Ecology• Ecology is the study of how organisms interact

with each other and with their environment• Structure • Function

Ecology

• Biotic = Living– Examples: Predation, competition,

reproduction (mating)

• Abiotic = Non-living (Energy & Inorganic)– Examples: Temperature, pH, light, water,

nutrients

Levels of Ecological Organization

• Populations• Communities• Ecosystems

Levels of Ecological Organization: Population

• A group of individuals of the same species living together in one area; interbreeding

Levels of Ecological Organization: Population

• A group of individuals of the same species living together in one area; interbreeding

Levels of Ecological Organization: Community

• Populations of different species living together in one area; naturally-occurring

Levels of Ecological Organization:Ecosystem

• Communities and non-living components of the environment in which they interact

Population Structure• Population size – number of individuals• Population density – number of individuals

per unit space (area)• Population dispersion – how individuals are

distributed

Population Size

• Number of individuals• Affects population’s ability to survive• Too small – population more likely to go

extinct• Too large – increased competition (may be

bad for individuals but can lead to natural selection!)

Population Density

• Number of individuals per unit area• Affects population’s ability to survive• Too dense – increased competition (e.g., for

resources, food, mates), can lead to reduced fitness, increased potential for disease

• Too sparse – can be problematic; must find mates! Also, there is safety in numbers, can lead to increased predation

Population Distribution

• How individuals are distributed• Nature is patchy!• Differences in light, moisture, temperature,

wind, etc. exist within a given ecosystem• Distribution may be random, uniform, or

clumped

Random• Random – position of each individual is

independent of the others• Dispersal depends on resource distribution• Not common in nature

Uniform• Uniform – individuals are evenly spaced• Usually a result of competition for resources• Spacing is frequently accomplished by territorial

behavior

Uniform

• Example: The uniform distribution of cactus resulting from allelopathy

http://www.marietta.edu/~biol/biomes/desert.htm

Clumped• Clumped – individuals are aggregated in

patches• Occurs frequently in response to uneven

distribution of resources• Common in nature• Many advantages

Clumped• Social interactions may also lead to clumped

distribution• Safety in numbers• Shared knowledge of others in group• Decreased energy in movement• Other advantages?

Metapopulation

• A network of distinct populations that interact with one another by exchanging individuals (immigration and emigration)

• May prevent long-term extinction• Source-sink metapopulations – population in

better-suited habitats (source) disperse new individuals to populations in poorer habitats (sink)

• Marine protected habitats; Whales?

Population Dynamics

• Survivorship – percentage of original population that survives to a given age

• Survivorship Curve – a graphical representation of the survivorship at each age

• Depicts age-specific mortality through survivorship

• Type I, II, and III

Survivorship Curves

• Plots the number of individuals of a particular age cohort against time

• Compares populations of one area, time, sex, or species with populations of another

• Three types of curves: Types I, II, and III

Survivorship Curve – Type I

• Convex• Typical of populations whose individuals tend

to live out their physiological life span (high degree of survival at all ages, but experience heavy mortality at the end of their life span)

• Examples – Red Deer, Humans, Annual Phlox (flowering plant)

Survivorship Curve – Type II

• Linear• Typical of organisms with constant mortality

rates (mortality does not change with increasing or decreasing age)

• Examples – adult stages of many birds, rodents and some plants

Survivorship Curve – Type III

• Concave• Typical of organisms with extremely high

mortality in their early stages of life• Examples – many species of invertebrates,

fish, some plants

Survivorship Curve – Type III

Survivorship Curves

Population Growth

• Populations typically maintain a constant size regardless of how many offspring are born

• No matter how rapidly populations may grow, they eventually reach their carrying capacity (K) – the maximum number of individuals that the population can support

• Limits include shortages of water, habitat, light, nutrients, etc.

Population Growth

• The rate of population growth (r) is defined as the difference between the birthrate (b) and the death rate (d) and corrected for the movement of individuals into (i) and out of (e) the population

• Population growth (r) = (b - d) + (i – e)

Population Growth

• Biotic potential – the rate at which the population will increase when no limits are placed on its rate of growth

• Defined as: dN/dt = riN N = number of individuals in the populationdN/dt = the rate of change in its numbers over timeri = the intrinsic rate of natural increase for the

population

Population Growth

• Biotic potential is exponential; the rate of increase remains constant, the actual number of individuals accelerates rapidly as the size of the population grows

• Result of unchecked exponential growth is a population explosion

Population Growth• As a population approaches its carrying capacity,

its rate of growth slows• Logistic growth equation:

dN/dt = rN ((K – N)/ K)

dN/dt = the growth rate of the populationr = the intrinsic rate of increaseN = the number of individuals present at any given time(K – N)/K = the unused carrying capacity

Two Models of Population Growth

• Human populations exhibit exponential growth

• Substantial growth occurred following the industrial revolution (late 1800’s) despite major ‘collapses’ in earlier history

Limits to Population Growth

• Resource limitation– Food, Habitat

• Predation• Parasitism, Disease• Anthropogenic Impacts – invasive species,

habitat destruction, pollution, hunting• How about a little exercise???

Community Ecology

• Study of interactions among populations• Niche

– the functional role of a species in the community, including activities and relationships

– total of all the ways an organism uses resources in its environment

– Food consumption, space utilization, temperature range, etc

Fundamental and Realized Niche

• Fundamental niche – the entire niche that a species is capable of using, based on its physiological tolerance limits and resource needs

• Realized niche – the actual set of environmental conditions, including the presence or absence of other species, in which the species can establish a stable population

Community Ecology• Habitat

– Physical location– Provides shelter, food, places

of rest– May involve imprinting – a

form of associative learning linking individuals to their place of birth or seasonal range

Competition

• Struggle between organisms to utilize the same resource when the resource is limited

• Can occur between species or within species• Especially strong when niches overlap• Can restrict the niche of species (and lead to

natural selection…)

Competition• Interference competition – physical

interactions over access to resources

http://www.flickr.com/photos/pixielove/2644810848/

Competition• Examples of competition:

– Fighting to defend territory– Displacing an individual from a particular location– A fraction of individuals obtain all the resources

they need; the remaining individuals get less and produce no offspring, or die

– Usually aggressive in animals– Shading of vegetation, production of toxins in

addition to consumption/use of resources

Interspecific Competition

• Interspecific competition – occurs when 2 different species attempt to use the same resource, and there is not enough of the resource to satisfy both

Competition• Intraspecific competition – competition within

a species• Elephant seal harems, Grasshoppers, trees

Competition for niche occupancy

• Classic study of barnacles by J.H. Connell• Scotland coast• Semibalanus balanoides inhabits deeper

water (lower level of substrate); Chthamalus stellatus inhabits shallower water (higher level of substrate)

• S. balanoides not usually exposed to air; C. stellatus frequently exposed to air at low tide

Competition for niche occupancy• When S. balanoides were removed, C. stellatus

was capable of occupying the deeper zone• When C. stellatus was removed, S. balanoides

was NOT capable of occupying the shallow-water habitat (exposure to air, higher temperatures)

• Functional niche of C. stellatus includes shallow and deeper water

• Functional and realized niche of S. stellatus identical

Competitive Exclusion

• Principle of Competitive Exclusion – when two or more species coexist using same resource, one must displace or exclude the other

• Hypothetical• Gradients exist within any (shared) habitat;

fundamental niches may overlap, but realized niches may differ

• Usually detrimental to both species over long term

Competition

• Niche overlap can lead to – Resource partitioning – the differentiation of

niches that enables two similar species to coexist within a community

– Character displacement – the tendancy for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species

• Hereditary changes evolve that bring about resource partitioning

Resource Partitioning

• Anoles lizards on Caribbean islands

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Character Displacement

• Allopatric species – species that do not occur together

• Sympatric species – species occur together

• Example – Darwin’s finches

Character Displacement• Niche overlap drives natural selection!• Brought about by selective effects of

competition• Differences among species are accentuated

where they co-occur• Can lead to sympatric speciation!

http://www.triplov.com/zoo_ilogico/peixes/agua_doce/pages/gasterosteus_aculeatus.htm

Predation• Predation – one living organism serves as a

food source for another• Prey evolves defenses, predators evolve

adaptations to overcome prey defenses, prey evolves further, and so on….

• An arms race ensues (and is in constant motion)

Predation• Plant defenses against herbivores (and visa versa)

– Morphological: thorns, spines, plant hairs– Chemical: Secondary compounds (allelopathy)

Predation

• Animal defenses against predators– Behavioral: Fleeing, Hiding, Self-defense, Noises,

Mobbing, “Flight or fight”– Physical: Camouflage – cryptic coloration,

deceptive markings– Mechanical: includes spines and shells– Chemical: Odors and toxins

Behavioral defenses

Physical defenses• Physical – Camouflage

(Pygmy seahorses)

Physical defenses• Physical – Camouflage (Shrimp fish,

cryptic frog, walking sticks)

Camouflage continued…

• Camouflage may occur seasonally (arctic fox, ermine, hare)

Physical - Disruptive markings

The Superstar of them all, the Mimic Octopus!

Mechanical defenses

Chemical defenses• Chemical – may be incorporated from food

they eat (milkweed and monarch butterfly)

Predation• Aposematic coloration – warning indicated by

coloration, anti-predator adaptation to advertise toxins

• Toxins don’t do you any good if you have to get eaten first!

• Sometimes warning is of unpalatability, and other times depicts danger

• Skunks advertise too!

Aposematic coloration

Mimicry• Batesian mimicry – a mimicry system in which

palatible organisms mimic the morphological characteristics of (an often unrelated) noxious species

• Named for Henry Bates; English naturalist who observed many mimics in Amazon

Mimicry• Toxic species must not kill predator• Predator must remember negative encounter

and learn from the experience• Mimics must overlap geographically with toxic

species

Mimicry• Monarch and Viceroy butterflies – Monarch

caterpillars feed on milkweed (toxic plant); incorporate toxins into body; Viceroy mimics

Mimicry• Müllerian mimicry – when two or more

unpalatable species resemble each other• All organisms are toxic and converge upon a

common morphological warning system• Minimizes losses by predation (predators have

memory)

Predation

• Can promote species diversity (drives natural selection)

• Predator feeds on superior competitor; most successful organism will be most abundant, predators will often select the most abundant prey, especially in plants

• Return of wolves to Yellowstone restored native vegetation, which benefits the elk population!

Predation• Predation reduces

competition• Without predators, sick

and weak individuals of the population will survive

• May pass on (weak) characteristics to offspring reducing fitness potential of the gene pool

Predation

• Keystone species – a species that has a disproportionate effect on its environment relative to its abundance

• Plays a critical role in its ecosystem by maintaining the structure of the ecological community

Predation

• Keystone species – Sea otter

http://www.flickr.com/photos/jimpatterson/815254101/

Keystone Species – Sea Otter

Coevolution and Interspecific Interactions

• Coevolution – reciprocal evolutionary adaptations of two interacting species

• When one species evolves, it exerts selective pressure on the other to evolve to continue the interaction

• May occur within species – if a female selects a male with an extreme characteristic, selection will fuel development of characteristic – may be detrimental!