The ecological and evolutionary significance of effectiveness landscapes in mutualistic interactions

Acorn dispersal effectiveness after 27 years of passive and active restoration in a Neotropical cloud forest

Ecological restoration can mitigate the negative effects of loss and degradation of natural ecosystems. However, the efficacy of restoration strategies (active vs. passive) in recovering crucial ecological interactions such as animal seed dispersal remains largely unknown. We evaluated the seed dispersal effectiveness of Quercus insignis, an endangered large-seeded oak of the cloud forest in a landscape in which passive (natural regeneration) and active (mixed plantations) restoration practices were implemented 27 years earlier. We compared acorn forager identity, acorn removal, seed fate (burial, post-dispersal predation), and dispersal distances of Quercus insignis under four vegetation conditions: cattle-grazed pasture with isolated trees (CP), 27-year-old forest under passive restoration (PR), 27-year-old forest under active restoration (AR), and mature forest (MF). We calculated seed dispersal effectiveness (i.e. the number of dispersed seeds that go on to establish as seedlings) for each vegetation condition and identified the main dispersal agents. We also examined the potential influence of masting and seed size on seed dispersal. We found that both restored forests presented similar acorn removal percentages and foragers, with Sciurus aureogaster as the main effective disperser. PR exhibited greater acorn burial and seedling establishment, and thus higher effective seed dispersal, than AR. Dispersal distance was similar in both restored forests. Masting modulated acorn dispersal patterns; in the non-mast year, PR, AR, and MF reached similar values but in the mast year, both restoration strategies had higher acorn removal and post-dispersal predation than those observed in CP and MF. Acorn removal and dispersal distance increased with acorn size. Our results suggest that passive restoration allows for higher effective acorn dispersal than active restoration, especially in the mast year. Nevertheless, natural acorn availability and seed size should be considered in restoration projects as they modulate the recovery and effectiveness of the seed dispersal process.

What drives seed dispersal effectiveness?

Seed dispersal is a critical phase in plant reproduction and forest regeneration. In many systems, the vast majority of woody species rely on seed dispersal by fruit-eating animals. Animals differ in their size, movement patterns, seed handling, gut physiology, and many other factors that affect the number of seeds they disperse, the quality of treatment each individual seed receives, and consequently their relative contribution to plant fitness. The seed dispersal effectiveness framework (SDE) was developed to allow systematic and standardized quantification of these processes, offering a potential for understanding the large-scale dynamics of animal-plant interactions and the ecological and evolutionary consequences of animal behavior for plant reproductive success. Yet, despite its wide acceptance, the SDE framework has primarily been employed descriptively, almost always in the context of local systems. As such, the drivers of variation in SDE across systems and the relationship between its components remain unknown. We systematically searched studies that quantified endozoochorous SDE for multiple animal species dispersing one or more plant species in a given system and offered an integrative examination of the factors driving variation in SDE. Specifically, we addressed three main questions: (a) Is there a tradeoff between high dispersal quality and quantity? (b) Does animal body mass affect SDE or its main components? and (c) What drives more variation in SDE, seed dispersal quality, or quantity? We found that: (a) the relationship between quality and quantity is mediated by body size; (b) this is the result of differential relationships between body mass and the two components, while total SDE is unaffected by body mass; (c)neither quality nor quantity explain more variance in SDE globally. Our results also highlight the need for more standardized data to assess large-scale patterns in SDE.

Common seed dispersers contribute most to the persistence of a fleshy-fruited tree

Mutualistic interactions are by definition beneficial for each contributing partner. However, it is insufficiently understood how mutualistic interactions influence partners throughout their lives. Here, we used animal species-explicit, microhabitat-structured integral projection models to quantify the effect of seed dispersal by 20 animal species on the full life cycle of the tree Frangula alnus in Białowieża Forest, Eastern Poland. Our analysis showed that animal seed dispersal increased population growth by 2.5%. The effectiveness of animals as seed dispersers was strongly related to the interaction frequency but not the quality of seed dispersal. Consequently, the projected population decline due to simulated species extinction was driven by the loss of common rather than rare mutualist species. Our results support the notion that frequently interacting mutualists contribute most to the persistence of the populations of their partners, underscoring the role of common species for ecosystem functioning and nature conservation.