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Community Ecology

Community Ecology

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Community Ecology

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  1. Community Ecology Reading: Freeman, Chapter 50, 53

  2. What is a community? • A community isan assemblage of plant and animal populations that live in a particular area or habitat. • Populations of the various species in a community interact and form a system with its own emergent properties.

  3. Pattern vs. Process • Pattern is what we can easily observe directly - vegetation zonation, species lists, seasonal distribution of activity, and association of certain species. • Process gives rise to the pattern- herbivory, competition, predation risk, nutrient availability, patterns of disturbance, energy flow, history, and evolution.

  4. Community ecology seeks to explain the underlying mechanisms that create, maintain, and determine the fate of biological communities. Typically, patterns are documented by observation, and used to generate hypotheses about processes, which are tested. • Not all science is experimental. Hypotheses tests can involve special observations, or experiments.

  5. Emergent Properties of a Community • Scale • Spatial and Temporal Structure • Species Richness • Species Diversity • Trophic structure • Succession and Disturbance

  6. Scale is the size of a community. • Provided that the area or habitat is well defined, a community can be a system of almost any size, from a drop of water, to a rotting log, to a forest, to the surface of the Pacific Ocean.

  7. Spatial Structure is the way species are distributed relative to each other. • Some species provide a framework that creates habitats for other species. These species, in turn create habitats for others, etc.

  8. Example: Trees in a rainforest are stratified into several different levels, including a canopy, several understories, a ground level, and roots. Each level is the habitat of a distinct collection of species. Some places, such as the pools of water that collect at the base of tree branches, may harbor entire communities of their own.

  9. Temporal structure is the timing of the appearance and activity of species. Some communities, i.e., arctic tundra and the decay of a corpse, have pronounced temporal species, other communities have less. • Example: Many desert plants and animals are dormant most of the year. They emerge, or germinate, in response to seasonal rains. Other plants stick around year round, having evolved adaptations to resist drought.

  10. Species Richness - is the number of species in a community. Clearly, the number of species we can observe is function of the area of the sample. It also is a function of who is looking. Thus, species richness is sensitive to sampling procedure

  11. Diversity is the number of species in the community, and their relative abundances. • Species are not equally abundant, some species occur in large percentage of samples, others are poorly represented. • Some communities, such as tropical rainforests, are much more diverse than others, such as the great basin desert. • Species Diversity is often expressed using Simpson’s diversity index:D=1-S (pi)2

  12. Example Problem • A community contains the following species: • Number of Individuals • Species A 104 • Species B 71 • Species C 19 • Species D 5 • Species E 3 • What is the Simpson index value for this community?

  13. Answer: • Total Individuals= (104+19+71+5+3)=202 • PA=104/202=.51 PB=19/202=.09 • PC=71/202=.35 PD=5/202=.03PE=3/202=.02 • D=1-{(.51)2+(.09)2+(.35)2+(.03)2+(.02)2} • D=1-.40=.60

  14. ClickerQuestion In the example above, what was the species richness? A. .60 B. 202 individuals C. 5 species D. .40 E. None of the above

  15. Succession, Disturbance and Change • In terms of species and physical structure, communities change with time. • Ecological succession, the predictable change in species over time, as each new set of species modifies the environment to enable the establishment of other species, is virtually ubiquitous.

  16. Example; a sphagnum bog community may persist for only a few decades before the process of ecological succession changes transform it into the surrounding Black Spruce Forest. • A forest fire may destroy a large area of trees, clearing the way for a meadow. Eventually, the trees take over and the meadow is replaced.

  17. Disturbances are events such as floods, fire, droughts, overgrazing, and human activity that damage communities, remove organisms from them, and alter resource availability.

  18. Some Agents of Disturbance • Fire • Floods • Drought • Large Herbivores • Storms • Volcanoes • Human Activity

  19. Disturbance, Invasion, Succession • Disturbance creates opportunities for new species to invade an area and establish themselves. • These species modify the environment, and create opportunities for other species to invade. The new species eventually displace the original ones. Eventually, they modify the environment enough to allow a new series of invaders, which ultimately replace them, etc.

  20. Invasion: • Disturbance creates an ecological vacuum that can be filled from within, from outside, or both. For example, forest fires clear away old brush and open up the canopy, releasing nutrients into the soil at the same time. Seeds that survive the fire germinate and rapidly grow to take advantage of this opportunity. At the same time, wind-borne and animal-dispersed seeds germinate and seek to do the same thing. • The best invaders have good dispersal powers and many offspring, but they are often not the best competitors in the long run.

  21. Succession • Disturbance of a community is usually followed by recovery, called ecological succession. • The sequence of succession is driven by the interactions among dispersal, ecological tolerances, and competitive ability. • Primary succession-the sequence of species on newly exposed landforms that have not previously been influenced by a community, e.g., areas exposed by glacial retreat. • Secondary succession occurs in cases which vegetation of an area has been partially or completely removed, but where soil, seeds, and spores remain.

  22. Early in succession, species are generally excellent dispersers and good at tolerating harsh environments, but not the best interspecific competitors. • As ecological succession progresses, they are replaced with species which are superior competitors, (but not as good at dispersing and more specialized to deal with the microenvironments created by other species likely to be present with them). • Early species modify their environment in such a way as to make it possible for the next round of species. These, in turn, make their own replacement by superior competitors possible.

  23. A climax community is a more or less permanent and final stage of aparticular succession, often characteristic of a restricted area. • Climax communities are characterized by slow rates of change, compared with more dynamic, earlier stages. • They are dominated by species tolerant of competition for resources.

  24. An Influential ecologist named F.E. Clements argued that communities work like an integrated machine. These “closed” communities had a predictable composition. According to Clements, there was only one true climax in any given climatic region, which was the endpoint of all successions. Other influential ecologists, including Gleason, hypothesized that random events determined the composition of communities. He recognized that a single climatic area could contain a variety of specific climax types.

  25. Evidence suggests that for many habitats, Gleason was right, many habitats never return to their original state after being disturbed beyond a certain point. • For example; very severe forest fires have reduced spruce woodlands to a terrain of rocks, shrubs and forbs.

  26. An incredibly rapid glacial retreat is occurring in Glacier Bay, Alaska. In just 200 years, a glacier that once filled the entire bay has retreated over 100km, exposing new landforms to primary succession. • Clements would have predicted that succession today would follow the sequence of ecological succession that has occurred in the past for other parts of Alaska. • In fact, three different successional patterns seem to be occurring at once, depending upon local conditions. Thus, Clements’ view of succession is somewhat of an oversimplification.

  27. AreClimax Communities Real? • Succession can take a long time. • For example, old-field succession may require 100-300 years to reach climax community. But in this time frame, the probability that a physical disturbance (fire, hurricane, flood) will occur becomes so high, the process of succession may never reach completion.

  28. Increasing evidence suggests that some amount of disturbance and nonequilibrium resulting from disturbance is the norm for most communities. • One popular hypothesis is that communities are usually in a state of recovery from disturbance. • An area of habitat may form a patchwork of communities, each at different stages of ecological succession. Thus, disturbance and recovery potentially enable much greater biodiversity than is possible without disturbance.

  29. Are biological communities real functional units? • Do communities have a tightly prescribed organization and composition, or are they merely a loose assemblage of species? • This is an unsolved problem in ecology. • Clements argued that communities are stable, functional units with a fixed composition-each integrated part needs the others. Every area should ultimately have the same species, given time. • Gleason argued that their composition is unstable and variable-they are more like assemblages of everything that can live together in one place

  30. The Kiddie Pool Experiment • Jenkins and Buikema conducted an experiment to see whether artificial ponds would develop predictable assemblages of freshwater microorganisms. • -if this were the case, it would support the notion that communities are real, integrated units. • -They set up 12 identical “ponds” and filled them with sterile water. Came back in year to study the composition of the resulting communities.

  31. Result-the ponds had very different compositions of species. • Accidents of dispersal, and different dispersal capabilities affected which species ended up in each pond. • The early arrival of certain competitors, and predators greatly affected the ability of later species to colonize later. • -Gleason’s view was supported. Composition of communities is dictated largely by chance and history.

  32. Trophic structure is the hierarchy of feeding. It describes who eats whom • (a trophic interaction is a transfer of energy: i.e., eating, decomposing, obtaining energy via photosynthesis). • For every community, a diagram of trophic interactions called a food web. • Energy flows from the bottom to the top.

  33. A Simple Food Web Killer Whales Sharks Harbor Seals Yellowfin Tuna Mackerel Cod Halibut Zooplankton Unicellular Algae and Diatoms

  34. Killer Whales Harbor Seals Mackerel Zooplankton Phytoplankton One path through a food web is a food chain.

  35. The niche concept is very important in community ecology. • A niche is an organism’s habitat and its way of making a living. • An organism’s niche is reflected by its place in a food web: i.e, what it eats, what it competes with, what eats it. • Each organism has the potential to create niches for others.

  36. Keystone species are disproportionately important in communities. • Generally, keystone species act to maintain species diversity. • The extinction of a keystone species eliminates the niches of many other species. • Frequently, a keystone species modifies the environment in such a way that other organisms are able to live, in other cases, the keystone species is a predator that maintains diversity at a certain trophic level.

  37. Examples of Keystone Species • California Sea Otters: This species preys upon sea urchins, allowing kelp forests to become established. • Pisaster Starfish: Grazing by Pisaster prevents the establishment of dense mussel beds, allowing other species to colonize rocks on the pacific coast • “Mangrove” trees: Actually, many species of trees are called mangrove trees. Their seeds disperse in salt water. They take root and form a dense forest in saltwater shallows, allowing other species to thrive

  38. Trophic Cascades • Species at one trophic level influence species at other levels; the addition or subtraction of species affects the entire food web. • This causes positive effects for some species, and negative effects for others. This is called a trophic cascade. For instance, removing a secondary consumer might positively affect the primary consumers they feed upon, and negatively affect the producers that are food for primary consumers.

  39. Top down vs. Bottom up • Most biological communities have both top-down and bottom-up effects on their structure and composition. • In a well known study of ponds by Matthew Leibold, it was demonstrated that the biomass of herbivores (zooplankton) was positively correlated to the biomass of producers (algae), indicating a top down effect. • He intentionally introduced fish to some ponds, The result was a decrease in zooplankton and increase in producers, indicating a top down effect.

  40. Badly scanned from Rose and Mueller (2006)

  41. Types of Interspecific Interactions • Effect on Effect on • Species 1 Species 2 • Neutralism 0 0 • Competition - - • Commensalism + 0 • Amensalism - 0 • Mutualism + + • Predation, - + • Parasitism, Herbivory

  42. Neutralism • Neutralism the most common type of interspecific interaction. Neither population affects the other. Any interactions that do occur are indirect or incidental. • Example: the tarantulas living in a desert and the cacti living in a desert

  43. Competition • Competition occurs when organisms in the same community seek the same limiting resource. This resource may be prey, water, light, nutrients, nest sites, etc. • Competition among members of the same species is intraspecific. • Competition among individuals of different species is interspecific. • Individuals experience both types of competition, but the relative importance of the two types of competition varies from population to population and species to species

  44. “Styles” of Competition • Exploitation competition occurs when individuals use the same limiting resource or resources, thus depleting the amount available to others. • Interference competition occurs when individuals interfere with the foraging, survival, or reproduction of others, or directly prevent their physical establishment in a portion of a habitat.

  45. Some specific types of competition • Consumptive competition • Preemptive competition • Overgrowth competition • Chemical composition • Territorial competition • Encounter competition

  46. Example of Interference Competition • The confused flour beetle, Triboleum confusum, and the red flour beetle, Triboleum castaneum cannibalize the eggs of their own species as well as the other, thus interfering with the survival of potential competitors. • In mixed species cultures, one species always excludes the other. Which species prevails depends upon environmental conditions, chance, and the relative numbers of each species at the start of the experiment.

  47. Outcomes of Competition • Exploitation competition may cause the exclusion of one species. For this to occur, one organism must require less of the limiting resource to survive. The dominant species must also reduce the quantity of the resource below some critical level where the other species is unable to replace its numbers by reproduction. • Exploitation does not always cause the exclusion of one species. They may coexist, with a decrease in their potential for growth. For this to occur, they must partition the resource. • Interference competition generally results in the exclusion of one of the two competitors.