The role of herbivores in a near future ocean: positive and negative effects of climate change on herbivore ecological function

Abstract

Earth’s climate is characterised by abrupt change through its history, yet human induced climate change is warming and acidifying our oceans at unprecedented rates. Such alterations in the seawater’s chemical and physical properties are anticipated to disrupt a multitude of ecological processes leading to potential reductions in productivity and biodiversity of marine systems. Functional groups such as marine herbivores are renowned for meditating competition between benthic organisms, affecting the physical structure and primary production in marine systems, countervailing the deleterious effects of global and local disturbances. Within this context, it is important to not only understand how herbivorous species respond to climate change, but also how their overall functional role are affected and how this might have cascading effects on other species. In this thesis, I reveal that whilst populations of many species are forecast to collapse due to the effects of future climate, some herbivorous species may capitalize on environmental change and boost their densities by increasing the carrying capacity of the environment by actively modifying the habitat under an otherwise stressful condition. I also show that the modifications performed by herbivorous species trough the strengthening of positive interaction under ocean acidification can assist other species to densify, stimulating species coexistence and ecosystem function, and perhaps mitigate the deleterious effect of CO2 enrichment expected at population and community level. Therefore, under ocean warming the functional role of herbivores is eroded releasing opportunistic algae from trophic control which can potentially lead marine systems to undergo structural modification. I show that loss of this functional role, reduces the capacity of the system to control the expansion of opportunistic algae. The identification of the circumstances as to whether herbivores functional role in marine systems will strengthen or decrease provides insights into the impacts of ocean warming and acidification at local scale and their potential management.