Mapping the Mangroves - MWL

Introduction

Mangroves have a number of unique adaptations that allow them to grow in an environment that is inhospitable to most plant species. The purpose of this lesson is to examine the difficulties of growing in an estuary and the adaptations of mangroves that allow them to live in this environment. Direct sun, heat, variable water levels due to tides, salt water conditions, currents, wave action, and anaerobic substrates are the environmental hurdles that will be examined.

• Direct sun and heat – Mangroves are a tropical/subtropical group of trees that are circumglobal and are exposed to direct sun and high daily temperatures. These two factors cause desiccation in mangroves. Water conservation strategies employed by the mangroves include; thicker leaves, waxy cuticle, and smaller leaves.
• Many mangroves grow in tidally influenced estuaries and shorelines. They experience periods of submersion on flood tides. The pneumatophores (snorkel roots) of the white and black mangrove, the buttress roots of the tea mangrove, and the prop roots of the red mangrove all aid in elevation aspects in dealing with high water levels.
• Waves in exposed shoreline areas, and currents on exposed shorelines and in estuaries are forces of erosion that work to dislodge mangroves. The buttress roots of the tea mangrove, and prop roots of the red mangrove both increase surface area, increasing stability; the spreading of the prop roots allows water to pass through them with minimal resistance, giving the red mangrove an advantage in current prone areas.
• Living is a salt-water environment is difficult for the cells that make up the mangroves. Salt water affects the osmosis of cells and will create hypertonic conditions causing the cell to shrink due to water loss, negatively affecting cellular function. Mangroves employ salt-water exclusion and extrusion strategies to maintain isotonic cellular conditions. The red and tea mangroves have membrane filters on its roots that help exclude salt and prevent it from entering the plant; also, any salt entering the plant will accumulate in the leaves, which are periodically dropped. The white, black and buttonwood mangroves all practice extrusion strategies where pores or nodes on the leaf give exude salts from the plant.
• Mangrove estuaries tend to have an anaerobic muddy substrate due to its low lying nature and saturated fine sediments. The root structures (prop, pneumataphore, and buttress) allow for oxygen to be taken in and cellular respiration to occur.

The mangrove’s abilities to live and thrive in the conditions described above allow them to take advantage of a nutrient rich zone and make those nutrients available to a larger community of organisms via photosynthesis and decomposition processes. In addition mangroves provide important habitat structure for aquatic organisms, terrestrial animals, and birds. The mangrove is a keystone species in this important coastal ecosystem.

Objectives

• Students will relate mangrove adaptations to the role that mangroves play in their ecosystems. 

Teaching Summary

There are a number of different observational activities and approaches that can be employed here; we will look at two, one that involves a trip to an estuary, and one that does not. If a visit to an estuary is possible, examining marine sediments and smelling an anaerobic muds is an experience students will not forget. Other observational activities such as examining salt crystal formation on the back of black mangrove leaves, and mangrove leaf comparisons are outlined in the activity below. If a trip is not possible to an estuary, Google images can be used in conjunction with relationship questions to explore mangrove ecosystem aspects, which are outlined below as well.  

Vocabulary:  Desiccation, Cuticle, Exclusion, Extrusion, Osmosis, Hypotonic, Hypertonic, Isotonic

Resources

Google images

Assessment Questions

Questions:

1. Why don’t mangroves grow in fresh water environments like lakeshores and stream banks?
2. What is a keystone species and why is a mangrove considered one?
3. Is there competition between the different mangroves for space within the estuary? Support and explain.

Answers:

1. Mangroves are outcompeted by other plants in fresh water environments; their adaptations allow them to dominate a salt water niche.
2. A keystone species is a species the ecosystem hinges on, without it the ecosystem would collapse or change drastically. The mangrove is considered a keystone species because it is the only primary producer that does well in the salt water estuary habitats. Without the mangrove, energy production for the system would be severely reduced.
3. Yes there is competition amongst the species. One example is the prop and buttress roots allow the red and tea mangroves live in strong current areas, where as the pneumatophores allow the white and black mangrove to dominate the flat intermediate tidal zones.