Temporal stability and spatial patterns of genetic diversity in populations of the climate-vulnerable fucoid Scytothalia dorycarpa.

Climate change is driving the loss of genetic diversity, potentially limiting species' capacity to adapt to environmental change. Detecting changes in genetic diversity requires replicated temporal data, which is lacking for most species. Here, we combined contemporary and historical specimens of the climate-vulnerable fucoid Scytothalia dorycarpa to assess genetic diversity across ~2700 km of its geographic range. We analyzed four conserved organellar markers (rbcL, CO1, cox3, and trnW-1) using newly collected specimens and herbarium material to reconstruct past diversity. Scytothalia dorycarpa is endemic to Australia and has experienced climate-mediated declines over recent decades. We found stability in haplotype diversity over the 16-year sampling period, as well as across additional historical herbarium collections (1800s, 1883, 1960). We identified several common contemporary and historical haplotypes across the sampled range, but diversity patterns varied between markers. Nonetheless, consistent trends emerged for certain populations, with high, unique haplotype diversity consistently present across all markers and timepoints in the Cape Naturaliste-Leeuwin region (Western Australia). Notably, both contemporary and historical (now extinct) warm-edge populations had unique haplotypes that were absent elsewhere in the sampled range. These results demonstrate strong temporal stability in S. dorycarpa genetic diversity, with limited haplotype turnover, highlighting the resilience of sampled populations. The presence of unique haplotypes in specific populations underscores their role as reservoirs of evolutionary potential. By documenting long-term stability alongside localized diversity losses, this study provides a critical baseline for understanding the processes shaping genetic variation in S. dorycarpa and predicting its responses to future climate change.