Google Earth, a cost-effective tool for mapping mangroves

Google Earth, a cost-effective tool for mapping mangroves

 

What are mangroves and why are they important?

Mangrove forests are coastal ecosystems in tropical and subtropical regions made up of trees and shrubs that live in the intertidal zone along the coasts, rivers and estuaries. Mangroves inhabit areas commonly flooded by tides with little wave influence. In the Galapagos, they can be found in lava rocks.  They range from narrow strips a few meters wide, to hundreds or thousands of hectares of forest, like the Sundarbans in Bangladesh and India where they can penetrate 80 km inland1. Mangroves are peculiar because they can tolerate high salinity levels, enabling them to live in waters 100 times saltier than any other plants can tolerate2. This is possible thanks to their incredible adaptations:

  • They can filter the sea water and excrete excess salt from their leaves.
  • They store water in their leaves, which are covered in wax to minimize evaporation.
  • They have aerial roots named pneumatophores that, besides stabilizing the plant in soft soils like sand or clay, capture oxygen in low tide. Without oxygen, the plant would drown.

These characteristics allow mangroves to create a crucial link between marine and terrestrial systems1; hosting an Eden of abundant biodiversity.

Muestra de manglares, hogar de muchas especies bajo y sobre el agua.Galapagos Mangroves are home to many species above and underwater. (Photo by Nicolás Moity/CDF)

Seventy three species and subspecies of mangroves exist in the world, four of which can be found in the Galapagos: button mangrove, white mangrove, black mangrove and red mangrove. Even though mangroves only cover 0.12% of the terrestrial surface, they are extremely important ecosystems. They generate an estimated US$1.6 billion annually in ecosystem services around the world3.  Mangroves are one of the most productive ecosystems on the planet: they store more carbon per hectare than other tropical forests4; they provide habitat for terrestrial species like the mangrove finch5;, they act as nursery grounds for socio-economically important species; they have scenic value; and they provide coastal protection from hazards like tsunamis4,6. Despite their widely recognized importance, mangrove forests are disappearing worldwide at alarming rates and are expected to functionally disappear in 100 years7,8. The main causes for the loss of mangrove ecosystems are mangrove forest clearing by shrimp and fish farming, firewood, creation of timber, agriculture, coastal development, pollution and tourism8. Fortunately, due to the strict protection of the Galapagos since 1959, there is no shrimp nor fish farming industry in the Archipelago and it is prohibited to clear mangroves for firewood.

Due to the mangrove’s importance, the Charles Darwin Foundation, in collaboration with SCRIPPS Institution of Oceanography – University of California San Diego, and with the support of the Galapagos National Park Directorate, started a project to estimate the ecosystem services of mangrove forests in Galapagos.

Imagen de especies marinas en manglar.

What we did and why we did it?

We needed to visit the principal Galapagos mangroves to study the fish that rely on the protection of this ecosystem. While planning the trip, we realized that the existing mangrove maps were inaccurate. Many of the mapped mangroves did not exist and others were not well located, which not only made it complicated to plan the trip, but it also meant we lost time to conduct our field work.

Due to the lack of good satellite images for the Galapagos, we decideddecided to use Google Earth’s High-Resolution Imagery (GE) to create a more reliable,new updated map of the mangroves in the archipelago. We wanted to determine whether high-resolution imagery from GE could be used to map mangroves in volcanic islands and used the Galapagos as a case study.

Imagen de muestreo en manglar.

Main Results

We developed a methodology to identify and delimit the mangrove patches of the Galapagos volcanic coasts using GE imagery. Our study demonstrates that mangroves cover 3690 hectares of the archipelago and that 85% of the actual recognized patches correspond perfectly to the GE imagery. Our fieldwork demonstrated that more than 90% of the mapped mangrove area identified with GE align with the mangrove forest. The resulting map has an estimated scale of 1:2500 ± 125 (1 cm in the map corresponds to 25 m in reality).

Compared with the previous maps, our maps provided a higher level of accuracy for estimating mangrove area and the identifying the location of mangrove patches. We also determined the level of detail/complexity of our maps, and found our maps to be 20 times higher than previous maps.

Comparación entre las capas existentes de manglar.

The polygons of this study (yellow) are more exact and complex than the previous maps. It more accurately reflects the actual mangrove distribution in the Galapagos.

We calculated that 35% of the coastline of Galapagos is covered by mangroves, i.e. ~ 700 km of coastline benefit from the protection of mangroves.
Our data demonstrates that GE imagery can be used to map mangrove forests in tropical and subtropical volcanic islands and we recommend this method. In addition, because GE VHR imagery is freely available, this method is particularly suited for mapping natural resources in developing countries and areas with constrained resources like the Galapagos National Park.

We conducted a cost-benefit analysis comparing the GE mapping of mangroves with satellite imagery of similar resolution and with images obtained via drone flights along the coastline. The GE option is much more cost effective (between 400 and 35 000% cheaper) than the other options and provides similar results. While mapping mangrove patches with drone images would be more precise, the need for logistical support from vessels to access the entire coastline of the islands would result in a higher carbon footprint.

Costos de mapeo de manglar en Galápagos.

Cost analysis of mapping mangroves in the Galapagos through several remote sensing images gathering technologies: Drone, unmanned aerial vehicle; Digital Globe, satellite images of very high resolution and global coverage; Google Earth, open access software that offers remote sensing high resolution image visualization and has global coverage.

Accurately mapping mangroves is fundamental because it can inform research and management, including fisheries, carbon sequestration, ecosystem services, biodiversity, conservation, restoration, tourism, coastal protection and climate change. The mapping of mangroves also provides an accurate baseline for time-series analyses, which is essential for the long-term monitoring of mangrove habitat, studying mangrove community dynamics, and better understanding the responses to various drivers like climate change.

References

  1. Hogarth, P. J. The Biology of Mangroves and Seagrasses. (Oxford University Press, 2015).
  2. Ball, M. C. & Pidsley, S. M. Growth Responses to Salinity in Relation to Distribution of Two Mangrove Species, Sonneratia alba and S. lanceolata, in Northern Australia. Funct. Ecol. 9, 77–85 (1995).
  3. Costanza, R. et al. The value of the world’s ecosystem services and natural capital. Nature 387, 253–260 (1997).
  4. Donato, D. C. et al. Mangroves among the most carbon-rich forests in the tropics. Nat. Geosci. 4, 293–297 (2011).
  5. Dvorak, M., Vargas, H., Fessl, B. & Tebbich, S. On the verge of extinction: a survey of the mangrove finch Cactospiza heliobates and its habitat on the Galápagos Islands. Oryx 38, 171–179 (2004).
  6. Alongi, D. M. Mangrove forests: resilience, protection from tsunamis, and responses to global climate change. Estuar. Coast. Shelf Sci. 76, 1–13 (2008).
  7. Duke, N. C. et al. A world without mangroves? Science 317, 41–42 (2007).
  8. Polidoro, B. A. et al. The loss of species: mangrove extinction risk and geographic areas of global concern. PloS One 5, e10095 (2010).