Why is fragility important?

• There is clearly growing awareness of the threats posed to ecosystems through human activity

• Applied conservation may need to focus on the most threatened ecosystems.

• Intuitively, “fragile” ecosystems are most at risk e.g Martorell and Peters (2009)

• But what do we mean by a fragile ecosystem?

• A good review of concepts is provided by Nilsson and Grelsson (1995)

What do we imagine when we hear the word fragility?

• The word “fragility” might suggest that ecosystems can “fail” completely.
• This rarely, if ever, happens as a result of a single shock.
• Ecosystems do often change, they can even change into different ecosystems
• However, they don’t typically totally break, through one single event.

What is the alternative to fragility? Is it stability?

• Ecosystems do always change, but they are still ecosystems
• Complete stability (lack of any change at all) is not very useful, either as a concept nor as a management goal
• A destructive change would lead to an ecosystem running downhill to a new and undesired state.
• This may lead to a loss its ability to effectively function (which we do want to avoid)
• Ecosystem decline tends to be the result of chronic (long term) stress
• The elements of ecosystems that can be influenced by practical, hands on management tend to be local, not global.

Stability? Or dynamic change with a point of attraction?

How do authors studying ecological systems view fragility?

Two main approaches to the concept of fragility.

1. Think about ecosystems that are naturally fragile because of large, natural, and internal successional changes. I.e. they change by themselves.
2. Think about ecosystems that change as a result of external pressures, usually caused by humans.

How do these two approaches differ?

• The first approach is more difficult to use.

• How do we really know exactly how an ecosystem might change naturally?

• How well are the complex dynamics of an ecosystem truly understood?

• The second approach is rather more intuitive. Think of changes that cause a potential shock.

What sort of changes might cause a shock?

• Fire regimes.
• Grazing
• Topsoil removal
• Hunting
• Pollution
• Pesticide use
• Invasive species
• Etc

Note that deliberate, planned land use change falls into a different category. This is habitat (or ecosystem) loss.

Is intuition an effective way of thinking about ecosystems?

• Ecosystems are very complicated systems.
• Not well understood
• Short term changes (the shocks) are very noticeable and so are newsworthy.
• However, longer term changes (recovery) may be much less observable.
• Surprises are rather more likely than predictable responses.

Are the two view complementary?

• It could be argued that the two approaches lead to the same conclusions.
• Ecosystems that are naturally fragile are more likely to be impacted by humans.
• Fragility ‘reflects the degree of sensitivity of habitats (ecosystems), communities and species to environmental change. It involves a combination of intrinsic and extrinsic factors’ Ratcliffe (1977).
• Fragile areas may be given legislative attention. EU law states that ’Areas being particularly sensitive from an ecological point of view should be given special concern".
• But how can such ecosystems be defined and identified?

What is the opposite of fragility?

Pimm (1991) distinguished three forms of lack of fragility.

• Persistence - Time a variable lasts before it is changed to a new value
• Resilience - Speed with which a system returns to its equilibrium after a change
• Resistance - Lack of consequences when a variable is permanently changed

Can you provide an example of a persistent system?

• Giant redwoods persist for 500 years.

Can you provide an example of a resistant system?

• Cork oaks in Portugal are resistant to fire.

Can you provide an example of a resilient system?

• Australian bush consistently recovers after fire.

Are mediterranean ecosystems fragile?

Is the arctic tundra fragile?

Are tropical forests fragile?

Are coral reefs fragile?

Are riparian habitats fragile?

Are riparian habitats fragile?

Is Wareham forest fragile?

• Practical exercise, field trip

How can stability be defined?

However stability (or fragility) is defined any measure of it must have a temporal component. In other words it must be based on measures taken at a minimum of two points in time.

• It is the differences between temporal measurements that determine whether a system is stable or not. However a reference point is needed.

What are the practical problems in defining fragility?

1. Different aspects of fragility are not necessarily correlated An ecosystem might be fragile in one respect and stable in another. Need to define terms.
2. Different aspects of fragility hard to quantify. Overall fragility even harder.
3. All depends upon scale. Temporal scale, spatial scale, level of taxonomic resolution, degree to which population numbers are included (over which temporal and spatial scales?)

Can you provide examples?

Ecosystems may appear stable for centuries, but highly fragile in the short-term.

• Coniferous forests can suddenly burn, then recover rather quickly..

• Grasslands may be affected by sudden drought then green up, when the rains come back.

• Flash floods may affect rivers, which then go back to more normal rates of flow.

• Deserts may change quickly after unusual rains then dry up again leaving the same vegetation in place.

• Hutchinson stated that ’lakes seem, on the scale of years or of human life spans, permanent features of landscapes, but they are geologically transitory, usually born of catastrophes, to mature and die quietly and imperceptibly

What is meant by spatial scale?

Ecosystems can appear fragile at small spatial scales may be stable at larger spatial scales.

• Shifting mosaics of grass, heath and shrub may be stable as a whole but change locally. Local disturbance. (e.g. gorse fires) may be necessary to maintain the habitat as a whole.
• Forests may have many successional stages
• Salt marshes consist of different zones with differnt vegetation types

What is meant by changes in relative abundance?

The abundances of some species in an ecosystem may change but the relative abundance of higher taxonomic, morphological and functional groups may be stable.

• If more widespread birds invade a distant island, and an equal number of endemic birds may go extinct, but the proportion of birds remains constant.
• This would be a concern for conservation, but the ecosystem may still function.

What is meant by temporal fluctuations?

Even if the abundances of species in an ecosystem fluctuates, the species composition (total species list) may remain constant over time.

• Monitoring of Lake Manyara National Park in northern Tanzania showed that the species composition, and the total biomass of large herbivores like buffalos and elephants remained constant through time although populations of each species changed.

What evidence is there of temporal changes in abundance?

(Owen-Smith 2019)

How can management of disturbances prevent fragility?

This could be thought of as having three dimensions (Wilson et al. 2005)

• Exposure

• Intensity

• Impact

The three dimensions

What is meant by exposure?

Exposure might be estimated either as the probability of a threatening process affecting an area over a specified time or the expected time until an area is affected. This is also often just called risk

What is meant by intensity?

Intensity could include measures of the magnitude, the frequency, and the duration of the risk. For biodiversity, the intensity of a threat can take many forms, including population sizes and fluctuations, amount of timber extracted per hectare of a forest type, density of an invasive plant species. Different exposures will have different intensities effects.

What is meant by impact?

Impact is the direct effects of some threatening process on particular features. Impacts might be referred to as outcomes or specific risks.

What is the intermediate disturbance hypothesis?

Can you outline the intermediate disturbace hypothesis in simple terms?

1. High levels of disturbance may simplify ecosystems due to stress
2. Low levels of disturbance may also simplify ecosystems due to competition.
3. Intermediate levels of disturbance lead to greater diversity.

Can concerns regarding fragility be counterproductive?

• Focussing on a single aspect of an ecosystem may always lead to unintended consequences.
• The aftermath of a “sharp short shock” disturbance tends to be ecosystem repair.
• Trying too hard may not be the best strategy.

How might managing only for risks be counterproductive?

Managers may be unaware of, or underestimate, the capacity of an ecosystem to rebound. I.e. Not take into account long term stability in terms of resistance, resilience and persistence. Management only focussed on risk may not be adaptive management.

An example from the literature

Carpenter et al. (2015)

• Management strategies that decrease variability and increase predictability may be preferred
• However, actions to decrease variance over short timescales (2-4 y), when applied continuously, may lead to long-term ecosystem changes with adverse consequences.
• Looked at lake eutrophication, harvest of a wild population, and yield of domestic herbivores on a rangeland.
• "In all cases, actions to decrease variance can increase the risk of crossing critical ecosystem thresholds, resulting in less desirable ecosystem states. Managing to decrease short-term variance creates ecosystem fragility by changing the boundaries of safe operating spaces, suppressing information needed for adaptive management, cancelling signals of declining resilience, and removing pressures that may build tolerance of stress. Thus, the management of variance interacts strongly and inseparably with the management of resilience. By allowing for variation, learning, and flexibility while observing change, managers can detect opportunities and problems as they develop while sustaining the capacity to deal with them.

How do forests vulnerability to loss of biodiversity vary geographically?

Chuvieco et al. (2014) The most vulnerable forests were found to be the rain forest of the Amazon Basin, Central Africa and Southeast Asia, the temperate forest of Europe, South America and north-east America, and the ecological corridors of Central America and Southeast Asia. The lowest vulnerability was observed in boreal regions, particularly those already affected by fires or having low biodiversity, agricultural regions of Australia, India, Latin America and Central Asia.

What might happen in a boreal forest?

• Changes may be surprising. For example (Payette et al. 2000)
• The impact of combined insect and fire disturbances translates into a dramatic decrease in post-fire tree regeneration of the forest inducing the shift to lichen woodland
• Lichen woodland may in fact be more “unique” and rich in species that the original forest.

Fragility of tundra

Tundra tends to be relatively stable.

For example. “Monitoring aimed at showing the fragility of an arctic tundra/goose grazing system concluded that the system was surprisingly stable”A limited effect of grazing on microbial respiration is consistent with a lack of significant differences in soil organic matter quantity and quality. The observed cycling was less than the natural variation within contrasting vegetation types" Strebel et al. (2010)

How might tropical ecosystems recover from disturbance?

• (Lugo 1999) and others involved on forestry have argued that selective logging is not a major threat.
• Attempting to convert tropical forests to agricultural land often fail
• Allowing time for the failures to become apparent and forests to recover may be a patient strategy for ecosystem repair.

“Major factors that constrain tropical soil fertility and sustainable agriculture are low nutrient capital, moisture stress, erosion, high P fixation, high acidity with aluminium toxicity, and low soil biodiversity” (Cardoso and Kuyper 2006)

How can productive alternative systems threaten intact ecosysystems?

In the Mediterranean, the development of aquaculture along the coasts appears as a source of disturbance to the littoral ecosystems, and in particular to Posidonia oceanica seagrass meadows.(Cancemi, De Falco, and Pergent 2003)

How may coral reefs be threatened?

• Human disturbances impact vulnerable life history stages, reducing reproductive output and the supply of recruits essential for recovery;

• Reefs can be vulnerable to the loss of few species, as niche specialization or temporal and spatial segregation makes each species unique

• Many foundation species have similar sensitivity to disturbances, suggesting that entire functions can be lost to single disturbances;

• Feedback loops and extinction vortices may stabilize degraded states or accelerate collapses even if stressors are removed. Mora, Graham, and Nystrom (2016)

Fragility after oil spills

Monitoring if Louisiana salt marsh vegetation showed both fragility and resilience after contamination by oil from the deep water horizon disaster. Silliman et al. (2012)

• Rapid salt-marsh vegetation recovery (high resiience)

• But also also permanent marsh area loss (low resistance)

• Thresholds of oil coverage associated with severity of salt-marsh damage, with heavy oiling leading to plant mortality

• Plant death on the edges of marshes more than doubled rates of shoreline erosion.

Fragility after oil spills

Fragility after oil spills

Recovery of vegetation in two years

Which factors may cause unnoticed long term decay?

• Subtle changes to nutrient cycles may be undetected.
• Loss of organisms that are difficult to monitor may go unnoticed.

Can changes outside protected areas affect them?

Hallmann et al. (2017) estimate more than 75 percent decline over 27 years in total flying insect biomass in protected areas

• Monitored small reserves in a fragmented landscape surrounded by agriculture.

Decline in insect biomass

Conclusion

• Fragility is easy to say but difficult to define in practice

• Each ecosystem varies in properties

• No simple solutions when ecosystems are being managed

• Both space and time are needed to prevent breakdowns