Ecological function in an age of extinction

by George Perry (School of Environment, University of Auckland)

Ecologists consider ecosystems as having three components -composition, structure and function. Structure and composition refer to what the components of an ecosystem are and how they are arranged, whereas function refers to the flow of energy and matter.

Most discussion of biodiversity tends to focus on composition i.e. the species present. This perspective is reinforced by the myriad of ways natural selection has come up with to deal with the challenges organisms face (such as lizards that shoot blood from their eyes) as well as the list of threatened and extinct species.

Seeing biodiversity as simply a catalogue of species provides only a partial view as it fails to consider function, and the process that bind ecosystem components together. So, in an age of biodiversity loss should we be at least as concerned about the ‘extinction of function’ as we are about the ‘extinction of composition’?

New Zealand is famously a land of birds but as Jared Diamond described it, all that remains is the ‘wreckage of an avifauna’. Given that birds in New Zealand are involved in seed dispersal, pollination, soil formation processes and nutrient transfer surely their decline has had functional implications for our ecosystems?

The function of seabirds as nutrient recyclers

Oceanic and terrestrial ecosystems are separate from each other; after all, it is obvious when you move from one to the other (not least that you get wet). In New Zealand’s offshore islands, however, a constant movement of material occurs from oceanic ecosystems to the land driven by seabirds in the form of defecated and vomited nutrients.

The elements so-moved include phosphorus, nitrogen, and bioavailable iron. In other ecosystems, these fluxes would also have been driven by animals such as andromadous fish which spend most of their adult lives at sea, and return to fresh water to spawn.

In a recent study, Doughty and colleagues1 tried to quantify the effects of the loss of marine animals (and especially megafauna) on these ocean-to-land fluxes. They came to two interesting conclusions: (i) globally the loss of marine animals has led to an up to 95% reduction in the movement of phosphorus from the ocean to the land, and (ii) although the flows are much reduced, northern New Zealand’s offshore islands are a global hotspot for ocean-land nutrient transfer by seabirds. This nutrient transfer, coupled with seabirds’ soil burrowing, was likely to have been a key shaper of forest communities in New Zealand’s pre-human island and coastal ecosystems.

Modern studies2 have shown that on New Zealand’s offshore islands invaded by rats (with negative effects on seabirds), foliar (leaf) nutrients and soil fertility are reduced, and belowground ecological processes altered.

A final question about seabird-driven nutrient movement is how far from the coast these fluxes extended? In coastal plant communities where seabird fluxes are intact, plants show high levels of nitrogen and phosphorus in their leaves. Research from the west coast of the South Island3 showed that the signal of these marine nutrients could be detected in kererū feathers, but that these nutrients were unlikely to be moved far beyond the colony itself.

However, extinct birds may have once transferred marine nutrients much further. In short, conservation of seabird populations by activities such as predator removal has the potential for flow-on effects through the ecosystem, and at locations well beyond seabird colonies.

Pollination and seed dispersal

A second example of the loss of ecological function following avifaunal decline in New Zealand is through pollination and seed dispersal. Many New Zealand’s plant species are either pollinated by, or have their seeds dispersed by birds, and a few even rely on the same bird species for both. These are crucial processes in the plant life-cycle -  fewer birds should have significant implications for the plant species involved.

A study published in Science in 2014  demonstrated the effects of pollinator loss on the plants that rely on them. The study looked at the plant taurepo (Rhabdothamnus solandri) which is pollinated by native birds such as hihi/stitchbird (Notiomystis cincta) and korimako/bellbird (Anthornis melanura) that are either now absent from or rare across mainland northern New Zealand.

The silvereye (Zosterops lateralis) which naturalised from Australia in the 1850s, may play a pollination role, but is much more important as a nectar robber (i.e. steals the reward without providing the pollination service). The presence of taurepo populations on the mainland where pollinators are either absent or very scarce, and on offshore islands where pollinators are present, provides an opportunity to assess how taurepo responds to pollinator decline/loss. The study showed that where the pollinators are absent, fruit set and the number of seed per fruit are lower than where the pollinators are present. The result is much lower densities of juvenile taurepo where the pollinators are absent. When taurepo seeds were sown at sites without the pollinators, a dramatic increase was seen in seedling abundance – this result is crucial as it suggests that a lack of seeds rather than some other environmental factor is responsible for the recruitment failure (i.e., seed germination, seedling survivorship, and seedling growth).

Similar ‘pollination limitation’ effects have been demonstrated in other New Zealand species such as toropapa (Alseuosmia macrophylla), kotukutuku (Fuchsia exorticata), and in forest ecosystems globally.

Extinction of ecosystem function

The key message is that the decline or extinction of a single species can reverberate throughout an ecosystem. Another important message is that the loss of a function in an ecosystem does not require the complete loss of the species that provide it and in fact, functional extinction is likely to precede complete loss of a species.

Notes

1 Doughty, C. E., Roman, J., Faurby, S., Wolf, A., Haque, A., Bakker, E. S., Svenning, J.-C. (2016). Global nutrient transport in a world of giants. Proceedings of the National Academy of Sciences, 113(4), 868–873.

2Fukami, T., Wardle, D. A., Bellingham, P. J., Mulder, C. P. H., Towns, D. R., Yeates, G. W., Williamson, W. M. (2006). Above- and below-ground impacts of introduced predators in seabird-dominated island ecosystems. Ecology Letters, 9(12), 1299–1307.

3Hawke, D. J., & Holdaway, R. N. (2005). Avian assimilation and dispersal of carbon and nitrogen brought ashore by breeding Westland petrels (Procellaria westlandica): a stable isotope study. Journal of Zoology, 266(4), 419–426.

4Anderson, S. H., Kelly, D., Ladley,  J. J., Molloy, S., & Terry, J. (2011). Cascading effects of bird functional extinction reduce pollination and plant density. Science, 331(6020), 1068–1071.

Environmental News Issue 39 Winter 2018