1. Resident species can facilitate invading species (biotic assistance) or inhibit their expansion (biotic resistance). Species interactions are often context-dependent and the relative importance of biotic assistance versus resistance could vary with abiotic conditions or the life stage of the invading species, as invader stress tolerances and resource requirements change with ontogeny. In northeast Florida salt marshes, the abundant dead litter (wrack) of the native marsh cordgrass, Spartina alterniflora, could influence the expansion success of the black mangrove, Avicennia germinans, a tropical species that is expanding its range northward.

2. We used two field experiments to examine how S. alterniflora wrack affects A. germinans success during 1) propagule establishment and 2) subsequent seedling survival. We also conducted laboratory feeding assays to identify propagule consumers and assess how wrack presence influences herbivory on mangrove propagules.

3. S. alterniflora wrack facilitated A. germinans establishment by promoting propagule recruitment, retention, and rooting; the tidal regime influenced the magnitude of these effects. However, over time S. alterniflora wrack inhibited A. germinans seedling success by smothering seedlings and attracting herbivore consumers. Feeding assays identified rodents – which seek refuge in wrack – as consumers of A. germinans propagules.

4. Synthesis: Our results suggest that the deleterious effects of S. alterniflora wrack on A. germinans seedling survival counterbalance the initial beneficial effects of wrack on A. germinans seed establishment. Such seed-seedling conflicts can arise when species stress tolerances and resource requirements change throughout development and vary with abiotic conditions. In concert with the tidal conditions, the relative importance of positive and negative interactions with wrack at each life stage can influence the rate of local and regional mangrove expansion. Because interaction strengths can change in direction and magnitude with ontogeny, it is essential to examine resident-invader interactions at multiple life stages and across environmental gradients to uncover the mechanisms of assistance and resistance during invasion.

This dataset includes all data necessary to re-create the figures and analysis in R.S. Smith, J.A. Blaze, and J. E. Byers. Dead litter of resident species first facilitates and then inhibits sequential life stages of range-expanding species. Journal of Ecology. 2021. These data represent measures collected during two field experiments – 1) propagule establishment experiment and 2) seedling survival experiment – and a series of laboratory invertebrate and rodent ‘no-choice’ and ‘choice’ feeding assays. All data was collected by R.S. Smith and J.A. Blaze. All data processing and analysis were performed in the statistical program R and are described in the manuscript text. Columns that contain emergent data have been defined with reference to the component columns in the associated meta-data.