Interactions Between Feral Animals and Rapid ʻŌhiʻa Death

Rapid ʻŌhiʻa Death (ROD) is a disease caused by two fungal pathogens. Ceratocystis lukuohia (destroyer of ʻōhiʻa) is the name of the more aggressive fungus and Ceratocystis huliohia (disruptor of ʻōhiʻa) is the less aggressive fungus.  Both of these fungal pathogens kill ʻōhiʻa, Hawaiʻi’s most abundant native tree and keystone species of great natural and cultural significance.

Unusual ʻōhiʻa mortality consistent with ROD was first reported in the Puna district of Hawaiʻi Island during 2009 to 2010, and in 2014 lab testing confirmed the identity and description of the emerging exotic pathogens of ʻōhiʻa. As the spread escalated, so had research and management efforts. Statewide aerial surveys began in 2016 and a multi-agency working group has dedicated significant resources to surveying, monitoring and mapping the disease's impacts.

Many observations have suggested how feral hooved animals have played a critical role in the destructive impacts this disease has on Hawaiʻi’s native forests.

For more information about Rapid ʻŌhiʻa Death, click  here. 

In the Kaʻū Forest Reserve, we recently observed how Rapid ‘Ōhi‘a Death can escalate very quickly. The magnitude of increased mortality is not nearly as large as within Kaiholena's fenced areas, where hooved animals are actively excluded. The previous section included a map of Kaiholena from a single date, and included both old and recent suspects. Here is that map again.

Now we show a zoomed in area that maps recent suspects in the same area on two different dates. The following slider map compares recent mortality mapped on 5/20/2022 and 2/3/2023, demonstrating how large-scale outbreaks can escalate quickly.

In Hawaiʻi Volcanoes National Park's Kahuku Unit, we see a similar pattern of greater ROD mortality in the unfenced area of Kaʻū Forest Reserve than within the park's fenced area. Here we see a different situation where the forest recovers over time.

It should be noted that there is a significant difference in forest type on both sides of the fence line. The Kahuku Unit was previously grazed by cattle prior to the National Park's acquisition of the property, and therefore the forest canopy is much more open. With fewer trees, one could reasonably expect less mortality. However, throughout the analyses you can see the positive impacts of having the area fenced.

Adjacent to Hawaiʻi Volcanoes National Park is the Kahaualeʻa Natural Area Reserve, where we find another example of a fenced area having no hooved animals and significantly less ROD suspects than the adjacent unfenced areas where there are many hooved animals. The following maps show imagery collected over a year apart.

In the previous section there was a clear distinction between ʻōhiʻa mortality inside and outside of the fenced area in January 2019, where hooved animals were actively excluded. The story of this fence unit in Hawaiʻi Volcanoes National Park takes a turn when the fence incurred damage and many feral pigs got inside. The following time series of maps show what followed.

It seems that not all hooved animals cause mortality at similar rates. In the Kapāpala Section of the Kaʻū Forest Reserve, there is a fence unit that excludes cattle. It is not pig proof and they are able to move freely in and out of the fenced area. We recently observed that the amount of ʻōhiʻa mortality is significantly reduced within the fence unit. There is still mortality, but nowhere near the catastrophic situation happening in the adjacent forest where wild cattle are destroying the forest at a very high rate.

The map shared in the previous section clearly shows the disparity of recent suspects inside and outside the cattle-proof fenced area. Here is the map again.

And here is just the imagery from a zoomed in area just southwest of the fence unit. You can see the gray fence line in the upper right corner. More importantly, you can see there is a lot of older mortality (seen as white and gray tree crowns) as well as recent mortality (seen as brown and red tree crowns).

We will continue to monitor this area, as it is yet known how this situation may play out in the long term. We see here and in other areas how feral cattle can destroy a forest very quickly, likely due to the size and severity of the wounds they cause. Will the persistence of feral pig activity over several years result in the same level of mortality, albeit at a slower rate?

Sometimes an area is so damaged that we don't need anything but the RGB imagery in natural color to understand. And that this is a multi-use area steeped in history and is also the headwaters of an important watershed, the impacts are felt deeply both by those lineally connected and the greater population.

This first image shows the forest consisting of a healthy canopy in 2013, before Rapid ‘Ōhi‘a Death had been discovered.

Five years later, in 2018, we see the beginnings of mortality resulting from Rapid ‘Ōhi‘a Death. You can see several red/brown canopies, especially in the center of the image, but the forest remain very much alive.

And here is how it appears in high-resolution satellite imagery today. This area that is greater than a square kilometer will require a lot of active intervention to restore its ecosystem services.

Seeing and quantifying mortality from a remote sensing perspective is very impactful, but first person perspective provides reality that we can all relate to.

It should be noted that these are areas where the more virulent species, Ceratocystis lukuohia, has been confirmed. These areas have experienced high rates of mortality.

These examples are just some of the many ROD outbreaks where there are interactions between feral hooved animals and ʻōhiʻa forests. There are similar observations of high rates of ʻōhiʻa mortality in every district on Hawaiʻi Island. Kauaʻi Island also has observed outbreaks with significant mortality.

There is still a lot to be learned about the relative damage hooved animals can cause to ʻōhiʻa forests. For example, feral cattle do appear to have the most significant immediate impact on mortality rates. On the other hand, not all cattle behave the same, and silvopasture systems with domestic cattle who are provided ample forage and water have not been observed having high impacts on the health of ʻōhiʻa. Remote-sensing research and on-the-ground observations have both shown that feral pigs create significant damage in ʻōhiʻa forests, often leading to high mortality rates. Pigs' behavior can act as a vector and may directly transmit the fungal pathogen, but we are not yet sure how significant this is. We have little evidence regarding the impacts of feral sheep or mouflon.

Even with all of these devastating losses, there still remains a lot of healthy ʻōhiʻa forests throughout the islands, and great efforts are being made to protect them.

There are available and effective management strategies to protect our forests from the most devastating impacts of Rapid ʻŌhiʻa Death due to feral hooved animals. Managing them through a variety of tools including fencing and hunting not only helps to protect our remaining ʻōhiʻa, but also helps to increase native forest regeneration and promotes effective restoration.

This is a ROD Working Group collaboration led by Brian Tucker (PCSU), Dr. Ryan Perroy (UH-Hilo) and Nai'a Odachi (UH-Hilo)

For further reading, click the link for the following peer-reviewed paper:

Perroy, R.L., Sullivan, T., Benitez, D., Hughes, R.F., Keith, L.M, Brill, E., Kissinger, K., and Duda, D.  Spatial Patterns of ‘Ōhi‘a Mortality Associated with Rapid ‘Ōhi‘a Death and Ungulate Presence. Forests 2021, 12, 1035.  https://www.mdpi.com/1999-4907/12/8/1035 

Special thanks to all the organizations, land managers and researchers that support this collaboration. This includes State of Hawaiʻi Division of Forestry and Wildlife, Department of Hawaiian Homelands, National Park Service, The Nature Conservancy, USDA Forest Service, USDA Agricultural Research Service, and U.S. Fish and Wildlife Service.

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