Week 3 - Species, Niches, and Communities
Week 3 Reading:
Chapter 3 of James H. Brown's Macroecology
Patzkowsky, M.E. and S.M. Holland. 2003. Lack of community saturation at the beginning of the Paleozoic Plateau: the dominance of regional over local processes. Paleobiology 29(4): 545 - 560.
Chapter 3 of James H. Brown's Macroecology
Patzkowsky, M.E. and S.M. Holland. 2003. Lack of community saturation at the beginning of the Paleozoic Plateau: the dominance of regional over local processes. Paleobiology 29(4): 545 - 560.
Chapter 3 of Brown’s Macroecology discusses and lays out what I suppose can be called the “units” and biological hierarchy of macroecology. Some ecologists probably wouldn’t like some of what is contained in this chapter, but it is totally appropriate for a macroecological (and incidentally, paleoecological) research agenda.
Species
In the first third of the chapter, Brown sets up a definition of species as “operationally useful biological entities.” This is necessary because species are the unit of measure for most macroecological applications (ecology, biogeography, etc.). In other words, while there are a number of contentious concepts on what a “species” is, this definition provides us with the essentials to do a macroecological analysis. He then proposes an alternative to the traditional hierarchy of biological organization. His new hierarchy puts species above organisms and populations and removes communities from the heirarchy. The rationale is that each level of this new hierarchy has consistent biological units (as opposed to abiotic units). He also points out that higher levels of the hierarchy can not necessarily be well studied by dissecting the lower levels (i.e. rebuilding a complex system from its individual parts).
Ecological Niche
In the next section, Brown defines what an ecological niche is. This is based largely off of Hutchinson’s concept and follows three things: 1) a niche is the multidimensional combination of abiotic and biotic variables required for an individual to survive or for a population to persist, 2) the exponential increase of a species is curtailed by abiotic factors, and inter-specific and intra-specific competition, and 3) the principle of competitive exclusion results in different species occupying the same area having different requirements. Brown points out that, because of all the abiotic factors and interactions between species, niches are extremely difficult to measure. To that end, a number of scientists or specialists would be required to measure the niche of even one species. Brown also argues that through this concept, niches are defined by the organism, which means there can never be “empty niches”. I feel like we should address this last point again during our discussion of the Patzkowsky and Holland paper.
Community
The final section of the chapter deals with the structure of communities. What I ultimately got out of this section was that there is a continuum of ways in which communities form. At one extreme end is MacArthurian (or Clementsian) structure and succession. At this end, community structure is controlled by abiotic conditions. One community is succeeded by another, and that community is succeeded by the next stage, etc. etc. At the other end of the spectrum is Gleasonian individualism, in which communities are built off of the adaptations resulting from biotic interactions (inter-specific and intra-specific competition, predation, etc.). The MacArthurian view is, as far as I can see, more macroecological because of the general pattern that is frequently observed with succession of communities. However, the real answer is likely to exist somewhere in the middle of this continuum. Brown points out that “The interactions of a species with other organisms are just as individualistic as its relationship with the physical environment” and that “the abiotic environment can cause patterns of community structure just as pronounced as those caused by interspecific interactions.”
Patzkowsky and Holland, Lack of community saturation at the beginning of the Paleozoic plateau: the dominance of regional over local processes
“As is also true in macroecology there is no single preferred scale of study in paleoecology; all scales matter and can inform about diversity structure through time.”
- Patzkowsky and Holland, 2003
Anyone who is familiar with the global diversity curve knows that global diversity during the Paleozoic is rather stable, then it shoots up during the Mesozoic, and generally increases up to the present. The Paleozoic plateau is a curiosity that has intrigued many; in their 2003 paper in Paleobiology, Patzkowsky and Holland present data which suggest that this period of time may actually have been unsaturated with species. They argue that regional processes (abiotic?) were the primary drivers for community structure, and that local processes (species interactions?) were less so. In addition to their plots in figure 5, they also provide supporting evidence that suggests that Ordovician communities were unsaturated for 13 Myr. “During times when oceanographic conditions changed little, the taxonomic composition of Ordovician communities was not stable.” Also, that “Saturated communities should also be resistant to invasion by exotic species”, which is not what we see in Ordovician communities.
So, then, what does all this say about empty niches? I kept this second part short, but it should stimulate a lot of discussion.
First off, wow - I don't think a better paper could have been chosen.
ReplyDeleteI don't know if empty niches and unsaturated local communities are necessarily synonymous. I realize now that I extrapolated a bit from the paper, but I came up with two reasons the niches would be unsaturated, but not empty: 1) abiotic factors could make two seemingly similar habitats vastly dissimilar (for purposes of comparing two habitat structures) and 2) if majority of the sea life were generalists (e.g., conodonts which are suspected to be filter feeders), then new, specialist species at the regional level would either cause the generalist to leave (extinction or migration) or cause the generalist to specialize. Although (2) sounds a but like interspecific competition, it is not restricted to the local level. These were the two ways I could conceive that all niches at t1 (prior to invasion) were occupied and then at t2 (after invasion) still occupied. That is, rather than trying to remain at some static homeostasis, the community went to a new steady state to retain community-level homeostasis.
The main difference I found between this paper and Brown's is that this paper includes communities, which have abiotic factors. I wonder if the paper focused on some other unit, such as monophyletic lineages, the paper would have come to a different conclusion (i.e., each ecosystem under study had same phylogenic distribution)?
So, I'm in the middle of reading this article, but I just can't wait to post this. I see an issue with the study carried out by Patzowsky and Holland: Unless they literally had annual varves in their sequence, there is necessarily going to be some time averaging of communities. Since community species composition probably varies over time, even if the fundamental structure stays the same, there is probably some species replacement within communities. If species are being replaced, you're necessarily going to overestimate the number of species in the community at one time. An overestimation might mask the signal of community saturation -- it might bring up the line in figure 1 closer to the regional diversity line. I'll write more after I finish the paper...
ReplyDeleteFor starters this chapter was a thorough analysis of species, niches, and communities. It is sometimes hard to grasp these concepts when you try to think of them in the mix with one another. However, I love the way that Brown explained their interactions and what each is individually. It is immensely overwhelming when we try to understand how these components of life interact with each other. At the simplest level they are easy to understand, but when thinking about every factor in the equation, I feel it can get hairy.
ReplyDeleteThe definitions Brown laid out, and the examples he gave did justice to how ecosystems interact and how they came to be. This chapter fits beautifully with the macroecological approach. I have several discussion points and questions I hope we can talk about next week. If not then I will post them for a blog discussion. I am about to get to the paper, I almost forgot about it so I am sorry this post is late!!!
So, I will start out with my criticisms an that way my comment will end with something sweet. I think that I may not be grasping this but I feel that their study area wasn't necessarily a thorough representation of biological diversity at the time. Or were they not concerned with a larger scale diversity estimate? Maybe I am missing something the authors said about this but thats one concern of mine. Secondly, how is it possible to tell whether or not the Nashville-Dome region was saturated at this time? Couldn't there be environmental factors that we are unaware of that played a large role in whether other marine species could migrate to the region or not? My knowledge of fossil records may be leading me astray but I am confused how the authors can draw these broad conclusions about the study area.
ReplyDeleteOverall, I enjoyed this paper more than I found it to be misleading. The idea that we can come to conclusions about the species in this region fascinates me. Being able to study the past helps us come closer to understanding the complex systems that govern life on this planet.
Another part-way-through the article comment. It looks to me like figure 5C does show saturation. Back for more in a bit!
ReplyDeleteSo, final thoughts on the Patzowsky and Holland paper: although I think the findings in this paper are interesting, I don't think they adequately address the time averaging problem, and I'd like to see more quantitative description of the local vs regional diversity curve. It seems they only used visual inspection to determine this last question, and I think quantitative models of proportional sampling vs. saturation might have found saturation to be more accurate in some of the curves.
ReplyDeleteI feel as though I'm missing something about this article, which is generally the case when I start screaming at my computer halfway through the discussion... Usually I get to the end of the paper and call myself an idiot for not understanding whatever I had been yelling about, but it just didn't happen with this article; here are some of my issues-
ReplyDelete1)How indicative of diversity can looking at braciopods really be? To my knowledge, brachiopods have been fairly stable since they first arose, like the cockroaches of the fossil record; wouldn't it be more useful to look at something less constant and more prone to be effected on a small scale? This strikes me as trying to look at the changes in annelid diversity to figure out how many cheetahs there are in an area.
2) Of course regional diversity increases with community diversity- regions are made up of communities. You could look at global diversity and get the same results.
3) How reliable is half a square meter in determining an entire community's diversity? Wouldn't that require that all the organisms live together in that same area?
4) How sure are we that organisms are really filling niches when they invade the community? How do we know they're not outcompeting other species for a previously filled niche over time? Does Brown's argument of no empty niche just throw this entire hypothesis out the window?
5) Changes in regional abiotic factors effect communal abiotic and biotic factors (if sea levels rise, a marine community of study may now be a much deeper marine community of study, which effects inter and intra-specific interactions). The same thing would happen if there was a localized change in water depth (I have no idea how it would happen, just go with it), so how can we say definitively that it was the regional change, and not just the resulting local biotic change that effected diversity?
Please clarify... my dog is still cowering in the corner waiting for me to stop twitching with anger and confusion.
Hmm I think I am confused on time averaging, maybe we could discuss this technique next class?
ReplyDeleteI really like the point Natalya brought up about there being larger environmental factors prohibiting migration.
I also am not so sure of what type of dispersal ability brachiopods had throughout history.
To address some of Kat's frustrations:
1) While brachiopods have been stable, brachiopods come in many different shapes and sizes. I think (but could be wrong) that using them as a measure of biodiversity is to look at how many different types of brachiopods are found (?)
2) This thought assumes that a) the larger, macro-level processes can be explained by micro-level processes and b) that the increase of regional vs. local is proportional (which I believe was the study's null hypothesis)
3) A square meter to measure diversity - I think that the question of if it is appropriate or not depends with the organisms in question. I think that it would be appropriate for looking at a benthic, marine community. It would be cool to discuss this in class.
Undisturbed bedding planes have been shown to represent relatively short periods of deposition, certainly short enough for the purposes of this study. If they weren't buried quickly, then then are eroded, which is something that can be seen in the rocks themselves. My experience, also, is that fossils occur in discrete horizons in undisturbed rock. Also, there wasn't any mixing, which would also be obvious.
ReplyDeleteIn short, I don't think time averaging was an issue here, not in this paper. I can definitely point to studies where this is a HUGE problem.
Folks - Melissa Pardi asked me if I wanted to add some comments to your discussion, so I hope everyone is OK with this. So far, no one has posted anything to regret.
ReplyDeleteFirst, a few comments about the fossil record. I think we have a good record of what lived in the area at that time. Jim Valentine once compared the Recent and Pleistocene molluscan fossil records from California and concluded that over 80% of Modern species in CA are known from the CA Pleistocene. There is no reason to believe that our situation is any different. Most of the skeletonized species that lived in the Nashville area during the Ordovician have been fossilized and reported. Brachiopods dominated the skeletonized taxa during the Paleozoic, so I think it is fair to use them to infer processes about biological diversity. It is not unlike an ecologist making similar inferences based in the study of species within a single higher taxon. Second, time-averaging is a fair issue to raise. I recall thinking long and hard about this at the time. Our best argument may have been that time-averaging of collections is probably on the scale of decades to a few hundred years, where as the individual time slices used for regional diversity were on the scale of a million years. Because we sampled sedimentary beds that were formed by similar processes, there is no reason to believe that the average amount of time-averaging varied through time. I think, then, that changes in local (time-averaged) diversity through time, must reflect largely the processes that control local diversity. We concluded that it was the regional processes that determined the size of the species pool.
Someone pointed out that of course regional diversity increases with community diversity. Well...it doesn't have to, but it gets at a serous criticism of this approach, which is the scale dependency of the observation. Similar to time-averaging, if the size of the local sample increases enough, it is bound to approach regional diversity. Our samples are very small relative to the size of the region, so we avoid this, but concern of scale-dependency is a good one. There are many similar kinds of criticisms of this approach that we try to deflect in the paper. I suggest you read the Srivastava paper that we cite as a good place to start. The same person also brought up the issue about water depth changes - another good point. We tried to hold any effects of sea level change constant by sampling the same range of water depth through time. We can do this by a detailed study of the sedimentary structures preserved in the sediments, which indicate processes that can be assigned to a general range of water depths.
I think there is other good evidence to suggest these communities are unsaturated. We recently looked at time series data through a biotic invasion that shows that regional diversity and habitat diversity increases with the biotic invasion suggesting unsaturated communities (Paleobiology, 2007, v. 33, 295-309). Interestingly, local diversity does not change, which makes me think that the scale of our local samples may be too small to show any pattern, which does call into question the specifics of the paper you are reading. These data are from another area, but at the same time.
Final comments - the main view that we have been arguing against in the paleontological literature is that marine communities show long intervals (millions of years) of stasis in taxonomic composition and relative abundance relationships. I think we are building the data to show that the extreme stasis view is incorrect, but what we do see is that turnover in communities over geologic time forms a spectrum from little change to lots of change. We would like to know what are the processes that control this variability in turnover? Jim Brown's Macroecology book has been very influential in my thinking along these lines. I just looked at my copy and Chapter 3 is filled with underlinings and side notes.
Thanks Mark!
ReplyDeleteThis is a very late comment, and it comes after so many good questions and even the author's response (Thank you Mark). I was confused about the issue of time averaging as well, but more for my own lack of understanding of how it compounds over such a time scale. I am used to very discrete measurements, even when I was doing ecology, this is going to be a major ethos change over the semester. When reading my main concern involved the sample size determining the local, which was brought up in the comments. I just have a hard time resolving both spatial and temporal elements of the samples. I am looking forward to the discussion.
ReplyDelete