Bottom‐up and top‐down human impacts interact to affect a protected coastal Chilean marsh

Abstract

Many ecosystems, even in protected areas, experience multiple anthropogenic impacts. While anthropogenic modification of bottom-up (e.g., eutrophication) and top-down (e.g., livestock grazing) forcing often co-occurs, whether these factors counteract or have additive or synergistic effects on ecosystems is poorly understood. In a Chilean bio-reserve, we examined the interactive impacts of eutrophication and illegal livestock grazing on plant growth with a 4-yr fertilization by cattle exclusion experiment. Cattle grazing generally decreased plant biomass, but had synergistic, additive, and antagonistic interactions with fertilization in the low, middle, and high marsh zones, respectively. In the low marsh, fertilization increased plant biomass by 112%, cattle grazing decreased it by 96%, and together they decreased plant biomass by 77%. In the middle marsh, fertilization increased plant biomass by 47%, cattle grazing decreased it by 37%, and together they did not affect plant biomass. In the high marsh, fertilization and cattle grazing decreased plant biomass by 81% and 92%, respectively, but together they increased plant biomass by 42%. These interactions were also found to be species specific. Different responses of plants to fertilization and cattle grazing were likely responsible for these variable interactions. Thus, common bottom-up and top-down human impacts can interact in different ways to affect communities even within a single ecosystem. Incorporating this knowledge into conservation actions will improve ecosystem management in a time when ecosystems are increasingly challenged by multiple interacting human impacts.

Human activities accelerated the degradation of saline seepweed red beaches by amplifying top‐down and bottom‐up forces

“…, Farina et al. , He and Silliman ). Bottom‐up and top‐down effects can act together to exert strong control over community dynamics, while how human activities modify these interactive forces remains unclear.…”

“…, Farina et al. ). Whether these two types of driving factors counteract or have additive or synergistic effects on ecosystems is poorly understood.…”

“…With the rapid growth and spread of the human population, human activities have introduced new types of disturbances into natural coastal wetland ecosystems, which can greatly shape plant composition in both top‐down and bottom‐up manners (Farina et al. ). A large body of studies have shown that multiple human impacts such as warming, eutrophication, and sea level rise in marine ecosystems can interact synergistically and exacerbate negative impacts mainly through bottom‐up processes (Crain et al.…”

“…A large body of literature has examined the dynamics of coastal salt marshes as a function of grazing pressure, drainage conditions, sedimentation accretion, and sea level rise (Leendertse et al 1997, Spivak et al 2007, He et al 2015, Elschot et al 2017). Human activities have constituted different types of disturbances to natural coastal wetland ecosystems, which can alter plant composition and distribution by influencing topdown and/or bottom-up mechanisms (Crain et al 2008, Farina et al 2016. Bottom-up and top-down effects can act together to exert strong control over community dynamics, while how human activities modify these interactive forces remains unclear.…”

“…In general, the relative importance of these two types of mechanisms may be scale-dependent and vary by spatial or temporal scale (Strong 1992, Gratton and Denno 2003, Meserve et al 2003, Hunt and Mckinnell 2006. However, few studies have examined the relative impacts of top-down and bottom-up forces and how they interacted with each other (Crain et al 2008, Farina et al 2016. Whether these two types of driving factors counteract or have additive or synergistic effects on ecosystems is poorly understood.…”