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Wildlife's hidden networks

Pete Wilton

What links different pollinating insects in your garden or sea otters and kelp?

In a recent paper in Royal Society B Becky Morris of Oxford University's Department of Zoology explored how species interactions create networks in which apparently unconnected organisms can affect one another.

I asked Becky about how these networks work and what the possible implications are for how we attempt to conserve biodiversity around the world...

OxSciBlog: How are species organised in networks?
Becky Morris: Species don’t exist in isolation; they are each linked to one or more other species, with which they interact in a variety of different ways, for example as predators or pollinators.

Think of them as being like web sites (the species) arranged on the World Wide Web (the network). For example, all the insects that pollinate flowers in your garden, along with the flowers that they pollinate, could be considered as a relatively discrete network (although they would also interact with species outside this network).

OSB: Why is understanding such networks important?
BM: Because species are organised in networks, any perturbation to one species is likely to have a knock-on effect on many other species in the network, including those with which it interacts only indirectly. Indirect interactions occur when one species affects a second species through one or more intermediate species. Through such indirect interactions, species that are not closely linked can affect each other, something that you would not predict without considering network structure.

A good example of this is in Pacific kelp forests, where sea otters feed on sea urchins. In areas where sea otters have been hunted to extinction, there has been a huge increase in their prey, sea urchins, which has resulted in the urchins overgrazing and decimating kelp forests. The sea otters have indirectly affected the kelp, via the intermediate sea urchins.

OSB: How might it change our approach to conserving biodiversity?
BM: Rather than focusing on conserving species it might be better to focus on conserving networks of species and their functions. In practice this would be extremely challenging. It is really only possible to ensure that the species that are involved in the networks are present; it is not possible to make them interact.

This has has been demonstrated in a recent study in restored heathlands in Dorset, where individual species of plants, bumble bee pollinators and their natural enemies were successfully restored, but the interactions between the bumble bees and certain natural enemies were not reestablished, so the network structure was not restored to how it was in ancient heathland sites.

However, a network approach can be used to evaluate the impacts of practices such as habitat management and biological control, and these practices indirectly affect biodiversity. A network approach will lead to much greater insight than if only species were monitored, but as yet we have no way of actually focusing our conservation efforts on the interactions between species, rather than the species themselves.

OSB: What impact could human activity have on these networks?
BM: Human activities such as deforestation, habitat fragmentation or climate change all have the potential to alter or disrupt network structure, which in turn will affect their susceptibility to species loss, and the functioning of ecosystems.

recent study led by Jason Tylianakis from Canterbury University in New Zealand revealed changes in network structure for insect communities in coastal Ecuador, along a gradient of habitat modification from the original forest habitat, through coffee agroforest, to the most modified pasture and rice fields. Surprisingly species richness did not change along this gradient of habitat disturbance, so if we looked only for changes in species richness we would not see any effect.

OSB: What future research is needed in this area?
BM: More experiments and theoretical studies are needed, taking into account the variability of the real world. We still don’t understand exactly how networks respond to perturbations, and whether there are critical thresholds at which the loss of species leads to the loss of network structure and functioning (and whether they can be restored). We need to be able to make general predictions, rather than have a case-by-case understanding.

Dr Rebecca Morris is a Royal Society University Research Fellow based at Oxford University's Department of Zoology.