Lec.3.sp. Richness

PATTERNS OF SPECIES RICHNESS • species richness is the number of species present at a site • a simple model of species richness: -Each species utilizes a portion n of the available resource dimension R, overlapping with adjacent species by an amount o. - ñ is the average niche breadth within the community - õ is the average niche overlap within the community

a)

o

n

R

R

b)

-More species, ñ smaller - species are more specialized in their use of resources

c)

- More species because each overlaps more with its neighbours (larger õ)

d)

- Contain fewer species when more of the resource continuum is unexploited

- More species because resource axis is more fully exploited (community more fully saturated)

•

if a community is dominated by interspecific competition, the resources are fully exploited, species richness will depend on:

iii. The range of available resources iv. The extent to which species are specialists v. The permitted extent of niche overlap • The role of predation in regulating species richness: • Keep species below their carrying capacities much of the time • Reduce the intensity & importance of direct interspecific competition for resources • Permit much more niche overlap • Permit a greater richness of species than in a community dominated by competition

Richness Relationships 3. Productivity • Productivity increase from pole to the tropics, • decrease at higher altitude in terrestrial environment, deeper water levels & arid environment a) Productivity

Species richness

• productivity leads to an increased range of available resources • in a community dominated by competition, an increase in the quantity of resources would allow greater specialization without the individual specialist species being driven to very low densities

b) Productivity

Species richness

• eg. cultural eutrophication in lakes, rivers • competitive exclusion • other factors varies in parallel with productivity, eg. disturbance • range of resource concentrations in productive environment less than in unproductive environment 2. Spatial heterogeneity • species richness increase in spatially heterogenous environment (a greater variety of microhabitats, a greater range of microclimates, etc) • richness & the heterogeneity of the abiotic environment

• animal richness & plant spatial heterogeneity (plant species diversity & structural diversity) • spatial heterogeneity arises from the abiotic environment or from the other biological components of the community capable of promoting an increase in species richness 3. Climatic variation • temporal niche differentiation in seasonal environments • in a seasonally changing environment, different species may be suited to conditions at different times of the year • climatic instability may increase or decrease richness

4. Environmental harshness • environment dominated by an extreme abiotic factor • any organism tolerating the harsh environment requires a morphological structure or biochemical mechanism which is not found in most related species • characterized by other features associated with low species richness • unproductive, low spatial heterogeneity

Species Diversity •

An expression of community structure

•

a measure of the variety of different animal & plant species of a community

•

Describe the number of species within that community & their relative abundances

•

species-abundance relationships as a single number

•

diversity indices are functions of the no. of species & the no. of individuals for each species

•

relationship between diversity & environmental quality

•

composed of 2 components:

ix. Species richness (total no. of species) x. Evenness (how the species abundances are distributed among the species)

• evenness is derived from the spread of individuals between the species • the equitability/evenness is a function of the species-abundance distribution displayed by the community •Species diversity as a measure of community stability & as an index of the maturity of a community •Indices of community structure (diversity, evenness, richness, similarity) have been used in biomonitoring • all measures of community structure depend on sample size

Measures of Species Diversity i. Margalef’s index: Da =

s-1 log N

ii. Menhinick’s index: Db =

s √N

Da and Db are inadequate because they do not differentiate between the same s and N Various diversity indices computed for hypothetical situations (random samples) of N individuals distributed among s species, with ni individuals in the ith species

Hypothetical examples Species

Abundance

A

B

C

D

E

1

n1

10

29

91

100

20

2

n2

10

19

1

100

20

3

n3

10

14

1

100

20

4

n4

10

11

1

100

20

5

n5

10

9

1

100

20

6

n6

10

7

1

100

7

n7

10

5

1

100

8

n8

10

3

1

100

9

n9

10

2

1

100

10

n10

10

1

1

100

s

10

10

10

10

5

N

100

100

100

1000

100

Da

4.50

4.50

4.50

3.00

2.00

Db

1.00

1.00

1.00

0.32

0.50

Ds

0.91

0.84

0.17

0.90

0.81

H’

1.00

0.86

0.22

1.00

0.70

H’max

1.00

1.00

1.00

1.00

0.70

J’

1.00

0.86

0.22

1.00

1.00

iii. Simpson’s index: If 2 individuals are taken randomly from a community, the probability that the 2 will belong to the same species is: L=

∑ni(ni – 1) N(N – 1)

• The quantity L is a measure of dominance (the concentration of N individuals among s species) • A collection of species with high diversity will have low dominance Ds = 1 - L Ds =

1 - ∑ni(ni – 1) N(N – 1)

A hypothetical set of species abundance data Species, i

Abundance, ni

Relative abundance, pi

1

50

50/85 = 0.588

2

25

25/85 = 0.294

3

10

10/85 = 0.118

s=3

N = 85

Ds = 1 -

50(49) + 25(24) + 10(9) 85(84)

= 1 – 3140/7140 = 1 – 0.44 = 0.56

Shannon & Weaver, 1963 H’ = - ∑ (Ni/N) log2 (Ni/N) Ni/N : the proportion of the sample comprised by the ith species Ni/N = Pi Log2 Pi = K Pi = 2K K=

Log10 Pi Log10 2

Evenness (J’) =

H’ H’max

=

H’ log2S

J’ should be max when all species in a sample are equally abundant

Species

N

i

P (= N /N) i i

P log P i 2 i

% Rel. abundance

A

3

0.029

-0.148

2.9

B

4

0.038

-0.179

3.8

C

5

0.048

-0.210

4.8

D

2

0.019

-0.109

1.9

E

1

0.009

-0.061

0.9

F

23

0.221

-0.481

22.1

G

2

0.019

-0.109

1.9

H

17

0.163

-0.427

16.3

I

6

0.058

-0.238

5.8

J

33

0.317

-0.525

31.7

K

7

0.067

-0.261

6.7

L

1

0.009

-0.061

0.9

∑ N = 104 i

H’ = 2.809

Factors promoting species diversity: c) The time hypothesis • Diversity is a function of time; all communities diversify with time • Increases in diversity due to immigration to an area, specialization of the existing species, evolution of new species etc • Diversity increase during succession b) Spatial complexity • Environmental complexity: a more complex & heterogenous environment – more niches are found – the fauna & flora more complex & diverse • An increase in species richness may cause a further increase in spatial complexity as animals & plants provide habitats for others

c) Productivity hypothesis • diversity increase or decrease with productivity • increase in abundance of all resources – overall increase in production – diversity increase • increase in only a small part of the total resource spectrum – diversity decrease (community become dominated by those species which are competitively superior in exploiting these augmented resources) • in a more severe environments – abiotic considerations have the dominant influence on community • in benign/stable environments – biotic interactions play a major role