Berkeley's Introduced-Plant Signal Peaks in the Urban Grid, Not Along a Simple Coast-to-Hills Gradient
Urban floras are often described as lists, but lists do not show how urban structure partitions native and introduced plant signals. I analyzed 12,137 research-grade iNaturalist plant observations from Berkeley, California, spanning 669 observed taxa, and compared taxa flagged as introduced by iNaturalist across four fixed west-to-east spatial strata: west flatlands, central urban grid, campus/creek corridor, and hill/canyon edge. Equal-effort resampling used the smallest stratum size, 102 observations, with 1,000 bootstrap draws per stratum. The strongest pattern was not a smooth urban-to-wildland gradient. The central urban grid had the highest introduced observation share (0.782; 95% bootstrap interval 0.696-0.853), while the hill/canyon edge had the lowest share (0.451) but was sparsely sampled. The campus/creek corridor was intermediate (0.639; 0.549-0.726), close to the west flatlands (0.616; 0.520-0.706), despite Berkeley's documented Strawberry Creek restoration history. These results suggest that Berkeley's plant records preserve a clear introduced-species core in the urban grid, but the available citizen-science data do not yet support a strong claim that the campus creek corridor has separated from the broader urban introduced-plant signal.
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This manuscript uses a sizeable iNaturalist dataset (12,137 research-grade plant observations; 669 taxa) to ask a clear, tractable question: whether “introduced” signal changes monotonically from Berkeley’s western flatlands toward the hills, or instead peaks in an intermediate urban core. The stratified comparison is straightforward and the main descriptive result is consistent across the reported rarefaction/bootstrapping: the central urban grid shows the highest introduced-observation share (~0.78 with a reasonably tight bootstrap interval), while the campus/creek corridor and west flatlands are similar (~0.62–0.64). The conclusion that the data support an “introduced core” in the urban grid (and not a simple coast-to-hills gradient) is broadly justified by the reported effect sizes. The main weakness is that inference is constrained by strong sampling imbalance and by treating iNaturalist “introduced” flags and opportunistic observations as if they were comparable ecological samples across strata. The hill/canyon edge stratum has only 102 observations total, forcing all strata to be rarefied down to 102 observations; as a result, the hill estimate has no resampling variability (the CI is degenerate) and the study’s effective sample size is dramatically reduced relative to what is available in the other zones. Additionally, “introduced observation share” is sensitive to observer behavior (where people go, what they photograph), horticultural plantings, and differential detectability/identification—factors likely structured by the same urban grid features under study. Given these limitations, the more cautious interpretation—patterns in records rather than in underlying abundance—should be emphasized, and stronger causal language about restoration or ecological separation should be avoided unless validated with independent survey data or modeling that accounts for effort and bias.