Although invasive species are a leading cause of extinction in modern ecosystems, our understanding of long-term effects of biotic invasions remains limited. Ecosystem functioning in response to changes in biodiversity and relative abundance are poorly understood. Yet to better predict ecosystem response and effectively manage resources, we must first understand how and why ecosystems respond to invasions. Do invasive species trigger ecosystem change, or do they simply take advantage of changing conditions, resulting from biotic or abiotic factors? Are there factors that make communities more susceptible to invasion, and can we identify factors that allow invasions to succeed, or cause them to fail? Are there characteristics of ecosystem structure that promote resistance to invaders, such as complexity or stability? The fossil record contains intervals of dramatic ecosystem changes, documented on evolutionary timescales that can be used to understand why and under what conditions past ecosystems have changed. To examine the effects of invasions on ecosystem structure, food webs were reconstructed from Late Ordovician marine ecosystems (~450 million years ago) before and after the Richmondian Invasion, a well-documented influx of invasive species. The invasion is thought to have resulted in community reorganization due to increasing biodiversity.
Results/Conclusions
Despite an increase in species richness, invasion may have led to a decrease in functional richness (number of guilds), thus a regional increase in biodiversity did not equate to an increase in the types of interactions between organisms. Connectance also increased post-invasion from 0.097 to 0.108, which may be correlated to the increased proportion of omnivores in the ecosystem from 48% to 53%. Furthermore, we observed a modest reduction in the number of trophic levels, modularity, link density, and average path length after invasion. Shorter trophic chain lengths could allow negative effects to spread rapidly through the network, indicating lowered resilience and stability after invasion, although additional quantitative tests are needed to assess stability and resilience. Our current understanding of community structural change is based largely on species counts and differences in relative abundance. However, these variables cannot capture changes in ecosystem complexity, trophic energy flow, or resilience. Using both food webs and data from the fossil record offers an alternative approach that yields promising results in expanding our understanding of invasion dynamics.