Communities and Ecosystems Lecture Flashcards

Introduction to Communities and Ecosystems

• Natural ecosystems provide significant value through several mechanisms:

  • Provision of natural resources.

  • Support for outdoor recreation.

  • Provision of natural services, including:

    • Buffering against hurricane damage.

    • Recycling nutrients.

    • Preventing erosion.

    • Pollinating crops.

• The study of ecosystems is divided into two primary dynamics:

  • Energy flow: The passage of energy through the components of the ecosystem.

  • Chemical cycling: The transfer of materials within the ecosystem.

Community Structure and Dynamics

• A community is defined as an assemblage of all the populations of organisms living close enough together for potential interaction.

• Community ecology focuses on factors that:

  • Influence species composition and distribution of communities.

  • Affect community stability.

• Communities are described by their species composition.

• The boundaries of a community are flexible and depend on the research question. Examples include:

  • The entire area of a pond.

  • The collection of intestinal microbes within a single pond organism.

Interspecific Interactions

• Interspecific interactions are relationships between individuals of different species in a community. These interactions significantly affect population structure and dynamics.

• Interactions are categorized by their effect on the populations involved:

  • Interspecific competition: Occurs when populations of two different species compete for the same limited resource. The effect on both species is $-/-$. Example: Squirrels and black bears.

  • Mutualism: An interaction where both populations benefit ($+/+$). Example: Plants and mycorrhizae; reef-building corals and photosynthetic dinoflagellates.

  • Predation: An interaction where one species, the predator, kills and eats another, the prey ($+/-$). Example: Crocodiles and fish.

  • Herbivory: Consumption of plant parts or algae by an animal ($+/-$). Example: Caterpillars and leaves.

  • Parasitism: The host (plant or animal) is victimized by parasites or pathogens ($+/-$). Example: Heartworms in dogs; Salmonella in humans.

Ecological Niches and Competition

• An ecological niche is defined as the sum of an organism’s use of the biotic and abiotic resources in its environment.

• Interspecific competition occurs specifically when the niches of two populations overlap.

• Competition lowers the carrying capacity ($K$) of competing populations because resources utilized by one population are rendered unavailable to the other.

Evolutionary Adaptations: Predation and Herbivory

• Predation Adaptations:

  • Predation provides a benefit to the predator but is lethal to the prey.

  • Prey species have evolved diverse protective strategies, including:

    • Camouflage (e.g., Seahorse camouflage).

    • Mechanical defenses (e.g., spines, shells).

    • Chemical defenses (e.g., toxins, often signaled by bright coloration).

• Herbivory and Coevolution:

  • Plants must expend energy to replace body parts lost to herbivores.

  • Defenses against herbivores include mechanical barriers like spines and thorns, as well as chemical toxins.

  • Coevolution describes a series of reciprocal evolutionary adaptations in two species, where a change in one species acts as a new selective force on another species.

Parasites and Pathogens

• A parasite lives on or in a host to obtain nourishment.

  • Internal parasites: Examples include nematodes and tapeworms.

  • External parasites: Examples include mosquitoes, ticks, and aphids.

• Pathogens are disease-causing microscopic parasites, including bacteria, viruses, fungi, and protists.

• Non-native pathogens can cause rapid, dramatic impacts because the ecosystem may lack natural defenses against them:

  • Chestnut blight (a protist) devastated the American chestnut.

  • A fungus-like pathogen is currently causing sudden oak death on the West Coast of the United States.

Trophic Structure and Food Webs

• Trophic structure represents the pattern of feeding relationships consisting of several levels.

• A food chain is the sequence of food transfer up these trophic levels, moving chemical nutrients and energy from producers upward.

• Producers = Autotrophs

• Consumers = heterotrophs

• Trophic Levels:

  • Producers: Autotrophs that support all other levels (e.g., Plants on land, Phytoplankton in water).

  • Primary Consumers: Herbivores (e.g., Grasshoppers on land, Zooplankton in water).

  • Secondary Consumers: Typically eat herbivores (e.g., Mice on land, Herring in water).

  • Tertiary Consumers: Typically eat secondary consumers (e.g., Snakes on land, Tuna in water).

  • Quaternary Consumers(predators): Typically eat tertiary consumers (e.g., Hawks on land, Killer whales in water).

• Detritivores and Decomposers:

  • Detritivores are any organism that derives energy from detritus (dead organic material).

  • Decomposers are mainly prokaryotes and fungi that secrete enzymes to digest organic material and convert it into inorganic forms through decomposition.

• Food webs are networks of interconnecting food chains. In a web:

  • Consumers may eat more than one type of producer.

  • Several species of consumers may feed on the same species of producer.

Species Diversity and Keystone Species

• Species diversity is defined by two components:

  • Species richness: The total number of different species in a community.

  • Relative abundance: The proportional representation of each species in a community.

• In a comparison of two woodlots (A and B):

  • If Woodlot A consists of $80\%$ Species 1, $10\%$ Species 2, $5\%$ Species 3, and $5\%$ Species 4, it has lower relative abundance diversity compared to Woodlot B where each of the four species represents $25\%$ of the population.

• Species diversity affects the diversity of animals and the impact of pathogens.

• Agricultural ecosystems are typically characterized by low species diversity.

• A keystone species is one whose impact on its community is much larger than its biomass or abundance would indicate.

  • They occupy a niche (role) that holds the rest of the community in place.

  • Examples: Pisaster sea stars, sea otters, long-spined sea urchins, hippos, lions, and wolves.

Community Disturbance and Succession

• Disturbances are events that damage communities (e.g., storms, fires, floods, droughts, overgrazing, human activity).

• Ecological Succession (Community Succession) is the process of colonization by a variety of species, which are then replaced by a succession of other species.

  • Primary succession: Begins in a virtually lifeless area with no soil (e.g., a new volcanic island).

  • Secondary succession: Occurs when a disturbance destroys an existing community but leaves the soil intact (e.g., an abandoned farm returning to a wild state).

  • Succession stages: Annual plants $→$ Perennial plants and grasses $→$ Shrubs $→$ Softwood trees (pines) $→$ Hardwood trees (climax community).

Invasive Species

• Invasive species are organisms introduced to non-native habitats by human actions that establish themselves at the expense of native communities.

• They often exhibit rapid population growth due to an absence of natural enemies.

• Examples:

  • Rabbits and Cane toads introduced into Australia.

  • Scotch broom and Himalayan blackberries in the Pacific Northwest (PNW).

  • European Starlings and English sparrows in the USA.

Ecosystem Ecology and Energy Budgets

• Primary production is the amount of solar energy converted to chemical energy by producers for a given area during a given time period.

  • Biomass: The amount of living organic material in an ecosystem.

  • Net primary productivity varies by ecosystem ($g/m^2/yr$). High productivity ecosystems include algal beds, coral reefs, and tropical rain forests. Low productivity ecosystems include deserts and the open ocean.

• Energy supply limits the length of food chains. For example, in a caterpillar:

  • $50\%$ of organic compounds are eliminated in feces.

  • $35\%$ is used in cellular respiration.

  • $15\%$ is used for growth.

• The Pyramid of Production shows that only about $10\%$ of energy stored at each trophic level is available to the next.

  • Example: $1,000,000\,kcal$ of sunlight $→$ $10,000\,kcal$ (Producers) $→$ $1,000\,kcal$ (Primary consumers) $→$ $100\,kcal$ (Secondary consumers) $→$ $10\,kcal$ (Tertiary consumers).

Ecological Cost of Meat

• The human population has approximately ten times more energy available to it when eating plants (as primary consumers) compared to eating meat from herbivores (as secondary consumers).

• Meat consumption is economically and environmentally expensive due to the $90\%$ energy loss at each trophic level.

Biogeochemical Cycles

• Ecosystems depend on a continual influx of energy (Sun, Earth's interior) and the recycling of chemical elements.

• Biogeochemical cycles include biotic and abiotic components and abiotic reservoirs (where chemicals stockpile outside organisms).

• The Carbon Cycle:

  • Major ingredient of organic molecules found in the atmosphere, fossil fuels, and dissolved in the ocean.

  • Balance: CO$_2$ removal by photosynthesis vs. return by respiration.

  • Impact: Burning wood and fossil fuels increases atmospheric CO$_2$.

• The Phosphorus Cycle:

  • Required for nucleic acids, phospholipids, and ATP.

  • No atmospheric component; rocks are the primary source.

  • Phosphate levels are often a limiting factor in aquatic ecosystems.

• The Nitrogen Cycle:

  • Required for proteins and nucleic acids.

  • Abiotic reservoirs: Atmosphere ($80\%$ N$_2$ gas) and soil.

  • Nitrogen fixation: Performed by bacteria to convert N$_2$ into usable compounds (e.g., ammonium ($NH_4^+$), nitrates ($NO_3^-$)).

Eutrophication and Ecosystem Degradation

• Eutrophication: The increase in primary production in standing water ecosystems due to accumulated nutrients (phosphorus and nitrogen).

• Rapid eutrophication causes:

  • Depletion of oxygen levels.

  • Decrease in species diversity.

• Sources of pollution include agricultural fertilizers, pesticides, sewage treatment facilities, and runoff from animal feedlots.

Ecosystem Services

• Human well-being depends on natural services from healthy ecosystems:

  • Supply of fresh water and food.

  • Recycling of nutrients.

  • Decomposition of wastes.

  • Regulation of climate and air quality.

• Increases in food production and human activities (e.g., deforestation) often occur at the expense of these essential services.