Inverse priority effects: A role for historical contingency during species losses

Communities worldwide are losing multiple species at an unprecedented rate, but how communities reassemble after these losses is often an open question. It is well established that the order and timing of species arrival during community assembly shapes forthcoming community composition and function. Yet, whether the order and timing of species losses can lead to divergent community trajectories remains largely unexplored. Here, we propose a novel framework that sets testable hypotheses on the effects of the order and timing of species losses-inverse priority effects-and suggests its integration into the study of community assembly. We propose that the order and timing of species losses within a community can generate alternative reassembly trajectories, and suggest mechanisms that may underlie these inverse priority effects. To formalize these concepts quantitatively, we used a three-species Lotka-Volterra competition model, enabling to investigate conditions in which the order of species losses can lead to divergent reassembly trajectories. The inverse priority effects framework proposed here promotes the systematic study of the dynamics of species losses from ecological communities, ultimately aimed to better understand community reassembly and guide management decisions in light of rapid global change.

Post-restoration grassland management overrides the effects of restoration methods in propagule-rich landscapes

Grassland restoration is gaining momentum worldwide to tackle the loss of biodiversity and associated ecosystem services. Restoration methods and their effects on ecological community reassembly have been extensively studied across various grassland types, while the importance of post-restoration management has so far received less attention. Grassland management is an important surrogate for natural disturbances, with which most ancient grasslands have coevolved. Thus, without the reintroduction of management-related disturbance, restoration targets are unlikely to be achieved in restored grasslands. In this study, we aimed to explore how 20 yr of management by mowing once a year or light cattle grazing affects restoration success in Palearctic meadow-steppe grasslands restored by either sowing native grasses (sown sites), applying Medicago sativa as a nurse plant (Medicago sites), or allowing spontaneous succession (spontaneous sites). We found that, following mowing, sown sites maintained long-lasting establishment limitation, while Medicago sites experienced a delay in succession. These limitations resulted in low total and target species richness, low functional redundancy, and distinct species and functional composition compared to reference data from ancient grasslands. Spontaneous sites that were mowed reached a more advanced successional stage, although they did not reach reference levels regarding most vegetation descriptors. Sown and Medicago sites that were grazed had higher total and target species richness than those that were mowed, and showed restoration success similar to that of spontaneous sites, on which grazing had only moderate further positive effects. Grazed sites, irrespective of the restoration method, were uniformly species rich, functionally diverse, and functionally redundant, and thus became important biodiverse habitats with considerable resilience. We conclude that an optimally chosen post-restoration management may have an impact on long-term community reassembly comparable to the choice of restoration method. Restoration planners may, therefore, need to put more emphasis on future management than on the initial restoration method. However, our findings also imply that if local constraints, such as potentially high invasive propagule pressure, necessitate the application of restoration methods that could also hinder the establishment of target species, the long-term recovery of the grassland can still be ensured by wisely chosen post-restoration management.

Community disassembly under global change: Evidence in favor of the stress-dominance hypothesis

Ecological theory suggests that communities are not random combinations of species but rather the results of community assembly processes filtering and sorting species that are able to coexist together. To date, such processes (i.e., assembly rules) have been inferred from observed spatial patterns of biodiversity combined with null model approaches, but relatively few attempts have been made to assess how these processes may be changing through time. Specifically, in the context of the ongoing biodiversity crisis and global change, understanding how processes shaping communities may be changing and identifying the potential drivers underlying these changes become increasingly critical. Here, we used time series of 460 French freshwater fish communities and assessed both functional and phylogenetic diversity patterns to determine the relative importance of two key assembly rules (i.e., habitat filtering and limiting similarity) in shaping these communities over the last two decades. We aimed to (a) describe the temporal changes in both functional and phylogenetic diversity patterns, (b) determine to what extent temporal changes in processes inferred through the use of standardized diversity indices were congruent, and (c) test the relationships between the dynamics of assembly rules and both climatic and biotic drivers. Our results revealed that habitat filtering, although already largely predominant over limiting similarity, became more widespread over time. We also highlighted that phylogenetic and trait-based approaches offered complementary information about temporal changes in assembly rules. Finally, we found that increased environmental harshness over the study period (especially higher seasonality of temperature) led to an increase in habitat filtering and that biological invasions increased functional redundancy within communities. Overall, these findings underlie the need to develop temporal perspectives in community assembly studies, as understanding ongoing temporal changes could provide a better vision about the way communities could respond to future global changes.

A casualty of climate change? Loss of freshwater forest islands on Florida's Gulf Coast

Sea level rise elicits short- and long-term changes in coastal plant communities by altering the physical conditions that affect ecosystem processes and species distributions. While the effects of sea level rise on salt marshes and mangroves are well studied, we focus on its effects on coastal islands of freshwater forest in Florida's Big Bend region, extending a dataset initiated in 1992. In 2014-2015, we evaluated tree survival, regeneration, and understory composition in 13 previously established plots located along a tidal creek; 10 plots are on forest islands surrounded by salt marsh, and three are in continuous forest. Earlier studies found that salt stress from increased tidal flooding prevented tree regeneration in frequently flooded forest islands. Between 1992 and 2014, tidal flooding of forest islands increased by 22%-117%, corresponding with declines in tree species richness, regeneration, and survival of the dominant tree species, Sabal palmetto (cabbage palm) and Juniperus virginiana (southern red cedar). Rates of S. palmetto and J. virginiana mortality increased nonlinearly over time on the six most frequently flooded islands, while salt marsh herbs and shrubs replaced forest understory vegetation along a tidal flooding gradient. Frequencies of tidal flooding, rates of tree mortality, and understory composition in continuous forest stands remained relatively stable, but tree regeneration substantially declined. Long-term trends identified in this study demonstrate the effect of sea level rise on spatial and temporal community reassembly trajectories that are dynamically re-shaping the unique coastal landscape of the Big Bend.

Woody plant richness does not influence invertebrate community reassembly trajectories in a tree diversity experiment

Understanding the relationship between plant diversity and diversity at higher trophic levels is important from both conservation and restoration perspectives. Although there is strong evidence for bottom-up maintenance of biodiversity, this is based largely on studies of simplified grassland systems. Recently, studies in the TreeDivNet global network of tree diversity experiments have begun to test whether these findings are generalizable to more complex ecosystems, such as woodlands. We monitored invertebrate community reassembly over 5 yr of experimental woodland restoration at the TreeDivNet Ridgefield site in southwest Australia, testing the effects of woody plant species richness and herb-layer manipulation on invertebrate community structure and ant species composition. From 2010 to 2014, we sampled ground-dwelling invertebrates using pitfall traps in herbicide vs. no-herbicide subplots nested within each of 10 woody plant treatments varying in richness from zero (bare controls) to eight species, which produced a total of 211, 235 invertebrates, including 98, 979 ants belonging to 74 species. In mixed model analyses, the presence of woody plants was an important driver of faunal community reassembly (relative to bare control plots), but faunal responses to woody plant treatment combinations were idiosyncratic and unrelated to woody plant richness across treatments. We also found that a herbicide-induced reduction in herbaceous plant cover and richness had a positive effect on ant richness and caused more rapid convergence of invertebrate community composition toward the composition of a woodland reference site. These findings show that woody plant richness did not have direct positive effects on the diversity and community reassembly trajectories of higher trophic levels in our woodland system. From a management perspective, this suggests that even low-diversity restoration or carbon sequestration plantings can potentially lead to faunal reassembly outcomes that are comparable to more complex re-planting designs.