The spread of invasive plants is mediated by interactions with resident communities. Characterizing the interactions of multiple, co-occurring invasive species with one another and with native species gives insight on how these interactions could drive invasive species’ spread. Specifically, invasive species may spread because they are better competitors than native species, or they may be facilitated by other invasive or native species. We constructed networks of plant species linked by competitive and facilitative interactions. Plant interactions have typically been studied in isolated pairwise relationships, and structuring these interactions in networks allows for community-level relationships to be quantified, such as the effect of a species on the community. Networks were constructed using interaction weights from a field experiment of an invaded old-field plant community in the Yale-Myers Forest in Connecticut, United States. We measured interactions among seven (three invasive) locally abundant woody species at multiple life stages, both between seedling pairs and between adults and seedlings. Interactions were measured with an index that uses the biomass of plants with and without neighbors. We also characterized functional traits of the study species, including specific leaf area, leaf N, and leaf C:N ratio, to look for relationships with species interactions.
Results/Conclusions
A majority of the interactions in the community were competitive, particularly in the seedling-seedling network. Facilitative interactions were also present, including facilitation between invasive species. At the community level, however, invasive species were not facilitated by either other invasive species or resident native species (community-level effects were measured as node strengths). Invasive species did not have stronger competitive effects on the community relative to native species in both networks. In general, variation in community-level interactions was attributable to species-level differences rather than to differences between invasive and native species, e.g., while nitrogen fixing invasive Elaeagnus was strongly facilitated by adults, invasive Rosa experienced competition from adults. Additionally, both networks were intransitive and did not follow strict dominance hierarchy, a structural feature that could promote coexistence in this community. Functional traits were not related to species interactions; and while there were differences in functional traits between native and invasive species, these were attributable to extreme traits in Elaeagnus. Among these woody invasive species, invasive success is likely not attributable to similar community-level competitive or facilitative effects or a shared suite of functional traits, and may instead depend on the interactions and traits unique to each species.