Rat control in a mainland reserve: can we infer a bird benefit?
A 400 ha mainland reserve has a ground-nesting bird species and introduced ship rats. Rat tracking indices were 32% before control and 4% after three years of 1080 bait stations. Ground-nesting bird counts rose from 12 to 26 adults, while bellbird counts stayed at about 40. Invertebrate pitfall trap catches fell from 85 to 55 per trap-night. Stoat sightings increased from 2 to 7 per year after rat control. The reserve also had a drier-than-usual spring in year 3, and a new walking track opened in year 2. Bird counts were done by five-minute call counts at six stations, once each November.
1. Explain two possible food-web mechanisms that could link rat control to the change in ground-nesting bird numbers.
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First, ship rats are predators and scavengers that can eat eggs and chicks of ground-nesting birds. If rat numbers fall from a 32% to a 4% tracking index, predation pressure on nests may drop, so more eggs hatch and more chicks survive to adulthood. That could explain the rise from 12 to 26 adults. Second, rats also eat invertebrates, and the fall in pitfall catches from 85 to 55 per trap-night may reflect reduced rat predation or other changes. For insectivorous birds, fewer invertebrates could reduce food supply and work against a bird increase, so the mechanism is not automatically beneficial. A third possibility is mesopredator release: stoats may prey more on birds when rats are scarce, and stoat sightings rose from 2 to 7 per year. This could offset some of the benefit of rat control. The food web therefore has several pathways, and the net effect on ground-nesting birds depends on which pathway is strongest.
2. Evaluate how confident you can be that rat control caused the increase in ground-nesting birds. Refer to at least three features of the monitoring and the reserve.
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Confidence should be limited because the evidence is a before-after comparison with no control site. The bird count rose, but other things also changed: a drier spring in year 3 could affect nesting success, and a new walking track in year 2 could change disturbance or predator movement. Stoat sightings increased, which may have increased predation on birds even as rat numbers fell. The monitoring method also has weaknesses. Five-minute call counts at six stations once each November may not detect all birds, and changes in calling behaviour or detectability could alter counts. Six stations is a small sample, and a single annual count does not show nesting success or survival. Invertebrate catches fell, suggesting the food web changed in ways not explained by rats alone. To strengthen the claim, the reserve would need replicated control and treatment areas, several years of data, nest survival monitoring, and indices for rats, stoats and birds collected in the same way. The pattern is consistent with a benefit from rat control, but causation is not established.