AP Syllabus focus:
‘Relationships among populations can be modeled as positive or negative interactions, such as predator–prey and cooperation.’
Interactions among species shape community structure by changing survival, reproduction, and resource access. Ecologists model these effects using sign conventions and simple equations that predict how one population’s growth changes when another population is present.
Why model species interactions?
Models translate observations into testable predictions about population change under different interaction types. In AP Biology, emphasis is on identifying positive, negative, and neutral effects and interpreting what they imply for population trends over time.
Interaction “signs” (net effects)
Practice Questions
FAQ
They separate immediate mechanisms (e.g., consumption) from mediated pathways (e.g., predator reduces a herbivore, indirectly benefiting plants).
Useful approaches include:
Removal/addition experiments
Path analysis or causal diagrams
Comparing outcomes across multiple community contexts
Yes. The interaction term is modified so predation pressure does not rise linearly with prey density.
Common outcomes:
Stabilised dynamics if refuges reduce predation at low prey density
Slower predator growth when predators saturate at high prey density
If conditions change so that one partner imposes costs exceeding benefits (e.g., resource scarcity, overcrowding, or partner “cheating”).
Models may shift coefficients from $+$ to $-$ as net fitness effects reverse.
You need demonstrated fitness changes attributable to the partner’s presence.
Evidence may include:
Increased survival/fecundity in paired treatments
Mechanistic links (e.g., shared defence behaviour)
Controls for abiotic variables and resource availability
Delays between prey increase and predator reproduction can create cycles even if average conditions are stable.
Time lags arise from:
Gestation/development time
Seasonal breeding
Delayed numerical response of predators to prey abundance
