Date of Award


Degree Type


Degree Name

Master of Science (MS)




Kari Segraves


coexistence;competition;mutualism;mutualistic dependence;species richness;synthetic ecology

Subject Categories

Biology | Life Sciences


Mutualism, or the beneficial interaction of species, is an important and widespread interaction type in nature. Mutualist species exchange goods or services that benefit the species involved. These benefits can include protection from predators, parasites, and disease, dispersal of individuals, offspring, or gametes, and provisioning of various nutrients. Well-known mutualisms include the pollination of flowering plants by insects, birds, and other animals and the interaction between clownfish and sea anemones. Many mutualistic associations are less visible than these examples, as organisms often form beneficial relationships with microorganisms, like the many beneficial bacteria that live in the human gut, or the soil bacteria and fungi that exchange nutrients with plant root systems. Like these examples, most mutualisms have more than two species, rather than being a pairwise relationship between only two species. When multiple mutualist species provide similar benefits to their partners, they are together called “guilds.” Species with overlapping nutritional and environmental needs will experience competition, which can cause one of the species to be excluded from a community entirely. Mutualist species that share mutualist partners compete for the benefits of mutualism. If the number of partners or the benefits gained from partners are in limited supply, one of the mutualist species that share the partner is at risk of being competitively excluded. Competitive exclusion is a phenomenon in which the population growth of two species is constrained by a common resource and therefore the species cannot coexist indefinitely. Limitation of mutualist species by the available quantities of partners or benefits gained from partners has been documented in many mutualisms, suggesting that natural systems of mutualisms that involve numerous mutualists that share partners must have additional mechanisms to prevent the competitively superior mutualist species from excluding all others. There are several ways that competitive exclusion could be avoided for mutualists that share partners. Species can coexist when differences in their individual needs counter-balance differences in how much the species reproduce. For example, mutualist species that share partners can coexist by having sufficiently different environmental or nutritional requirements, differences in foraging behaviors that avoid competitors when searching for mutualist partners, or if they are active at different times of the year. Mutualistic dependence is one characteristic of mutualist species that has been hypothesized to influence the severity of competition for shared mutualist partners. Mutualistic dependence is the degree to which survival and reproduction of a mutualist species relies on the interaction with the mutualist partner(s). Obligate mutualists have high dependency and cannot survive or reproduce without interacting with a mutualist partner, whereas facultative mutualists have lower dependency and do not need to participate in a mutualistic interaction to survive or reproduce. Facultative mutualists fall in a range of benefitting to some degree from interacting with a mutualist partner. In this thesis, I have examined the effect of mutualist dependence on competition within mutualist guilds. I predicted that lower mutualist dependence would decrease the severity of competition between mutualists, leading to lower rates of competitive exclusion in facultative mutualist guilds. I conducted a series of experiments to test the idea that obligate mutualist guilds experience greater competitive exclusion than facultative mutualist guilds. First, I constructed artificial communities of mutualists, using brewer’s yeast (Saccharomyces cerevisiae) strains that have been genetically modified to form a mutualism. One type of mutualist makes an excess amount of adenine and these are called “AdeOP strains.” Additionally, AdeOP strains require external sources of lysine to survive and reproduce. They are partnered with “LysOP strains” that make an excess of lysine but need adenine to survive and reproduce. The two types of strains form an obligate mutualism when grown together in a nutrient medium that contains all the nutrients the yeast require except adenine and lysine. I created facultative mutualisms by adding small amounts of adenine and lysine to the nutrient medium where the yeast can uptake them. I created mutualisms that were pairwise, with just one AdeOP and one LysOP strain, as well as mutualisms with four AdeOP and four LysOP strains, to create mutualist guilds with multiple species. As I maintained the mutualisms, I observed the exclusion of strains from the AdeOP and LysOP guilds over time. By the end of the initial twenty-week experiment, seventy percent of the cultures that started with four AdeOP and four LysOP strains had lost all but one strain in each guild. This result suggested that there was severe competition that was leading to high rates of competitive exclusion. I looked at the patterns of extinction in the mutualist guilds and did follow-up experiments to determine whether competition was the root cause of the extinction of strains and whether there were differences in the severity of competition between facultative and obligate mutualisms. From an analysis of the timing and pattern of extinctions in the mutualisms with eight strains, I found evidence that competition was likely the cause of the observed strain extinctions and that the extinctions were more severe in obligate mutualisms. The extinction rates differed between the strains in both the AdeOP and LysOP mutualist guilds. Some strains appeared to be competitively superior, as they survived in more mutualisms than did other strains. Additionally, the rate of exclusion was greater for obligate mutualisms than for facultative mutualisms, supporting my hypothesis that competition was more severe in obligate mutualisms. To validate the results of the initial experiment and learn more about competition within the AdeOP and LysOP mutualist guilds, I conducted a series of pairwise competition assays. The pairwise competition assays were assembled by putting the final, or winner, strain from one experimental mutualism with one of the strains that was excluded from the same guild in the same mutualism. I grew pairwise competition assays in facultative medium, where the two strains could grow without a mutualist partner, and with an added mutualist partner, to determine whether I could replicate the same outcomes as observed in the initial experiment. The results showed the same patterns of exclusion that were observed in the LysOP guild. Notably, though, the results of this assay were dependent on the AdeOP mutualist partner strain also being present. The winner LysOP strain did not consistently outcompete the other strains when grown without a mutualist partner. This difference in competitive outcomes is likely a result of the winner LysOP strain being better adapted to mutualism with the AdeOP strains, as opposed to taking up available environmental adenine. In contrast to the results for the LysOP strains, the assays for the AdeOP strains did not consistently replicate the exclusion results from the initial experiment. One possible explanation is that the exclusions in the AdeOP guild occurred quickly, and the strains used in the competition assays were the strains left over after the many early extinctions. The strains that quickly went extinct were likely to be weaker competitively but were excluded too quickly to be included in the competition assays. Alternatively, the AdeOP assays may not have been able to validate the previously observed exclusions because the test was saddled with a lack of statistical power. Because fewer AdeOP strains remained in the mutualisms, there were fewer strains to use in conducting the competition assays and the ability to draw conclusions from statistical analysis is dependent on having enough tests. To learn more about competition between the strains in the AdeOP and LysOP mutualist guilds, I measured two traits of the yeast strains that I predicted would influence competition in my mutualisms. I predicted that the strains’ rates of reproduction and how well the strains convert adenine and lysine from partners into reproduction, also called resource use efficiency, could mediate competition because competition among microorganisms is generally decided by which species can most quickly reproduce, given the available resources. Even though the rate of reproduction is thought to be important in microbial competition, I found that there was no correlation between rates of reproduction and whether a strain was competitively excluded. Resource use efficiency also did not correlate with whether a strain won in competition or was competitively excluded. Although neither of these measurements appear to have had a significant effect on competitive exclusion, I did find that the traits changed over time. Contrary to my expectations, AdeOP strains developed decreased reproductive rates in facultative mutualisms with multiple strains present, as well as in obligate mutualisms with just one AdeOP and one LysOP strain. The reproductive rates of AdeOP strains in pairwise facultative mutualisms and obligate multistrain mutualisms did not change from the ancestral reproductive rates. LysOP strains in facultative mutualisms evolved to have a faster reproductive rate, whereas LysOP strains in obligate mutualisms did not. Resource use efficiency decreased across all AdeOP strains regardless of the dependency or how many strains were present in the mutualism. Resource use efficiency did not evolve for the LysOP strains. Traditional expectations for microbial competition led me to predict that reproductive rates and/or resource use efficiency would increase but only the reproductive rates of LysOP strains in facultative mutualisms increased, whereas all other examples did not change or decreased. Given these results, I conclude that another trait may drive patterns of competitive exclusion in the yeast mutualisms, or a number of traits may together be responsible for the observed exclusion patterns. The experiments that I present here provide support for the fundamental importance of competition for determining whether the species in mutualist guilds will coexist or experience competitive exclusion. Indeed, I found that mutualistic dependence influenced the strength of competition within mutualist guilds and the rates at which species were excluded from mutualisms as a result. Pairwise competition assays demonstrated that mutualist species influence the competitive outcomes between their mutualistic partner species, which may be of particular importance for coexistence in the species rich facultative mutualisms that are common in nature. Facultative mutualists that compete for environmental resources, rather than mutualist partner species, may experience different competitive outcomes than the species that compete for mutualist partners, which could aid in the continued coexistence of the competing species. Competitive dynamics within mutualisms are complex and influenced by many factors but here I have shown that how much mutualist species depend on their mutualist partners has a significant effect and should be considered in studies of competition within mutualisms. Competition among mutualists is an important current topic, as novel species are increasingly being introduced into new communities and coming into competition with species in native mutualist guilds.


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