Basics of Ecology

How Organisms Respond to Environmental Changes

• survival of organism: depends on their ability to respond to changes in the environment

  • responses to changes can be behavioral or physiological

• stimulus: change in the environment that triggers a response

• behavioral response: refers to how animals cope or deal with changes in their environment by changing their behavior

• physiological response: refers to how animals cope or deal with changes in their environment by changing their physiological processes/habits

• example:

  • stimulus: extension of day length

  • behavioral response: many bird populations tend to migrate elsewhere

  • physiological response: some animals slow their metabolism to conserve energy

• stimulus can be communicated between organisms to trigger changes in behavior with other organisms; they can communicate with:

  • audible signals: birds use audible signals to send warnings to other birds or to attract mates; some primates use vocalization to assert dominance or warn of the presence of predators

  • chemical signals (pheromones): can be released by some organisms to illicit a response in other organisms; responses can be mating, warning others, to scare off predators, etc.

  • electrical signals: sharks and rays can send electrical signals throughout the water to locate prey species

  • tactile signals: touching between primates can be used to express affection or to indicate dominance; some plants curl up to protect sensitive parts of their bodies from damage

  • visual signals: some species use warning coloration to scare off predators

    • aposematism: the use of warning coloration to inform potential predators that an animal is poisonous, venomous, or otherwise dangerous

• signals can be used to establish hierarchy, find mates, and find resources

  • natural selection will favor signals and responses that increase survival and the chance of successful reproduction; over time this can lead to evolution of the population

• cooperative behaviors: can lead to increased fitness of individuals and populations; it is cooperative if it is beneficial to another organism, the recipient, and is selected, at least partially, due to the benefits to the recipient

  • example: meerkats take care of each other; they huddle together for warmth, groom each other, forage together, fend off predators together in groups, etc.

Energy Flow Through Ecosystems

• organisms use energy to reproduce, grow, and maintain organization; different species have different adaptations for maintaining energy levels (and body temps.)

• endotherms: use thermal energy generated from their metabolism to maintain body temp. (ex. mammals, birds)

• ectotherms: do not have internal mechanisms for maintaining body temp.; absorb heat from the environment; their behaviors will change depending on their body temp. (ex. fish, reptiles, amphibians)

• metabolic rate: the total amount of energy an organism uses per unit of time

  • smaller organisms: higher metabolic rate because they have greater SA:V ratios and lose heat more quickly; higher metabolic rates compensate for the quicker loss of heat

  • larger organisms: lower metabolic rate because they lose heat less quickly

• net energy gain: can result in energy storage (ex. fat tissues of animals)

• net energy loss: can result in loss of mass or death of organism

• changes in energy availability can affect population sizes (less energy = smaller population supported)

• trophic levels: represent steps in the food and energy transfers between organisms in an ecosystem

  • organisms are classified into trophic levels based on their food and energy sources

• food chains: show the transfer of energy between trophic levels; direction of arrows in food chain show direction of energy flow between organisms

• food webs: show the interconnections between organisms in different food chains; provide a more complete representation of energy transfers in ecosystems

  • autotroph (producers): get energy from physical or chemical sources in the environment

    • photoautotrophs: get energy from sunlight (ex. plants)

    • chemoautotrophs: obtain energy from small inorganic molecules (ex. bacteria)

  • heterotroph: get energy from carbon compounds made by other organisms; obtain energy from carbs, lipids, or proteins by breaking down the macromolecules using hydrolysis (ex. animals)

  • decomposers: break down dead organic material to allow for the recycling of nutrients in the ecosystem (ex. fungi)

    • detritivores: organisms that obtain energy by consuming organic waste of dead plants/animals (ex. earthworm)

• kleptoplasty: an unusual strategy for obtaining energy; when a heterotroph consumes an autotroph that it uses as a food source but then removes the chloroplasts from the autotroph’s cells to incorporate them into their own cells

• energy is lost as it moves through trophic levels; it can be lost as heat or being consumed for necessary metabolic processes in organisms

  • more energy is required but harder to obtain in higher trophic levels, resulting in a smaller population (ex. there are less bears (top level) than there are bacteria (bottom level))

  • biomass: the total quantity or weight of organisms in a given area or volume.; increases as you go down trophic levels (producers have greatest biomass)

Regulation in Ecosystems

• bottom-up regulation: in which the population size of producers decreases, so the population of subsequent (higher) trophic levels also decreases

• top-down regulation: when animals at higher trophic levels limit population sizes at lower levels

  • if the predators (top level) are removed, the trophic levels in between predators and producers could increase due the lack of population control of them

• availability of food and energy resources affects organisms’ reproductive strategies

  • organisms in unstable environments: produce a larger number of offspring to increase chance of survival

  • organisms in stable environments: produce fewer offspring because likelihood of survival is increased

Population Ecology, Community Ecology, and Biodiversity

Population Ecology

• population: made up of individual organisms that interact with one another and with their environment in complex ways

  • population sizes and population growth are affected by how organisms obtain the energy and matter they need to survive

  • population growth depends on:

    • population size (N)

    • birth rate (B)

    • death rate (D)

• population growth rates are calculated as the change in population size over the change in population time: dN / dt

  • calculating population growth can be with: dN / dt = B - D

    • if birth rate exceeds death rate, population size will increase

    • if birth rate is less than death rate, population size will decrease

  • exponential growth: if there are no limiting factors (ex. abundance of food) on a population, it will experience exponential growth and can be described with the following equation:

    • N: population size

    • rmax: maximum per capita growth rate of the population

    • the larger the population size, the higher the growth rate and the faster it will grow

• some populations eventually exceed the resources of their environment, meaning the growth is limited by resource availability

  • density-dependent factors: disease, predation, competition (for food, habitat, mates, etc.)

  • density-independent factors: temp., precipitation, natural disasters

  • both result in logistic growth of population, with the following equation

    • 

    • N: population size

    • rmax: maximum per capita growth rate of population

    • K: carrying capacity of environment

      • carrying capacity: the maximum population that can be supported by the available resources

    • as population size increases (N gets closer to K), the rate of population growth decreases

K-Selected vs. r-Selected Populations

• K-selected: possess relatively stable population sizes at or near the carrying capacity of the environment

  • k-selected populations usually reproduce more than once per lifetime, with few offspring per reproductive cycle

    • invest greater levels of parental care in their offspring, resulting in higher survival rates of offspring

  • usually experience logistic growth and are sensitive to density-dependent factors

  • examples: most mammals, birds

• r-selected: live in unstable environments

  • usually reproduce at a younger age and only once per lifetime with large numbers of offspring

    • invest less time in offspring, resulting in lower rate of survival for offspring

  • experience “boom or bust” periods, in which periods of exponential growth lead to populations that far exceed the carrying capacity (“boom”), resulting in rapid decreases of population ('“bust”)

  • usually not sensitive to density-dependent factors

  • examples: many fish, amphibians

Community Ecology and Simpson’s Diversity Index

• community: a group of interacting populations living in the same habitat; described by their species composition and species diversity

  • species composition: number of species in a given area

  • species diversity: reflects the number of species in an area and the number of members in each of those species

    • species diversity gives a more accurate assessment of the variety of organisms found in an area

• simpson’s diversity index: way of representing species diversity with the following equation:

  • 

  • n: total number of organisms of a particular species

  • N: total number of organisms of all the species

  • the higher the SDI, the more diverse the community

Relationships Within Communities

• interactions can change over time

  • changing interactions can influence how many members of the community access the matter and energy they need to survive

• competition: organisms compete for resources (ex. food, habitats, mates)

  • interspecies competition: between members of two different species

  • intraspecies competition: between members of the same species

  • any competition can lead to the demise of an organism in an ecosystem

• predator/prey: predator species eat prey species and depend upon prey populations for food

  • insufficient numbers of prey species will lead to a decline in the predator species

  • increasing number of prey species will lead to an increase in predator species

  • fluctuation in the number of predators will generally follow fluctuations in the number of species

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• niche partitioning: competing species may coexist if they use the resources available in their habitats differently

  • dietary partitioning: where organisms eat different prey/sources of food, so they can co-exist peacefully

• trophic cascades: refers to the far-reaching effects of the reduction of one trophic level in a food web

  • example: decrease in sea otters lead to an increase in sea urchins because sea otters eat sea urchins

• parasitism: a symbiotic relationship where one species benefits from the relationship but the other is harmed; negative relationship

• commensalism: one species benefits and the other species neither benefits nor is harmed by the relationship; neutral relationship

• mutualism: both species benefit from the given relationship between them; positive relationship

Biodiversity

• biodiversity: refers to the variety of living organisms in an ecosystem

  • high biodiversity: increased resilience and more adaptable to changes in the environment

  • low biodiversity: less resilience and cannot adapt as well to changing environments

• biodiversity depends on abiotic and biotic factors:

  • abiotic: non-living factors; influence the types and the number of species that can survive in the environment

    • ex. climate, water availability, etc.

  • biotic factors: living factors; limit how many consumers can survive in the environment

    • ex. number of producers

• biodiversity of an ecosystem will influence the structure of food chains and food webs found in that ecosystem

• keystone species: have a disproportionately large effect on an ecosystem compared to their population size

  • when a keystone species is removed from the environment, the ecosystem may collapse

Disruptions to Biodiversity

• invasive species: a type of disruption to the ecosystem; a non-native species that causes struggle within the populations of the native species (ex. increases competition, introduces disease, etc.)

• human impacts:

  • habitat destruction (as urbanization increases)

  • diseases (humans can come in contact with new species and pass off pathogens to them)

  • pollution

• geological events: natural disasters (ex. volcanic eruption, flood) can decrease plant and animal diversity; prolong droughts can change biodiversity

• disruptions in ecosystems can lead to evolution in populations

  • changed environments will select for adaptations that provide advantages in the environment

  • adaptations are generate by mutations, which are random, but selection for them is not random

• rapid changes in environments can accelerate the pace of evolution