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Competition Interactions between organisms. Acorn trees have mast years where they produce a huge crop of acorns - causes an increase in mice, deers come to the area to eat the acorns, both of these have a capacity of being vector for ticks, and there is also an increase in the pathogen causing Lyme disease, Borelia, which can be passed onto humans. Lyme disease gives arthritic problems in later moth. There is a decrease in the gypsy moth population as they are eaten by the mice feeding on the acorns (and also being parasitized by the ticks). The decreased levels of gypsy moth larvae is beneficial to the health of the forest, so the absence of the ticks would not be ideal. Types of interactions: Predation/herbivory where one species is eating all or part of another; Parasitism where one organism benefits to the detriment of another; Diseases/pathogens close association; Commensalisms either one or both species are benefiting from the interaction and Competition where two or more species use the same resources to the detriment of some or all parties involved. Usually between species but can also occur within species - individuals competing within a species for a resource and if the access to the resource is determined by some genetic trait then evolution occurs within the species. Resource competition units (species/individuals) are using a common resource that is limited. Also called scramble or exploitation competition. Don't necessarily have any direct interactions between individuals that are competing with each other and the costs are likely to be symmetrical - everybody is a loser. They may do it at different locations or at different times of day but they use it up so there is less for everyone. Can include: o Food (moose that got onto an island and ate every plant until they all died) o Water o Space (eg mussels and barnacles) o Plants - access to light Resource competition and Lotka-Volterra models Carrying capacity is the maximum number of individuals that can be supported in the area - just a model, changes with time. This is for one population
For two populations, the following equations are given. What if the two species have different carrying capacities? Just like squirrels and deer. So K is not equal for the two species. We deal with that by having a weighting factors. So if a squirrel is worth 1/20 of a deer, then multiply by 20 to get the equivalent number of squirrels. Expressing numbers of one species in terms of the other species. Now that can convert numbers of a into b, can now look at the increase of populations, taking away from carrying capacity of that species a, the amount that is used by the other species b converted into a factor so that it is equivalent to a. Describe the competition using that model. At equilibrium, this is equal to zero. There is no net change in population size of that species. The idea behind this is that you can look at the species on the graph and determine whether they will compete with other species and how much. Arrows refer to how changes in population size affect the population growth - they will move towards the isocline - the point at which the species are at equilibrium. All of these things are happening at the same time - constant changes in population sizes. Can put isoclines together to ascertain whether species will co-exist or out-compete each other. NB. Species will always move towards each others isoclines. If the isoclines overlap, then stable ecosystems exist. Gause's experiments Gause experimented on yeasts to produce many growth curves. The volume of the yeast as a function of time are displayed on a graphs and for saccaromyces has a
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