The niche is a basic concept in ecology, and seems like something that everyone should be able to easily define – and to define consistently. Yet, recent conversations have made me realize that there’s a lot more variation than I would have expected in how the niche – especially the realized niche – is defined.
The crux of the debate seems to come down to whether people say the realized niche is limited by just competition, or whether they say it’s influenced by all interspecific interactions. There’s also a debate on whether dispersal plays a role. Once I realized this and started talking to others about it, there were two main kinds of reactions:
1) “OMG, yes, isn’t it weird that some people define it that other way? Clearly they are wrong.”
2) “Wait, what? Not everyone agrees that X is what determines the realized niche?”
To back up: The original Hutchinson “Concluding remarks” paper focuses on competition as the factor that limits organisms’ distribution; that is, in that paper, the realized niche is smaller than the fundamental niche due to competition. Since then, though, there has been a lot of work updating the niche concept. This includes relatively recent work, and work that adds in mutualism, which can make it so that the realized niche is larger than the fundamental niche.
When I teach about niches, I teach that the fundamental niche is where an organism can occur based on abiotic conditions, and the realized niche is where it actually does occur, given the presence of interacting species. I tend to gloss over the effect of mutualists on the niche in my Intro Bio course, but I used to focus on that more when I taught Ecology. This mostly matches what the textbook we use (Morris et al’s How Life Works) says:
The fundamental niche comprises the full range of climate conditions and food resources that permits the individuals in a species to live. In nature, however, many species do not occupy all the habitats permitted by their anatomy and physiology. That is because other species compete for available resources, prey on the organisms in question, or influence their growth and reproduction, reducing the range actually occupied. This actual range of habitats occupied by a species is called its realized niche (Fig. 47.2).
The main thing I think is left out of that definition is the possibility for facilitation to make the realized niche larger than the fundamental niche, but, again, that is not a topic I tend to emphasize at the freshman level. The way I teach about the niche also matches what is in Begon et al., which is my go-to reference for checking on this sort of thing:
“Usually, a species has a larger ecological niche in the absence of competitors and predators than it has in their presence. In other words, there are certain combinations of conditions and resources that can allow a species to maintain a viable population. This led Hutchinson to distinguish between the fundamental and realized niche. The former describes the overall potentialities of a species; the latter describes the more limited spectrum of conditions and resources that allow it to persist, even in the presence of competitors and predators.” (page 31 of the 4th edition)
So, given all that, I was surprised to learn that others (including some of my colleagues here at Michigan) hold the view that the realized niche is only influenced by competition. In recent conversations I’ve had with others about this, several people at other institutions said they shared my view but also have colleagues who hold the view that the realized niche is only influenced by competition. This has me wondering what the split is in terms of how many people hold the different views. Hence this post.
I can see that it might be possible that people might think one thing but teach another in the interest of trying to keep things simpler. So, I’m also wondering:
At the risk of biasing the poll, I will say that I find the view that only competition restricts the realized niche to be surprising. What, then, explains why really large bodied Daphnia don’t co-occur with fish? It’s not that they’re not good competitors.
Finally, another issue that has come up during these discussions is whether the realized niche is influenced by dispersal. Let’s consider this clicker question that I’ve used in class:
60 common starlings were released in Central Park in NYC in 1890 by someone trying to introduce all the birds mentioned in Shakespeare’s plays to the New World. There are now millions of starlings in the US, and they are common in Ann Arbor. Prior to 1890, Ann Arbor was:
A) part of the realized niche of starlings but not their fundamental niche.
B) part of the fundamental niche of starlings but not their realized niche.
C) part of the fundamental and realized niches of starlings.
D) not part of either the fundamental or realized niche of starlings.
I give the correct answer as B. But some colleagues of mine argue that dispersal does not influence the realized niche. Everyone is in agreement that Ann Arbor was within the fundamental niche of starlings prior to 1890. The question is whether it’s okay to say that the realized niche of starlings was extended by them being introduced into Central Park. I say yes, but others say no. What do you think?
I can understand how this is more of a gray area – if we are saying that the realized niche is influenced by interspecific interactions, then it’s not clear what to do about the influence of dispersal. But, then again, how else would you characterize the change in distribution of starlings? I’d love to hear people’s perspectives in the comments.
All of this is reminding me again of one of the things that I really like about teaching – it forces me to think harder about things that I thought I knew. I will be interested in hearing more about what others think about the realized niche, and how they teach about it!