---
title: what is a spillover effect in nature
canonical_url: https://ensurance.app/guide/what-is-a-spillover-effect-in-nature
markdown_url: https://ensurance.app/guide/what-is-a-spillover-effect-in-nature.md
subtitle: the stock stays inside the line. the flow does not
category: natural-capital
---

# what is a spillover effect in nature

*the stock stays inside the line. the flow does not*

A **spillover effect** in nature is a benefit that leaves the piece of land or water that produced it and lands on someone else's. Fish bred inside a reserve are caught outside it. Pollinators that nest in a hedgerow work the field beside it. Shade from a park cools the block around it. The forest that holds a slope keeps the sediment out of an intake miles below. The thing that made the benefit stays where it is. The benefit crosses the line. Ecologists mean this sense, not the disease sense of a virus jumping species.

That line is the point. A natural asset is a **stock**: a reef, a stand of forest, a line of trees, a park, a wetland, with a boundary someone drew. Its **flows**, the fish, the pollinators, the shade, the clean water, do not stop at the boundary. They exist whether or not anyone pays for them. [**Ensurance**](/?from=guide) is how the people on the far side of the line can pay the stock. It is not the stock.

:::johnson
**the stock stays inside the line. the flow does not.** — Four literatures have measured that crossing for fifty years. The source is rarely the one that gets paid.

[see the flows a stock emits →](/natural-capital/flows?from=guide)
:::

## stocks, flows, and the line between them

The framework behind this guide uses two words for natural capital. A stock is what accumulates: an ecosystem, with an extent and a condition. A flow is what circulates: the service that ecosystem produces each year. [The manual](/manual/natural-capital) puts it in one sentence: natural capital is in, under, on, and flows across natural assets.

"Flows across" is spillover. The stock has a boundary because someone drew one: a property line, a reserve edge, a park fence, a harvest unit. The flow was never asked. Water, pollinators, predators, shade, seeds, fuel moisture, and genes cross the line at distances that depend on what is moving, from tens of meters for shade to a whole watershed for water. [Does protecting one parcel help the land around it](/guide/does-protecting-one-parcel-help-the-land-around-it?from=guide) carries the evidence that they do, and the seven mechanisms with their reach. This page does not repeat it.

The manual's own example shows the scale of what leaves. A 100-acre wetland in the manual carries a stock value of $294,250, the cost of the land, and flows valued at $1,449,703 a year, a natural cap rate of 493%. Much of that value does not stay on the hundred acres. The flood storage lands on the town below. The clean water lands at the intake. The habitat lands on every species that passes through.

## five names for one crossing

Four literatures have been measuring this crossing for about fifty years, each in its own vocabulary, and sustainability accounting has since given it a fifth name. All five describe the same event.

| the literature | its word | what it measures |
|---|---|---|
| Ecology | Spillover effect | Organisms and conditions crossing a boundary; the reserve effect inside, what the press calls the halo outside |
| Economics | Positive spatial externality | A benefit produced here and received there, unpriced |
| Real estate | Capitalization effect | The benefit showing up in the neighbor's price; hedonic pricing recovers it |
| Risk modeling | Avoided damages | The loss that did not happen because the stock was there |
| Sustainability accounting | Ecosystem service flow | The annual service a stock produces, wherever it lands |

The risk modeler's version is the easiest to feel. Coastal wetlands in Hurricane Sandy's path kept an estimated $625 million of damage from happening. The Otter Creek wetlands above Middlebury, Vermont, cut flood damage to the town by 54 to 78 percent. Neither wetland sent a bill. The Sandy figure is a modeled estimate for one storm; averaged over the storms to come, avoided loss becomes a yearly flow. It landed on the buildings.

Inside each vocabulary are sharper terms worth knowing. In marine biology, **ecological spillover** is fish moving out of a reserve and **fishery spillover** is the catch that rises outside it. In agroforestry, **overyielding** means a mixed system out-produces the same land in monoculture. In real estate, the **proximity premium** is what a parcel near a park sells for over an otherwise similar one that is not. In conservation, **leakage** is the inverse: harm pushed across the line instead of benefit.

## three shapes the crossing takes

Not every spillover has the same geometry, and the shape decides what kind of instrument can ever pay for it.

| shape | where value is made | where it is captured | closest wrapper today |
|---|---|---|---|
| Spatial, at the same time | Here, now | Next door, now | Land-value taxes, special assessment districts; hedonic prices show it in the neighbor's sale |
| Temporal, delayed | Now | Later, by someone else | Endowments, perpetual easements; permanence is the multiplier |
| Network, emergent | Across many nodes at once | Diffusely, by a whole system | No clean instrument. The flows have to be bundled, not sold one at a time |

The first shape is already priced, implicitly, every time a house near a park clears for more. The second is priceable with a wrapper durable enough to outlast the people who signed it. The third is why [one stock, many flows](/guide/one-stock-many-flows?from=guide) argues against stacking: take the network apart to price its pieces and the thing being priced stops existing. [The web is the asset](/guide/the-web-is-the-asset?from=guide) makes the same point from the ecology side.

## four cases, read from the stock

The rest of this series takes one stock at a time and follows the flow across the line.

| stock | the flow that leaves | who is on the far side | the post |
|---|---|---|---|
| A marine reserve | Fish, and the catch they become | The boats, the ports, the town | [do marine protected areas help fisheries](/guide/do-marine-protected-areas-help-fisheries?from=guide) |
| The trees left standing in a harvest | Predators, pollinators, fungal networks, shade, water, fuel moisture | The next rotation, the mill, the intake below | [what retention forestry actually does](/guide/what-retention-forestry-actually-does?from=guide) |
| A windbreak or hedgerow | Shelter, pollinators, pest control, held soil | The field beside it, and the creek below | [what agroforestry does for the field beside it](/guide/what-agroforestry-does-for-the-field-beside-it?from=guide) |
| A city park and its canopy | Cooling, air, the view, a place to be | Every parcel within half a mile, and the tax roll | [do parks increase property values](/guide/do-parks-increase-property-values?from=guide) |

In every case the stock stays put and the flow lands somewhere it can be counted: a catch record, a yield, a sale price, an assessment. The last post, [what a positive externality is](/guide/what-a-positive-externality-is?from=guide), asks why the counting always happens on the far side of the line, and what a wrapper that reaches back to the source would have to look like.

## why the source cannot see it

The stock's own accounts show none of this. The reserve has no catch record. The windbreak has no yield. The park has no sale price. [The premium went next door](/guide/the-premium-went-next-door?from=guide) explains how markets capitalize these flows into someone else's asset, and [what is a keystone parcel](/guide/what-is-a-keystone-parcel?from=guide) names the stocks that carry the most for others. This page only adds the frame: the price shows up where the flow arrives, not where it started.

We did not discover spillover. The four literatures did, and their methods are mature. Some wrappers already reach back to the source in places: perpetual easements, endowments, water funds. What is still rare is a standing unit on the source itself: an account addressed to the stock, that the far side of the line can pay. That is the series' last question, and the one this protocol is built to answer.

## frequently asked questions

### what is a spillover effect?

A spillover effect is a benefit produced inside one boundary and received outside it. Harm pushed across the line is the reverse, a negative spillover, also called leakage. In nature it is the fish, pollinators, shade, clean water, held soil, or avoided damage that a stock produces and that lands on the land, water, and people around it.

### is spillover the same as an externality?

Mostly. A positive spillover is a positive spatial externality: a benefit produced here and received there without a price. A benefit absorbed into the neighbor's sale price is priced for the neighbor and still external to the source. [External to what](/guide/external-to-what?from=guide) follows the word back to where it came from.

### what is the difference between a stock and a flow?

A stock is the ecosystem itself, what accumulates: a wetland, a forest, a reef. A flow is the service the stock produces each year, what circulates: clean water, pollination, cooling, flood storage. The natural cap rate is the yearly flows value divided by the stock's cost.

### is a spillover always good?

No. Harm crosses lines too: a pest reservoir, an invasive seed source, clearing pushed onto the next parcel. Conservation calls displaced harm leakage. [Does protecting one parcel help the land around it](/guide/does-protecting-one-parcel-help-the-land-around-it?from=guide) shows how often each happens and how far each mechanism reaches.

## next

The first case starts at sea: [do marine protected areas help fisheries](/guide/do-marine-protected-areas-help-fisheries?from=guide).

If your operations sit on the far side of one of these lines, [corporations](/solutions/corporations?from=guide&topic=flows-across) shows how we map and value the services a business depends on. If you can already name the stock your business stands on, [tell us which](/contact?from=guide&topic=flows-across).

## sources

[Natural capital, the ensurance manual](/manual/natural-capital) — stocks, flows, the natural cap rate, and the 100-acre wetland example.

[Narayan, S. et al. 2017. The value of coastal wetlands for flood damage reduction in the northeastern USA. *Scientific Reports*.](https://doi.org/10.1038/s41598-017-09269-z) — An estimated $625 million in Sandy damages avoided.

Watson, K.B. et al. 2016. Quantifying flood mitigation services: the economic value of Otter Creek wetlands and floodplains to Middlebury, VT. *Ecological Economics* 130, 16–24. — Flood damage reduced 54–78%.

## the series

1. [what is a spillover effect in nature](/guide/what-is-a-spillover-effect-in-nature?from=guide)
2. [do marine protected areas help fisheries](/guide/do-marine-protected-areas-help-fisheries?from=guide)
3. [what retention forestry actually does](/guide/what-retention-forestry-actually-does?from=guide)
4. [what agroforestry does for the field beside it](/guide/what-agroforestry-does-for-the-field-beside-it?from=guide)
5. [do parks increase property values](/guide/do-parks-increase-property-values?from=guide)
6. [what a positive externality is](/guide/what-a-positive-externality-is?from=guide)