Agroecosystem Diversity and Stability - Agroecology: The Ecology of Sustainable Food Systems - page 220

RHIZOBIUM BACTERIA, LEGUMES, AND THE NITROGEN CYCLE

One important way of taking advantage of ecological diversity is to introduce nitrogen-fixing legumes into the
agroecosystem. As a result of the mutualistic relationship between the leguminous plants and bacteria of the genus
Rhizobium, nitrogen derived from the atmosphere is made available to all the biotic members of the system. The
ability of a system to supply its needs for nitrogen in this way is an emergent quality made possible by biotic diversity.
Rhizobium bacteria possess the ability to capture atmospheric nitrogen from the air in the soil and convert it to
a form that is usable by the bacteria and also by plants. These bacteria can live freely in the soil; however, when
legume plants are present, the bacteria infect the plants' root structure. A bacterium moves into an internal root cell,
causing it to differentiate and form a nodule in which the bacterium can reproduce. The bacteria in a root nodule
begin to receive all the sugars they need from the host plant, giving up their ability to live independently; they
reciprocate by making the nitrogen they fix available to the host. The interaction provides an advantage to both
organisms: the plant is able to obtain nitrogen that would otherwise not be available to it, and the bacteria are able
to maintain a much higher population level than they can in the soil. A great deal more nitrogen fixation occurs
with nodulated legumes, therefore, than with free-living Rhizobium alone. When the host plant dies, the bacteria
can revert to an autotrophic lifestyle and reenter the soil community.
Because nitrogen is often a limiting nutrient, a legume's relationship with Rhizobium allows it to survive in soil
that may contain too little nitrogen to support other plants. And if the legume is returned to the soil after it dies,
the bacterially fixed nitrogen it incorporated into its biomass during its life becomes part of the soil, available for
other plants to use.
This mutualism has been historically important in agriculture. The legume-Rhizobium symbiosis is the primary
source of nitrogen addition in many traditional agroecosystems, and was one of the only methods used to incorporate
environmental nitrogen into many crop systems before the development of nitrogen fertilizer. Legume crops have
been intercropped with nonlegumes, as in the corn-bean-squash polyculture common in Latin America, and legumes
are used as cover crops and green manure crops in the U.S. and other regions to improve soil quality and nitrogen
content. Legumes have also been an important part of managed fallow systems. All of these systems take advantage
of the legume-Rhizobium symbiosis, using biological nitrogen fixation to make usable nitrogen available to the
entire plant community, and ultimately to humans.

On a still larger scale is gamma diversity, which is a
measurement of the species diversity of a region such as
a mountain range or river valley.
The difference between the three types of diversity
can be illustrated with a hypothetical 5-km transect. It is
possible to measure alpha diversity at any location along
the transect by counting the number of species within,
say, 10 m of a specified point. A measure of beta diversity,
in contrast, includes at least two points along the transect
in different but adjacent habitats. If the species makeup
of these two locations is very different, beta diversity is
high; if the species makeup changes little as one moves
between the two habitats, beta diversity is low. A measure
of gamma diversity is made along the entire length of the
transect, taking into account both the total number of
species and the variation in their distribution. In principle,
the distinction between alpha, beta, and gamma diversity
can be extended to other dimensions of ecological diver-
sity, such as structural and functional diversity.
Alpha, beta, and gamma diversity are helpful con-
ceptual distinctions because they allow us to describe
how different ecosystems and landscapes vary in the
structure of their diversity. For example, a highly
diverse natural grassland that stretches for hundreds of
kilometers in every direction is likely to have high alpha

TA B L E 1 6 . 2
Dimensions of Ecological Diversity in An Ecosystem

Dimension Description

Species Number of different species in the system

Genetic Degree of variability of genetic information in the

system (within each species and among different
species)

Vertical Number of distinct horizontal layers or levels in

the system

Horizontal Pattern of spatial distribution of organisms in the

system

Structural Number of locations (niches, trophic roles) in the

system organization

Functional Complexity of interaction, energy flow, and material

cycling among system components

Temporal Degree of heterogeneity of cyclical changes (daily,

seasonal, etc.) in the system

forest is different from the species diversity across the
river valley's different communities.
Species diversity in a single location is often called
alpha diversity. This is simply the variety of species in a
relatively small area of one community. Species diversity
across communities or habitats — the variety of species
from one location to another — is called beta diversity.