Population dynamics of an ecologically important range-extender: kelp beds versus sea urchin barrens

Abstract

The barrens-forming sea urchin Centrostephanus rodgersii (Diadematidae) has recently undergone poleward range-extension to Tasmania where grazing of diverse and economically important macroalgal beds has occurred. We compared growth, biometric, dietary and morphometric characteristics of C. rodgersii between macroalgal habitat and adjacent barrens to inform population dynamics of the sea urchin within the newly extended range. The age structure of C. rodgersii in macroalgal habitat and adjacent barrens is similar, suggesting that both habitats have been influenced by similar recruitment events. However, there are clear differences in body size, body mass, spine length and diet of sea urchins in the 2 habitat types. We identified 2 broad morphologies of C. rodgersii. The first is of relatively large size with thick test, rapid growth and short spines, that grazes macroalgal beds (macroalgal eco-morph). The second ecomorph persists on the barrens habitat and demonstrates smaller body size, slower growth and thinner test, with gut contents dominated by closely cropped filamentous and/or coralline algae; but occurs at higher density, possesses longer spines and appears to enhance the persistence stability of barrens habitat once it is formed (barrens ecomorph). Invading dense, swell-prone macroalgal habitat of eastern Tasmania, the phenotypic plasticity displayed by this sea urchin appears to be an important mechanism facilitating colonisation of reef habitats within the extension region. Importantly, habitat-specific patterns in population dynamics are broadly consistent with those observed from within the historical range, suggesting that patterns in macroalgal-sea urchin dynamics and the ecological importance of this species will be similar across the newly extended range.KEY WORDS: Centrostephanus rodgersii · Climate change · Invasion · Morphology · Growth · Reproduction · Seaweed beds · Sea urchin barrens Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 374: [113][114][115][116][117][118][119][120][121][122][123][124][125] 2009 algal-dominated habitat, which results in widespread and persistent sea urchin barrens throughout the species' historical range in New South Wales (NSW) (reviewed by Andrew & Byrne 2001). Indeed, such is the dominant ecological effect of C. rodgersii that, within its historical range, the sea urchin has formed and maintains barrens over approximately 50% of all near-shore rocky reefs (Andrew & O'Neill 2000).Since its first detection on the Tasmanian mainland coast in 1978, the sea urchin's abundance has increased, and evidence of barren areas now occur in some Tasmanian locations (Johnson et al. 2005). The potential for broad-scale ecological shifts from luxuriant macroalgal beds to sea urchin barrens in Tasmania therefore poses a major threat to local biodiversity and valuable reef-based fisheries that depend on macroalgal habitat (Johnson et al. 2005, Ling 2008. Hence, understanding the characteristics of Centrostephanus rodge...

Forming sea urchin barrens from the inside out: an alternative pattern of overgrazing

“…As with invariant nocturnal patterns in movement, there were no major differences in movement between incipient and widespread barrens, as might be expected if C. rodgersii exhibited the kinds of behavioural shifts demonstrated in strongylocentrotid sea urchins (Mattison et al 1977, Dean et al 1984, Lauzon-Guay & Scheibling 2007a, Scheibling & Hatcher 2007. Importantly, observations of homing tendency around localised shelter sites on widespread barrens over short time scales are consistent with our observations of high levels of site fidelity over much longer time scales, as evident from up to 45% recovery of chemically tagged individuals within unfenced 8 × 8 m areas on extensive barrens over a 14 mo time period (Ling & Johnson 2009). …”

“…While many foraging species display classic Lévy flight movements (i.e. local random movement with occasional large 'jumps' to new sites), there is no evidence to suggest that Centrostephanus rodgersii exhibits this mode of behaviour, as indicated by high recovery of tagged urchins from circumscribed sites after 12 to 14 mo (Ling & Johnson 2009). Using this evidence, combined with a moderate to strong homing tendency across all habitats (average net displacement < 0.6 m) (Fig.…”

Annual reversible plasticity of feeding structures: cyclical changes of jaw allometry in a sea urchin

“…Differences in the relative size of jaws (demi-pyramids) of Aristotle's lantern or the entire lantern have been reported for a large number of sea urchin species in field studies, including S. purpuratus [20], Mesocentrotus (Strongylocentrotus) franciscanus [21,22], Strongylocentrotus droebachiensis [23,24], Echinometra mathaei [25,26], Diadema setosum and Diadema antillarum [25,27], Sterechinus neumayeri [28], Evechinus chloroticus [29], Arbacia punctulata [30], Centrostephanus rodgersii [31] and Heliocidaris erythrogramma [32]. Changes in jaw and diameter allometry also have been induced under laboratory conditions of food manipulations in S. purpuratus [18,33,34], S. droebachiensis [24,35], M. franciscanus [36,37], D. antillarum [27], Paracentrotus lividus [38] and Lytechinus variegatus [39].…”

Population Dynamics of Edible Sea Urchins Associated with Variability of Seaweed Beds in Northern Japan

“…The sea urchins in macroalgal habitats are larger with faster growth rates. In contrast, sea urchins in barrens have smaller body size and slower growth rates (Ling and Johnson 2009). Wing et al (2003) examined E. chloroticus growth throughout the Doubtful-Thompson Sound complex in New Zealand.…”

“…As characteristic of the phenotypic plasticity in sea urchins, Centrostephanus rodgersii adjust its body growth with food availability (Ling and Johnson 2009). The sea urchins in macroalgal habitats are larger with faster growth rates.…”