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Chapter 53

Unit 9 - Chapter 53 Bio 1M03 .docx

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James S Quinn

Bio 1M03 Unit Nine: Ecology Chapter 53: Community Ecology  Community – consists of interacting species, usually living within a defined area 53.1 Species Interactions  The fate of a particular population is tightly linked to the other species that share its habitat because the species interact constantly o Eat one another o Pollinate each other o Exchange nutrients o Compete for resources o Provide habitats for each other  Fitness – the ability to survive and produce offspring  Categories of interaction between species and their fitness o Competition  -/- o Consumption  +/- o Mutualism  +/+ o Commensalism  +/0  Three Key Themes 1. Species interactions may affect the distribution and abundance of a particular species  Changes in species interactions often explains short-term changes in population size and distribution 2. Species act as agents of natural selection when they interact  Arms race – natural selection favors that prey run fast; predators have to be fast and natural selection favours fast predators as well  Coevolution – changes in species interaction that leads to long-term changes in the characteristics of populations 3. The outcome of interactions among species is dynamic and conditional Competition  -/- interaction  Occurs when different individuals use the same resources and when those resources are limiting; lack of access to resources prevents individuals from surviving better and having more offspring  Intraspecific Competition – competition that occurs between members of the same species o Intensifies as population density increases o Major cause of density-dependent growth  Interspecies Competition – individuals from different species use the same limiting resources o Consumptive Competition – when individuals consume the same resources  Eg/ Trees competing for water and nutrients o Preemptive Competition – hen one species makes space unavailable for other species  Eg/ Space preempted by barnacles is unavailable to competitors o Overgrowth Competition – when one species grows above another  Eg/ Shrub grows and shades other individuals o Chemical Competition – when one species produces toxins that negatively affect another species  Eg/ Few plants grow under Salvia shrubs o Territorial Competition – when a mobile species protects its feeding or breeding territory against other species  Eg/ Grizzly bears drive off black bears o Encounter Competition – when two species interfere directly for access to specific resources  Eg/ Spotted hyenas and vultures fight over a kill  Niche – the range of resources that the species is able to use or the range of conditions it can tolerate  Interspecific competition occurs when the niches of two species overlap Bio 1M03  Competitive Exclusion Principle – it is not possible for species within the same niche to coexist (G.F. Gause) o Symmetric Competition – each of the interacting species experiences equal decrease in fitness o Asymmetric Competition – one species suffers a greater fitness decline that the other species  If niches have completely overlapping niches – stronger competitor is likely to drive other species to extinction  If niches do not overlap completely – weaker competitor can retreat to an area of non-overlap o Fundamental Niche – combination of resources or areas used or conditions tolerated when competition occurs o Realized Niche – the portion of resources or areas used or conditions tolerated when competition occurs o Eg/ Paramecium caudatum and Paramecium Aurelia  When placed different species in separate laboratory cultures – both species exhibited logistic growth  When placed different species in same culture – P. aurelia exhibits logistic growth, P. caudatum is driven to extinction  Asymmetrical Competition  Experimental Studies of Competition o Joseph Connell o Barnacle larvae are mobile o Barnacle adults live attached to rocks o Two species of barnacles with distinctive distributions  Adults of Chthamalus stellatus – occurred in an upper intertidal zone  Adults of Semibalanus balanoides – restricted to a lower intertidal zone  Young of both species were found together in the lower intertidal zone o The upper tidal zone is a more severe environment for barnacles – exposed to air for longer periods at low tide each day Bio 1M03  Mechanisms of Coexistence: Fitness Trade-offs and Niche Differentiation o The ability to compete for a particular resource is only one aspect of an organisms niche  If individuals are extremely good at competing for a particular resource, they are probably less good at enduring drought conditions, warding off disease or preventing predation o Eg/ Semibalanus and Chthamalus growing in intertidal zones  The fitness trade-off is  Semibalanus – rapid growth and success in competing for space; large  Chthamalus – the ability to endure the harsh physical conditions of the upper intertidal (long exposure to air, intense sun); grow slowly o Fitness trade-off limits the ability of superior competitors to spread o Strong natural selection on both species to avoid competition, as it is -/- interaction o Niche Differentiation (resource partitioning) – an evolutionary change in traits reduces the amount of niche overlap and the amount of competition  Character Displacement – the change in species traits o Competition exerts natural selection; the characteristics of species change in a way that reduces competition o Galapagos Finches  Dramatic increase in average beak size and body size of Geospiza fortis was examined during a drought  Major food source available during the drought was fruit from a plant Tribulus cistoides – only large beaked individuals were able to crack these fruits and eat the seed inside  G. magnirostris arrived; about twice the size of G. fortiz and also ate T. cistoides  Another drought occurred but had different outcome – only small beaked G. fortis survived  G. magnirostris outcompeted G. fortis for large seeds; only G. fortis who could eat extremely small seeds could survive Consumption  Three types of consumption 1. Herbivory – when herbivores (plant eaters) consume plant tissues  Eg/ Caterpillars chew leaves 2. Parasitism – when a parasite consumes relatively small amounts of tissue or nutrients from another individual (host)  Often occurs over a long period of time  Not necessarily fatal  Parasite normally small relative to host  Doesn’t always involve consumption  Eg/ Social parasites in birds and insects lay eggs in host and induce them to raise the young  Eg/ Ticks parasitize cattle and other large mammals 3. Predation – when a predator kills and consumes all or most of another individual (prey)  Eg/ Woodpeckers eat bark beetles  Prey Defense Against Predators o Run, fly, swim away or hide Bio 1M03 o Safety in numbers – confuse predator o Standing/Constitutive Defenses – when prey defends itself while being present  Sequester or spray toxins, employ weaponry (shark spines or kicking hooves)  Mimicry – occurs when one species closely resembles another species  Mullerian Mimicry – when harmful prey species resemble each other o Eg/ Paper wasp, bumblebee and honey bee o The existence of similar-looking dangerous prey in the same habitat increases the likelihood that predators will learn to avoid them o Should reduce the likelihood of dangerous individuals being attacked  Batesian Mimicry – when a harmless species resembles a harmful species o Eg/ Wasp beetle, hornet moth, hoverfly  Energy expensive; resources must be devoted to producing and maintaining these defenses  Adaptations in response to natural selection exerted by predators that reduce likelihood of becoming victims o Inducible Defenses – defensive traits that are produced only in response to the presence of a predator  Decline if predators leave the habitat  Efficient energetically, but are slow – takes time to produce them  Ex/ Mussels and Crabs  Observations – predation on mussels by crabs was o High in an area of the estuary with relatively slow tidal currents (low flow area) o Low in an area of the estuary with relatively rapid tidal currents (high flow area)  Hypothesis – mussels increase investment in defense in the presence of crabs o Inducible defenses should be more frequent in low flow area where predation pressure is high  Experimental Setup o Mussels downstream from no crabs o Mussels downstream from crabs  Results o Shell thickness was higher in areas downstream from crabs  Conclusion o Mussels increase investment in defense when they detect crabs o Shell thickness is an inducible defense  Efficiency of Animal Predators o Predators play a role in density-dependent growth of prey populations o Prey are typically smaller than predators, have larger litters or clutch sizes and tend to begin reproduction at a younger age – have a larger intrinsic growth rate o Predators reduce the size of prey population  Why Don’t Herbivores Eat Everything? – Why is the World Green? 1. Top-Down Control Hypothesis – predation or diseases  Herbivore populations are limited by predation and disease  Predators and parasites remove herbivores that eat plants 2. Poor-Nutrition Hypothesis – plant tissues could offer poor or incomplete nutrition  Plants are a poor food source in terms of the nutrients they provide for herbivores  Plant tissues have less than 10% of the nitrogen found in animal tissues, by weight  If the growth and reproduction of herbivores are limited by the availability of nitrogen, then their populations will be low and the impact of herbivory relatively slight  Herbivores cold eat more plant material to gain nitrogen, but at a cost – exposing self to predators and expend energy processing food 3. Plant-Defense Hypothesis – plants could defend themselves effectively against attack  Plants defend themselves effectively enough to limit herbivory  Thorns, prickles, hairs, potent poisons (nicotine, caffeine, cocaine)  Adaptation and Arms Races o When predators and prey or herbivores and plants interact over time, coevolutionary arms race result  Consumers evolve traits that increase their efficiency Bio 1M03  Prey evolve traits that make them unpalatable or elusive  Selection occurs on consumers that counter the prey adaptation  Cycle  Eg/ Humans and Plasmodium parasite  Plasmodium causes malaria  Natural selection favors alleles that allow Plasmodium cells to infect hosts efficiently and multiply inside  Cells in the human immune system destroy liver cells and red blood cells  Natural selection favors alleles that allow mosquitoes and humans to resist infection  HLA-B53 allele – protection against malaria o HLA-B53 proteins on the surface of a liver cell infected by Plasmodium display a parasite protein called c26 – causes immune system to kill cell before Plasmodium can divide  Plasmodium populations have a variety of alleles for the protein that bind to HLA-B53 and trigger an immune response in the host, but others escape detection  In some cases, the recognition step by HLA-B53 breaks down when certain strains are found together  Parasites can manipulate their hosts o Eg/ Parasite flatworms burrow into snails tentacles and wriggle  Snails normally avoid sunlight; but they go into the sun and wriggle tentacles  Birds spot them and consume them – worms have been transmitted to bird Mutualism  +/+ interactions  Eg/ Ants and Acacia Trees o Ants live in bulbs at the base of acacia thorns and feed on small structures that grow from tree branches o Ants protect the tree by attacking and biting herbivores and by cutting vegetation from the ground below the host tree  Eg/ Cleaner Shrimp and Fish o Shrimp pick external parasites from the jaws and gills of fish, which taking residence in the gills of fish  Does not involve altruism, even though both species benefit  The outcome of the interaction depends on current conditions Type Of Interaction Fitness Short Term Impact Distribution and Long-Term Impact: Coevolution Effects Abundance Competition -/- Reduces population size of both species; if Niche differentiation via selection to competition is asymmetric, competitive reduce competition exclusion reduces range of one species Consumption +/- Impact on prey population depends on prey Strong selection on prey for effective density and effectiveness of defenses defense; strong selection on consumer for trait that overcome defenses Parasitism +/- Impact on host population depends on Strong selection on host for effective parasite density and effectiveness of defensdefense; strong selection on parasite for traits that overcome defenses Mutualism +/+ Population size and range of both species arStrong selection on both species to dependent on each other maximize fitness benefits and minimize fitness costs of relationship Commensalism +/0 Population size and range of commensal may Strong selection on commensal to depend on size and distribution of host increase fitness benefits in relationship;
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