ANIMAL ECOLOGYChapter 2

Chapter opener 02

Ecology

Definition

• Ecology – Study of the relationship of organisms to their

environment

2-3

Ecology• Necessary to understand the physiological and

behavioral mechanisms of organisms to understand their ecological relationships

• Animals in nature coexist with others of the same species as reproductive units are called populations– Population has properties that cannot be

discovered by studying individuals alone• Populations of many species live together in complex

communities• The number of different species present in a

community is measured as species diversity

2-4

Ecology• Many species interact with each other such as predation,

parasitism, competition• Predators obtain energy and nutrients by killing and

eating prey• Parasites drive similar benefits but do not kill the host• Parasitoid is a parasite that kills its host organism• Competition occurs when resources become limited• Mutualism occurs when both members of a pair of

species benefit from their interactions, usually avoiding negative interactions with other species

• Larger units or ecosystems allow study of the community and the physical environment

• Biosphere is the land, water, and atmosphere that envelops the planet and supports all life on earth

2-5

Ecology

Environment and the Niche

• Abiotic factors (nonliving) – Space, energy forms including sunlight, heat, wind and

water currents, and the soil, air, water and chemicals• Biotic factors (living)– Include other organisms as food, or competitors,

predators, hosts or parasites• Resources – Environmental factors that an animal uses directly

2-6

Ecology

• Some resources are expendable– Food, once eaten, is no longer available and must be

continuously replenished

• Space is not consumed by being used and is therefore nonexpendable

• Habitat – Physical space where an animal lives and is defined by the

animal’s normal activity

2-7

Ecology

• Niche– The life requirements of an organism define its niche– A niche includes the animal’s limits of temperature,

moisture, food, and other factors– Addition of important factors such as salinity or pH

describes a complex multidimensional niche– The niche of a species undergoes evolutionary changes

over successive generations

2-8

Ecology• Some animals are generalists– Wide niches– Can tolerate a wide range of salinity or eat a wide range of

foods• Other animals are specialists – Have narrow dietary requirements or limited tolerance to

temperature changes, etc.• Fundamental niche– Describes animal’s potential role to live within a wider

range of conditions• Realized niche– The narrower subset of suitable environments that an

animal actually experiences

2-9

Hierarchy of Ecology

Populations

• Population Factors– A population is a reproductively interactive group of

animals of a single species

– A geographically and genetically cohesive population that is separable from other such populations is a deme • Share a gene pool

– Movement among demes provides some evolutionary cohesion among species

2-10

Ecology– Local environments may change unpredictably • Can cause a local deme to become depleted or

eliminated– Immigration • Source of replacement among demes within a

region– Interaction among demes in this manner called

metapopulation dynamics

2-11

Ecology• Survivorship Curves define the age pattern of a

population from birth to death of the last member– Type I Survivorship Curve

• All individuals die at the same time, occurs rarely in nature– Type II Survivorship Curve

• Rate of mortality as a proportion of survivors is constant across ages– Characteristic of birds that care for their young– Humans fall between Type I and Type II depending on nutrition and

medical care– Type III Survivorship Curve

• Represents many species that produce huge numbers of young but experience rapid and sustained mortality

• Explains the need for high reproductive output of many animals

2-12

2-13

2-14

Ecology

• Semelparity is the condition in which an organism reproduces only once during its life history

• Iteroparity is the condition in which an organism reproduces mutiple cohorts of offsprings that may mature and reproduce while their parents are still alive and reproductively active

• Populations of animals containing multiple coherts such as robins, box turtles, and humans exhibit age structure

2-15

Ecology• Population growth – Difference between rates of birth and death

• Populations have the ability to grow exponentially at the intrinsic rate of increase (r)

• Forms a steeply rising curve • Unrestricted growth is not prevalent in nature• Growing population eventually exhausts food or space– Planktonic blooms and locusts outbreaks both end when

resources are exhausted

2-16

Ecology

• Among many resources, one will be depleted first – Limiting resource

• The largest population that the limiting resource can support is the carrying capacity (K)

• A population slows growth rate in response to diminishing resources

2-17

2-18

2-19

Ecology

• Carrying Capacity of the Human Species– Humans have the longest record of exponential population

growth • About 5 million around 8000 BC before the Agricultural Revolution• Human population rose to 500 million by 1650• 1 billion by 1850• 2 billion by 1927• 4 billion by 1974• 6 billion by October 1999• Will reach 8.9 billion by 2040

– Food production cannot keep pace with exponential population growth indefinitely

2-20

Ecology

• Extrinsic Limits to Growth– Density-independent Factors • Abiotic factors reduce populations by floods, fires,

storms and severe climate fluctuations• These agents kill young or other members of a

population regardless of the size of population• Cannot truly regulate population growth because they

are unrelated to population size

2-21

Ecology– Density-dependent Factors • Biotic factors that respond to density of the population – Include predators and parasites

• As a population increases in number and individuals live closer together– Effects of parasites and disease are more severe

• Competition between species for a common limiting resource – Lowers the effective carrying capacity for each species

2-22

Community Ecology

Interactions Among Populations in Communities• In a community, populations of different species interact• The number of species that share a habitat

– Known as species diversity• Species interactions may benefit or harm the species involved• In a predator-prey interaction

– Predator benefits and the prey is harmed• Parasitism benefits the parasite and harms the host• Herbivory benefits the animal and harms the plant

2-23

2-24

Community Ecology• In commensalism– One species benefits and the other neither benefits nor is

harmed• Some mutualistic relationships become obligatory

mutualism – Neither can survive without the other

• Competition between two species – Reduces the fitness of both– Asymmetric competition (amensalism) affects one species

less than the other

2-25

2-26

Community Ecology

Competition and Character Displacement

• Competition occurs when two or more species share a limiting resource

• If resource is not in short supply– Sharing the resource does not demonstrate competition

• Niche overlap – Portion of the niche’s resources that are shared by two or

more species

2-27

Community Ecology• Competitive exclusion – Principle that no two species will occupy the same niche

for a long time– Eventually one will exclude the other

• To coexist, two species can specialize by partitioning a shared resource

• Specialization involves character displacement– Differences in organismal morphology or behavior related

to exploitation of a resource– They do not compete with each other directly

2-28

2-29

Community Ecology

• Where two species coexisted – Competition between them led to evolutionary

displacement to diminish the competition• When two or more species reduce niche overlap to

share the same general resources, they form a guild

2-30

2-31

Community Ecology

Predators and Parasites• Many animals and plants are in co-evolutionary

relationships– Each in a race with the other

• If a predator relies primarily on a single prey species– Populations tend to fluctuate cyclically with each other

• Predator-prey relationship– Led to development of mimicry• Harmless species mimic models that have toxins or

stings

2-32

2-33

2-34

Community Ecology• Another mimicry complex consists of many different

species, all with noxious or toxic factors, that evolve to resemble each other

• A keystone species is so pervasive to a community that its absence drastically changes the entire community

• Keystone species reduce competition and allow more species to coexist on the same resource

• Periodic natural disturbances also allow more species to coexist in diverse communities

2-35

2-36

Community Ecology• Ectoparasites– Host provides nutrition from its body and aids dispersal of

parasite• Endoparasites – Lost ability to choose habitats – Must have tremendous reproductive output to ensure

offspring will reach another host• Coevolution between parasite and host is expected to

generate an increasingly benign, less virulent relationship if host organisms are uncommon and/or difficult for a parasite to infest– Death of the host also ends or shortens a parasite’s life– Exceptions occur when alternative hosts are available

2-37

EcosystemsTrophic Levels

• Primary producers– green plants or algae, fix and store energy from sunlight

• Herbivores – First level of consumers that eat plants

• Carnivores – Feed on herbivores or other carnivores

• Decomposers – Mainly bacteria and fungi– Break dead organic matter into mineral components for

reuse by plants to start the cycle over again

2-38

2-39

Ecosystems

Energy Flow and Productivity

• Every organism has an energy budget and must obtain enough energy to grow, reproduce, etc.

• Gross productivity (Pg) – Total energy assimilated or taken in– Some used to maintain metabolism

• Net productivity (Pn) – Energy stored in the animal’s tissue as biomass – Available for growth of the animal and for reproduction

2-40

Ecosystems• Energy is limited and can be represented as

Pn = Pg - R where R is respiration• Energy budget of every animal is finite• Much energy is lost when it is transferred between

trophic levels in food webs• More than 90% of the energy in an animal’s food is

lost as heat• Less than 10% is stored as biomass• Each trophic level contains only 10% of the energy of

trophic level below it

2-41

Ecosystems• Ecological Pyramids– Eltonian pyramid • Based on numbers of organisms at each trophic level• Does not indicate mass of organisms at each level

– Pyramid of biomass • Total bulk or “standing crop” of organisms at each trophic

level• Energy pyramids – Depicts rate of energy flow between levels– Never inverted

• Amount of energy transferred from each level is less than what entered it

– Gives best overall picture of community structure because it is based on production

2-42

2-43

Ecosystems

Life Without the Sun • From 1977 to 1979, dense communities were first

discovered on sea floor thermal vents• Producers in the vent communities are

chemoautotrophic bacteria that oxidize hydrogen sulfide

• Tubeworms and bivalve molluscs form trophic communities that rely on this non-photosynthetic source of nutrients

2-44

Ecosystems

Nutrient Cycles • Decomposers feed on remains of animals and plants

and on fecal material and return substances to the ecosystem

• Biogeochemical cycles involve exchanges between living organisms, rocks, air and water

• Continuous input of energy from sun keeps nutrients flowing and the ecosystem functioning

• Synthetic compounds challenge nature’s nutrient cycling

2-45

2-46

Ecosystems• Pesticides in food webs – May be concentrated as they travel up through succeeding

trophic levels– Kills non-target species– Some chemicals remain in the environment for long

periods of time• Genetic engineering of crop plants aims to improve

resistance to pests and lessen the need for chemical pesticides

2-47

Extinction and Biodiversity

Biodiversity• Rates of speciation slightly exceed rates of extinction• Approximately 99% of all species that have ever lived

are extinct• Extinction events killing at least 5% of existing

species have occurred almost continuously throughout geological time

• Human activity clearly has induced numerous species extinctions

• Humans must avoid making the present time rival the great extinction crises of earth’s geological history

2-48