Even the mention of parasites may be enough to cause some people’s skin to crawl. But for recent UC Santa Barbara PhD graduate Dana Morton, these scary creatures occupy important ecological niches, fulfilling roles that she believes have too often been overlooked.
That’s why Morton has just published the most extensive ecological food web that includes parasites. After eight years of creation, the dataset includes more than 21,000 interactions between 942 species, all carefully annotated. The detailed description, published in the journal Scientific data, is a boon for basic research, conservation efforts and resource management.
Understanding who eats who, or trophic interactions, in an ecosystem is critical information for biologists. These relationships alone can say a lot about a system, its complexity, and even its overall health. However, environmentalists often overlook parasites when studying these interactions, perhaps because parasitology has only recently entered the realm of ecology, emerging from the medical sciences.
“But you can’t ignore parasitic interactions once you know them,” Morton said. “If you ignore half of the interactions in the system, you don’t really know what’s going on in that system.”
Previous work led by his mentors, Armand Kuris and Kevin Lafferty of the Department of Ecology, Evolution and Marine Biology, revealed that parasites are common in estuarine food webs. But Morton wanted to tackle a more diverse ecosystem. Considering all of the research on California kelp forests, she thought it would be quite easy to simply add parasites and small free-living invertebrates to an existing network. But she quickly realized that the previous food webs compiled for the kelp forest were too coarse to develop. They focused on big fish eating small fish, but paid less attention to mammals, birds, and invertebrates. She would need to start from scratch.
An exhaustive effort
First Morton has compiled a list of species that inhabit the kelp forest. She and her co-authors used virtually any credible source they could find. They looked at reviews of the literature and obtained data from long-term research projects, such as the Santa Barbara Coastal Long-Term Ecological Research Program and the National Park Kelp Forest Monitoring Program. of the Channel Islands. She also researched other divers, and when that wasn’t enough, Morton and his team did their own field sampling.
Morton especially acknowledged the help she received from student volunteers and undergraduate experts throughout the process, including Milton Love, Bob Miller, Christoph Pierre, Christian Orsini and Clint Nelson of UC Santa Barbara. ; Mark Carr of UC Santa Cruz; Ralph Appy at the Cabrillo Marine Aquarium; and David Kushner at Channel Islands National Park.
The authors’ next task was to discern all interactions, which mainly fell into three types: predator-prey, parasite-host, and predator-parasite. Morton’s general rule was that every animal had to eat something, and every node had to have at least one connection.
It soon became clear that adults and juveniles often have different roles in food webs, requiring more detail than other food webs usually contain. It was also a comprehensive task that required browsing academic literature and databases, conducting field observations and dissections, and discussing with expert researchers.
By combining information on predator-prey and parasite-host relationships, Morton was able to deduce certain relationships based strictly on logical reasoning. For example, it helped determine whether an ingested parasite was likely to die or infect the predator that ate its host.
Each node in the food web – corresponding to a particular species or life stage – had a reference in its entry. In fact, Morton made sure the entire web was filled with metadata. “We don’t want food webs to be just these black boxes where you don’t know how they were put together, so you don’t know how to use them appropriately,” she said.
She was particularly alert to uncertainty and estimated her confidence in each of the tens of thousands of putative relationships. For example, some parasites may appear in only one or two specimens simply because they are rare, rather than due to specialization. Unobserved but real interactions between hosts and parasites create a false negative in the food web.
Morton therefore estimated the probability of false negative links for each potential host-parasite interaction. If an absent interaction had more than a 50% change from a false negative, it assigned it as a link in the network. It also removed parasite species that were particularly prone to false negatives, in order to reduce the overall error.
She also included an estimate of her confidence for each of the tens of thousands of putative relationships.
A major challenge Morton faced was simply knowing when the project was finished. There are few sharp divisions in the ocean; The ecosystems are incredibly interconnected, and many species that live in the kelp forest inhabit other ecosystems in Southern California as well. This project could have squeezed its way into an account of the entire Eastern Pacific.
To prevent it from ballooning, Morton limited the study to the rocky reef in the giant kelp depth range. It also did not attempt to include viruses and bacteria, nor did it specify the many species of phytoplankton. Eventually, the food web reached a point where the additions did not change the overall structure of the network, indicating that the web was converging to a full account.
A complex system
Morton’s years of work produced a complete food web comprising 492 free species and 450 parasites. Taking into account specific life stages brings the total number of nodes to 1,098, with 21,956 links between them.
“This is the first food web for a truly structurally complex marine ecosystem, which is truly dynamic and open,” Morton said. She was amazed at the extent to which the network grew after taking into account often overlooked groups of organizations. The inclusion of small free invertebrates doubled the size of the network. The addition of parasitic interactions further doubled it.
The results highlight something she suspected all along: “Whether or not you decide to create a food web (which I wouldn’t recommend),” she joked, “you might still be thinking about parasites. who could participate in the system. If you are missing half of the interactions that you[areprobablymissingalargepartofthepicture”[‘reprobablymissingahugepartofthepicture”[manquentprobablementunegrandepartiedel’image”[‘reprobablymissingahugepartofthepicture”
Parasites were even more prevalent in the kelp forest food web than in the estuarine food webs that inspired his project. While a food web filled with parasites might seem unhealthy, according to Morton, this is actually a good sign because parasites often need complex food chains to complete their life cycle. “The discovery of a large number of parasites indicates that there are intact trophic structures and a great diversity of species,” she said.
Parasites are only present because the kelp forest offers them so many opportunities. Kelp forests are well-known biodiversity hotspots, particularly those in the Santa Barbara Canal, which lie at the confluence of cold water communities north of Point Conception and warm water communities in the south and south. Baja California.
“This new look at kelp forest food webs puts fish in the background,” said co-author Kevin Lafferty, Morton’s advisor at the Marine Science Institute. “Most of the action is with invertebrates. And most of those invertebrates were parasites.”
Morton was surprised to find a large number of parasites that use birds and mammals as final hosts. This suggests that birds and mammals have a greater presence in the kelp forest ecosystem than they thought.
As for next steps, Morton has already gotten to work comparing his kelp forest food web to the few other intertidal and lake food webs in the literature that include parasites. She also plans to study how the food web of the kelp forest might change as the ocean warms. But the main point of publishing her data, she said, was to inform conservation efforts and resource management in forest kelp ecosystems.
When studying ecosystems, there is often a large cloud of unknowns that lead to great variability in data. “My hope in doing this was to provide people with the resources to get a more mechanistic understanding of what they’re seeing,” Morton said, “because now they basically have a map of all the things that could possibly happen. in this ecosystem. . “