Extreme Events

Before we round up our investigation into the numerous effects of climate change on flora and fauna and begin to look at the big picture of what the future holds for ecosystems and biodiversity across the planet, there is one more thing I would like to discuss with you.

Extreme events.

While I’m not here to talk to you about the X Games or something organised by Red Bull wherein someone jumps off a very tall mountain on a very small BMX, what I do have to say is nonetheless of the utmost importance. As all third-year geographers worth their salt know, climate change brings with it an increased frequency of extreme events. These range from drought, storms, flooding, wildfires and freezing all the way up to hurricanes, and can have devastating impacts for local ecosystems. As Shen and Ma (2014) point out, these extreme events affect the biodiversity of ecosystems directly through environmental changes to habitats, which can upset the balance within and between trophic levels and can cause dramatic shifts and changes to occur within food webs.

The ways in which the many varieties of extreme events affect ecosystems are too numerous to discuss at length here. Instead, I would like to leave you with an example, that, whilst not giving you a breadth of knowledge of the ways in which extreme events can manipulate ecosystems, will hopefully give you an understanding into how these changes may occur and the significance of the threat to biodiversity that extreme events create. I would like, then, to discuss Beaver et al.(2013)’s paper into the effects of hurricanes on a subtropical lake (Lake Okeechobee), specifically those upon its phytoplankton inhabitants.

Now, I know that yet again I am giving an example that lies outside Europe, however this time I can explain myself. There are only a small amount of studies that investigate the effects of increasing frequency of extreme events on biodiversity, and of the few I could find, Beaver et al. (2013) painted the best picture of the severe nature of these effects. As I learnt from Beaugrand et al. (2003) when constructing my previous blog, “A Codding Mess” (well worth a read), plankton and phytoplankton are hugely sensitive to climatic triggers, and hence offer one of the best examples when explaining impacts of changing climates. 

Between 2004 and 2005, Lake Okeechobee, a large freshwater lake in Florida, experienced three hurricanes which had dramatic effects on the biological communities that make the lake their home. Prior to the hurricanes, the phytoplankton communities in the lake were dominated by cyanobacteria (an algae of sorts that produces energy by way of photosynthesis). These organisms flourish under conditions of adequate access to light and a high nutrient content, which Beaver et al. (2013) consider the two fundamental determinants of the composition of phytoplankton communities.

The team of academics found however, that after the hurricanes had hit the lake, these cyanobacteria were replaced as the dominant biota (in terms of abundance) by meroplankton diatoms. They observed that post-hurricane, cyanobacteria of genera such as Anabaena and Planktolyngbya decreased in abundance by around an order of magnitude (tenfold), whilst meroplankton diatoms such as Aulacoseira spp. declined significantly less, by around 20%.

Although tiny, phytoplankon (and all forms of plankton) make up a significant portion of most marine ecosystems, and are depended on by predators all up the trophic level, as they are often among the primary producers in a food web.

Beaver et al.(2013) theorise that the variation in response to the hurricanes stems largely from a change in the environmental suitability of the lake for cyanobacteria in lieu of several habitat changes caused by the hurricane winds.  They point out that for lakes, hurricanes can often lead to mass incidents of sediment resuspension, alongside almost total destruction of aquatic vegetation, both of which contribute to the floating matter and causes a significant reduction in transparency. Increases in the availability of nutrients are also commonplace. In fact, for Lake Okeechobee, Beaver et al. (2013) observed that concentrations of NO₂ and NO₃ nearly doubled in the post-hurricane period, and Secchi disk transparency fell from 0.43m to just 0.21m (meaning a fall in water transparency). As cyanobacteria require high light attenuation to generate food, they find themselves somewhat at a loss in darker waters, hence why they suffered such significant reductions post hurricane. Conversely meroplankton diatoms, and diatoms in general, are more successful under conditions of low light and considerable turbidity, and so suffered considerably less decline.

Another effect of this increased turbidity and lower light penetration is that of increased predation. Beaver et al. (2013) argue that cloudier conditions supposedly offer more cover for crustacean zooplankton grazers, who otherwise would be more vulnerable to predation from fish and other biota gained a relative degree of concealment, benefiting population size and hence creating greater predation pressures for phytoplankton communities.

The impacts on biodiversity of these changes are quite severe. Beaver et al. (2013) calculate several indices of biodiversity for both pre- and post-hurricane Lake Okeechobee. The Shannon-Beaver biodiversity index (also known as Shannon’s H, which represents species richness) fell from 7.91 to 5.56 and the Simpson biodiversity index (which portrays the relative abundance of species) fell from 5.32 to 3.95.

It is clear then that extreme events can be devastating for ecological communities. As we have seen, extreme events can have massive effects on the environmental regime in an ecosystem, and can have significant effects on the fitness of species within the food web. This can damage biodiversity, as we have seen in Lake Okeechebee, reducing species richness and relative abundance. As we learn from Jones (2014) (and will discuss further next week), diverse and rich ecosystems are essential for promoting ecological resilience, so any reduction in either richness or abundance can put ecosystems at greater risk. 

Further study into the effects of extreme events therefore is necessary. This information could help us to understand how ecosystem dynamics will change in an anthropocene world where extreme events have greater frequency. It is clear these events will become significant structuring factors for many ecosystems, which will become more prone to food web disturbances and trophic reorganisations such as the ones we have discussed today. Comprehension is key for any form of management or restoration.

Next week, as we begin to round off our discussion, I would like to begin discussing the big picture for biodiversity and ecosystems in a climate change age. It is time to bring all we have discussed together, and look at what may be a potentially grim outlook for the future of our planet’s biosphere.