Establishment of Broad-leaved Thickets in Serengeti, Tanzania: The Influence of Fire, Browsers, Grass Competition, and Elephants1

Direction of plant-soil feedback determines plant responses to drought

Droughts exarcerbate Plant-soil feedbacks (PSFs) making positive PSFs more positive and negative PSFs more negative. Alterations in PSFs that droughts induce could relate to the rooting depth of the tested plants. We present some rare evidence on how a driver of global change will alter a biotic interaction.

Analyses of three-dimensional species associations reveal departures from neutrality in a tropical forest

The study of community spatial structure is central to understanding diversity patterns over space and species co-occurrence at local scales. While most analytical approaches consider horizontal and vertical dimensions separately, in this study we introduce a three-dimensional spatial analysis that simultaneously includes horizontal and vertical species associations. Using tree census data (2000 to 2016) and allometries from the Luquillo forest plot in Puerto Rico, we show that spatial organization becomes less random over time as the forest recovered from land-use legacy effects and hurricane disturbance. Tree species vertical segregation is predominant in the forest with almost all species that co-occur in the horizontal plane avoiding each other in the vertical dimension. Horizontal segregation is less common than vertical, while three-dimensional aggregation (a proxy for direct tree competition) is the least frequent type of spatial association. Furthermore, dominant species are involved in more non-random spatial associations, implying that species co-occurrence is facilitated by species segregation in space. This novel three-dimensional analysis allowed us to identify and quantify tree species spatial distributions, how interspecific competition was reduced through forest structure, and how it changed over time after disturbance, in ways not detectable from two-dimensional analyses alone.

Rare plant species are at a disadvantage when both herbivory and pollination interactions are considered in an alpine meadow

Rare plant species often suffer less damage than common species because of positive density-dependent herbivory, and it has been suggested that this 'rare species advantage' fosters plant species coexistence. However, it is unknown whether rare species have an advantage when pollination interactions are also considered. We hypothesized that a 'positive density-dependent pollination success' across plant species would result in common plants experiencing higher seed set rates compared to rare species, and that positive density-dependent effects would negate or even override the positive density-dependent damage due to herbivory resulting in higher seed loss rates in common plant species. We tested this hypothesis by concurrently examining a plant-predispersal seed predator system and a plant-pollinator system for 24 Asteraceae species growing in an alpine meadow community (Sichuan Province, China). Having previously reported a positive density-dependent effect on seed loss rates due to seed predators, we here focus on the density-dependent effects on pollination success by investigating pollinator species richness, visitation frequencies and seed set rates for each plant species. We also estimated the seed output rate of each plant species as the product of seed set rate and the rate of surviving seeds (i.e. 1 - the seed loss rate). Consistent with our hypothesis, a positive density-dependent effect was observed for pollinator species richness, visitation frequencies and seed set rates across plant species. Moreover, the positive effect overrode the negative density-dependent effect of herbivores on seed production, such that common species tended to have a higher seed output rate than rare species (i.e. we observed a 'rare species disadvantage'). These results indicate that the low seed output rate of rare species might result from a pollination limitation, and that both mutualistic and antagonistic interactions should be examined simultaneously to fully understand plant species coexistence in local communities.

How frequency-dependent selection affects population fitness, maladaptation and evolutionary rescue

Frequency-dependent (FD) selection is a central process maintaining genetic variation and mediating evolution of population fitness. FD selection has attracted interest from researchers in a wide range of biological subdisciplines, including evolutionary genetics, behavioural ecology and, more recently, community ecology. However, the implications of frequency dependence for applied biological problems, particularly maladaptation, biological conservation and evolutionary rescue remain underexplored. The neglect of FD selection in conservation is particularly unfortunate. Classical theory, dating back to the 1940s, demonstrated that frequency dependence can either increase or decrease population fitness. These evolutionary consequences of FD selection are relevant to modern concerns about population persistence and the capacity of evolution to alleviate extinction risks. But exactly when should we expect FD selection to increase versus decrease absolute fitness and population growth? And how much of an impact is FD selection expected to have on population persistence versus extinction in changing environments? The answers to these questions have implications for evolutionary rescue under climate change and may inform strategies for managing threatened populations. Here, we revisit the core theory of FD selection, reviewing classical single-locus models of population genetic change and outlining short- and long-run consequences of FD selection for the evolution of population fitness. We then develop a quantitative genetic model of evolutionary rescue in a deteriorating environment, with population persistence hinging upon the evolution of a quantitative trait subject to both frequency-dependent and frequency-independent natural selection. We discuss the empirical literature pertinent to this theory, which supports key assumptions of our model. We show that FD selection can promote population persistence when it aligns with the direction of frequency-independent selection imposed by abiotic environmental conditions. However, under most scenarios of environmental change, FD selection limits a population's evolutionary responsiveness to changing conditions and narrows the rate of environmental change that is evolutionarily tolerable.

Rare genotype advantage promotes survival and genetic diversity of a tropical palm

Negative density dependence, where survival decreases as density increases, is a well-established driver of species diversity at the community level, but the degree to which a similar process might act on the density or frequency of genotypes within a single plant species to maintain genetic diversity has not been well studied in natural systems. In this study, we determined the maternal genotype of naturally dispersed seeds of the palm Oenocarpus bataua within a tropical forest in northwest Ecuador, tracked the recruitment of each seed, and assessed the role of individual-level genotypic rarity on survival. We demonstrate that negative frequency-dependent selection within this species conferred a survival advantage to rare maternal genotypes and promoted population-level genetic diversity. The strength of the observed rare genotype survival advantage was comparable to the effect of conspecific density regardless of genotype. These findings corroborate an earlier, experimental study and implicate negative frequency-dependent selection of genotypes as an important, but currently underappreciated, determinant of plant recruitment and within-species genetic diversity. Incorporating intraspecific genetic variation into studies and theory of forest dynamics may improve our ability to understand and manage forests, and the processes that maintain their diversity.

Impala, Aepyceros melampus: does browse quality influence their use of sites originally utilised as short-duration kraals in a southern African savanna?

In eastern and southern Africa, some ranch owners are now keeping cattle overnight in temporary corrals (hereafter referred to as kraals) within rangelands for short durations to improve grass production. However, this has profound effects on the woody plant community. For instance, cattle break woody plant stems and strip them of foliage, initiating resprouting. The resprouts produced have high foliar nitrogen (N) and reduced condensed tannin (CT) concentrations, making them attractive to herbivores. The aim of this study was to determine the key nutrient-quality parameters of resprouts that make previously kraaled sites attractive to impala soon after cattle removal at Debshan Ranch in central Zimbabwe. We determined resprout length, foliar N, phosphorus (P), potassium, CT, fibre and rumen fermentation of three browse species, viz. Grewia monticola Sond., Terminalia sericea Burch. ex DC. and Dichrostachys cinerea (L.) Wight and Arn., and related them to impala use of previously kraaled sites. We used impala dung density to determine the use patterns of previously kraaled sites 2, 4, 12 and 24 weeks after cattle removal and compared them with the surrounding vegetation. Impala use of previously kraaled sites was highest 4 weeks after cattle removal and lowest in the surrounding vegetation. Resprout length increments were 6-fold over a 10-week growth period in all three woody species. Foliar N and P were generally higher, whereas CT was lower, in previously kraaled sites than the surrounding vegetation in all three of the plant browse species. Impala use of previously kraaled sites showed a strong negative relationship with foliar CT. We conclude that kraaling initiates strong resprout responses by woody plants soon after cattle removal, to produce resprouts of high nutrient quality, which attract herbivores such as impala.

Negative density dependence is stronger in resource-rich environments and diversifies communities when stronger for common but not rare species

Conspecific negative density dependence is thought to maintain diversity by limiting abundances of common species. Yet the extent to which this mechanism can explain patterns of species diversity across environmental gradients is largely unknown. We examined density-dependent recruitment of seedlings and saplings and changes in local species diversity across a soil-resource gradient for 38 woody-plant species in a temperate forest. At both life stages, the strength of negative density dependence increased with resource availability, becoming relatively stronger for rare species during seedling recruitment, but stronger for common species during sapling recruitment. Moreover, negative density dependence appeared to reduce diversity when stronger for rare than common species, but increase diversity when stronger for common species. Our results suggest that negative density dependence is stronger in resource-rich environments and can either decrease or maintain diversity depending on its relative strength among common and rare species.

The effects of seed ingestion by livestock, dung fertilization, trampling, grass competition and fire on seedling establishment of two woody plant species

The increasing rate of woody plant encroachment in grasslands or savannas remains a challenge to livestock farmers. The causes and control measures of woody plant encroachment are of common interest, especially where it negatively affects the objectives of an agricultural enterprise. The objectives of this study were to determine the effects of gut passage (goats, cattle), dung (nutrients), fire, grass competition and trampling on establishment of A. nilotica and D. cinerea seedlings. Germination trials were subjected to the following treatments: 1) seed passage through the gut of cattle and goats and unpassed/ untreated seeds (i.e. not ingested), 2) dung and control (no dung), 3) grass and control (mowed grass), 4) fire and control (no fire), 5) trampling and control (no trampling). The interaction of animal species, grass and fire had an effect on seedling recruitment (P < 0.0052). Seeds retrieved from goats and planted with no grass and with fire (6.81% ± 0.33) had a significant effect on seedling recruitment than seeds retrieved from goats and planted with grass and no fire (2.98% ± 0.33). Significantly more D. cinerea and A. nilotica seeds germinated following seed ingestion by goats (3.59% ± 0.16) than cattle (1.93% ± 0.09) and control or untreated seeds (1.69% ± 0.11). Less dense grass cover, which resulted in reduced grass competition with tree seedlings for light, space and water, and improved seed scarification due to gut passage were vital for emergence and recruitment of Acacia seedlings. These results will contribute considerably to the understanding of the recruitment phase of woody plant encroachment.

Did the elephant and giraffe mediate change in the prevalence of palatable species in an East African Acacia woodland?

We report on a 2009 survey of Acacia woodlands in the Seronera area of central Serengeti, Tanzania, and compare the results to previous surveys from the 1970s–1980s. We document a substantial change in woodland structure and composition. From 1978 to 2009, woody plant density (mean ± SD) declined from 255 ± 35 trees ha−1 to 147 ± 14 trees ha−1. Canopy cover declined from 15.1 ± 1.9% to 5.7 ± 0.6%. Canopy volume declined from 1810 ± 207 m3 ha−1 to 1410 ± 121 m3 ha−1. A 19% increase in the relative canopy volume of Acacia robusta, a species avoided by the giraffe, represents a sharp decline in the quality of the giraffe's food supply, which was last comprehensively assessed in 1978. We examine these changes in the context of Pellew's 1983 woodland dynamics model describing the impacts of elephant, giraffe and fire on an Acacia tortilis tree population and determine that our data are qualitatively consistent with this model. We hypothesize that selective elephant and giraffe browsing has contributed to an increase in the relative dominance of unpalatable species, consistent with Pellew's predictions.

Spatial genetic structure in four understory Psychotria species (Rubiaceae) and implications for tropical forest diversity

• Premise of the study: Tropical forests are the most species-rich terrestrial communities on Earth, and understory trees and shrubs comprise a large fraction of their plant species diversity, especially at high rainfalls. The mechanisms responsible for generating such high levels of diversity remain unknown. One hypothesis is that fleshy-fruited understory species should have limited seed dispersal due to the sedentary nature of their avian dispersers, resulting in restricted gene flow, population differentiation at small spatial scales, and ultimately, high rates of allopatric speciation.• Methods: We sampled four species of the hyperdiverse tropical shrub genus Psychotria (Rubiaceae) on Barro Colorado Island (BCI) and two nearby sites in Panama. We genotyped each species with AFLPs, assessed genetic differentiation among populations, and determined patterns of fine-scale spatial genetic structure in the BCI population. Measures of spatial autocorrelation and population density were used to estimate the dispersal distance parameter σ.• Key results: Regionally, Φ_PT values ranged from 0.13 to 0.28, reflecting local population differentiation and suggesting that Lake Gatun/Rio Chagres has posed a relatively strong barrier to gene flow. Fine-scale spatial genetic structure on BCI was stronger than in most canopy trees, and estimated distances of gene flow were unusually low for endozoochorous tropical woody plants, with dispersal distance σ = 9-113 m.• Conclusions: These results demonstrate comparatively limited gene flow in bird-dispersed understory species, supporting a hypothesized mechanism for generating high levels of plant species diversity in tropical rain forests, in one of the largest genera of flowering plants on Earth.

Termites facilitate and ungulates limit savanna tree regeneration

Both large herbivores and termites are key functional groups in savanna ecosystems, and in many savanna areas, large termite mounds (termitaria) are associated with distinct woody clusters. Studies on the effect of large mammals on tree regeneration are few, and the results are conflicting. Large herbivores have been found to be important seedling predators in some areas, but facilitate tree regeneration by outcompeting small mammals and reducing grass cover in other areas. Through the use of the experimental fencing of termite mounds and adjacent savanna areas in this study, we investigated how termites and large herbivores influence tree regeneration. Termite mounds had a higher number of seedlings, more species richness, more alpha diversity (OD) and lower evenness (E) than savanna plots. Large herbivores did not significantly affect overall seedling density, species richness, OD or E. Beta diversity was higher in savanna areas than on termitaria, and beta diversity decreased in savanna areas when herbivores were excluded. Herbivore exclusion increased the density of the 12 (40%) most common seedling species, representing 79% of all seedlings, and fenced plots had relatively taller seedlings than open plots. Thus, termites were the main determinants of tree regeneration in our study area, but large mammals regulated the most common species. Although our study confirms previous work suggesting that large herbivores affect tree regeneration, we found that termites were an even more important determinant. Termite impacts on tree regeneration deserve increased attention by savanna ecologists.

Heavy impact on seedlings by the impala suggests a central role in woodland dynamics

Research has increasingly established that mesoherbivores influence the regeneration of woody plants. However the relationship between mesoherbivore density and degree of impact, and the spatial component of this impact, has not been well established. Using a novel sampling design, we assessed in iMfolozi Park, South Africa, the impact of impala (Aepyceros melampus) across the full complement of woody species within the home range, evaluating its spatial component and relationship to impala density. We used four GPS collars, in separate breeding herds, and a GIS to detect zones of different density of impala in the landscape, thus defining a fine-grain browsing gradient. We assessed impact on woody recruits (≤ 0.5 m height) across this gradient by means of 1600 random 1 × 1-m quadrats. Densities of woody seedlings, and mean percentage of remaining canopy, were significantly less in areas of high impala density versus low-density areas. There was a significant correlation between increasing impala density and decreasing density of favoured woody recruits. We propose a hypothesis of impala-induced patch dynamics. It seems likely that the ubiquitous impala may create and sustain a shifting mosaic of patches, and thus function as a key determinant of landscape heterogeneity.

Potential aboveground biomass in drought-prone forest used for rangeland pastoralism

The restoration of cleared dry forest represents an important opportunity to sequester atmospheric carbon. In order to account for this potential, the influences of climate, soils, and disturbance need to be deciphered. A data set spanning a region defined the aboveground biomass of mulga (Acacia aneura) dry forest and was analyzed in relation to climate and soil variables using a Bayesian model averaging procedure. Mean annual rainfall had an overwhelmingly strong positive effect, with mean maximum temperature (negative) and soil depth (positive) also important. The data were collected after a recent drought, and the amount of recent tree mortality was weakly positively related to a measure of three-year rainfall deficit, and maximum temperature (positive), soil depth (negative), and coarse sand (negative). A grazing index represented by the distance of sites to watering points was not incorporated by the models. Stark management contrasts, including grazing exclosures, can represent a substantial part of the variance in the model predicting biomass, but the impact of management was unpredictable and was insignificant in the regional data set. There was no evidence of density-dependent effects on tree mortality. Climate change scenarios represented by the coincidence of historical extreme rainfall deficit with extreme temperature suggest mortality of 30.1% of aboveground biomass, compared to 21.6% after the recent (2003-2007) drought. Projections for recovery of forest using a mapping base of cleared areas revealed that the greatest opportunities for restoration of aboveground biomass are in the higher-rainfall areas, where biomass accumulation will be greatest and droughts are less intense. These areas are probably the most productive for rangeland pastoralism, and the trade-off between pastoral production and carbon sequestration will be determined by market forces and carbon-trading rules.

Large herbivores may alter vegetation structure of semi-arid savannas through soil nutrient mediation

In savannas, the tree-grass balance is governed by water, nutrients, fire and herbivory, and their interactions. We studied the hypothesis that herbivores indirectly affect vegetation structure by changing the availability of soil nutrients, which, in turn, alters the competition between trees and grasses. Nine abandoned livestock holding-pen areas (kraals), enriched by dung and urine, were contrasted with nearby control sites in a semi-arid savanna. About 40 years after abandonment, kraal sites still showed high soil concentrations of inorganic N, extractable P, K, Ca and Mg compared to controls. Kraals also had a high plant production potential and offered high quality forage. The intense grazing and high herbivore dung and urine deposition rates in kraals fit the accelerated nutrient cycling model described for fertile systems elsewhere. Data of a concurrent experiment also showed that bush-cleared patches resulted in an increase in impala dung deposition, probably because impala preferred open sites to avoid predation. Kraal sites had very low tree densities compared to control sites, thus the high impala dung deposition rates here may be in part driven by the open structure of kraal sites, which may explain the persistence of nutrients in kraals. Experiments indicated that tree seedlings were increasingly constrained when competing with grasses under fertile conditions, which might explain the low tree recruitment observed in kraals. In conclusion, large herbivores may indirectly keep existing nutrient hotspots such as abandoned kraals structurally open by maintaining a high local soil fertility, which, in turn, constrains woody recruitment in a negative feedback loop. The maintenance of nutrient hotspots such as abandoned kraals by herbivores contributes to the structural heterogeneity of nutrient-poor savanna vegetation.

Browsing and fire interact to suppress tree density in an African savanna

Disturbances from fire and herbivory strongly affect savanna vegetation dynamics. In some savannas, fire especially may be instrumental in preserving the coexistence of trees and grasses. The role of herbivory by large mammals is less clear; herbivory has been shown variously to promote and to suppress tree establishment. Here we ask how interactions between herbivory and fire act to shape savanna vegetation dynamics via their effects on tree populations in Hluhluwe iMfolozi Park in KwaZulu Natal, South Africa, a savanna with a full complement of native large mammals. We examined the effects of herbivore exclusion on tree growth, mortality, and seedling establishment from 2000 to 2007 at 10 sites located in areas of low and high herbivore pressure throughout the park. Results were analyzed statistically and using Leslie matrix models of population dynamics. Herbivory and fire acted primarily to suppress sapling growth rather than on sapling mortality or seedling establishment. This indicates that browsing, like fire, suppresses tree density by imposing a demographic bottleneck on the maturation of saplings to adults. Model results suggest that, while browsing and fire each alone impacted growth, a combination of browsing and fire had much greater effects on tree density. Only fire and browsing together were able to prevent increases in tree density. These results suggest that, while soil resources, including nutrients and moisture, are probably instrumental in determining tree growth rates, disturbances from fire and herbivory may be instrumental in limiting tree cover and facilitating the coexistence of trees and grasses in savannas.

Serengeti birds maintain forests by inhibiting seed predators

Of fundamental interest in conservation ecology are the regulatory mechanisms that maintain communities. We document a mechanism that maintains forests in the Serengeti ecosystem, Tanzania, and the destabilization when disturbance opens forest canopy. Forest birds, by consuming seeds, protected them from beetle attack. Consumption increased the germination rate and the density of seedlings and recruits, which was sufficient to maintain the forest. Opening of the canopy resulted in loss of birds, increased beetle attack, and loss of germination. Thus, frugivorous birds are necessary for the maintenance of forests. Their absence could have resulted in the observed forest decline since 1966.

Food-web structure and ecosystem services: insights from the Serengeti

The central organizing theme of this paper is to discuss the dynamics of the Serengeti grassland ecosystem from the perspective of recent developments in food-web theory. The seasonal rainfall patterns that characterize the East African climate create an annually oscillating, large-scale, spatial mosaic of feeding opportunities for the larger ungulates in the Serengeti; this in turn creates a significant annual variation in the food available for their predators. At a smaller spatial scale, periodic fires during the dry season create patches of highly nutritious grazing that are eaten in preference to the surrounding older patches of less palatable vegetation. The species interactions between herbivores and plants, and carnivores and herbivores, are hierarchically nested in the Serengeti food web, with the largest bodied consumers on each trophic level having the broadest diets that include species from a large variety of different habitats in the ecosystem. The different major habitats of the Serengeti are also used in a nested fashion; the highly nutritious forage of the short grass plains is available only to the larger migratory species for a few months each year. The longer grass areas, the woodlands and kopjes (large partially wooded rocky islands in the surrounding mosaic of grassland) contain species that are resident throughout the year; these species often have smaller body size and more specialized diets than the migratory species. Only the larger herbivores and carnivores obtain their nutrition from all the different major habitat types in the ecosystem. The net effect of this is to create a nested hierarchy of subchains of energy flow within the larger Serengeti food web; these flows are seasonally forced by rainfall and operate at different rates in different major branches of the web. The nested structure that couples sequential trophic levels together interacts with annual seasonal variation in the fast and slow chains of nutrient flow in a way that is likely to be central to the stability of the whole web. If the Serengeti is to be successfully conserved as a fully functioning ecosystem, then it is essential that the full diversity of natural habitats be maintained within the greater Serengeti ecosystem. The best way to do this is by controlling the external forces that threaten the boundaries of the ecosystem and by balancing the economic services the park provides between local, national and international needs. I conclude by discussing how the ecosystem services provided by the Serengeti are driven by species on different trophic levels. Tourism provides the largest financial revenue to the national economy, but it could be better organized to provide more sustained revenue to the park. Ultimately, ecotourism needs to be developed in ways that take lessons from the structure of the Serengeti food webs, and in ways that provide tangible benefits to people living around the park while also improving the experience of all visitors.

The savanna tree Acacia polyacantha facilitates the establishment of riparian forests in Serengeti National Park, Tanzania

Forests are being converted to grasslands and croplands across Africa and natural regeneration of forests is typically poor. In Serengeti National Park, Tanzania, the savanna tree species Acacia polyacantha established in riparian grasslands and forest trees subsequently established within these stands. We examined the conditions for establishment of: (1) A. polyacantha and (2) riparian (non-Acacia) forests. Fire was excluded from three grassland areas for 5 y allowing A. polyacantha to establish during 1999 when dry-season rainfall was high. The seedlings of forest tree species did not establish in grasslands, but were found in large A. polyacantha stands (> 0.3 ha) with reduced grass cover (< 10%), higher cover of herbs (> 80%) and thorny shrubs (> 90%). Seeding survival was high in large stands (0.87 y−1), but declined in artificial canopy gaps due to the ingrowth of grasses (0.21 y−1) and subsequent fires (0.07 y−1). Shrub removal also reduced seedling survival (0.46 y−1) due to browsing by antelope. We propose that: (1) A. polyacantha establishes in pulses perhaps as infrequently as twice per century, and (2) riparian forests in Serengeti have established via facilitation under larger stands where shade excludes grass, and therefore fires and thorny shrubs exclude browsers.

Insect-damaged fossil leaves record food web response to ancient climate change and extinction

Plants and herbivorous insects have dominated terrestrial ecosystems for over 300 million years. Uniquely in the fossil record, foliage with well-preserved insect damage offers abundant and diverse information both about producers and about ecological and sometimes taxonomic groups of consumers. These data are ideally suited to investigate food web response to environmental perturbations, and they represent an invaluable deep-time complement to neoecological studies of global change. Correlations between feeding diversity and temperature, between herbivory and leaf traits that are modulated by climate, and between insect diversity and plant diversity can all be investigated in deep time. To illustrate, I emphasize recent work on the time interval from the latest Cretaceous through the middle Eocene (67-47 million years ago (Ma)), including two significant events that affected life: the end-Cretaceous mass extinction (65.5 Ma) and its ensuing recovery; and globally warming temperatures across the Paleocene-Eocene boundary (55.8 Ma). Climatic effects predicted from neoecology generally hold true in these deep-time settings. Rising temperature is associated with increased herbivory in multiple studies, a result with major predictive importance for current global warming. Diverse floras are usually associated with diverse insect damage; however, recovery from the end-Cretaceous extinction reveals uncorrelated plant and insect diversity as food webs rebuilt chaotically from a drastically simplified state. Calibration studies from living forests are needed to improve interpretation of the fossil data.