Mechanisms of ecosystem stability for kelp beds in urban environments

Ecologists have long been interested in determining the role biotic relationships play in natural systems. Even Darwin envisioned natural systems as \bound together by a web of complex relations‚ÄövÑvp noting how complex and unexpected are the checks and relations between organic beings‚ÄövÑvp (On the Origin of Species1859 pp 81-83). Any event or phenomenon that alters the implicit balance in the web of interactions to any degree can potentially facilitate a re-organisation in structure that can lead to a wholescale change to the stability of a natural system. As a result of the increasing diversity and intensity of anthropogenic stressors on ecosystems previously well-understood biotic interactions and emergent ecological functions are being altered requiring a reappraisal of their effects. A key challenge lies in understanding whether and how continuing anthropogenic change alters the relative importance of biotic interactions in determining community dynamics or whether it creates entirely novel interactions that modify patterns in the distribution and abundance of species at the scale of landscapes. Along temperate coasts worldwide shallow sub-tidal reefs are usually dominated by canopy-forming large brown alga commonly referred to as kelp. These kelps are important foundation species supporting high production providing habitat for many associated species and delivering numerous ecosystem services valued in the range of billions of dollars annually. There are four classes of events well recognised to influence the nature of important biotic interactions and result in considerable alteration to shallow temperate reef ecosystems: 1) depletion or 2) increase of a foundation species 3) addition of non-native species and 4) the addition and interaction of multiple direct human-stressors. The kelp Ecklonia radiata is arguably the single most important foundation species on southern Australian sub-tidal reefs. This thesis examines the dynamics of this species on rocky reefs in Port Phillip Bay adjacent to the city of Melbourne (Victoria Australia). In Chapter 2 I examine how herbivory and sedimentation influence survival at different life-history stages of this species and the impact of herbivory and sedimentation on the capacity of kelp beds to recover from depletion of foundation species. I identify evidence of critical bottlenecks in the kelp's life-history which act to reduce its capacity for recovery (thus reducing kelp bed resilience) due to altered strengths of competitive interactions. Chapter 3 builds on the understanding that E. radiata plays a key role in structuring the benthos on Australian sub-tidal reefs and examines whether an invasive kelp (Undaria pinnatifida) can fulfill a similar ecological function concluding that the invasive species cannot be a functional equivalent of the native kelp. Chapter 3 also shows that sea urchin (Heliocidaris erythrogramma) grazing is leading to native kelp bed (E. radiata) decline in Port Phillip Bay facilitating dominance by turf algae. Even though invasive U. pinnitifida establishes once the native kelp is disturbed the ecological function performed by the exotic species in clearing away turf is weaker than that of E. radiata. Chapter 4 examines the influence of multiple urban stressors ‚Äö- sediments and nutrients ‚Äö- on the resilience and resistance capacity of kelp beds. This multifactorial field experiment over 14 months showed that the response to elevated loadings of sediments and nutrients were context-dependent dependent on the local compliment of algal taxa. Chapter 5 combines the mechanistic understanding of drivers affecting kelp bed resilience and resistance (derived from chapters 2-4) to explain patterns of loss and fragmentation of kelp bed patches at the reefscape (100 m scale). Specifically loss and fragmentation of kelp beds is driven by urchin overgrazing at kelp bed edges which drives a rapid increase in the ratio between perimeter (edge) and area (interior) of kelp patches. Urchin overgrazing at the edge of kelp patches causes significant flow-on effects including breakdown of kelp recruitment increased invasibility of non-native kelp and development of dense turfs that inhibit kelp recruitment. The combined results from this intensive field research highlights the importance of kelp itself for maintaining strong intrinsic and extrinsic interactions through positive feedbacks that result in a system that is both resistant and resilient. Yet kelp is still being lost which is fundamentally driven by high densities of the sea urchin H. erythrogramma in this temperate southern Australian reef system. The interaction between nutrient enrichment and loss of kelps via urchin grazing shifts the system to a state dominated by opportunistic algae such as turfing species and U. pinnatifida. Moreover once the kelp is lost and the system becomes dominated by these other species ‚Äö- particularly turf algae ‚Äö- it appears difficult for the kelp to recover suggesting a strong hysteresis in the system. The suite of field experiments and surveys defining this thesis span multiple-spatial scales (from sub-metre to 10s of kilometres) include multiple stressors of urbanisation and consider multiple life-history stages to build a comprehensive understanding of management actions available to minimise loss of native kelp beds and maximise the recovery potential of kelp beds under highly urbanised regimes. These include removal of urchin biomass kelp bed restoration and conservation efforts focused on maintaining diverse functional groups of algae to contribute to local species pools. These management actions can work in concert to maintain stability of this essential ecosystem that supports important services to human society."