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Paleoecology
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High herbivore density associated with vegetation diversity in interglacial ecosystems
Abstract The impact of large herbivores on ecosystems before modern human activities is an open question in ecology and conservation. For Europe, the controversial wood–pasture hypothesis posits that grazing by wild large herbivores supported a dynamic mosaic of vegetation structures at the landscape scale under temperate conditions before agriculture. The contrasting position suggests that European temperate vegetation was primarily closed forest with relatively small open areas, at most impacted locally by large herbivores. Given the role of modern humans in the world-wide decimations of megafauna during the late Quaternary, to resolve this debate it is necessary to understand herbivore–vegetation interactions before these losses. Here, a synthetic analysis of beetle fossils from Great Britain shows that beetles associated with herbivore dung were better represented during the Last Interglacial (132,000–110,000 y B.P., before modern human arrival) than in the early Holocene (10,000–5,000 y B.P.). Furthermore, beetle assemblages indicate closed and partially closed forest in the early Holocene but a greater mixture of semiopen vegetation and forest in the Last Interglacial. Hence, abundant and diverse large herbivores appear to have been associated with high structural diversity of vegetation before the megafauna extinctions at the end of the Pleistocene. After these losses and in the presence of modern humans, large herbivores generally were less abundant, and closed woodland was more prevalent in the early Holocene. Our findings point to the importance of the formerly rich fauna of large herbivores in sustaining structurally diverse vegetation in the temperate forest biome and provide support for recent moves toward rewilding-based conservation management.
Combining paleo-data and modern exclosure experiments to assess the impact of megafauna extinctions on woody vegetation
Abstract Until recently in Earth history, very large herbivores (mammoths, ground sloths, diprotodons, and many others) occurred in most of the World’s terrestrial ecosystems, but the majority have gone extinct as part of the late-Quaternary extinctions. How has this large-scale removal of large herbivores affected landscape structure and ecosystem functioning? In this review, we combine paleo-data with information from modern exclosure experiments to assess the impact of large herbivores (and their disappearance) on woody species, landscape structure, and ecosystem functions. In modern landscapes characterized by intense herbivory, woody plants can persist by defending themselves or by association with defended species, can persist by growing in places that are physically inaccessible to herbivores, or can persist where high predator activity limits foraging by herbivores. At the landscape scale, different herbivore densities and assemblages may result in dynamic gradients in woody cover. The late-Quaternary extinctions were natural experiments in large-herbivore removal; the paleoecological record shows evidence of widespread changes in community composition and ecosystem structure and function, consistent with modern exclosure experiments. We propose a conceptual framework that describes the impact of large herbivores on woody plant abundance mediated by herbivore diversity and density, predicting that herbivore suppression of woody plants is strongest where herbivore diversity is high. We conclude that the decline of large herbivores induces major alterations in landscape structure and ecosystem functions.
This has been posted on the forum elsewhere but it's honestly probably my favourite paper ever. Well worth the read this, fascinating stuff.
Megafauna and ecosystem function from the Pleistocene to the Anthropocene Abstract Large herbivores and carnivores (the megafauna) have been in a state of decline and extinction since the Late Pleistocene, both on land and more recently in the oceans. Much has been written on the timing and causes of these declines, but only recently has scientific attention focused on the consequences of these declines for ecosystem function. Here, we review progress in our understanding of how megafauna affect ecosystem physical and trophic structure, species composition, biogeochemistry, and climate, drawing on special features of PNAS and Ecography that have been published as a result of an international workshop on this topic held in Oxford in 2014. Insights emerging from this work have consequences for our understanding of changes in biosphere function since the Late Pleistocene and of the functioning of contemporary ecosystems, as well as offering a rationale and framework for scientifically informed restoration of megafaunal function where possible and appropriate.
Ecological and evolutionary legacy of megafauna extinctions
ABSTRACT: For hundreds of millions of years, large vertebrates (megafauna) have inhabited most of the ecosystems on our planet. During the late Quaternary, notably during the Late Pleistocene and the early Holocene, Earth experienced a rapid extinction of large, terrestrial vertebrates. While much attention has been paid to understanding the causes of this massive megafauna extinction, less attention has been given to understanding the impacts of loss of megafauna on other organisms with whom they interacted. In this review, we discuss how the loss of megafauna disrupted and reshaped ecological interactions, and explore the ecological consequences of the ongoing decline of large vertebrates. Numerous late Quaternary extinct species of predators, parasites, commensals and mutualistic partners were associated with megafauna and were probably lost due to their strict dependence upon them (co‐extinctions). Moreover, many extant species have megafauna‐adapted traits that provided evolutionary benefits under past megafauna‐rich conditions, but are now of no or limited use (anachronisms). Morphological evolution and behavioural changes allowed some of these species partially to overcome the absence of megafauna. Although the extinction of megafauna led to a number of co‐extinction events, several species that likely co‐evolved with megafauna established new interactions with humans and their domestic animals. Species that were highly specialized in interactions with megafauna, such as large predators, specialized parasites, and large commensalists (e.g. scavengers, dung beetles), and could not adapt to new hosts or prey were more likely to die out. Partners that were less megafauna dependent persisted because of behavioural plasticity or by shifting their dependency to humans via domestication, facilitation or pathogen spill‐over, or through interactions with domestic megafauna. We argue that the ongoing extinction of the extant megafauna in the Anthropocene will catalyse another wave of co‐extinctions due to the enormous diversity of key ecological interactions and functional roles provided by the megafauna.
There are three images in this video (which very briefly describes the study above: Combining paleo-data and modern exclosure experiments to assess the impact of megafauna extinctions on woody vegetation), which show how fencing off megafauna grazers means woodland prevails, and that they are an important suppressive force for dense vegetation. I would directly post these but wildfact doesn't let the post go through so you'll have to watch it (only 2 minutes)
These clues tell us that the extinction of the megafauna at the end of the pleistocene likely resulted in significant biome changes in many areas. We expect that without the suppressive forces of megafaunal herbivores, woodland and accompanying varieties of more dense vegetation would occur. Therefore we can conclude the pleistocene epoch was one characterised with more open ecosystems such as grasslands, steppes, and savannah's in comparison to today.
Unraveling the consequences of the terminal Pleistocene megafauna extinction on mammal community assembly
Abstract: Recent studies connecting the decline of large predators and consumers with the disintegration of ecosystems often overlook that this natural experiment already occurred. As recently as 14 ka, tens of millions of large‐bodied mammals were widespread across the American continents. Within 1000 yr of the arrival of humans, ∼ 80% were extinct including all > 600 kg. While the cause of the late Pleistocene (LP) extinction remains contentious, largely overlooked are the ecological consequences of the loss of millions of large‐bodied animals. Here, we examine the influence of the LP extinction on a local mammal community. Our study site is Hall's Cave in the Great Plains of Texas, which has unparalleled fine‐grained temporal resolution over the past 20 ka, allowing characterization of the community before and after the extinction. In step with continental patterns, this community lost 80% of large‐bodied herbivores and 20% of apex predators at the LP extinction. Using tightly constrained temporal windows spanning full glacial to modern time periods and comprehensive faunal lists, we reconstruct mammal associations and body size distributions over time. We find changes in alpha and beta diversity, and in the statistical moments associated with periods of climate change as well as with the LP extinction event. Additionally, there is a fundamental change in the composition of herbivores, with grazers being replaced by frugivores/granivores starting about 15 ka; the only large‐bodied grazer remaining today is the bison Bison bison. Moreover, the null model program PAIRS reveals interesting temporal patterns in the disassociation or co‐occurrence of species through the terminal Pleistocene and Holocene. Extinct species formed more significant associations than modern ones, and formed more aggregated pairs than do modern species. Further, negative species associations were about three times stronger than positive ones, suggesting that competitive interactions or environmental filtering are a strong force in community structure.
Biotic responses of canids to the terminal Pleistocene megafauna extinction
Abstract: Trophic downgrading is a major concern for conservation scientists. The largest consumers in many ecosystems have become either rare or extirpated, leading to worry over the loss of their ecosystem function. However, trophic downgrading is not a uniquely modern phenomenon. The extinction of 34 genera of megafauna from North America ∼13 000 yr ago must have led to widespread changes in terrestrial ecosystem function. Studies that have examined the event address impacts on vegetative structure, small mammal communities, nutrient cycling, and fire regimes. Relatively little attention has been paid to community changes at the top of the food chain. Here, we examine the response of carnivores in North America to the Pleistocene extinction. We employ fossil data to model the climatic niche of endemic canids, including the extinct dire wolf Canis dirus, over the last 20 000 yr. Quantifying the abiotic niche allows us to account for expected changes due to climate fluctuations over the Late Quaternary; deviations from expected responses likely reveal influences of competition and/or resource availability. We quantify the degree of niche conservatism and interspecific overlap to assess species and community responses among canids. We also include in our analyses a novel introduced predator, the domestic dog Canis lupus familiaris, which accompanied humans into the New World. We find that endemic canid species display low fidelity to their climatic niche through time, We find that survivors increasingly partition their climatic niche throughout the Holocene and, surprisingly, do not expand into niche space presumably vacated by the extinction of very large carnivores. These results suggest that loss of megaherbivores and competition with humans likely outweighed advantages conferred from the loss of very large predators. We also find that wolves and dogs decrease their niche overlap throughout the Holocene, suggesting a distinctive relationship between dogs and man.
Body mass‐related changes in mammal community assembly patterns during the late Quaternary of North America
Abstract: The late Quaternary of North America was marked by prominent ecological changes, including the end‐Pleistocene megafaunal extinction, the spread of human settlements and the rise of agriculture. Here we examine the mechanistic reasons for temporal changes in mammal species association and body size during this time period. Building upon the co‐occurrence results from Lyons et al. (2016) – wherein each species pair was classified as spatially aggregated, segregated or random – we examined body mass differences (BMD) between each species pair for each association type and time period (Late Pleistocene: 40 000 14C–11 700 14C ybp, Holocene: 11 700 14C–50 ybp and Modern: 50–0 yr). In the Late Pleistocene and Holocene, the BMD of both aggregated and segregated species pairs was significantly smaller than the BMD of random pairs. These results are consistent with environmental filtering and competition as important drivers of community structure in both time periods. Modern assemblages showed a breakdown between BMD and co‐occurrence patterns: the average BMD of aggregated, segregated and random species pairs did not differ from each other. Collectively, these results indicate that the late Quaternary mammalian extinctions not only eliminated many large‐bodied species but were followed by a re‐organization of communities that altered patterns of species coexistence and associated differences in body size.
Figures from: Reintroducing extirpated herbivores could partially reverse the late Quaternary decline of large and grazing species posted in the Pleistocene rewilding thread
 *This image is copyright of its original author Changes in the body mass and diet of herbivore species from the Late Pleistocene to the present‐day. (a) Change in body mass of herbivore species globally since the Late Pleistocene. (b) Global species richness declines of grazers, mixed feeders, and browsers since the Late Pleistocene  *This image is copyright of its original author Changes in the body mass and diet structure of a western European herbivore assemblage. Schematic showing changes in the body mass and diet structure of a western European herbivore assemblage during the late Quaternary and the consequences on assemblage trait structure when reinstating herbivore species using different restoration baselines. The animal silhouettes illustrate the species present at different time points in the past (blue bars) or in different future scenarios (green bars)
From the study posted above. It makes sense that the reduction in the mammoth steppe and warming temperatures were detrimental to grazers and mixed feeders, and as such browsers weren't as heavily impacted.
"We find that extinctions have removed the largest herbivores since the Late Pleistocene (Figure 1a), a trend also observed for megafauna in general (Smith et al., 2018). Consequently, current herbivore assemblages are down‐sized compared to the present‐natural assemblages that would have existed in the absence of human impacts (Figure 2a–b). Extinctions have also led to a decline in grazing herbivores. Even though they have lost fewer species in absolute terms, grazers have suffered relatively more extinctions than either mixed feeders or browsers (χ2 test of independence, χ2 = 9.18, df = 2, p = .01, Supporting Information Appendix Table S4.1). In total, grazers have lost 53% of their total species richness since the Late Pleistocene (48/90 species), whereas mixed feeders have lost 37% (65/177 species), and browsers have lost 36% (90/251 species) (Figure 1a). Most of these extinctions happened during the Late Pleistocene (Figure 1b). Grazers and mixed feeders, when compared to browsers, also possess a larger difference between their current and present‐natural‐ranges (Kruskal–Wallis, χ2 = 22.71, df = 2, p < .001, Supporting Information Appendix Table S4.2). On average, the current range of a grazing herbivore was 991,767 km2 smaller than their present‐natural range (median difference between current and present‐natural ranges). The range of a mixed feeder was 493,555 km2 smaller and the range of a browser was 65,186 km2 smaller. This suggests that grazers and, to a lesser extent, mixed feeders have experienced proportionally larger anthropogenic range restrictions than browsers have. As a result of the late Quaternary extinctions and extirpations, current wild herbivore assemblages are mostly dominated by herbivores with browsing diets (Figure 2c). In fact, wild grazers are absent from most of world’s terrestrial ecosystems (Supporting Information Figure S3.2)—contrasting strongly with the present‐natural scenario. Most present‐natural assemblages would have a more or less equal composition of grazing and browsing (Figure 2d). Wild herbivore assemblages where grazing is common still exist, but are rare and restricted to eastern Africa, Patagonia, parts of Central Asia, and Australia (Figure 2c). Australia appears to be an outlier, though. In contrast to most places, the continent has primarily lost browsing species (Figure 2c–d). Historically it also possessed relatively few large grazers (Australian herbivore species ≥ 500 kg: browsers: 6, mixed feeders: 1, grazers: 2)."
"The evidence we review suggests that Steller’s sea cows exerted substantial direct and indirect influences on kelp forests, likely affecting the physical ecosystem structure, productivity, nutrient cycling, species interactions, and export of nutrients to surrounding ecosystems. This suggests that kelp forest dynamics and resilience were already significantly altered prior to the influence of more recent and well-known stressors, such as industrial fishing and climate change, and illustrates the important ecological roles that are lost with megafaunal extinction."
The ghost of a giant – Six hypotheses for how an extinct megaherbivore structured kelp forests across the North Pacific Rim |
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