Learning target: I can use resource data to find how many elk a fenced reserve can support in wet, average and drought years, identify the limiting resource in each, and design and justify a drought plan that meets criteria and constraints.
Work with a partner. Read the real case first, then the problem at Ridgeback Reserve. In Part A, calculate what the reserve can support with no plan. In Part B, test each option by itself. In Part C, design your own plan, check it against every criterion and constraint, and defend it. Show every calculation with units. When you divide to find a number of elk, round down, because the reserve cannot feed part of an elk.
You will need: This worksheet, Pencil, Calculator
At Point Reyes National Seashore, north of San Francisco, a herd of tule elk at Tomales Point lived behind a fence that kept the elk off neighboring ranch land. In the dry years of 2012 to 2015, the National Park Service's counts of that herd fell from about 540 animals to about 280. Park biologists found no evidence that the elk died of thirst; there was not enough good forage inside the fence for that many elk. (Some groups argued that water ran short too, and the Park Service disagreed.) A free-roaming herd can walk to wherever water and food remain. A fenced herd cannot, so it depends entirely on the resources inside the fence.
Ridgeback Reserve is an imaginary coastal reserve with the same problem. Its 300 elk live inside a fence. Elk need two resources the reserve can run short of: forage (grasses and other plants they eat) and summer water, because most ponds dry up by the end of a dry summer. The managers expect a drought within the next few years and have asked for a plan. The numbers below are planning values for teaching, not measurements from a real reserve.
| Kind of year | Forage grown (kg per hectare) | Ponds that still hold water at the end of summer |
|---|---|---|
| Wet | 1800 | 6 |
| Average | 1200 | 3 |
| Drought | 700 | 1 |
| Planning value | Amount |
|---|---|
| Area inside the fence | 1,000 hectares |
| Forage one elk eats in a year | about 1,500 kg |
| Share of each year's forage that must be left ungrazed (criterion C4) | one half |
| Elk that one summer pond can water | about 120 |
| Elk in the herd now | 300 |
| Option | What it does | Estimated cost | Notes |
|---|---|---|---|
| A. Water trough fed by a solar-powered well pump | Waters about 100 more elk through a dry summer | $12,000 each, one time | Only 4 sites have groundwater, so at most 4 troughs; each lasts 20 years or more |
| B. Hay brought in during a drought year | Replaces the forage one elk would otherwise lack that year | $200 per elk, every drought year | Must be bought again in every drought; elk crowd around feeding sites, where disease spreads more easily |
| C. Move elk to other public lands | Removes elk from the herd | $600 per elk | Receiving sites can take at most 60 elk this year; capture is stressful for the animals |
| D. Open a corridor of neighboring park land | Adds 300 hectares of forage and one pond that holds water even in droughts | $45,000, one time | Deer and hikers already use the corridor; the half-forage rule applies there too |
| Usable forage (kg) | Elk the forage can feed | Elk the ponds can water | Carrying capacity | Limiting resource |
|---|---|---|---|---|
| Wet year | ||||
| Average year | ||||
| Drought year |
| Cost | Elk the forage (plus any hay) can feed | Elk the water can supply | Drought carrying capacity | Lasts for future droughts? |
|---|---|---|---|---|
| 1 trough | ||||
| 2 troughs | ||||
| 4 troughs | ||||
| Corridor (option D) | ||||
| Hay for 70 elk |
| Met? (yes, partly or no) | The number or reason that shows it |
|---|---|
| C1: drought capacity at least the herd you keep | |
| C2: at least 250 elk | |
| C3: keeps working in future droughts | |
| C4: half the forage left ungrazed | |
| Budget: $60,000 or less this year | |
| No more than 60 elk moved |
Step 1: forage grown on the whole reserve = kg per hectare × 1,000 hectares. Step 2: usable forage = half of that. Step 3: elk the forage can feed = usable forage ÷ 1,500 kg, rounded down. Step 4: elk the water can supply = ponds × 120 (add 100 for each trough). The carrying capacity is the smaller of the Step 3 and Step 4 numbers. For the drought year, Step 1 is 700 × 1,000 = 700,000 kg. For the corridor, repeat Steps 1-3 with 300 hectares and add the result to the reserve's number.
Suppose four droughts happen in the next 20 years. Compare the total 20-year cost of a plan built on the corridor and one trough with a plan built on two troughs and drought hay, assuming the herd is 300 at the start of each drought. Then write a paragraph about whether the cheapest plan this year is the cheapest plan overall, and name one cost that is not measured in dollars (for example, the stress of capture or the spread of disease at hay piles).