# Community Assembly and Turnover

#### Gilles Colling

#### 2026-06-14

## Introduction

A species list at a single site says little about how a community came to be. The signal lives in how composition shifts from one site to the next, and at what spatial scale that shift accelerates or stalls. Two regions can share an identical mean richness yet differ sharply in turnover: one a smooth gradient where neighbours resemble each other, the other a mosaic where adjacent plots hold almost disjoint species sets. Community assembly leaves its fingerprint in that spatial structure rather than in any single count.

Three questions organise the analysis. The first asks how fast compositional similarity fades with geographic distance. The second asks how the pool of shared species shrinks as more sites are compared at once, and whether that shrinkage is gradual or abrupt. The third asks whether the observed pattern departs from what a randomised community would produce. spacc answers each with a dedicated function: `betaDecay()` and `distanceDecay()` for the first, `zetaDiversity()` for the second, and `ses()` for the third. The functions return typed objects with `print()`, `summary()`, `plot()`, and `as.data.frame()` methods, so output can be inspected, plotted, and extracted into a data frame for downstream modelling.

The three lenses each answer a different question. Distance-decay collapses the whole study region onto a single curve of dissimilarity against separation, which makes it sensitive to the dominant turnover scale but blind to whether that turnover is driven by common or rare species. Zeta diversity keeps the multi-site structure that pairwise measures discard, so it can separate the contribution of widespread generalists from that of narrow specialists, at the cost of needing more sites to sample higher orders reliably. Null models add the inferential layer the other two lack: they convert a descriptive curve into a test against an explicit counterfactual. Reading the same dataset through all three guards against the overinterpretation that any one curve invites.

This vignette walks through the three lenses on a single simulated dataset with known structure. Simulating the data fixes the ground truth: the strength of spatial aggregation is set by one parameter, so the analyses can be checked against an answer that is known in advance. Real survey data never come with such a guarantee, which is precisely why a controlled example is worth the detour. When a method recovers the planted signal, its output on a field dataset can be trusted to mean what it claims. The closing section gives concrete guidance on permutation counts, sample sizes, the choice of zeta order, and the situations where each lens misleads rather than informs.

## The three lenses

### Distance-decay

Distance-decay summarises how compositional similarity falls off with separation. Let \(S(d)\) denote the similarity of two assemblages \(d\) units apart. The classic form is exponential,

\[ S(d) = S_0 \, e^{-d/h}, \]

where \(S_0\) is similarity at zero distance and \(h\) is the half-distance scale, the distance over which similarity drops by a factor of \(e\). A small \(h\) marks steep turnover; a large \(h\) marks a community that stays compositionally coherent across the region. Working with dissimilarity \(\beta = 1 - S\) instead, the exponential model `betaDecay()` fits is

\[ \beta(d) = 1 - a \, e^{-b d}, \]

with the half-life reported as \(d_{1/2} = \ln(2)/b\), the distance at which the similarity component \(a\,e^{-bd}\) reaches half its initial value. The package also fits a power model \(\beta = a\,d^{b}\) and a linear model \(\beta = a + b d\), then selects among the three by AIC. Power decay implies scale-free turnover with no characteristic distance; linear decay is the least structured of the three and often wins when curvature is weak.

The choice of dissimilarity index shapes what the decay measures. Sorensen dissimilarity, the default, weights shared species twice in the denominator and so emphasises the species two assemblages have in common; Jaccard treats shared and unshared species symmetrically and reads as a stricter turnover measure. For a given pair of sites Sorensen never exceeds Jaccard, so a decay curve fitted to Jaccard sits higher and saturates sooner. Neither is correct in the abstract: the question is whether the analysis cares about the overlap between assemblages (Sorensen) or the fraction of the combined species pool they fail to share (Jaccard). The half-distance changes with the index, which is one more reason to report the index alongside any decay parameter.

Two mechanisms generate distance-decay, and the curve alone cannot tell them apart. The first is environmental: sites that are far apart tend to differ in climate, soil, or habitat, so their species differ because their conditions differ. The second is dispersal limitation: even under identical conditions, species fail to reach distant sites, so composition drifts with distance through spatial processes alone. Both produce a falling \(S(d)\), and separating them requires either environmental covariates or the kind of randomisation the null models below provide. The decay curve quantifies the pattern; it does not name the cause.

### Zeta diversity

Zeta diversity counts shared species across more than two sites at once. For an order \(i\), \(\zeta_i\) is the mean number of species common to all \(i\) sites in a sampled set (Hui & McGeoch 2014). By construction \(\zeta_1\) is mean per-site richness, \(\zeta_2\) is the mean count of species shared by a pair, and \(\zeta_i\) declines monotonically as \(i\) grows. The shape of that decline carries the signal. The diagnostic is the ratio of successive orders,

\[ R_i = \frac{\zeta_i}{\zeta_{i-1}}. \]

A roughly constant ratio means \(\zeta_i\) follows a geometric (exponential) sequence, \(\zeta_i \propto c^{\,i}\), which arises when species occur independently with fixed probabilities. That is the signature of stochastic, dispersal-driven turnover. A ratio that climbs toward 1 with order means the decline is power-law, \(\zeta_i \propto i^{-\gamma}\), which a few widespread generalists produce when they persist across many sites while specialists drop out early. Power-law decline is read as evidence of niche-based, deterministic assembly. The contrast between the two regimes is the reason the ratio plot deserves as much attention as the decline curve.

The reason the ratio is more informative than the raw curve is that the absolute zeta values confound richness with turnover. A species-rich region and a species-poor region can share the same assembly mechanism yet show \(\zeta_i\) curves separated by a vertical shift, because \(\zeta_1\) alone differs. Dividing successive orders cancels that shift and isolates the rate of decline, which is the quantity assembly theory speaks to. A geometric sequence has the property that the ratio of consecutive terms is constant by definition; departures from a flat ratio are therefore departures from the independent-occurrence model, and the direction of the departure points to the mechanism.

The two regimes also differ in how they treat rarity. Under exponential decline, rare species drop out of the shared set at the same proportional rate as common ones, so no part of the species pool is privileged. Under power-law decline, a small set of widespread species dominates the high-order zeta values while the long tail of rare species contributes only to \(\zeta_1\) and \(\zeta_2\). The ecological reading follows: power-law turnover implies a structured pool with clear winners across the region, the kind of asymmetry that environmental filtering and competitive sorting produce. Sampling `knn` neighbourhoods rather than random sets sharpens the contrast, because nearest neighbours share the local environment and any niche structure surfaces at lower orders than under random sampling.

### Standardised effect sizes

Raw turnover values cannot say whether a pattern is non-random on their own, because aggregation, richness gradients, and sampling all leave marks that mimic assembly. A null model strips out the targeted structure by randomising the species matrix while holding chosen features fixed, then measures how far the observed value sits from the randomised cloud. The standardised effect size is

\[ \mathrm{SES} = \frac{\text{obs} - \overline{\text{null}}}{\mathrm{sd}_{\text{null}}}, \]

the number of null standard deviations between the observed statistic and the null mean. Under a two-tailed convention, \(|\mathrm{SES}| > 1.96\) flags a departure at roughly the 5% level when the null distribution is approximately normal. Positive SES on a richness accumulation curve means more species accrue than the null predicts (spatial overdispersion); negative SES means fewer accrue (clustering, or redundant encounters of aggregated species). `ses()` also returns a permutation p-value from the rank of the observed value among the null draws, which does not lean on the normal approximation.

The choice of null model is the substance of the test, not a technical detail. A null randomises some features of the species matrix while pinning others, and the pinned features define the alternative the test cannot detect. Holding species frequencies fixed but shuffling their placement asks whether composition is spatially structured given those frequencies. Fixing both row and column totals asks the stricter question of whether species co-occur more or less than expected once richness and frequency are both accounted for. Spatial nulls go further and preserve autocorrelation, so they test for structure beyond what proximity alone explains. Two nulls applied to the same data can disagree, and the disagreement is informative: it localises which feature of the matrix carries the signal. An SES reported without its null model is therefore an incomplete result, no more interpretable than a p-value without its test statistic.

## Simulating a structured community

The simulation places 60 sites at random in a 100-by-100 arena and gives each of 25 species a centre. Occurrence probability decays with distance from that centre, so each species occupies a patch rather than scattering uniformly. The decay scale of 30 units is the range parameter: it sets how broad each species’ patch is relative to the arena. At 30 units a species is common near its centre and rare beyond about twice that distance, which builds genuine spatial turnover into the data without making patches so tight that sites stop sharing any species.

    library(spacc)
    set.seed(123)

    n_sites <- 60
    coords <- data.frame(
      x = runif(n_sites, 0, 100),
      y = runif(n_sites, 0, 100)
    )

Each column of the species matrix is drawn from a Poisson whose mean falls off exponentially with distance from the species centre. The factor of 3 scales the peak abundance near a centre, keeping per-site richness in a realistic range while the range parameter governs the spatial footprint.

    n_species <- 25
    species <- matrix(0, nrow = n_sites, ncol = n_species)
    centers_x <- runif(n_species, 0, 100)
    centers_y <- runif(n_species, 0, 100)

    for (sp in seq_len(n_species)) {
      dists <- sqrt((coords$x - centers_x[sp])^2 + (coords$y - centers_y[sp])^2)
      prob <- exp(-dists / 30)
      species[, sp] <- rpois(n_sites, lambda = prob * 3)
    }
    colnames(species) <- paste0("sp", seq_len(n_species))

The accumulation curve below is reused by `ses()`, which needs an existing `spacc` object to know which analysis to re-run on each randomised matrix. Ten seeds give a stable mean curve while keeping the later permutation loops fast.

    sac <- spacc(species, coords, n_seeds = 10, progress = FALSE)

The ground truth is now fixed. Patches span roughly 30 to 60 units, so turnover should be moderate, the zeta decline should track the geometric signature of independent-ish occurrences, and richness accumulation should fall below a co-occurrence null because aggregated species are encountered redundantly. The three lenses are checked against those expectations in turn.

The range parameter deserves a closer look, because it is the single knob that controls how much spatial signal the data carry. A very small range, say 5 units, would shrink each species to a tight cluster of sites; neighbouring plots would then share almost nothing, distance-decay would be steep, and zeta would collapse to zero by order 3. A very large range, say 200 units, would spread every species across the whole arena; sites would share most of their species regardless of separation, distance-decay would be nearly flat, and zeta would decline slowly. At 30 units the simulation sits between these extremes, which is the regime where all three lenses have something to measure. The same reasoning applies to field data: a lens is only as informative as the turnover present in the sampling design, and a design that samples too coarsely or too finely relative to the real patch scale will flatten the signal the lens is built to detect.

## Distance-decay

`betaDecay()` computes Sorensen dissimilarity for every site pair, regresses it on geographic distance, and fits the exponential, power, and linear models before choosing among them by AIC. The plot overlays the raw pairwise cloud, binned means in orange, and the selected model in red.

    decay <- betaDecay(species, coords, progress = FALSE)
    plot(decay) + transparent

The printed summary lists every fitted model with its coefficients and AIC, names the AIC winner, and reports the exponential half-life. On this dataset the linear model edges out the others by AIC, which signals that curvature in the decay is weak over the observed distance range.

    summary(decay)
    #> $n_sites
    #> [1] 60
    #>
    #> $n_pairs
    #> [1] 1770
    #>
    #> $distance_range
    #> [1]   2.133552 122.675695
    #>
    #> $dissimilarity_range
    #> [1] 0.1351351 1.0000000
    #>
    #> $mean_dissimilarity
    #> [1] 0.5569438
    #>
    #> $coefficients
    #>                    model         a           b        AIC
    #> a            exponential 0.6639603 0.008131670 -1925.4191
    #> (Intercept)        power 0.2005184 0.256130232   481.6219
    #> (Intercept)1      linear 0.3656503 0.003674776 -1943.9352
    #>
    #> $best_model
    #> [1] "linear"
    #>
    #> $half_life
    #> [1] 85.24045
    #>
    #> $index
    #> [1] "sorensen"

The coefficients are easiest to read through the `coef()` method, which returns the slope and intercept of whichever model AIC selected. Here the linear slope is about \(0.0037\) dissimilarity units per distance unit, so two sites at opposite corners of the arena differ in roughly a third more of their species than two neighbours.

    coef(decay)
    #>           a           b
    #> 0.365650265 0.003674776
    decay$half_life
    #> [1] 85.24045

The half-life of about 85 units comes from the exponential fit even when the linear model wins overall. It says that the similarity component would need roughly 85 distance units to halve, which is large relative to the 30-unit patch scale because Sorensen dissimilarity saturates slowly when patches overlap. The half-distance and the AIC ranking answer different questions: the first gives a scale, the second gives a shape. Reporting both, rather than forcing one model, is the honest summary when curvature is shallow.

A second, complementary view comes from `distanceDecay()`, which is a different operation despite the shared name. Where `betaDecay()` works on the dissimilarity between site pairs, `distanceDecay()` drops focal points into the arena and counts how cumulative richness grows as the radius around each focal point expands. The return object is `spacc_decay`, not `spacc_beta_decay`, and the y-axis is species count rather than dissimilarity.

    dd <- distanceDecay(species, coords, n_seeds = 30, progress = FALSE, seed = 1)
    plot(dd) + transparent

Cumulative richness climbs from about 12 species within the smallest radius to the full pool of 25 once the radius spans the arena. The steepness near the origin reflects how quickly new species appear as the neighbourhood grows, which is the species-area face of the same turnover that `betaDecay()` reads through pairwise dissimilarity. The two functions agree when turnover is real: a steep accumulation near focal points corresponds to high dissimilarity between near neighbours.

The complementarity is worth stating plainly. `betaDecay()` answers a comparative question, how different are two assemblages as a function of their separation, and its output is a dissimilarity scale. `distanceDecay()` answers a cumulative question, how many species does a growing neighbourhood contain, and its output is a richness curve with a confidence band from the focal-point seeds. A region with strong turnover shows both a steep `betaDecay()` slope and a steep `distanceDecay()` rise; a region with weak turnover shows a shallow slope and a curve that reaches the species pool only at large radii. Reporting both gives a reader the rate of compositional change and the rate of richness accrual from the same data, which are distinct facets of the same spatial process.

## Zeta diversity

`zetaDiversity()` samples sets of \(k\) sites and counts species shared across all \(k\), sweeping \(k\) from 1 to 10. The default `knn` method draws each set as a focal site plus its nearest neighbours, which targets spatial turnover directly; the `random` method would instead give a non-spatial baseline. Fifty samples per order keep the standard deviations tight enough to read the trend.

    zd <- zetaDiversity(species, coords, orders = 1:10, n_samples = 50,
                        progress = FALSE, seed = 1)
    plot(zd) + transparent

The decline curve starts near \(\zeta_1 \approx 12\) shared-by-one species (mean per-site richness) and falls steeply: by order 5 fewer than two species are shared across all five neighbours, and the count approaches zero by order 10. The shaded band is one standard deviation across the sampled sets and widens at low orders where the per-set counts vary most.

The ratio plot is the diagnostic that separates the two assembly regimes. A flat ratio points to exponential, stochastic decline; a ratio rising toward 1 points to power-law, niche-driven decline carried by a few widespread species.

    plot(zd, type = "ratio") + transparent

On this dataset the ratios stay in a band around 0.5 to 0.8 without a clear upward march, and the printed object confirms the exponential model fits better than the power-law by AIC (\(R^2 \approx 0.97\) versus \(0.91\)). That matches the simulation: species were placed independently with patchy but unstructured occurrences, so the turnover is closer to stochastic than niche-partitioned. The numbers are extracted with `as.data.frame()`, which returns one row per order with the zeta value, its standard deviation, and the ratio.

    zeta_tab <- as.data.frame(zd)
    head(zeta_tab)
    #>   order  zeta       sd     ratio
    #> 1     1 11.60 3.862905        NA
    #> 2     2  6.38 3.415989 0.5500000
    #> 3     3  4.68 2.743750 0.7335423
    #> 4     4  4.00 2.294625 0.8547009
    #> 5     5  1.82 1.624933 0.4550000
    #> 6     6  1.16 1.267490 0.6373626

The interpretation hinges on the ratio column rather than the raw zeta values. A community engineered with a few dominant generalists and many narrow specialists would push those ratios upward with order, because the generalists keep being shared while the specialists vanish; the absence of that climb here is the evidence for stochastic turnover, not the steepness of the decline alone.

## SES null models

`ses()` takes the existing `spacc` object, randomises the species matrix under a chosen null model, re-runs the accumulation, and repeats. The `curveball` algorithm holds both row totals (site richness) and column totals (species frequency) fixed while shuffling the co-occurrence structure, so it tests whether the spatial arrangement of species, not their marginal abundances, drives the observed pattern. Nineteen permutations is a deliberately small count for a fast illustration; the guidance section explains why real tests need many more.

    result <- ses(sac, species, coords,
                  null_model = "curveball", n_perm = 19,
                  progress = FALSE, seed = 1)
    print(result)
    #> Standardized Effect Size (SES)
    #> ------------------------------------
    #> Input class: spacc
    #> Metric: richness
    #> Null model: curveball (19 permutations)
    #> SES range: [-7.59, 1.13]
    #> Mean SES: -0.48
    #> Significant (p < 0.05): 0 / 60 values (0%)

The SES curve traces the standardised effect size along the accumulation steps. The dashed lines mark the \(\pm 1.96\) thresholds and the solid line marks zero; points where the permutation p-value falls below 0.05 are highlighted in red.

    plot(result, type = "curve") + transparent

The curve drops increasingly negative as more sites enter the accumulation, from near zero at the first step to strongly negative deeper into the curve. Negative SES means observed richness accumulates more slowly than the curveball null predicts. The mechanism is spatial aggregation: when a species occupies a compact patch, the nearby sites that the accumulation visits early carry redundant copies of it, so fewer genuinely new species appear than a co-occurrence-shuffled community would deliver. The effect compounds with each step, which is why the deficit deepens rather than holding flat.

The histogram summarises the SES values across all accumulation steps in one view, with the \(\pm 1.96\) reference lines marking the conventional significance band.

    plot(result, type = "histogram") + transparent

The bulk of the distribution sits at or below zero, and a substantial tail reaches past \(-1.96\), the visual counterpart of the clustering signal. The permutation p-values should be read alongside the SES magnitudes: with only 19 permutations the finest achievable two-tailed p-value is \(2/20 = 0.1\), so no step can reach \(p < 0.05\) no matter how extreme the SES. That ceiling is an artefact of the permutation count, not evidence of a weak pattern, and it is exactly the failure mode the guidance section warns against.

## Comparing null models

The conclusion from a null-model test can hinge on what the null holds fixed. Running `ses()` under a second algorithm makes that dependence explicit. The `frequency` null shuffles each species column independently, preserving species frequencies but breaking site richness and all co-occurrence structure; it asks whether composition matters given the observed frequencies. The `curveball` null preserves both marginals and is the stricter test of co-occurrence.

    res_freq <- ses(sac, species, coords,
                    null_model = "frequency", n_perm = 19,
                    progress = FALSE, seed = 1)
    c(curveball = mean(result$ses), frequency = mean(res_freq$ses))
    #>  curveball  frequency
    #> -0.4835156 -0.6467084

The `frequency` null yields a more strongly negative mean SES than `curveball`. The reason is that `frequency` randomises away more structure, including the site richness totals, so its null cloud sits further from the spatially aggregated observation. The stricter `curveball` null retains the marginals and therefore gives a more conservative, smaller-magnitude effect. A pattern that survives the `curveball` test is the more defensible claim, because fewer alternative explanations remain available to the null. The contrast is a reminder that an SES value is meaningful only in reference to a named null model.

These three lenses answer complementary questions, summarised below. Distance-decay gives a scale and a shape for pairwise turnover; zeta diversity distinguishes stochastic from niche-driven multi-site turnover; SES tests whether either pattern exceeds a randomised baseline.

Approach | Question | Key output | Reads turnover via
--- | --- | --- | ---
Distance-decay | How fast does similarity decline with distance? | Decay rate, half-distance, AIC-best model | Pairwise dissimilarity vs distance
Zeta diversity | Stochastic or niche-driven multi-site turnover? | Zeta decline, ratio, exp/power fit | Species shared across \(k\) sites
SES null models | Is the pattern non-random? | SES, p-value, null model | Observed vs randomised accumulation

Used together they cross-check one another. Steep distance-decay, exponential zeta decline, and negative richness SES tell one coherent story on this dataset: moderate, stochastic turnover driven by spatial aggregation rather than niche partitioning.

## Practical guidance

Each lens has a sample-size floor below which its output is noise dressed as signal, and each has a regime where it is the wrong tool entirely. The rules below use specific numbers tied to the functions in this vignette.

Permutation count for SES. The smallest two-tailed p-value a permutation test can return is \(2/(n_{\text{perm}} + 1)\). To resolve \(p < 0.05\) at all, use at least 199 permutations; with 19 the floor is \(p = 0.1\) and nothing can clear the 0.05 bar, exactly as seen above. For 99 permutations the floor is \(0.02\), which resolves 0.05 but leaves p-values coarse. Use 999 for any result that will be reported, and treat anything under 199 as a plumbing check rather than a test.

Sample count for zeta. The `n_samples` argument sets how many \(k\)-site sets are drawn per order. With 50 samples the standard deviations at low orders are wide; 500 to 1000 samples tighten the decline curve enough that the exponential-versus- power AIC comparison is stable. Below roughly 100 samples the ratio plot jitters and can fake an upward trend that vanishes with more sampling, so do not read assembly mode off a thinly sampled ratio.

Maximum zeta order. Set the top order no higher than the point where \(\zeta_i\) is still comfortably above zero; once shared counts hit zero the ratio is undefined and the fit gains nothing. A practical ceiling is the order at which \(\zeta_i\) drops below about 1, which on this dataset is near order 6 to 8. Pushing to order 15 on 60 sites mostly adds zeros and noise.

Minimum sites and site-pairs. Distance-decay needs enough pairs to bin: with \(n\) sites there are \(n(n-1)/2\) pairs, so 30 sites give 435 pairs, a reasonable floor, and below about 20 sites (190 pairs) the binned means become unstable. Zeta of order \(k\) needs many more than \(k\) sites to sample distinct sets; aim for at least \(5k\) sites at the top order. SES on an accumulation curve needs enough sites that the curve has shape, which is again roughly 20 or more.

The decision table below maps a question to the lens that answers it and flags the common failure.

If the goal is to … | Use | Watch out for
--- | --- | ---
Find a turnover distance scale | `betaDecay()` half-distance | Shallow curvature: linear may win AIC, report half-life anyway
Distinguish stochastic vs niche turnover | `zetaDiversity()` ratio | Too few `n_samples` fakes an upward ratio
Test richness against co-occurrence | `ses(null_model = "curveball")` | Too few `n_perm` caps the p-value
Test composition given frequencies | `ses(null_model = "frequency")` | Breaks richness totals, less conservative
See richness grow with neighbourhood | `distanceDecay()` | Not a dissimilarity measure despite the name

When not to use each lens. Distance-decay is uninformative when sites span a narrow distance range, because the model is then extrapolating a slope from a short arm of the curve; if the maximum pairwise distance is only a few times the minimum, skip it. Zeta diversity is the wrong tool when richness is so low that \(\zeta_2\) is already near zero, since every higher order is then a string of zeros with no shape to fit. SES null models should not be the headline when the marginal totals themselves are the pattern of interest: a null that holds richness fixed cannot detect a richness gradient, so a `curveball` SES near zero does not rule out structure that lives in the marginals a frequency or richness null would expose. Match the null to the hypothesis, and never read a single SES value without naming the model that produced it.

## References

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- Nekola, J.C. & White, P.S. (1999). The distance decay of similarity in biogeography and ecology. Journal of Biogeography, 26, 867-878.
- Soininen, J., McDonald, R. & Hillebrand, H. (2007). The distance decay of similarity in ecological communities. Ecography, 30, 3-12.
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- Strona, G., Nappo, D., Boccacci, F., Fattorini, S. & San-Miguel-Ayanz, J. (2014). A fast and unbiased procedure to randomize ecological binary matrices with fixed row and column totals. Nature Communications, 5, 4114. ```