Biological Communities and Species Interactions


“Today we are faced with a challenge that calls for a shift in our thinking, so that humanity stops threatening its life-support system.”

~Wangari Maathai

Clear cutting in Amazon, Brazil

When seeking to find solutions to the problems of how we manage our natural resources, one need only look to Nature for wisdom and inspiration to find millenia-worth of infallible ecological design.  Agroecology utilizes this  by observing and understanding the relationships between organisms and their environment can teach us how to better care for the ecosystems that support us.

Two words that spring forth immediately when one thinks of a pristine natural environment, such as a virgin rainforest or an estuary or coral reef, are abundance and diversity.  But what do these words mean?  And how are they related to each other? According to William P. and Mary Ann Cunningham (2012), these two concepts are closely interlinked.  “Abundance is an expression of the total number of organisms in a biological community, while diversity is a measure of the number of different species, ecological niches, or genetic variation present” (Cunningham 2012).  When an ecosystem has an abundance of a particular species (such as an redwood forest) the diversity of the total ecological community present tends to be limited to a few species.  Conversely, when an ecosystem has great biodiversity (such as a virgin rainforest), the abundance of one particular species is usually limited in population due to the sheer number of other species sharing the same available resources.

                  

In 1927, British ecologist  Charles Elton observed what indigenous people have always known:  that each individual species had a specific role within a community of species, or ecological niche which determines the way it obtains food, the relationships it has with other species, and the ecological services provided to its community (Cunningham 2012).  In fact, as American limnologist G.E. Hutchinson would later observe, “every species exists within a range of physical and chemical conditions (temperature, light levels, acidity, humidity, salinity, etc.) and also biological interactions (predators and prey present, defenses, nutritional resources available, etc.)”(Cunningham 2012).  As each species habitates their ecological niche, a closely interconnected community emerges, or guild of species that mutually benefit or support each other.  This highly interconnected community has the potential to form a very elaborate food web, greatly increasing the resiliency and stability of an ecosystem.

        

Modern agriculture is a far cry from this symbiotic relationship and bases its simplistic designs on what is referred to as the competitive exclusion principle, which states that “no two species can occupy the same ecological niche for long. The one that is more efficient in using available resources will exclude the other” (Cunningham 2012).  Therefore, with this mentality, one cannot plant two different crops together as one’s yield will be greatly reduced due to the out-competition of available resources.  However, what modern agriculture does not take the time to observe is that resource partitioning “allows several species to utilize different parts of the same resource and coexist within a single habitat” (Cunningham 2012).  This is to say that if one plants two species with similar root structures and nutritional needs, one species will certainly out-compete the other for available resources.  However, if one plants two species with complimentary root structures, growth habits and nutritional needs (such as corn with beans and squash) a symbiotic relationship often enhances the survival of one or both species.

                           

Squash covers the ground to prevent weeds,and the corn provides a trellis for the beans.  This technique, called the Three Sisters method, has been used by indigenous populations in Mesoamerica for thousands of years.

                     

The propensity for mutually beneficial relationships in natural ecosystems is seen in many different contexts.  In Central and South America, acacia trees (Acacia collinsii) and ants (Pseudomyrmex ferruginea) hold a well-known close relationship with each other.  The acacias provide shelter and food for the ants, who in turn fiercely defend their territory, driving away herbivorous insects that would feed on the acacias.  

Ants and acacia; Source:  http://science.kennesaw.edu/~jdirnber/Bio2108/Lecture/LecEcology/EcologyComm.html

Another commonly seen example of ecological symbiosis is the relationship between over-story trees that also fix nitrogen in the soil (Erythrina sp.) and under-story species that produce fruit or nuts (i.e. Coffea sp.).  The overstory tree provides key nutrients for the plants below, while providing shade and organic  material that decomposes to fertilize the soil.

Commensalism is yet one more type of symbiosis commonly found in natural ecosystems, in which “one member clearly benefits and the other apparently is neither benefited nor harmed” (Cunningham 2012).  Mosses and epiphytes, such as bromeliads and orchids, that grow on trees in the moist tropics are perfect examples.  The production of vanilla in agroforestry systems utilizes this relationship without harm to its parent tree.  In addition, the incorporation of this species in a fruit or nut orchard in the tropics greatly diversifies the harvest available throughout the year.

 What is most interesting to observe as a global ecological pattern is that as we move from the equator toward either pole, “diversity decreases but abundance within species increases” (Cunningham 2012).  The rich, complex diversity of the forests of the Equatorian Amazon gradually becomes simpler as we move North and find ourselves in thick Pine forests or ancient Oak forests or coastal Redwood forests, with but only a handful of species cohabitating.  Even desert ecosystems, with the limited resources available, hold a dominant species (i.e. cactus) with limited bio-diversity.  And yet, one may travel the world over and find an increasingly mono-cultural form of modern agricultural technique producing human food that defies all ecological rules.  The rich diversity of highly elaborate symbiotic native rainforests are plowed to make room for acreage of palm oil or sugarcane or banana monocrops.  The brittle tropical soil, unprotected by the plethora of root forms, dries quickly in the tropical heat and is easily blown away by winds and rains.  This massive erosion creates a need for increased nutrition to feed the mono-crops, and modern agriculture has declared it necessary to pump the soils and foods we eat with high doses of chemically produced fertilizers rather than enjoying the natural accumulation of biomass that decomposes to provide rich compost for the understory plants.  When the monocrop weakens due to the harsh environment they are being forced to grow in, pests and diseases plague the crop and take advantage of their inability to fight off with immunity.  In a natural ecosystem, the complexity of the food web creates a strength that enhances each organism as a stronger whole.

When modern agriculturists argue that massive mono-cropping increases their productivity, they are erroneously combining industrial business terminology associated with profits with living ecological systems.  Productivity within a factory setting indicates a high quantity of product in a short amount of time, yielding a net profit.  In ecological terms, primary productivity is “the rate of biomass production”, which directly indicates the rate of solar energy being converted to chemical energy in photosynthesis (Cunningham 2012).  Any energy left after the respiration of the plants is considered net primary production.  Ecosystems with high productivity are tropical forests, coral reefs, and estuaries, because of their abundant supplies of all these resources.  Deserts, on the other hand, lack water which limits photosynthesis and reduces plant growth, as do the low temperatures experienced in the  Arctic tundras or high mountainous environments.  High yields from any plant in these conditions can not be sustained for long.

Ecological complexity within plant communities and species interconnectedness are important ecological indicators of a productive environment. Our modern agricultural systems base their production on man-made industrial concepts and create unsustainable ecological deserts where once diversity reigned strong.  As humans continue to experience increased destruction of precious resources we rely upon, such as virgin forests, estuaries, and coral reefs, our very survival is being challenged with each diminishing species.  Survival of the fittest may no longer refer to the domination of the strongest species, as was once assumed, but it is increasingly evident that the survival of organisms (including humans) relies upon those who can live cooperatively together.

Work Cited

Cunningham, William P. and Mary Ann.  Environmental Science:  A Global Concern, Twelfth Edition.  New York:  McGraw-Hill Companies, Inc., 2012.  76-93. PRINT.

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