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Archive for January, 2019

The long shadow of Humboldt

January 15th, 2019 No comments

Behind the paper “Humboldt and the reinvention of nature” [1]. Text simultaneously published here and in

It all started when I was reading an excellent book by Andrea Wulf entitled The Invention of Nature: Alexander von Humboldt’s New World. The book provides many details about Humboldt’s fascinating life and the wide-ranging influence he had on science and society. When reading the book, you can easily understand the unquestionable role Humboldt played in the history of ecology and biogeography. One of his many contributions to science was to set the basis for explaining how environmental factors affect species distribution; for example, he demonstrated that vegetation systematically varies across the world with climate and showed the ecological similarities between altitude and latitude.

Fig. 1. Vegetation of Chimborazo (Ecuador) by Humboldt and Bonpland (1807).

A question that came to my mind was not covered by the book; namely, to what extent does Humboldt’s view bias our vision of nature? This is relevant because many classical naturalists and ecologists, such as Henry David Thoreau, Charles Darwin, George Perkins Marsh, John Muir, Rachel Carlson, Frederic E. Clements, and Henry A. Gleason, were all inspired by Humboldt. By spreading a vision, they shaped what is today mainstream ecology and the environmental movement. The current emphasis on the role of climate and soil in many ecological and evolutionary studies, the emphasis on forests as the potential and most important vegetation, and the difficulties many researchers have accepting the ecological and evolutionary role of disturbances at broad scales, suggest that we are still largely viewing nature through the eyes of Humboldt.

After reading the book, I was lucky enough to be in London for a conference and met William Bond in a pub just next to Kew Gardens. We stayed hours talking about many things, mainly fire, grazing, alternative stable states, and all the wonders of ecosystems maintained by disturbances. Our conversation jumped from one country to the other, from one biome to the other, and from one continent to another. And when talking about the overwhelming role that many researchers attribute to the environment when explaining broad temporal and spatial vegetation patterns [2], we glimpsed the long shadow of Humboldt. That conversation in a quiet London pub was the seed of this paper (and of another to come). Talking with William is always enjoyable because of his enthusiasm, experience, and creativity.

Now that we are approaching the 250th anniversary of Humboldt’s birth, it is instructive to evaluate his legacy of climate and soil as primary factors explaining broad vegetation patterns. There is increasing evidence that many open, non-forested ecosystems (savannas, grasslands, and shrublands) cannot be predicted by climate and soil – and are ancient and diverse systems maintained by fire and/or vertebrate herbivory. Paleoecological and phylogenetic studies have shown the key role of fire and grazing at geological time scales (Fig. 2). In this paper [1], we propose moving beyond the legacy of Humboldt by embracing fire and large mammal herbivory as key factors in explaining the ecology and evolution of world vegetation. This implies understanding grasslands, savannas, and shrublands as ancient and diverse ecosystems that require conservation, including the processes that maintain them (grazing and wildfires).

Fig. 2. Changes of the drivers related to plant consumers (fire and herbivory), together with the evolution of different vegetation types, and some plant traits (serotiny and thick bark of pines, epicormic resprouting in eucalypts), along the evolutionary history of plants. Upper pointing triangles are peaks of O2 atmospheric concentration and fire activity; lower-pointing triangles are megafauna extinction events, also associated to fire activity peaks. Note that modern fire regimes are very recent, and at this scale they are almost a point. From [1].

Fig. 3. Alexander von Humboldt and Aimé Bonpland on the foot of Chimborazo, painting by Friedrich Georg Weitsch (1810)

[1] Pausas J.G. & Bond W.J. 2019. Humboldt and the reinvention of nature. J. Ecol. [doi | jecolblog | pdf]

[2] Pausas J.G. & Lamont B.B. 2018. Ecology and biogeography in 3D: the case of the Australian Proteaceae J. Biogeogr. 45: 1469-1477. [doi | pdf]

Global change in the Mediterranean basin

January 9th, 2019 2 comments

The paleartic region with mediterranean climate (southern Europe and northern Africa; the Mediterranean Basin; Fig. 1) is a hotspot of biodiversity, a hotspot of climate change (warming of the region is above global average), and a hotspot of human population (a highly populated area and a top tourist and retirement destination). In addition, the Mediterranean Sea is the world’s largest inland sea, and climatic disruptions in the region have consequences in the large catchment area that includes central-eastern Europe (Fig. 1). That is, environmental changes and disruptions of the water cycling in the Mediterranean region have consequences affecting a large human population [1].

Fig. 1. Area with mediterranean climate (green) and limits of the Mediterranean catchment (red).  The European catchment limit based on Cortambert (1870). From [1].

The region, as all the planet, is subject to global warming. In addition there are three main local processes (not directly related to global warming) that are very important in understanding dynamic changes in the region [1]:

a) Rural abandonment in an environment depauperate of native herbivores; this increases wildlands (greening) but also the abundance and continuity of fuels that feed wildfires [2]

b) Increasing the wildland-urban interface; this increases biodiversity degradation (e.g., alien species), fire ignitions, and the vulnerability of the society to fires

c) Coastal degradation enhances drought (browning) through negative feedback processes; that is, the desiccation of coastal marshes, the deforestation for agriculture, and more recently, the explosive coastal urbanization, have drastically reduced the original ecosystems and thus the water available for the sea breeze that was once feeding the rain in the upper part of the mountains [1].

All these mechanisms act in different directions (greening, browning), and the current balance is still towards greening, as land abandonment is buffering the browning drivers; however, it is likely to switch with global warming. The challenge is to mitigate the browning processes. The good news is that the importance of small-scale drivers suggests that local policies and actions can make a difference in reducing overall impact on the landscape and society.

Mechanisms acting at a fine scale, together with global drivers (CO2 enrichment and climatic warming) interact and drive current vegetation changes in Mediterranean landscapes. Any model aiming to predict the future of our vegetation and climate must consider these local mechanisms; and failing to consider them at an appropriate scale is likely to produce inconclusive predictions.

Fig. 2. The disruption of the natural fire and drought regimes in Mediterranean landscapes is driven by global and local drivers. Increased fire activity is a response to the fuel amount and landscape homogeneity generated by rural abandonment (fire hazard) in an environment depauperated of herbivores and with increasing human ignitions (fire risk) and droughts (fire weather). The increased dry conditions are the consequence of global warming, but also of storm losses caused by the disruption of the water cycle generated by the coastal degradation. WUI: wildland-urban interface. From [1].


[1] Pausas J.G. & Millán M.M. 2019. Greening and browning in a climate change hotspot: the Mediterranean Basin. BioScience [doi | OUP | pdf]  

[2] Pausas J.G. & Fernández-Muñoz S. 2012. Fire regime changes in the Western Mediterranean Basin: from fuel-limited to drought-driven fire regime. Climatic Change 110: 215-226. [doi | springer | pdf]  



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