There is always conflict over who gets the “best” chocolate in the tub of Quality Street, Roses or Heroes. Not only are there favourite sweets, but these are often in short supply. There is never an even distribution of the types of chocolates in the tub. While each tub of Quality Street might have 11 different kinds of sweets, the abundances differ from sweet type to type.
Imagine a tub where 90 per cent were toffee fingers with only a single individual of the other 10 kinds. While the richness of sweet types is the same as a normal tub (11 types), this would not be a pleasing and diverse collection.
In this low-diversity box of chocolates you would be most likely to get a toffee finger from a random selection, thus disproving the conclusion of Forrest Gump’s line “You never know what you’re gonna get”. Only in a tub with an even distribution of sweet types would a random selection result in an equal probability of getting any one of the 11 types.
This example shows the difference between “richness”, the number of species of chocolates, and a diversity measure, which takes the evenness of the distribution of chocolate species into account. The science of biodiversity has several different measures of the diversity of life, which range from simple counts of numbers of species through to measures such as evenness. Intuitively we can grasp that an even number of each species in an ecosystem makes it more diverse than an ecosystem that is dominated by one or a few species.
Simplified environments
When we simplify our environments, by conversion to a single land use or by managing many areas in a similar way, we provide fewer opportunities for the diversity of life to flourish and we can end up with ecosystems dominated by very few species.
Simplification of the environment can lead to individual species dominating as they exclude other species through competition for a limited number of resources. Interestingly, the introduction of a predator that preys on the dominant species can enable other species to increase. This is the science behind trophic rewilding, where the (re)introduction of apex predators can increase biodiversity.
Joshua Twining at Queen's University Belfast provides a great example of the role that predation and habitat complexity can play in rebalancing competition between native red and non-native grey squirrels. Recovering numbers of the predatory native pine marten are leading to an increase in red squirrel populations. Non-native grey squirrels are more vulnerable to pine marten predation because they lack an evolved "fear" response. Red squirrels move out when they smell pine marten urine but greys do not.
There is a twist to the tale, however: the positive effects of pine marten on red squirrels disappear in non-native conifer plantations. Red squirrels benefit much more strongly in native broadleaf woodlands, because of the availability of refuges for the red squirrels and alternative prey for the pine martens. This example shows the importance for intact, functioning native ecosystems for the recovery of declining species like the red squirrel.
Space for nature
These ecosystems are in very short supply on the island of Ireland. Practically every square centimetre of our land surface has been modified by humans in some way, providing precious little space for wild nature and disrupting interactions between species. Short, tidy hedgerows, lush green fields of perennial rye grass and forests of non-native conifers are not diverse, functional native ecosystems.
The recently announced Citzens’ Assembly on Biodiversity will enable deliberation on the state of nature in Ireland and what we and our leaders need to do to protect, restore and rehabilitate biodiversity.
It is time we rebalanced our land use to provide space for nature and for the complex interactions between species to play out. It is not enough to prevent species from going extinct, we need a sufficient abundance of each kind to make up a functional ecosystem. We need true diversity across our landscape.
Yvonne Buckley is an ecologist, Irish Research Council laureate and professor of zoology at Trinity College Dublin