Chap 55

• Biological Communities

  Community - Species that occur at any particular locality. Characterized by

  • Species richness - Number of species present.
  • Primary productivity - Amount of energy produced.
• Ecotones - places where the environment changes abruptly

  (a transition area between two biological communities, where two communities meet and integrate)
• Niche - the total of all the ways an organism uses the resources of its environment. (Space utilization, Food consumption, Temperature range, etc)
• Ecological Niche: Competition

  Interspecific competition - when two species attempt to use the same resource and there is not enough resource to satisfy both.

  Interference competition - Physical interactions over access to resources (Lions Fighting for resources, displacing another, etc)

  Exploitative competition - Consuming the same resource (limits food for others)
• Types of Ecological Niches

  Fundamental niche - Entire niche that a species is capable of using, based on physiological tolerance limits and resource needs (niche that is potentially occupied by a species)

  -

  Realized niche - Actual set of environmental conditions, presence or absence of other species, in which the species can establish a stable population(the portion of the fundamental niche that the organism actually occupies)
• If two species are competing for a limited resource, the species that uses the resource more efficiently will eventually eliminate the other locally.
• Resource Partitioning - Subdivided niche to avoid direct competition

  (Thought to result from the process of natural selection)
• Character displacement - Differences in morphology evident between sympatric species; May play a role in adaptive radiation.

  (favoring those who are different in a species because they aren't competing for the same things as their peers)
• Prey populations can have explosions and crashes

  • White-tailed deer in Eastern U.S.
  • Introduction of rats, dogs, cats on islands.
  • New Zealand: Stephen Island wren extinct because of a single cat
• Predation and coevolution

  • Predation provides strong selective pressure on the prey population.

  • Features that decrease the probability of capture are strongly favored. • Coevolution race may ensue.
• Plants as Prey

  • Plants adapt to predation (herbivory) by evolving mechanisms to defend themselves •
  • Chemical defenses: secondary compounds; Oils, chemicals to attract predators to eat the herbivores, poison milky sap, and others.
  • Herbivores coevolve to continue eating the plants.
• Chemical Defenses in Animals

  • Monarch butterfly caterpillars feed on milkweed and dogbane families
  • Monarchs incorporate cardiac glycosides from the plants for protection from predation
  • Poison-dart frogs produce toxic alkaloids in the mucus that covers their brightly colored skin
  • Defensive Coloration
  • Insects and other animals that are poisonous use warning coloration
  • Camouflage or cryptic coloration help nonpoisonous animals blend with their surroundings.
• Mimicry

  Mimicry allows one species to capitalize on defensive strategies of another

  • Batesian mimicry - Mimics look like distasteful species (mimics poisonous animals/Sheep in Wolfs Clothing)
  • Müllerian mimicry - Several unrelated but poisonous species come to resemble one another (group defense/ predators learn not to eat any animals that look a certain way)
• Symbiosis

  • 2 or more kinds of organisms interact in more-or-less permanent relationships.
  • Potential for coevolution.
• Three major types of symbiosis

  1. Commensalism - benefits one species and is neutral to (does not hurt or help) the other
  2. Mutualism - benefits both species
  3. Parasitism - Parasitism benefits one species and hurts the other
• Mutualism (type of symbiosis)

  • Coevolution: flowering plants and insects.
  • Ants and acacias.
  • Acacias provide hollow thorns and food; Ants provide protection from herbivores
  • Ants and Acacias can be parasitic. In Kenya, several species of ants live on acacias.
  • One species clips the acacia branches to prevent other ants from living in the tree.
  • Clipping branches sterilizes the tree.
• Parasitism (type of symbiosis)

  • External parasites
  • Ectoparasites: feed on exterior surface of an organism.
  • Parasitoids: insects that lay eggs on living hosts. (wasp)
  • Internal parasites
  • Endoparasites - live inside the host
  • Extreme specialization by the parasite as to which host it invades
  • Structure of the parasite may be simplified because of where it lives in its host
  • Life cycles involving more than one host
• Parasitism (type of symbiosis)

  Internal Parasites

  • A flatworm that lives in ants as an intermediate host with cattle as its definitive host
  • To go from the ant to a herbivorous mammal, it changes the behavior of the ant
  • Causes the ant to climb to the top of a blade of grass to be eaten with the grass
  • Makes the ant get eaten by a herbivore so the worm can get to where it really wants to be, in a herbivore tummy.
• Commensalism (type of symbiosis)

  • Oxpeckers and grazing animals
  • Oxpeckers eat parasites off of grazers.
  • Sometimes pick scabs and drink blood.
  • Grazers could be unharmed by the insects the oxpeckers eat.
• Effects of Interaction

  • Ecological processes have interactive effects
  • Predation reduces competition.
  • Predators choice depends partly on relative abundance of the prey options.
  • Superior competitors may be reduced in number by predation.
  • This allows other species to survive when they could have been outcompeted
• Parasitism and Competition

  • Parasitism may counter competition
  • Parasites may affect sympatric species differently, changing the outcome of interspecific interactions.
  • Flour beetles and a competition experiment.
  • Without a parasite: T. castaneum is dominant.
  • With the parasite: T. confusum is dominant.
• Indirect Effects - presence of one species may affect a second by way of interactions with a third species
• • Keystone Species - Species whose effects on the composition of communities are greater than one might expect based on their abundance
  • Sea star predation on barnacles greatly alters the species richness of the marine community
  • Keystone species can manipulate the environment in ways that create new habitats for other species
  • Beavers are keystone species
• Beavers as a Keystone Species

  Beavers construct dams and transform flowing streams into ponds, creating new habitats for many plants and animals
• Succession and Disturbance

  Succession - Communities have a tendency to change from simple to complex.

  • Primary succession occurs on bare, lifeless substrate.
  • Open water, Rocks.
  • Organisms gradually move into an area and change its nature.
  • Secondary Succession occurs in areas where an existing community has been disturbed but organisms still remain
  • Field left uncultivated.
  • Forest after a fire.
  • Succession happens because species alter the habitat and the resources available in ways that favor other species entering the habitat
• Why Succession Happens

  Three dynamic concepts in the process

  • Establishment: early successional species are characterized by r-selected species tolerant of harsh conditions.
  • Facilitation: early successional species introduce local changes in the habitat K selected species replace r-selected species.
  • Inhibition: changes in the habitat caused by one species inhibits the growth of the original species. (species hinder the establishment and growth of other species, community becomes more crowded, competition for resources like light, space, and nutrients intensifies),
• Succession in Animal Communities

  Animal species in a community can also change over time

  • Krakatau island.
  • Volcanic eruption.
  • Fauna changed in synchrony with the vegetation.
  • Changes in animals affect plant occurrences; pollination, animal dispersion.
• Change in Communities

  • Communities are constantly changing as a result of
  • Climatic changes, Species invasions and Disturbance events.
  • Nonequilibrium models that emphasize change rather than stability are used to study communities and ecosystems
• Intermediate Disturbance Hypothesis

  • Communities experiencing moderate amounts of disturbance will have higher levels of species richness than communities experiencing either little or great amounts of disturbance
  • Patches of habitat will exist at different successional stages.
  • May prevent communities from reaching the final stages of succession
• Role of Disturbance

  • Disturbance is common, rather than exceptional in many communities
  • Understanding the role that disturbances play in structuring communities is an important area of ecology
• When species occupy the same niche, both competitive exclusion and resource partitioning are common outcomes.
• Moderate disturbance often leads to an increase in species richness, while severe disturbance usually leads to a decrease in species richness.
• Other causes of niche restriction are predator absence or presence and absence of pollinators.
• G.F. Gause’s classic experiment on competitive exclusion show that If two species are competing for a limited resource, the species that uses the resource more efficiently will eventually eliminate the other locally
• Character displacement - Differences in morphology (form) evident between sympatric (living in same area) species. (occupy similar niches differentiate in order to minimize niche overlap and avoid competitive exclusion like changing beaks)

  • May play a role in adaptive radiation