Protecting biostructure
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That biodiversity is in sharp decline is no longer in question, but scientists still heatedly debate the functional consequences of this loss. Attempts to tackle the problem have mainly involved trying to establish a direct link between species diversity and the sustainability of ecosystems. But in taking this approach, scientists have concentrated on diversity at the expense of ignoring the biological structure that maintains ecosystems. This is akin to the physiologist cataloguing animal parts and ignoring the anatomical structure that connects them. Clearly it is the underlying architecture, not just the parts by themselves, that maintains the bodily functions necessary for life. Analogously, the network of interactions between organisms, not diversity per se, breathes life into ecosystems. To understand the implications of biodiversity loss, it is crucial to monitor changes to the underlying ‘biostructure’. Perhaps the main reason why researchers have tended to focus on diversity is that it is easier to count species than to document their interactions. Empirically mapping biological networks such as food webs is no small chore. Like realist painters, some intrepid scientists have attempted to render the topology of these wildly complex webs by meticulously piecing together stomach-content analyses and many hours of field observations. But stomach contents are difficult to decipher, so it is often impossible to quantify energy fluxes that pulse through these networks. Recently, ecologists have begun to use stable isotopes to trace the flow of energy through food webs. Because the concentration of the N isotope tends to increase by a certain amount with each step of the food chain, patterns in N fractionation provide a wonderfully simple measure of a species’ position in the web. Similarly, signatures in C can be used to determine a given predator’s prey — although only prey organisms that have very different carbon sources can be differentiated (for example, C3 versus C4 plants). A fuzzy but fluid empirical sketch of a food web emerges from stable-isotope analyses. Importantly, these ‘sketches’ can reveal changes in major network attributes across ecological gradients; for example, increased omnivory by fish as lake size decreases. On the horizon, DNA-barcoding may soon provide a more highly resolved and precise understanding of both network topology and energy flux, by allowing ecologists to rigorously identify and quantify prey species taken from a predator’s stomach. Here, bits of mitochondrial DNA are used to identify species, much like a barcode is used to identify the price of an item in a grocery store.