Storms, Species and Ecosystem Stability

An interesting lecture last Friday, from Prof. Anson Mackay, about biodiversity and landscape change got me thinking. He talked about the value of biodiversity and various ecosystem services that are at risk due to recent global changes. Changes such as CO2 levels, urban expansion, deforestation, ocean acidification, sea ice loss, habitat loss, marine ecosystem over-exploitation and more, are all effecting biodiversity and their associated ecosystem services at both global and local scales.

I was left with one question: Being an ex-weather man, I couldn’t help but wonder about how storms can affect biodiversity. Can a single storm or series of storms lead to extinction of a species? If they can, then in a warmed future world where storms may be more severe, possibly more frequent in places, and perhaps moving their tracks to hit places that are not well-adapted to such storms, is future storminess a more significant threat to biodiversity than it is today? This also needs to be put in the context of human population to growth continuing through this century (UN predictions of around 11 billion people by 2100), which will no doubt continue to put more pressure on natural habitats through urban expansion with increasing numbers of people living in megacities.

Species on the brink

What evidence is there for storms affecting biodiversity? From personal experience, I can think of one species of bird, which may be put under more pressure from increased storminess and as well as human activity: the Bermuda Longtail. After living for three years on the small island of Bermuda, I only saw one or two, but these stunning marine birds that live on the wing are the subjects of keen conservation efforts. 

Source: Bermuda Goverment

Roughly 50% of the breeding pairs in the North Atlantic nest in Bermuda’s cliffs. The Longtails are however quite rare and are prone to pressures from:

• ·       storms and floods,
• ·       coastal development and human activity,
• ·       predation from new species alien to the island such as rats, crows and domestic cats,
• ·       competition for suitable nesting sites from pigeons, all only present since humans inhabited the island.

Lots of pressures, and surely an example of one of many species in a similar situation. In general, my instincts tell me that certain vulnerable species should be prone on a local scale. As in the case of the Longtail, when a vital breeding ground is as isolated as Bermuda, it seems to be reasonable to expect that species that are already on the brink, and are vulnerable to abrupt changes of landscape like the erosion and flooding after a storm or change in land use of a coastline, could be pushed towards extinction by a single event at a critical time, for example during breeding.

Coral destruction

Teixidó et al. in 2013 looked at the impact of severe storms in the NW Mediterranean Sea on biodiversity on the sea bed (benthic region) in coral producing species of marine life. The case study looks at a storm that hit the study region on December 26^th 2008 and was considered to be the strongest in 50 years at the time. The study examined the benthic community composition from data gathered from the preceding couple of years, and also from surveys during the years after the storm. The damage was severe largely due to this storm generating huge waves that smashed and scoured the coral outcrops and areas of shallow sea bed. Surveys after the event revealed extensive damage to corals and sea-bed communities, including some species that are relatively long-lived like some sponges, sea fans, and anenomes. The study makes the interesting point, backed up by numerous citations, that species that exhibit little change in populations and few community changes over time, due to a lack of disruption, are particularly susceptible to the impact of low frequency, high impact events. This compounded by evidence from studies that show the Mediterranean to be a potential hotspot for climate change (e.g. Giorgi and Lionello 2008). This means that we may see certain species in the Med under increasing pressure from extinction in the future. If we are going into an Antropocene, partly characterised by greater extremes of climate, then long-lived species with limited adaptability could well be the first to become extinct.

Disturbance biodiversity boosts

Storms can also trigger an ecological process in which species take advantage of the openings presented by a disturbance, seeking to fill any ecological niche that opens up and temporarily increasing biodiversity. This is known as “gap phase succession”. A non-storm related example can be found in many forest and grassland ecosystems with fire being a key trigger. Many species actually rely on these events, however, the question of whether they can adapt to greater ferocity of fires in a future warmer world, with longer and deeper drought turning grasslands and forests into tinder just waiting for a spark (normally from human activity or lightning), remains. This idea is similar in concept to the intermediate disturbance hypothesis, first introduced by Joseph Connell in 1978. The diagram below highlights this idea, but basically, it’s how biodiversity can be maximised by disturbances that are neither too frequent, nor too rare.

Source:  The intermediate disturbance hypothesis (data from Connell 1978).

But back to storms, the huge numbers of trees that get blown over during winter storms across Europe will continue to provide a bonus for biodiversity in the short term (downed trees provide food and habitat for new invertebrates, fungi and lichen). However, again it does appear these ecosystems have developed in such a way as to take advantage of the gaps presented by rare infrequent events. Change the frequency and severity of shocks on a system that has developed based on past experience over a long period of stable extremes, and who know what will happen! We can certainly hypothesise.

Goodbye equilibrium, hello climate change

This study by Backlund et al. in 2008 paints a rather pessimistic picture assessing broad impacts of climate change, and for the purposes of this blog, it describes how ecosystems are likely to be pushed further in to alternate states due to climate change. It considers that established predator/prey or pollinator/plant interactions may be put under additional stresses. It seems that this can potentially leave them vulnerable to the impacts of single rare events, such as hurricanes, which could lead to system failure! Stressed systems are generally less able to bounce back from big shocks.

If climate change is modifying the severity of storms, and potentially frequency and storm tracks for some regions, then we have lots of work to do in working out where we have vulnerable species and where we need to build in extra resilience to prepare for future severe events, which outside of the past experience. Climate change is a slow, creeping effect, which can easily catch us off-guard (and is arguably already doing so). In some cases however, ecosystem management may be the most effective route to help build our societal resilience to a future of more intense storms, as highlighted in a recent Royal Society report (see recommendation 4).

Stormy times ahead

It seems from my research for this blog that storms do have a part to play in maintaining our biodiversity, and the benefits that diverse biological systems bring. Storms (and climatic extremes) are important for the ecosystems that have already evolved to fit the past frequency and severity of rare events, but in an uncertain future due to climate change, they may also have the potential to exert significant extra pressure on local ecosystems, and could result in local extinctions of threatened species.

Enhanced focus on vulnerable and keystone species, tightly dependant ecosystems structures, and isolated communities should help guide our conservation and resilience building efforts in the context of a changing climate.