Lecture 19: Energy Flow and Food Webs Flashcards
Trophic Interactions
- what they eat and what eats them
- Can change over time
Trophic Levels
- Describe the feeding positions of groups of organisms in ecosystems
- All organisms are either consumed by other organisms or enter the pool of dead organic matter (detritus)
- First (primary producers - autotrophs), second (primary consumers - herbivores), third (secondary consumers - primary carnivores), fourth (tertiary consumers - secondary carnivores)
Detritus (dead organic matter)
- part of the first level
Detrivores (equivalent to herbivores)
- part of the second level
Allochthonous
- external energy inputs
- Found in water systems
- Inputs can be important in stream ecosystems (99.8%)
Autochthonous
- Energy produced by autotrophs within the system
- Energy input increases from the headwaters toward the lower reaches of a river
As you Go Downstream
- velocity decreases
- nutrient concentrations increase
Energy flow among Trophic Levels
- Amount of energy transferred from one trophic level to the next depends on food quality and consumer abundance and physiology
- Energy will decrease with each trophic level
Trophic Pyramid
- Portrays the relative amounts of energy or biomass of each trophic level
- Very large at base and gets smaller as you go to the top
- Due to some of the biomass at each level not being consumed, so it is dispersed in the transfer to the next level
Terrestrial Ecosystems
- Energy and biomass pyramids are similar
- Biomass is closely associated with energy production
Aquatic Ecosystems
- Biomass pyramid inverted
- Live and die quickly, energy is produced and then leaves the ecosystem
Inverted biomass pyramids
- more common where productivity is lowest, such as nutrient-poor regions of the open ocean.
- Low productivity system = inverted pyramid
terrestrial vs aquatic biomass
- terresteral consume a much lower proportion of autotroph biomass than herbivores in most aquatic ecosystems
- On average 13% of terrestrial NPP is consumed
- Aquatic ecosystems average of 35% NPP is consumed
Herbivores are Constrained by Predators
- never reach carrying capacity
- Predator removal experiments support this
Autotrophs have defences against herbivory
- such as secondary compounds, spines, etc
- Plants of resource-poor environments tend to have stronger defences than plants from resource-rich environments
Phytoplankton are more nutritious for herbivores than terrestrial plants
- Terrestrial plants have structural components such wood (with few nutrients)
- Freshwater phytoplankton have carbon
- nutrient ratios closer to those of herbivores than to those of terrestrial plants
Trophic Efficiency
amount of energy at on trophic level divided by the amount of energy at the trophic level immediately below it
Trophic Efficiency incorporates three types of efficiency
- Consumption efficiency
- Assimilation efficiency
- Production efficiency
Consumption Efficiency
-Proportion of available energy that is consumed
- Higher in aquatic ecosystems than in terrestrial ecosystems
- Tends to be higher for carnivores than herbivores
Assimilation efficiency
- Proportion of ingested food that is assimilated
- Determined by food quality and the physiology of the consumer
- Food quality of plants and detritus is low because of complex compounds such as cellulose, lignins, and humic acids that are not easily digested and low concentrations of nutrients such as nitrogen and phosphorus
Production efficiency
- Proportion of assimilated food that goes into new consumer biomass
- Is strongly related to the thermal physiology and size of the consumer
Assimilation Efficiencies of herbivores and Detritivores
20-50%
Assimilation Efficiency of Carnivores
80%
Endotherms
- Digest food more completely than ectotherms
- Thus have higher assimilation efficiencies
- Allocate more energy to heat production
- Have less for growth and replication than ectotherms
- Body size affects heat loss ( as body size increases, the surface area-tovolume ratio decreases)
Some Herbivores Have Mutualistic Symbionts
- Help them digest cellulose
- EX. (cattle, deer, camels) have a modified foregut with bacteria and protists that break down cellulose
- They have higher assimilation efficiencies than other herbivores
Change in food quality
- Impacts trophic efficiency and can determine consumer population size
- Steller sea lion populations in Alaska declined by about 80% over 25 years
- Smaller body size and decreased birth rates suggested food quantity or quality might be a problem
- Prey quantity was not declining
- Sea lions however had shifted from a diet of mostly herring (high in fats) to more cod and pollock (half as much fat)
- Change in diet reflected a shift in the fish community
Trophic Cascades
Changes in the abundances of organisms at one trophic level can influence energy flow at multiple trophic levels
Bottom Up Control
production at bottom drives rest of ecosystem
Top Down Control
energy flow governed by energy consumption – predators’ impact how many plants get eaten
- highest level affects the trophic levels below it
What determines # of trophic levels in an ecosystem?
Number of trophic level may change due to:
- Addition or loss of a top predator
- Predator in the middle of the food chain
- Omnivore may change food preference
Several factors may control the number of trophic levels:
- Amount of energy entering via primary production
- Frequency of disturbances
- Ecosystems size
Amount of energy entering via primary production
more production should allow more trophic levels
Appears to be important in resource poor ecosystems
Frequency of Disturbances
- higher trophic levels depend on lower levels, and take time to re-establish after disturbance
- If disturbance is frequency, higher level may never establish
Food Webs
- conceptual models of the trophic interactions of organisms in an ecosystem
- an important tool for modelling ecological interactions
- As more organisms are added, the complexity increases to reflect the complexity of real ecosystems
- Static descriptions of energy flow and trophic interactions
- Normally don’t include other types of interactions, such as pollination
Stability Of Food Webs
- Measured by how much the populations change over time
- How an ecosystem responds to species loss or gain is strongly related to the stability of food webs
How does diversity at one trophic level affect stability at other trophic levels?
- > plant diversity = > plant biomass and stability.
- > plant biomass and stability = > arthropod diversity.
- > arthropod diversity = a greater portfolio effect, in which variation in one herbivore population can cancel out variation in that of another.
Toxins in Remote Places
- Arctic has been thought of as one of the most remote and pristine areas on Earth
- Studies of inuit women showed they have PCB concentration in their breast milk
- Seven times higher than in women in Quebec
PCB’s
- Belong to a group of industrial chemical compounds called persistent organic pollutants (POPs)
- because they remain in the environment for a long time
How do these toxins make their way to the Arctic?
- POPs produced at low latitudes are in gaseous form and enter the atmosphere
- Atmospheric circulation patterns carry them to the Arctic
where they condense to liquid and fall from the atmosphere
Manufacture and Use of POPs
- banned in North America but some developing countries still use them
- Emissions of POPs have decreased, but they can remain in Arctic snow and ice for decades
- Everytime the snow melts you get contaminants in the air during spring and summer
Corelation between POPs and Diet
- Communities that eat marine mammals tend to have the highest levels of POPs
- Communities that eat herbivorous caribou tend to have lower levels
Understanding Energy Flow in Ecosystems
Important in understanding the effects of POPs
Bioaccumulation
- Some chemicals are not metabolised or excreted
- Become progressively more concentrated in tissues over and organisms life
Biomagnification
- Concentration of these compounds increases in animals at higher trophic levels
- as animals at each trophic level consume prey with higher concentrations of the compounds
