Background

The streams in the foothills of Alberta have long been home to the Bull Trout (Salvelinus confluentus). However, due to anthropogenic interference, it now finds itself accompanied by two introduced species: Brook Trout (Salvelinus fontinalis) and Brown Trout (Salmo trutta). While sharing many similarities, these three species are not the same, and as a result, the question of whether and by how much they compete is still not fully answered.

Aquatic invertebrates and trout are closely interconnected in the food webs in which they are found together, such as in the foothills of Alberta (Hunt & Krokhin, 1975). The objective of this project is to determine if there is a clear relationship between beta diversity of aquatic invertebrate and trout abundance. If such a relationship is found, it could potentially enable new ways of assessing habitat quality for trout, either as broader category or species specific, which could be used both in restoration and management projects.

As mentioned above, a key component of all three species diets are aquatic invertebrates and as such, this makes them an interesting subject when looking at the potential competition between the trout species. The component being looked at in this project is species diversity, including the previously mentioned beta diversity.

But what exactly is beta diversity?

Beta Diversity

Beta diversity, when calculated with either the Jaccard or the Sørensen and Simpson indices, shows how, and by how much, biological communities differ from each other. This can in turn aid in the understanding of how the species composition and diversity of aquatic invertebrates, affects other ecosystem components such as the abundance of Trout.

Beta diversity in its simplest form (multiplicative beta diversity or strict sense beta diversity) is the ratio between local/alpha diversity and regional/gamma diversity (Whittaker, 1960). This can be useful, but hard to compare, as the results are dependent on the number of different sites. Different indices have been developed to deal with this. The Sørensen and Jaccard indices both remove the influence of the number of sites, but they still cannot tell us about the drivers behind the observed beta diversity. When comparing the two indices, we can see that the Sørensen index has a greater focus on the more commonly found species while the Jaccard index is more influenced by species that are only found at a singular sampling site or event, although they are closely related.

To determine the drivers of beta diversity, we must introduce the Simpson dissimilarity index. By itself it gives us the replacement, also called turnover, but in conjunction with the Sørensen index it can also be used to calculate the degree that nestedness, the loss/gain of species between different sites, plays in the overall beta diversity. If a stream's beta diversity was purely driven by nestedness, then there would be one site containing all the species present, while the other sites would merely be subsets of this. In other words, when a species is lost from one site compared to another, it is not replaced, and the species that are present there are also present in the original site. This means that a stream with only nestedness as a driver of beta diversity would have quite a varied number of species between sites, since when a species is absent from a site, it is not replaced by a new species.

Species turnover or replacement is what makes up the rest of the beta diversity. Turnover describes how the loss of a species in a subsequent site is replaced by a new species not found in the previous site. This means that a stream with only turnover as a driver of beta diversity would have the same number of species at all of the sites, as when a species is not found at a site, it is replaced by a new species instead.

I hypothesize that total beta diversity and replacement will have positive impacts on total trout abundance as well as individual species abundance, as this would indicate that trout are more abundant in streams where the aquatic invertebrate communities differ along it, but without the species richness decreasing. If it does not have a positive effect it could be due to nestedness being the largest component as I am also hypothesizing that nestedness has a negative effect on total trout abundance as well as individual species abundance. Another reason this could have no effect is is simply if the total level of beta diversity is not great enough to affect the trout. Finally, it could have no effect if the trout are highly specific and ignore most aquatic invertebrate taxa in favour of a few and as such are not likely be affected by the species changing unless it is their preffered taxa that changes.

Alpha Diversity & Abundance

In order to see if other factors played a role, I looked at the relation between both the abundance of aquatic invertebrates overall, as well as their alpha diversity. Alpha diversity is at its simplest a measure the diversity of species in one place (Thukral, 2017). However, there is more to it than simply the species richness, as it also takes into account the abundance of the different species that are found in the area. This can be done so in a number of different ways, and with different indices, two of the most common ones being the Shannon-Wiener index and the Simpson diversity index, both off which will be used in this project.

I hypothesize that both abundance and alpha diversity will have a positive effect on trout abundance as it has been documented that aquatic invertebrates are an important food source for trout (Hunt & Krokhin, 1975). If this does not have an effect, it could possibly be due to the presence of other abundant food sources or interspecific competition between the three trout species in the areas where alpha diversity and abundance are high.