Crab on beach under a twig

Many living organisms can expect to have some freeloaders: other organisms that live on or inside and may or may not prove harmful to it. When that tends to be a positive relationship for both parties, it is called a symbiotic relationship, whereas a negative impact for either party would make it a parasitic relationship.The differing biologies of host and parasite can affect the success of either organism, such as their performance and fitness under certain climatic conditions. Scientists are now studying host and parasite systems in order to better understand how things may change as the climate changes.

A host and its parasites

Parasitic relationships have existed for many millennia, but we still do not seem to know the whole story when it comes to host-parasite ecology. Parasite is a term that denotes a negative relationship from the point of view of the host. The host doesn’t receive a benefit from the coexistence, and may even suffer negative impacts in terms of survival and performance. However, simply because these organisms have coexisted and potentially coevolved together does not mean that will they respond similarly to environmental changes.

Parasites often breed and feed on hosts, which can damage a host’s chances for survival. A large parasitic load (i.e. lots of parasites on one host) can even kill a host before it has the chance to reproduce and pass down its genes. The two populations often find some kind of balance, however, because the parasites will not be able to persist without a host population and it would not do well for them to kill off their host species!

Change brought by climate

Scientists are asking how these relationships will be affected by climate change, specifically rising temperatures in temperate areas of the world. The main question here is how will climate change affect the thermal ecology of hosts and parasites, meaning the relationships between the two species’ populations in the context of environmental temperature.

In a study by Gehman, Hall and Byers, the researchers wanted to find out how potential increases in water temperature (as could happen due to climate change) would affect the ecology of the ectotherm hosts and its parasites in a specific ecosystem in coastal southeastern United States. Will changes in temperature affect both species equally? Will the parasites or the hosts benefit more from increasing temperatures?

An ectotherm is an organism that depends on the ambient temperature of the environment to determine the internal temperature of the organism. In this study, the researchers focused on the ectotherm Eurypanopeus depressus, which is an oyster reef dwelling crab, and its parasite Loxothylacus panopaei, a barnacle type organism. In this system, the parasites are not native to the study area, meaning that they had not evolved in this location with these specific hosts but were brought or travelled there in recent history.

The big questions

There are a few parts to this question:

One, how does a higher ambient temperature affect the survival and functioning of the ectothermic host species?

Two, how does a higher ambient temperature affect reproduction of the parasite species?

Three, how do the potential changes in the parasite species at higher temperature affect the host species?

Examining these questions may give us some answers as to what to expect should the average ambient temperatures increase for these species.

How this was examined

The scientists modeled host abundance and parasite prevalence for various scenarios of uniform increase in ambient temperature and of increases that were variable by season. In the southeastern United States where this study system is, the changes in ambient temperature due to climate change are more pronounced in winter months. The dynamics between the two populations may change along different performance and survival parameters such as survival, reproduction, and transmission/infection.

The researchers also ran experiments in a laboratory setting with higher environmental temperatures and used that data to inform a mathematical model of the host and parasite survival. From these models, they determined optimal temperatures for host performance in different states: susceptible, exposed, and infected.

And the winner is?

In this case, it seems like increasing environmental temperatures may in some ways benefit the host species, the crabs. The parasites may not be able to survive in areas just outside of the northern edge of the range because winter temperatures may still be too low. However, because the parasites may achieve higher reproduction during warmer months, the parasite load and infection of the host crabs may increase and lead to greater host mortality. This study suggests that the parasites impact the hosts by reducing their chances of survival at higher temperatures. At higher temperatures, this effect may be so intense that the parasite population becomes locally extirpated (brought down to zero) because they have killed off their individual hosts before transmission can happen.

Taking this one step further, annual cycles of the crab and barnacle populations will go through peaks and troughs. The heights of the peak will depend on the host survival and barnacle reproduction and transmission under warming conditions. We may see long-term impacts on the crab populations, but that type of prediction will require more in-depth data collection. Going forward, it will be important for similar future host-parasite studies to predict the thermal performance of both species under warming scenarios and host performance as it is affected by parasite infection.

For what is becoming one of the most pressing global issues of our time, climate change has far-ranging and diverse impacts. Complex interactions and relationships such as those between hosts and parasites will change. Being able to understand how those changes occur may enable us to make better predictions on how climate change will impact wildlife populations.



By Chia Yi Hou



Gehman, A.L.M., Hall, R.J. and Byers, J.E., 2018. Host and parasite thermal ecology jointly determine the effect of climate warming on epidemic dynamics. Proceedings of the National Academy of Sciences, p.201705067.