The Big Picture - Biodiversity and Resilience

We have talked a lot over the past couple of months about the drastic effects of climate change on individual species and their ability to survive and flourish in their habitats. From range shifts, phenological fitness changes, invasive species, extreme events all the way to ecosystem desynchronization, we have seen just how big the threat of global warming is to species across the globe.

But now we need to look at the big picture. What is the significance of these challenges to species? What affect will it have on ecosystems worldwide? Will it just be a case of losing some cherished species, or is the problem more significant for ecosystems around the globe?

What I have learnt throughout this blog is that the effects of climate change on biota are never isolated. Rather, they feed into global, local and regional systems, changing interactions within and between ecosystems. Specifically, climate, through its penetrative direct and indirect effects on the biodiversity of communities, is having significant effects on the overall resilience of ecosystems.

What is Resilience?

But what does ecosystem resilience mean? Côté and Darling(2010) provide a nice summary, describing it the ecosystem’s capacity to ‘absorb disturbance without shifting to an alternative state and losing function and services’. Similarly, Jones (2014) describes it as a measure of an ecosystem’s persistence in the face of change, whilst still maintaining the same relationships both with biotic and abiotic variables. Folke et al. (2004) support these views, summarising that resilience is the ability of ecosystems to retain their identity, alongside structure, function and feedbacks.

Ecosystem resilience plays an incredibly important role in preserving the integrity of the biosphere by preventing ecosystems from incurring irreversible damage in the face of changing environmental, biological or nutritional conditions. Resilience acts almost as an “insurance policy” or “buffer” and prevents what is commonly referred to as “regime change”, whereby the make-up of an ecosystem changes dramatically from one form to another. These regime shifts by nature are sudden and irreversible; it is very rare for ecosystem change to be a gradual or smooth progress. These usually occur after a threshold has been passed causing drastic change to feedbacks within ecosystems and changing its entire form and function (Folke et al. 2004). Resilience, then, plays a critical role in preventing this from occurring.

Resilience and Climate Change

Ecosystem resilience is driven largely by biodiversity at a variety of scales. It is this biodiversity that allows ecosystems to weather disturbances without drastic change, and to recover to optimum conditions after such disturbances have occurred. “Resilience”, according both to Folke et al. (2004) and Jones (2014), can be theorised as made up of three general components. These are; functional-group diversity, functional-response diversity and renewal and reorganisation.

Unfortunately, all three are highly susceptible to climate change. Biodiversity, as we have seen, suffers greatly as temperatures warm at unprecedented speeds and precipitation regimes become highly variable and uncertain. And, as biodiversity is central to resilience, it is unavoidable that ecosystems find themselves more vulnerable and pushed further and further towards breaking point. 

Functional-Group Diversity

Functional group diversity in essence refers to a group of organisms that complete a certain service or function within an ecosystem. These “functional groups” can range from pollinators and grazers to those that generate soils or moderate water flows, and their presence and persistence within ecosystems are essential for performance and sustainability (Folke et al. 2004). The loss of a functional group then means that ecosystems can become unbalanced and critical processes are lost, reducing resilience and pushing the system closer to its threshold. As we have explored over the past few months, climate change causes mass declines in species and could potentially lead to significant loss, meaning it seems inevitable that functional groups will disappear and resilience damaged beyond repair.

To expand on this point, we will look at Jones' (2014), exploration of the consequences of the loss of the top predator, a key functional group. He suggests that such an occurrence can trigger what is referred to as a “trophic cascade” whereby the removal of the apex predator removes suppressive effects on those lower in the food chain, which can in turn increase predation on producers and have unpredictable and uncertain effects on the food web of ecosystems. Jones (2014) offers the example whereby removal of sea otters in a marine community led to a dramatic rise in numbers of sea urchins which predate on kelp. This, in turn, reduced the numbers of those species who relied on kelp for cover and protection. Removal of any functional group then can have unforeseen consequences for all members of the ecosystem and can subsequently affect balance and stability.

Climate change, as we have seen, also encourages invasive species by increasing their potential distributions. This can affect resilience whereby an ecosystem that originally lacked a functional group finds it filled by an invasive species. This can be devastating for an ecosystem, affecting both relationships and environmental conditions and causing further disturbance. Take for instance the spread of Myrica faya (also known as the firetree) into Hawaii; this plant, being a nitrogen fixer (an organism that converts nitrogen from the atmosphere into ammonia), introduced a new function to the ecosystem, resulting in nitrogen inputs increasing threefold and consequently facilitating the spread of further invasives to the area which were better adapted to nitrogen rich conditions, whilst making conditions considerably more hospitable for native species adapted to low nitrogen conditions (Folke et al. 2004).

Climate change hence can have significant effects on the functional-group diversity of ecosystems. Climate change not only can result in the removal of functional groups, reducing resilience and destabilising ecosystems, but can also create new functional groups that push ecosystems towards a new state.

Functional-Response Diversity

Also essential for resilience is what is referred to as “functional response diversity”. This refers to species within a particular functional group having a diversity of responses to environmental changes or disturbances that may undermine dominant actors in said group. This creates resilience as if one species were to suffer, there would be others that contribute to the same ecosystem function who could take their place. It is in this way that higher biodiversity (specifically species abundance) buffers an ecosystem against change, and the greater number of species that can perform an ecosystem function and have diversity in responses, the greater the functional redundancy (that is, the amount of species that can replace a dominant functioning species) (Jones2014).

Climate change has a negative impact on resiliance then if it reduces the biodiversity of an ecosystem. By making species less fit or removing them from an ecosystem entirely, functional redundancy is reduced and hence functional-response diversity contributes less to resilience.

Renewal and Reorganisation

According to Folke etal. (2004), the ability of an ecosystem to recover to an equilibrium state after disturbance is also a key determinant of ecosystem resilience. Although referred to by Folke et al. (2004) as “renewal and reorganisation”, this is sometimes referred to as the “back loop of ecosystem development”. 

This re-building, if you will, is conducted mainly through the functional groups of both biological legacies and what Folke et al. (2004) refer to as mobile link species. Biological legacies, for instance, are those species that are sufficient at weathering disturbances and re-establishing populations afterwards. Large trees, for instance, can serve as biological legacies in forest ecosystems after storms or fire have ravished the ecosystem. In the tundra, this job is taken by those species that create seed banks or make use of vegetative propagules. Mobile link species, on the other hand, are those that bring in seeds and other organic material from surrounding ecosystems, such as birds and other animals that eat matter that, after passed, can be used to reintroduce species. In this way, it is not only local biodiversity that is important for resilience, but also biodiversity at larger scales such as those of landscape and region, as this can ensure key species are reintroduced to local systems.

Flying foxes (of the genus Pteropus) are a fine example of a species that exists in the mobile link functional group. As fruit eaters, they readily transport seeds between ecosystems and habitats, making them excellent recolonisers of vegetation. (Folke et al. 2004)

Once again, climate change can damage resilience by making it harder for ecosystems to redevelop after disturbances. If key species suffer at the hands of the climate (through methods we have discussed), this can limit the ability of an ecosystem to rebuild, reducing its resilience and pushing it closer towards total regime change. 

Conclusion

Over the past few months we have explored in detail the range of consequences that climate change can have for species and ecosystems alike. Today we see the ramifications of these changes. If climate change continues as is, and biodiversity continues to decline, then the resilience of ecosystems could become completely subverted. This would make these communities vulnerable to disturbances that previously could have been absorbed and mitigated with ease by these systems, and could eventually lead to thresholds being crossed and irreversible changes to our planets biosphere.

Widespread collapse of ecosystems seems inevitable if climate change continues on this path. Next week I would like to look at the potential ramifications of this, and what the future really does hold for biodiversity. Are we moving towards a 6^th mass extinction?

The evidence really doesn't seem very optimistic.