Dramatic species-specific differences exist in the expected numbers of colonies remaining in the post-disturbance assemblage following larger disturbance events for the present-day strength scenario (a
, solid curves). For instance, a direct hit by a category 1 cyclone (S1) is expected to dislodge almost 50% of the existing A. hyacinthus
population, but only 5% of the A. palifera
population. These trends are driven by the way in which species characteristically distribute colony shape above the substrate as they grow (Madin & Connolly 2006
), whereby top-heavy colonies with smaller attachment areas are much more prone to hydrodynamic dislodgement than colonies with wide bases and low profiles. For instance, the table-shaped morphology of A. hyacinthus
is particularly good at ‘overtopping’ (growing up and over) and outcompeting neighbours (Baird & Hughes 2000
) and provides more habitat structure for reef organisms; however, it is disproportionally affected by hydrodynamic force. Consequently, this population is expected to display dramatic decreases in colony size following relatively small disturbance events (b
), resulting in increases in relative dominance (abundance and cover) of the other two species (c
Figure 2 Forecasted ecological changes in post-disturbance coral assemblages. Solid curves represent assemblage changes based on the present-day substrate strength; dashed curves represent halving of strength due to ocean acidification. (a) The proportion of colonies (more ...)
Owing to the hierarchy in mechanical vulnerability among these species, each is expected to dominate the post-disturbance assemblage at some point over the disturbance continuum (d). Larger colonies of A. gemmifera are more vulnerable to hydrodynamic force than A. palifera due to their narrower bases relative to their vertical mass distribution. Therefore, at greater disturbance intensities, the A. gemmifera population declines along a similar trajectory to that of the A. hyacinthus population. The mean colony size of A. palifera colonies actually tends to increase following larger disturbances, because larger colonies tend to have wider bases and lower profiles and are therefore more likely to survive. Reducing substrate strength by half, i.e. simulating acidification conditions 50–100 years into the future, exacerbated changes in assemblage structure by causing assemblage trends to shift (leftwards) to lower disturbance intensities than are predicted under the present-day conditions (, dashed lines). Consequently, populations are expected to lose up to 20% more colonies than under the present-day strength conditions (a), and switches in the dominance hierarchy happen at much lower disturbance intensities (d). These shifting trends will give mechanically adapted species a substantial advantage in the post-disturbance assemblages. Furthermore, the shifts caused by acidification will be compounded by predicted changes in the future storm intensity regimes.
Relative per cent cover, relative abundance and colony size are critical for the recovery of both colonies and populations following disturbances. For example, larger colonies tend to be more resistant to physical abrasion and disease, have greater competitive and reproductive potential, and command greater proportions of limited substrate space (Jackson 1979
; Hughes 1984
). Increases in available space and smaller (competitively weaker) colonies facilitate growth of other benthic reef-dwelling species, including fast-growing algae and soft corals, which can preclude coral population recovery (McCook et al. 2001
). Moreover, the number of associated species supported by a colony scales with both colony size and morphological complexity (Luckhurst & Luckhurst 1978
). For example, many reef fishes require spatial heterogeneity for their nursery habitats (Almany 2004
), including juvenile parrotfish, a functional group that is important for freeing surfaces from algae for coral settlement (Mumby 2006
). The post-disturbance dominance of mechanically robust, morphologically simple, coral species is expected to lead to decreased whole-reef diversity and functional redundancy with cascading ecosystem effects.
These mechanistic expectations, which are based on the physical constraints imposed by the environment, elucidate the fundamental boundaries within which other important factors operate to structure communities (e.g. herbivory, competition, bleaching, disease). An important next step will be to expand upon this work to include demographic rates (i.e. recruitment, growth, background mortality) so as to track coral population growth and post-disturbance recovery in relation to different climate change scenarios. Understanding physical constraints and their effects on coral assemblage structure in the context of pressing environmental factors (acidification and hydrodynamic disturbance regimes) provides a useful basis for assessing reef vulnerability.