Can intraspecific differences lead to ecosystem differences?

Sara Lindsay Jackrel and J. Timothy Wootton. 2013. Local adaptation of stream communities to intraspecific variation in a terrestrial ecosystem subsidy. Ecology. Online early.

It’s funny how complex outcomes can arise from simple realizations. For example, it is plausible that when there are differences among individuals of a species (like when local populations are adapted to the local environment), these could implications for function on the ecosystem scale. But while there is increasing evidence for the importance of intraspecific variation for ecological interactions within communities, the question of how intraspecific diversity scales up to ecosystem functioning is still ambiguous.

Sara Jackrel and Timothy Wootton explore this question in “Local adaptation of stream communities to intraspecific variation in a terrestrial ecosystem subsidy”. The basis for their study was simple: local adaptation is common, and populations/genotypes/ecotypes tend to be best adapted to the particular conditions of their locale. For example, “spatial variation in prey and predators can lead to a geographic mosaic of co-evolutionary interactions”. Further, these localized interactions can affect the greater ecosystem, if individuals or materials move between ecosystem boundaries.

In particular, the authors note that there is evidence that tree species composition riverside can alter the composition of the local aquatic community. This occurs via leaf litter fluxes into the river: the type and amount of leaf litter that falls into streams varies, and so the type of macroinvertebrates in the recipient stream also varies in response. These macroinvertebrates break down the leaf litter via shredding, collecting, and filtering, playing an important role in nutrient cycles. Leaf litter is carried from a given tree by wind or water and may decompose near or far away, creating a connection between ecosystems. The question then is whether macroinvertebrate compositional shifts will occur in response to intraspecific differences in leaf (i.e. trees), and what the implications might be for ecosystem functions such as leaf decomposition. To explore this question, Jackrel and Wootton performed reciprocal transplants of leaf litter material between eight sites along rivers in the Olympic Peninsula of Washington.

All eight of these sites were early successional forests dominated by red alder. The authors collected fresh leaves from alder trees, bagging leaves from each tree separately. These bags of leaves were either placed in the river adjacent to the trees they were taken from, or in a more distant site. Non-adjacent sites were either in the same river as the home site, or in different river all together. Leaf packs were weighed before and after spending 17-18 days in the river. This would allow comparison of how decomposition rates vary between home and away sites, and between home and away rivers.

Their results suggested a few interesting things. First, the identity of a tree affects the rate of decomposition of its leaves: individual alder trees’ leaves were highly variable in the rate of decomposition. Second, the combined identities of trees at a site seem to have affected the composition of the decomposer community at the home river site: put leaves from that site in another river with a new community of decomposers, and the decomposition rate drops significantly. In general, leaves decomposed significantly more rapidly when in their home river, regardless of whether at the home site or elsewhere along the river. But if they put leaves upstream from the home site, but in the same river, the rate of decomposition also dropped. Upstream decomposer communities were apparently much worse at breaking down leaves from novel communities of alders. However, if you put the leaves in sites downstream from home, the decomposition rates are not significantly different than in the home site. This is likely because of the directional movement of a river, such that downstream locations receive leaf litter from all upstream sites, and so downstream decomposer communities experience a greater variety of leaf litter than upstream sites. This might lead to upstream sites being more closely adapted to the individual trees in their neighbourhood than downstream sites, which receive inputs from a wide variety of trees. These results suggest that individual differences in trees at different spatial locations can matter, both locally, across trophic levels, and even across ecosystems.

Admittedly there is not a lot you can infer about the mechanisms at play from this preliminary experiment. One interesting follow up would be to measure compositional differences in aquatic macroinvertebrates at very fine scales in correspondence with differences in trees. Another important question is whether these communities differ via phenotypic plasticity, adaptation to local sites, or species sorting. But this paper does hint at one way in differences among individuals can shape local ecosystems and even structure distant ecosystems (e.g. downstream decomposer communities) through fluxes across boundaries. And that is a rather complicated implication from a logical and simple starting point.

Research in dangerous times

Research is full of disasters. Some are small – there’s nothing on the gel; someone published it first. Others are larger – ATVers just crushed the plants you were monitoring; you lost your lab notebook. But there are also real disasters – floods, fires, hurricanes, tornadoes, earthquakes.

The university where I am a postdoc, University of Colorado at Boulder, suffered through a 100-year flood this week. Though the university dealt with less water than many other parts of Boulder County, one-quarter of campus buildings have flood damage of some sort, there were power outages and the campus was closed and fairly inaccessible for several days. It sounds like my building got through okay – little to no water got in other than some roof leaks, and the power didn’t even go out. The water crept up into the parking lot, but never made it the rest of the way. But given the extent of the damage, undoubtably some other CU labs weren’t so fortunate. (And many people in Boulder County were not lucky at all).

What this experience reminded me of were the stories I had heard about university laboratories in New Orleans after Hurricane Katrina. The floodwaters there destroyed laboratories, took out research computers and servers, and killed 8000+ research animals in populations at Louisiana State University and Tulane University. This probably represents the loss of years of work and millions of dollars of funding. Data kept on damaged servers were gone. Research is generally so reliant on things we take for granted – like a reliable power source to maintain environmental conditions, and safe locations for irreplaceable data. When campuses are closed down, it may be impossible to reach laboratory populations of species that rely on regular care; routine measurements that may have been occurring for months or more are often disrupted; all the samples stored in a -80°C freezer might thaw out. And these are not uncommon outcomes when disasters hit universities. After Hurricane Sandy, criticisms were levelled at NYU for housing research animal populations in a basement, despite the potential for flooding. As a result, flooding killed thousands of mice, including strains representing a decade of research on forebrain development.

The personal impacts of these kind of losses are the saddest. The loss of months or years of data or experiments can be the nail in the coffin for grad students, who are reliant on time-limited funding as it is. A large enough setback may be the difference between finishing and dropping out for many students. Researchers, especially without tenure, face setbacks that could range between demoralizing and debilitating.

Like it or not, most predictions suggest disasters are going to be more common in the future. Often research labs rely on the disaster preparedness plans of the institution, and don't have specific plans for an individual lab. Every group (individual labs, departments, colleges, the institution as a whole) to some extent should consider disaster management plans. For most ecology labs, the considerations would not even be too onerous – consider how to maintain power to equipment that must remain on (e.g. freezers, experimental gear); ensure data is backed up in more than one physical location; if water damage is a possibility, be careful what you keep in the basement lab. Also, be sure to have contact information (including phone numbers) for all lab members - it's surprising how many labs neglect this - this way you can coordinate and ensure labmates and students are safe. Sometimes, no matter how prepared you are, loss and damage will be unavoidable. But hopefully, you are lucky and all of these preparations will be for nothing.

When enemies catch up – declining invasive impacts in hogweed?

Dostál P., Müllerová J., Pyšek P., Pergl J. & Klinerová T. (2013). The impact of an invasive plant changes over time. Ecology Letters, Early View.

Invasive species are a major ecological issue in this age of global connectivity. Many ecosystems are unrecognizable today after invasion, and invasive species have considerable impacts on agriculture, tourism, aquaculture, forestry, and native biodiversity. Some invasive species have been present in novel environments for decades or centuries, but many are more recent introductions. What we are still trying to understand is whether invasive species impacts are constant through time. Do ecosystems and communities adapt to invasive species, and how do they do so? Some examples of how the impacts of invasions can decline through time exist: for example,  the efficacy of allelopathic chemicals produced by garlic mustard (e.g. Alliaria petiolata) may lessen as native species adapt. Another hypothesis is that specialized pathogens or herbivores, absent at the start of the invasion, will increase with time, reducing populations of invasive species and allowing the recovery of native species.

Do specialized pathogens
find escaped species eventually?

If specialist enemies eventually catch up with invaders, this could reduce invasion impacts eventually, but long-term empirical data showing this type of effect is almost nonexistent. Dostal et al. (2013) provide some of the first  strong evidence that specialist pathogens catch up with and reduce the impact of giant hogweed (Heracleum mantegazzianum), a species from the Caucasus now found throughout Europe and North America. Hogweed invades successfully for a number of reasons, including its ability to form large, dense stands that reduce light availability, as well as through the production of allelopathic chemicals.

Giant hogweed was first introduced to the Czech Republic as an ornamental in 1863, but became invasive when grassland management ceased during WWII. The authors took advantage of aerial photographs taken since 1964 to locate hogweed presence. They identified a chronosequence of sites invaded for at least 48 years, 42 years, 28 years, 11 years or never (hogweed-free sites). These sites were on average 1.3km apart, and shared similar management history and environmental conditions. The authors then surveyed the sites, recording current hogweed percent cover, native biomass, and native species richness.

The researchers also collected soil samples from these sites. These samples were used in a common garden experiment to test whether soil pathogens that affected hogweed success might differ between the sites with different ages of invasion. Soil from the sites with different ages of invasion history was either sterilized to kill living pathogens, or left unsterilized. To these different types of soil (sterilized or not, 0, 11, 28, 42, or 48 years of invasion), they planted 1) 1 hogweed seedling, 2) a mix of native species seed, or 3) 1 hogweed seedling and the mix of native species together. They then looked at how well the hogweed seedlings survived.

Results from the observational data showing changes in native productivity and richness,
and hogweed cover as a function of time since initial invasion. (Dostal et al. 2013)

Hogweed showed decreases in biomass as the length
 of invasion history in the soil increased, but
 only if the soil was not sterilized. (Dostal et al. 2013) 

Hogweed, as expected, had very negative effects on the native species in these sites: on average, hogweed invasion decreased native species richness by ~10 species, and similarly decreased native biomass. These two sets of experiments however, showed clearly that as hogweed invasion age increased, hogweed cover declined. Further, native species richness and biomass increased once invasions were more than 28 years old. The common garden experiment suggested that this might be due to hogweed-specific soil pathogen accumulation through time. Non-sterilized soil from sites with longer histories of invasion had a significantly more negative effect on hogweed biomass than the sterilized soil. This suggests that living components of the soil -presumably pathogens - differed between soils with different invasion histories. Directly quantifying or identifying soil pathogens would be the obvious and important next stage.

Dostal et al. (2013) suggest that for these sites in the Czech Republic, hogweed dieback due to increased specialist pathogen load will prove key for native species recovery. They make it clear that this is not the same as advocating for a lack of response to the invasion. Ignoring established sites is providing a source for seed for new sites, puts rare native species at risk, and may leave management concerns such as declining ecosystem functions untouched. The other issue is that density dependent, stabilizing processes like increases in specialist pathogens, may lead to boom and bust cycles – initial rapid invasion might be followed by declining invasive populations as specialist enemies increase, but declining invasive populations would lead to declines in specialist enemies, and increased invasive pressures could start anew. Such a situation wouldn’t lead to a general decline in invasive species, but rather a new (non-equilibrium) state for the community.

The evolution of evolution, LEGO in the lab and other Science-y links

My week is coming to an early end as I head off to some friends' wedding tomorrow, so in lieu of another post, here are some interesting science links from around the internet this week :)

This infographic explores how thinking about evolution has changed since Darwin. It shows pretty clearly the circuitous path that science takes, the way ideas converge and diverge, and ultimately become more nuanced and complicated.

A theoretical physicist blogger answers the question "should you write a science blog?". She mentions the basic, but undeniably key points - do you have time? do you really have time? do you like writing? I also like her advice: "don't be afraid of your readers".

Mental illness can be exacerbated or first show up during grad school. Even in liberal academia, talking about mental health issues can be a bit taboo, something that doesn't help anyone. A blog post from Nash Turley considers the issues and implications.

Another serious issue, related to issues of gender in science: an article in the Economist presents evidence that women authors tend to be less cited than male authors, and this was in part due to less self-citation by women.

Also, LEGO now has its first female scientist character, and with her short hair, goggles, lab coat and gloves, she's a great lab safety role model too ;)

Of course, LEGO has many other science-related applications :)

Edit - the link about blogging was initially incorrect, should be correct now.