Ecological criteria for evaluating coral reefs and their implications for managers and researchers

Coral Reefs (1995) 14:183-192

Reports Ecological criteria for evaluating coral reefs and their implications for managers and researchers T. J. Done

Australian Institute of Marine Science, PMB 3, Townsville MC, Queensland 4810, Australia

Accepted: 14 June 1995

Abstract. A three step decision-making process is advocated for coral reef managers which (1) evaluates an area at risk, (2) quantifies the risk, and (3) assesses recoverability and consequences in terms of ecological succession and bioconstruction. The judgements required at each stage of the decision-making process should be based on a clear understanding of their ecological and geomorphological implications. 'Biodiversity' and 'bioconstruction' criteria are used to evaluate locations on a five point scale. Ecological risk assessment assigns likelihoods for various damage scenarios. Acceptable change is scale and context dependent and ranges from zero to complete, depending on ecological value and alternate values. Three questions need to be addressed in relation to recoverability of damaged sites: (1) effects on future suitability for settlement, growth and/or repair, (2) certainty of supply of appropriate propagules; and (3) identifiable, 'on site ecological factors' (such as predators, competitors, diseases) which may prevent recovery.

Introduction

The sorts of ecological principles which may be usefully applied to management of coral reefs vary greatly according to the present status and use of reefs. For example, in nations where the reefs are important sources of day to day sustenance, the application of any ecological principles may be a purely academic exercise. At most, those ecological principles which treat coral reefs as 'resource systems' for sustained extractive use by human communities are relevant (e.g. many papers in Munro and Munro 1994). The physical appearance of the reef, let alone details of its benthic communities, may be irrelevant unless they can be shown to influence the reefs capacity to produce protein for human consumption. However where the reefs have high 'intrinsic' or 'existence' value, or where they are recognized as presently or potentially a basis for economic advancement of the nation through tourism, benthic

communities and physical structure do matter, and decisions which affect their quality will be made. It is the application of ecological principles to these types of decisions which I discuss here.

The Great Barrier Reef (GBR), is a large multiple use area whose management ideals are guided by a 25 year vision and strategic plan (GBRMPA 1994). The shared vision includes not only a GBR which sustains major shipping routes and commercial and recreational fishing, but also a continuation of the present rapid growth of visitor members, population, industrialization, coastal development and land use on the adjacent coast (Driml 1994). Over 100 years of exploitation and expanding human land use may have already degraded some GBR reefs and waters in various ways (Moran 1986; Endean and Cameron 1990a, b; Bell 1991, 1992; Brodie 1992). For the purposes of this paper, however, the specific historical precedents which have led to the current 'state' of the GBR's reefs, its waters, and its sea floor, are not the primary issue.

However, some of the ideas developed here do apply to a retrospective analysis of the current status of coral reefs. Both decisions support and retrospective approaches are currently being developed within a large multi-institution research program based in Townsville (CRC 1994), but none of what follows has so far been applied.

Maintenance of bioconstruction and biodiversity as goals for ecologically sustainable development ( ESD )

In the affluent context of the GBR and the 25 year vision, it is reasonable to manage for a goal of'sustainable bioconstruction', (relevant time scales of decades to centuries, Buddemeier and Hopley 1988) and to place a high value on old benthic organisms and 'pristine' communities. Equally, it is important to manage for the 'sustainable biodiversity', (of genes, species, communities, habitats, Grassle et al. 1990), responsible for ecosystem processes, reef structures and harvestable secondary production

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(Done et al. 1995). Different strategies and actions are appropriate for local, whole reef and regional scales.

At the local scale (typically ~104-105 m2), managers and tourism operators, for example, act to preserve the beauty of the reefscape, the corals, the fish and the other biota, in order to maintain the quality of the visitor experience, and the viability of individual tourism operations. These activ- ities may also have importance in the preservation of genetic and species diversity.

At the scale of a whole reef (which, on the GBR, can be anything from 0.5 to 30 km in length), a long-term (decades) goal for managers should be the maintenance of the reef's structure and architectural complexity (i.e. maintenance of net bioconstruction) and of the diverse populations associated with it (see biodiversity, later). The abundance and vitality of the reef's benthic populations of calcifying plants and animals are affected by water quality (Birkeland 1987), and ecological factors such as grazing, predation, disease, and the frequency, intensity and chro- nology of history of human- and natural disturbance (e.g. Hughes 1994, and see Discussion). These fluctuations mean there will be time windows, equivalent to those under which management operates (years to decades), during which individual reef zones and whole reefs may have sub-par performance, not to mention an unattractive appearance (Buddemeier and Hopley 1988).

Framework areas can represent the result of up to hundreds or even thousands of years of uninterrupted accumulation of living corals and associated reef-building organisms on the standing skeletons of their predecessors (Davies 1983; Scoffin 1987). Even currently living corals can represent the results of perhaps 1000 years or more of ecological succession (e.g. Done and Potts 1992). Such areas are often spectacular, and are irreplaceable in human time scales. In that sense, they are more valuable than those reefs or parts of reefs whose structure and biota are more ephemeral.

At the regional scale, with or without human impact, a proportion of reefs, and sites within reefs, are expected to be in 'degraded', 'recovering' and 'healthy' states as a result of previous natural disturbances. The issue for managers then becomes, how much (or how much more) can the mosaic of reefscapes be modified by humans? For scientists, it becomes, how are these states recognized, and how should their proportions differ in the different bio- logical (Done 1982) and geomorphological (Hopley et al. 1989) classes of reef, and how certain and fast is recovery of degraded sites in different environmental and historical contexts?

A three step decision-making process

Consider a proposed development, at a reef or on the adjacent coast or catchment, which it is thought might degrade one or more coral reefs or sites. 'Sites' (areas of say 104-105 m 2) include stands of corals, macro-algae and/ or seagrasses on reef slopes or reef-tops; lagoonal or reef flat sand areas; rubble banks). The responsible management agency would undertake a process which (1) evaluates the area which is to be placed at risk by the activity,

Step 1: assuming some risk of total loss

Value Risk of total loss

Low High

High Go to step 2

Low Develop, because there is little risk and not much to lose

Preserve, because of high value and high risk of loss

Develop if total loss of low value site acceptable

Step 2: assuming partial loss of high value reef or site

Value Predicted loss

Low High

High Develop if recovery Preserve, because will quickly replace recovery time of damage - this high value site go to step 3 is too long

Step 3: recoverability of partially damaged high value site

Loss Recoverability

Low High

High Do not develop Develop if recovery time is short enough

(2) quantifies the risk, and (3) assesses the consequences of total or partial loss in terms of recoverability of the biodiversity and bioconstruction values.

If a target area (site, reef or group of reefs) has a high value (by any criterion), the activity might only be allowed if the risk of partially or completely damaging it is accept- ably low. If, by contrast, its total value is low, (i.e. an area in which natural processes could replace its present benthic composition, diversity, cover and size frequency distribution very quickly - e.g. a turf-covered rubble accumulation, or an area newly colonized by corals following a severe storm), the management agency and the community at large may have few qualms about sacrificing it for the sake of permitting an economically valuable activity. (Such a consensus would be without precedent, and would need to be 'sold' to the community through a convincing education process which demonstrated the loss was truly localized and transient in human time-frames). Thus a manager, if informed about site value and risk of total loss, need sometimes proceed no further than step 1. However, the more common scenario of some risk of partial loss or damage leads to step 2.

Here, a decision to proceed with an activity which will cause a low level of loss to a high value areas, is conditional

upon the affected area having high capacity to quickly recover to its pre-loss condition. The recoverability of the affected area therefore needs to be taken into account (step 3).

The three steps thus include eight outcomes, of which four are unqualified 'yes/no' and the remaining four require further information and judgement. The next section considers concepts of value, risk and recoverability, and how these criteria might be defined, calculated, and called on in making these judgements.

S t e p 1 - as ign a va lue

A reef might, by virtue of its very existence at a particular location, be deemed to be of very high value to the community. On this basis, a decision-maker may choose to allocate a high value without reference to its ecological attributes. In other cases, a site, reef or region may be evaluated ecologically according to the commonness or rarity of its biota ('biodiversity value'), and its successional age, maturity and importance in bioconstruction ('bioconstruction value' - see below).

Biodiversity value

A suggested biodiversity value (Vb) indicating the unique- ness of the area of interest in the regional context is:

v,, = x % �9 aO (1)

where c = the proportion of colonies, plants or bottom cover (~s appropriate) in category j with j = commonness index for regional species pool, and with j = 1 for common; j = 2 for rare, and j = 3 for previously unreported and ot = a constant, here arbitrarily set at 10 so as to produce a maximum V b of 1000 (i.e. when 100% of colonies, plants or bottom cover in the area are previously unreported).

Thus, a site with a species list typical of the area would score 10, a site with equal abundance of common, rare and unreported would score 366, and a site with a unique composition (all species previously unrecorded in the region) would score 1000.

Bioconstruction value

'Time for replacement' is a 'natural ' currency for bioconstruction value, since longevity and large size equate with mass and structural importance. Also, like 'old growth forests', the age of the community may be used as a surrogate for other ecosystem and community attributes (such as maturity, complexity and diversity of their biota and ecosystem processes) which accompany old age. Thus the site may be assigned two values:

Unweighted value V = age of the oldest sessile benthos

(be it coral, algae, sponge or soft coral) (2)

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Alternatively, taking into account the abundance of benthos of different ages,

Area-weighted value V~ = Z (a i �9 m i) years (3)

where a i = age class i (in years) mi = proportion of individuals or of defined area covered by individuals of age class ae.

Both indices assign a zero weighting to bare sand, and low weighting to young benthos (<5 y old e.g. the algal turfs or pioneer corals on rubble or other newly-disturbed areas). Both assign a value of 1000 to a site completely covered by 1000 year-old coral heads.

In a worked example (Fig. 1), there are two sites. Each has the same bottom coverage of corals (~80%), and the size frequency distribution of 'slow' and 'medium' growth rate corals is the same. However site 1 is dominated by large 'fast' growing corals (Fig. la), while site 2 is dominated by large, 'very slow' growing corals (Fig. lc). Both sites have the same V, of 400 years (oldest coral colony present), but (Fig. lf) , site 2 has a higher V,~ (123) than site 1 (29.5) due to the abundance of more very old corals in site 2 (Fig. ld) than site 1 (Fig lb).

Composite value

Five point scales (Fig. 2) could be adopted to categorize quantitative assessments of Vb and V~,, based on detailed mapping, sampling and inventories. The criterion chosen to assign a final value for use in step 1 [i.e. (a) biodiversity, (b) bioconstruction, or (c) composite, in Fig. 2] would depend on the context. For example, biodiversity might take precedence over bioconstruction if a site or reef at risk was faunistically or floristically unusual. However the reverse may be true if there was a choice to be made among which of a series of faunistically and floristically equivalent sites of different ages or successional stages was to be degraded or placed at risk. Finally, recall that in many nations, a high value might be assigned based simply on the existence of any reef at a place.

Decrease in value

The proposed activity might cause a mixture of death and injury in populations of the benthos. For decision making purposes, a simple index, A Vw is suggested.

A V, = 100 �9 (V~,. (before) - Vw (after)) / V,,, (before)% (4)

where V,,, (before) is the weighted value as calculated in Eq. (3), and V~. (after) is as it is re-calculated based on predicted losses to different types and ages of benthos.

In the worked example (Fig. If), there are two damage scenarios. Both moderately reduce the value of site 1 whereas damage scenario 2 caused almost 50% reduction of the highly valuable site 2 and damage scenario 1 caused negligible reduction. In constructing such scenarios, one can be guided by assessments made at various times after a similar disturbance or permitted activity in comparable locations (but see also step 2 below).

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a) Site 1

2O

6 8

!4 0

e) Site 2 20

o C) 12

4

b) Site 1 Very slow Slow Medium Fast (I cngyr) (2 cm/yr) (5 cm/yr) (10 crrdyr) m

I 5 n i t } I I _ ~

Age Class (y) Diameter Class (cm)

11 , In I l l i . i ln �9 . . I 1 . 1 1 . . I . . l : l l l : l : L ! _ , l : : ,

e) Damage

n s $ ~ , - , s~855 n o $ $ $ n a s 8 , ~ s o _ ~ . ~ o n . . . . $ 8 g $

Diameter Class (cm) f) Value Age Class (y)

Site V w A V w 0% loss i ]

50 % loss Scenario 1 Scenario 2

i 1 29.5 23 % 27 %

5 0 % lOSS [ 0 % fOSSil 2 123 1% 49 %

Scenario 1

Scenario 2 [

Fig. la-f . Two hypothetical examples of sites used in worked example (from a spreadsheet) referred to in the text. Panels a and c show field data categorized into four classes based on colony growth rates. Panels b and d show the show the distribution into age classes, assuming growth rates shown in a. The index V~ is simply the sum of the percent cover of each age class x the age. In the worked example,

the lower limit of the size class is used, which makes the estimate of V~. conservative. Panel e shows two damage scenarios, 'scenario 1' killing 50% of 'medium' and 'fast' growing corals; 'scenario 2' killing 50% of 'very slow' and 'slow' growing corals. Panel f shows initial V~, which is recalculated for each scenario to obtain an estimate of A V,~ using Eq. 4

1000

100

10

a) Biodiversity criteria

yw=i? @i Old @ and rare

@ @ @

. . . . . . . . . . . . . . . . . . . . . . . . i . . . . . . . . .

Young @ ! @ Oldbut ~ and ommoni common

10 100 1000 Vw(years)

c) 1000

100

10

b) Bioconstruction criteria 1000

100

10

0 0

Composite criteria

Ygr'g? | Old @ and rare

@ | |

Young | :i | ~ Old bo~ and communi common

0 10 100 1000 Vw(years)

bYu~Ur~rg? @ Old ( ~ and rar~

........ ( 5 ........ @ . . . . . . . .

~o.== @ ! @~0,dUut Q and commoni common

10 100 1000 Vw(years) Fig. 2. Suggested 5 point scales for reef value based

on a biodiversity as sole criterion for value; b bioconstruction as sole criterion, and e a composite of biodiversity and bioconstruction as joint criteria. The two indices of value are: V b (based on the abundance, in the area of interest, of taxa which are endemic or rare either regionaIly or globally); Vw, based on both the age structure and the percent cover of taxa in the area (Eq. 2). Value for composite criteria e are the mean of those in a and b

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Table 1. Stepsinriskassessmentf•rwavedis••dgment•fidea•isedc•ra•sbased•nP•ritesheads••eadingt•estimate•f̀ •ifeexpectancies••fc•ra•s in different habitats and latitudes (Massel and Done 1993)

Step Description

1. Identify en@oint (or designated response in the coral community).

2. Determine age-specific response threshold

3.

4.

Calculate probability of exceeding threshold

Calculate site-specific cumulative probability of persistence (analogous to conventional survivorship curves of population biology).

The dislodgment by waves of 'idealized corals' of standard shapes and strengths of attachment.

The hydrodynamic force necessary to cause the dislodgement of the idealized corals. This was calculated from first principles of fluid dynamics. These forces change each year, as the coral increases in volume and mass.

The probability that the response threshold would be exceeded at any designated latitude and depth of interest.

The probability that a coral settling at a particular latitudes and depth would grow to various ages without being dislodged.

How a manager might use the assessments is very context dependent. In Australia, one can envisage a spec- trum of acceptable reduction in value ranging from 0% (e.g. every coral sacrosanct) to 100% (permanent destruction, such as channel dredging, change of reef-flat drain- age, or shading by a pontoon) if the destruction would enhance some other value or activity.

Recovery time

The decrease in value could be converted to estimates of recovery time using age-structured models (e.g.. transition matrices) and 'best-case scenarios' for direction and rate of succession (Done 1992) and for demographic performance (Done 1988). The damage scenarios need to be defined not as percent cover, but as categories defined by size (or age) class and injury (e.g. Done 1987, 1988; Done et al. 1989). For example predicted recovery times for Porites populations at 21 sites damaged by crown-of-thorns starfish ranged from 9 to > 100 years (Done 1988).

S t e p 2 - a s se s s the r isk

It is easy to project hypothetical scenarios of damage onto field data describing the target community (e.g. Fig. le), but difficult to assign relative likelihoods to these scenarios. Ecological risk assessment (ERA - S u t e r 1993) provides the tools, but they have rarely been applied in a coral reef ecology context (c.f. Massel and Done 1993 and Table 1). The damage scenarios would be identified as 'end points' (Table 2). The specifications for the proposed development would be used to estimate the characteristics of the effluent or impact at its source. Where water quality issues are involved, ' transport and fate' models and eco- toxological procedures would be used to assign risks and durations of exposure to levels which are deleterious to key indicator species. Where physical impact is involved, calculation of likelihood of exceeding key geomechanical and biomechanical thresholds of reefs and reef benthos would be required (e.g. Massel and Done 1993). However, it is suggested that a formal ERA would not be necessary for

low value sites with a high frequency of natural disturbance. (Such sites were eliminated from the decision making process in step 1).

Predicted loss in the context of unmanageable and/or natural loss

The worst case and most likely scenarios need to be evaluated against unmanageable and/or natural impacts. The change (i.e. reduction in value, such as Vw) which is predicted to accompany proposed use of a reef site must be evaluated against the change that could be caused by a cyclone, flood or crown of thorns outbreak, and the probability that one of these events might occur, at that place, within a specified time window (e.g. the proposed life-time of the development, or a critical time period for bioconstruction). If, for example, the human impact is 'one off' and merely brings forward damage that may even- tually happen from natural causes, the choice comes down to how likely that natural impact is at that site within the specified time frame. This question is particularly relevant for areas which are disturbed on average every one to several decades. For areas older than this, their very age is evidence that destructive impacts are unlikely to occur within most time frames relevant to managers and commercial operators (i.e. one to a few years).

If the human impact is recurrent, i.e. it changes chronic stress levels or the frequency or intensity of impacts, the consequences for benthic community dynamics need to be assessed. There is a wide range of natural disturbance regimes across the range of habitats within a single reef, and across the biogeographic regions where coral reefs occur (Fig. 3a). These are reflected in the maximum ages reached by individual benthic organisms, and on the maximum stages reached in benthic community succession and in situ framework accretion. Coral larvae which settle in a shallow intertidal section of a reef may have a life expectancy of < 1 year (Fig. 3b, line A) because of the habitat 's exposure to air in one or two daytime spring low tides per year (Fig. 3a). However the same larvae may settle in a site (C or D in Fig. 3) where it has some chance of surviving to 100 (C) or 1000 years of age (D). A

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Table 2. Description of steps in site specific (= reef or part of reef) assessment of risk of retrogression in coral communities as a result of flood plume, crown-of-thorns starfish (COTS) or eutrophication

Step Flood plume COTS Eutrophication (diffuse source)

1. Identify endpoint (or designated response in the coral community).

2. Determine taxon- and age-specific response threshold

3. Calculate probability of exceeding threshold

4. Calculate site-specific cumulative probability of persistence

Death of corals due to osmotic stress > natural replacement

Duration of exposure to reduced salinity which causes death

Annual probability that flood plume will envelop reef for longer than threshold duration

The probability that a site will change along algae- coral trajectory

Death of corals due to predation > natural replacement

Rate of predation which is > coral population's rate of growth, repair and recruitment

Annual probability of presence of COTS population with predation rate exceeding threshold

The probability of coral size-frequency distributions reaching stability

Enhanced abundance of non-reef building benthos, at the expense of corals, in putative coral areas

Demographic performance non- corals > corals

Annual probability of exposure to water which causes demographic performance non-corals > corals

The probability that a coral dominated site will be replaced by a non-coral dominated benthos

a) Max. ~ A ~ B

Min. ~ ', j j 1 10 i ',100 1000

i Return interval (Years)

b) ~';~=~-~o'~100

0 1 10 100 1000

Age (Years)

Fig. 3. Relationship between disturbance regime a and longevity b of uninterrupted succession or maximum coral age in five habitats (A-D). The 'threshold' in a indicates an intensity of disturbance which limits successional age, coral age, or framework age. This threshold is exceeded most frequently in habitat A (more than 1.y-i) and least frequently in habitat D (less than 1.1000 y-~). Part b shows the differing probabilities of succession, individual organisms, or reef framework reaching ages of 1-1000 + years in A, B, C and D habitats

manager may choose to go ahead with an activity which increases disturbance frequency and/or intensity in a type B or A habi tat (because it will have a low Vu and V) . However any projected shift f rom a baseline condition of D or C may be resisted because it would have greater consequences with respect to bioconstruction and aes- thetic qualities. (For example, a shift f rom D to C to B equates with a shift in max imum viable Porites height

f rom 10 to 1 to 0.1 metres, or a shift f rom a f ramework accumulating (D) mode to a sediment source mode (B). Note that even a chronic pollutant, which kills corals and favours ephemeral corals and/or non-reef building benthos (A) appears on this scheme as an episodic disturbance with a return interval of anything f rom micro-seconds to months.

Step 3 - a s ses s recoverabi l i ty

A decision to permit an activity (step 2) is conditional upon establishing that recovery time is short enough. However estimates of recovery time would usually be based on a "best case scenario' for recoverability, i.e. that all conditions necessary for recovery will continue to exist after the permit ted activity. Three questions which need to be addressed in relation to recoverability are: (1 )what effects will the proposed activity have on the future suitability of the affected area for settlement, growth and/ or repair of the relevant megabenthos, (2) is there an assured, adequate and timely supply of appropr ia te propagules, and (3) are there identifiable "on site ecological factors ' (such as predators, competitors, diseases) which may prevent recovery?

Four types of assessments based on these questions are indicated in Table 3. A manager could permit an activity based on a type b assessment. For assessments a, c and d, the manager could suggest modifications to the proposals to minimise or remove impacts on site suitability (type a and b) or supply (type c and d), or prescribe a rehabilitation p rogram (type a,c,d - assuming a 'one-off ' impact).

Consider, for example, an activity in a catchment likely to damage the coral-dominated flats of reefs within a river's influence. (The delineation of that area in itself is amenable to risk analysis based on historical river flow statistics (Lough 1994) and plume modelling (Wolanski

Table 3. Range of assessments of recoverability of sites placed at risk of damage by an activity

Supply Suitability

Bad Good

High, a) The activity would b) The activity would fre- degrade suitability not degrade either quent but not affect a suitability

supply of propagules or supply

Low, c) The activity would d) The activity would infre- reduce supply and not degrade suit- quent degrade ability would effect

suitability supply

et al. 1995; see also Table 4). For each reef flat, the assessor would need to predict whether corals would be exposed to lethal loadings of fresh water, sediments, nutrients, and/or pollutants in the plume; whether the accumulated rubble f rom the dead coral would appreciably raise the reef 's elevation and hence its exposure to air during low tides; whether the substra tum would become clogged or covered in silt, especially at critical times for coral settlement and

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early growth; whether the area showed evidence of having history of adequate densities and diversity of coral settlement and recruitment; whether there are populat ions in the area (e.g. macro-algae; stoloniferous soft corals; corallivorous gastropods or starfish; some territorial damselfish) likely to subvert a direct return to coral dominance (see later).

These assessments may simply require some minor additions to site assessments, or could form the basis of detailed studies. For example, the quantity, regularity and diversity of supply of broadcasting and locally recruiting corals can be inferred f rom field observations on patterns of density and sizes of visible coral recruits. Possible consequences of permanent changes in site suitability might be also be inferred by analogy with patterns of communi ty structure and zonat ion on reefs along strong natural gra- dients of turbidity (Kleypas 1995; Van Woesik 1994). However to build predictive capacity on regional scales, investigation of hydrodynamic patterns and processes (Burrage et al. 1994; Wolanski 1994), especially during spawning seasons, can be undertaken to provide the basis for assessing both supply (connectivity of target areas with ups t ream source populations) and suitability (e.g. risks of exposure of reefs at various distances f rom river mouths to flood plumes (Wolanski et al. 1995) or tidally resuspended sediments (Kleypas 1995).

Table 4. Assessment and research needs and tools common to both pro-active management and retrospective analysis of current status of coral reefs

Decision maker's question a) pro-active; Assessment needed Research needs and tools for improving b) retrospective assessments

la) What geographic area wiI1 be affected by the proposed activity?

lb) What geographic area has been affected by human activity?

2a) What is its present value? 2b) What is its present value?

3a) How much will the activity reduce its present value?

3b) How much has human activity reduced its previous value?

4a) What are the affected area's prospects for recovery and over what time period?

4b) What are the affected area's prospects for recovery and over what time period?

5a) What is the likelihood of a natural disturbance causing equivalent damage within 1,5 or 10 years?

5b) What is the likelihood that a natural disturbance has caused the perceived degradation?

Specifications of effluents or impacts of activity, their spatial distribution within the GBR, and the responses of reefs to exposures

V and V

Scenarios (zX Vu and A Vw) for 'minor, 'medium' and 'severe' impact.

1,5 and 10 year projections of benthic composition and V, based on scenarios for AV, and AV w

Return interval for disturbance exceeding threshold needed for 'minor', 'medium' or 'severe' scenario (or present perceived degradation).

Plume models to predict reef exposures to fresh water, nutrients and sediments

GIS Exposure/response relationships (one off and cumulative)

Age structure surveys of benthic communities Age/size curves for dominant biota Facies descriptions and aging, especially frameworks

Results of post-impact monitoring elsewhere Experimental studies Reconstructions ofpre-European community structure

Field studies of 'site suitability', 'supply' and 'on site ecological factors'

Demographic performance of key benthic species Lessons from overseas

Long term climatological and hydrological records from instruments and ancient coral skeletons

Transport and fate models for effluents from point sources

Exposure/response relationships for benthos and floods, cyclones, UV, pollutants, nutrients

Probability of exceeding key thresholds, of same

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D i s c u s s i o n

Implications for managers and developers

The criteria and processes developed here suggest a clear set of questions for reef and land-use managers, both in their role as planners for the long-term well-being of the GBR (GBRMPA 1994; Kelleher 1994), and in dealing with existing perceptions that the GBR is already severely degraded (e.g. Bell 1991, 1992). Table 4 (based on the requirements of the three step decision-making process) shows that both roles raise similar questions and call for similar assessments and similar research outputs and tools. A program of studies of these types forms a major part of the Townsville-based Cooperative Research Cen- tre for the Ecologically Sustainable Development of the GBR (CRC t994).

The concept of 'value' may be used as the basis for design of environmentally benign infrastructures and land-use practices. Already, proponents, for on-reef development are required to develop infrastructure designs and usage patterns which minimize threats to corals. This essay provides an ecological rationale for this intuitively reasonable requirement. (Examples include regulating fin and anchor impacts at a particular site; design of on-site pon- toons and moorings; building of bund walls to preserve natural regimes of reef-flat ponding.)

A less obvious and more far-reaching application is the manipulation of land-use and run-off in order to protect coral reefs (e.g. manipulation of flood impacts by construction of dams; changes to land-use (e.g. agricultural or industrial practices); river-bank restorations, or other catchment manipulation). This scenario clearly has major social, economic and political implications, and in Queens- land, is being implemented in pursuit of the goal of inte- grated catchment management, not coral reef protection specifically. However if manipulation of catchment were ever to be advocated with a prime motivation of improving water quantity and/or quality for the benefit of the GBR's coral reefs, it would be necessary to have a much clearer understanding of the area of influence of specific rivers (Hallock and Schlager 1986; Lough 1994), their historical and cumulative effects, and the potential use- fulness of the manipulation in terms of biodiversity, bioconstruction, or other coral reef values.

Resource managers should be aware of a number of circumstances which can cause recovery of disturbed sites to 'go wrong' (Fig. 4). Chance settlement events can direct succession towards persistent megabenthos other than reef-building corals, and a previously framework-dominated area may cease bioconstruction indefinitely. The ambient physical, chemical and nutritional environment determines the reef's suitability for phototrophic bioconstructors (corals; coralline algae) as opposed to non- bioconstructors (macro-algae, heterotrophic, non-reef building benthos such as worms, zooanthids, sponges etc. - Birkeland 1987, and Fig. 4a). Human-induced eutrophication of lagoonal waters of the GBR (Bell 1991; 1992) would amount to an extension of the geographic areas affected by water types to the right of the figure, and a contraction of those to the left. 'Connectivity' (James and

ScandoI 1992, and Fig. 4c) is a measure of the extent to which a reef or a part of a reef has access to propagules which replenish its populations (e.g. algal spores; fish, coral and other invertebrate larvae). Ecologists no longer assume that the ocean is a soup of coral larvae just waiting for a space to settle upon. Settlement may be a very sporadic event, and may be of the 'wrong type' (e.g. non- reef building types in key framework zone). 'On-site ecological factors' are exemplified by the relationship between grazing and benthic community structure (Fig. 4b). There can be 'too little' (Hughes 1994) or 'too much' grazing pressure (McClanahan 1994) for net bioconstruction. It is often tempting to equate high benthic algal biomass with eutrophication (Fig. 4a), but the inverse relationship of algal biomass with grazing intensity (Fig. 4b, d) needs also to be considered. The complete destruction of reef-building capacity and fish production on Jamaica's reefs by a combination of natural catastrophes and overfishing is well known to coral reef ecologists, and should be man- datory reading for managers (Hughes 1989; 1994).

Application in other areas

The ideas and suggestions in this paper are based on conceptual models in which natural disturbance and recovery have primacy (after Connell 1978) and which 'make sense' in the GBR context of cyclones, rivers and crown-of-thorns starfish. In other countries, especially those in less stormy latitudes, and away from rivers and crown-of-thorns starfish, the region-scale disturbances and the assumption of ubiquitous 'ephemeral' benthic communities which are central ideas of 'age as value' may not apply. In that case, calculations of Vw of all sites may be so clustered around the high end of the scale, that ecological discrimination among sites may be based much more on biodiversity criteria (e.g. Vb) than age criteria (e.g. Vw). Again there may be many cases where a reef is afforded a high value to society based simply on its existence in a particular location.

Implications for assessors and researchers

Coral reef scientists rarely undertake research with a view to making intentional value judgements, or assessing risks. The three step decision-making process could be used by managers or proponents to specify more precisely their management objectives and their requirements from their 'assessors'. This would involve not only a 'coral survey', but a valuation of each site based on coral composition and ages as well as total cover, and an assessment of recoverability; not only a study of 'effects of natural disturbance', or 'effects of enhanced nutrients', but an assessment of the likelihood of different reefs and site's exposure to disturbances or nutrient loadings which exceed key thresholds leading to significant change.

The range of inputs and skills to make such a wide range of quantitative assessments is far beyond that pos- sessed by individual scientists or assessors, and the benefits of long-term and interdisciplinary collaborations are clear

191

a)

c)

<72Z/ benthic filter feeders

Oligotrophic ~-. ~ Eutrophic

nearby distant self source source seeding

b)

d)

/ coral/coralline 7Uffe~xcavate d

Low ~ Grazing Pressure ~ High

fishes A overfishing I mPasast~g~l]tyl

~ . "--"""ll~D/aOJe 2 / / ~

c/a Diadema

1492 1980 1983 1995

Fig. 4a-d. Four conceptual models relevant to 'recoverability' of a site damaged by human or natural disturbance or degradation of the environment, a 'Suitability' - equilibrium benthic community composition as a function of loadings of sediment, organic matter and nutrients in the water column (after Birkeland 1987). b Example of'on site ecological factor' - equilibrium benthic community composition as a function of the intensity of grazing, c 'Supply' - connectivity among reefs showing difference in importance of 'broadcast spawners' versus 'local propagators', d Example from Jamaica of 'on-site ecological factors' changes in coral/algae ratio (c/a) as a result of cen-

turies of exploitation of fish predators and competitors of the grazing sea urchin Diadema antillarum causing it to undergo a population explosion which made it vulnerable to a pathogen which caused a sudden crash, allowing proliferation of algae (i.e. decrease in c/a ratio). Dates represent the beginning of European presence in the Caribbean (1492); Hurricane Allen impacting Jamaican reefs (1980); Diadema die-off (1983); time of writing (1995). Diagram after Done et al. (1995), based on original by Jackson (1994) and on synthesis by Hughes (1989; 1994)

(e.g. Ogden 1987; CRC 1994). Endpoints and thresholds (Table 2), for example, are based on physiological studies (e.g. osmotic stress; nutrient response; sediment tolerance), and on demographic and feeding studies (predators, grazers, coral, algae). Calculation of the probabilities of exceeding thresholds requires application of climatological, oceanographic, and statistical models, tools and skills. Estimating cumulative probability of persistence depends on the linking of the properties of exceeding thresholds with acceptable models of population dynamics (Done 1987) and communi ty succession (e.g. Connell 1978; Done 1992).

Implications for human communities

The system of values and assessment proposed in this paper, if adopted, could make the ecological criteria con- tributing to managers ' decision making processes more rigorous and more transparent. In the broad Australian community, the Grea t Barrier Reef is revered, and notions of ecologically sustainable development and biodiversity are widely valued, if not well understood. I f they can be shown to be at the core of decisions which are made about the present and future well-being of the Grea t Barrier Reef, the communi ty support which is essential will continue to be given. Elsewhere, especially where coral reefs' greatest values are their day to day sustenance of local communities through provision of food and/or building materials, it is difficult to see how the values discussed here

could be translated into useful action at the communi ty level. This would require clear demonstrat ion that the benthic communi ty has a significant role in providing services which the human communities can relate to, such as more sustainable fish harvests, and protection of shore- lines f rom erosion.

Acknowledgements. Thanks are due to the following people: Simon Woodley, Jon Brodie at the Great Barrier Reef Marine Park Authori- ty for showing me the reef manager's perspective; Brian Lassig and Steve Hillman (GBRMPA), Colin Creighton of the Queensland Department of Primary Industries, Chuck Birkeland of the University of Guam, and Russ Reichelt, Lyndon Devantier and Stan Massel of the Australian Institute of Marine Science for invaluable comments on various drafts of this paper; Kayt Raymond, facilitator of the Twenty Five Year Strategic Plan, for teaching me that being a stake- holder has nothing to do with barbeques; Keith Nielson of the tourism industry for a different perspective; Chris Crossland, Director of the Cooperative Research Centre for the Ecologically Sustainable Devel- opment of the Great Barrier Reef (CRC Reef Research Centre for short) and Russ Reichelt Director of AIMS for the opportunity to write this paper. This is AIMS contribution no. 756, produced with support from the Commonwealth of Australia's Cooperative Re- search Centre Program.

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Ecological criteria for evaluating coral reefs and their implications for managers and researchers