Classical metapopulation model Area effects How does island area - - PDF document

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Classical metapopulation model

Fraction of patches occupied Rate Colonization Extinction

Equilibrium metapopulation size

Area effects

P Ss Sl

How does island area affect the number species on the island? Cs Es El Island species number Rate Cl How does size affect colonization rate? Ss Sl Ssmall < Slarge

Distance effects

How does distance from the mainland affect the number species on the island? How does size affect colonization rate?

P Sn Sf

Cf E Cn isn Rate Sf Sn Sfar < Snear S

What explains total diversity in a community?

Keystone species can influence diversity Equilibrium theory of island biogeography Disturbance

Disturbance

An event that moves the community away from its equilibrium state

Disturbance events can be caused by biotic (e.g. disease) and abiotic factors (e.g. fire)

An event that moves the community away from its equilibrium state

Disturbance Equilibrium state?

In Southeast = long-leaf pine forest In Midwest = tall grass prairie

Disturbance vs. Equilibrium

When disturbances are very frequent, then equilibrium may never happen. Some species specialize on disturbance

Furbishs lousewort

Succession

the process by which a community moves towards its equilibrium state following a disturbance

What is the relationship between disturbance and species diversity?

How does diversity vary with the frequency of

• disturbance?

How does species diversity vary with the time since disturbance? How does species diversity vary with the magnitude

• f disturbance?

What explains total diversity in a community?

Keystone species can influence diversity Equilibrium theory of island biogeography Disturbance Productivity

Productivity

amount of sunlight turned into primary producers (plants, algae, etc), often estimated using biomass

• f primary producers

Many empirical studies have found a hump-shaped relationship between the productivity of a system and the number of species in that system

Productivity vs Species richness

Biomass Species richness

Habitat heterogeneity: the relationship between species richness and biomass varies among microhabitats

Productivity and Biodiversity

Habitat heterogeneity: the relationship between species richness and biomass varies among microhabitats Competition

Productivity and Biodiversity Number of species Biomass

Increase in productivity allows for coexistence

• f more

species Competition begins to remove less competitive species

What explains total diversity in a community?

Keystone species can influence diversity Equilibrium theory of island biogeography Disturbance Productivity Habitat heterogeneity

Habitat heterogeneity and Biodiversity

Ecosystems with more heterogeneous habitats have more potential niches, allowing the coexistence of more species.

Habitat heterogeneity Number of species

Habitat heterogeneity and Biodiversity

Foliage height diversity Bird species diversity

• MacArtur and MacArthur (1961)

found that the bird diversity of a habitat increased with the complexity of the habitat’s vegetation

• Similar relationships have been

demonstrated in other taxa

What explains total diversity in a community?

Keystone species can influence diversity Equilibrium theory of island biogeography Disturbance Productivity Habitat heterogeneity

Is biodiversity important for ecosystem structure and function?

Do ecosystems with high species diversity “function” better? Are ecosystems with high species diversity more stable? Species richness Ecosystem function Species richness Ecosystem function Species richness Ecosystem function Species richness Ecosystem function

No relationship More diversity is more stable Redundancy Idiosyncratic

Do ecosystems with more species function better?

Empirical evidence shows that in many ecosystems there is a positive relationship between productivity and species

• richness. But some studies show that

there is either no correlation or a negative correlation.

Do ecosystems with high species diversity “function” better ?

What do the empirical data tell us?

• 1. Experiments in The Ecotron:

The Ecotron is facility designed to establish simplified experimental communities

Do ecosystems with high species diversity “function” better ?

What do the empirical data tell us?

• 1. Experiments in The Ecotron

Naeem et al. (1994) created communities with 3 levels of biodiversity (low, medium, and high) and examined the relationship between biodiversity and ecosystem function in these artificial communities.

Do ecosystems with high species diversity “function” better ?

What do the empirical data tell us?

• 1. Experiments in The Ecotron

low medium high

Time % Change in vegetation cover

• High biodiversity communities

had denser canopies and higher photosynthetic rates

• low diversity communities also

consumed less CO2

Are ecosystems with more species more stable?

Tilman and Downing (1994) Biodiversity and stability in grassland. Nature 367: 363-365.

METHODS

Experimental design. In a 7-ha field at Cedar Creek Natural History Area, Minnesota, USA, we controlled the number of plant species in 168 plots, each 9 m £ 9 m. Plots were randomly assigned to be seeded with 1, 2, 4, 8 or 16 perennial grassland species, with 39, 35, 29, 30 and 35 replicates, respectively, of the diversity levels. The composition of each plot was randomly chosen from a set of 18 perennials (four C4 grasses, four C3 grasses, four legumes, four non- legume forbs and two woody species). All plots received 10 g m22 of seed in May 1994 and 5 g m22 in May 1995, with seed mass divided equally between species.

Are ecosystems with more species more stable?

Biomass varies less from year to year in plots with high species richness

Minnesota grassland plot experiment

Coefficient of variation = standard deviation mean

Are ecosystems with more species more stable?

Minnesota grassland plot experiment

Relationship between drought resistance of vegetation in a Minnesota grassland and plant species richness prior to the

• drought. Drought resistance was

measured as the log of the ratio

• f plant biomass at the height of

the drought to plant biomass before the drought. Data are shown as means + SE (redrawn from Tilman and Downing 1994).

Are ecosystems with more species more stable?

Minnesota grassland plot experiment: resource usage

Species richness Resource availability

Tilman et al (1996, 1997) examined the effect of species diversity on productivity and soil nutrients.

Are ecosystems with more species more stable?

Minnesota grassland plot experiment: resource usage Plots with more species less nitrogen in their soil lower resource availability

Are ecosystems with more species more stable?

Species richness Resource availability Species richness Resistance to invasions Hypothetical relationship between species richness and invasion resistance Species rich communities are less susceptible to invasion because they use more of the available resources.

Are ecosystems with more species more resistant to invaders?

Species Diversity and Invasion Resistance in a Marine Ecosystem

John J. Stachowicz, Robert B. Whitlatch, Richard W.

• Osman. 1999. Science 286:1577-1579

Theory predicts that systems that are more diverse should be more resistant to exotic species, but experimental tests are needed to verify this. In experimental communities of sessile marine invertebrates, increased species richness significantly decreased invasion success, apparently because species-rich communities more completely and efficiently used available space, the limiting resource in this system. Declining biodiversity thus facilitates invasion in this system, potentially accelerating the loss of biodiversity and the homogenization of the world's biota.

Is biodiversity important for ecosystem function?

Some studies show that species rich communities are more productive, but some other studies show alternative interpretation. Some studies show that species rich communities are more stable and recover from disturbances faster and are less vulnerable to invasive species. More studies are needed to allow generalizations beyond some model systems.