Beaver

Interesting as an animal that is most often cited as an example of an ecosystem engineer and a terraformer even.

The impact on biodiversity seems to be generally positive. Beavers do create wetlands and were prevalent in North America as well as in other places in the Northern hemisphere.

However, it is complicated and there are multiple simultaneous negative impacts. Here, negativity is stakeholder-specific rather than abstracted conceptual positivity/negativity to the whole ecosystems, however measured.

Beavers produce a relatively rapid local impact. If they are successful, they spread and modify their ecosystems significantly (any modification takes out previous systems and beings). If they are not successful (the environment constrains them) they suffer in the process (hunger, injury, illness, death, fewer offspring, inferior diet, predation, etc.)

Agents harmed:

Trees that beavers kill
Fish (low oxygen water, etc.)
Plants, insects, etc. (in the submerged land, overwatered land, during river redirections, etc.)
Other beavers during territorial competition

Other effects:

permafrost degradation
erosion from sudden release
increased evaporation
damage to biodiversity by selective tree cutting
severe damage when out of place: In Tierra del Fuego, beavers introduced 75 years ago destroyed 30% of the forests, in the Northern hemisphere, they are limited by predation and other factors that they cannot experience as comfortable

For a detailed situated narrative, see Разбираем бобров: дикие инженеры-терраформеры в действии

For an example of tree/beaver conflict see: Beavers Versus Old Growth: The Tough Reality of Conservation

Examples:

"Beavers have a sustained need for forage acquisition, but the pond riparian zone may become depleted of suitable forage when browsing is intensive and/or prolonged."
"Selective beaver herbivory near beaver ponds and beaver-occupied lakes can create a “bathtub ring” of avoided conifers around their periphery."

Johnston, Carol. Beavers: Boreal Ecosystem Engineers. New York: Springer, 2017.

References

Rozhkova-Timina, Inna, Victor Popkov, Peter J. Mitchell, and Sergey Kirpotin. ‘Beavers as Ecosystem Engineers – a Review of Their Positive and Negative Effects’. IOP Conference Series: Earth and Environmental Science 201 (2018): 012015. https://doi.org/10/gqm9zr.

Brazier, Richard E., Alan Puttock, Hugh A. Graham, Roger E. Auster, Kye H. Davies, and Chryssa M. L. Brown. ‘Beaver: Nature’s Ecosystem Engineers’. WIREs Water 8, no. 1 (2021): e1494. https://doi.org/10/ghqdhj.

Müller-Schwarze, Dietland. The Beaver: Natural History of a Wetlands Engineer. 2nd ed. 2003. Reprint, New York: Comstock, 2011.

References