Rats and Seabirds Project - Project Summary
Background
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| Rakitu, a rat-invaded island (T. Fukami) |
Invasions of native ecosystems by alien organisms are a major component of human-induced global change. Some alien species cause large changes in the community structure of native organisms through a range of biotic mechanisms, notably competition and predation. However, what remains less well understood is how these changes affect the function of native ecosystems.
Islands provide a unique opportunity for investigating biological drivers of ecosystem function. Different islands in an archipelago may contain different combinations of organisms, and each island operates as a spatially discrete entity, which enables a level of true replication of independent ecosystems that can be achieved in no other way. If islands in an archipelago differ in alien biota, this provides a powerful tool for assessing impacts of alien organisms on ecosystems.
Objectives
Our work is aimed at understanding the ecosystem-level consequences of predation of native organisms by alien organisms in natural habitats.
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| View from Aiguilles, a rat-invaded island, with the boat waiting for our return from fieldwork (T. Fukami) |
Our system involves small offshore islands around Coromandel Peninsula of northern New Zealand. On these islands, seabirds naturally operate as ecosystem drivers by burrowing and cultivating soil as well as by transporting nutrients from the sea to land. Some of these islands have been invaded by introduced mammals, notably the rats Rattus rattus and Rattus norvegicus, which devastate seabird populations. Large declines in seabird populations caused by rats may in turn greatly reduce the ecosystem-level effects of these birds.
We are seeking to provide new basic knowledge on several topical areas in ecology, including:
- How above- and below-ground subsystems are interlinked
- How native 'ecosystem engineers' (seabirds) affect ecosystem function
- How alien top predators (rats) indirectly affect ecosystem function
- How sea-to-land nutrient fluxes affect ecosystem function
- How New Zealand's mainland ecosystem used to function (Prior to human and rat arrivals, much of coastal regions likely supported large populations of seabirds.)
Hypotheses
Hypothesis 1
Reduction of seabirds by rats causes significant shifts in soil food web structures, by favouring fungal-based over bacterial-based energy channels, decomposer fauna with larger body sizes over smaller ones, and increases in decomposer diversity. Soil disturbance is known to promote bacterial-based energy channels and soil microfauna over fungal-based energy channels and soil mesofauna and macrofauna. Further, evidence suggests that the diversity of most groups of soil organisms is reduced by disturbance. We therefore expect that seabirds promote soil food webs characteristic of disturbed systems owing to their burrowing and nutrient subsidisation activities, and that reduction of seabirds by rats reduces these effects.
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| Some potential interactions on islands (T. Fukami) |
Hypothesis 2
Rat predation on seabirds causes retardation of litter decomposition rates, reduced release rates of nutrients from litter, and increased soil sequestration of carbon and nutrients. We expect that seabirds promote litter decomposition by (1) facilitating soil food webs that promote more rapid decomposition (hypothesis 1); (2) cultivation by burrowing, resulting in incorporation of plant litter into the soil; and (3) creating a positive feedback, in which the vegetation produces litter with a higher quality for decomposers.
Hypothesis 3
Seabird predation by rats, and corresponding retardation of plant litter decomposition rates, results in a reduced supply rate from the soil of available nutrients for plant growth.
For other potential interactions depicted in the figure, but not covered in our hypotheses, see Christa Mulder's web site.
Approach
We have selected 20 islands that range between a few hectares and a few hundred hectares. Some of them have never been invaded by rats, while others currently have rats. Since all islands were probably burned by people from about 800 years ago onward, we restrict our sampling to well-developed forest to enable comparability across islands. In February-April 2004, we established two 10 x 10 m plots in forest on each island to examine burrow densities, vegetation structure, soil microbiota and invertebrates, soil resource base and litter and wood decomposion. We have also brought soil samples from each island for laboratory experiments on plant nutrient supply.
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| Forest floor on Aiguilles, a rat-invaded island (T. Fukami) |
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| Forest floor with seabird burrows on Tawhiti Rahi, a rat-free island (T. Fukami) |