Fenced areas are often illustrative of the role of herbivores in shaping vegetation [1]. Here is an example from Schönbuch Nature Park (south west of Stuttgart, Germany), where you can see how the vegetation is shaped by different densities of red deer (Cervus elaphus). Is this park-like landscape (Fig. 2; inside the enclosure) an example of the prehistoric lowland landscapes in Central Europe?
Fig. 1. Fence separating the area with absence (left) / presence (right) of red deer
Fig. 2. Within the enclosure where the density of red deer is highest. Is this park-like landscape an example of the prehistoric lowland landscapes in Central Europe?
Fig. 3. Hervivores of central Europe, from [2].
References [1] Pausas JG & Bond WJ. 2020. Alternative biome states in terrestrial ecosystems. Trends Pl Sci, 25(3), 250–263. [doi | pdf]
[2] Vera FWM. 2000. Grazing ecology and forest history. CAB International.
I have recently been in Minas Gerais, Brazil (in the Cerrado region). I visited different biomes (forests, savannas, grasslands) occurring in the same climate, i.e., Alternative Biome States [1,2]. The sharp boundaries that separate the different biomes (photos blow) suggesting the existence of strong feedbacks [3]. Savannas and grasslands are maintained by frequent fires (flammable or pyrophilic communities) in climates where dense forest can occur; frequent fires maintain those open ecosystems dominated by light-demanding grasses, and woody plants with traits for fire survival (thick corky bark [4], epicormic resprouting [5], belowground organs [6]). In contrast, forest rarely get burn (non-flammable or pyrofobic communities), as the low light inhibit grasses and generate microclimatic conditions that are not favorable for fire (no grasses, high humidity, low wind, etc.) but favorable for shade-tolerant forest trees.
Forest-grassland mosaic in Serra da Canastra, Minas Gerais, Brazil
Forest-grassland mosaic in Serra da Canastra, Minas Gerais, Brazil
Savanna dominated by Vochysia thyrsoidea (Vochysiaceae; the large tree), in Serra da Canastra, Minas Gerais, Brazil. Brazilian savannas are often termed “cerrado”.
References
[1] Pausas JG & Bond WJ. 2020. Alternative biome states in terrestrial ecosystems. Trends Pl Sci 25: 250-263. [doi | sciencedirect | cell | pdf]
[2] Dantas VL, Hirota M, Oliveira RS, Pausas JG. 2016. Disturbance maintains alternative biome states. Ecol Lett 19: 12-19. [doi | wiley | pdf | supp.]
[3] Pausas JG & Bond WJ. 2022. Feedbacks in ecology and evolution. Trends Ecol Evol 37: 637-644. [doi | sciencedirect | pdf]
[4] Pausas JG. 2017. Bark thickness and fire regime: another twist. New Phytol 213: 13-15. [doi | wiley | pdf]
Ecology and evolutionary biology have focused on how organisms fit the environment. Less attention has been given to the idea that organisms can also modify their environment, and that these modifications can feed back to the organism, thus, providing a key factor for their persistence and evolution [1]. We propose that there are at least three independent lines of evidence emphasising these biological feedback processes at different scales (figure below): niche construction (population scale); alternative biome states (community scale); and the Gaia hypothesis (planetary scale). Flammability is an example of niche construction [2], and the forest-savanna mosaics are an example of the alternative biome states [3] (figure below).
The importance of feedback processes make us rethink traditional concepts like niche and adaptation. For instance, the idea of evolution as a process of adaptation to fit a pre-existing environment needs to be replaced by a ‘co-evolutionary’ species-environment approach. An implication is that the concept of species niche, and niche occupancy, is less relevant than traditionally thought. That is, organisms do not adapt to a pre-existing environment (available niche), they construct their environment and then both ‘co-evolve’. A higher level of fitness is the result of this coevolution. Feedbacks also provide an alternative framework for understanding spatial and temporal patterns of vegetation that differ from those based on gradual changes (e.g., gradient analysis and succession), and suggest that multi-stability and abrupt transitions in a given environment are common [3]; this also has implications for species’ niche modelling [4].
Earth is in transition to a new and warmer state due to anthropogenic forcing, and feedback thinking may help us understand the process. We suggest that incorporating feedback thinking and understanding how feedbacks may operate at different scales may help in opening our minds to key processes contributing to the dynamics and resilience of our biosphere.
Fig. 1. Examples of eco-evolutionary feedbacks at different organising levels: Niche construction (population; e.g. flammability), alternative biome states (community; forests and savannas) and Gaia (biosphere). The signs of the feedbacks indicate the most common type of feedback for each example. Evolutionary feedbacks represent the evolutionary processes at the different scales (from selection at the micro-evolutionary scale to the acquisition of key macro-evolutionary innovations). From [1].
References
[1] Pausas J.G. & Bond W.J. 2022. Feedbacks in ecology and evolution. Trends Ecol. Evol. [doi | pdf]
[2] Pausas J.G., Keeley J.E., Schwilk D.W. 2017. Flammability as an ecological and evolutionary driver. J. Ecol. 105: 289-297. [doi | wiley | pdf]
[3] Pausas J.G. & Bond W.J. 2020. Alternative biome states in terrestrial ecosystems. Trends Plant Sci. 25: 250-263. [doi | sciencedirect | cell | pdf]
[4] Pausas J.G. & Bond W.J. 2021. Alternative biome states challenge the modelling of species’ niche shifts under climate change. J. Ecol. 109: 3962-3971. [doi | wiley | pdf]
The role of large herbivores in explaining broad-scale ecological pattern has often been underestimated [1]. Plants have defenses against large herbivores (e.g., spines, high wood density [2]). And many continents had abundant large herbivores (megafauna) that were extinguished in Pleistocene (except in Africa). In a recent paper [3] we asked, to what extent the past distribution of extinct magafauna explains current geographical distribution of plant defense traits in the Neotropics (South & Central America). We fond that a significant proportion of the variance in the distribution of wood density, leaf size, stem spines, and leaf spines are explained by variable related to past megafauna (richness and body mass).
We defined 3 antiherbiomes in South America, that is, regions with characteristic plant defenses, environmental conditions, and Pleistocene megafauna, as follows: Small-Leaves-Thorny (SLT): thorny and small-leaved plants, in arid, cold and nutrient-rich ecosystems, containing numerous extinct and extant large grazers. Intermediate-Leaves-Woody (ILW): intermediate leaf sizes and levels of chemical defenses, and very high wood density, in moist and hot climates, and extremely nutrient-poor soils; and a high extinct megafauna richness, especially in relation to small browsers and mixed-feeders. Broad-Chemically-defended-Leaves (BCL): very large leaves with chemical defenses, mostly associated with moist climates and intermediate fertility soils, with few but large extinct megafauna species, especially browsers. Similar antiherbiomes can be observed in current Africa. These antiherbiomes represent one of the most striking broad-scale anachronisms in ecology.
We estimated that in South America, savannas occupied about 10 millions of Km2 during the Pleistocene, ca. 63% of them were converted to forests (44% to moist forests, 19% to dry forests) after the megafauna extinction (biome shifts [4]), and ca. 37% remains as savanna (stable). This suggests that South America was a savanna-dominated continent, much more similar to Africa than today, and that a large proportion of South American forests are the result of megafauna extinctions.
Overall our results suggest that past (extinct) large herbivores explain an important proportion of the variability of current plant traits and community assemblies.
Fig. 1. Left: Distribution of the 3 anti-herbiomes. Right: Hypothesized distribution of savanna during the Pleistocene (coloured areas; based on the distribution of extinct megafauna), that currently are savanna (in yellow), moist forests (dark green) and dry forests (light green). From [3]
Fig. 2. Reconstruction of Pleistocene savanna (ILW antiherbiome) with Taxodon platensis (a mixed feeder) next to the tree Bowdichia virgilioides (sucupira-preto; Fabaceae), and a Notiomastodon in the background. Artist: Júlia d’Oliveira
Fig. 3. Additional reconstructions of the Pleistocene Brazilian savannas from [5]. Artist: Júlia d’Oliveira
References
[1] Pausas JG & Bond WJ. 2019. Humboldt and the reinvention of nature. J. Ecol. 107: 1031-1037. [doi | jecol blog | jgp blog | pdf]
[2] Dantas V & Pausas JG. 2020. Megafauna biogeography explains plant functional trait variability in the tropics. Glob. Ecol. & Biogeogr. [doi | pdf | data:dryad | blog ]
[3] Dantas V., Pausas J.G. 2022. The legacy of Southern American extinct megafauna on plants and biomes. Nature Comm. 13: 129 [doi | pdf | data & codes] – New!
[4] [2] Pausas JG & Bond WJ. 2020. Alternative biome states in terrestrial ecosystems. Trends Plant Sci. 25: 250-263. [doi | sciencedirect | cell | pdf]
[5] Pansani et al. 2019. Isotopic paleoecology (δ13C, δ18O) of Late Quaternary megafauna from Mato Grosso do Sul and Bahia States, Brazil. Quat Sci Rev, 221, 105864.
The question of whether species are organised as collectives of integrated interacting assemblages (Clements’ community concept) or behave individualistically (Gleason’s community concept) is a century-old debate in ecology that is still unresolved. In a recent article, we are reconciling the two approaches [1].
The Gleasonian view suggests that communities are assembled by species that respond individualistically along environmental gradients and thus cannot form bounded units (Fig. 1A). However, in many world landscapes, for a given climate, strikingly different biomes with sharp boundaries co-occur forming landscape mosaics. These mosaics are typically formed by a closed biome (forests) and open (non-forest) biome (e.g., grassland, savanna, shrublands). These two alternative biome states (ABSs [2]) are maintained by different feedback processes and have radically different species with contrasting shade and disturbance tolerance traits [2].
Under the individualistic view of species along climatic gradients, the overlapping response curve along a climate gradient (Fig. 1A) may indicate plant coexistence (and potentially competitive interactions); however this is true only if they occur in the same biome (Fig. 1B). That is both Gleason’s individualistic view (within biome) and Clements’s organismic view (across biomes) are complementary; both perspective of community remain useful in ecology.
The consequence is that fitting species distribution models or using climate limits in modelling for projecting future species distributions are inappropriate for extensive regions with alternative biome states. One way to improve these predictions would be to consider the presence or absence of forest shade in the modelling [1].
Figure 1. Classical (Gleasonian) pattern of species response curves along a climate gradient (A), and the alternative pattern along the same climatic gradient when there are ABSs (B). Note that in the driest and the wettest section of the gradient, we find open (e.g., grassland) and closed (forest) biomes, respectively; but at intermediate levels of the gradient, both are possible depending mainly on the disturbance regimes and feedback processes [2). Thus, under the intermediate levels of the gradient, species that may seem to coexist when considering climate only (A) are not really coexisting but occurring in drastically different biomes (B). From [1].
References
[1] Pausas J.G. & Bond W.J. 2021. Alternative biome states challenge the modelling of species’ niche shifts under climate change. J. Ecol. 109: 3962-3971 [doi | pdf]
[2] Pausas J.G. & Bond W.J. 2020. Alternative biome states in terrestrial ecosystems. Trends Pl. Sci. 25: 250-263. [doi | sciencedirect | cell | pdf]
Plants are the largest biomass component of most terrestrial ecosystems, and litter decomposition is considered the dominant process by which nutrients return to plants. In a recent paper [1] we show that in terrestrial ecosystems, there are three major pathways by which plant biomass is degraded into forms that release nutrients again available to plants: microbial decomposition; vertebrate herbivory; and wildfires. These processes act at different spatial and temporal scales, have different niches, and generates different ecological and evolutionary feedbacks. The three processes can occur in a given ecosystem (competing for the same resource, biomass), but the relative importance of each varies with the micro- and macro-environmental conditions (see Figure below).
Wildfires and herbivory are two powerful biomass consumers; they generate feedback processes that maintain vegetation at states of lower biomass than would be expected from the physical environmental conditions (alternative vegetation states [2]). In addition, wildfires and herbivory also select for light-loving species with a set of adaptive traits to persist under these consumers [3,4]. That is, both herbivory and fire can influence the mix and attributes of plant species, while the mix and attributes of plants also influence the fire and grazing regimes. These ecological and evolutionary feedbacks make fire and herbivory distinct from other abiotic disturbances such as cyclones, landslides, avalanches, volcanoes, or floods, where plants may respond – but the disturbance will not change in response to these plant responses. That is, wildfires, herbivory, and microbial decomposition can be viewed as biotic processes that structure our ecosystems and the biosphere, at different temporal and spatial scales.
This holistic view in which microbes, herbivores, and wildfires play a joint role in the functioning of ecosystems contributes to a better understanding of the diversity of mechanisms regulating the biosphere.
Figure: Plant biomass and vegetation structure in terrestrial ecosystems are determined by three feedback processes: vertebrate herbivory (H), microbial decomposition (D), and wildfires (W). These three processes also interact with each other (e.g., competition for biomass; but positive interactions also exist). Relative importance of each of the three ecosystem pathways varies in the environmental space (niche), here defined by the water availability and soil fertility. Illustration by Dharmaberen Studio. From [1].
References
[1] Pausas J.G. & Bond W.J. 2020. On the three major recycling pathways in terrestrial ecosystems. Trends Ecol. & Evol. [doi | pdf]
[2] Pausas J.G. & Bond W.J. 2020. Alternative biome states in terrestrial ecosystems. Trends Plant Sci. 25: 250-263. [doi | sciencedirect | cell | pdf]
[3] Keeley J.E., Bond W.J., Bradstock R.A., Pausas J.G. & Rundel P.W. 2012. Fire in Mediterranean Ecosystems: Ecology, Evolution and Management. Cambridge University Press. [the book]
[4] Bond, W. J. 2019. Open Ecosystems: Ecology and Evolution Beyond the Forest Edge. Oxford University Press.
There is growing interest in the application of alternative stable state (ASS) theory to explain major vegetation patterns in tropical ecosystems [1] and beyond [2]. In a recent paper [3] we introduced the theory as applied to the puzzle of non-forested (open) biomes growing in climates that are warm and wet enough to support forests (alternative biome states, ABSs; Fig. 1). Long thought to be the product of deforestation, diverse lines of evidence indicate that many open ecosystems are ancient. They have also been characterized as ‘early successional’ even where they persist for millennia. ABS is an alternative framework to that of climate determinism and succession (Table 1 below) for exploring forest/nonforest mosaics. Within climatic and edaphic constraints, consumers (fire and herbivores) can produce vastly different ecosystems from the climate potential and have done so for millions of years [4]. This framework explains not only tropical forest–savanna landscapes, but also other landscape mosaics across the globe (Fig. 2).
Fig. 1. Generalized feedback processes in fire-prone landscapes where open and closed biomes (e.g., a grassland and forest) are alternative stable states maintained by stabilizing feedbacks, while perturbations generate abrupt transitions among states (destabilizing factors). From: [3].
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Fig. 2. Examples of multibiome landscape mosaics where closed forests alternate with open biomes (grasslands) that are maintained by mammal herbivory and fire. From: [3].
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Table 1. Comparison of the three main dynamic processes assembling disturbance-prone communities and landscapes: classical (facilitation) succession, autosuccession, and ABS. From: [3].
References
[1] Dantas V.L., Hirota M., Oliveira R.S., Pausas J.G. 2016. Disturbance maintains alternative biome states. Ecol. Lett. 19: 12-19. [doi | wiley | pdf | suppl.]
[2] Pausas, J.G. 2015. Alternative fire-driven vegetation states. J. Veget. Sci. 26:4-6. [doi | pdf | suppl.]
[3] Pausas J.G. & Bond W.J. 2020. Alternative biome states in terrestrial ecosystems. Trends Plant Sci. [doi | sciencedirect| pdf]
[4] Pausas J.G. & Bond W.J. 2019. Humboldt and the reinvention of nature. J. Ecol. 107: 1031-1037. [doi | jecol blog | jgp blog | pdf]
A recent paper suggested that fire-vegetation feedback processes may be unnecessary to explain tree cover patterns in tropical ecosystems and that climate-fire determinism is an alternative possibility [1]. This conclusion was based on the fact that it is possible to reproduce observed broad scale patterns in tropical regions (e.g., a trimodal frequency distribution of tree cover) using a simple model that does not explicitly incorporate fire-vegetation feedback processes. We argue that this reasoning is misleading because these two mechanisms (feedbacks vs fire-climate control) operate at different spatial and temporal scales [2]. It is not possible to evaluate the role of a process acting at fine scales (e.g., fire-vegetation feedbacks) using a model designed for reproducing regional-scale pattern; i.e., there is a mismatch between the scale of the question and the scale of the approach for addressing the question. While the distribution of forest and savannas are partially determined by climate, the most parsimonious explanation for their environmental overlaps (as alternative states) is the existence of feedback processes [3,4], as has been shown in many ecosystems, not only tropical ones [4]. Climate is unlikely to be an alternative to feedback processes; rather, climate and fire-vegetation feedbacks are complementary processes acting at different spatial and temporal scales [2].
Figure: Fire activity (based on remotely sensed data) for savannas and forests located in the range of environmental conditions where both occurs, for Africa and South America (Afrotropics and Neotropics, respectively). From [2,3].
References
[1] Good, P., Harper, A., Meesters, A., Robertson, E. & Betts, R. (2016) Are strong fire–vegetation feedbacks needed to explain the spatial distribution of tropical tree cover? Global Ecol. and Biogeogr. 25, 16-25.
[2] Pausas J.G. & Dantas V.L. 2017. Scale matters: Fire-vegetation feedbacks are needed to explain tropical tree cover at the local sacle. Global Ecol. and Biogeogr. [doi | wiley | pdf]
[3] Dantas V.L., Hirota M., Oliveira R.S., Pausas J.G. 2016. Disturbance maintains alternative biome states. Ecology Letters 19: 12-19. [doi | wiley | pdf | suppl | blog]
[4] Pausas, J.G. 2015. Alternative fire-driven vegetation states. J. Veget. Sci. 26:4-6. [doi | pdf | suppl.] | blog]
It is becoming more and more evident that climate alone does not explain spatial and temporal patterns of the world vegetation, and that disturbance regimes explain an important part of the variability in vegetation and biome composition and distribution [1]. This has been suggested specially in tropical ecosystems where alternative vegetation states (e.g., forests and savannas) are possible for a given climatic conditions [2]. For instance, in dry years, surface fires may enter in forests and kill fire-sensitive trees and select for fire-resistant woody species with open crown architectures that generates well lit communities with a flammable grassy understory. Forest trees and savannas trees have a marked difference in bark thickness (thinner in the former) and thus a contrasted sensitivity to surface fires [3]. Thus, a switch to a forest state from a savanna depends on a sufficiently long fire interval or high resource availability allowing the outcompetition of shade-intolerant savanna trees and grasses (i.e. the inhibition of fires) by means of a closed canopy of forest trees. Similarly, herbivory can also exert a control on woody biomass and favour herbivory-resistant shrubs and grasses. However, empirical (field-based) evidence for alternative sates were very limited. In a recent paper [4] we used field data to show that, for a wide range of environmental conditions (in South America and Africa), fire feedbacks maintain savannas and forests as alternative biome states in both the Neotropics and the Afrotropics. In addition, wooded grasslands and savannas occurred as alternative states in the Afrotropics, depending on the relative importance of fire and herbivory feedbacks. That is, we found evidence for a disturbance-driven bistability in the Neotropics and a disturbance-driven tristability in Afrotropics (figure below).
Fig. Top: Frequency distribution of basal area in afrotropical (tristability) and neotropical (bistability) ecosystems. Bottom: The discontinuous pattern of basal area along the resources gradient for both afrotropical and neotropical ecosystems (red: wooded grasslands; orange: savannas; green: forests). Note that there are regions of the gradient where two alternative vegetation types are possible; they are maintained by different disturbance regime (see [4]).
References:
[1] Pausas, J.G. 2015. Alternative fire-driven vegetation states. J. Veget. Sci. 26: 4-6. [doi | pdf | suppl.]
[2] Dantas V., Batalha MA & Pausas JG. 2013. Fire drives functional thresholds on the savanna-forest transition. Ecology 94:2454-2463. [doi | pdf | appendix]
[3] Pausas, J.G. 2015. Bark thickness and fire regime. Funct. Ecol. 29: 317-327. [doi | pdf | suppl.]
[4] Dantas V.L., Hirota M., Oliveira R.S., Pausas J.G. 2016. Disturbance maintains alternative biome states. Ecology Letters 19:12-19 [doi | wiley | pdf |supp.] – New!
[5] Update (a new relevant paper): Pausas J.G. & Dantas V.L. 2017. Scale matters: Fire-vegetation feedbacks are needed to explain tropical tree cover at the local sacle. Global Ecol. Biogeogr. 26: 395–399. [doi | pdf | post ]
One of the clearest pieces of evidence for the role of fire in shaping vegetation is the occurrence of alternative vegetation types maintained by different fire regimes in a given climate. The different flammability of alternative communities generates different fire feedback processes that maintain contrasted vegetation types with clear boundaries in a given environment; and fire exclusion blurs this structure. This has been well documented in tropical landscapes (e.g., [1]) that are often mosaics of two alternative stable states – savannas and forests – with distinct structures and functions and sharp boundaries. Currently, there is an increasing evidence that alternative fire-driven vegetation states do occur in other environments, including temperate forests ([2, 3] and figure below). That is, the existence of alternative fire-driven vegetation states may be more frequent than previously thought, although human activities may favour one of the states and mask the original bistability.
Figure: Factors determining the transition between two alternative vegetation states (fire sensitive forest and fire resilient shrubland) in a temperate landscape in Patagonia. Human factors (global warming, increased ignitions, and livestock grazing) favour transition to shrublands. From [2].
References
[1] Dantas V., Batalha MA & Pausas JG. 2013. Fire drives functional thresholds on the savanna-forest transition. Ecology 94:2454-2463. [doi | pdf | appendix]
[2] Pausas, J.G. 2015. Alternative fire-driven vegetation states. Journal of Vegetation Science 26: 4-6 [doi | pdf | suppl.]
[3] Paritsis J., Veblen T.T. & Holz A. 2014. Positive fire feedbacks contribute to shifts from Nothofagus pumilio forests to fire-prone shrublands in Patagonia. J. Veget. Sci., 26.
