Guidance: Habitat

The Habitat sub-goal is a component of the Biodiversity goal and assesses the condition of habitats that support many species.

Habitat-based goals (Storage, Coastal Protection, and Biodiversity) should be considered together during the data gathering process because the same data underly these goals.

Practical Guidance

STEP 1: Do you need a habitat subgoal?

Habitat is included in the Biodiversity goal to provide a more complete picture of the condition of the species in the study area. For most places, only a portion of the species are assessed and consequently, habitat condition may improve estimates of biodiversity based on the assumption that healthier habitats mean healthier species. If the region has comprehensive species assessments you probably will not need to include a Habitat sub-goal.

STEP 2: Where are the habitats?

You will first determine the relevant habitats in your study area, both in the coastal regions and offshore. In the global assessments, data were available for mangroves, coral reefs, seagrass beds, salt marshes, sea ice, or subtidal soft-bottom habitats, but there are likely other important habitats in your assessment area.

At this point you will want to consider whether and how far a habitat goes offshore and inland and how to deal with this. For example, in global assessments mangrove area within 1 kilometer of the coastline were included, but this distance could be different in OHI+ assessments.

The next step is to determine the location and the area of habitats located in each region. Often, the contribution of each habitat to the final Habitat status is weighted by the relative areas of the habitats within each region. Even if all habitats are equally weighted (regardless of area, as is done in the global assessment) the area estimates will be useful for other goals/subgoals.

TIP: Use local data whenever possible! Ideally local maps describing habitat locations and spatial extent will exist. Alternatively, you can use summarized habitat data, specific to your regions, that exist in tables or other sources. If local data are not available, global data may work for some habitats/regions, depending on the resolution and coverage of the data. For example, the mangrove and sea-ice data used in the OHI global assessment may be appropriate to use at smaller scales.

STEP 3: Estimating habitat status requires a good reference point

Next, you will need to determine how to estimate status (i.e., current condition relative to a reference point) given the conservation objectives of the assessment area.

The reference point is often based on historic habitat area coverage and/or health, with the assumption that habitat destruction has been and still is occurring and the target is to return to some point in the past. The challenge is to find a reference point that is both ambitious and realistic, using the data available. If data allow, it will be possible to set a reference point that is more ambitious than that used in global assessments.

Alternatively, the reference point could be guided by a policy target. For example, are there any climate change policies in your area, with defined targets and objectives? Are there any restoration or carbon storage projects in your area? Do any organizations offer guidance on the amount of carbon storage your management policies should be aiming for?

STEP 4: Find the data

You will need to find data that can be used to measure current condition and recent changes to condition (i.e., trend). The condition of habitats can be measured in different ways, and will probably depend on the data available.

In addition to longer-term historical data needed to estimate current condition, you will want to gather data collected more recently (e.g., within 5 to 10 years) in order to calculate trend. Ideally, there will be enough years of data to directly calculate the current condition and the recent trend in condition.

One way to estimate condition is based on the current area relative to a historical area.

TIP 1: If maps showing current habitat distribution and historical habitat distribution are available, this is a good way to assess condition. This will require historical data from satellites, published papers, or even hand-drawn maps. In the U.S. West Coast assessment (2014), researchers went to the local public library to find hand-drawn maps of historical salt marsh and sand dune extents in California.

In addition to changes in habitat area, other indicators of condition can be used, such as habitat density, susceptibility to pathogens, or change in species composition or growth rates from impacts such as overgrazing.

TIP 2: Look for studies assessing habitat integrity or condition in your regions. For the global assessment, coral condition was based on current percent “living cover” relative to the percent living cover observed at the same site during a reference period from 1985 to 1987. This was possible because estimates of coral cover were regularly estimated along transects over a relatively long time period. Even with high levels of sampling, many sites did not have enough data to adequately assess condition. Consequently, we pooled observations from different locations.

Data for habitat condition can be difficult to find. In some cases, longer-term historical data can be found but more recent data needed to estimate trend is impossible to locate (or, vice-versa). In these cases, proxies or other estimates must be used.

TIP 3: If no data are available to estimate condition, as a last resort you can use a single estimate of condition across all regions based on nearby, or global, assessments. This is not ideal, but it is usually better than not including the habitat or leaving the habitat condition blank (which will result in a default score of 1 or 0, depending on how the scores are coded). For the global assessment, global data describing recent changes in seagrass cover were unavailable to estimate trend. In this case, we applied a single global estimate of recent seagrass trend to all countries (which is not very satisfying, but the best we could do).

STEP 5: Habitat model formulation

Finally, you will want to consider how to combine the estimates of condition for all the habitats to arrive at a single Habitat status value. Often, the contribution of each habitat is weighted by the relative areas of the habitats within each region. In other cases, they are averaged without consideration to area. Another alternative is to weight their contribution by a species richness metric.

Examples of the Approach