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Chapter 54

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  1. Chapter 54 Community Ecology

  2. Reminder: Earth Day April 22nd, 2010

  3. Earth Day 2010 (40th anniversary of Earth day)

  4. Iowa DNR 2009 self-assessment The DNR also rated water quality at C-. Numerically the water quality rated a D, but they gave themselves extra credit for effort!

  5. Iowa’s water quality (DNR self-assessment)

  6. A biological community • Is an assemblage of populations of various species living close enough for potential interaction The animals and plants surrounding this watering hole are all members of a savanna community in southern Africa

  7. Community Interactions • A community’s interactions include competition, predation, herbivory, symbiosis, and disease • Populations are linked by interspecific interactions that affect the survival and reproduction of the species engaged in the interaction

  8. Interspecific Interactions

  9. Competition

  10. Competition • Interspecific competition • Occurs when species compete for a particular resource that is in short supply • Strong competition can lead to competitive exclusion • The local elimination of one of the two competing species

  11. The Competitive Exclusion Principle • The competitive exclusion principle • States that two species competing for the same limiting resources cannot coexist in the same place

  12. Ecological Niches • The ecological niche • Is the total of an organism’s use of the biotic and abiotic resources in its environment • So if we take the niche concept into account • Then the competitive exclusion principle can be reworded as: • Two species cannot coexist in a community if their niches are identical

  13. EXPERIMENT RESULTS Ecologist Joseph Connell studied two barnacle speciesBalanus balanoides and Chthamalus stellatus that have a stratified distribution on rocks along the coast of Scotland. When Connell removed Balanus from the lower strata, the Chthamalus population spread into that area. High tide High tide Chthamalus Chthamalusrealized niche Balanus Chthamalusfundamental niche Balanusrealized niche Ocean Ocean Low tide Low tide In nature, Balanus fails to survive high on the rocks because it isunable to resist desiccation (drying out) during low tides. Its realized niche is therefore similar to its fundamental niche. In contrast, Chthamalus is usually concentrated on the upper strata of rocks. To determine the fundamental of niche of Chthamalus, Connell removed Balanus from the lower strata. CONCLUSION The spread of Chthamalus when Balanus wasremoved indicates that competitive exclusion makes the realizedniche of Chthamalus much smaller than its fundamental niche. • Ecologically similar species can coexist in a community. If there are one or more significant difference in their niches • Fundamental niche (the niche potentially occupied) versus realized niche (the niche actually occupied)

  14. As a result of competition • A species’ fundamental niche may be different from its realized niche

  15. A. insolitususually percheson shady branches. A. ricordii A. insolitus A. distichus perches on fence posts and other sunny surfaces. A. alinigar A. christophei A. distichus A. cybotes A. etheridgei Resource Partitioning • Resource partitioning is the differentiation of niches that enables similar species to coexist in a community

  16. G. fortis G. fuliginosa Beak depth Santa María, San Cristóbal 40 Sympatric populations 20 0 Los Hermanos Percentages of individuals in each size class G. fuliginosa, allopatric 40 20 Daphne 0 40 G. fortis, allopatric 20 8 10 12 14 16 0 Beak depth (mm) Character Displacement • In character displacement there is a tendency for characteristics to be more divergent in sympatric (geographically overlapping) populations of two species than in allopatric (geographically separate) populations of the same two species • The allopatric populations have similar beaks and use similar resources. • The sympatric populations would potentially compete for resources and their characteristics are more divergent

  17. Predation

  18. Predation • Predation refers to an interaction where one species, the predator, kills and eats the other, the prey • Adaptations • Feeding adaptations of predators include claws, teeth, fangs, stingers, and poison • animals also display a great variety of defensive adaptations to conteract predation

  19. Predation (Hawk-snake)

  20. Cryptic coloration, or camouflage makes prey difficult to spot

  21. Aposematic coloration: warns predators to stay away from prey

  22. Warning coloration in poisonous frogs (movie)

  23. Mimicry • In some cases, one prey species • May gain significant protection by mimicking the appearance of another • Batesian mimicry • Müllerian mimicry

  24. (b) Green parrot snake (a) Hawkmoth larva • In Batesian mimicry: A palatable or harmless species mimics an unpalatable or harmful model

  25. (a) Cuckoo bee (b) Yellow jacket • In Müllerian mimicry • Two or more unpalatable species resemble each other

  26. Fooling predators (movie)

  27. HerbivoryParasitismDiseaseMutualismCommensalism

  28. Herbivory • Herbivory • Has led to the evolution of plant mechanical and chemical defenses and consequent adaptations by herbivores

  29. Parasitism • In parasitism, one organism, the parasite • Derives its nourishment from another organism, its host, which is harmed in the process • Parasitism can have a significant effect on survival, reproduction, density, and productivity of a population • Effects can be direct or indirect Tomato Hornworm covered in cocoons of pupating parasitic wasp

  30. Disease • Pathogens (disease-causing agents) have effects on populations and communities that are similar to that of parasites Deer with Chronic Wasting Disease

  31. Mutualism • Mutualistic symbiosis, or mutualism is an interspecific interaction that benefits both species

  32. Mutualism

  33. Mutualism

  34. Commensalism • In commensalism: One species benefits and the other is not affected • Difficult to positively document in nature, because in any association between organisms both are probably affected Cattle egrets and water buffalo

  35. Interspecific Interactions and Adaptation • Evidence for coevolution • Which involves reciprocal genetic change by interacting populations, is scarce • However, generalized adaptation of organisms to other organisms in their environment appears to be a fundamental feature of life

  36. Species in the Structure of a Community • In general, a small number of species in a community exert strong control on that community’s structure. • Dominant and Keystone species (more on these soon)

  37. Species Diversity • The species diversity of a community • Is the variety of different kinds of organisms that make up the community • Has two components: species richness and relative abundance • 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

  38. A B C D Community 1 A: 25% B: 25% C: 25% D: 25% Community 2 A: 80% B: 5% C: 5% D: 10% • Two different communities can have the same species richness, but a different relative abundance

  39. A community with an even species abundance • Is more diverse than one in which one or two species are abundant and the remainder rare

  40. Trophic Structure • Trophic structure • Is the feeding relationships between organisms in a community • Is a key factor in community dynamics

  41. Quaternary consumers Carnivore Carnivore Tertiary consumers Carnivore Carnivore Secondary consumers Carnivore Carnivore Primary consumers Zooplankton Herbivore Primary producers Plant Phytoplankton A terrestrial food chain A marine food chain • Food chains link the trophic levels from producers to top carnivores

  42. Humans Smaller toothed whales Baleen whales Sperm whales Elephant seals Leopard seals Crab-eater seals Squids Fishes Birds Carnivorous plankton Copepods Euphausids (krill) Phyto-plankton Food Webs • A food web is a branching food chain with complex trophic interactions

  43. Limits on Food Chain Length • Each food chain in a food web • Is usually only a few links long • There are two hypotheses that attempt to explain food chain length • The energetic hypothesis suggests that the length of a food chain is limited by the inefficiency of energy transfer along the chain • The dynamic stability hypothesis proposes that long food chains are less stable than short ones

  44. Species with a Large Impact • Certain species have an especially large impact on the structure of entire communities • Either because they are highly abundant or because they play a pivotal role in community dynamics

  45. Dominant Species • Dominant species • Are those species in a community that are most abundant or have the highest biomass • Exert powerful control over the occurrence and distribution of other species

  46. One hypothesis suggests that dominant species • Are most competitive in exploiting limited resources • Another hypothesis for dominant species success • Is that they are most successful at avoiding predators (human introduced invasive species as an example)

  47. Keystone Species • Keystone species • Are not necessarily abundant in a community • Exert strong control on a community by their ecological roles, or niches

  48. With Pisaster (control) 20 15 Number of species present 10 Without Pisaster (experimental) 5 0 1963 ´70 ´71 ´73 ´64 ´65 ´69 ´66 ´72 ´67 ´68 (b) When Pisaster was removed from an intertidal zone, mussels eventually took over the rock face and eliminated most other invertebrates and algae. In a control area from which Pisaster was not removed, there was little change in species diversity. (a) The sea star Pisaster ochraceous feeds preferentially on mussels but will consume other invertebrates. • Field studies of sea stars exhibit their role as a keystone species in intertidal communities

  49. 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 6 Number per 0.25 m2 4 2 0 1972 1985 1989 1993 1997 Year (c) Total kelp density Food chain after killerwhales started preyingon otters Food chain beforekiller whale involve-ment in chain • Observation of sea otter populations and their predation shows the effect the otters have on ocean communities