Biological Communities and Species Interaction

Important Concepts:

• Critical Environmental Factors• Adaptation• Natural Selection• Speciation• Ecological Niche• Population Dynamics• Community Properties• Succession• Introduced Species

Types of Species Interactions• Competition• Predation – Trophic levels• Mutualism• Community Structure• Succession

Critical Environmental Factors

• Single factor in shortest supply relative to demand is the critical determinant in species distribution.

• Each environmental factor has both minimum and maximum levels, tolerance limits, beyond which a particular species cannot survive. No humans permanently above 5 km

Tolerance Limits

Limits of Range• Physical Barriers

Oceans (humans, cattle egrets, marsupials) Mountains (house finch) Ice (humans in the Americas)

• Climatic• Altitude• Food• Water• Competitors

Expanding Human Range

Critical Environmental Factors• For many species, the interaction of several

factors, rather than a single limiting factor, determines biogeographical distribution. Altitude = oxygen, temperature, food May be a specific critical factor that mostly

determines abundance and distribution.• Species requirements and tolerances can

also be used as useful indicators. Environmental indicators

Adaptation

Adaptation is used in two ways:• Individual (moving from

Alabama to Wisconsin)• Population (evolution)

Natural Selection

• Natural Selection - Members of a population best suited for a particular set of environmental conditions survive and produce offspring more successfully than their competitors. Acts on pre-existing genetic diversity. Limited resources place selective

pressures on a population.

Speciation

• Given enough geographical isolation or selective pressure, members of a population become so different from their ancestors that they may be considered an entirely new species.

• Alternatively, isolation of population subsets, preventing genetic exchange, can result in branching off of new species that coexist with the parental line.

Divergent vs. Convergent Evolution

• Divergent Evolution - Mutations and different selective pressures cause populations to evolve along dissimilar paths.

• Convergent Evolution - Unrelated organisms evolve separately to cope with environmental conditions in the same fashion. Look alike - Act alike Usually means some physical basis

Ecological Niche

• Habitat - Place or set of environmental conditions where a particular organism lives.

• Ecological Niche Role a species plays in a biological community

(e.g. large grassland herbivore) Total set of environmental factors that

determines a species’ distribution. Generalists - Broad niche Specialists - Narrow niche

• When generalists and specialists collide, generalists usually win.

Competition

Law of Competitive Exclusion

• No two species will occupy the same niche and compete for exactly the same resources for an extended period of time.

• One will either migrate, become extinct, or partition the resource and utilize a sub-set of the same resource.

• Given resource can only be partitioned a finite number of times.

Resource Partitioning

Predation

• Feeds directly upon another living organism, whether or not it kills the prey in doing so. Mosquitoes prey on humans

• Prey most successfully on slowest, weakest, least fit members of target population. Reduce competition, population

overgrowth, and stimulate natural selection.

Co-evolution (arms race)

Co-Evolution and Disease

• If a disease kills too quickly, it can’t spread• Disease can moderate while host becomes

more resistant (measles)• Disease can be lethal but messy (cholera,

ebola)• Disease can be lethal but slow-acting (AIDS)

Keystone Species• Keystone Species - A species or group of species

whose impact on its community or ecosystem is much larger and more influential than would be expected from mere abundance. Large predators Critical food organisms (bamboo and pandas) Often, many species are intricately

interconnected so that it is difficult to tell which is the essential component.

Competition

• Interspecific - Competition between members of different species.

• Intraspecific - Competition among members of the same species. Often intense due to same space and

nutritional requirements.- Territoriality - Organisms defend specific

area containing resources, primarily against members of own species.

- Resource Allocation and Spacing

Mutualism

Intimate living together of members of two or more species.

• Commensalism - One member benefits while other is neither benefited nor harmed. Cattle and Cattle Egrets

• Symbiosis - Both members benefit. Lichens (Fungus and cyanobacterium)

• Parasitism - One member benefits at the expense of other. Humans and Tapeworms

Commensalism: Epiphytes:

Symbiosis - Lichens

Defensive Mechanisms

• Batesian Mimicry - Harmless species evolve characteristics that mimic unpalatable, dangerous or poisonous species Viceroy and Monarch butterfly

• Mullerian Mimicry - Two unpalatable species evolve to look alike Bees and Wasps

• Camouflage• Advertising and warning (coral snake)• Attracting prey, pollinators, mates, etc.

Abundance and Diversity

• Abundance -Total number of organisms in a community.

• Diversity - Number of different species, ecological niches, or genetic variation. Abundance of a particular species often

inversely related to community diversity. As general rule, diversity decreases and

abundance within species increases when moving from the equator to the poles.

Productivity• Primary Productivity - Rate of biomass

production. Rate of solar energy conversion to chemical energy. Net Primary Productivity - Energy left after

metabolism Highest in rain forest, estuaries, reefs Decreases toward poles Open oceans very low

Trophic Level (Food Chain)

• A pond Phytoplankton Zooplankton Small Fish Larger Fish Higher predators (birds, mammals)

• Organisms are at same trophic level if they get their food from similar sources

Trophic Level (Food Chain)

• A forest Decaying organic matter Insects Small mammals and birds Higher predators (owls, foxes, bears)

• A Pasture or Grassland Grass Herbivore Higher predators

Trophic Level (Food Chain)

• At each level, some matter goes into biomass

• Most goes into energy and metabolism• Hence each level needs about 10x as much

energy, has fewer individuals• Bio-Accumulated chemicals get more

abundant higher up the food chain

Food Requirements• Warm-blooded organisms require more food

than cold-blooded Predator/prey ratio higher for cold-blooded Indication that some dinosaurs may have

been warm-blooded• Large organisms eat less in proportion to

their mass than small ones Shrew: 100%+ per day Human: 1% per day

Improbable Movie Biology

• Things that eat people (Morlocks, The Time Machine)

• Really huge carnivores (The Phantom Menace)

• Huge carnivores in empty environments (Empire Strikes Back, Return of the Jedi)

• Ultra-voracious carnivores (Jaws, Alien, Anaconda, Jurassic Park)

Complexity and Connectedness

• Complexity - Number of species at each trophic level, and the number of trophic levels, in a community. Diverse community may not be complex if

all species are clustered in a few trophic levels.

Highly interconnected community may have many trophic levels, some of which can be compartmentalized.

Resilience and Stability

• Constancy (Lack of fluctuation)• Inertia (Resistance to pertubation)• Renewal (Ability to repair damage)

MacArthur proposed complex, interconnected communities would be more stable and resilient in the face of disturbance.

- Controversial

Edges and Boundaries

• Edge Effects - Important aspect of community structure is the boundary between one habitat and others.

• Ecotones - Boundaries between adjacent communities. Sharp boundaries - Closed communities Indistinct boundaries - Open communities

COMMUNITIES IN TRANSITION

• Ecological Succession Primary Succession - A community begins

to develop on a site previously unoccupied by living organisms.

- Pioneer Species Secondary Succession - An existing

community is disrupted and a new one subsequently develops at the site.

Terrestrial Primary Succession

Ecological Succession

• Ecological Development - Process of environmental modification (facilitation) by organisms.

• Climax Community - Community that develops and seemingly resists further change. Equilibrium Communities (Disclimax

Communities) - Never reach stable climax because they are adapted to periodic disruption.

Introduced Species

• If introduced species prey upon or compete more successfully than native populations, the nature of the community may be altered. Human history littered with examples of

introducing exotic species to solve problems caused by previous introductions.

- Mongoose and Rats in Caribbean

Summary:

• Critical Environmental Factors• Adaptation• Natural Selection• Speciation• Ecological Niche• Population Dynamics• Community Properties• Succession• Introduced Species