This is the ultimate bit of research into the negative ecological impacts of brumbies on indigenous ecosystems in the Alps.
It is explained in three reports, and the primary author is Graeme L. Worboys. A range of other researchers were involved in the work. It is peer reviewed and based on observations by the author in the Australian Alps protected areas that covers a period of 42 years.
From the introduction:
The iconic Australian Alps National Parks and Reserves are a National Heritage Listed Place in recognition of their “outstanding heritage significance for Australia”. Few Australian natural areas attain this special status and the Alps share their listing with other famous protected areas such as the Great Barrier Reef Marine Park and Uluru–Kata Tjuta, Kakadu and Blue Mountains national parks. The Alps are special because the vast majority of Australia’s alpine and subalpine environments are found there. Relative to the Australian continent, it is very small, and for many Alps species it is the only place in the world where they can exist.
Some of the National Heritage inscription values for the Alps parks include: “Glacial and periglacial features; fossils; karst features; biological heritage; moth feasting; transhumance grazing; scientific research, water harvesting; and recreation values”
Many of these heritage listed Alps values are being degraded and in particular the biological and water harvesting values. A non-Australian (introduced) pest animal, the wild horse is causing serious, landscape scale impacts in the Alps and especially to its wetlands and their associated plants and animals. This statement identifies the devastating impact of these wild horses on the Alps fragile wetlands. It shows that Australian heritage values are being lost and the consequent negative implications for the Alps catchments in a climate change world. Through many images and maps, it illustrates the problem at hand.
You can find the reports here.