By calculating connectance as a measure of relative complexity of food webs, infer which of the following statements is correct?
(1) Connectance increases as species richness increases.
(2) Connectance decreases as species richness increases.
(3) Connectance is constant regardless of species richness.
(4) Relationship between species richness and connectance is stochastic.

Introduction

Understanding the complexity of food webs is essential for ecologists studying ecosystem stability, resilience, and biodiversity. One important measure of food web complexity is connectance, which quantifies the proportion of possible feeding links that are actually realized in a food web. This article explores how connectance changes with species richness and which statement about their relationship is correct.

What Is Connectance?

Connectance is defined as:

Connectance=Actual number of links in food webPotential number of links in food webConnectance=Potential number of links in food webActual number of links in food web

For a food web with $n$ species, the potential number of links (ignoring cannibalism and self-loops) is $n(n-1)$ for directed links.

How Does Connectance Change with Species Richness?

As the number of species in a food web increases, the potential number of links grows quadratically (n2n2). However, the actual number of links observed in real food webs does not increase at the same rate. This means that as species richness increases, the proportion of possible links that are realized—connectance—generally decreases.

Empirical Evidence

• Real food webs: Studies of natural ecosystems show that connectance tends to decrease as species richness increases. This is because it becomes increasingly difficult for each species to interact with every other species as the community grows12.

• Constant connectance model: Some theoretical models assume connectance is constant, but empirical data rarely support this. Instead, connectance usually declines with increasing species richness12.

Why Does Connectance Decrease?

• Biological constraints: Species have limited diets and cannot realistically interact with all other species in large communities.

• Resource limitation: As the number of species increases, competition and specialization limit the number of possible interactions.

• Sampling effects: In larger communities, it is harder to sample all possible interactions, but even accounting for this, the trend remains.

Common Misconceptions

• Connectance increases with species richness: This is incorrect. The actual number of links does not keep pace with the rapid increase in possible links, so connectance decreases.

• Connectance is constant: This is rarely true in nature, though it is sometimes assumed in theoretical models.

• Relationship is purely stochastic: While there is variability, the general trend is a decrease in connectance with increasing species richness.

Implications for Ecosystem Stability

Connectance is a key factor in determining the stability and resilience of ecosystems. Lower connectance in species-rich communities may make them more resistant to disturbances, as the loss of one species affects fewer others. However, the relationship between connectance, species richness, and stability is complex and influenced by many factors47.

Conclusion

The correct statement is that connectance decreases as species richness increases. This is because the number of possible links grows much faster than the number of actual links in real food webs.

Correct answer:
(2) Connectance decreases as species richness increases.