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Chapter 53

Chapter 53. Communities. What is a biological community?. An assemblage of species living close enough together so that there are potential interactions. What causes a community to have a certain assemblage of species?. Two contrasting views: INDIVIDUALISTIC HYPOTHESIS

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Chapter 53

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  1. Chapter 53 Communities

  2. What is a biological community? • An assemblage of species living close enough together so that there are potential interactions.

  3. What causes a community to have a certain assemblage of species? • Two contrasting views: • INDIVIDUALISTIC HYPOTHESIS • INTERACTIVE HYPOTHESIS

  4. Individualistic Hypothesis • Chance assemblage of species that come together because they have similar abiotic requirements. • Proposed by H.A. Gleason the early 1900’s at the University of Chicago.

  5. Interactive Hypothesis • Communities are such because the organisms that make up the community are locked into an association by mandatory biotic interactions. • Communities are an integrated unit. • A “super organism” • Proposed by F.E. Clements in 1900’s

  6. Both hypotheses are debated today. • Research in community ecology is done from the point of view from one of these hypotheses. • Individualistic followers promote their research by studying individual species within the context of the community. • Interactive followers promote the study of the community as a whole and their research reflects such thinking.

  7. Robert Whittaker addressed these questions in plant communities.

  8. So plant communities are generally individualistic, but what about animals? • Refinement of the interactive model in animals  Rivet model  All species are like rivets in the wing of an airplane. If you remove enough “rivets” you don’t want to fly that plane.

  9. Resurfacing of the Individualistic model for animal communities. • Redundancy model: The web of life is very loose. An increase or decrease has little effect on other species. • Ex: If a predator disappears another one will take its place. The community has a certain “redundancy.”

  10. INTERSPECIFIC INTERACTIONS • Relationships between species • Let’s start simple: Examine the relationship between just two species. • COMPETITION • PREDATION (and Parasitism) • MUTUALISM • COMMENSALISM

  11. Comparison of interspecies interactions

  12. COMPETITION • Occurs only when resources are limited. • Competitive Exclusion Principle: Two species cannot occupy the same area if one species has a distinct advantage over the other. The species with the advantage will eventually differentially reproduce and “out compete” the other if given enough time to occur.  Experiments by G.F. Gause with paramecium.

  13. The Ecological Niche • The sum total of a species’ use of the biotic and abiotic resources in its environment. • Analogy: If an organisms habitat is its address its niche would be that habitat and its occupation. • It’s an organisms role or where it fits in to the environment. • Another way of stating the competitive exclusion principle is that two species cannot occupy the same niche as each other in a particular environment. • Two species can coexist if their niches are slightly different.

  14. Two possible outcomes that arise from competition of two species that have identical niches: • The less competitive species will be driven to extinction OR • One of the species will evolve through natural selection to use a different set of resources  Resource Partitioning

  15. Barnacles and the competitive exclusion principles

  16. Character Displacement is a principle that illustrates the competitive exclusion principle • Character Displacement: Sympatric populations of two species show more divergence in there physical features than two allopatric populations. • What does this suggest with regard to the competitive exclusion principle?

  17. Character displacement is evidence for the competitive exclusion principle Take these two finch species on the Galapagos… On separate islands they have similar beak sizes. When they are on the same island they have divergent beak sizes What does this suggest with regard to the competitive exclusion principle?

  18. PREDATION • We think of a lion eating an antelope. • But predation also refers to herbivory (bison eating grass) or parasitism (mosquito sucking your blood).

  19. Predator Adaptations • Acute senses in order to identify what is prey and where to find it. • Refined mechanisms for capturing and subduing prey; claws, teeth, fangs, poisons, etc. • What would be some of the adaptations that a predator that hunts and chases it prey have? • How about those that ambush their prey?

  20. Prey Defenses • Plants may have thorns or chemical defenses. Peppermint, clove, cinnamon are all distasteful to insects. • Animals flee vigorously energetically costly. • Some animals use hiding or use alarm calls. • Some rely upon camouflage Cryptic coloring

  21. Aposematic coloration • When an animal displays warning signs that it is toxic bold, bright colors.

  22. Batesian mimicry • The art of disguising oneself so that you resemble a more dangerous or distasteful counterpart. Green parrot snake Hawkmouth larva

  23. Mullerian mimicry • When two more unpalatable species resemble each other so that each species gains additional advantage because by pooling numbers, predators learn more quickly to avoid this type of critter with this coloration. Yellow jacket Cuckoo bee

  24. Parasites and pathogens as predators • Symbiotic relationship where one individual derived nourishment (parasite) and the other is harmed in the process (host). • May live within the host Endoparasites • May live on the external surface of the host  Ectoparasites • Parasitoidism  Insects lay eggs on the host and the larva feed on the body of the host.

  25. What’s the form of parasitism?

  26. Mutualism South American Acacia trees and Pseudomyrmex ants • Both species benefit The ants live in the thorns and consume nectar and they bite any vertebrate or invertebrate when they try to eat the leaves. The ants also release a odor which drives away predators. So what happens when bees try to pollinate the flowers? Don’t they get attacked?

  27. Commensalism • One benefits and the other gains nothing

  28. Food chains vs. Food webs

  29. Dominant Species The species that has the most biomass in a community, if removed may not have a great effect. Redundancy model. Ex: Sugar maple Keystone Species Not abundant in a community, exert strong control over community structure. If removed their impact is obvious. Ex: Sea otters in N. Pacific eat sea urchins, which keep eat kelp. Dominant species vs. Keystone Species

  30. Ecological Succession • Primary Succession  called this if the area was relatively lifeless to begin. Nothing more than bacteria and rocks; Ex: Glacier retreats or new volcanic island Bacteria break down rocks to soil lichens and moss from windblown spores grow Soil develops even more decomposition and first organic nutrients in soil seeds from plants grow  grasses, weeds then ultimately trees and animals.

  31. Secondary Succession • An existing community has been cleared by some sort of disturbance but the soil has been left intact. • Ex: Fire, eruption etc. Early arrivals facilitate, inhibit or tolerate later arrivals.

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