The barnacle species, Chthamalus stellatus (CS), is found only in the high intertidal zone whereas Balanus glandula (BG) is found only in the low intertidal zone. A researcher transplanted CS from the high to low (T-CS), and BG from the low to high (T-BG) intertidal zones. Additionally, they allowed the species to grow alone or in competition with each other, and quantified survival.
Which one or more of the following inferences is/are consistent with the experimental results shown below?
This experiment tests the relative importance of abiotic factors versus interspecific competition in determining the realized niche (actual distribution) of two barnacle species, CS (Chthamalus stellatus) and BG (Balanus glandula).
Analysis of Experimental Results
1. Abiotic Limitations (High Zone vs. Low Zone, Alone)
- BG in High Zone (T-BG alone): Survival is low ($\approx 40\%$). Since BG survives well in its native Low Zone, this low survival in the High Zone indicates severe limitation by abiotic factors (e.g., desiccation, temperature stress).
- CS in Low Zone (T-CS alone): Survival is high ($\approx 95\%$). CS can tolerate the abiotic conditions of the Low Zone.
2. Competition Effects (Together)
- BG in High Zone (BG + CS): Survival remains low ($\approx 40\%$). Since the presence of CS does not further reduce BG survival below the abiotic limit, BG's distribution limit here is set by abiotic conditions.
- CS in Low Zone (CS + BG): Survival drops drastically ($\approx 40\%$), down from $\approx 95\%$ when alone. This major reduction in survival is due to the presence of BG, indicating strong interspecific competition (typically physical exclusion by the faster-growing BG).
Evaluation of Options
Option 1: Only abiotic conditions increase mortality of BG in the high intertidal zones.
- BG survival is low even when grown alone in the high zone ($\approx 40\%$), demonstrating high mortality due to abiotic stress. When CS is added, survival remains low, indicating that abiotic factors dominate BG's mortality in this zone.
- CONSISTENT (Correct).
Option 2: Only abiotic conditions increase mortality of CS in the low intertidal zones.
- CS survival is high when grown alone ($\approx 95\%$). Its mortality increases significantly when BG is present ($\approx 40\%$). This mortality is clearly caused by competition (biotic factors), not just abiotic ones.
- INCONSISTENT (Incorrect).
Option 3: Interspecific competition increases mortality of BG in the high intertidal zone.
- BG survival is already limited by abiotic factors ($\approx 40\%$) when alone. Competition does not cause a significant additional increase in mortality.
- INCONSISTENT (Incorrect).
Option 4: Interspecific competition increases mortality of CS in the low intertidal zone.
- CS survival drops from $\approx 95\%$ (alone) to $\approx 40\%$ (together with BG) in the low zone. This significant drop is caused by competition from BG.
- CONSISTENT (Correct).
The correct inferences are Option 1 and Option 4.
Important Questions from Competition
- Ecologists have studied the distributions of birds on archipelagos. Some studies found an interesting 'checkerboard' pattern within island groups. Sister species were never present on the same island, even when the islands were very close to each other. Which one of the following processes best explains this checkerboard pattern?
- Two bird species, A and B, are found on a single mountainside. A is a low-elevation species, found between 500 m and 1500 m Above Sea Level (ASL), while B is a high-elevation species, found between 1000 m and 2000 m ASL. At 1250 m ASL, species A and B have very different bill morphologies, but the bill morphology of species A at 500 m is very similar to the bill morphology of species B at 2000 m ASL. Which one or more of the following explain(s) the difference in bill morphology at 1250 m ASL?
The figure panels below show population growth in two species (solid circles and open circles), when they are grown alone, and when they are grown together. The interaction between these species is an example of
- In an assemblage of coexisting wild cat species, the size of canine teeth was found to be strikingly different between these species. Which one or more of the following statements explain(s) this observation?
Species 1 and 2 are sympatric, but species 2 has a wider physiological tolerance than species 1. The two species simultaneously invade a new environment that has an average temperature of 22 degrees C. What are the expected outcomes for species 1 and 2 ?
