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Dr. Mark A. McGinley Texas Tech University BIOL 5311 Summer 2011 Competition:MBEA Activity
Competition Competition is an ecological interaction in which both participants are harmed. http://www.eoearth.org/article/Competition Competition can occur between members of the same species Intraspecific competition Modeled in the logistic growth equation http://www.eoearth.org/article/Intraspecific_competition Competition can occur between members of different species Intraspecific competition http://www.eoearth.org/article/Interspecific_competition?topic=58074
Competition Competition occurs when resources are limited. Resources Food Water Soil nutrients Light Space shelter Mates
Mechanisms of Competition There are two mechanisms of competition Interference competition Exploitative competition http://www.eoearth.org/article/Exploitative_competition
Interference Competition Two male deer fighting over females Interference competition between lion and hyena
Exploitative Competition Exploitative competition occurs when one individual consumes an resource so it is no longer available for consumption by another individual. Exploitative competition is the most common mechanism of competition.
Exploitative Competition Explotative competition for light in rainforest Exploitative competition for berries in cardinals
Effects of Intraspecific Competition Intraspecific competition can affect Population sizes Patterns of spatial dispersion
Intraspecific Competition Intraspecific competition for resources can be an important factor influencing population size Modeled by the logistic growth equation As N increases b decreases and d increases so r decreases
Spatial Dispersion Spatial dispersion describes how individuals are located across space even , clumped, random
Competition and Dispersion Competition may lead to even patterns of dispersion Individuals that are two close to each other compete and die. Common in some desert shrubs
Effects of Interspecific Competition Interspecific competition may affect Population size Biodiversity phenotypes
Studying Competition Ecologists have studied competition using theoretical models, observations, and experiments
Studying Competition: Observations Observational studies can be used to look for evidence of competition Negative associations between presence/absence of species Negative correlation between abundance of species
Studying Competition: Experiments in the Field In the Chihuahuan Desert in Arizona researchers have studied competition between desert rodents and ants by setting up experiments where they experimentally manipulated the population size of one species and observed changes in other species population size in response.
Modeling Competition Relatively simple to modify the Logistic Growth Model to include the effects of interspecific competition The Lotka-Volterra Model of competition examines competition between 2 species
Lotka-Volterra Model of Competition Can examine the results of this model graphically
Results Three possible outcomes of competition between two species One species wins and drives the other extinct Coexistence, unstable Coexistence, stable The stable coexistence result is the one that we are most interested in Two species can coexist only if the strength of intraspecific competition is greater than the strength of interspecifccompetition
Competition Competitive Exclusion Principle If two species share exactly the same niche then they will not be able to coexist One species will win and the other will go extinct Thus, species can only coexist if the have different niches Niches differentiation. http://www.eoearth.org/article/Competitive_exclusion_principle
Niche “The Niches is a bi**c!!” There are many different definitions of niche. In this discussion I am talking about “feeding niche” which describes what, when, and where an organism eats.
Niche Differentiation Organisms can differentiate their niches by Eating different foods E.g., insects and seeds Feeding in different places E.g., feeding on seeds found under desert shrubs or in the open E.g., feeding on bugs on the top or bottom of trees Feeding at different times E.g., feeding on insects that are active during the day versus those that are active at night.
Niche Differentiation Often feeding niches is influenced by the size Either of organisms Trophic structures (the structures used to capture food) Animals with the same sized trophic structures often eat the same food Therefore two organisms of the same size might compete too much to be able to coexist
Law of Limiting Similarity There is a limit to how similar two niches can be in order to allow two species to coexist.
Niche Differentiation Therefore species that are of similar sizes may not be able to coexist. If two species compete because their niches overlap then natural selection might cause their niches to vary so that they no longer overlap Character Displacement
Insectivorous Bats Most of the bats captures in Krau Wildlife Reserve are insectivores (the rest are frugivores and nectarivores) Insectivorous bats use their echolocation system to locate flying insects and they capture their prey in flight.
Insectivorous Bats Insectivorous bats can reduce competition by Feeding on different sizes of insects Size of insects that bats can eat appears to be correlated with their size Bigger bats can eat bigger bugs Feeding in different parts of the forest E.g., feed in in the open spaces above the forest versus feeding in the canopy
Current Research Ongoing research conducted by Dr. Kingston, her graduate students Julie Sewani and AinNural, and colleagues from Germany examines- Diet of the bats Examine fecal samples Relationship between morphology and diet Effect of jaw size on bite force Effects of morphology on foraging ability Relationship between size and diet Relationship between wing shape and diet Relationship between morphology and foraging location E.g., how close to objects can bats catch a prey?
Strength of Competition Because a bat’s feeding strategy influenced by body size, wing shape, and echolocation system we expect members of the same species to compete most strongly (they are all similar in size). Because of similarities of members of the same genus we would expect that members of the same genus should compete more strongly than members of different genera.
Activity 1 Question: Do members of the same genus compete for resources? If members of two species are competing for resources then we predict Negative association between the presence of both species in a sample Negative correlation between the abundance of the two species
Activity #1 Use the data from 2009 MBEA to 1. test for an association between the presence of R. sthenoand the presence of R. lepidusin a sample 2. test for a correlation between the abundance of R. sthenoand R. lepidusin a sample
Niche Differentiation Because there was no indication of a negative association between the presence/absence of the two species or a negative correlation between the abundance of the two species it appears that the species are not competing strongly for resources. Is there evidence for niche differentiation between these two species? Assume that diet is influenced by body size Is there overlap in body size between the two species.?
Activity 2 Draw the frequency distribution for body size (you can use either mass or forearm length) for both species Do we see niche overlap or niche differentiation?
Activity 3 Use the correct statistical test to test for difference between the mean size of the two species.