc

eto

n, 6

7-1

00

Southern England

New Mexico

Papua New Guinea

Competition rare – vacant niches:

Implies competition unimportant

Maybe widespread amongst herbivoresWHY?

Whilst competition is obviously important for sessile species

It may not be

important for all

species

Structuring power of competition

Even if competition intense, species concerned may co-

exist and competition need not determine species

composition of communityUnpredictable environments

Patchy resources

Aggregated distributions

At the same time – even if competition is not obvious at the

time of observation, does not mean it is not, or was not,

important in determining composition of community

“Ghost of competition past”

Species may compete rarely – when seasonally abundant

One approach to examining the role of competition in

structuring communities is to predict what they should look

like if inter-specific competition played/plays a part in

shaping them and then to compare the results with real

communities.

One approach to examining the role of competition in

structuring communities is to predict what they should look

like if inter-specific competition played/plays a part in

shaping them and then to compare the results with real

communities.

Predictions:

Potential competitors that coexist should exhibit niche

differentiation

Niche differentiation by species may take form of

morphological differentiation

Within a community, potential competitors with little

differentiation would be unlikely to coexist – negative

associations

Taking each of these in turn……………………..

Potential competitors that coexist should exhibit niche

differentiation – evidence from community patterns

Anemonefish - Amphiprion

Nine species off PNG –

each typically associated

with just one species of

anemone that is

aggressively defended

Anemones limiting

resource – results of

translocation experiments

Surveys at three sites – replicated

Surveys in four depth zones

- nearshore (N)

- mid-lagoon (M)

- outer barrier (O)

- offshore (OS)

Results show each anemonefish associated with particular

anemone AND characteristic preference for a particular zone

1 – niche complimentarity

Ellio

tt & M

ariscal (20

01) Ma

rine B

iolo

gy 138

: 23-36

Amphiprion perculaAmphiprion perideraion

Amphiprion clarkiiAmphiprion chrysopterus

Amphiprion leucokranos

Amphiprion melanopus

Amphiprion clarkii

Amphiprion chrysopterusAmphiprion sandaracinos

Amphiprion leucokranos

This example

suggests that

WITHIN A GUILD

niche

differentiation

involves several

dimensions – and

species that

occupy a similar

position along one

dimension

(anemone species)

tend to differ

along another

dimension (zone

occupied)

2 – niche differentiation in space

11 species of Macaranga in Borneo

Species

Percentage of individuals in each of five crown illumination classes

Marked differentiation

in light requirements

Shade tolerant species

(e.g. K) small, persist in

understorey, rare in new

gaps

High light species (G),

pioneers of large new

gaps

Intermediate light

specialists (T), small

gap specialists

Species

Species also differentiated along a second niche axis – soil

type (moisture and/or nutrients)

Evidence of

complimentarity –

species with similar

light requirements

differ in terms of soils

– especially in the case

of shade-tolerant

species

Davies et al (1998) J of Ecology 86: 662-673

Dick

ie et al (2002

) New

Ph

ytolo

gist 156

: 527-535

3 – Are patterns real or not? The NULL MODEL approach

There is a temptation to interpret differences as confirming

the existence of competition

BUT…

are the differences big or regular enough to be different

from those expected by chance? Need to construct a null

model

A null model is like a null hypothesis – it provides a set of

“random” data that can be used to test observations

against.

A null model of a community must retain certain

characteristics of the community under investigation but

reassemble components at random – specifically excluding

the consequences of biological interactions.

Lizard communities in North America

Lawlor (1980) examined the dietary overlap between lizards

in ten communities and then asked if these differed from

those that would be expected by chance alone. HOW?

1) Calculated electivities for each diet item for each species

in each community (range from 0-1)

Species A Species B Species A Species B1 0 4 0 0.162 5 4 0.2 0.163 9 3 0.36 0.124 3 0 0.12 05 0 5 0 0.26 0 9 0 0.367 8 0 0.32 0

Diet ItemELECTIVITYNUMBER

2) Calculated dietary overlap between every pair of species

in each community

3) Calculated mean

dietary overlap between

species in each

community

Community No

No Lizards in

Community

Mean Dietary Overlap

1 4 0.065

2 5 0.30

3 5 0.29

4 6 0.12

5 6 0.16

6 7 0.11

7 8 0.28

8 9 0.19

9 9 0.21

10 9 0.20

La

wlo

r (19

80

) Am

eric

an

Na

tura

list 1

16

: 39

4-4

08

Four null models used that retained different aspects of the

food environment

Model 1 Minimal amount of initial structure retained

Only original number of species and number of dietary items

retained. Otherwise, all electivities, including zeros,

assigned a random number between 0-1. Repeated 100 times

Species A Species B Species A Species B1 0.65 0.38 0.18 0.162 0.13 0.06 0.04 0.023 0.57 0.23 0.16 0.104 0.48 0.80 0.13 0.335 0.87 0.06 0.24 0.036 0.44 0.32 0.12 0.137 0.52 0.54 0.14 0.23

Diet ItemELECTIVITYNUMBERIndividual overlap

between species in a

community then

calculated, as too mean

overlap per community

No species in community

Mean

dietary o

verlap

Null Model data

Real data

Niche breadth and overlap increased wrt observed

Model 2

Original number of species and number of dietary items

retained: ONLY electivities > 0 assigned a random number

between 0-1. Repeated 100 times

Species A Species B Species A Species B1 0 19 0.00 0.282 12 13 0.22 0.193 24 18 0.44 0.264 17 0 0.31 0.005 0 4 0.00 0.066 0 15 0.00 0.227 2 0 0.04 0.00

Diet ItemELECTIVITYNUMBER

Null Model Data

Individual overlap between species in a community then

calculated, as too mean overlap per community

No species in community

Mean

dietary o

verlap

Null Model data

Real data

Niche breadth and overlap increased and different wrt observed

Model 3

Original number of species, number of dietary items and

electivities retained: just randomly reassigned to different

diet items. Repeated 100 times

Individual overlap between species in a community then

calculated, as too mean overlap per community

Species A Species B Species A Species B1 0 4 0 0.162 5 4 0.2 0.163 9 3 0.36 0.124 3 0 0.12 05 0 5 0 0.26 0 9 0 0.367 8 0 0.32 0

Diet ItemELECTIVITYNUMBER

Species A Species B Species A Species B1 3 3 0.12 0.122 9 4 0.36 0.163 5 0 0.20 0.004 0 0 0.00 0.005 0 9 0.00 0.366 8 5 0.32 0.207 0 4 0.00 0.16

Diet ItemELECTIVITYNUMBER

Original Data

Null Model Data

No species in community

Mean

dietary o

verlapNull Model data

Real data

Original number of species, number of dietary items and

electivities retained: non-zero values just randomly

reassigned to other non-zero diet items. Repeated 100 times

Species A Species B Species A Species B1 0 4 0 0.162 5 4 0.2 0.163 9 3 0.36 0.124 3 0 0.12 05 0 5 0 0.26 0 9 0 0.367 8 0 0.32 0

Diet ItemELECTIVITYNUMBER

Original Data

Null Model DataSpecies A Species B Species A Species B

1 0 3 0.00 0.122 8 9 0.32 0.363 3 4 0.12 0.164 9 0 0.36 0.005 0 5 0.00 0.206 0 4 0.00 0.167 5 0 0.20 0.00

Diet ItemELECTIVITYNUMBER

Individual overlap between species in a community then

calculated, as too mean overlap per community

No species in community

Mean

dietary o

verlapNull Model data

Real data

Model 4

This model retains the greatest amount of the

original structure in the system

Taken overall then – there is a significant difference

between the observed patterns and those simulated by

the null models – implying that biological interactions (in

this case interspecific competition) have played a part in

structuring the communities observed

Niche differentiation may take form of morphological

differentiation: evidence from community patterns

Where niche differentiation results in morphological

differentiation, niche spacing should be reflected in

morphological differences between species in a guild

In animal guilds, adjacent species tend to show regular

differences in body size or in size of feeding structures

Ratios of 2.0 for mass and 1.3 for length

Cuckoo doves – 1.9 (mass)

Bumblebees – 1.32 (proboscis length)

Weasels – 1.23 (canine length)

Fossil brachiopods – 1.48-1.57 (body outline length)

How do you test if patterns real?

Construct a null model……..

74 fossil brachiopod taxa – random

sample of four drawn and size ratios

between adjacent species calculated.

Repeated 100 000 times. Results compared to actual

observations – null hypothesis (observed ratios were a

chance event) rejected, support idea of limiting similarity

Herm

oy

ian et al (20

02) Geo

log

y 30: 15-18

Inter-specific competition may often act through a process

of selective extinction: too similar species fail to persist

together

Over the period 1860-1980, 18 pairs of introduced con-

generic passerine birds were present on the same Hawaiian

Island at the same time

Of these, six pairs persisted together, three pairs both went

extinct, and in the other nine, one of the two species went

extinct

When one of the two species went extinct, the species pair

were morphologically more similar to each other than when

neither species went extinct: 9% difference in bill length as

oppose to 22% difference.

Mo

ulto

n &

Pim

m (1986

) In: C

om

mu

nity E

colo

gy, D

iamo

nd

& C

ase (E

ds

), Harp

er & R

ow

, 80-97

Serinus species

Within a community, potential competitors with little

differentiation would be unlikely to coexist – negative

associations: evidence from community patterns

Checkerboard distribution of two cuckoo-doves in the Bismarck archipelago.

NO island has both species!

Diam

on

d (19

75) In: E

colo

gy &

Evo

lutio

n o

f Co

mm

un

ities, C

od

y & D

iamo

nd

(Ed

s), Belkn

ap, 342-4

44

How do you test if patterns real?

Construct a null model……..

Compare the pattern of species co-occurrences at a suite of

locations with that which would be expected by chance

Index of association between all pairs of native and

(separately) alien plant species found on 23 islands in

Lake Manapouri (New Zealand) constructed:

dik = (Oik – Eik) / SDik

dik = Association Index

Oik = Observed number of island shared by species I and k

Eik = Expected number of islands shared by species I and k

SDik = Standard deviation of expected number

For example………

Wilson (1988) J of Ecology 76: 1030-1042

The distribution of these values was then compared to the

distribution of values obtained if the species were

randomly distributed amongst the islands (keeping number

of species on each island, and the total number of islands

occupied by a species, at the observed level)

Native Introduced

Sig

nifica

ntly d

iffere

nt; N

ot sig

nifica

ntly d

iffere

nt

Sig

nifican

tly differe

nt : N

ot sig

nific

antly d

ifferen

tNeutral model results

Native Species

Significantly more negative associations than expected by

chance – competitive exclusion

Significantly more positive associations than expected by

chance – probably based on common microhabitats

The distribution of these values was then compared to the

distribution of values obtained if the species were

randomly distributed amongst the islands (keeping number

of species on each island, and the total number of islands

occupied by a species, at the observed level)

Native Introduced

Sig

nifica

ntly d

iffere

nt; N

ot sig

nifica

ntly d

iffere

nt

Sig

nifican

tly differe

nt : N

ot sig

nific

antly d

ifferen

tNeutral model results

Alien Species

No significant differences between observed and expected

distribution of associations – interactions have not yet

reached equilibrium OR generally weedy and generalist

nature

Role of interspecific competition – an appraisal

Possible and plausible explanation for some organisation in

some communities – BUT not all

Why? Current competition not widely demonstrated

“Ghost of Competition Past” too easy to uncritically invoke to

explain patterns

Communities chosen for study may not be typical –

subjective. Studies in which competition not demonstrated

may not have been published!

Patterns may have alternative explanations

Patterns may have arisen by chance!

Role of competition will vary from community to community –

important in species rich, stable vertebrate communities

unimportant in phytophagous insects

Other interactions may also play a role

THE END

Image acknowledgements – http://www.google.com