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Chapter 54

Chapter 54. Community Ecology. A biological community is an assemblage of populations of various species living close enough for potential interaction. 54.1: Community interactions are classified by whether they help, harm, or have no effect on the species involved.

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Chapter 54

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  1. Chapter 54 Community Ecology

  2. A biological community is an assemblage of populations of various species living close enough for potential interaction

  3. 54.1: Community interactions are classified by whether they help, harm, or have no effect on the species involved • Ecologists call relationships between species in a community interspecific interactions • Competition • Predation • Herbivory • Symbiosis (parasitism, mutualism, and commensalism) • Can affect the survival and reproduction of each species

  4. Competition • (–/– interaction)occurs when species compete for a resource in short supply • Example?

  5. Competitive exclusion principle • **The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same place**

  6. Niche • Ecological niche- The total of a species’ use of biotic and abiotic resources • an organism’s ecological role • Ecologically similar species can ONLY coexist in a community if there are one or more significant differences in their niches • Why??????????????????????????

  7. Resource partitioning • differentiation of ecological niches • enabling similar species to coexist in a community • Example? • Species can change niches when there is overlap with another species • Fundamental vs. realized niche

  8. Fig. 54-2 A. distichus perches on fence posts and other sunny surfaces. A. insolitus usually perches on shady branches. A. ricordii A. insolitus A. aliniger A. christophei A. distichus A. cybotes A. etheridgei

  9. Fig. 54-3 EXPERIMENT High tide Chthamalus Chthamalus realized niche Balanus Balanus realized niche Ocean Low tide RESULTS High tide Chthamalus fundamental niche Ocean Low tide

  10. Character Displacement • Thetendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species • In other words: • two species are MORE different from each other when they live in the same area than when they live apart from each other.

  11. Predation • (+/– interaction) refers to interaction where one species, the predator, kills and eats the other, the prey • Predator adaptations include____ • Prey adaptations include______

  12. Cryptic coloration- camouflage, makes prey difficult to spot • Aposematic coloration- Bright warning coloration that animals with effective chemical defense often exhibit as a warning

  13. Mimicry • Batesian mimicry- a harmless species mimics an unpalatable or harmful model • Müllerian mimicry, two or more unpalatable species resemble each other

  14. Fig. 54-5c (c) Batesian mimicry: A harmless species mimics a harmful one. Hawkmoth larva Green parrot snake

  15. Fig. 54-5d (d) Müllerian mimicry: Two unpalatable species mimic each other. Cuckoo bee Yellow jacket

  16. Herbivory • (+/– interaction) refers to an interaction in which an herbivore eats parts of a plant or alga • It has led to evolution of plant mechanical and chemical defenses and adaptations by herbivores

  17. Fig. 54-6

  18. Symbiosis • Symbiosis is a relationship where two or more species live in direct and intimate contact with one another

  19. 3 • parasitism (+/– interaction) one organism derives nourishment from another organism, which is harmed in the process • the parasite • the host • What do you think is the difference btwn an endoparasite and an ectoparasite?

  20. Why are many/most parasites NOT extremely deadly?

  21. Mutualism • (+/+ interaction), is an interspecific interaction that benefits both species • Obligate, where one species cannot survive without the other • Facultative, where both species can survive alone

  22. Fig. 54-7 (a) Acacia tree and ants (genus Pseudomyrmex) (b) Area cleared by ants at the base of an acacia tree

  23. Commensalism • (+/0 interaction), one species benefits and the other is apparently unaffected • Commensal interactions are hard to document in nature because any close association likely affects both species

  24. Fig. 54-8

  25. 54.2: Dominant and keystone species exert strong controls on community structure • In general, a few species in a community exert strong control on that community’s structure

  26. Species Diverstiy • Species diversity of a community is the variety of organisms that make up the community • Species richness is the total number of different species in the community • Relative abundance is the proportion each species represents of the total individuals in the community

  27. Fig. 54-9 A B C D Community 1 Community 2 A: 80% B: 5% C: 5% D: 10% A: 25% B: 25% C: 25% D: 25%

  28. Two communities can have the same species richness but a different relative abundance • Diversity can be compared using a diversity index • Shannon diversity index (H): H = –[(pAlnpA) + (pBlnpB) + (pClnpC) + …]

  29. Trophic Structure • Trophic structure is the feeding relationships between organisms in a community • Food chains link trophic levels from producers to top carnivores • REMEMBER WHICH WAY THE ARROWS GO!

  30. Fig. 54-11 Quaternary consumers Carnivore Carnivore Tertiary consumers Carnivore Carnivore Secondary consumers Carnivore Carnivore Primary consumers Herbivore Zooplankton Primary producers Plant Phytoplankton A terrestrial food chain A marine food chain

  31. Food Webs • A food web is a branching food chain with complex trophic interactions • Species may play a role at more than one trophic level • How is a food web diff from a food chain?

  32. Fig. 54-12 Humans Smaller toothed whales Sperm whales Baleen whales Elephant seals Leopard seals Crab-eater seals Squids Fishes Birds Carnivorous plankton Euphausids (krill) Copepods Phyto- plankton

  33. Limits on Food Chain Length • Each food chain in a food web is usually only a few links long • Two hypotheses attempt to explain food chain length: • the energetic hypothesis • the dynamic stability hypothesis

  34. The energetic hypothesis suggests that length is limited by inefficient energy transfer • The dynamic stability hypothesis proposes that long food chains are less stable than short ones • Most data support the energetic hypothesis

  35. Species with a Large Impact • Certain species have a very large impact on community structure • Such species are highly abundant or play a pivotal role in community dynamics

  36. Dominant species • Species that are the most abundant or have the highest biomass • Maybe dom b/c most exploitive, competitive • Maybe b/c they avoid predation • Biomassis the total mass of all individuals in a population

  37. Invasive species (what do you think that means?) • typically introduced to a new environment by humans, often lack predators or disease

  38. Keystone species exert strong control on a community by their ecological roles, or niches • What’s “keystone” mean to you? • In contrast to dominant species, they are not necessarily abundant in a community • Often, we figure out “keystone” species b/c when they are removed… whoa changes!

  39. Fig. 54-15b RESULTS 20 With Pisaster (control) 15 Number of species present 10 Without Pisaster (experimental) 5 0 ’73 1963 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 Year

  40. Fig. 54-16 100 80 60 Otter number (% max. count) 40 20 0 (a) Sea otter abundance 400 300 Grams per 0.25 m2 200 100 0 (b) Sea urchin biomass 10 8 Number per 0.25 m2 6 4 2 0 1972 1985 1989 1993 1997 Year (c) Total kelp density Food chain

  41. Foundation Species (Ecosystem “Engineers”) • Cause physical changes in the environment that affect community structure • beaver dams can transform landscapes on v. large scale

  42. Some foundation species act as facilitators that have positive effects on survival and reproduction of some other species in the community • *small distinction btwn “facilitators” and “keystone”

  43. Fig. 54-18b 8 6 Number of plant species 4 2 0 With Juncus Without Juncus (b)

  44. How are these communities controlled? • Bottom-up model • Top-down model

  45. Bottom up model • Unidirectional influence from lower to higher trophic levels • Presence or absence of mineral nutrients determines community structure, including abundance of primary producers

  46. Top-down model • Control comes from the trophic level above • also called the trophic cascade model • Predators control herbivores, which in turn control primary producers

  47. Long-term experimental studies have shown that communities vary in their relative degree of bottom-up to top-down control • Next slide: biomanipulation is used to fix the effects of pollution

  48. Fig. 54-UN1 Polluted State Restored State Fish Abundant Rare Zooplankton Rare Abundant Algae Abundant Rare

  49. 54.3: Disturbance influences species diversity and composition • Decades ago, most ecologists favored the view that communities are in a state of equilibrium • Recent evidence of change has led to a nonequilibrium model, which describes communities as constantly changing after being buffeted by disturbances

  50. Characterizing Disturbance • A disturbance is an event that changes a community, removes organisms from it, and alters resource availability • Fire is a significant disturbance in most terrestrial ecosystems • It is often a necessity in some communities

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