It seems surprising to me that as you glance over the current issues of the most popular science journals, very little is done about plants in general, let alone plant ecology. Of course, I look through the Journal of Plant Ecology and that’s all it is. But, very few of those experiments are natural experiments. There is such a heavy focus on manipulations and complex experimental designs the natural experiments seem to get lost in them.
For about 8 months, there have been three issues of Bioscience sitting on the coffee table in our lab. I was feeling very “blog-uninspired” today and thought –what the heck, I might as well flip through these and see if there is anything I could use. The first article I came across had to do with Biodiversity losses and ecosystem function in freshwaters. My first thought was- nope and I turned the page. But what I found right in the middle of this article was a beautiful chart that outlined my whole entire idea for starting a blog about natural experiments. Needless to say I read this paper, and have some interesting things to report!
Caryn C. Vaughn took a very in-depth look into recent research that has focused on how biodiversity losses may affect ecosystem functioning in aquatic systems and noticed some trends developing that complicate the ability to do so.
Species traits determine ecosystem function
There have been numerous studies which address how biodiversity influences ecosystem function. Experimental set-ups, manipulations, mesocosm experiments, natural experiments, surveys, you name it-it’s been done. The problem with these studies Vaughn claims is that they only examine specific, single traits i.e.: primary productivity or breakdown of organic matter. The problem with focusing on single traits is that a species’ role is not that simple and this oversimplification likely means that the estimates of the impacts of species losses are likely lower than they are in reality.
Species within functional groups are not necessarily ecological equivalents
Species are often put into groups based on the role that they play in ecosystems- function groups. Recently, many studies have been focused around the diversity of plant functional groups and the effects of removing functional groups or species from functional groups (Hooper at el. 2005). Several studies also suggest that if one species from each group remains an ecosystem can still function so long as the other species increase in abundance. This is known as functional redundancy.
Functional redundancy is a well-known and researched topic, but as Vaughn suggests, there are problems with using redundant species within functional groups to simply the study of ecological systems. Specifically this is a problem in aquatic ecology as freshwater species are often placed into groups based on life history traits, behaviour, habitat, etc. These traits likely do not translate into shared ecological function. Also, it is nearly impossible to understand and interpret functional redundancy when the environmental variables are changing.
Biodiversity losses include declines in the abundance of common species
Most studies focus on what happens if a species becomes extinct- this is definitely a popular subject given the effects of climate change. But rarely do we consider the decline of common species. This is especially a problem in aquatic ecology. For example dominant fish play important roles in ecosystems and often link benthic and pelagic areas. Also, they tend to occupy higher trophic levels. For these two main reasons, and many more it would be difficult for these dominant fish to be replaced by a functionally equivalent species. Special attention should be brought to these key members of the ecosystem.
Biodiversity losses affect whole food webs
Studies of single trophic levels essentially tell us nothing about the functional consequences of biodiversity decline. Natural ecosystems have many trophic levels and changes at any level can lead to cascading effects.
Effects of biodiversity losses depend on context
The effects of a loss in biodiversity can depend on the abiotic and biotic factors. For example: in Venezuelan rivers the magnitude of top-down effects of grazing fish on organic materials is a function of seasonal changes in water level. Temporal and Spatial scales can play a huge role in studies of biodiversity loss and are often minimized or not considered.
So, Vaughn did a great job informing us all about the problems that complicate our ability to predict biodiversity losses. Her last point is what I found most interesting and is the reason I wrote this lengthy blog today.
UNDERSTANDING HOW BIODIVERSITY LOSSES INFLUENCE ECOSYSTEM FUNCTION REQUIRES THE COMBINATION OF APPROACHES AND SCALES
Vaughn stresses that multiple empirical approaches are necessary to effectively and efficiently study the connection between ecosystem function and biodiversity (loss). These multiple approaches should span across different temporal and spatial scales. There are advantages and disadvantages to any one approach at any different scale. The figure below was in Vaughn’s paper and neatly describes some of these trade-offs.
As you can see, comparative field experiments can cover large temporal and spatial scales. This is not possible in a laboratory setting. In a lab you are limited to small spatial and temporal scales.
For example long term studies offer a good understanding for how environmental changes can affect biodiversity. For example: Mary Power studied the Eel River system and found that algal blooms are controlled by the hydrologic regime and impacts the rest of the food web. This was evident after 18 years of data collection and observation. Data from a single year would have led to entirely different conclusions. In the lab you can also create unrealistic species assemblages whereas in the field complete species assemblages can be obtained. The ability to control environmental factors is advantageous in certain instances but in general field experiments offer the ability to study in a natural habitat. Finally, a snapshot in time is what you see from field studies whereas in a lab you can track changes between different levels. This can be advantageous.
Vaughn takes no side in the natural vs. Experimental debate, or in her case the field vs. Lab experiment debate. And she is right! You can’t understand any one process entirely in ecology without looking at it in as many ways as possible. Another chart from Vaughn’s paper below outlines the different approaches and scales that contribute to the understanding of the influence of mussel biodiversity on nutrient recycling in streams.
As I continue to develop my research project I am taking the ideas of all the studies I have blogged about into account. This one really sticks out in particular as it defines the importance of multiple empirical approaches which is something that has recently come up in my project. This paper really defines the basis of my blog in general and outline how there is no way to know what goes on in ecology without looking at it from all angles including both with and without clothes.
Source: Vaughn, C.C. 2010. Biodiversity losses and ecosystem function in freshwaters: emerging conclusions and research directions. BioScience. 60(1): 25-35.
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