Population regulation of wild horses in the Burragorang Valley: a Doctoral research project

PhD student: Andrea Harvey (veterinarian).

Supervisor: Dr Daniel Ramp, University of Technology Sydney


The management of wild horses is a controversial topic that is positioned around two central ideas: a) that wild horses negatively impact on ecosystems; and b) that their populations grow rapidly.  Combined, these ideas suggest that prevention of population growth is a key management goal for regions where conservation values are viewed as being impacted.

Despite previous research on wild horses throughout the world, there remains a considerable knowledge gap on how wild horses interact with, and impact upon, other species, particularly in ecosystems where horses are novel and recent constituents. While the documentation of how, when and why wild horses impact on conservation values is important, how populations grow and are regulated is vital for understanding their ecology and behaviour. Regulation in wild horses, however, remains an elusive component of wild horse research, as there is very little scientific data on how populations regulate themselves under different circumstances. Regulation processes should differ between arid and temperate regions, between areas of varying habitat availability and quality, and when regulation is influenced by trophic interactions. The outcome of these processes should determine the difference between maximum annual potential fecundity and lifetime reproduction success, a limitation that should vary spatially and temporally. This information is currently missing from conservation decision-making and wild horse management.

Limitations to population growth in wild animal populations can be driven by environmental and climatic heterogeneity and competition and predation from other fauna that shape the trophic cascade. No research has been performed in an Australian setting regarding the roles of bottom-up regulation through sociality and resource availability and top-down regulation through predation in horses. Clearly, knowledge of the behavioural and population ecology of wild horses is essential to maintain confidence in population projections and potential targets for management intervention. Robust quantitative methods are required to evaluate the effectiveness of any interventional modality aimed at limiting population growth.


Horses in the Burragorang Valley

The horses occupying this region are thought to have originated from the Shetland Islands, and were used as pit ponies in past mining operations. Previous culling kept the total population density low, but culling is currently prevented for political reasons. Their range currently comprises several nearly-isolated populations related to the topography of the landscape, with geographical constraints on movement restricting movement centred around riparian corridors. The landscape is heterogeneous and access to resources varies spatially. The current population is estimated to be approximately 60 horses consisting of about 10 herds. There is currently no information on what limitations, if any, are influencing reproduction. Hence, the capacity of the high quality riparian corridors to act as source habitat to re-populate surrounding habitats is currently unknown.

Study aims and objectives

This doctoral research project proposes to examine the population and behavioural ecology of wild horses in the Burragorang Valley. Assessments will be made to evaluate the carrying capacity of the area, quantifying limitations to population growth, interactions with other species, and the influence of habitat availability and quality on density dependent habitat selection. The goal will be to develop an understanding of metapopulation structure that can be used as a basis for robust decision-making. As Burragorang is a water catchment of significance, this research will determine the importance of the riparian corridors and flood-out zones for the horse population, enabling an assessment of the contribution of horses to nutrient loading. This will provide information on potential influences on water quality.


Wild horses will be surveyed using a combination of on-ground surveys, camera traps, and faecal samples to assess:

  1. Population-level and herd-level demographics and con-specific interactions;
  2. Distribution, movement, and habitat selection;
  3. Distribution and density of faeces as an estimate for relative abundance and habitat use ;
  4. Carbon isotope signatures and DNA metabarcoding of faeces to investigate dietary dynamics to determine the proportion of grazing that occurs in riparian corridors;
  5. Nitrogen isotope values and multi-elemental analysis of faeces to provide insights into the effects of horse faeces on ecosystem nutrient cycles; and
  6. Presence of faecal pathogens.

Parallel to this research in the Burragorang Valley, a veterinary study will be carried out on semi-wild horses housed at a sanctuary to evaluate the efficacy and behavioural effects of an immunocontraceptive agent as a means of providing additional regulation control where necessary. Data from the behavioural ecology study will be combined with the immunocontraceptive study to model metapopulation trajectories for a range of management alternatives.

Project update April 2017

Fieldwork results

  • Fieldwork was initiated in December 2015. Forty camera traps were placed throughout Kedumba valley in strategic locations (on the Kedumba (Mt Solitary) side of Lake Burragorang as WaterNSW has not yet granted access to the other side).
  • Ongoing observations and camera trapping has continued with 2-3 day field trips every 1-2 months for ongoing monitoring of foaling rates, mortalities, changes in herd structures and movement and correlation of these parameters with body condition and other signs of health.
  • In August 2016, 27 horses had been identified within 4 different mobs, with the density of horses being approximately 1 horse per 2.5km2.
  • Foaling rates and mortality rates are being recorded, as well as signs of health and detailed body condition. In 2015/16 overall foaling rate (number of foals/number of sexually mature mares) was very low at 0.28 (4 foals from 14 sexually mature mares), compared to previously published foaling rates in wild horses throughout the world of 0.7-0.9.
  • On the most recent field trip April 22-24, only 14 horses were directly observed, with an extra 4 horses possibly present as observed on camera traps. It is clear that horse mortality due to lack of food is regulating the population. Assessments based on body condition, reproductive rate and vegetation suggest that the area is unlikely to be able to hold more than 30 horses.
  • Collection of fresh faecal samples has been ongoing approximately each month, for faecal parasitology analysis including for zoonotic pathogens (e.g. Cryptosporidium). No pathogens detected to date.
  • From April 2017, NPWS has reinitiated trapping in the upper Kedumba area where horses are overlapping schedule 2 recreation areas of Warragamba Catchment.

Any inquiries: Rosalie Chapple r.chapple@bmwhi.org.au

Project update March 2016

  • The project was initiated on 19th December 2015
  • 5 field trips totalling 13 days have been undertaken to date
  • All work undertaken has been in accordance with the conditions of access consent and Safe Work Plans, and no health and safety issues have been encountered
  • Preliminary on-ground surveys have been completed for all areas in Kedumba Valley, and further on-ground surveys will be continued to determine distribution and density of horse faeces throughout the area
  • All water access points have been determined during on-ground surveys.
  • Camera traps have been placed so far at about 80% of these water access points, and all the identified water access points will be covered by camera traps by the end of this month. This should allow identification of every horse in the catchment area so that accurate population numbers can be obtained, as well as time spent at water access areas
  • Further camera traps have been distributed throughout specific areas in order to obtain further demographic information as well as detailed information on distribution, movement and habitat selection throughout the area.
  • A total of 40 camera traps have been placed to date, yielding approximately 15,000 images thus far. Images will be fully analysed over the coming months. Mark-recapture techniques are being used to identify individual horses and herds, and monitor movement and habitat selection
  • Camera traps have also provided additional surveillance information by capturing illegal activities such as bushwalkers in Schedule 1 areas, and trail bikers. Any such images are always passed promptly to NPWS and WaterNSW
  • Collection of fresh faecal samples has been initiated with the aim of collecting 200 faecal samples over the next 12 months which will be analysed for zoonotic Cryptosporidum spp
  • Faecal samples are also being stored to later undertake carbon and nitrogen isotope signatures, multi-elemental analysis and DNA metabarcoding