Two things

• Elephants

And

• The new book

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Chapter ( - continued - Population

and community structure and food

web structure

• Are there food web structures that are more stable than others?

• What is stability?

• A resilient vs a resistant community

• Resilient: returns rapidly to former state

• Resistant: little change

• A fragile vs a robust stability

• Fragile: remains unchanged but alters completely in large disturbance

• Robust: remains roughly the same in the face of larger disturbances

• Stability: in the face of disturbance

• Disturbance – loss of one or more populations from a community

Keystone species

• removal of this species would lead to significant changes throughout the food web, producing a community with a very different species composition

• one whose impact is disproportionately large relative to its abundance

• Can occur at any trophic level (not just predators)

• Conventional wisdom: increased complexity = increased stability

• But mathematical models do not support this argument• Conclusions differ depending on

whether we focus on individual populations within a community or on aggregate properties of the community (such as their biomass or productivity)

Food webs…

1. Number of species they contain

2. Connectance of the web (fraction of all possible pairs of species that interact directly with one another)

3. Average interaction strength between pairs of species

• Increases in # of species, increases in connectance, and increases in average interaction – all tend to decrease the tendency of individual populations within the community to return to their former state following a disturbance (resilience)

• Thus – community complexity leads to population instability

• What is found in real communities – and not in models?

• Keep in mind that:• The only

communities we can observe are those that are stable enough to exist

• Data on interaction strength for whole communities are unavailable – so assume contant

• Recent studies:• Connectance may

decrease with species number …or…

• May be independent of species number … or …

• May even increase with species number

• Conclusion?• Stability argument

does not receive consistent support from food web analyses either

Summary of this chapter

• there are multiple determinants of the dynamics of populations

• Dispersal, patches and metapopulation: movement can be vital factor in determining and/or regulating number

• There are temporal patterns in community composition

• No predator-prey, parasite-host or grazer-plant pair exists in isolation

Chapter 10

-- review --

• Every population exists within a web of interactions• with other populations • across several trophic levels

• Each population must be viewed in the context of the whole community

• Populations occur in patchy and inconstant environments in which disturbance and local extinction may be common

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…to add…

• Need to appreciate that the world’s biological diversity is becoming increasingly important

• Need to understand why species richness varies widely across the face of the Earth

• Why do some communities contain more species than others? Are there patterns or gradients in biodiversity? If so, why?

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Species richness…• The number of species in a community

• Why is it difficult to count or list the species in a community?• Taxonomic inadequacies• Only a proportion of the organisms can usually be counted

• Number of species recorded…• Depends on the number of samples that have been taken or on

the volume of the habitat that has been explored• But…the most common species are likely to be represented in the

first few samples… more samples -> rarer species will be added• ? – at what point does one stop taking further samples?

• Until the number of species reaches a plateau

• Species richness of different communities should be compared only if• They are based on the same sample sizes

• Area of habitat explored• Time devoted to sampling• Number of individuals included in the samples

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Diversity indices

• Species richness• two communities with ten species…but

• Diversity indices• Combines both species richness and

evenness or equitability of the distribution of individuals among those species

• Calculated by determining for each species, the proportion of individuals or biomass that that species contributes to the total in the sample

• Diversity then increases with equitability, and for given equitability, diversity increases with species richness

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Add to those factors…

• (1) interspecific competition• If a community is dominated by interspecific

competition, the resources are likely to be fully exploited• Species richness will then depend on the range of

available resources (specialists and niche overlap)

• (2) predation• Exerts contrasting effects• Predators can exclude certain prey species

community can be less than fully saturated (unexploited resources) predation may reduce species richness

• Or predation may keep species below k – reducing intensity and importance of competition for resources more niche overlap and a greater richness of species

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…and other factors…

• Factors that vary spatially• Productivity• Predation intensity• Spatial heterogeniety• Environmental ‘harshness’

• Factors that vary with time (temporal variation)• Climatic variation• Disturbance• Evolutionary age

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Spatially varying factors

Productivity and resource richness

• For plants: productivity of the environment can depend on most limiting element (nutrient or condition)

• Animals: broadly speaking, productivity of environment for animals follows the same trends as for plants• Why?• Due to changes in resource levels at the

base of the food chain

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Productivity and richness

• Expect species richness to increase with productivity• Supported by an analysis of species richness of trees in

North America in relation to a measure of available environmental energy, potential evapotranspiration (PET)• Amount of water that under prevailing conditions would

evaporate or be transpired under a saturated surface• Water and energy

• Energy: heat and light• Higher energy inputs lead to more ET and more water

• Southern African trees – species richness increased with water availability (annual rainfall) but first increased and then decreased with available energy

• What about animals?• Animal species richness positively correlated with crude

atmospheric energy

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Productivity and species richness

• Why?

• Not sure… but• For an ectotherm (eg – a reptile) – extra

atmospheric warmth increase intake and use of food resources

• For an endotherm (eg – bird) – extra warmth less use of resources in maintaining body temp and more resources for growth and reproduction

• In both cases faster individual and population growth larger populations

• Warmer environments might -> allow species with narrower niches to persist and such environments may support species

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Productivity and species richness

• May also be a direct relationship between animal species richness and plant productivity

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• Strong positive correlations between species richness and precipitation for both seed-eating ants and seed-eating rodents; more species of very large ants and very small ants. Why?

• Species richness of fish also increases with lake’s phytoplankton productivity

• Note: increase in production does not always lead to increase in diversity

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Predation intensity

• Predation may increase richness• By allowing competitively inferior species

to coexist with their superiors (predator-mediated coexistence)

• Predation may reduce richness• Intense predation may drive prey species

to extinction

• Overall: predation intensity and species richness: humped relationship• Greatest richness at intermediate

intensities

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• Removed starfish from an 8 m long and 2 m deep shoreline

• Other species became dominant (barnacles and then mussels)

• Later – reduction in species richness

• Predator-mediated coexistence

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Wolves…• The gray wolf was reintroduced to Yellowstone National

Park in the spring of 1995, after a 70-year absence. In the past 6 years, the population has grown from 31 released animals to more than 100 individuals, as wolves have exploited an abundant elk population. Consequently, elk that had previously experienced significant mortality primarily in the late winter because of starvation now face mortality throughout the year. When an elk is killed by wolves, its carcass is partially consumed by the wolves and then is scavenged extensively by eight other carnivore species (coyote, bald eagle, golden eagle, grizzly bear, black bear, raven, magpie, and red fox) and less intensely by up to 20 other species. Field observations indicate that the infusion of wolf-killed ungulate carrion throughout the year has created an abundant and dependable food source for these other carnivores.

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Spatial heterogeneity

• Environments – more heterogeneous – more potential for species richness. Why?

• Some studies: positive relationship between plant diversity and heterogeneity• Substrate…slope… drainage … soil pH

• Most studies: related species richness of animals to structural diversity of plants in their environment• Due to experimental manipulation of plants• Or through comparisons of natural communities

that differ in plant structural diversity• Or plant species richness

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Environmental harshness

• Environments dominated by extreme abiotic factor (harsh environments) more difficult to recognize than apparent

• Anthropocentric view. What is cold for us might seem benign to a penguin

• ‘let the organism decide’ harshness factor

• Environment extreme – if organisms, by their failure to live there, show it to be so. Circular definition.

• Environment extreme: one that requires, of any organism tolerating it, a morphological structure or biochemical mechanism that is not found in most related species and is costly, either in energetic terms or in terms of the compensatory changes in the biological processes of the organism that are needed to accommodate it• Example: plants living in highly acidic soil (low pH). Need specific

structures/mechanisms

• Harsh environments: low species richness• Hot springs. Caves. Highly saline water (Dead Sea)• Each also categorized by low productivity and low spatial heterogenity

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Temporally varying factors

• Climatic variation• Is the variation predictable? (relative time scales) …

• temporal niche differentiation• Opportunities for specialization in a non-seasonal

environment

• Is the variation unpredictable? – climatic instability• Could increase species richness …or not

• Does species richness increase as climatic variation decreases?

• Disturbance

• Environmental age: evolutionary time

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Temporally varying factors

• Disturbance• Reminder: In a dominance-controlled community

community succession• Very frequent disturbances: keep most patches in

early stages of succession (few species) + rare disturbances allow patches to be dominated by best competitors (few species)

• Conclusion: intermediate disturbance hypothesis• All communities are subject to disturbances that

exhibit different frequencies and intensities

• Environmental age: evolutionary time

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Temporally varying factors

• Environmental age: evolutionary time• Communities may differ in species richness because some are

closer to equilibrium and therefore more saturated than others• Some argued that tropics are richer than temperate in part

because tropics have existed over long and uninterrupted periods of evolutionary time (temperate regions – recovering from glaciations)• But tropical areas were also disturbed during the ice ages by

associated climatic changes that saw tropic forest limited to a small number surrounded by grassland

• Another explanation: species evolve faster in the tropics because of higher rates of mutation in these warmer climes• Rates of evolution of woody plant species twice as fast in the

tropical species

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Gradients of species richness

• Habitat area and remoteness: Island biogeography

• Latitudinal gradients

• Gradients with altitude and depth

• Gradients during community succession

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Island biogeography

• Number of species on islands decreases as island area decreases. Species-area relationship

• What is an island?• Islands of land in a sea of water• Lakes - Islands in a sea of land• Mountaintops – high altitude islands in a low

altitude ocean• Gaps in a forest canopy – island in a sea of trees• Islands of particular geological types, soil types,

or vegetation types

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Island biogeography

• Species-area relationships: one of the most consistent of all ecological patterns

• Is the impoverishment of species on islands more than would be expected in comparably small areas of mainland?

• Why do larger areas contain more species?• Encompass more different types of habitat –

not enough of an explanation• Island size and isolation: number of species

on an island determined by a balance between immigration and extinction; this balance is dynamic; and extinction rates may vary with island size and isolation

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Island biogeography

1. The number of species on an island should eventually become roughly constant through time

2. This should be a result of continual turnover of species, with some becoming extinct and others immigrating

3. Large islands should support more species than small islands

4. Species number should decline with the increasing remoteness of an island

- MacArthur and Wilson’s theory’s predictions

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Latitudinal gradients• Increase in species

richness from the poles to the tropics

• Why?

• No clear explanation.

• Latitudinal gradient intertwines components previously discussed…

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Gradients with altitude and depth

• Decrease in species richness with altitude – akin to that observed with latitude - -in terrestrial environments

• Other studies: increase with altitude; other studies: hump-shaped patterns

• Productivity and temperature? Productivity and growing season? Stress with extremes?

• In aquatic environments: change in species richness with depth strongly similar to terrestrial gradient with altitude

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• Skip – patterns in taxon richness in the fossil record

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Summary

• Richness and diversity

• Productivity and resource diversity

• Predation intensity

• Spatial heterogeneity

• Environmental harshness

• Climatic variation

• Disturbance

• Environmental age

• Island biogeography

• Gradients in species richness

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