EVOLUTION, BIOLOGICAL COMMUNITIES, & SPECIES

INTERACTIONS

OBJECTIVES

• Examine specific ways organisms are limited by the physical aspects of their environment.

• Discuss how members of a biological community interact.

• We will answer questions like why does a particular species live where it does? How is it able to live there? How does it deal with the physical resources of its environment? How does it interact with others? What gives one species an edge over another?

Lamark’s Theory of Evolution

• Believed that organisms acquired characteristics throughout life and then passed these onto their offspring.

• “Inheritance of acquired characteristics”

• If giraffes stretch during life to reach trees, then their offspring will have long necks.

• Does a body builder produce a baby with bulky muscles?

Darwin’s Theory of Evolution• Collected specimens on voyage of

the Beagle.• Returned to England & over 40

years developed his theories on natural selection & evolution.

• Used Thomas Malthus’ Essay on the Principle of Population to help him develop his theories.

• His theory of evolution by natural selection is…– Most organisms produce more

offspring than can survive.– Those with better attributes will

survive, others will die– Those that survive pass on fit traits

to offspring resulting in entire population with that fit trait.

• Presented his theory in book, On the Origin of Species in 1859.

How would Darwin describe Giraffe evolution?

• Some giraffes had long necks, some short

• Those with long necks reached the food, those with short starved

• Long necks mate and produce population of long necked giraffes.

What do Darwin’s theories assume?

• Different traits or properties must be present in a population

• These traits must affect reproductive success

• The traits must be genetic• Some selective pressure

must favor these traits differently.

• We now know that these traits can be the result of mutations in DNA

What types of selective pressures can influence a population?

• Limiting factors determine distribution and population size of different species

– Temperature, moisture, nutrient supply, soil, water chemistry, predation, competition

• Tolerance limits- minimum & maximum limits for an organism

• Critical factor- usually one specific limiting factor that limits populations

– Ex: Saguaro cactus very sensitive to low temperatures.

• Many species of plants & animals are so sensitive to environmental change, they can be used as environmental indicators to determine health or components of an ecosystem

– Ex: trout sensitive to pollution so presence or absence of them indicates water quality; Lichens indicate air pollution/acid rain problems

Can evolution be forced by humans?

• Selective breeding for traits in plants & animals

• Pesticide usage has led to the rapid evolution of resistance in insects

• Overuse of antibiotics has led to evolution of microbes

Now that we know why organisms are different, lets discuss how they

interact…• Everything has a habitat- place

where it lives.• Everything has a niche- or role in

its habitat. How it gets food, interacts with other populations, or service it provides to community.

• Generalists- wide range of food & habitats (raccoons, cockroaches)

• Specialists- specific food sources & habitats (pandas); less resilient to changes in environment

What is the Law of Competitive Exclusion?

• No two species will occupy the same niche and compete for exactly the same resources in the same habitat for very long.

• Eventually, one species will outcompete the other and push the weaker species out of the habitat, or cause its extinction.

• Could result in two insectivores feeding in same tree, just at different times of the day.

More on competition…• Compete for food, chemical nutrients in

soil, space, mates, etc.• Intraspecific- competition within a

species– Trees provide shade so their own

seedlings can’t grow underneath them so no competition for sun

– Caterpillars eat leaves while adult butterflies eat nectar. Reduces competition for food sources

– Some create a territory to prevent competition

• Interspecific- competition between two different species– Ex: when different weeds produce

numerous seeds & race to cover most ground

– secrete substance that inhibits root growth

– different herbivores on savanna compete for grasses

Besides competition, how else do species interact?

• PREDATOR-PREY• Predator- an organism

that feeds directly upon another living organism, may or may not kill prey

• Includes herbivores, carnivores, omnivores

• Help force evolution of prey species

• Leads to coevolution- exert selective pressure on each other- evolve in response to each other

How else do species interact?

• PARASITE-HOST• Parasites feed directly

on another living thing but do not kill it in the process.

• Ex: ticks suck blood from dog

How else do species interact?

• COMMENSALISM• one member benefits, the

other is not substantially benefited or harmed.

• Ex: – cattle egrets feed on

insects stirred up by grazing cattle

– Clownfish gets protection from sea anemone which is not helped or harmed by fish.

– Bromeliads grow on tree branches to reach sunlight

How else do species interact?

• MUTUALISM• Both members benefit

significantly from the relationship

• Ex:– Lichens- combo of fungus

(provides water & place for algae to grow) and algae (provides food for both)

– Insects & their pollinators– Ants and acacia trees

How else do species interact?

AMENSALISM

• is a relationship in which a product of one organism has a negative effect on another organism

• interaction in which one organism is harmed, while the other is unaffected, and not benefited

How else do species interact?

• KEYSTONE SPECIES• A species or group of species whose

impact on a community are much larger than would be expected from mere abundance.

• Ex: sea otters & kelp beds; tropical figs & frutivores (fruit eaters)

• Keystone species increase niche diversity because without them other species cannot survive- may either move to new habitat or become locally extirpated

How do organisms defend themselves against predation & parasitism?

• Chemical defense- smell bad or taste bad

• Batesian mimicry– Harmless species mimics

harmful– Ex: monarch butterfly &

viceroy butterfly; moth has eyespots, looks like owl

• Mullerian mimicry– Two species that are harmful

develop same patterns/colors– Most wasps have yellow/black

banding patterns

• Camouflage- look like dead leaves or twigs or even bird poop

What are some fundamental properties of biological communities?

• Productivity

• Diversity

• Complexity

• Resilience

• Stability

• Structure

Productivity• Rate of biomass production which

indicates the rate of conversion from sun to food.

• Energy left after respiration (breaking down food to get energy) is called net primary productivity

• Needs for productivity:– High light levels– High temperature– High moisture– High nutrient availability

• See Figure 4.18• Tropical rainforests, estuaries, and

coral reefs all have high levels of productivity because all have met requirements listed above.

Abundance & Diversity

• Abundance- total # of organisms in a biological community

• Diversity- # of different species in a community.

• Usually this is an inverse relationship…

• Rainforest is very diverse, but only a few individuals of each species.

• Tundra is overflowing with flying insects, but usually only a couple of different species.

Complexity

• Number of species at each trophic level and number of trophic levels in a community

• Diverse community may not be complex if all species are located at 1 or 2 trophic levels

• Some complex communities have specially adapted populations which helps to reduce competition– Ex: herbivores are grouped

based on what part of plant they eat- fruit, seed, root, leaf

Resilience & Stability

• 3 kinds– Constancy- lack of fluctuation– Inertia- resistance to change– Renewal- ability to repair

damage after disturbance

• The more complex & interconnected a community is, the more stable and resilient it will be when faced with a disturbance. Should be able to recover easily from disturbance.

• Can diversity make an ecosystem less resilient rather than more?

Community Structure• Structure- spatial distribution of

individuals & populations & how they relate to environment

• Random distribution-– Live where resources are available

• Clumped distribution– Populations are clustered– Protection, reproduction, mutual

assistance– Ex: hunt in packs, schools of fish,

mating season causes clumping• Uniform distribution

– Usually result of competition & territoriality; nesting sites, roots of sagebrush release toxins to keep other plants away from it

• Vertical distribution– Animals feed at different levels to

reduce competition– Tropical rainforest, African savanna,

aquatic ecosystems

Community Structure

• Ecotones- boundary between one habitat and another.

• From forest moving toward field- closed community

• If edge is less distinct, animals move in & out of each habitat- open community

Community Structure

• Shape of environment is as important as size.

• Some animals need a large “core” for protection, establish territories, feeding, etc.

• Some environments are irregularly shaped- fragmented- which reduces cover for animals

How do communities come to exist?

• Succession- process by which organisms occupy a site and gradually change environmental conditions by creating soil, shade, shelter, or increasing humidity.

Primary Succession

• Occurs when a community begins to develop on a site previously unoccupied by living organisms

• Ex: volcanic island, glacier retreating

• Pioneer species- lichens, moss, microbes that don’t need soil to grow.

Secondary Succession

• When an existing community is disrupted and a new one develops at the site

• Ex: deforestation, fire, flooding, mining, natural disaster, fallow crop land

• Pioneer species- grasses, annuals

Primary vs. Secondary• Primary succession

– Lichens, mosses, break down rocks create soil

– Annuals, wildflowers that have lightweight seeds, tolerate sun & exposed soil, soil thickens

– Perennials, grasses, shrubs, outcompete annuals which die, soil thickens

– Sun-loving evergreen trees move in, outcompete grasses for sun, soil thickens

– Evergreen seedlings cannot compete with parent trees, shade-loving deciduous trees move in, outcompete evergreens and take over.

– Climax community- community that resists change; has all plants & animals can hold.

– Takes a long time because soil has to be established

• Secondary Succession– Soil already established– Annuals, wildflowers that have

lightweight seeds, tolerate sun & exposed soil, soil thickens

– Perennials, grasses, shrubs, outcompete annuals which die, soil thickens

– Sun-loving evergreen trees move in, outcompete grasses for sun, soil thickens

– Evergreen seedlings cannot compete with parent trees, shade-loving deciduous trees move in, outcompete evergreens and take over.

– Climax community- community that resists change; has all plants & animals can hold.

– Takes less time- soil is already formed

Abiotic factors & succession

• As plants move in, the abiotic factors within an ecosystem change- this leads to changes in more biotic factors.

• Biomass accumulates & the ecosystem becomes more rich, able to capture more moisture, sheltered from wind/climate, biologically more complex.

• As plants colonize, animals are attracted.

Fire-climax community

• Community that is shaped or maintained by fires

• Grasslands, chaparral, coniferous forests

• Often called equilibrium communities or disclimax communities

How are organisms adapted to fire climax communities?

• Resistant to some degree to fire

• Reseed quickly after fire

• Need fire to pop open seed coating (jack pine)

• Reduces competition

What can threaten biological diversity?

• Introduced species- species not native to a particular area.

• Usually outcompete or prey on native species

• Usually do not have a natural predator to limit their populations.

Introduced Species

• Sometimes escape from ships onto islands…

• Livestock outcompete native animals & plants

• Trample & compact soil• Native island organisms

have not evolved defensive mechanisms against predators

• EX: goats, cats invading bird nests, pigs & rats digging up sea turtle eggs, invade nests

In Hawaii, Silversword plant became endangered when feral pigs, goats, & sheep overconsumed the plant

Introduced Species

• Sometimes introduced to solve problems in an ecosystem- but cause problems instead.

• Mongoose introduced in Hawaii to kill rats, rats are nocturnal, mongoose is diurnal so they did not interact. Instead mongoose contributed to problem by killing native birds instead of rats.

Introduced Species

Cane Toads-• Introduced in Australia to

control sugar cane beetles in 1935

• Prolific breeder• Has poison glands so kills

or sickens anything that might prey on it.

• Has outcompeted native amphibians, killed native predators.

Other invasive species…

• Zebra mussels• Snakehead fish• Kudzu• Dandelions

Zebra mussels covering crayfish