Loss of a dominant nitrogen‐fixing shrub in primary succession: consequences for plant and below‐ground communities

Abstract

Summary 1. Many ecosystems are gaining some species and losing others, leading to large shifts in community composition. Plants that support nitrogen (N)‐fixing symbionts (hereafter N‐fixers) are major ecosystem drivers but human‐induced environmental changes can lead to their loss; the impacts of this loss have seldom been studied. In New Zealand, the gain of some invasive herbivorous mammals leads to the loss of N‐fixing shrubs that are pioneers during primary succession. 2. We studied primary successional surfaces caused by river floods and landslides in a remote valley in New Zealand's South Island. We set up and maintained a removal experiment over 10 years, with N‐fixing Carmichaelia odorata (hereafter Carmichaelia) either left intact or removed to represent the effects that invasive herbivorous mammals could cause under high densities. 3. Above‐ground effects of 10 years of Carmichaelia removal included a 7‐fold reduction of woody plant biomass, but had no effect on the shorter non‐woody vegetation. Foliar N concentrations of two woody species were also reduced. Carmichaelia removal reduced local or α‐diversity but enhanced β‐diversity of the remaining vegetation at the functional group but not the species level. 4. Below‐ground effects of 10 years of Carmichaelia removal included large reductions in soil carbon (C) and N levels, and biomass or abundance of several groups of soil biota occupying three consumer trophic levels as well as the root‐, bacterial‐ and fungal‐based energy channels. Invertebrate bacterial consumers were more adversely affected than were fungal consumers. Carmichaelia loss reduced α‐diversity of some below‐ground groups, while β‐diversity was unresponsive or enhanced. 5. Synthesis. Our results reveal that loss of a single functionally distinct plant species, such as occurs through herbivore invasion, can cause substantial effects both above‐ground and below‐ground. This may affect the trajectory of the ecosystem over successional time, especially in primary seres that otherwise have very low soil carbon (C) and nitrogen (N) reservoirs. More generally, our results support the view that the simultaneous gains of some organisms (e.g. invasive herbivorous mammals) and resultant losses of others (e.g. palatable N‐fixing plants) are a major element of human‐induced global change that may be transforming many communities and ecosystems world‐wide.