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. 2019 Jan 25;14(1):e0210220.
doi: 10.1371/journal.pone.0210220. eCollection 2019.

Resilience and Stability of Kelp Forests: The Importance of Patch Dynamics and Environment-Engineer Feedbacks

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Free PMC article

Resilience and Stability of Kelp Forests: The Importance of Patch Dynamics and Environment-Engineer Feedbacks

Cayne Layton et al. PLoS One. .
Free PMC article

Abstract

Habitat forming 'ecosystem engineers' such as kelp species create complex habitats that support biodiverse and productive communities. Studies of the resilience and stability of ecosystem engineers have typically focussed on the role of external factors such as disturbance. However, their population dynamics are also likely to be influenced by internal processes, such that the environmental modifications caused by engineer species feedback to affect their own demography (e.g. recruitment, survivorship). In numerous regions globally, kelp forests are declining and experiencing reductions in patch size and kelp density. To explore how resilience and stability of kelp habitats is influenced by this habitat degradation, we created an array of patch reefs of various sizes and supporting adult Ecklonia radiata kelp transplanted at different densities. This enabled testing of how sub-canopy abiotic conditions change with reductions in patch size and adult kelp density, and how this influenced demographic processes of microscopic and macroscopic juvenile kelp. We found that ecosystem engineering by adult E. radiata modified the environment to reduce sub-canopy water flow, sedimentation, and irradiance. However, the capacity of adult kelp canopy to engineer abiotic change was dependent on patch size, and to a lesser extent, kelp density. Reductions in patch size and kelp density also impaired the recruitment, growth and survivorship of microscopic and macroscopic juvenile E. radiata, and even after the provisioning of established juveniles, demographic processes were impaired in the absence of sufficient adult kelp. These results are consistent with the hypothesis that ecosystem engineering by adult E. radiata facilitates development of juvenile conspecifics. Habitat degradation seems to impair the ability of E. radiata to engineer abiotic change, causing breakdown of positive intraspecific feedback and collapse of demographic functions, and overall, leading to reductions in ecosystem stability and resilience well before local extirpation.

Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Experimental patch reef array.
Clockwise from top left: a 7.68 m2 patch reef with transplanted kelp; a 7.68 m2 patch reef during construction; a 1.08 m2 patch reef with no transplanted kelp, and; an aerial photo of the grid formation of experimental patch reefs (yellow scale bar represents 25 m).
Fig 2
Fig 2. Effects of Ecklonia radiata patch size and kelp density on sub-canopy abiotic factors.
(a) Water flow (expressed as dissolution of plaster clod-cards relative to the paired above-canopy clod-card); (b) irradiance levels (expressed as light relative to the paired above-canopy measurement, where mean ambient irradiance was 558 ± 26 μmol photon m-2 sec-1 , n = 112, ± SE); (c) sediment deposition (expressed as percentage of sediment deposition relative to the deposition recorded in a paired sediment trap above the canopy), and; (d) depth of accumulated sediments in turf-sediment matrices growing on the reef substratum (mm).
Fig 3
Fig 3. Differences in survivorship of transplanted juvenile Ecklonia radiata sporophytes with patch size, season, and kelp density.
Fig 4
Fig 4. Growth rates of macroscopic juvenile Ecklonia radiata sporophytes.
Measurements taken 90 days after transplanting to (a) patches at normal kelp density (8.3 kelp/m2) across three seasons, and (b) patches of varying kelp density during spring. Note the different y-axis scale.
Fig 5
Fig 5. Recruitment and survivorship of microscopic sporophytes on experimental patch reefs.
Mean number (± SE) of (a) naturally recruited microscopic Ecklonia radiata sporophytes, dependent on patch size and kelp density; and of (b) surviving microscopic sporophytes grown on slides 42 days after outplanting, dependent on patch size and kelp density. Note the different y-axis scales and, that recruitment at patch size 1.08 m2 (Fig b) was in the medium density treatment.
Fig 6
Fig 6. Recruitment of macroscopic juvenile Ecklonia radiata sporophytes on different sized patch reefs at end of study.
Expressed as (a) absolute number of juveniles per patch and (b) density of recruits (no. individuals/m2). Tests were conducted using only density of recruits.

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Grant support

This study was funded by an Australian Research Council Discovery Grant (DP130101113; www.arc.gov.au) awarded to JTW and CRJ, and a Holsworth Wildlife Research Endowment (www.ecolsoc.org.au/awards-and-prizes/holsworth-wildlife-research-endowment) awarded to CL. CL was also supported by an Australian Government Postgraduate Award (www.education.gov.au) administered through the University of Tasmania. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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