Friendly Invaders

This short article by Carl Zimmer challenges the common view of invasive species as a threat to biodiversity. The prevailing view is that invasive species outcompete endemic species and drive them to extinction. Zimmer reports the findings of two scientists, Dov Sax, an ecologist from Brown University, and Steven D. Gaines, a biologist from U.C. Santa Barbara. The two published an article in PNAS which documented extinctions of native plant species in New Zealand due to invasive species. Here are the numbers: 2,065 native plants, 22,000 non-native plants, 2,069 of which have naturalized, leading to 3 total extinctions. Sax and Gaines argue that invasive species are not all bad. In fact many times the extinctions they are predicted to cause never actually happen. They say that the increased competition creates a selective pressure that spurs the evolution of greater diversity. Often invasives will hybridize with native plants to form new and wonderful species like the common cordgrass (See awesome picture below).

The two scientists say that often there is more than enough room for invaders and the native species. There is also mention of invasions throughout biological history that have led to increases in biodiversity rather than extinction. They give an example of pacific mussels invading north atlantic mussels. Rather than causing the atlantic mussels to go extinct the two species hybridized forming a new species, thus increasing biodiversity.

Critics of Sax and Gaines, such as Anthony Ricciardi from McGill argue that invasive species are a greater threat than ever due to their rapid spread by humans and the pressure already placed on native species due to climate change. He says that the extinctions could be eminent.

So maybe invasive species aren't so bad....think again. I say invasive species are hurting America. We need to put up a wall to keep out the invasives so that they stop taking niches away from hard working native species. I know what you're thinking, they do the jobs no one else wants, if we didn't have honey bees we'd be pollinating all those flowers ourselves, and no one wants to do that. But that's the kind of laziness that led to this:

THE ASIAN CARP!!!!!!

These fatties can grow up to 3 feet long and weigh 100 pounds. They eat everything in sight (consumptive competition) causing native fish to starve. They are threatening to destroy the $4.5 billion great lakes fishing industry (http://www.glu.org/asiancarp). Not only are they threatening to kill off fish, they are threatening to kill us. Asian carp are easily startled and jump out of the water. One fisherman was witness to the dangers of the fish, "One of our guys got hit in the (groin). It's insane," said Bob Bennington, of Streater, Ill."

http://blog.mlive.com/muskegon_chronicle_extra/2007/08/asian_carp_pose_dire_threat_to.html

It certainly doesn't end with the Asian Carp. There are a number of economically disastrous invasive species. They are costing the US billions of dollars in damage. (http://www.mnn.com/lifestyle/pets-animals/stories/invasive-exotic-animals-costing-us-billions-of-dollars). The USDA keeps a comprehensive list of all invasives, check it out at: http://www.invasivespeciesinfo.gov/

Gotelli and Colwell, Quantifying biodiversity: procedures and pitfalls in the measurement and comparison of species richness

Species richness is a common measurement in describing diversity and community structure. It is an important tool in conservation efforts, which frequently aim to maximize an area’s richness and thereby, increase its biodiversity. Despite its simplicity as a concept, species richness, or the number of species present, is often a very difficult measurement to accurately obtain and is even harder to properly apply to an ecological study. Gotelli and Colwell describe some more methods for obtaining species richness data as well as common pitfalls in their utilization.

Presented in Figure 1, are four alternate taxon sampling curves based on two dichotomies. The first being the unit of measurement: individual- or sample-based; and the second being the method of collection: accumulation or rarefication curves. Individual-based simply determines the number of species looking at randomly decided individuals in each plot while sample-based sets random samples and determines the number of species in each and pools the collected data. There exists also a “hybrid” method (not shown in Fig. 1), “m-species list”, in which the first m number of species encountered in a sampling area are listed and is repeated for multiple samples, plotting the number of new species against the sample number. Accumulation curves function similarly in that with each new sampling effort, the new species found are plotted until, ideally, no new species are being recorded and the curve reaches an asymptote. Alternatively, rarefcation curves repeatedly sample the collection of a certain number of samples/individuals, recording the number of species observed with consecutively increasing (or decreasing) sample numbers. When plotted, this too will ideally reach an asymptote, as a some number of increasing samples should ultimately contain all species present.

These curves, which have the potential to accurately describe the species richness of the area in question, need to be applied with caution. Figure 2 shows that a rarefication curve, when rescaled from samples to individuals, gives conflicting data. It is important to compare richness based on individuals rather than samples given the possibility of datasets to differ in their mean number of individuals per sample. There are pitfalls associated with the “m-species list” method as well when comparing species rich to species poor sites (Figure 3). This method requires a greater sampling effort in the species- poor site in order to reach the required number of m species. And should ideally level out sooner than the species rich site. However, in a case where sampling stops before an asymptote is reached, both sites will appear to have similar richness.

Category-subcategory ratios, such as species to individual ratios, are often used to make richness comparisons. Figure 4 shows two examples of pitfalls in using this value. This ratio is dependent on species density, and as seen in 4(a), samples with equal species richness may still show different ratios. Also, the ratio assumes that richness increases linearly with abundance so that it is possible to obtain equal ratios when the richness is in fact different (4(b)). Figure 5 and 6 explain the pitfalls in using a species/genus ratio. This ratio will be greater for a larger sample than for a smaller sample. Traditionally, a low ratio is attributed to strong intergenic competition, but more recent evidence suggests that this is not always the case. Figure 7 compares two rarefication curves (one individual and one sample based), which are used to determine species richness or density. In the individual based curve (a), both treatments must be “rarefied” to the same number of individuals (white dot) to compare the richness as the complete samples (black dots) give the species density, while in the sample based curve (b), the complete samples give the richness, so the treatments must be rarified to the same number of samples to compare the species density.

One final issue of importance is the use of asymptotic estimators in instances where exhaustive sampling is impractical. Extrapolating from rarefication curves does not provide a reliable estimate, as it is essentially a tool for interpolation. Nonparemetric estimators are a better option. One example is based on the number of rare species- a larger amount indicates a greater likelihood of species not represented by the dataset. There are some asymptotic estimators, which have proven accurate when used on small datasets for which the richness is known. In applying these estimators to larger datasets, which have not begun to level off, however, an asymptote is not always reached. In these cases the estimator serves as a lower bound estimate of richness.

Hawkins et al 1997

The authors used life tables of insects to quantify levels of enemy induced mortality. Three factors were analyzed for differences in assocation: enemy type, (predator, parasitoid, or pathogen), the developmental stage of the insect (egg, early larva, mid-larva, late larva, and pupa) when killed, and ecological characteristics of the herbivores (feeding biology, invasion status, and the cultivation status and latitudinal zone of the habitat).

Major Findings:

-Mortality by predators is greater in the later developmental stages.

-Parasitoids kill more than predators or pathogens.

-Fewer endophytes are killed by pathogens and predators than exophytes.

-Within endophytes: leaf miners had the greatest mortality by parasitoids. Galler/borers/root feeders have the lowest mortality by parasitoids.

-Mortality caused by enemies is similar in natural and cultiviated habitats.

-Exotic and native insects do not suffer different enemy induced mortality rates.

- Tropical/subtropical habitats suffer more predation and pathogens.

-Temperate habitats suffer more from parasitism.

In a previous, similar study, the authors had classified mortality into 6 sources. Each of these sources was grouped by ecological category and developmental stage in the life table. They found that source of mortality 1. changed as the herbivores developed and 2. was strongly influenced by feeding biology. They also found that enemy attach was the most frequent cause of death for the herbivores, but were unable to distinguish the different types of enemies. For the present study, one of their goals was to the examine enemy induced mortality in greater detail.

Some issues discussed in class:
-Medians were used as a measure central tendency because the data were not distributed normally.
-The authors cited themselves often, probably to justify why they published two papers.
-For Bonferroni analysis, the accepted p values was very small. The reason for this is because it was a meta-analysis and many studies can accumulate a lot of error.

Gurevitch et al 2000... or... The Sequel: Bigger, Better, and More Amazing?

"Ecologists working with a range of organisms and environments have carried out manipulative field experiments that enable us to ask questions about the interaction between competition and predation (including herbivory) and about the relative strength of competition and predation in the field. Evaluated together, such a collection of studies can offer insight into the importance and function of these factors in nature."

Thus are the opening lines of the abstract of Jessica Gurevitch, Janet A. Morrison, and Larry V. Hedges' April 2000 Paper in The American Naturalist (Vol 155 No 4 pp 435-453). These two sentences seem to state some simple realities about the science of ecology, and ways to look at and think about data collected to come to a few simple (if not somewhat broadly overgeneralized) understandings of the interactions between organisms and between populations. A seemingly easy enough premise to make some sense out of, yes?

No. Nineteen pages later, I'm not sure my understandings of the importance of competition and predation are any more refined or concrete.

I want to respect the integrity of any paper published in a journal like The American Naturalist, but I find it quite easy to get a bit cynical about the meta analysis work of Dr Gurevitch and her collaborators (a feeling shared by at least a few other individuals in this class). The abstract of this paper goes on to say that a "new factorial meta analysis technique" will be employed to address the interactions between predation and competition, but many of the stumbling blocks encountered in Gurevitch's 1992 paper in the same journal, this time co authored by L. Morrow, A. Wallace, and J. Walsh, are encountered in this 2000 paper.

In analyzing the studies of other scientists, the playing field must be leveled so as to compare apples to apples and oranges to oranges. Published summaries of studies may not have included all of the parameters needed by Gurevitch et al to normalize the data for their studies, and we all know that with the fear of arm chair ecologists stealing our lives' works, raw data can be hard to come by. Gurevitch et al are only able to find 20 useable articles on 39 field experiments, a considerably smaller sample size than the 1992 paper, which points out the differences in meta analysis results when looking at different trophic levels and different communities (marine, freshwater, terrestrial, etc). The smaller sample size in the 2000 paper makes conclusions reached and arguments made that much less convincing.

While the sample size and breadth is an issue, some of my problems with the paper are even more basic in their nature. Competition and predation. When we think of one something murdering and eating one something else, the difference would seem obvious. On a population level, however, both are interactions between organisms where one is negatively effected by the other. Interactions where one organism or group just lowers the fitness of another can be quite the sticky wicket to parse out; herbivory and allelopathy in plants would seem to be two of many particularly slippery slopes in these respects. We have toyed with the idea in class that competition might be thought of as mostly between trophic levels, while competition is within a trophic level, but where on that spectrum would cannibalism lie? My point here is that in order to address a question of competition versus predation, one must first define these two classes to a tee. In my humble opinion, Dr Gurevitch and her collaborators do not succeed at this task.

Mention of equilibrium and non equilibrium theories and states of communities and their structuring are also referenced multiple times, but concise definitions of these terms are also lacking. Are all ecological processes not in a constant state of change on some level? At what point do Gurevitch et al define a community or even a species pair as being at equilibrium?

All of this nitpicking over words aside, does the paper come to any amazing conclusions or revelations? The opening line of the Discussion section states "The patterns that emerge from this factorial meta-analysis are strikingly clear, make sense, and are relevant to larger issues of community structure." Bold statement; but less than three paragraphs later, there is a line that is colored a bit differently in my opinion. "Our meta-analysis cannot address these types of questions, but it shows how consistently important predation can be as a factor that diminishes competitive interactions in the field." I hate to parse things a la Fox News to get at my point, but words like "can be", "a factor" (of many?) and "diminishes" are quite relative and wishy washy for something that is "strikingly clear". The ambiguity continues two pages later when Gurevitch et al states "At a very basic level, our conclusions agreed with some of those of Sih et al (1985); they reported that competition and predation were both generally important in field experiments." Independent of the need for an adverb like "generally", it is quite confusing to say in one's discussion that a pair of factors were both important when one of the main points of the paper was to parse out the importance of each factor.

Are Jessica Gurevitch, Janet A. Morrison, and Larry V. Hedges amazing scientists? Undoubtedly. Does this particular paper show all of their amazingnesses in a good (comprehensible) light to a budding ecologist like myself? I unfortunately have to say perhaps not so much. Are there more convincing uses of these meta analysis techniques? Probably. Let's look them up when we find some free time... in July maybe?

Gurevitch et. al 1992... Some Thoughts

Competition definitely plays a role in community structure, but how much and are these effects homogeneous? Gurevitch concludes that at all trophic levels are subject to competition however they are effected at different levels. Carnivorous and deposit feeders and primary producers all have a mean effect size of below 0.4 demonstrating that their is a low level response, whereas filter feeders and herbivores were show to have >1.0 mean effect size demonstrating a large response. However, it is interesting to note that although Filter Feeders and Herbivores were shown to respond to competition stronger most of the studies were done on these trophic categories. This may be bias, or it may be that studies looking at competition in Carnivores, deposit feeders and primary producers have not shown publishable results. So, the take home message is competition plays a role, but the extent of its role is dependent upon trohpic level.

Who is the better competitor?
http://mozey.files.wordpress.com/2007/10/112lion.jpg

Further Thoughts on “Which has a Stronger Impact on Community Structure?” Time?

Hello blog!!!!

So I suppose this makes me one of those terrible persons who lays awake on a Friday night reflecting on a community ecology class instead of… well, what do most twenty-somethings in the Bronx do on a Friday night?

I am trying, however, to (a) get my head around and (b) figure out if I agree with something Mr… well let’s call him Bob… said in class on Thursday (the 18th) towards the end of our discussion on “which has a stronger impact on community structure (and evolution): competition or predation?” His comment, as I understood it, was that time was a bigger/underlying driving force, and competition and predation would both be happening and contributing to changes in various populations anyways, but not driving evolution or community structure per sey. The examples supporting this idea included a sort of island biogeography colonization type thing, with it taking time for more and more species to arrive on an island, and alter the dynamic of the community, and also comparing the tropics to temperate forests, and a notion that the tropics are more diverse and dynamic and the critters in the tropics are more specialized (within a very specific niche) and eccentric and extreme because they have been available to colonize and evolve and specialize for longer time periods than temperate areas, which were covered in thick sheets of ice in geologically recent times.

Interesting idea, which seems to be supported by those examples, but when I think of the idea in a different frame of mind, it does not work out so well. Thinking of a typical, modern, successional cycle in temperate North America, and thinking more of plants and animals you can see without a magnifying glass (I know, what a terrible thing for an ecologist to do), a climax(ish) stage is not typically the most diverse (or species rich for that matter) or even the most specialized in terms of niche availability and use. Little or no edge habitats, reduced understory dynamic, etc etc etc.

So, does the idea that time is the biggest underlying driving force in community structure and dynamic only work with the unique (specific and simplified) parameters of island biogeography (colonization) and the big question of why the tropics are so much richer and more diverse than temperate climates? Or is there something about how I am framing my example and understanding (with climax communities within temperate regions only) that makes it unamenable to ‘Bob’s’ idea?

Seed mimicry in plants

The kind of mimicry I mentioned yesterday, with weeds growing to look like crops, is called Vavilovian mimicry; the wikipedia article has a couple of neat examples. A brief paper on this, which is still apparently one of the longer treatments, is Barrett, S. (1983) Crop Mimicry in Weeds. Economic Botany. 37:255–282. There's a horrible scanned pdf of it available here.