6.2 Data Analyses
6.2.3 Data presentation - graphs, tables and maps
In your report, data can be presented as tables, graphs and maps. Graphs are usually easier to interpret than tables unless there are only a few lines of data. Maps are extremely useful for interpreting site level data and any large changes on reef ecosystems in a specific area are easily and quickly seen such as proximity to a village or known fishing grounds.
The type of graphs and maps to create for interpretation of data, reporting and communication will very much depend on the specific questions of managers at your site, the number of sites and years of data and any interesting trends or patterns that occur at your site. Therefore this protocol cannot prescribe a comprehensive list of tables, graphs and maps that will be suitable for all MPAs. However, a suggested a list of graphs is provided that will show the status and trends of fish and benthic communities in the different management zones over time.
Data can be presented in different ways. Graphs can be produced that illustrate a general representation of the MPA, by averaging across all the sites (Figure 4, 6, & 7). However, there may be an interest in the trends and status of individual sites.
Graphs with variables plotted for individual sites are included for the benthic data (Figure 5);
this approach can also be applied to fish data. These graphs are also a good complement to the general MPA graphs because they help to identify which sites are contributing to the overall observed trends. Graphs can also be produced to illustrate the “difference-in-difference” to compare the performance of the different management zones relative to a baseline (Figure 8).
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Trends observed in summarized data from the output variables can be very informative (such as an increase in fish biomass), however, sometimes graphs and average values can be misleading if there is no understanding of the variability in the data. Variability can be shown by including error bars or values in the figures and tables, so this is strongly recommended.
The following provides some examples of the suggested graphs to include in reporting. Each graph is followed by some notes on how those particular graphs could be interpreted.
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Figure 4. Average percent cover (±SE) for main benthic categories within No Take and Use Zones in an MPA 2009-2011
*The composition of the benthic community in each zone and each year of sampling is shown in Figure 4. Comparisons of changes in benthic communities over time in different zone types show there were few differences among zones but some differences among sampling years.
This is to be expected because the zoning plan was not implemented and enforced until October 2011, after the last sampling in March 2011, and reefs are very similar throughout this MPA. The main features of the benthic communities include:
Hard coral cover was higher at sites in No Take Zones compared to Use Zones.
Macroalgal cover was low at all sites.
The amount of available substrate increased each year in No Take Zones. In Use Zones available substrate increased from 2009-2010 but was then stable. There was less mobile substrate (rubble) in No Take Zones compared to Use Zones and levels were stable over time in both zone types.
With implementation of the zoning plan, we may expect that benthic communities in No Take Zones will increase in cover of hard coral and available substrate.
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Figure 5. Cover of hard coral showing cover of branching and tabulate forms at each site (shown by transect #s) in No Take and Use Zones in a) 2009; b) 2010 and c) 2011
*The composition of the coral community varied among sites with the proportion of branching and table life forms comprising between 10-70% of the coral community. Hard coral cover and branching and tabular cover were generally lower in 2010 compared to 2009 and 2011. This may have been due to changes in the monitoring team in 2010. However, some sites such as 2227 and 2228 maintained similar benthic communities throughout the sampling period. Sites with a high proportion of branching and table species will provide good habitat for fish and other organisms. These species however are also highly susceptible to bleaching and disease and should be a priority for monitoring during thermal stress events.
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Figure 6. Average herbivore fish biomass (±SE) of different fish families within sites in different management zones in an MPA from 2009-2011
* In this graph you can see that there is a change in herbivore biomass in (a), but finer taxonomic analyses can tell you that both (b) Acanthuridae and (c) Scaridae are making up the most of the change in herbivore biomass. It is important when interpreting the graphs, to pay careful attention to the scale of the y axes, as the data are plotted on different scales. Another observation that could be made here is that the three fish families show a similar pattern in the No Take Zone with a decrease in populations following 2009. The interpretation might be that there was some disturbance that had a negative effect on all three fish families or that this No Take Zone is not enforced.
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Figure 7. Average carnivore fish biomass (±SE) across sites within a No Take and a Use Zone in an MPA from 2009-2011
*This graph shows a small increasing trend of carnivore biomass over the years 2009-2011 in both the No Take and Use Zones. Lutjanids make up most of the carnivore biomass and also show a small increase over the three years of sampling. Because Lutjanidae represents most of the biomass, then it can mask trends in other fish families if we just look at the overall carnivore biomass. In this case, the increasing trend in overall fish biomass and fish in the family Lutjanidae are similar. However, different patterns are observed in the fish family Serranidae and Haemulidae. Haemulidae have low populations in the No Take Zone with variable populations in the Use Zone. Serranidae have fairly constant
populations in both No Take and Use Zones. In the short time frame that this MPA has been monitored, it appears that there has been a positive impact on Lutjanidae populations throughout the MPA, although the families Haemulidae and Serranidae have no discernible trends in biomass over time.
Analyzing this data with a finer taxonomic resolution demonstrates that different fishes are going to respond differently to management.
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Figure 8. “Difference-in-Difference” graph showing another example of how data can be analyzed to illustrate management effectiveness. The x axis represents the year sites were monitored and the y axis is the average % change in coral cover. The graph shows differences in values for different zones relative to a baseline (2009). Each line represents a regime (No Take, Use, Control). The baseline values represent the average of all data for the variable in the first year of sampling (in this case it is 2009). The baseline value will be different for each
management regime (unless they happen to have the same average value) but they will all be normalized to “0” or the baseline (see Box 6).
To further explain the “difference-in-difference” concept, the graph is further explored in graph (b). The first “difference” is the difference within a management regime between the baseline and the following years that the sites were monitored. Each of the lines on the graph shows this trend for each zone. These differences will then be compared to the difference between
management regimes for the second “difference”. Two examples of “difference-in-difference”
are illustrated in graph (a), though it is important to understand that this can be constructed for every year (relative to baseline) comparing both No Take and Use Zones to the Control. The second panel is used to demonstrate the concepts, but graph (a) is more similar to what would be included in a report. This is a hypothetical example and could represent any selected output variable (i.e. % coral cover, fish biomass).
The variable % Hard Coral (HC) is used in this example to demonstrate how this is applied mathematically to calculate “difference-in-difference”:
= (Ave. HC No TakeBaseline – Ave. HC Cover2011) – (Ave. HC Control Baseline – Ave. HC Control 2011)
A positive value would indicate there is a positive impact on coral cover due to the management intervention in the No Take Zone (in this case it would be positive) and vice versa.
One thing to point out is that in this case just examining the data without the control sites would indicate a slight increase in the Use Zone in hard coral in 2011, but because conditions decreased outside the MPAs, this does indeed show that there are positive impacts occurring inside the Marine Protected Area.
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