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.
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.
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.
The role of large herbivores in explaining broad-scale ecological pattern has probably been underestimated [1]. For instance, they are important in maintaining many landscapes and biomes across the world [2]. In a recent paper we show that the different history of megafauna abundance and extinctions in different continents has shaped the dominance of many plant traits [3]. Tropical Africa (paelotropics) and tropical South America (neotropics) are a clear example of contrasting megafauna history under similar environmental conditions. By comparing plant traits of woody species in different biomes (savannas and forests) and for the two different continents, we found that continent explain better the differences in plant traits than biome, climate, or soil, and that the differences between continents are consistent with the higher impact of large vertebrates in Africa than in South America. For instance, plants in African savannas tend to be more thorny and to have higher wood density, i.e., traits related to defense against megaherbivores. In contrast, South American savannas (Cerrado) harbor more species with underground bud banks (geoxyles [4]), and thicker protective barks, i.e., traits related to protect from wildfires [4,5].
Megafauna was certainly present in South America before the Holocene overkilling by humans, however, it is unlikely they live in the brazilian savannas (cerrado); their weak and vulnerable stems (low height growth, low wood density, and lack of spines), are unlikely to have evolved in regions with abundant browsers. We hypotheses that megafauna in South America was distributed in: 1) an open version of the current seasonally dry tropical forests (SDTF, e.g., Chaco) as the proportion of thorny species is similar to African savannas (and much higher than the cerrado); and 2) the subtropical grasslands, as they currently need to be maintained by humans due to the missing megaherbivores (landscape anachronism [1]).
Overall our results suggest that variation in plant traits in the tropics is unlikely to be fully understood without considering historical factors, especially the direct and indirect impacts of megafauna. Looking at plants and thinking on their megafauna history may provide novel insights for understanding vegetation patterns across the globe [1].
The effects of megafauna history (left: absence, right: present of megafauna) on plant functional traits of tropical woody species in different biomes (savannas and forests). Arrows between the dominant driver(s) (boxes) and traits (circles) indicate positive (blue) and negative (red) effects. From [3]
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] Pausas JG & Bond WJ. 2020. Alternative biome states in terrestrial ecosystems. Trends Plant Sci. 25: 250-263. [doi | sciencedirect | cell | pdf]
[3] Dantas V & Pausas JG. 2020. Megafauna biogeography explains plant functional trait variability in the tropics. Glob. Ecol. & Biogeogr. [doi | pdf | data:dryad]
[4] Pausas JG, Lamont BB, Paula S, Appezzato-da-Glória B & Fidelis A. 2018. Unearthing belowground bud banks in fire-prone ecosystems. New Phytol. 217: 1435–1448. [doi | pdf | suppl. | BBB database]
[5] Pausas JG. 2017. Bark thickness and fire regime: another twist. New Phytol. 213: 13-15. [doi | wiley | pdf] & Pausas, J.G. 2015. Bark thickness and fire regime. Funct. Ecol. 29:317-327. [doi | pdf | suppl.]
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]
