Functional types

Functional types are groups of species that have similar functional roles in ecosystem processes due to similar responses to multiple environmental factors. Since

CH 1: Introduction

recognized as far back as Walther (1888; according to Ginsburg and Schroeder (1973, p. 605); not seen by EW). Walther recognized corals as being separated into three main functional types: ‘frame-building’, ‘frame-binding’ and ‘sediment- producing and trapping.’ Klement (1967), recognizing the importance that an erect morphology had on slowing down or baffling currents allowing sedimentation to drop from the water column, added a fourth functional type: ‘bafflers.’

Fagerstrom (1991) presented a hierarchy of rather subjective criteria for partitioning corals into ‘Constructor’, ‘Baffler’ and ‘Binder’ functional types similar to those presented by Walther and Klement (see Table 2). Although Fagerstom presents his three groups as guilds, they are not guilds, as the groups are not competing for a common resource. The misuse of the term guild by Fagerstrom and others in reef ecology has been reviewed by Precht (1994). Rather than sharing a common resource, the constructor, baffler, binder groups are describing reef accretion, which is important to the sedimentologists and geologists that most frequently use these groupings to describe modern and ancient reefs.

Table 1.2 Fagerstroms checklist for assigning corals from both modern and ancient reef to one of three functional types based on how they contribute to reef construction adapter from Fagerstrom, 1991). The traits biostratonomy and skeletal packing density were not included as they are mainly of interest to palaeontologist. Criteria Constructor Baffler Binder

Dominant growth

direction Upwards Upwards Lateral

Colony morphology Massive, domes, branches, cups, columns Cylinders, cones, blades Sheets, lenses, runners, webs, plates, umbrellas Skeletonization Well skeletonization, stron, rigid Poorly skeletonized, mostly as skeletal fragments Well-skeletonized

Colony size Large Small Medium

CH 1: Introduction 1.4.2.6.Specific response groups

Specific or functional response groups are groups of species that exhibit similar responses to specific environmental factors (i.e. bleaching resistant corals). A specific response group is not a functional group in the strictest sense because its member species do not process resources similarly nor do they necessarily impact the same on ecosystem processes. Rather specific response groups can be thought of as groups of species that pass through environmental filtering events (i.e. storms, warming events, disease outbreaks) with similar success rates. I identified three studies in the coral literature that used specific response groups based on coral traits. Disease can be considered a filtering event through which groups of corals can pass with varying degrees of success. Diaz and Madin (2011) identified a specific response group to disease (although they did not identify it as such) which they simply termed corals with ‘disease potential’. A coral was observed to have ‘disease potential’ if it had been observed in the literature in a diseased state. Diaz and Madin used a general linearized model to examine the influence of 9 coral ‘traits’ on membership in the disease potential groups. They found most of the traits had some influence on membership to the ‘disease potential group’ when examined alone, however, when analysed together predator diversity, geographical range size, and characteristic local abundances were the main predictors for disease potential. The ‘traits’ that Diaz and Madin used are summarized in Table 1.3.

CH 1: Introduction

Table 1.3 The nine coral ‘traits’ and respective attributes used by Diaz and Madin (2011) to identify which coral traits were the greatest predictors of a corals susceptibility to disease.

Trait Attribute type Attributes Source

Characteristic local

abundance Categorical Common Uncommon 1,2 Corallite size Unspecified Unspecified 1,4-6 Wave exposure Categorical Protected

Exposed Broad (protected and exposed) 1 Preferred water clarity Categorical Turbid Clear Both 1 Geographic range Continuous Area 1 Colony growth form Categorical Solitary Encrusting Massive Columnar Foliaceous Digitate Branching Tabulate Corymbose 1,3 Shallowest depth found Unspecified Unspecified 2 Reproductive mode Categorical Brooder

Spawner

7 Number of

predatory species

Unspecified Unspecified 8

1_{Veron and Stafford-Smith, 2002;} 2_{Carpenter et al., 2008;} 3_{Wallace, 1999;} 4_Veron

and Pichon, 1976; 5Veron and Pichon, 1980; 6Veron et al, 1977; 7Baird et al., 2009;

8_{Diaz and Madin, 2011}

In trait-based ecology there is a growing consensus that the term ‘trait’ should only be used to refer to “features measurable at the individual level, without reference to the environment or any other level of organization” (Violle et al., 2007). Using this trait definition Diaz and Madin use only three true coral traits: corallite size, colony growth form, and reproductive mode. Diaz and Madin use the term ‘trait’ for characteristic local abundance, wave exposure preference, preferred water clarity, and shallowest depth. These distribution patterns are really reflections of a corals niche, which in turn is determined by the overall individual fitness of the coral, which in turn is determined by traits. The number of predatory species that a coral

CH 1: Introduction

has is also not a trait since its measurement relies on the presence of a predator and can therefore not be measured at the individual level without reference to its environment. The ‘palatability’ of a coral however is a true trait since it relies on species-level measurable attributes such as corallite width, cynidae type, tentacle length etc. Finally the disease potential is not in itself a trait since it was calculated entirely using traits. Clear definitions of what are and are not traits are needed when applying trait-based ecology to coral reefs.

Another environmental filtering event for corals are warming events. Riegl and Purkis (2009) modelled the persistence of six specific response groups to repeat bleaching events in 1996, 1998, an 2002 which caused mass mortality at two study sites in the Arabian/Persian gulf. While they referred to their groups as both ‘guilds’ and ‘functional groups’, I would argue that a more useful term here would be ‘specific response group’ since the intent was to examine how these species groups responded to a specific stressor (warming events). Coral species were sorted into 6 groups based on genera (Acropora, faviids, and Porites) and life stage/size (small, large).

Quantitative species-specific traits were not used to define the Riegl and Purkis six groups; instead the groups were identified using genus level growth rates and percentage coverage data for corals from 1995 when the system had presumably reached climax. As coral trait data is compiled and trait-based methodology becomes more commonplace in coral ecology, response groups should be defined using the traits they possess and then validated by observing community composition before and after filtering events rather than using field observation to define the groups and then making assumptions about the traits that define the groups. This in turn will eventually allow us to predict reef responses to stress events based on traits alone. Storm events are also environmental filters, which coral response groups can, or cannot, pass through. Different mortality levels were found for different coral morphologies after the occurrence of the 1967 cyclones on the Heron Island reef (Hughes and Connell, 1999). The morphology groups found to be the most storm

CH 1: Introduction

asexual reproduction by fragmentation, and a more detailed description of morphology (such as surface to volume ratio) would aid in refining the storm response groups further.

It is important to remember that different environmental filter events (stressors) may be operating simultaneously, or in the case of pulse events, at different frequencies. Therefore simply identifying a single response group does not allow prediction of future reef coral compositions. Response groups to a plethora of filters must be identified and their interactions with one another studied. This is a key future research area as the size and frequency of stress events on reefs increase.