Patterns of connections are important. Think about a grid of electricity connections where substations are connected to different numbers of houses. Like electricity networks, species are connected to each other — predators eat prey, pollinators introduce pollen from one flower to another and fungi barter for nutrients with plants. These ecological partnerships or conflicts are embedded in complex food webs.
Food webs are composed of species joined by links in a network. Some species in the network are highly connected and linked to many others, a generalist pollinator like a honey bee can pollinate many species of plants so has many links. A specialised fig wasp can only pollinate a single fig species and has very few links.
Knocking out a highly connected electricity substation will cause a widespread blackout, whereas disruption to a more remote and less well-connected substation will only affect a few houses. Similarly, species extinctions do not just cause a single node in the network to disappear but also affect links to other species.
Just like highly connected electricity substations, it seems that mammal species disappearing from food webs have been disproportionately better connected, leading to ‘blackouts’ of species interactions in the food webs
Prof Jens-Christian Svenning from Aarhus University in Denmark, speaking at the Ecology & Evolution Ireland conference recently, pointed out that over the past 130,000 years humans have severely disrupted global food webs. Prof Svenning and colleagues have estimated that while just 6 per cent of mammal species have gone extinct over the past 130,000 years, this has led to the disappearance of more than half of all food web links between mammal species.
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Just like highly connected electricity substations, it seems that mammal species disappearing from food webs have been disproportionately better connected, leading to “blackouts” of species interactions in the food webs.
While there are fascinating examples of fossils which provide snapshots of animals interacting as predator and prey, interactions between animals are generally poorly documented in deep time. So how do we know how food webs were structured in the past and how can we estimate change? The ecologists used a large database on over 1,000 modern species of mammals with information on their interactions together with an Artificial Intelligence model.
Predators are likely to prey on species that are relatively similar to each other in traits such as size, lifespan and hibernation status. Data on predator-prey interactions was used to build a “deep learning” model, a type of Artificial Intelligence, which could “learn” the likelihood of particular food web interactions from the traits of the species involved.
The food web model was trained using data from modern species and then applied to communities of mammals that lived in the past, which we only know from fossil remains. Time and again, for examples of food webs from different time periods and places, the model showed that extinctions and contractions in the area occupied by mammals led to a disproportionate simplification of food webs.
Large mammals are much more likely to go extinct and these species also tend to be the most highly connected in food webs. Downsizing of mammals through extinction and range contraction, therefore, leads to downsizing of the complexity of their food webs. This matters because complex food webs are much more likely to maintain their function and are more likely to recover through “rewiring” of lost connections than simple food webs. The downsizing and simplification of mammal food webs make them much more vulnerable to future disruption.
While species extinctions were responsible for much of the simplification of food webs another important component was the reduction in the area occupied by mammal species that still persist. This raises the possibility that by reinstating mammals, particularly large ones, to areas where they existed previously we can restore food web complexity and function.
Despite 130,000 years of food web simplification and the extinction of iconic mammals such as mammoths, giant sloths, sabre-toothed tigers, and giant wombats, the restoration of some complex food webs is still within our grasp
There has been a marked resurgence in large mammals in Europe over the past couple of decades as conservation activities and changes to land use have enabled lynx, wolves, and bears to extend their range, even in areas populated by humans.
Despite 130,000 years of food web simplification and the extinction of iconic mammals such as mammoths, giant sloths, sabre-toothed tigers, and giant wombats, the restoration of some complex food webs is still within our grasp.
More than 200 scientists attended the Ecology & Evolution Ireland conference, supported by the Irish Ecological Association; British Ecological Society, Journal of Ecology, EPA, Marine Institute, UCD Earth Institute, Trinity College Dublin and the Chartered Institute of Ecology and Environmental Management.
- Yvonne Buckley is an ecologist, Irish Research Council laureate and professor of zoology at Trinity College Dublin