TROPHIC STRUCTURE – PRODUCTIVITY AND FOOD WEBS
STUDY OF ECOSYSTEMS
Several different methods to measure importance of components of an ecosystem (p. 542-543):
1) Biomass – collect and obtain the mass of all living material in the study area
what is produced as a result of organism life
biomass stays constant if the ecosystem is at equilibrium
2) Chemical flow – measure flow of various elements or molecules through system
most common are N, P, C, O
these are cycled through an ecosystem
3) Energy flow – measure amount of energy in system
energy is not cycled, in as sun light, out as lost heat
Ecosystem components are studied from a functional point-of-view (fig 22.2)
producers – plants and other photosynthetic organisms
consumers – animals eating other animals
herbivores (plants), carnivores (animals), omnivores (both)
decomposers – consume dead material
macro (insects) – physical decomposition
micro (bacteria & fungi) – chemical decomposition
(these 3 are part of the biotic component, or living)
soil – inorganic + organic material
(part of the abiotic component)
All of the above are part of a natural cycle within an ecosystem
PRIMARY PRODUCTIVITY
Producers are the most important part of an ecosystem (p. 454-457)
Over 99% of all biomass is plant material
their production is referred to as primary productivity
Trophic Structure, p. 2
Measured at two levels:
1) Gross Primary Productivity = energy of photosynthesis
a number of ways to measure this include:
oxygen production
measure of radioactive CO2 uptake
2) Net Primary Productivity = energy in plant biomass
reduced by plant respiration & consumers
easiest method is to measure is by biomass or energy on combustion
Measures have revealed that specific biomes are more productive (note Table 22.1):
Most diverse are the tropical forests.
A variety of reasons for differences:
precipitation amounts (dry vs wet)
growing season length within the biome
Primary Productivity is limited by a number of abiotic factors (pp. 458-470):
A) Aquatic systems:
1) light penetration
2) nutrient levels (especially in oceans), especially nitrogen and iron
exceptions are in areas of upwelling:
+ west coast South America (Peru-Chile)
+ east coast Africa
+ coast off Antarctica
B) Terrestrial systems: (fig 22.15, p. 467)
1) evapotranspiration – precipitation & temperature (climate)
2) proportion of C3 and C4 plants, especially grasses (C4 are heat tolerant)
3) nutrients (soil chemistry)
Problems with measuring plant primary productivity:
1) impractical to sample frequently enough to catch all plant parts (flowers & leaves fall off)
2) hard to get underground biomass; especially perennial plants
3) hard to correct for herbivore removal
possibility of using excluding fences, but some plants stimulated by grazing
4) problems with parasites & symbionts
Trophic Structure, p. 3
SECONDARY PRODUCTION
Primary production may be moved into the remainder of the ecosystem cycle by:
1) consumption by a consumer
2) death and decomposition
Measurement of animal assimilation of energy from plants can be quite difficult
note schematic in fig 23.1
In a simple form:
Assimilation rate = respiration rate + net productivity
We can then measure each
a) respiration (CO2 produced)
confine animal in cage then measure O2 consumption, CO2 output & heat production
simple relationship between body size and metabolic rate (see fig 23.2)
b) production (biomass)
measure growth of individuals (biomass change) + reproduction (natality)
a second method is population biomass change + losses due to mortality or emigration
Problems develop:
1) tropic level assignment is hard – many species are omnivores
2) what is done with detritus? – is this undigested plant?
3) ecosystems are constantly changing – not at equilibrium
One question is that of production efficiency of energy transfer from one organism to another
Stated another way: how well can a species assimilate biomass?
for a species efficiency this can be measured as:
Net productivity of species
Production efficiency = ---------------------------------------Assimilation of species
Collected data has shown
warm-blooded animals have values of 0.5-3.% (very low)
cold-blooded animals have higher values of 25-55%
For an ecosystem we can group into trophic levels to get trophic efficiency:
Trophic Structure, p. 4
Net production at tropic level i+1
Trophic efficiency = ---------------------------------------------Net production at tropic level i
In aquatic systems the values range 2% to 24% (average at 10%)
In terrestrial systems the values are lower, < 10%
Some suggest that 97% of primary production (plants) goes to decomposition
(think large tree trunks in forests)
ANOTHER AREA OF RESEARCH IS THE KEYSTONE SPECIES
Keystone species = a species which has a profound influence on a community
* influence is greater than it's abundance or biomass
Several different categories have been created:
keystone predator (ex starfish in NW)
keystone prey (palm trees in tropics)
keystone habitat modifier (beaver)