The Enemy of My Competitor is My Friend
Specifc parasites indirectly influence niche occupation of non‑hosts community members (2018) Fernandes Cardoso et al., Oecologia, https://doi.org/10.1007/s00442-018-4163-x
One of the oldest questions in community ecology is why do some species seem to co-occur with one another, while others don’t? Two hypotheses have been put forward to explain why this happens: environmental filtering and niche partitioning. Environmental filtering is when some abiotic feature of a given environment – such as the temperature or oxygen levels – prohibits some species from ever living in the same location as another. A very broad (and overly simplistic) example of this is that you would never see a shark living in the same habitat as a lion, because the shark needs to live in the ocean and the terrestrial Savannah of Africa where lions are found “filter” the sharks out. Niche partitioning, on the other hand, involves species adapting to specialize on a given part of the environment, thus lessening competition for a niche by dividing it up. You can see this with some of Darwin’s Finches, which adapted differently-sized beaks to feed on differently-sized seeds. They all still eat seeds, but they are not eating the same seeds.
Interactions with other organisms, either direct or indirect, can also influence which species co-occur. If one species can out-compete another, they likely won’t be able to co-occur because the better competitor will take most of the resources, forcing the other out. This can all change, however, if a third organism affects the competitive ability of the superior competitor, allowing the inferior competitor to persist despite its lesser ability.
Today’s authors used two spider species to study community assembly and how it may be affected by a fungal parasite. Chrysso intervales (hereafter inland spiders) builds webs further away from rivers, while Helvibis longicauda builds webs close to the river (hereafter river spiders). Interestingly, only the river spiders are infected with the fungal parasite, thus they investigated how interactions between the two spiders may be mediated by this fungal parasite.
What They Did
The researchers created transects parallel to the river at three different distances, 0, 10 and 20 meters from the river in both summer and winter. Within the transects, they counted the number and species of spiders that they found, and for each spider they noted what plant they were found on, the height of the web, and if the spiders were infected by fungus or not.
To test if environmental filtering explained the species occurrence, the authors moved river spiders away from the river and moved inland spiders next to the river, in addition to setting up spiders in their usual habitats. Doing this allowed them to record what (if any) effect the habitat had on web establishment success. To test if niche partitioning explained the species occurrence, the authors noted what prey items were found in and around the webs of the spiders in their respective habitats.
Did You Know: Coexistence vs. Co-occurrence
Despite the terms coexistence and co-occurrence being used to sometimes describe the same pattern (two species living in the same habitat as one another) they are two different phenomena. Co-occurrence is when one species is found in the same area as another, but there is no evidence of it being a stable pattern. Coexistence, on the other hand, is a stable phenomenon whereby the population of Species A grows faster when it is rare, while the population of Species B does not because it is competing more with members of its own species than it is with members of Species A. This advantage when rare allows Species A to persist, despite possible disadvantages in other areas.
What They Found
The river and inland spiders had distinct times at which they were found in transects throughout the summer and winter, though there was a large overlap in their timing of occurrence. Although more spiders of both species were found in the summer than in the winter, the river spiders were mostly found next to the river, while the inland spiders were mostly found further away from it.
For the transplant experiment, 13% of the river spiders established webs near the river, but none of the river spiders established webs away from it. In contrast 7% and 1% of the inland spiders established webs away from and near the river, respectively. While there was not a significant difference in the types of prey items consumed by the two spider species, the river spiders consumed both more prey items and had a broader diet than the inland spiders.
Only the river spiders were killed by the fungal parasite, with more deaths occurring in the winter than in the summer. Interestingly, more inland spiders were found near the river in winter than in the summer, and in some cases inland spiders were found in sites containing a river spider that had been killed by the fungal parasite.
In the discussion, the authors point to a variety of results that indicate the two spider species appear to coexist with one another. While this may indeed be true, the authors never analyzed coexistence, they looked at co-occurrence. These two patterns, though they sound similar, are actually different (see the Did You Know). From talks with colleagues and friends that study community ecology this improper use of the term “coexistence” is not uncommon, and it is important in science that we use the correct terminology when we discuss certain things.
Despite being closely related and occurring within close proximity to one another, the inland and river spiders display fine-scale habitat differentiation from one another. This is interesting in and of itself, because these spiders are segregated in a horizontal fashion i.e. they are living in areas next to one another. This is not common, despite the frequent observations of different spiders segregating themselves in a vertical fashion, i.e. living in habitats on top of one another.
What’s more, fungal parasites appeared to release the inland spiders from competition in winter due to the higher mortality of river spiders, allowing the inland spiders to colonize areas closer to the river. These results serve to highlight the complexity of the natural world, where it’s never as simple as one species affecting or competing with another.
Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and his work for Ecology for the Masses here, or follow him on Twitter here.