Variance and Rate‐of‐Change as Early Warning Signals for a Critical Transition in an Aquatic Ecosystem State: A Test Case From Tasmania, Australia

Abstract

Critical transitions in ecosystem states are often sudden and unpredictable. Consequently, there is a concerted effort to identify measurable early warning signals (EWS) for these important events. Aquatic ecosystems provide an opportunity to observe critical transitions due to their high sensitivity and rapid response times. Using palaeoecological techniques, we can measure properties of time series data to determine if critical transitions are preceded by any measurable ecosystem metrics, that is, identify EWS. Using a suite of palaeoenvironmental data spanning the last 2,400 years (diatoms, pollen, geochemistry, and charcoal influx), we assess whether a critical transition in diatom community structure was preceded by measurable EWS. Lake Vera, in the temperate rain forest of western Tasmania, Australia, has a diatom community dominated by Discostella stelligera and undergoes an abrupt compositional shift at ca. 820 cal yr BP that is concomitant with increased fire disturbance of the local vegetation. This shift is manifest as a transition from less oligotrophic acidic diatom flora (Achnanthidium minutissimum, Brachysira styriaca, and Fragilaria capucina) to more oligotrophic acidic taxa (Frustulia elongatissima, Eunotia diodon, and Gomphonema multiforme). We observe a marked increase in compositional variance and rate‐of‐change prior to this critical transition, revealing these metrics are useful EWS in this system. Interestingly, vegetation remains complacent to fire disturbance until after the shift in the diatom community. Disturbance taxa invade and the vegetation system experiences an increase in both compositional variance and rate‐of‐change. These trends imply an approaching critical transition in the vegetation and the probable collapse of the local rain forest system.

Biogeochemical Responses to Holocene Catchment‐Lake Dynamics in the Tasmanian World Heritage Area, Australia

“…Long‐term disturbance from fire in fire‐sensitive vegetation, such as rainforest, can result in substantial ecosystem transformation, such as altered species composition, soil formation, and nutrient loss that can have important implications for associated water bodies (Ball et al, ; Huvane & Whitehead, ; Korhola et al, ; Leys et al, ; Morris et al, ; Smith et al, ). Fires in temperate environments, such as western Tasmania, mostly occur in the period from September to March (from spring to early autumn) and are often followed by heavy rain events (Bridle et al, ; Pemberton, ), which can remove the soil layer into water bodies, altering water geochemistry and nutrient availability (Beck, Fletcher, Kattel, et al, ; Boerner, ) and precipitating an aquatic ecosystem response (Beck, Fletcher, Gadd, et al, ). Disturbance from repeated fire in the rainforest of Tasmania, for example, are associated with the destruction and complete erosion of highly organic soil profiles, localized plant species extinctions, and invasion by fire‐promoting vegetation that can radically alter fire‐vegetation‐soil dynamics.…”

“…Disturbance from repeated fire in the rainforest of Tasmania, for example, are associated with the destruction and complete erosion of highly organic soil profiles, localized plant species extinctions, and invasion by fire‐promoting vegetation that can radically alter fire‐vegetation‐soil dynamics. Investigation on the effects of these changes in the dystrophic aquatic environments of Tasmania has revealed a tight coupling between fire‐driven changes in the terrestrial environment and aquatic ecosystem dynamics, via the influence of organic soils over aquatic trophic status (Beck, Fletcher, Kattel, et al, ; Fletcher et al, ) and the influence of terrigenous input over lake water pH (Beck, Fletcher, Gadd, et al, ). While the recent work by Beck, Fletcher, Gadd, et al () reveals a tight coupling between terrestrial and aquatic ecosystem changes in response to fire, there is little empirical evidence for the influence of long‐term changes in fire and vegetation over important within‐lake processes, such as lake mixing, light availability changes, and redox conditions.…”

Can we detect ecosystem critical transitions and signals of changing resilience from paleo‐ecological records?

“…be conducted. Multi-proxy studies would also be fruitful as examining changes in the functional composition of the community as a whole may provide better diagnostic information of an approaching critical transition than any one proxy (Doncaster et al 2016, Beck et al 2018). This would also allow to test whether regime shifts and critical transitions could be upscaled to the whole aquatic ecosystem, that is, whether resilience indicators at one trophic level are detectable in other components of a food web .…”

“…Paleo-ecological records thus represent a valuable opportunity to extend our temporal perspective and enhance our ability to detect critical transitions retrospectively (Dakos et al 2008, Lenton et al 2012, Spanbauer et al 2014, Belle et al 2017, Beck et al 2018. Paleo-ecological records thus represent a valuable opportunity to extend our temporal perspective and enhance our ability to detect critical transitions retrospectively (Dakos et al 2008, Lenton et al 2012, Spanbauer et al 2014, Belle et al 2017, Beck et al 2018.…”

“…In general, a deeper understanding of the causes of regime shifts and whether these are critical transitions requires high-resolution and lengthy times series; often monitoring records do not meet these requirements. Paleo-ecological records thus represent a valuable opportunity to extend our temporal perspective and enhance our ability to detect critical transitions retrospectively (Dakos et al 2008, Lenton et al 2012, Spanbauer et al 2014, Belle et al 2017, Beck et al 2018. However, to the exception of varved sediments which provide annual integration, sediment core records have their own biases and shortcomings, such as sediment focusing and irregular temporal integration (Carstensen et al 2013, Frossard et al 2015.…”