Ecology and Climate

 

 

Ecosystems and climate change are the subjects of a paper by Malhi et al. (2020) which forms part of a Royal Society publication on the “threats, opportunities and solutions” pertaining to this area. The authors explore “how ecosystems respond to climate change”, how their resilience can be enhanced and how they can “assist in addressing the challenge of a changing climate.” A number of papers on the interaction between climate change and the biosphere are introduced, five of which are outlined below.

Turner et al. (2020) are concerned with abrupt changes in ecological systems and the research agenda needed for diagnosing them. As examples of such changes they list coral bleaching, changes to kelp forests, ice loss, soaring tree mortality, plummeting forest carbon uptake, and more frequent and severe fires. Such changes have profound consequences for ecosystems and human wellbeing and it is important to understand them.

Changes to ecosystems can be driven by many factors other than climate change: these drivers include changes in land cover, land use, nutrient fluxes, harvesting of living resources, ocean circulation, predation, pathogen outbreaks, urbanization, water-borne diseases and fungal infections. Five insights from studies of abrupt changes in ecological systems (ACES) are highlighted.

First, an “abrupt change in one dimension of an ecosystem does not necessarily imply an abrupt change in others”, and different drivers affect different ecosystems. Changes in climate and circulation may produce abrupt shifts in open-ocean ecosystems, whereas changes in harvesting or predator control may produce abrupt shifts in local aquatic ecosystems.

Second, ACES are more likely to be triggered by climate extremes than by trends in mean climate, while the extremes themselves are expected to become more frequent as the climate warms. Extreme drought in 2018 “set the stage for large wildfires in western North America” and affected grassland and forests in central Europe. Climate extremes are however still rare events and not all of them produce abrupt changes, so that understanding of how they affect ACES “is still nascent”.

Third, ACES can result from the interaction of multiple drivers. “Ecosystems seldom respond to drivers in isolation” and ACES are more likely to result from interactions among them. This view is supported by palaeoecological records, and today habitat patterns can be seen which appear to be caused by local feedbacks “initiated among soils, snowdrifts and forest bands” in Colorado, USA.

Fourth, ACES depend on contingencies, many of which are not well understood. Four types of contingency are listed: Ecological memory, which “refers to the adaptations, individuals and materials that persist during and after a disturbance event”; Linked disturbances, the interaction between successive disturbances; Compound disturbances, the synergistic effect that may result from two disturbances within a short time period; and Pre-conditioning, which may exist where an ecosystem has been able to adapt following earlier disturbance.

Fifth, ACES often result from tipping points “where strong positive feedbacks within an ecosystem lead to self-sustaining change”. This can be the result of interactions between slowly and rapidly changing processes: for example a slow change in the level of a lake, and a rapid growth of harmful algae. The points discussed lead the authors to propose “a research agenda for diagnosing ACES across a wide range of systems and scales.” The many research questions listed include the applicability of early warning signs, the identification of the ecosystems most sensitive to climate extremes globally, and whether tipping points necessarily follow a particular sequence.

França et al. (2020) review the effects of events such as storms, floods, heatwaves, and droughts on tropical ecosystem recovery. They focus on tropical forests, which are host to more than two-thirds of all terrestrial species, and coral reefs which hold “the highest species diversity of any marine ecosystem” and are of direct importance to more than 500 million people.

Climate change is held to be responsible for more frequent and intense hurricanes, cyclones and typhoons, which affect both reefs and forests; storm damage and ecological consequences are cited in the Great Barrier Reef and in the forests of the Caribbean and Central America. Heatwaves and droughts have affected both corals and associated fish communities, particularly in 2015 and 2016, but also gave rise to “unprecedented and large-scale wildfires in tropical forests”. In some forests species composition has shifted towards more drought tolerant species.

Climate change, extreme climatic events and stress resulting from human activity can interact to affect the resilience of ecosystems, and “only a small subset of the original species pool is likely to respond positively to multiple stressors”. An example is the combination of stress from climate change and from land-use change such as deforestation for food production. “Most remaining tropical forests are currently subject to some form of anthropogenic disturbance” including selective logging and wildfires, resulting in greater canopy openness and drier understoreys; and in combination with longer and hotter dry spells, tropical forests, which have few fire-resistant species, become more susceptible to fire. The ecosystems of coral reefs are subject to “overharvesting, landbased pollution, diseases, sedimentation and nutrient loading”. The increasing effects of marine heatwaves result in chain effects that are “pushing coral communities towards their physiological stress limits”.

Guarding biodiverse ecosystems against the impacts of “multiple interacting threats” will require both local and global action, and “a new focus on functional and climate connectivity”, allowing species to shift their range along climate gradients. The majority of tropical forests are unlikely to allow adequate species movements, and so there is a need for “viable patch-linkages and habitat corridors” to allow vulnerable species to move along climate gradients. This could involve protection of new areas, including private land, and new strategically placed marine and forest reserves, requiring “the collective effort of a broad range of stakeholders at distinct levels”.

Seddon et al. (2020) discuss the potential for nature-based solutions to “deliver both climate change mitigation and adaptation.” The authors describe nature-based solutions (NbS) as “working with and enhancing nature to help address societal challenges”. Examples include “protection and management of natural and semi-natural ecosystems”, the inclusion of parks, trees, fields, ponds, wetlands, floodplains etc. (green and blue infrastructure) in urban areas, and the “application of ecosystem-based principles to agricultural systems.” NbS is an umbrella concept for ecosystem-based adaptation and mitigation, “eco-disaster risk reduction and green infrastructure.”

The challenges to be addressed are the “three central challenges of the Anthropocene: mitigating and adapting to climate change, protecting biodiversity and ensuring human wellbeing”, and the authors believe that they must be addressed together, and that there is growing awareness of the potential to mitigate the impacts of climate change while “slowing further warming, supporting biodiversity and securing ecosystem services.”

Three barriers are listed that hinder the adoption of NbS: there is uncertainty regarding their cost-effectiveness; investment in them is deterred by “poor financial models and flawed approaches to economic appraisal”; and “inflexible and highly sectoralized forms of governance” tend to prefer better-known methods of climate mitigation. These are usually engineering approaches such as dams, seawalls, roads, pipes or water treatment plants, collectively known as grey infrastructure. 

Some references to specific examples of NbS are given, most of which relate to reducing exposure to the effects of climate change: reduction of soil erosion through forest management in China; inland flood protection through reforestation in Canada and the USA; protection of coasts at many sites globally by mangroves, seagrass and kelp beds, and in the Gulf of Mexico by oyster reefs; moderating urban heatwaves by canopy cover and greenspaces; managing flooding in urban Italy through establishing upstream wetlands and green spaces; and increasing productivity with agroforestry in Panama and in Europe. The authors go on to argue that instead of framing NbS as an alternative to engineered approaches, “we should focus on finding synergies among different solutions.” Making full use of NbS requires a shift in focus from “infinite economic growth to a recognition that the energy and material flows needed for human wellbeing must remain within safe biophysical limits.”

Hobbie and Grimm (2020) discuss nature-based approaches to managing climate change impacts in cities. Warming is increased by the effect of urban heat islands, and many cities are at flood risk because of their proximity to rivers or the sea, or because of high rainfall on their impervious surfaces. The authors categorised impacts as Social, Ecological and Technical. Social impacts include physical health problems arising from heat stress, flood borne pollution, and contamination of drinking water; mental health and behavioural issues include aggression, criminal behaviour, suicides, mood disorders and dementia. Ecological impacts include reduction of urban tree cover, and algal blooms and harmful cyanobacteria in ponds and lakes. Technical impacts include damage to urban infrastructure such as power lines, sewer pipes and water supply due to flooding.

Opportunities for reducing exposure to climate change hazards using nature-based strategies are discussed under the above categories of threats and impacts. Conservation and restoration of shore habitats such as reefs, kelp and seagrass beds, dunes, saltmarshes and mangrove forests can offer protection from the sea. Extreme heat can be mitigated by increasing the urban tree canopy, deployment of green roofs, and parks and open space. Runoff during heavy rain storms can be reduced by green roofs, stormwater ponds, bioswales, raingardens and retention basins. Social impacts can be lessened to some extent by adequate green space in cities. The vulnerability of urban ecosystems to climate change can be reduced through a variety of approaches such as planting climate-adapted and pest- and pathogen-resistant species. The built infrastructure can receive nature based protection in some situations, for example from protective wetlands along coasts.

The authors prioritise research into the effectiveness of nature-based strategies in reducing the impacts of climate change hazards; their costs and benefits relative to alternatives; and their equitable distribution within and across cities.

Lenton (2020) introduces his paper on tipping points by referring to the range of complex systems which can exhibit them: these include individual humans, societies, ecosystems, the climate system and the Earth system. Tipping points occur when a small perturbation in a system can produce a large response sending it into a different future state. The author addresses “how we think about tipping points and could use them to guide action”. Tipping points are often discussed in the context of the environment, and the associated emotions are generally negative, because we “rightly fear abrupt changes” away from conditions in which humanity has flourished. This is the case for the tipping points associated with parts of the West Antarctic ice sheet, the collapse of Labrador Sea convection, and the loss of tropical coral reefs. There have however been tipping points in the past with positive connotations, such as the Great Oxidation and the rise of plant life on Earth, and at the present time we see tipping points in society and in our collective awareness regarding the conservation of our common resources. Although we cannot avoid some damaging tipping points “avoiding others will require finding and triggering positive tipping points towards sustainability.”

The definition of tipping points and various categories of tipping point are discussed, as well as the interactions between them: there are cases where the tipping of one system causes the tipping of another, and also cases where tipping in one reduces the likelihood of tipping in the other. Causal tipping effects are possible from large scale to small scale systems and also the converse. Positive feedback is characteristic of systems tipping, and the positive feedbacks between new technologies, capitalism and an expanding labour force which occurred during the industrial revolution can be linked to climatic tipping points. In the reverse direction, the threat of badly damaging climate tipping “could be enough to tip human social dynamics” into “a coordinated effort to pool the necessary societal resources to avert climate change disaster”.

Some kinds of tipping points “carry generic early warning signals” opening the possibility of taking action to avoid them, and this implies the need for continuous monitoring. Information on warning signals can be gleaned from the study of systems which have recovered from repeated disturbance, for example the “recovery of tidal marshes from inundation events”. Where a system approaching a tipping point has more than one possible response, deliberate disturbance may be used to induce the most favourable outcome. Knowledge of early warning signals may also help in deciding how best to restore a system which has already tipped into a degraded state. Lenton concludes that “we need to be thinking and acting in a more systemic way”, monitoring the dynamics of complex systems, addressing the associated challenges of ‘big data’ and ‘big analysis’ and of using models to offer understanding and predictability.

 

References

França, F. et al., 2020, Climatic and local stressor interactions threaten tropical forests and coral reefs, Phil. Trans. R. Soc. B, online, accessed 24 December 2021

https://royalsocietypublishing.org/doi/full/10.1098/rstb.2019.0116

Hobbie, S., and Grimm, N., 2020, Nature-based approaches to managing climate change impacts in cities, Phil. Trans. R. Soc. B, online, accessed 24 December 2021

https://royalsocietypublishing.org/doi/full/10.1098/rstb.2019.0124

Lenton, T., 2020, Tipping positive change, Phil. Trans. R. Soc. B, online, accessed 23 December 2021

https://royalsocietypublishing.org/doi/10.1098/rstb.2019.0123

Malhi, Y. et al., 2020, Climate change and ecosystems: threats, opportunities and solutions, Phil. Trans. R. Soc, online, accessed 18 December 2021

https://royalsocietypublishing.org/doi/10.1098/rstb.2019.0104

Seddon, N., et al., 2020, Understanding the value and limits of nature-based solutions to climate change and other global challenges, Phil. Trans. R. Soc. B, online, accessed 24 December 2021

https://royalsocietypublishing.org/doi/full/10.1098/rstb.2019.0120

Turner, M., et al., 2020, Climate change, ecosystems and abrupt change: science priorities, Phil. Trans. R. Soc. B, online, accessed 24 December 2021 

https://royalsocietypublishing.org/doi/full/10.1098/rstb.2019.0105