The line on the graph goes up. And up. And up. Then it falls off a cliff It's one of those things that adds up..
If you've taken a high school biology or environmental science class in the last thirty years, you've seen this graph. Maybe you had to plot the points yourself. Maybe you just stared at the photocopied worksheet and wondered why the teacher cared so much about some reindeer on a rock in the Bering Sea And that's really what it comes down to..
Here's the thing: this isn't just a worksheet. It's one of the clearest warnings ecology has ever handed us.
What Happened on St. Matthew Island
St. No predators. No trees. That said, matthew Island sits alone in the Bering Sea, about 200 miles west of mainland Alaska. Just wind, fog, and thick mats of lichen — reindeer candy, essentially Simple, but easy to overlook..
In August 1944, the U.Because of that, s. In real terms, coast Guard released 29 reindeer there as an emergency food source for the 19 men stationed at a loran navigation station. The war ended. Because of that, the men left. The reindeer stayed.
No wolves. No bears. No hunters. Just lichen, growing back slower than anyone realized.
By 1957, the herd had grown to 1,350. Six thousand animals on an island 32 miles long and 4 miles wide. So by 1963, it hit 6,000. The lichen was gone — trampled, eaten down to dirt, buried under reindeer waste.
Then winter came. The carrying capacity had been exceeded years earlier. The 1963–64 winter was brutal, but the population crash wasn't just weather. The reindeer had been living on borrowed time, eating the principal instead of the interest.
When biologists returned in 1966, they found 42 survivors. All female. Because of that, no calves. By the 1980s, the last reindeer on St. Matthew Island was dead.
Why This Case Study Shows Up in Every Textbook
You're not learning this because your teacher likes sad animal stories. The St. Matthew Island reindeer are the textbook example of exponential growth overshooting carrying capacity — and the crash that follows.
Most populations in nature don't do this. But remove every check? Give a species unlimited resources and zero predation? Also, predators, disease, territorial behavior, resource dispersion — something usually kicks in before the environment is stripped bare. The math takes over.
The growth curve follows a classic J-shape. In practice, the doubling time shrinks. Worth adding: it looks like success. Then the curve steepens. On the flip side, slow at first. Each generation produces more breeders, who produce more breeders. It feels like success. Right up until it isn't.
So yes, the worksheet deserves the attention it gets. That's why to calculate the growth rate. But to see the lag between resource depletion and population response. It forces you to sit with the numbers. To understand that overshoot isn't a theory — it's a body count.
The Core Concepts Your Worksheet Is Testing
Carrying Capacity (K)
This is the number the environment can support indefinitely. And not "right now while the lichen is thick. " Not "during a good year." Indefinitely.
On St. Matthew Island, the carrying capacity for reindeer was probably around 1,000–1,500 animals — maybe fewer. Here's the thing — that lag is critical. On the flip side, the herd blew past that in the late 1950s and kept growing for years because the lichen took time to disappear. It's why overshoot is so dangerous: by the time you feel the pain, the damage is done The details matter here..
Worksheet tip: if a question asks for carrying capacity, don't just pick the peak population. Think about it: carrying capacity is lower. The peak is overshoot. Often significantly lower.
Exponential vs. Logistic Growth
Exponential growth: dN/dt = rN. But the rate of increase is proportional to the current population. Graph looks like a J.
Logistic growth: dN/dt = rN(1 - N/K). Growth slows as N approaches K. Graph looks like an S Simple, but easy to overlook..
Real populations should follow logistic growth. On the flip side, matthew reindeer followed exponential growth right up until they didn't — because the feedback mechanism (starvation) was delayed. Your worksheet will almost certainly ask you to identify which model fits which phase. Early years = exponential. So naturally, the St. The crash = not a model, that's reality breaking the model Nothing fancy..
Lag Time and Overshoot
This is the concept students miss most. That said, the reindeer didn't stop growing the day they exceeded carrying capacity. Because of that, it gets grazed, regrows slower, gets grazed again. Because of that, they kept growing for years because the lichen doesn't vanish instantly. The system has inertia The details matter here..
By the time the population curve bends down, the resource base is already wrecked. On top of that, the new carrying capacity after the crash? The crash goes below carrying capacity because the environment has been degraded. Lower than before. The lichen mats take decades to recover — if they ever do.
Dieback vs. Stable Equilibrium
In a logistic model, the population levels off at K. Matthew herd didn't stabilize at 1,000. The St. Populations that overshoot often crash past K, sometimes to extinction. Smooth. Think about it: clean. In reality? It hit 6,000, then 42, then zero Nothing fancy..
Your worksheet might ask: "Why didn't the population stabilize at carrying capacity?" Answer: because overshoot destroys the very resources that define carrying capacity. But it's not a thermostat. It's a bank account you overdrew until the bank closed.
Common Worksheet Questions — And How to Think Through Them
"Calculate the average annual growth rate (r) for the period 1944–1957."
You'll need the formula: r = (ln(Nt) - ln(N0)) / t
N0 = 29. Nt = 1,350. t = 13 years. Also, do the math. But also think about what that number means. Think about it: an r of ~0. 3 means the population grows ~35% per year. That's fast. But it's not "uncontrolled" — it's just what happens when large herbivores have unlimited food and no predators.
"Graph the population data. Label the exponential growth phase, the overshoot, and the dieback."
Don't just plot points. Label why. Think about it: the exponential phase ends around 1957–1960, not 1963. The overshoot is the gap between the peak and the estimated K.
The dieback is the rapid decline that follows once the lichen canopy can no longer support the herd’s metabolic demands. Because the vegetation has been stripped to a point where its regenerative capacity is severely compromised, the population plummets far below the original carrying capacity, often overshooting into a new, lower equilibrium—or, as happened on St. Even so, matthew Island, to local extinction. Recognizing this sequence helps students see that the logistic curve is an idealization; real ecosystems exhibit hysteresis, where the path of decline does not simply retrace the path of growth Turns out it matters..
Additional Worksheet Prompts and Strategies
“Estimate the new carrying capacity after the crash, assuming the lichen biomass recovers at 5 % yr⁻¹.”
Start by estimating the residual lichen fraction after the dieback (field studies suggest ≈10 % of the original mat survived). Apply exponential recovery: B(t) = B₀ e^{0.05t}. Solve for the time when B(t) reaches the level needed to sustain the pre‑crash population (≈1 000 reindeer). The resulting t gives a projection of when the island could again support a herd of that size, illustrating how recovery timescales can dwarf the boom‑bust cycle Which is the point..
“If a small predator (e.g., Arctic fox) were introduced in 1950, how might the trajectory have changed?”
Introduce a top‑down term to the logistic equation: dN/dt = rN(1 − N/K) − pN, where p is the per‑capita predation rate. Even a modest p (≈0.02 yr⁻¹) reduces the effective growth rate, lowering the peak and potentially preventing overshoot. Discuss how predator‑mediated top‑down control can stabilize herbivore populations, contrasting with the bottom‑up starvation that drove the actual crash.
“Critique the assumption of a constant r in the exponential phase.”
Highlight that r itself can be density‑dependent even before K is reached—e.g., due to social stress, increased competition for micronutrients, or disease spread. Encourage students to look for subtle curvature in early‑year data; a slightly concave‑down trend would signal the onset of density‑dependent feedbacks before the classic “J‑shape” ends Not complicated — just consistent..
“Design a management intervention that could have averted the dieback.”
Possible answers include: supplemental feeding during harsh winters, controlled culling to keep N below a safe threshold (perhaps 0.6 K), or translocation of excess individuals to other islands with unused lichen reserves. make clear that any intervention must consider lag times; acting only after the population shows visible decline is often too late.
Connecting the Case to Broader Ecological Theory
The St. Matthew reindeer episode is a textbook illustration of ecological overshoot and alternative stable states. It shows how a system can be pushed past a threshold where internal feedbacks (plant regeneration) shift from negative to positive, driving the system toward a different attractor—a barren landscape incapable of supporting the former herbivore density. The concept of hysteresis—where the return path requires a different set of conditions than the departure path—is crucial for conservation practitioners assessing restoration feasibility.
Conclusion
By working through the calculations, graph annotations, and conceptual questions outlined above, students gain more than rote proficiency with exponential and logistic formulas; they develop an intuitive grasp of why real populations frequently deviate from simple models. The St. Matthew reindeer saga reminds us that carrying capacity is not a fixed ceiling but a dynamic property shaped by the very organisms that depend on it. Recognizing the delays, feedbacks, and potential for irreversible change equips learners to better interpret wildlife data, evaluate management strategies, and appreciate the fragility of ecosystems facing rapid anthropogenic change But it adds up..
