Chasing Mangroves: When the Map and the Field Tell Different Stories

Written by Onome Ofoman (Guest Writer)

Maps give us direction and a sense of understanding of a location before our feet even touch the ground. As ecologists, we often begin our work scrolling through satellite imagery, identifying landscape patterns to guide our research [1]. 

This was exactly how I began planning my fieldwork in Lagos state, which is situated along Nigeria’s southwestern coast. It is surrounded by a complex network of lagoons, wetlands and creeks.

I was using GIS skills to assess potential study sites for Rhizophora racemosa. From above, the eastern and western peninsulas appeared to show large patches of mangroves. It looked like a promising study system, rich with opportunity.

But there is an unspoken rule in field ecology: never trust the map without walking the land.

Sometimes what is above isn’t necessarily what is below

My first stop was the western peninsula, where reality was aligned with the imagery. Mangroves stood where expected, making it feel like a straightforward case of a simple validation exercise. 

Then came the eastern peninsula. 

Maps suggested a substantial patch of flooded vegetation, some of which we expected to be mangroves, yet we found almost no R. racemosa trees!

Confused, we turned to the local community. Their responses added uncertainty; some suggested the few R. racemosa individuals we saw weren't even native, and others couldn't recall dense mangrove stands.

Flooded vegetation on the eastern peninsula with no mangroves in sight (Photo credit: Onome Ofoman)

When satellite imagery tells half the story: The Eastern Peninsula

During a seven-hour boat journey through the eastern peninsula, the environment revealed itself. The water was covered in invasive water hyacinths, which form dense layers that block sunlight and oxygen. 

For mangrove propagules, which require suitable mud to anchor and grow [2], the hyacinths likely act like a physical barrier, preventing germination. Additionally, the eastern peninsula sits further inland, where less frequent tidal exchange likely creates an environment less favorable for R. racemosa [3] and better suited for other vegetation. 

Satellite imagery had captured "flooded vegetation" broadly and not mangroves specifically. Further verification on Mangrove Watch corroborated what was on the ground. The reality was clear - there were barely any mangroves left on the eastern peninsula.

Sentinel-2 Land Cover map showing "flooded vegetation" class in green in the west and eastern part of Lagos State (Photo credit: ESRI Living Atlas)

A contrasting observation in the Western Peninsula

The western peninsula revealed a different challenge. R. racemosa mangroves were present, but they were clearly under pressure. We observed boatloads of cut mangrove trunks being transported to markets, and large dredging vessels extracting sand from the lagoon.

These activities significantly burden mangroves since they cannot regenerate as quickly as they are removed [4]. Here, the map was spot-on about the mangroves' existence but could not convey the active exploitation threatening their future.

Signs of mangrove harvesting on the western peninsula (Photo credit: Onome Ofoman)

Satellites speak, but the ground whispers the truth

These experiences highlight a critical lesson: remotely sensing is not always understanding. 

While satellite imagery is a powerful tool for monitoring large areas, it cannot fully capture species composition, complex ecological interactions, or the human activities shaping the land in real time. 

It cannot replace the experience of being physically present.

Fieldwork fills these gaps, allowing us to notice inconsistencies, ask new questions, and connect ecological patterns with the invaluable insights of local communities. The story of these two peninsulas is a reminder that ecosystems are dynamic, shaped by both natural processes and human action. 

Ultimately, good science requires the willingness to question our tools, revisit our assumptions, and step into the field to see for ourselves. 

True understanding lies at the intersection of satellite data, field observation, and lived experiences.

References

[1] Soti,V., Lelong, C., Goebel, F-R. & Brévault, T. (2018). Designing a field sampling plan for landscape-pest ecological studies using VHR optical imagery. International Journal of Applied Earth Observation and Geoinformation, 72, 26-33. https://doi.org/10.1016/j.jag.2018.05.016. 

[2] Balke, T., Webb, E. L., van den Elzen, E., Galli, D., Herman, P. M., & Bouma, T. J. (2013). Seedling establishment in a dynamic sedimentary environment: a conceptual framework using mangroves. The Journal of applied ecology, 50(3), 740–747. https://doi.org/10.1111/1365-2664.12067. 

[3] Tomlinson, P. B. (2016). The Botany of Mangroves (2nd ed.). Cambridge: Cambridge University Press.

[4] Dale, P.E.R., Knight, J.M. & Dwyer, P.G. (2014). Mangrove rehabilitation: a review focusing on ecological and institutional issues. Wetlands Ecol Manage 22, 587–604. https://doi.org/10.1007/s11273-014-9383-1.

About the Author

Onome Ofoman is a first year PhD student at the Rubenstein School of Environment and Natural Resources and a Graduate Fellow at the Gund Institute for Environment, both at the University of Vermont. She is interested in understanding how forested landscapes impact and are impacted by the biotic, including human, communities living within them. Her current research is focused on understanding the implications of mangrove diversity metrics on the ecosystem services they provide.