Our research evaluates the relationships and feedbacks between consumers, nutrient cycling, biodiversity, and productivity. We take an integrative approach to ecology, considering linkages between processes ranging from organism to ecosystem scales, and asking how these perspectives can enhance our understanding of species interactions, distributions, and abundances.
In the Marine Biodiversity Lab, we work to understand the causes and consequences of changes in marine biodiversity, especially in the context of the current biodiversity crisis. This includes research to understand not only how and why diversity is changing but also the consequences of species losses for how marine systems function. For example, we have shown that nonrandom species losses – e.g., those due to biological invasions or physical stress – can have very different effects on nutrient uptake than random losses (Bracken et al. 2008, Ramsay-Newton et al. 2017). Increasingly, we are exploring how human-mediated changes in biodiversity – those associated with aspects of global change such as invasive species, climate warming, and increases in both atmospheric and oceanic CO2 levels – are affecting ecosystem functioning (Bracken et al. 2018).
The loss of consumer species can transform communities and ecosystems, and these effects are not limited to changes in consumption. For example, herbivores do not just eat primary producers, they can also dramatically affect access to critical limiting nutrients such as nitrogen. We have explored this question in a variety of marine systems over the past two decades, recently focusing our research on methods for partitioning the negative top-down effects of herbivores (e.g., via consumption) from their positive effects (e.g., via nutrient recycling) (Bracken et al. 2014). Here in Southern California, we have found that as grazer abundances increase, photosynthetic biomass also increases. These observations are supported by our current NSF-supported experiments, which show that where grazer abundances are high, removal of grazers results in declines in algal abundance. Subsequent experiments will cross grazer manipulations with experimental nutrient additions and warming across natural gradients in temperature and nutrient availability spanning the California coast.
The results of our research collectively demonstrate the importance of an integrative approach to understanding how biological systems – from organisms to ecosystems – work, especially in the context of human-caused changes in nutrient loading, biodiversity, and ecosystem functioning.