Do species matter for biogeophysical functioning of ecosystems?

In the broad context of the history of life on Earth, extinction of species is a dominant process that contributes to the current and future diversity of life. Previous mass extinctions were associated with cataclysmic transformations of the global environment, such as the cometary impact inferred to have caused the extinctions at the end of the Cretaceaous. In the Holocene, human populations have altered large portions of the landscape in many biomes around the world, and the rate of landscape change has accelerated in the past 100 years. In addition to changes in land use, biogeochemical cycles of carbon and nutrients have been altered and organic contaminants and trace metals have been introduced and mobilized in the environment. The current extinction of species and loss of biodiversity in many taxonomic groups is an unintended, but not surprising, consequence of the large scale transformations of many biomes. Species that are most vulnerable to extinction are rare species that occur in restricted habitats, species that require large expanses of protected land to forage or hunt, and species that are impaired due to exposure to high doses of contaminants through bioaccumulation in their supporting food web.

An alpine site with many weathered pocket gopher mounds.

Regulations to protect "endangered species" have become part of the fabric of environmental legislation in the US and many countries, and concern for preserving species and the genetic diversity of life influences international environmental agreements. Conservation efforts are often focused on particular species, especially large mammals, although recent conservation plans have emphasized an ecosystem approach. At the same time, ecologists have been actively studying the relationship between species diversity and ecosystem productivity and stability.

Studies of many ecosystems have demonstrated that the presence or absence of a "keystone" species can control not only the trophic structure (foodweb interactions) of an ecosystem, but also important biogeochemical and biogeophysical interactions. For example, in the alpine tundra of the Colorado Rocky Mountains, the pocket gopher burrows through soil under the winter snow cover, and their piles of reworked soils provide new soil for colonizing plants contributing to spatial patchiness and plant species diversity in the tundra. The reworking of soils by the gophers also contributes to the mobilization of particulate soil organic matter and nutrients during snow melt, important processes in the harsh environment of the alpine tundra because of the slow rates of soil accumulation and slow growth of plants.

The pocket gopher is an important herbivore in the alpine tundra. Pocket gophers burrow while eating roots and leaves mounds of exposed soil.	Fresh pocket gopher mounds in alpine tundra.	Tunnels created by pocket gophers burrowing in winter under the snowpack.

Several general concepts and hypotheses have been proposed for the relationship between stress, biodiversity and ecosystem function. Ecosystem function refers to rates of processes such as photosynthesis, respiration, or production within a group of similar organisms, such as bottom-dwelling insects in a stream, which can be referred to as a functional group or trophic level. The redundancy hypothesis proposes that the as species succumb to a stress and are removed from an ecosystem, other species will take over that function, and ecosystem processes will continue unabated until all species within a trophic level are eliminated. Another hypothesis proposes that species diversity enhances ecosystem function, using the analogy that the diverse species are like "rivets" holding together an ecosystem, and proposes that function decreases as species are eliminated due to stress. Given the current large scale changes in land use, changes in elemental cycles, and potential environmental stress from changing climate regimes, better understanding of the fundamental relationships between biodiversity and ecosystem function could help to inform land and water resource management in the future.