Date Thesis Awarded


Document Type

Honors Thesis

Degree Name

Bachelors of Science (BS)




Helen Murphy

Committee Members

Jelen Pantel

Randolph Chambers

Leah Shaw


Communities are structured by interactions between species and their environment and between one another. Because resources are typically limited in nature, competition (sensitivity to the presence of other individuals of the same or of another species) is an important determinant of whether or not species can coexist, and is also an important process to understand biodiversity (Tilman, 1987; Freckleton et al., 2009 ). Numerous studies have measured how the presence of competitors alters growth and survival (Connell, 1983; Ascheoug et al., 2016), and researchers are currently focused on effectively translating experimental measures of competition to the coexistence and biodiversity patterns observed in natural communities (Freckleton et al. 2009). Resource competition has been demonstrated to show sensitivity to temperature and other factors that influence productivity (Goldber et al., 1999), and it is therefore likely that biodiversity patterns associated with competition will shift as a result of anthropogenic environmental changes such as altered rainfall patterns (Hautier et al.,2009; Clark et al., 2011) and urbanization (Shochat et al., 2010).

The effects of competition can be quantified in numerous ways (Weigelt et al., 2003). An effective method for quantifying competitive ability is to estimate parameter values from models that describe the effects of competitors for growth of populations (May & Leonard, 1975; Beverton & Holt, 1957). This method allows a precise, quantitative definition of competitive ability for a given species that can be compared across different species or conditions, which differs from approaches such as experimental removal of focal species (e.g. Oksanen et al. 2006). Generally, the models include a growth rate term, a term for intraspecific competition (interaction with individuals of the same species), and a term for interspecific competition (interaction with individuals of other species).

Despite the numerous models available for describing population dynamics in the presence of competitors, Chesson (2000; 2012) has shown that common determinants for competitive ability and coexistence emerge from these models. Across the range of models for competition, competitive dominant species are those that combine a high growth rate in the absence of competition and an ability to tolerate competition from both conspecific and heterospecific individuals in their shared location (Hart et al. 2017). If the parameters of these competition models are correctly estimated, it is possible to quantify competition, determine competitive hierarchies, and determine expected coexistence patterns for groups of potentially co-occurring species (Hart et al. 2017).

One important assumption for these models is that the parameters are traditionally treated as fixed for each species. This implies there is no intraspecific variation for the traits that underlie these parameters, which is unlikely to be the case in nature. The population growth rate parameter used in ecological models of population growth, r, is ultimately the same as an individual’s realized fitness (Coulson et al. 2006), and fitness varies in the context of genes and environment. Furthermore, traits that influence competition (e.g. Jung et al. 2010; Edwards et al 2013; Vogt et al. 2013; Kunstler et al. 2016) are often heritable and vary among populations (Ehlers et al., 2016). These findings are important for accurately understanding biodiversity because community dynamics can be influenced by by the genetic composition of resident species (Vellend 2006). For example, one study demonstrated that the genetic composition of one species altered the colonization success of other immigrant species (De Meester et al 2007), and another study found that rapid adaptation for one species in response to different environmental conditions caused entirely different zooplankton communities to assemble (Pantel et al 2015). Another study in a plant- microbe system, found that coevolution of microbes with their host Brassica rapa also led to microbial communities with distinct composition patterns (terHorst et al 2014).

While studies have measured the consequences of genetic variation for competition, community assembly, and coexistence, it is currently not known whether coefficients for competition models demonstrate heritable intraspecific variation. The aim of this study was to determine whether the strength of competition between two species is a heritable trait. We used freshwater zooplankton as a model system to investigate this and had three main goals: (1) to estimate genetic variation in functional traits that might influence competitive ability; 2) to determine whether there is genetic variation for competitive ability itself (both intraspecific and interspecific competition); 3) and to determine if variable competition strength affects community dynamics and species coexistence in experimental mesocosms. We combined three experiments to achieve these goals. The first wass measurement of grazing rates in multiple clones of two zooplankton species in a common garden environment, The second was a common garden experiment to quantify pairwise competition coefficients for multiple clones of each species. The third was a mesocosm experiment to determine whether intraspecific genetic variation in competition strength altered the outcome of community dynamics and whether this effect was temperature-dependent.

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