One reason why the niche concept is difficult is that in ordinary use "niche" often means a place. It's tempting to think of a species' niche as the same thing as its geographic distribution. That view contains two subtle misconceptions. The first has to to with a theoretical perspective. Most contemporary ecological niche concepts, starting with Hutchinson (1957), consider a niche to be a mathematical space (the multivariate distribution of resources and conditions within which a species population can persist), not a geographic space, per se. The fact that real resources and conditions occur in geographic spaces makes this idea hard to get one's mind around. The second misconception has to do with the emerging recognition that populations can occur in places--maybe temporarily, and perhaps only as long as they receive immigrants from elsewhere--where environmental variables lie outside the species' niche. Populations also might not occur in places where they populations could sustain themselves just fine, if they could only get there.
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| Sorry, little ash trees. Just because you germinated in a pavement crack doesn't mean you can sustain a population there. |
H. Ronald Pulliam's "On the relationship between niche and distribution" (2000) does a masterful job of explaining the above distinctions. Among other routes, Pulliam does this by showing that demographic processes and dispersal link the niche to distributions. A species' niche is defined as the multivariate environmental set in which populations' finite rates of increase, lambda, are greater than or equal to 1. Dispersal from locations with high growth rates may maintain populations in "sinks" where lambda < 1. Dispersal limitation may keep populations from occupying sites that where lambda (would be) > 1.
The article has three sections. The first sets up the problem and introduces a perspective that makes sense of it. I've reproduced this section's key figure below. A key point is that in Fig. 1, the points are not in geographic space.
The article has three sections. The first sets up the problem and introduces a perspective that makes sense of it. I've reproduced this section's key figure below. A key point is that in Fig. 1, the points are not in geographic space.
Pulliam then analyzes a simulation model (NICHE) in which a species with a two-dimensional niche (in mathematical space) and a defined dispersal ability is introduced into a two-dimensional landscape grid (the geographic space), Grid cells are assigned, at random, values of the two niche dimensions, values that may or may not lie within the species' niche. Pulliam explores the model to identify situations in which distributions reflect niches closely and those in which they don't. (While I love this article, I wish Pulliam also had explored a few more realistic landscape structures with some degree of spatial autocorrelation in the environment: patchiness.)
Finally, Pulliam reviews the limited evidence (as of 2000) on the extent to which natural species distributions correspond to most of, more than, or less than the geographic distribution of suitable habitat, and he identifies data worth collecting. My colleagues and I are trying to collect the right data in long-term studies of distribution, demography, and adaptation in the California annual plant, Clarkia xantiana (my ResearchGate page). But that's another story for another blog.
What does Pulliam's perspective add to studies of organism distribution and abundance, the focus of my Ecology students' independent projects on the Grinnell College campus? I had my students read Pulliam's paper earlier in the semester. I hope they'll think about whether, for their study organisms (e.g., ground beetles; hardwood tree saplings; "ovivores" such as crows and raccoons; ragweed and sweet clover), the campus represents a source environment or a sink environment, and how their dispersal ability (possibly human-assisted) affects their small-scale distributions. In a rapidly urbanizing world, increasing attention to the niches and distributions of urban occupants makes sense.
Finally, Pulliam reviews the limited evidence (as of 2000) on the extent to which natural species distributions correspond to most of, more than, or less than the geographic distribution of suitable habitat, and he identifies data worth collecting. My colleagues and I are trying to collect the right data in long-term studies of distribution, demography, and adaptation in the California annual plant, Clarkia xantiana (my ResearchGate page). But that's another story for another blog.
What does Pulliam's perspective add to studies of organism distribution and abundance, the focus of my Ecology students' independent projects on the Grinnell College campus? I had my students read Pulliam's paper earlier in the semester. I hope they'll think about whether, for their study organisms (e.g., ground beetles; hardwood tree saplings; "ovivores" such as crows and raccoons; ragweed and sweet clover), the campus represents a source environment or a sink environment, and how their dispersal ability (possibly human-assisted) affects their small-scale distributions. In a rapidly urbanizing world, increasing attention to the niches and distributions of urban occupants makes sense.
You're right. I neglected to mention Hutchinson's key insight that interacting species can limit (if they're enemies) or expand (if they're facilitators or mutualists) the portion of a species' niche that the species actually occupies. Fig. 1B. What can I say? It was easier to admit the omission here than to integrate the idea into an already long-ish post.
ReplyDeleteEleven months later, I sees only flods in mines piece. Not evens linings of silvers. Or even also even any comments, even also even from troes.
ReplyDeleteHutchinson and Pulliam didn't mention mutualists/facilitators.
ReplyDeleteThat is to say, neither Hutchinson nor Pulliam did.
ReplyDelete