Lunch in the tree-tops for the birds and the bugs

A few weeks ago, our laboratory published a paper in PeerJ (an open-access journal) titled “Vertical heterogeneity in predation pressure in a temperate forest canopy“. This work resulted from a project by former Master’s student Kathleen Aikens. She graduated a little while ago, and although we published one of her thesis chapters in 2012, it took another year to get this paper out, in part because Kathleen and I both become too busy.  Thankfully, post-doc Dr. Laura Timms agreed to help us finish up the paper, and she worked with me and Kathleen to re-analyze the data, re-write some sections, and whip it into shape.

As is now traditional for my laboratory, here’s a plain-language summary of the paper:

Tree canopies, including those in deciduous forests in southern Quebec, are important for many different animals, including insects and spiders. These small, marvelous creatures crawl up and down trees with regularity, feed upon the leaves of trees, feed upon each other, and are food for animals such as birds and bats. Past research has shown that many species of insects and spiders live in tree canopies, and in general, more insects and spiders are found closer to the ground compared to the very tops of the trees. This makes sense, since deciduous tree canopies often need to be recolonized each spring, and tree canopies are relatively harsh environments – they are windy, hot, and often-dry places as compared to the forest floor.  What we don’t know, however, is whether the insects and spiders avoid the tree canopies because they may be eaten more frequently in the canopy as compared to the understory. The objective of this research was to test this question directly, and find out whether insects and spiders are arranging themselves, vertically, because predators may be preferentially feeding on them along this vertical gradient. This is a very important area of study since biodiversity is highly valued and important in forests, but we cannot fully appreciate the status of this diversity without discovering what controls it.

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Our mobile aerial lift platform. TO THE CANOPY!

We did this work by using two experiments that involved manipulating different factors so we could get at our question in the most direct way possible. In the first experiment, we made ‘cages’ out of chicken wire and enclosed branches of sugar maple trees in the cages. We did this at the ground level all the way to the tops of trees, using a ‘mobile aerial lift platform’. These cages acted to keep out large predators, such as birds, but allowed insects and spiders to live normally on the vegetation. We counted, identified, and tracked the insects and spiders both within these cages, and in adjacent branches that did not have cages (the ‘control’). By comparing the control to the cage, we could find out whether feeding activity by larger vertebrate predators affected insects and spiders, and whether this differed when comparing the ground to the top of the trees. In the second experiment, we used small pins and attached live mealy worms (larvae of beetles) to the trunks of trees, and we did this in the understory all the way up to the canopy. We watched what happened to these mealy worms, and compared what happened during the day and overnight. This is called a ‘bait trial’, and let us figure out what sort of predators are out there in the environment, and in our case, whether they fed more often in the canopy compared to the ground-level. This second experiment was designed for seeing the effects of insect and spider predators along a vertical gradient whereas the first experiment was focused more on vertebrate predators (e.g., birds).

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Munch munch. Carpenter ants feeding on mealworms.

Our results from the first experiment showed that the cages had an effect: more insects and spiders were found when they were protected from predation by birds. Birds are playing a big role in forest canopies: they are feeding on insects and spiders, and in the absence of vertebrate predators, you might speculate more insects and spiders would occupy trees. Our second experiment showed that ants were important predators along the tree trunks, and overall, the most invertebrate predators were found in the lower canopy. Both experiments, together, confirmed that the understory contained the most insects and spiders, and was also the place with the highest amount of predation pressure.  The take-home message is that there is an effect of predation on insects and spiders in deciduous forests, and this effect changes if you are in the understory as compared to the top of the canopy. We also learned and confirmed that insects and spiders remain a key element of a ‘whole tree’ food web that includes vertebrates such as birds, and that predators in trees tend to feed on insects and spiders along a gradient. Where there is more food, there is more predation pressure! Our work was unique and novel because this is the first time a study of predation pressure was done along a vertical gradient in deciduous forests. It will help better guide our understanding of forest biodiversity, and the processes that govern this diversity.

A more detailed discussion of this work is posted on the PeerJ blog.

Assessing five decades of change in a high Arctic parasitoid community

As my colleague Terry Wheeler mentioned on his blog, our Northern Biodiversity Program team is thrilled to see post-doc Laura Timms‘s paper about Arctic parasitoid wasps published in Ecography!  Our team worked on Ellesmere Island, Nunavut, in 2010, and compared parasitoid wasps to historical collections from the same site that were made in 1961-65, 1980-82, and 1989-92. Parasitoid wasps are at the top of the insect food chain: they lay eggs inside or on top of other arthropods and the wasp larvae emerge after consuming their hosts – a gruesome but very common lifestyle for many types of wasps.  Species at higher trophic levels, such as these parasitoid wasps, are often the first to respond to new environmental pressures, including the climate change that is occurring rapidly in Arctic systems.

Laura identified a LOT of wasps, recorded the type of host attacked (e.g. plant-feeding hosts versus hosts that are predators), and the body size of two species of wasps that were commonly collected in all time periods.  We found no clear pattern of change in most aspects of the parasitoid wasp community on Ellesmere Island over past 50 years, even though temperature and precipitation have increased significantly during the same period. However, there were some signs that parasitoids of plant-feeding insects may be more affected more than other groups: one common parasitoid species that was abundant in 1960s hasn’t been collected since then, and the community in the 2010 study contained fewer parasitoids of plant-feeding insects than previous studies.

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Some members of the Northern Biodiversity Program working in the Yukon in 2012. (l-r, Chris Buddle, Laura Timms, Crystal Ernst and Katie Sim)

Laura takes it as a good sign that no major changes in the ecology of the high Arctic parasitoid community have been observed, but isn’t taking it for granted that the community will remain unaffected for long.  At 82°N, Ellesmere Island is relatively isolated, but other research has found that parasitoid communities further south are changing dramatically (Fernandez-Triana et al 2011).

Laura has the following comment about our work: “We hope that our findings will be used as baseline data for ongoing monitoring on Ellesmere Island”, said Timms.  “We know so little about these high Arctic insect communities, we should learn as much as possible about them while they are still intact.

References

Timms, L., Bennett, A., Buddle, C., & Wheeler, T. (2013). Assessing five decades of change in a high Arctic parasitoid community Ecography DOI: 10.1111/j.1600-0587.2012.00278.x

Fernandez-Triana, J., Smith, M., Boudreault, C., Goulet, H., Hebert, P., Smith, A., & Roughley, R. (2011). A Poorly Known High-Latitude Parasitoid Wasp Community: Unexpected Diversity and Dramatic Changes through Time PLoS ONE, 6 (8) DOI: 10.1371/journal.pone.0023719

Taxonomic sufficiency in biodiversity research: Is it always necessary to identify species?

It’s been a successful few weeks in the lab!  Two weeks ago I promoted an exciting paper about spider silk and herbivory and just after that paper come out, another publication from our lab was published, titled: “Does species-level resolution matter? Taxonomic sufficiency in terrestrial arthropod biodiversity studies“.  This paper evolved out of a past graduate-level class in Forest Entomology at McGill, and was re-worked and re-written by post-doc Laura Timms, former Phd student Joseph Bowden, and my colleague Keith Summerville.

Let me provide a plain language summary of this work and I will also touch upon some of the controversy that has arisen because of this paper:

Biodiversity science is about the discovery and description of all the different kinds (species) of organisms living on our planet.  It is a vitally important area of research because different species play important roles in our ecosystems, and as a consequence, are important to us.  The different number of species in an area can also inform us about how we might be harming or helping ecosystems.  This is an active area of study in the context of forestry, since some forest practices (for example, cutting all the trees down in an area) can cause changes in the number of species (and whether they are rare or common) and these changes can inform us about whether our forestry practices are harming our ecosystems.  All of this kind of work, however, depends on the ability of scientists to collect, sort, and identify different kinds of species.  Since most described species on the planet are Arthropods (e.g., spiders, insects, and their relatives), these animals are often used as a way to indicate how biodiversity might be affected by environmental change.  However, there is a problem: it takes a very long time to identify different arthropods, and it is costly and difficult – requiring highly specialized training, by people known as taxonomists.  In our research project, we asked whether not you always need to know the exact differences between insects and spiders  in order to tell if a disturbance is affecting biodiversity.  We did this by looking at a series of data-sets about beetles (Coleoptera), moths & butterflies (Lepidoptera), and spiders (Araneae). These data-sets were from past research projects about how forest disturbance affects biodiversity.

Here is how we did the work: Different kinds of organisms are classified using a two-part name:  the genus and the species.  There can be many different species within one genus.  You can then classify different genera (the plural of genus) into grouping called Families.  For example, all wolf spiders are in the Family Lycosidae.  A common genus within this family is Pardosa – there are dozens of species of Pardosa in Canada; Pardosa mackenziana, Pardosa moesta, Pardosa hyperborea, etc.  We first took our big data-sets and using the lowest level of naming (the species) we asked whether forest disturbance affected biodiversity.  We then grouped all our species into their respective genera -this meant that the data-sets got smaller (i.e., there are necessarily fewer genera than species).  We did the same analysis to see if we could still get a signal about the effects of disturbance on biodiversity, but now with the ‘reduced’ data.  We did this again at the family level.  We did this because we wanted to know if you could take a short-cut. Stated another way, if you don’t have the time or ability to figure out all the species in your research project, can you still see if there is an effect of forestry on biodiversity?

A wolf spider (Lycosidae)

A wolf spider – do you need to know its name?

Our results showed that in most cases, you do not need to know the species identity to see the effects of forestry practices on the biodiversity of spiders, beetles and moths & butterflies – you do not get as clear answers when things were grouped into Families, but the datasets with species grouped into genera were almost as good as when you group things into species.  This was surprising, because an assumption in biodiversity science is that species-level identifications are necessary and should be the ‘gold standard’ for this kind of research.  We showed that in many cases, you can get your answer by identifying arthropods to the generic level:  this can save you a lot of time (and money).   Some researchers (including taxonomists) may not be thrilled with this result as it might suggest that species are not important, and specialized taxonomic knowledge is not essential to complete biodiversity research.  This is certainly not the case, which leads me to the caveats:

1) Our results do not mean species are not important!  Instead, we are saying that if there are logistical and financial constraints, you might be able to answer your research question without having to identify all the species.   If you have a project about large-scale disturbance and are looking to see whether there are any broad affects on biodiversity, our approach might work.   However, you might miss some subtle effects, so this approach must be taken with caution.  Although our suggestion is a short-cut, it would still be important to save all the samples, and at a later time (as money and expertise permits) the species could be determined.

2) Our study is specifically geared towards research about insects and spiders in relation to large-scale forestry disturbances.  We are not saying that this will work in all situations and with all different kinds of organisms! The context is important.  Related to this, if an overarching research question is about species in an ecosystem, species-level identifications are essential.  Everything depends on the research question and the research context.

3) This general approach that we have discussed is highly dependent on what kind of organisms you are studying.  If you are working with a group of organisms that do not have too many different species within a genus, our approach may work.  If, however, there are many species within a single genus, our suggestion will not work as well.  Therefore, a researcher should look at the general relationship between the number of species per genus for their study organisms and use this ratio as a guide when thinking about taking the short-cut that we discussed in the research.

In sum, we are quite excited about this research – we think it will provide more opportunities for biodiversity projects to get done, and will help answer certain research questions when there are substantial constraints on time and money.  This is one way to be pragmatic about biodiversity research.

Please share your thoughts!

Reference:

Timms, L., Bowden, J., Summerville, K., & Buddle, C. (2012). Does species-level resolution matter? Taxonomic sufficiency in terrestrial arthropod biodiversity studies Insect Conservation and Diversity DOI: 10.1111/icad.12004