Climate Change Alterations to Ecosystem Dominance: How Might Sponge‐Dominated Reefs Function?

Abstract

Study DescriptionAnthropogenic stressors are impacting ecological systems across the world. Of particular concern are the rapid changes occurring on coral reefs, with future reefs being likely to function differently to current coral-dominated ecosystems. Our research explores the impacts of changing sponge dominance on coral reefs using qualitative modelling. We found that that changing sponge dominance due to increased sponge abundance will have different outcomes for other trophic levels compared with increased sponge dominance as a result of declining coral abundance. Addressing knowledge gaps we have identified will facilitate the development of more complex models that assess functional attributes of spongedominated reef ecosystems.

Will coral reef sponges be winners in the Anthropocene?

“…Bell, Rovellini, et al () describe five specific questions that should be answered to better understand how sponge‐dominated reefs would occur, function, and be maintained. We agree with the directions for future research on sponge ecology proposed by Bell, Rovellini, et al (), and encourage the inclusion of novel approaches consistent with the anthropogenic‐based changes we are now faced with (e.g., Williams et al, ).…”

“…While phytoplankton biomass declines with decreasing NPP, autotrophic picoplankton have been predicted to increase (Flombaum et al, ). Picoplankton are important trophic resources for sponges, particularly on deep reefs where POC concentrations exceed those of shallow reefs (Lesser, ; Lesser & Slattery, ), so increases in this resource should select for sponges as “winners” under predicted changes in climate (Bell, Bennett, et al, ; Bell, Rovellini, et al, ). But a recent holistic model of autotrophic picoplankton abundances, incorporating temperature, chlorophyll biomass, nutrients, and irradiance (photosynthetically active radiation [PAR] and UVR), predicts that Synechococcus , Prochloroccocus , and picoeukaryote abundances will actually decline by 32%, 18%, and 42%, respectively, with a 2°C increase in ocean warming, subsequent stratification, and the associated decrease in nutrient delivery (Agusti, Lubían, Moreno‐Ostos, Estrada, & Duarte, ).…”

“…A decrease in NPP will also result in a decrease in the amount of DOM released by the phytoplankton community, and subsequently the abundance of heterotrophic picoplankton available as a food resource for sponges (de Goeij et al, ; Lesser & Slattery, ). This climate change‐mediated scenario predicts that diminishing trophic resources for sponges will have significant effects on the distribution, growth, and abundance of sponge populations, which will ultimately affect their functional role on coral reefs relative to nutrient cycling and the sponge loop pathway (e.g., Bell, Rovellini, et al, ; de Goeij et al, ).…”

“…As a result of these ecological changes, coral reefs have become the “poster child” for ecosystems experiencing profound ecological changes now, and predicted to worsen into the future in the Anthropocene, where high biodiversity coral reefs will probably exist in very few places (Hoegh‐Guldberg et al, ). Sponges, in particular, have been predicted to become “winners” under these climate change scenarios when phase shifts in community structure occur (Bell, Bennett, Rovellini, & Webster, ; Bell, Davy, Jones, Taylor, & Webster, ; Bell, Rovellini, et al, ). But there is not widespread support for a general increase in sponge populations on shallow reefs; site‐specific increases (McMurray, Finelli, & Pawlik, ) and decreases (Wulff, ) have been reported for sponge populations monitored over extended time periods.…”

“…In their hypothesis on climate change and sponge resilience, Bell, Bennett, et al () and Bell, Rovellini, et al () correctly note that understanding the ecological impacts of increased sponge populations on coral reefs must address their effects on carbon fluxes, and the potential for food limitation. Here, we describe a scenario where sponge populations may actually be constrained rather than promoted by climate change due to decreases in their primary trophic resources.…”