What if a habitat lost its water supply?

Rain and streams give water to the land. Plants drink it and grow. Deer eat the plants.

What if the water got much less? Would the deer still have enough food?

A habitat is the home where an animal finds everything it needs: food, water and shelter. In many habitats, rain, snowmelt and streams water the plants, and the plants feed the animals.

So what happens if a habitat's water supply shrinks, in a long dry spell called a drought? Less water means less plant food. In this lab you can cut the food in a model habitat and watch what a herd of deer does.

Drought is generally defined as a deficiency of precipitation over an extended period, usually a season or more, resulting in a water shortage. For a plant-eating population, its most direct effect is on primary production: fewer plants grow, so the habitat can support fewer animals.

This lab models that link in the simplest way: the drought lowers the food supply, which lowers the carrying capacity, and the herd must adjust. How fast does it adjust, and to what?

In a normal year, Olly's land grows lots of plants.

There is food for 500 deer, and 500 deer live there. Everyone has enough!

In an ordinary year, our model land grows 50 tonnes of plant food. Each deer needs a tenth of a tonne a year, so there is enough for 500 deer, and 500 deer live there.

The herd is exactly the size its food can feed. Each year new deer are born and others die, but the total stays level, and every deer gets all the food it needs.

The model's carrying capacity is K = food ÷ need = 50 ÷ 0.1 = 500. Starting the herd at N = 500 with no response delay puts it at equilibrium: dN/dt = rN(1 − N/K) = 0.

The food share readout, min(100%, K ÷ N), stays at 100 percent. In the model, food regrows to the same amount each year and is never damaged by grazing, so this balance could last indefinitely.

Now a dry time comes. Less rain falls, and fewer plants grow.

There is only food for 200 deer now. Fewer baby deer are born, and the herd slowly gets smaller until it fits its food.

Now make a drought: the land grows only 20 tonnes of food, enough for 200 deer. At first, the 500 deer get just 40 percent of the food they need.

In the model the herd shrinks quickly at first and then more slowly: about 270 deer after one year, about 230 after two, and close to 200 after about six years, when there is enough food for every deer again.

Dropping food to 20 tonnes sets K = 200. With N = 500 the food share falls to 40 percent and the growth term rN(1 − N/K) turns strongly negative: about 274 deer after 1 year, 228 after 2, 212 after 3, and within half a percent of K after about 6 years.

With the model's default one-year response lag, the decline overshoots: the herd falls to about 102 deer at 1.8 years, rebounds to about 222 at 5 years, and settles at 200. A lagged response can push a shrinking population below its new carrying capacity.

Plants need water to grow. Deer need plants.

Less water means less food. Less food means the land can feed fewer deer.

The water a habitat gets sets how many plants grow there, and the plants set how many plant-eaters it can feed. When there is not enough food for every deer, fewer babies are born and survive, and the herd shrinks until it matches its food again.

The model keeps only that chain: food in, deer out. It has no predators, no disease and no seasons, and its numbers were chosen to show the pattern, not measured from a real herd. It also leaves out drinking water itself.

The model encodes drought only as a lower food supply, K = food ÷ need. It assumes food regrows fully each year, so grazing during the drought never damages future growth; no predators, disease, hunting or migration; and a smooth population with one optional lag standing in for breeding seasons.

It also omits several real drought effects: animals needing open water, moving away to find it, crowding at remaining water sources, and lasting effects on young born to mothers in poor condition. Its growth rate and food values are illustrative, not measured.

In Utah, one of the driest states, deer and birds have less food and water in dry years.

People help! They build water tanks that catch rain and melting snow, so animals can drink.

Utah's wildlife agency says drought can severely affect deer. With less food and water, fewer fawns are born and fewer survive their first year, so deer numbers fall. Drying wetlands lose the small water animals and plants that ducks, pelicans and herons eat.

People help in several ways. In dry areas where natural water has disappeared, Utah and its partners build guzzlers: devices that collect rainwater and snowmelt and store it as drinking water for wildlife. Biologists also watch range conditions during droughts.

The Utah Division of Wildlife Resources reports that limited food and water during drought reduce the number of fawns born and surviving their first year, and lower overall deer numbers. Poor nutrition can stop does becoming pregnant or affect fetal growth, with effects that last a deer's whole life. Wetlands with less water lose the invertebrates and vegetation that waterfowl and shorebirds depend on.

Its responses include monitoring range conditions and competition between elk and deer, and guzzlers: built systems that collect and store rainwater and snowmelt where natural water sources have disappeared. Drought.gov describes drought as a 'creeping phenomenon' that is hard to predict and to date.