Community Ecology

Communities A community is a group of organisms of different species that live in a particular area

Individualistic Hypothesis vs. Interactive Hypothesis Individualistic Hypothesis: A community is a chance group of species found in the same area because they have similar abiotic requirements Integrated (Interactive) Hypothesis: A community is a group of closely linked species locked together in mandatory biotic interactions that cause the community to function as an integrated unit

Interspecific Interactions Interspecific interactions are interactions that occur between populations of different species living together in a community There are 4 major interspecific interactions: Predation (and parasitism) Competition Commensalism Mutualism

Predation- Parasitism

Predation (and Parasitism) (+ -) The interaction is beneficial to one species and detrimental to the other Predation: When a predator eats its prey Example in picture:

Predation (and Parasitism) Parasitism: Predators that live on or in their hosts, usually feeding off their body tissues or fluids Usually do not kill their hosts Examples in picture (tick, leech)

Parasitism One organism (the parasite) gets its nourishment from another organism (the host), which is harmed in the process Endoparasites: Live within host tissues (tapeworms) Ectoparasites: Feed on external surfaces (mosquitoes) Parasitoidism: Insect lays eggs on or in a host. The eggs feed on the host... eventually killing it

Disease Pathogens are similar to parasites (typically bacteria, viruses or fungi)

Plant Defenses Against Herbivores “Plants Fight Back!” Plants have 2 major mechanisms by which they defend themselves against being eaten Mechanical Defenses Thorns, hooks, etc. Chemical Defenses Poisons

Plant Defenses Against Herbivores Chemical Defenses Produce chemicals that are distasteful or harmful to an herbivore Morphine (opium poppy) Nicotine (tobacco)

Animal Defenses Against Predators Animals defend themselves against predators passively (hiding) or actively (fleeing) Cryptic coloration (camouflage) makes prey difficult to spot Aposematic coloration (warning coloration)warns predators not to eat animals that may be toxic or may sting.

Animal Defenses Against Predation Mimicry When one species “imitates” or “mimics” another Batesian mimicry When one edible or harmless species mimics an bad- tasting (unpalatable) or harmful species Example: hawkmoth mimics a snake

Animal Defenses Against Predation Mimicry Mullerian mimicry Two species, both of which are unpalatable (taste bad) or harmful, resemble each other Example: monarch butterfly (unpalatable) and queen butterfly (unpalatable) resemble each other

Competition

Interspecific Competition (-/-) Competition between organisms of different species for a particular limited resource The Competitive Exclusion Principle: Two species with similar needs for the same limiting resources cannot coexist in the same place Niches may overlap but they may not be identical. Niche: sum total of a species’ use of the biotic and abiotic resources in an environment

Ecological Niches An organism’s niche is the specific role it plays in its environment All of its uses of biotic and abiotic resources in its environment Example: oak tree in a deciduous forest Provides oxygen to plants, animals, etc. Home for squirrels Nesting ground for blue jays Takes water out of the soil Etc., etc.

Fundamental v. Realized Niche Fundamental Niche includes resources an organism could theoretically use (if no competition) Realized Niche includes resources it actually does use given competition from other species.

Resource Partitioning Similar species develop ways to partition/divide resources in order to coexist.

Character Displacement Evidence for competition can sometimes be determined by looking at closely related species Allopatric (geographically separate): are morphologically similar and use similar resources Sympatric (overlapping geography): show different morphology and use different resources Character displacement: the trend of sympatric species to be more divergent than allopatric species

Symbiosis A close relationship between two organisms.

Symbiosis Parasitism (+,-) Already discussed Commensalism (+,neutral) Mutualism (+,+)

Commensalism (+0) relationship One partner benefits, the other is not affected Examples: Sea anemone and clownfish Clownfish gets a place to live, sea anemone is not affected

Mutualism (++) relationship Both partners benefit from the relationship “You scratch my back, I’ll scratch yours” Examples: Ants & acacia tree tree provides high protein food in beltian bodies & habitat for nests inside thorns; ant protects against predators Mycorrhizae-fungal extentions on plant roots Plant gets increased water/nutrition, fungi gets food Hummingbirds & flowers Hummingbirds get food, flowers can reproduce

SUMMARY Relationship Organism #1Organism #2 Commensalism +0 Mutualism++ Parasitism+- Predator Prey+- Competition--

Evolutionary component Many of the relationships discussed could be a result of coevolution Each species influences the heritable traits of another,closely associated, species

Community Structure

Community structure Community structure describes the make up and interactions of the species in a community. Many times this is a result of 2 factors: Species diversity Feeding relationships

Species diversity Species diversity if made of two components: Species richness is the total number of different species in a community Relative abundance is the proportion of each species that makes up the community

Who eats who? A trophic structure describes the feeding relationship between organisms in a community Feeding relationships always start with some sort of primary producer (generally a photosynthetic organism) Then you will have primary consumers (herbivores) and various secondary and tertiary consumers (carnivores) Eventually, the cycle ends with decomposers

Food chains Food Chains- A single pathway of energy relationships among organisms in an ecosystem

Energy transfer The arrows DO NOT merely show what gets eaten The purpose of the arrows is to show where the energy is going Scientists refer to eating as an energy transfer, because when one organism eats another, the main goal is to get energy from the organism. SO, the arrow points at the organism that GETS the ENERGY (the organism doing the eating)

Limits of food chain length A food chain is usually only a few links long Can be as few as 2 to as many as 5 OR more Why? One hypothesis is the energetic hypothesis: the length of a food chain is limited by the inefficient transfer of energy from one organism to the next (only about 10%) Food chains with more photosynthetic organisms should be longer because you have MORE starting energy

Limits of food chain length Second hypothesis is the dynamic stability hypothesis: long food chains are less stable than short food chains The more organisms involved in a food chain the more potential for variation Extinction, migration, climatic changes With more species, you have more chances to disrupt the food chain

Food web Food chains are a very inaccurate depiction of feeding relationships in an ecosystem…Food webs are more accurate Food webs are interrelated food chains of an ecosystem

Large impact In communities, certain species may have a larger impact on the community structure than other They may be highly abundant Play a pivotal role in maintaining the balance in a community

Dominant Species Dominant Species: Species in a community that have the highest abundance or highest biomass These species have a powerful effect on the distribution and eating patterns of all other species in a community Possible reasons for a dominant species Dominant species is most competitive in acquiring limited resources Dominant species is most successful at avoiding predators OR disease This may be the reason invasive species can take over a community that lack their natural predators and pathogens

Keystone Species Keystone Species: Important to a community because of their ecological roles (niches), not by numbers When these species become extinct, or scarce, the entire community changes and usually many other species are affected

Sea otters and sea urchins

Keystone Species- Pisaster When a species of starfish (pisaster) that feeds on mussels was removed from an intertidal zone, the mussel began to dominate and eat other species (decreasing biodiversity)

Foundation Species Some organisms exert their influence by altering the environment This changes the landscape and alters the structural dynamics of the environment They may act as facilitators that have a positive affect on the community (protect from salt variations, maintain soil cohesion) They may also be harmful... What species do you think has been altering the environment the most?

Species interactions Two simple ways to explain the effects of organisms in a community Bottom-up model – lower levels of the food web influence the levels above them (producers are the most influential and the higher levels, top consumers, have the least influence) Top-down model – the opposite of above There are also many intermediates of these two

Ecological Disturbances

Disturbance Classic view of communities: Communities are in a state of equilibrium unless seriously disturbed by outside influences (they are stable) Constant composition of species Newer model is the nonequilibrium model Communities are constantly changing in response to disturbances Disturbance: anything that changes the community, removes organisms, or alters the natural resources (shocker: humans have the highest impact)

Ecological Succession Ecological succession is a change in the species that live in a given area over a period of time One community replaces another Primary succession = occurs in places where soil is not yet formed (bare bedrock) Secondary succession = occurs in places where there is soil, but where some disturbance has eliminated the previous community (fire, tidal wave, natural disaster)

Ecological Succession

The first organisms to inhabit an area undergoing succession are known as pioneer organisms These are usually small organisms (bacteria, lichens, algae, etc.) The ecosystem goes through a number of stages, with each new stage usually consisting of larger organisms than the last one Once a community has become stable and is not changing much, it is known as a climax community

Causes of Ecological Succession There are 3 major causes of ecological succession: 1. Human Activities - logging, mining, development, etc. 2. Natural Disasters/Disturbances - fires, volcanic eruptions, etc. 3. Natural Competition Among Species - Fictitious example: - turtles and frogs both eat crickets - frogs are faster, turtles are slower - frogs eat more crickets, turtles starve - turtle population dies out, frog population gets bigger

Biodiversity

Two factors are usually associated with species diversity: Geographic location Geographic size In general, fewer organisms on islands, than on main continent

Tropical diversity In general, there is a larger diversity of species in tropical regions than in temperate or polar regions Possible reasons: Evolutionary history – in general, tropical regions are generally older than other biomes (more consistent climate means fewer major disturbances)

Tropical diversity Climate is another reason for more diversity in the tropics Fairly consistent solar input Abundant rainfall Evapotranspiration measures how much water remains in a biome as opposed to the loss due to transpiration In the tropics, much of the water remains in the biome