For hundreds of years, humans have been causing large-scale changes in the abundance and distribution of large mammalian herbivores throughout the world. Many species have been driven extinct, become imperiled or introduced to new regions outside their native range. These alterations are of great concern because large mammalian herbivores can have major effects on vegetation dynamics, biological invasions and prevailing disturbance regimes. Such changes can also trigger cascading effects on the abundance and composition of other co-occurring animal taxa, as well as a wide range of ecosystem-level processes.
In response to these changes, a wealth of programs have been initiated to reintroduce extirpated mammals into parts of their historical range. While these efforts have often been a resounding success, and extensive research has been conducted to understand the dynamics of reintroduction, remarkably little attention has been focused on how the return of long-absent mammals has affected biological invasions, plant and animal communities and the ecosystem-level processes in the recipient landscapes.
Here, we use a 19-year-old exclosure experiment to assess the population, community and ecosystem consequences of reintroducing tule elk (Cervus canadensis nannodes) into a coastal landscape in northern California. Through their activities as consumers, disturbance agents and fertilizers, we have investigated the degree to which elk have 1) mediated the dynamics of plant invasions, 2) altered the structure, composition and functional traits of plant communities, 3) changed soil characteristics and processes, and 4) triggered cascading effects on arthropods and small mammals in this system.
Our research has shown that reintroduced elk have a wide range of effects on the vegetation of this coastal landscape: they increase the abundance of exotic annual grasses, decrease cover of a dominant exotic perennial grass, reduce biomass of herbaceous thatch, and reduce cover of native shrubs. Elk also have influences that cascade through the system to decrease the abundance of small mammals and increase the abundance of arthropods, although both effects vary among habitat types, soil formations and seasons. In addition, elk compact the soil and reduce infiltration rates. Elk also have a negative effect on soil nitrate availability as the proportion of coarse sand in the soil increases. Conversely, they have a positive effect on the ammonium availability as the proportion of silt and clay in the soil increases. Collectively, these results indicate that successful efforts to reintroduce elk populations have caused a complex and sometimes undesirable range of effects on this coastal landscape.