Questions: Source-Sink Population Dynamics

Question 1

A conservation survey finds birds occupying 20 forest patches across a fragmented landscape. Detailed demographic analysis reveals that only 3 patches have positive local population growth (births exceed deaths). A highway is proposed that would destroy one of those 3 productive patches. Based on source-sink theory, what outcome should the conservation biologist predict?

• A. Minimal impact — 17 other occupied patches remain, so the species retains most of its range
• B. Gradual collapse across the whole network — destroying a source will starve the sink patches of immigrants, causing them to decline toward extinction one by one
• C. The sink patches will become self-sustaining once immigration from the lost source stops, because competition pressure will be reduced
• D. Only the patches directly adjacent to the destroyed source will be affected; distant sinks receive immigrants from multiple sources and will be buffered

Answer: (B)

The 17 unproductive patches are sinks — they persist only because of immigration from the 3 sources. Eliminating a source removes the demographic subsidy those sinks depend on. With fewer immigrants arriving, mortality continues to exceed births in the sink patches, and populations decline until extinction. The survey finding 'birds in every patch' gives a false impression of habitat quality. The correct conservation priority is identifying and protecting sources, not just any occupied patch.

Question 2

Which definition correctly describes a sink habitat in source-sink population dynamics?

• A. A habitat patch with low population density due to strong interspecific competition from neighboring patches
• B. A habitat patch where local mortality exceeds reproduction, so the population persists only through continued immigration from better-quality patches
• C. A habitat patch that receives more immigrants than it exports, causing local density to exceed carrying capacity
• D. A habitat patch where individuals reproduce successfully but emigrate before local density builds, keeping the resident population artificially low

Answer: (B)

The defining feature of a sink is negative intrinsic population growth: locally, deaths exceed births, and the population would decline to zero without immigration. The population is 'rescued' demographically by a continuous supply of dispersers from source patches. Crucially, this means abundance alone tells you nothing — a sink can be dense and appear healthy while being entirely subsidized. Only demographic data (birth rates, death rates, immigration rates) distinguishes sources from sinks.

Question 3

A sink population can appear occupied and even dense while being entirely dependent on immigration from a source population for its long-term persistence.

• T. True
• F. False

Answer: True

This is the central, counter-intuitive insight of source-sink theory. Because immigrants from the source continuously replenish the sink, the sink can maintain a substantial, apparently stable population. A naive count of individuals would suggest healthy occupancy. Only a demographic analysis showing that local births fail to replace local deaths reveals the sink's dependence. This insight has major conservation implications: abundance is a misleading indicator of habitat quality when immigration is occurring.

Question 4

In source-sink landscapes, sink populations are generally smaller and less dense than source populations, making sources relatively easy to identify through census data alone.

• T. True
• F. False

Answer: False

Sink density can be high — sometimes even higher than source density — if immigration rates are large. The presence of abundant immigrants can maintain sink populations well above what local reproduction would support. This is precisely why source-sink dynamics are easy to misread: conservation surveys based on abundance data alone may protect the wrong patches. Identifying sources and sinks requires demographic rates (per-capita birth and death rates), not just counts.

Question 5

Why is immigration from a source important not just demographically but also genetically to a sink population, and what conservation management practice does this motivate?

Model answer: Demographically, immigrants prevent the sink from collapsing by replacing individuals lost to excess mortality. Genetically, immigrants introduce alleles from the larger, more genetically diverse source population, counteracting the genetic drift and inbreeding depression that threaten small isolated populations. This motivates maintaining or restoring connectivity — habitat corridors, stepping-stone patches, or translocation programs — between source and sink patches so that both demographic rescue and genetic rescue can operate.

The dual benefit of connectivity (demographic + genetic rescue) makes it especially valuable in conservation planning. A sink that is demographically stable but genetically isolated will still slowly accumulate inbreeding effects, reducing fitness over generations. Landscape-level management that preserves the source patches and the movement corridors linking them to sinks addresses both problems simultaneously. Fragmentation that severs these connections can lead to extinction even when sink habitats appear intact.