ECOLOGY STUDY GUIDE COMPETITION Competition is a interaction both parties are negatively affected CAUSE Occurs when a shared resource is limited EFFECT Results in decreased individual growth survival and or reproduction rates that ultimately alter population growth rates Two types of competition o Interspecific between species and intraspecific within species Intraspecific competition Self thinning population decline as a result of intraspecific competition o Experimental example M sativa tarweed was planted in high initial densities Over time the density of plants plants per m2 decreased and the mean dry weight or average size of individual plants increased There were less plants at the end of the experiment but they were larger because the larger individuals used up all the resources and the smaller ones were unable to survive Populations experiencing intraspecific competition level off at K carrying capacity But interspecific competition can keep population size N of either species below K Interspecific competition Niche concept Fundamental niche Physical conditions that a species can persist in Realized niche Fundamental niche when altered shrunk by species interactions e g predation disease competition If you plot resource state x axis vs resource utilization efficiency y axis peaks on the graph correspond to niches o d is the distance between peaks and w is the width of the peak o Large d w less intraspecific competition and higher probability of coexistence o Species packing If resource availability remains constant adding a new species a new peak will decrease w species will become specialists o Limits to species packing As you add more species the probability of another species establishing decreases This has applications for biological invasions Experimental Example Survivorship of invasive tunicates decreases as the native species richness number of species increases In other words resistance to invasion increases as a result of heavier species packing o Most systems have multiple shared resources and the niches are less Fuzzy Niches clear o Experimental Example Silica is limiting for Diatom 1 and phosphate is limiting for Diatom 2 Diatom 2 can outcompete in lower silica higher phosphate areas Diatom 1 wins in high silica low phosphate conditions In intermediate conditions species can coexist Competitive exclusion principle Two species that use the same resource cannot coexist overlapping niches will force one to exclude the other to extinction Experimental example Paramecium o Two species show logistic growth patterns when grown independent of one another P caudatum P aurelia o When grown together P aurelia always outcompetes P caudatum o For two different species P bursaria and P caudatum they exhibit logistic growth curves when grown separately but when grown together neither is driven to extinction o This means that their niches are not the same Experimental example Barnacles o Barnacles Cthamalus sp and Balanus sp o Cthamalus sp Can grow in lower intertidal without Balanus sp o With Balanus sp Cthamalus sp was outcompeted o Lower limit of Balanus sp is set by physiological tolerance Competitive displacement Competitive exclusion does not always mean extinction Experimental Example o Snail 1 is a competitive dominant to Snail 2 o But Snail 2 is not driven to extinction it s just relegated to a less desirable niche living on mud flats Types of interspecific competition Interference competition direct confrontation or prohibition e g competition for space territoriality Exploitative competition indirect resource depletion What one uses another may not e g competition for food nutrients o Experimental example Red smaller bryozoan and blue larger Blue one is getting all the food so the red one doesn t really get much food exploitative Eventually blue one will overgrow the red one direct Red bryozoans that are closer to blue ones exhibit a higher feeding rate Types of interspecific competition a different classification system Consumption i e competitive dominant consumes more nutrients Preemption i e spores of Algae species A try to settle on a turf of Algae species B and are unable to because the space is already occupied Overgrowth i e o Pseudolithophyllum overgrows Lithothamnion Chemical Algae secretes allelochemicals that suppresses growth of another one Territoriality mobile species only Animals defend space for the resources in that space Encounter mobile species only Fight over resources that is not space Like consumption but for mobile species Lotka Volterra Competition Model For interspecific competition Competition coefficients Per capita effect of one species on the growth of another species 1 interspecific competition is stronger than intraspecific competition intraspecific competition is stronger than interspecific competition intraspecific competition interspecific competition 1 1 Converting logistic growth to include competition No competition Logistic growth equation o For species 1 dN1 dt r1N1 1 N1 K1 Including competition o For species 1 o For species 2 dN1 dt r1N1 1 N1 12N2 K1 dN2 dt r2N2 1 N2 21N1 K2 o We added the term 12N2 the population size of SPECIES 2 N2 and the effect on the growth of species 1 by species 2 12 Assumptions of LV Competition Model 1 Resources are limited 2 Density dependence is linear 3 Competition coefficients and carrying capacity and K are constant Equilibrium population sizes are no longer changing Three types o Stable equilibrium Can return to equilibrium if it is displaced o Unstable equilibrium Cannot return to equilibrium after a slight o Neutral equilibrium Stays in displaced position after slightly displacement disturbance Isoclines for LV Competition Isocline of zero population growth IZPG The conditions under which population will not grow or shrink FOR ISOCLINES WE RE LOOKING ONLY AT WHEN THE TERM 1 N2 21N1 K2 EQUALS ZERO Simplify LV Equation For species 1 N1 12N2 K1 For species 2 N2 21N1 K2 Four outcomes of competition shown at left are totally dependent on the N and K of both species o Outcome 1 Species 1 always wins When K1 12 K2 and K1 K2 21 o Outcome 2 Species 2 always wins When K1 a12 K2 and K1 K2 a21 o In other words For the first two outcomes in which the isoclines do not intersect whichever species isocline is farthest from the origin is the dominant competitor o IF THE ISOCLINES DO NOT INTERSECT COEXISTENCE IS NOT POSSIBLE o Outcome 3 Unstable coexistence Isoclines of species 1 and 2 cross with species