A mechanism on why communities of exotic species are less diverse than communities of native species

Plant communities dominated by exotics tend to be less diverse than plant communities dominated by natives. Apparently, few people have been curious enough to plan an experiment to try to further understand why this is the case. A recent paper in ecology letters Brian Wilsey and collaborators showed the results of an experiment designed to explore this. What they did is to create monocultures of a series of exotics and natives species, and mix cultures of exotics (a mix of 9 exotics, zero natives ) and mix cultures of natives (9 natives, zero exotics). They found that large exotics (plants with high aboveground biomass) tended to be even bigger when growing in mix cultures than in the monocultures, so big plants got bigger, which tend to reduce plant richness since it may displace other plants. On the other hand, for natives, small plants tended to get bigger, which is a mechanism for promoting biodiversity (communities may be more even). This research highlights the importance of understanding the mechanisms of plant coexistence and the fact that exotic species may behave very differently than native species.

Wilsey, B., Teaschner, T., Daneshgar, P., Isbell, F., & Polley, H. (2009). Biodiversity maintenance mechanisms differ between native and novel exotic-dominated communities Ecology Letters, 12 (5), 432-442 DOI: 10.1111/j.1461-0248.2009.01298.x

The evolutionary meaning of autumn colors

As a kid growing up in Ontario, Canada, I have vivid memories of vast expanses of forests set ablaze by their autumn colors. Whole landscapes look like the canvas of a painter whose love of red, orange, gold and yellow are readily apparent. But, like most biologists, I had been taught that these colors are simply the by-product of leaf senescence, nothing more than a biochemical accident. I was amazed to read Marco Archetti's recent work showing that there may actually be adaptive benefits to changing leaf color in autumn and for particular colors. Generally the adaptive benefits involve either protection against abiotic factors or as a response to plant-animal interactions. One of his interesting results is that autumn coloration has evolved repeatedly and cannot be explained by being related to an ancestor who changed colors, rather that there must be some other evolutionary or adaptive explanation. While he suggests a large number of candidate hypotheses, some more plausible than others, I'll list five for example:

1) Sunscreen: Pigments provide photoprotection against photooxidation during the recovery of nutrients.

2) Leaf warming: Colors absorb light and warm the leaves during cooling temperatures.

3) Coevolution: Tells overwintering insects that the tree is not suitable (poisonous or low nutrition) for hibernation.

4) Camouflage: Many insects lack red photoreceptor, making leaves difficult to see -thus protecting trees from overwintering pests.

5) Unpalatability: Pigments (e.g., red -anthocyanins) are unpalatable.

So, we may quibble about particular hypotheses, but the point for me is that there may be deeper explanations as to why certain species produce the vivid colors they do. At a minimum, Archetti provides ammunition to experimental botanists and evolutionary biologists for testing new hypotheses. I'll never look at an autumn forest the same again.

Archetti, M. (2009). Classification of hypotheses on the evolution of autumn colours Oikos, 118 (3), 328-333 DOI: 10.1111/j.1600-0706.2008.17164.x

Archetti, M. (2008). Phylogenetic analysis reveals a scattered distribution of autumn colours Annals of Botany, 103 (5), 703-713 DOI: 10.1093/aob/mcn259

Archetti, M., Döring, T., Hagen, S., Hughes, N., Leather, S., Lee, D., Lev-Yadun, S., Manetas, Y., Ougham, H., & Schaberg, P. (2009). Unravelling the evolution of autumn colours: an interdisciplinary approach Trends in Ecology & Evolution, 24 (3), 166-173 DOI: 10.1016/j.tree.2008.10.006

Being a clover isn’t always so lucky

Happy St. Patrick’s Day! I thought that covering an article about Trifolium (clover) seemed very appropriate. In a recent paper, Matthias Schleuning and colleagues examine the population dynamics of Trifolium montanum, a species in decline in central Germany. They examined the relative threats of habitat fragmentation and degradation on T. montanum’s population dynamics. They found that both degradation and fragmentation were having serious negative impacts. Degraded habitats in this system mean the shift away from nutrient-poor conditions and include the invasion of taller species that are better light competitors. T. montanum is a poor light competitor and maintains larger populations in mown or grazed habitats that keep taller invaders out. This species also faces the double whammy of fragmented habitats resulting in isolated populations. These isolates have lower reproductive output likely due to greater inbreeding and less genetic transfer, via pollinators, among different populations.

I always think of Trifolium species as being particularly common and widely distributed, but there are some that are threatened and potentially tell us about the threats faced by imperiled plant populations. In fact, while a number of North American Trifolium species have successfully invaded North America, but T. montanum is not, according to the USDA Plants Database. These results reveal that these negative effects affect plants at different stages of their life cycle (growth to maturity vs. recuitment) and that log-term persistence of these populations requires management activities that ameliorate both of these effects.

SCHLEUNING, M., NIGGEMANN, M., BECKER, U., & MATTHIES, D. (2009). Negative effects of habitat degradation and fragmentation on the declining grassland plant Trifolium montanum Basic and Applied Ecology, 10 (1), 61-69 DOI: 10.1016/j.baae.2007.12.002

The incredible spreadable weeds

Research into the spread of non-native species usually assumes a long time lag between introduction and rapid spread, and many studies cite 50 years as the lag time. The reason for believing this is that it is thought that there needs to be sufficient time for adaptations to fine tune the fit between the exotic and its new environment, or that densities are so low to start with, finding mates and buffering populations from stochasticity (i.e., Allee effects) takes time. However, Curtis Daehler at the University of Hawaii, collected information on purposeful plant introductions into Hawaii in the 1920s. 23 of those planted have become serious invaders and the herbacious species showed a lag time of 5 years and 14 years for woody species. Knowing that lag times can be much shorter then we previously thought means that monitoring and management activities need to much more aggressive. It seems we can no longer assume a period of relative safety after a new species in introduced, new records of non-natives needs to be followed active assessment and perhaps intervention.

Curtis C. Daehler (2009). Short Lag Times for Invasive Tropical Plants: Evidence from Experimental Plantings in Hawai'i PLoS ONE, 4 (2) DOI: 10.1371/Journal.pone.0004462

Who is Naïve, the invaders or the natives?

Why some species can invade natural ecosystems and many others cannot, is a question that doesn’t have an answer. Many ideas have been proposed to explain this, with relative success, but very low predictability. Most of the ideas have been focused on the factors that promote invasion (i.e. why successful invaders are successful). In a recent ideas paper Koen Verhoeven and collaborators propose a different approach. They ask the question, how ecological mismatches between natives and exotics can explain invasion? They propose a series of predictions based on plant defenses and plant enemies (herbivores, pathogens). They propose that the mismatches between exotic plants and their new enemies could explain their success or failure. For example, if a plant presents a new type of toxic chemical compound that the local herbivores have never encounter and cannot deal with, it would be a clear advantage for the plant (this is related to the novel weapons idea). On the other hand, if the plant has defenses that need to be trigger by a particular enemy (for example an induced defense triggered by a specific chewing insect) that could be a disadvantage for it. They propose that biotic resistance (when the native community resist the invasion) and enemy release (when an exotic invades due to experiencing less pressure by enemies than in its native range) are not oppose ideas but could be the different outcome of these mismatches.

This paper propose a very interesting approach to study some cases of successful and failed invasions, and I look forward for empirical tests of this idea.

Koen J. F. Verhoeven, Arjen Biere, Jeffrey A. Harvey, Wim H. van der Putten (2009). Plant invaders and their novel natural enemies: who is naïve? Ecology Letters, 12 (2), 107-117 DOI: 10.1111/j.1461-0248.2008.01248.x

Grazers chew, cereal gets sick

Managing plant disease is a major part modern agricultural practice, so it is important to understand the basic ecological dynamics of plant diseases. Some theoretical studies have found that the prevalence of plant diseases can be affected by the amount of herbivory in a system. Given that human land-use and the removal of top predators from many ecosystems has fundamentally changed the abundance and distribution of many herbivores, the repercussions of herbivory can have important cascading consequences throughout foodwebs –including disease dynamics. In the first experimental study of the interaction between herbivory and plant disease, the forthcoming paper in PNAS by Elizabeth Borer and colleagues, shows that increased exposure to large herbivores (e.g., mule deer) resulted in higher disease prevalence in the plant community. The disease they studied, barley and cereal yellow dwarf virus (shown in the photo), is transmitted by aphids, so herbivory does not cause increased transfer. Rather, herbivores changed community composition resulting in higher abundances of very susceptible species, creating a feedback where higher abundances resulted in higher infection rates due to a larger pool of potential hosts near by. These results are important for two reasons. First, this particular virus is an important agricultural disease. Secondly, we need to take a whole-community approach to understanding disease dynamics because these dynamics are not only a property of host-vector-pathogen interactions, but are subject to direct and indirect effects from interactions with other community members.

E. T. Borer, C. E. Mitchell, A. G. Power, E. W. Seabloom (2009). Consumers indirectly increase infection risk in grassland food webs Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0808778106

How to plan an experiment that could last 99 years

For a number of reasons, including the fact that most grants only allow for research over a time span of 1-3 years, ecologists and evolutionary biologists usually plan experiments that last few years (with notable exceptions, such as the LTER). A usual approach to study long term phenomena is to take advantage of “natural” experiments. This allows us to understand about processes over long time periods, but usually with limited control on the initial conditions.

In a recent paper by Thomas Bruns and collaborators I learned about another way. They study spores viability of an important genus of ectomycorrhizal fungi, symbiont of Pinaceae: Rhizopogon. Pinaceae (the family of pines and other conifers) need ectomycorrhizal fungi to survive and usually spores and seeds are dispersed independently. It was not known how long their spores can last, which has very important implications, for example for colonization of areas not previously colonized by Pinaceae, or colonization after large scale disturbances, since if seeds cannot find mycorrhizae they have really few chances of survival. Now we know, based on this research that spore banks can be build and last probably decades.

What they did is really interesting, and was inspired on a previous study on seeds. They planted known number of spores of several species of Rhizopogon in terracotta pots, that were later planted into the ground (to mimic natural conditions). They planted 16 replicates, and they plan to open and analyze them later in the century based on the spore viability (for example, if in a few years most spores seem to be not viable that may reduce the expected length of the experiment to increase resolution). This paper found that after 4 years the inoculum potential of these spores seems to be increasing with time. I found the approach used in this experiment really fascinating and I look forward to see what happens in the next years!

Thomas D. Bruns, Kabir G. Peay, Primrose J. Boynton, Lisa C. Grubisha, Nicole A. Hynson, Nhu H. Nguyen, Nicholas P. Rosenstock (2009). Inoculum potential of
spores increases with time over the first 4 yr of a 99-yr spore burial experiment
New Phytologist, 181 (2), 463-470 DOI: 10.1111/j.1469-8137.2008.02652.x

Whence diversity?

It is a truism to say that ecological communities are diverse. They often contain dozens or hundreds or thousands of species that represent many of the deep origins in the tree of life. A recent paper by Prinzing and colleagues published in Ecology Letters tested the hypothesis that communities of plants that include more of the ancient divergences from the evolutionary tree of plants should also contain a greater diversity of physical traits. They examined over 9000 plant communities and found that those that contain fewer evolutionary lineages actually had greater trait diversity than those randomly assembled from more lineages. This result reveals that when communities are assembled from a few lineages (likely due to strong environmental selection -e.g., drought tolerance) those members tended to have evolved large differences. That is, while species may be constrained to certain habitat types due to their evolutionary heritage, successful coexistence depends on maximizing differences.
Andreas Prinzing, Reineke Reiffers, Wim G. Braakhekke, Stephan M. Hennekens, Oliver Tackenberg, Wim A. Ozinga, Joop H. J. Schamine, Jan M. van Groenendael (2008). Less lineages more trait variation: phylogenetically clustered plant communities are functionally more diverse Ecology Letters, 11 (8), 809-819 DOI: 10.1111/j.1461-0248.2008.01189.x