Interspecific Interactions and Competition: Ecological Principles and Examples
Allelopathy
Initially, people sought a single explanation for plant community structure, including a potential "wheel pattern."
Early allelopathy experiments were often poorly designed.
Example of poor design: Extracting allelopathic chemicals in isopropyl alcohol (common lab alcohol) and treating plants with it.
Problems with this design:
Alcohol itself can be toxic to plants, requiring a well-designed control to mitigate this.
This scenario does not occur in nature; ecological experiments should mimic natural processes.
Better experimental design (textbook example):
Control Setup: A suspected allelopathic plant (e.g., barley grass) grows in soil. Separately, target plants (e.g., apple tree seedlings) grow in the same type of soil, but without direct contact, ensuring no chemical transfer.
Treatment Setup: The suspected allelopathic plant grows in soil. Water is run through this soil, collected, and then used to water the target plant seedlings. Stunted growth in the treated seedlings compared to the control would indicate allelopathy.
Second Control (Addressing Pathogens): Soil from the same location (without the grass) has water run through it and collected. If target plants are unaffected by this water, it helps rule out other harmful soil components (e.g., pathogens) as the cause of growth inhibition.
Allelopathy is a form of interspecific competition where both species are harmed by reduced resource availability.
Competition and Coexistence
Complete Competitors Cannot Coexist: A principle stating that two species with overly similar resource requirements cannot permanently inhabit the same location; one will outcompete the other.
This often leads to selection pressure for species to diverge and use different resources, if possible.
Resource Partitioning (Niche Partitioning):
Selection favors individuals using different resources to avoid competition.
Resources are conceptualized as a continuum (e.g., seed size from small to large).
Competing species optimize their use of resources in distinct ways, minimizing overlap (e.g., different species feeding on different seed sizes).
Also known as Niche Shift if there is evidence of a species changing its resource use over evolutionary time.
This minimizes interspecific competition.
Community Assembly Theory: A subfield of ecology attempting to explain how species groups interact and why certain species coexist.
A core idea is that species with minimal overlap in environmental requirements can coexist more effectively than those with high overlap.
Short-term manifestation: Resource partitioning.
Long-term (evolutionary time) manifestation: Niche shift.
Fundamental vs. Realized Niche
Fundamental Niche: The full range of resources a species could potentially use under ideal conditions.
Realized Niche: The subset of resources a species actually uses in the environment, often restricted by competition or other biotic/abiotic factors.
Character Displacement: Over evolutionary time, competition can lead to morphological differences in closely related species living in the same area, reflecting adaptations to different resource subsets.
Example: Small wildcats in the same region may have different dentition adapted to different prey types despite similar size and coloration.
Niche Breadth: A species without competitors (in isolation) is predicted to have a greater niche breadth (using a wider range of resources). In the presence of competitors, its niche breadth narrows due to partitioning.
Examples of Niche Partitioning in Nature
Spatial Separation: Robert MacArthur's 1960s study of five warbler species foraging on conifer trees.
Warblers inhabiting the same geographical range used different regions/strata of the tree (e.g., top, middle, bottom) to avoid direct competition or aggressive interactions.
Avoiding aggression conserves energy, which is evolutionarily beneficial.
Temporal Separation: A 1960s study of African grazers (zebras, wildebeest, Thompson's gazelle).
These species (differing significantly in size) graze at different times of the year, allowing vegetation to resprout between species.
This avoids direct competition for food resources by not being in the same place at the same time.
Similar patterns are observed in nocturnal animals using game cameras, foraging at different times of day/night.
Resource Specialization (Morphological): Small wildcats (e.g., sand cats in Israel) show differences in tooth diameters linked to their specific prey species.
Behavioral Shift: A 1974 study on Anolis lizards.
In isolation (bottom graph), Anolis lizards consumed a wide range of prey sizes.
When coexisting with a competitor, the focal Anolis species shifted its foraging to focus specifically on mid-range prey sizes.
Ghosts of Competition Past
Strong competitive exclusion is rarely observed directly because species often adapt to avoid it over time.