This page is meant to be a brief overview of energy flow and community structure in desert ecosystems. It should be a useful supplement to our discussion of how human development is impacting deserts in the Southwestern United States.
Primary production is the basis for all energy flow into biological systems. In deserts, primary production of above ground vegetation varies from 30 to 200g/m^2 (Ricklefs 1993). This is lower than many other habitat types (Table 1). Primary productivity is most often limited by water availability and water use efficiency in deserts (Noy-Meir 1973). However, nutrients, especially nitrogen and phosphorus, can be limiting is some cases (Smith 1996). Energy flow diagrams have been useful for modeling in some ecosystems, however, it might be more useful in a desert environment to model water flow between different parts of the ecosystem (Noy-Meir 1973). The flow of water from one trophic level to the next may be just as important and informative as the flow of energy.
|Habitat type||Net Primary Production (g/m^-2/yr^-2)|
|Swamp and Marsh||2500|
|Temperate Grass Land||500|
|Tundra and Alpine||140|
The Great Basin desert plant community is dominated by sage brush and associated
grasses (Brewer 1988). Similarly, the vegetation of the hot deserts south
of the Great Basin, which include the Mojave, Sonoran, and Chihuahuan are
all dominated by creosote bush (Brewer 1988). Desert shrubs like
sagebrush and creosote bush play important roles in community structure.
For instance these shrubs tend to create islands of fertility beneath themselves
where nutrient rich plant material tends to collect and enrich the soil
(Smith 1996). They also create isolated pockets of shade that reduce
temperature and evaporation. These shrubs are
often regularly spaced due to competition for water and/or nutrients or
due to inhibition by allelopathic substances. There appears to be
very little change over time in the basic structure of desert plant communities
suggesting that succession is very slow or nonexistent (Brewer 1988).
An important factor in determining the structure of desert communities are the presence of seed eating granivores especially ants and rodents (Smith 1996). Although these seed eaters do not appear to consume a large proportion of the above ground biomass, they can consume up to 87% of seeds. This profoundly effects community structure and the removal of seed predators has been shown to increase the density of grasses and related grassland species.
Due to the presence of ephemeral desert herbs and drought avoiding animals, community structure can change quickly and dramatically with rain fall (Noy-Meir 1973, Noy-Meir 1974). The flowers at right, from a desert in South America, have sprung up quickly after a heavy rain. This dynamic in the desert ecosystem requires high flexibility between active and dormant periods. It can also help to explain how desert ecosystems reach some level of stability in the face of highly variable precipitation (plants that store water and/or tap ground water are also very important in ecosystem stability). An important model in understanding the ephemeral element in desert ecology is the pulse and reserve model (Noy-Meir 1973). A trigger (rain fall) signals the pulse (rapid growth and reproduction in desert ephemerals) which replenishes the reserve (seed bank) so that the system can respond to another trigger.
One might hypothesize that the harsh conditions of the desert would keep
population sizes so low that competition would be unimportant. This
however does not seem to be the case in most desert ecosystems (Noy-Meir
1973). In fact, the lack of water seems to increase competitive interactions
in the desert. Evidence for competition can be found in the regular
spacing of desert plants and in the wide cover of extensive root systems.
Competition is also present in higher trophic levels and has been demonstrated
for some guilds such as the granivores (Smith 1996). Competition
for water in plants can be reduced when plants are able to tap ground water.
Competition in animals can be reduced by the partitioning of resources.
The picture to the left shows the relatively regular spacing of desert plants.
desert food chain is better described as a web. In desert ecosystems
the occurrence of strict carnivores is low and most secondary consumers
are omnivorous (Noy-Meir 1974, Smith 1996). A good example
of this is the coyote (below). One hypothesis is that the lack of
free standing water forces predators to gain most of their moisture through
their food (Noy-Meir 1974). If prey items are not supplying enough
water then it would be adaptive for predators to be able to consume some
plant material. Another hypothesis is that there simply aren't enough
prey items to support a strict carnivore and therefore it is adaptive to
be omnivorous. Additionally, many desert herbivores are generalists
and opportunists feeding on many types of vegetation (Smith 1996).
The hypotheses for increased omnivory can also explain an increase in herbivore
generalists. A result of complex food chains is that degradation
of one species could have wide reaching effects across the ecosystem.
It might also allow for more ecosystem stability by decreasing the dependence
of one species on another.