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Chapter 53: Community Ecology

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1 Chapter 53: Community Ecology
Trophic Structure, Predation, and Competition

2 Interspecific interactions:
Biological Community Assemblage of populations of various species living close enough for potential interaction Interspecific interactions: Symbiosis Competition Disease Herbivory Predation Interspecific Competition: species compete for a resource that’s in short supply

3 Competition Competitive Exclusion Principle 2 species competing for same resource can’t coexist in same place at same time Ecological Niche Sum of species’ use of biotic & abiotic resources Resource partitioning: differentiation of niches, so similar species coexist in same community Character Displacement More divergent characteristics in sympatric populations of 2 species than in allopatric populations of same species sympatric: geographically overlapping, need diff. niches to survive; allopatric: separate, can have same niches

4 Predation, Parasitism, Herbivory
+/- interaction; predator kills & eats prey +/- interaction; herbivore eats plant, kills it Prey Predator Herbivore Plant Self-defense mechanisms: camouflage, bright coloration, mimicry Have senses that enable them to locate prey Have special sensors to recognize appropriate food Chemical weapons/ thorns protect them +/- interaction; one organism (parasite) gets nourishment from other organism (host), which is harmed parasitoidism ectoparasite endoparasite Insect lays eggs in/on living host, leaving the larvae to feed on body, and kill it Feed on external surface of host Live within host’s body

5 Disease, Mutualism, Commensalism
Inflict lethal harm, can limit populations +/0 interaction; 1 species benefits, & the other is not affected; hard to find true example in nature +/+ interaction; both species help each other Pathogen: disease-causing agent; unlike parasite: microscopic, lethal; +/- interaction Species Diversity Variety of diff. kinds of organisms that make up the community Species Richness Total # of different species Relative abundance Proportion each species represents of total individuals in the community

6 Trophic Structure Feeding relationships between organisms Food Chain:
Eventually to decomposers Feeding relationships between organisms Food Chain: Energetic Hypothesis: length of food chain is limited by inefficiency of energy transfer along chain Dynamic Stability Hypothesis: long food chains are less stable than short food chains; population fluctuations at lower levels are more profound at higher levels, causing potential extinction of high level predators Food Web: food chains are linked together; 1 species, such as plant, is eaten by several species

7 Species With Large Impacts
Biomass: total mass of all individuals in a population Dominant species Keystone species Not necessarily abundant; exert strong control on community by their ecological roles Species that: collectively have highest biomass OR most abundant “foundation species”; facilitators; cause physical changes that benefit community, by increasing survival and reproduction Ecosystem engineers

8 Bottom-Up and Top-Down Models
Top-Down (Trophic Cascade) Model Bottom-Up Model Model of community organization Predation controls community organization; predators control herbivores, which control plants, which control nutrient levels Mineral nutrients control community organization; nutrients control plant #’s, which control herbivore #’s, which control predator #’s Polluted State Restored State Fish Abundant Rare Zooplankton Algae Biomanipulation

9 Disturbance An event that changes a community, removes organisms from it, and alters resource availability storm Not always negative fire flood drought overgrazing Burned trees release nutrients; no trees to block sunlight for new plants Human Disturbance: harmful, reduces species diversity & has contributed to famines by overgrazing land

10 Ecological Succession Factors Correlated With a Community’s
Transition in species composition of a biological community, usually following a disturbance of the community Primary Succession: occurs in virtually lifeless area, with no organisms or soil; only autotrophic prokaryotes are present Secondary Succession: occurs where an existing community was cleared by disturbance, that leaves soil intact; first plants to recolonize: grow from wind-blown/animal-borne seeds Factors Correlated With a Community’s Species Diversity Size Geographic Location Greater area=more diverse habitats, more species Tropical habitats have the most species; tropical habitats are older (long growing season), and greatest evapotranspiration

11 Island Equilibrium Model (immigration
rate=extinction rate) Factors Affecting # of Species That Will Inhabit Island New Species Immigration Rate Influenced By: Species Extinction Rate # of Species Already Present Island Size Island’s Distance from Mainland new colonizers: unlikely to reach small island = low immigration rate, high extinction rate (less resources) # Species immigration rate (those reaching island are less likely to represent species not already there) Extinction rate (competitive exclusion) Islands near mainland: high immigration rate, low extinction rate (new colonizers sustain presence of species)

12 Integrated and Individualistic Hypotheses
Integrated Hypothesis (Clements) Individualistic Hypothesis (Gleason) Describes a community as an assemblage of closely linked species, locked into association by mandatory biotic interactions States that species are found in the same area simply because they happen to have similar abiotic requirements These interactions cause the community to function as an integrated unit

13 Rivet and Redundancy Models
Rivet Model Redundancy Model Suggests: most of the species in a community are associated tightly with particular other species in a web of life Exact opposite of the rivet model Suggests: species in a community are redundant; species operate independently, & aren’t affected if one species increases/decreases Ex. – if one pollinator disappears, then another species will do the job **the Ehrlich’s agree with Clements **Walker agrees with Gleason

14 DDT Affects Trophic Structure
DDT and other toxins, when in tissues and fats of an organism, can accumulate and be passed onto the next level of the food chain, affecting many organisms. Rachel Carson states in Silent Spring that the concentration of DDT in a low level organism’s fat is a lot lower than that of a higher level organism on the food chain Zooplankton & other primary consumers, secondary consumers, etc. are affected If phytoplankton is infected Everything occurring at low food chain levels is magnified at high food chain levels, causing possible extinction of top level predators

15 Pollution Affects Trophic Structure
Similar effect of DDT; If organism is affected by pollution, it can spread through food chain, greatly affecting top-level predators If water is polluted and filled with garbage, the fish living there drink bad water and aren’t getting the right nutrients Make fish sick, and all those who eat them: fish’s predators, fish’s predators’ predators, and even human who eat fish for dinner **If one organism is infected by either DDT, pollution, or by several other possibilities, the harms done to that animal certainly don‘t stop there**

16 DDT Impacts Predation Top-Down Model
The spraying will cover the soil and the nutrients it contains. The producers absorb the DDT and are eaten by the consumers. The predators will continue to consume the DDT laden prey, accumulating the DDT in their body fats. imbalance of predators to prey will cause the ecosystem to go out of balance The reduction of 4th level predators, for example, will cause: overabundance of the 3rd level prey, which consume so much 2nd level prey Top-Down Model Decrease 2nd level prey, so there’s not as many organisms to eat primary producers

17 Effects of Global Warming on Predation
As the environment becomes increasingly warmer each year, organisms might not be able to adapt or migrate to other areas The introduction of organisms disrupts and changes the ecosystem of the environment. This migration will bring new predators and new prey, which would alter the trophic structure. If organisms die, their predators would starve, and there would be an increase in the dying organisms’ prey Altering the trophic structure would change the predator-prey relationships; the top level carnivores may not be the top level carnivores anymore, having an effect like the top-down model

18 DDT Affects Competition
DDT can increase competition DDT may kill many organisms of a species that two other species compete for. Because the resource is in short supply, interspecific competition between the two competing species will increase, hurting both species eventually eliminating one of the two competing species (competitive exclusion).

19 DDT can also lessen competition
DDT Affects Competition DDT can also lessen competition Reproduction Rates DDT influences reproduction rates Ex.: Eagles in Silent Spring by Rachel Carson -DDT stored in eagles’ fatty cells cause the deposition of calcium in their eggshells to be hindered. Biological Magnification DDT’s concentration with each link in the food chain. By eating DDT-contaminated food, DDT accumulates in carnivores, which may be enough to kill the carnivores. Less carnivores; some organisms’ populations from lower trophic levels (top-down model), providing more of those organisms for the surviving carnivores (more food for them). Weak eggs, so many parent eagles accidentally crushed the egg during incubation; some eggs also didn’t hatch eagle population; the population of eagles’ prey increased, so competition for those prey isn’t as intense. less competition among remaining carnivores for resources.

20 Global Warming’s Impact on Competition
Some organisms may not be able to adapt fast enough to the rapidly changing conditions of the world Species may become extinct; its predators may starve/be eliminated & its prey is more abundant Have edge over other species to get resources, reproduce rapidly; lead to local elimination of interior competitor (not able to obtain resources) Some organisms may adapt well to the new conditions caused by global warming If species is extinct or adapts, a competitor is still eliminated, leading to less competition for resources Survivors reproduce, pass genes onto offspring; over evolutionary time, may evolve to new species

21 Presented By: Tom Doherty
Kelly Terlizzi Kerena Thomas Andrew Tom A.P. Biology Pd. A 9/15/08


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