Species Richness

1
Species Richness

• Chapter 10

2
Species Richness

• The number of species in a community
• Some species are common, others are rare
• Easy to count common species, more difficult for
  rare

3
Species Richness

• Richness provides one aspect of community, but
  ignores another important factor abundance
• Diversity considers both richness and abundance

4
Diversity Indices

• Diversity indices based on number of species
  present, as well as distribution of individuals
  among those species
• High diversity requires many different species
  plus even distribution of individuals among them

5
Diversity Indices

• Low diversity produced by low number of species
  and uneven distribution of individuals among the
  species
• Examples Shannon diversity, Simpson diversity

6
Diversity

• Most communities have a few common species and
  many rare ones
• Often depicted in rank-abundance diagrams
• Steeper line lower diversity

7
Species Richness Models

• Greater range of resources
• More specialization
• More overlap among species
• Resource range more fully exploited

8
Species Richness Productivity

• Greater productivity may lead to greater range of
  resource availability, greater species richness
• Fertilized plot experiments show opposite trend
  fewer species with increasing productivity

9
Species Richness Productivity

• Species richness can also be highest at
  intermediate productivities - hump-shaped pattern
• All possible patterns have been observed

10
Species Richness Competition?

• Can rules of interspecific competition be used
  to predict how many species should be present?
• Competitive exclusion principle and niche
  differentiation

11
Species Richness Competition?

• Niche differentiation can/should lead to
  morphological differentiation
• Hutchinsons ratio rules

12
Hutchinsons Ratio Rules

• Adjacent species along resource dimension exhibit
  regular differences in body size
• Weight ratio of 2.0
• Length ratio of 1.26 (cube root of 2.0)

13
Regional Woodpeckers
Red-headed woodpecker 7.5 (7.24) Red-bellied
woodpecker 8.5 (9.13) Flicker 10.5-11
(11.5) Pileated woodpecker 15 (14.49)
Nuthatch 4-5 (4.56) Downy woodpecker 5.75
(----) Hairy woodpecker 7.5 (7.24) Y.-b.
sapsucker 7.75 (7.24)
14
Species Richness Predation

• Predator-mediated coexistence
• Generalist predator may crop many different types
  of prey, keeping numbers of all suppressed at
  same time

15
Species Richness Predation

• Net effect reduce competition between different
  prey types
• Usually leads to increased species richness
  because competitive dominants reduced
• Lawnmower, rabbit

16
Species Richness Predation

• Increased predation eventually reduces species
  diversity, as rarest species are eliminated
• Selective predators have varying effects,
  depending on prey consumed (dominant or inferior)

17
Species Richness Spatial Heterogeneity

• More heterogeneous environments provide greater
  variety of microhabitats, microclimates, hiding
  places, and so on
• More species, since it increases the extent of
  the resource spectrum

18
Species Richness Environmental Harshness

• Harsh environments are dominated by some extreme
  abiotic factor temperature, pH, salinity,
  pollution, and so on
• Few species have evolved to tolerate these
  conditions

19
Species Richness Climatic Variation

• Predictable, seasonal changes in climate may
  allow more species to persist (different species
  during different seasons)
• But more constant environments may allow for more
  specialization, and greater niche overlap

West Coast of North America
Range in mean monthly temperature
20
Species Richness Habitat Area

• Number of species on islands decreases as island
  area decreases
• Species-area relationship holds for true islands
  (a-plants on cays)
• Also other island habitats (b-birds in lakes,
  c-bats in caves, d-fish in springs)

21
Species Richness Habitat Area

• Simple explanation larger areas should have more
  species because they have more habitat types
• Larger resources spectrum (more habitat
  diversity), more niches

22
Species Richness Habitat Area

• Both habitat diversity and habitat area appear to
  be important
• One may be more important than the other, but
  which is most important varies among groups

Beetles vs. area, plants
Different species groups
23
Island Biogeography

• Equilibrium theory of island biogeography by
  MacArthur Wilson (1967)
• Island size and isolation both play important
  roles in determining number of species present on
  islands
• Number of species is a balance between
  immigration and extinction, which vary with
  island size and isolation

24
Island Biogeography Predictions

• Number of species should eventually become
  constant through time
• Continual turnover of species, extinction vs.
  immigration
• Large islands should support more species than
  small islands
• Species number should decline with remoteness
  (isolation) of an island

25
Island Biogeography
26
Island Biogeography

• Remoteness a strong influence (bird species more
  impoverished on far rather than near islands)

27
Island Biogeography

• But it takes time to establish the species
  equilibrium (new island being slowly colonized by
  new species)
• Local evolution, speciation processes also must
  be considered (fruit flies on Hawaiian islands -
  more important than immigration, extinction)

28
Species Richness Latitude

• Increase in species richness from poles to
  tropics (marine bivalves, butterflies, lizards,
  trees)
• Pattern same in terrestrial, marine, freshwater
  habitats

29
Species Richness Latitude

• Explanations
• More predation in tropics
• Increasing productivity in tropics
• Climatic stability in tropics
• Greater evolutionary age of tropics
• No perfect explanation

30
Species Richness Altitude

• Decrease in species richness with altitude
• Widespread pattern, but not universal

31
Species Richness Depth

• Decrease in species richness with depth
• Changes in light, temperature, oxygen
  availability
• Coastal regions may have lower peak - more
  environmental predictability here

Megabenthos in ocean off Ireland
32
Species Richness Fossils
marine inverts land plants insects

• Cambrian increase (predator-mediated coexistence)
• Permian decline (loss of habitats during Pangea
• Competitive displacement among plant types

amphibians reptiles mammals
33
Species Richness Fossils
Large mammalian herbivores
Africa
Australia
N. Amer.
Mad.-New Z.
34
Species Richness Alien Species
Alien species dominate many habitats
Alien flora of British Isles