Larger crayfish are better at defending shelters than smaller crayfish, regardless of what species they are. Small crayfish can escape competition by occupying small rocks that are not sought after by larger crayfish. The size distribution of the two species is shown in the frequency versus size graph.

What is the most likely outcome of competition between species A and B? Competitive coexistence

The two species have evolved different body sizes. This is an example character displacement , an evolutionary outcome of competition.

Question

Larger crayfish are better at defending shelters than smaller crayfish, regardless of what species they are.  Small crayfish can escape competition by occupying small rocks that are not sought after by larger crayfish.  The size distribution of the two species is shown in the frequency versus size graph.

What is the most likely outcome of competition between species A and B? Competitive coexistence

The two species have evolved different body sizes. This is an example character displacement , an evolutionary outcome of competition.

BlackTom AI · Accepted Answer

The question describes two crayfish species with size-based distribution and a graph showing their size frequencies, along with a statement that larger individuals defend shelters better and smaller ones escape competition by occupying smaller rocks. This sets up a scenario about competition and its ecological or evolutionary consequences.

Option 1: Competitive coexistence
- In ecology, competitive coexistence occurs when two species share a habitat but use different resources or occupy different niches in a way that allows them to persist together over time.
- The description that large crayfish defend shelters better while small crayfish use smaller rocks to avoid competition suggests resource partitioning by size, reducing direct competition for the same shelter resource.
- If each species predominates at different size classes and these size-structured niches minimize overlap, both species can persist in the same area rather than one excluding the other.
- A common misstep would be to assume complete niche separation is necessary for coexistence; however, partial resource partitioning across the size axis can still support stable coexistence even if some overlap remains.
- The key idea supporting coexistence here is that size-based niche differentiation reduces direct competition for identical resources, allowing both species to inhabit the same environment.

Option 2: Character displacement
- Character displacement is an evolutionary process where species diverge in morphology or other traits because of interspecific competition, often leading to reduced overlap in resource use over generations.
- The question text mentions that the two species have evolved different body sizes, and labels this as an example of character displacement—an evolutionary outcome of competition.
- The graph showing historical or current size distributions could illustrate how species have diverged to minimize competition, which aligns with the concept of character displacement.
- A potential pitfall is assuming character displacement is the only or primary driver of all observed niche differences; while divergence in size can be a result of character displacement, coexistence can also arise from ecological mechanisms without having completed evolutionary divergence.
- In this context, noting that the two species have evolved different body sizes supports the idea of character displacement, but one should recognize that coexistence can occur via ecological niche partitioning even if the divergence is not fully complete or solely due to competition.

In summary, both concepts are relevant: competitive coexistence describes how the species can live side by side through resource partitioning, while character displacement explains the evolutionary change (divergent body sizes) that can accompany or enhance such coexistence by reducing competition over time.

生物学 1001 (001-013) 第24课学习测验

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