Predators Decline First From Human Land Use

Predators decline in numbers before prey animals whenever wild places are developed for human land use. Find out why this is a concern for our global ecosystem.

I’ve spent more than 40 summer vacations at our family cottage in the woods, so I’ve picked up a few things about how nature works. One obvious point is that there are far more plants than animals in the wild.

When it comes to animals, forests contain far more prey species than predators. It’s pretty easy to spot a chipmunk, a squirrel or a rabbit.

When it comes to a coyote, a rattlesnake or a red fox, even though they’re wandering in the wilderness “out back”, they’re rarely close to the road. The number of predators declines as you move from the deep woods into the cottage properties..

MOST PREDATORS ARE TECHNOPHOBES

Ecologists use the word technophobes for animals who don’t care for people or their developments. Most predators fall under that group.

It’s easy to forget that most animals in the wild are insects and arachnids. We might not think of them as predators but spiders, black flies and mosquitoes are carnivores. Timothy C. Winegard gives us a light-hearted history of how we fall prey to mosquitos in his book, The Mosquito: A Human History of our Deadliest Predator.

It serves them right that the food web also has carnivores at the next step in the food chain that survive by eating these predatory pests. These are the predator populations of sweet-sounding warblers for which our area is famous.

FIRST OF ITS KIND STUDY

My views about how predators decline in built-up areas are just something I’ve noticed on my own. Even so, a first of its kind study came out last week that backs me up.

The study came under the PREDICTS project. PREDICTS stands for Projecting Responses of Ecological Diversity in Changing Terrestrial Systems. That’s a mouthful but in plain terms, PREDICTS looks at the ecological effects of human land use on the diversity of life.

PREDICTS has put together a database that stores a wide-ranging collection of biodiversity samples from places all over the world. The environmental sites they chose had all been impacted by human land use. 

OVER A MILLION RECORDS IN 460 SCIENTIFIC PAPERS

The team went through over a million records of the numbers of animals collected in 460 scientific papers. They sorted the animal species by size, what they ate and whether they were warm or cold-blooded.

Then they looked at habitats. They classed each habitat as primary vegetation, secondary vegetation, plantation forest, cropland, pasture and urban. They also grouped them into three levels of human impact; minimal, light and intense.

This may sound like a big sample, but it isn’t. The study’s lead author, Dr. Tim Newbold of University College London explained, “We were able to get information for more animals than ever before, but this was still only around 1 out of every 100 animals known to science.” 

STILL ONLY AROUND 1 OUT OF EVERY 100 ANIMALS

The team also had trouble pulling together an unbiased sample. Most of this type of research seems to be done in natural and rural areas.

There aren’t as many animal counts from more urban settings. The team found that ecologists tend to work where there are plenty of wild animals to study and seem to overlook towns and cities.

Insects and birds seemed to be studied a lot more often than other animal species. For some reason, there also wasn’t much data from Asia.

PREDATORS DECLINE BEFORE PREY ANIMALS FROM LAND USE

The team set out their findings in the journal Functional Ecology from the British Ecological Society. Their studies show that the headcount of predators declines before prey animals when we take over wild spaces for land use.

The team saw predators decline most severely among tiny invertebrate predators like spiders and ladybirds (we call them ladybugs in North America). The study also saw large carnivores decline among animals higher up on the food chain like reptiles, amphibians, birds and mammals.

“Normally when we think of predators, we think of big animals like lions or tigers.” Dr. Newbold clarified. “We find small predators – such as spiders and ladybirds – to show the biggest declines.” Dr. Newbold added that activities like hunting appear to do more damage to large predators than land use does.  

“SMALL PREDATORS LIKE SPIDERS SHOW BIGGEST DECLINE”

Animals at the top of the food chain don’t have to be big predators. They just have to have no other predators chasing after them.

This seems to tell us that the world’s ecosystems are being badly messed up by our land use. Top predators decline as we tear up the plants that grow naturally on the land. That happens whether it’s for farms or to build new factories or subdivisions.

Dr. Newbold continued, “We know that different types of animals play important roles within the environment – for example, predators control populations of other animals.” Dr. Newbold warned that these roles go unfilled when we develop natural spaces for human use.

PREDATORS LIMIT PREY ANIMALS IN HABITATS

When predators decline in an ecosystem, it ruins what ecologists call the “trophic cascade.” Predators limit how many prey animals a habitat contains and how they act, which filters down the food chain.

For example, when we convert a natural area to agriculture, the predators decline. This often leads farms to be overrun by prey species such as flies, mice, rabbits or deer.

We humans seem to feel alienated from the natural world. We tend to think of fields, towns and cities as our domain and of the wilderness as foreign to us. The truth is that there is no distinction.

WE CAN’T ESCAPE NATURE BY CONVERTING WILD SPACES

We can’t escape nature. When we try to do that by converting wild spaces for our own narrow purposes, it tosses nature out of balance, affecting us and all the other species around us.

Now that the team knows that predators decline first when we change land use, they want to look at it from another angle. They’d like to find out how the loss of habitat affects animals that help us with agriculture.

These include pollinators like bees, ants, butterflies and flower beetles and animals that do pest control such as barn owls and hawks. We need to find out if these species are declining too and, if so, what that means for the global food system.

We always have more to learn if we dare to know.
British Ecological Society
Global effects of land use on biodiversity differ among functional groups
Food Ethics: An Embarrassment of Choices
Agricultural Biodiversity Under Threat Worldwide
Finding New Ways to Share the Land
Biodiversity Always Wins
Mass Extinction Happening Again
Nature Emergency – Time to Make It Official

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