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.
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.
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
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.
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’
burrowing, was likely to have been a key shaper of forest
communities in New Zealand’s pre-human island and coastal
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
Seabirds off Aotea Great Barrier Island.The mainland of New Zealand was once
home to millions of seabirds.
Photo: Dr E Cronin
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
showed that the signal of these marine nutrients could be detected
feathers, but that these nutrients were unlikely to be moved far
beyond the colony itself.
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.
Taurepo (Rhabdothamnus solandri)
is pollinated by native birds that are absent or rare from the
mainland. Photo: W Bennett
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
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
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
‘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.
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),
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.
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),
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.