Phase Shifts, Alternative States, and the Unprecedented Convergence of Two Reef Systems

Abstract

Abstract. Initial conditions can generate differences in the biotic composition of spatially disjunct communities, but intense, large-scale perturbations have the potential to reduce or eliminate those historical differences. The latter possibility is of particular concern with respect to coral reefs, which have undergone dramatic changes in the last 25-30 years. This paper reports a case in which two reef systems with different biotic histories were recently perturbed to a single, novel state.We compared millennial-scale records of species dominance from reefs in Bahia Almirante, a coastal lagoon in northwestern Panama, to previously published records from reefs in the shelf lagoon of Belize. Reef cores extracted from Bahia Almirante at 5-10 m water depth revealed that the Panamanian reefs were persistently dissimilar from the Belizean reefs for at least 2000-3000 years prior to the last several decades. The Panamanian reefs were dominated continuously by branching finger corals, Porites spp. (primarily P. furcata). Shifts from the Ponfes-dominated state to dominance by other coral species were rare, were restricted to small areas, and lasted for decades to centuries. The Belizean reefs were dominated continuously by the staghorn coral Acropora cervicornis in the same depth range during the same period. Excursions from the Acropora-dominated state were again rare and spatially localized. Populations of Ac. cervicornis in the Belizean lagoon were nearly extirpated by an outbreak of white-band disease in the late 1980s, and changes in water quality were apparently detrimental to branching Porites in Bahia Almirante in recent decades. These large-scale perturbations caused the two reef systems to converge on a third, historically unprecedented state: dominance by the lettuce coral Agaricia tenuifolia. Ag. tenuifolia possesses life-history attributes and environmental tolerances that enabled it to become dominant in both disturbed ecosystems. Although the two phase shifts to Ag. tenuifolia differed in both their general mechanisms and specific causes, they had the effect of eliminating the salient difference in benthic composition between the Panamanian and Belizean reefs. The changes in species composition thus obliterated the influence of several thousand years of reef history.

Molluscan subfossil assemblages reveal the long-term deterioration of coral reef environments in Caribbean Panama

“…Compared to previous estimates of rubble accumulation rates from Almirante Bay of 1-1.75 cm yr À1 (Aronson et al, 2004;Hilbun, 2009) which predict a possible age range of 80-140 years for lagoonal pits, the average span of calibrated midpoint values from the top and bottom layers was 137 years (Table 1). Two lagoonal pits encompassed at least 400 years, and clearly had lower sedimentation rates (and higher degrees of Table 3 Significant differences in molluscan environmental and community parameters between lagoonal and offshore reef environments.…”

“…However, comparison within each pit of the oldest possible age from the 60-80 cm layer with the youngest possible age of the 0-20 cm layer produced an estimate of the maximum possible number of years encompassed by an individual pit: 239 years and 59 years on average for lagoonal and offshore pits respectively (Tables 1 and 2). By comparison, coral rubble accumulation rates measured from lagoonal sites in Bocas and Belize predict a possible age span of 80-140 years for lagoonal pits (Aronson et al, 2004;Hilbun, 2009). …”

“…Benthic surveys have revealed an up to 10% decline in live coral cover from 1999-2002, the first four years of ongoing monitoring in this region (Guzmán, 2003). A recent reef coring program conducted at fringing reefs in Bocas' Almirante Bay found a general transition in coral dominance from Porites furcata to Agarica tenuifolia that occurred sometime after 1960 and was unprecedented over a millennial scale (Aronson et al, 2004(Aronson et al, , 2005. The analysis of subfossil coral assemblages excavated from large pits below modern reefs in Bocas confirmed that recent reef deterioration has deeper historical roots: the loss of the regionally co-dominant staghorn coral Acropora cervicornis occurred before 1960 and in some cases likely as far back as the early 1900s in lagoonal reefs, contemporaneous with the onset of land clearing for industrial agriculture (Cramer et al, 2012;Cramer, 2013).…”

Tropical dead zones and mass mortalities on coral reefs

“…Changes in coral community structure were still evident nearly 4 y after the hypoxic event, with no apparent recovery in the cover of live coral (P > 0.10). Moreover, these changes occurred in an ecosystem where the coral community had already shifted toward more stress-tolerant species in recent centuries in association with human impacts, including fisheries exploitation and land-use change (30)(31)(32). The long-term effects of hypoxia are potentially different from, and more substantial than, those of other disturbances on coral reefs because hypoxia affects a broad range of taxa including consumers, habitat formers, and pathogens.…”

Evidence of size-selective evolution in the fighting conch from prehistoric subsistence harvesting

“…Almirante Bay (figure 1) was flooded sometime before 7.2 ka when modern fringing reefs began to develop [24]. Following the historical and more recent declines of the branching coral Acropora cervicornis [24,25], the fringing reefs of Bocas del Toro became dominated by a mixture of Porites and Agaricia [26].…”