BIODIVERSITY & ECOSYSTEM FUNCTION
The effect of biological diversity on ecosystem function has been one of the most active and controversial areas of research over the past two decades. While numerous studies have shown that ecosystem productivity (i.e., the amount of carbon fixed into aboveground plant biomass) increases with increasing numbers of species, the question of why has not been adequately answered. The key to understanding how diversity influences productivity, is to account for ecological similarities and differences among species. One powerful way to do this is by accounting for the evolutionary relationships between species. In a number of high-profile publications, I, along with collaborators, have shown that phylogenetic diversity provides a strong correlation with biomass production (Cadotte et al. 2012, Dinnage et al. 2012, Cadotte 2013, 2015, Liu et al. 2015, Cadotte et al. 2017b). I have also applied evolutionary models to predict how different ecosystem functions and services are influenced by diversity (Cadotte et al. 2017b).
Our most recent work examines how species differences contribute to the mechanisms that link biodiversity to ecosystem function. Specifically, we can measure species direct contributions to function and identify those that disproportionately influence function (selection effects) versus the additional benefit of multiple species that utilize differing resources (complementarity). We have shown that selection effect appears best explained by a few traits that are linked to competition, while complementarity is explained by multiple traits, across a number of different trait axes, that link to complex niche differences (Cadotte 2017, Huang et al. in review).
These findings have had broad impact on how researchers think about the relationship between biodiversity and ecosystem function. Further, these results have important implications for how we restore habitats (Hipp et al. 2015) and build urban habitats and green infrastructure (MacIvor et al. 2016) and provide a way to design these management activities to maximize the delivery of ecosystem services (MacIvor et al. 2018). We are currently investigating how biodiversity influences ecosystem function in human impacted landscapes (Jia et al. in review, Livingstone et al. in review, Carboni et al. in review, MacIvor et al. 2017).
biodiversity in Urban systems
Cities represent the most drastic transformation of the natural world, into one that represents the future of the Anthropocene. And we need to understand the nature of cities in order to best manage them for conservation and the delivery of ecosystem services. We work on a variety of projects to understand how urbanization influences coexistence and community assembly mechanisms -thus impacting biodiversity, as well as the how urban biodiversity influences ecosystem function.
We examine how biodiversity is maintained and functioning on different facets of urban green space. We examine how biodiversity influences ecosystem function in urbanized areas (Livingstone et al. in review, Carboni et al. in review, MacIvor et al. 2017). We ask basic questions about the impact of species invasion on biodiversity in city green spaces (Cadotte et al. 2017 -Biol. Inv, Sodhi et al. 2019, El-Barougy et al. 2017), about the value of urban trees and urbanization impacts on tree health, and we examine the value of increasing biodiversity on green roofs (MacIvor et al. 2019, Macivor et al. 2016).
Species invasions are widespread in human-dominated landscapes, where large economic costs are often incurred due to altered ecosystem services, impacts on human health, and from control efforts. Frameworks, concepts and paradigms for understanding and managing invasions are based largely on insights from natural habitats, yet urban ecosystems differ radically in that the environmental impacts of human activity are detrimental for many species and so maintaining native biodiversity and the ecosystem services that benefit people in cities can be difficult and expensive. We recently created the Global Urban Biological Invasions Consortium (GUBIC) to oversee a network of projects and collaborations to determine the magnitude of invasion economic and ecosystem impacts in cities around the world. The main objectives of this consortium and this funding proposal are: 1) to assess the influence of urban to rural gradients in human impact, economics, and environment within cities on invasive species population sizes and diversity; 2) to determine how political, economic, trade, and environmental differences among cities influence the invasibility of cities; 3) to quantify ecosystem service and disservice provided by non-native species within and among cities; and 4) to evaluate invasive species urban policy and management decision triggers in different socio-economic conditions. GUBIC is a multidisciplinary global analysis of how urbanization shapes and is shaped by the movement of species around the world. We aim to uncover how environment, city structure, biogeography, history, trade, economics and governance all shape the biodiversity of cites and the environmental benefits to people. GUBIC consists of more than 80 collaborators from more than 45 cities in 21 countries. GUBIC provides a platform to share data and ideas, and to get researchers together for collaboration and discussion.
Understanding Ecological assembly
Much of our work and the major focus of community ecology is on how communities assemble. However, most of the literature on community assembly examines static community composition patterns sampled at a single place only one time. Understanding the mechanisms driving community assembly require that we observe dynamic processes across scales and trophic levels (Cadotte and Tucker 2017, Seibold et al. 2018). While a number of studies do examine communities of different ages, this is not sufficient to understand how communities are structured. Work from my lab has shown, definitively, that we are simply not able to fully understand assembly mechanisms unless we account for species colonization and extinction in local communities. Along with my former PhD student, Shaoping Li, and our collaborator, Scott Meiners, we utilized 480 vegetation plots sampled continuously for the past 50 years to examine patterns of invasion and extinction and have gain fundamental insights into the roles of competition versus correlated responses to the environment for shaping community change over time (Meiners et al. 2014, Li et al. 2015a, Li et al. 2016).
We examine how different mechanisms of ecological assembly structure communities in a number of different contexts. But what unites this work is our belief that patterns of species richness or small-scale experiments are not sufficient to test assembly mechanisms, but rather we require large-scale studies that examine how environmental gradients shape patterns of species similarity and difference (Luo et al. 2016, Gianuca et al. 2017, Kuang et al. 2017, Uchida et al. 2019, Arnillas & Cadotte 2019).
Further, studies that employ species ecological traits to make inferences about assembly mechanisms frequently rely on species mean trait values. However, this overlooks the ecological importance of among-individual, within-species trait variation. Recent work from my lab has shown that by accounting for within-species variation, we can better understand the relative contribution of the different assembly mechanisms that structure communities (Luo et al. 2016, Carscadden et al. 2017, Luo et al. 2018).
Ecophylogenetic theory and methods
Over the past decade the use of phylogenetic information for testing basic hypotheses in community ecology and for making conservation recommendations has become widespread. I have been deeply involved in the creation of new diversity metrics that combine phylogenetic distances with observed species abundances. With, colleagues, I have now created a freely available package in the R programming language called Pez (Pearse et al. 2015), which calculates metrics as well as performing associated null model analyses. Complicating the value of such approaches is the fact that ecologists use both traits and evolutionary phylogenies to estimate species similarity with a poor understanding of how to integrate these two types of distances. Individual traits may show evolutionary patterns that are poorly explained by specific evolutionary models. I am currently actively working on a paper that will be an important step towards understanding how to use and compare these types of measures. Earlier, along with Steve Walker and Cecil Albert, we developed a new method to integrating traits and phylogeny to explain ecological patterns (Cadotte et al. 2013a). This method combines weighted distance vectors (trait and phylogenetic distances) for all species pairs in an assemblage. The weighting can be allowed to vary, and maximum likelihoods can be estimated for weightings. From this analysis, researchers can assess the relative contributions of phylogenetic and trait distances, and whether these are complementary or redundant. This method has been employed a number times and has revealed how these different facets of biodiversity respond to natural environmental gradients (Bässler et al. 2016, Bassler et al. 2016, Gianuca et al. 2017) and to human impacts (Bassler et al. 2014, Thorn et al. 2016).
Different disciplines have adopted phylogenetic methods largely in isolation from one another, resulting in the creation of a plethora of different ecophylogenetic measures and statistical tests, and misunderstanding about how these measures link with hypotheses. Currently, there are more that 70 such measures in existence. On top of this, it is not clear that researchers have been matching the correct metrics to specific types of questions. A working group I had funded focused on this problem and using multiple approaches, including: logical frameworks, mathematical assessment, and simulation tests, we produced the definitive paper that provides researchers with a framework for choosing appropriate metrics for their analyses (Tucker et al. 2017).
Community ecology has gone through a paradigm shift over the past ten years, and my work has been an integral part of this shift. Classically, community ecology progressed by developing theoretical models that modelling species interactions and used these to generate hypotheses about ecological patterns. Most ecological theory has been tested with field observations or experiments that ultimately measure the number of species. In this simple narrative, there is an important disconnect -ecological theory has most often been constructed with mechanisms based on species similarities and differences, while empirical evaluations that measure species number implicitly assume all species are potentially equally different. This is a fundamental disconnect that has limited scientific advance in ecology. Species richness has been the incorrect ecological measure to evaluate ecological theory. I have been at the vanguard of change in ecology by arguing for the need to use distance-based trait or evolutionary measures to better align theory with ecological patterns in two key papers (Cadotte et al. 2013b, Cadotte and Davies 2016, Cadotte et al. 2017a). The move towards a distance-based ecology that accounts for species differences, has resulted in researchers adopting simple and readily available statistical routines, but not necessarily considering the critical assumptions that come along with such measures. Perhaps as a signal of this field’s maturing status, I have co-authored a book on the history, assumptions, methods of ecophylogenetic analyses (Cadotte and Davies 2016), and have further published a couple of papers that focus in on critical assumptions that resulted in researchers misinterpreting their findings, or not appreciating the limitations on inference (Cadotte et al. 2017a, Cadotte and Tucker 2017, Cadotte et al. 2019, Tucker et al. 2019). Thus, while my work has become synonymous with these new methods and hypotheses, I have also become a voice to steer and temper rampant misuse of these approaches.