Some theoretical notes on agrobiodiversity: spatial heterogeneity and population interactions 

Diego Griffon and Maria-Josefina Hernandez

Ecological interactions are fundamental in ecological pest management, and these interactions form networks. The properties of these networks, where interactions of all possible nature (positive, neutral, negative) coexist, are key for management, but little is known about them. The main reasons for this lack of knowledge are the difficulties in obtaining empirical evidence. These problems may be partially bypassed using a theoretical approach. Here, by means of mathematical models that represent networks of ecological interactions in agroecosystems, we characterize some architectural features that promote the self-regulation of population densities in these networks. The results show that the key features are: spatial heterogeneity and a high proportion of positive interactions.

Biodiversity and spatial heterogeneity strongly benefit agricultural landscapes. Among others, these benefits are related to population regulation of organisms that feed on cultivated plants (Duflot et al. 2015; Fahrig et al. 2011; Letourneau et al. 2011; Rusch et al. 2016, 2010; Tscharntke et al. 2002, 2012; Vandermeer 1989). However, there is a need for further theoretical development to help us understand the processes behind these empirical observations, particularly from a mechanistic point of view. In a very general sense, in the agroecological literature it is proposed that increasing agricultural biodiversity involves an increase in the number of trophic interactions of the ecological community, which in turn promotes the stability of the whole system (Altieri 1983; Altieri and Nicholls 2000, 2004; Nicholls and Altieri 2002). On the other hand, we acknowledge that the ecological evidence concerning the relationship between the number of species (richness) and the number of trophic interactions in natural ecosystems is ambiguous (Hall and Raffaelli 1997) and, that from a theoretical point of view, the relationship between complexity and stability is an issue far from resolved (Allesina and Tang 2015; Bersier 2007; Ings et al. 2009; Namba 2015). However, when it comes to contrasting a monoculture with a multidiverse agroecosystem, these topics may have clearer answers (Griffon and Hernández 2014; Griffon and Rodríguez 2017; Rusch et al. 2016, 2010; Tscharntke et al. 2012).

In a conventional monoculture, the system is explicitly designed and managed to reduce as much as possible the unplanned associated biodiversity(typically by using insecticides, herbicides, etc.). Paradoxically this may contribute(among other things) to the long term establishment of phytophagous organisms in the system, eradicating at the same time their biological controllers (Jonsson et al. 2015; Landis, Wratten, and Gurr 2000; Levins and Vandermeer 1990). In this type of farming system most species are related directly to one (the monoculture) by a victim-exploiter relationship (i.e., predation, parasitism, parasitoidism and herbivory), where the monoculture species (the crop) typically plays the role of the victim. So, the system has a star-like architecture (i.e., many nodes connected to a central hub) with the monoculture in the center (Griffon and Torres-Alruiz 2008), which is a structure that favours the occurrence of pest situations and crops losses (Griffon and Hernández 2014; Griffon and Rodríguez 2017).

Alternatives to the ecological oversimplification of monocultures are companion crops. One of the aims of this cropping strategy is the population regulation of phytophagous and phytopathogenic species by means of ecological interactions (Altieri and Nicholls 2004). The success of this approach not only depends on the occurrence of a more complex trophic web, but also on the occurrence of other ecological interactions (competition, mutualism, amensalism and commensalism) that together make up the ecological network system, i.e., a network consisting of all types of ecological interactions (Ings et al. 2009). We have very little information on the structure of ecological networks (Pocock et al. 2012) and we also lack knowledge on how ecological networks promote the regulation of phytophagous population densities. Given the need of information, coupled with the difficulty and effort involved in achieving it in the field, this paper addresses the issue from a theoretical perspective. In order to do this, we build and numerically evaluate mathematical models that simulate networks of hypothetical ecological interactions associated with agricultural ecosystems. This is done with the objective of finding patterns that can provide guidelines on architectural features associated with self-regulation in populations.

Another related topic must be considered. There is abundant field information that shows the positive effects of spatial heterogeneity (Batáry et al. 2011; Duflot et al. 2015; Fahrig 2013; Fahrig et al. 2011, 2015; Jonsson et al. 2015; Landis, Wratten, and Gurr 2000; Rusch et al. 2016; Tscharntke et al. 2012; Tuck et al. 2014) on the maintenance of associated agricultural biodiversity (sensu Vandermeer and Perfecto 1995; Altieri et al. 2005). In some cases, space heterogeneity may even play a more important role than intra-farm diversity in the regulation of phytophagous population densities (Fahrig et al. 2011, 2015). But surely the two components (intra and inter farm diversity) relate synergistically.

For the spatial heterogeneity to have a positive effect on the internal dynamics of agroecosystems it is necessary, on the one hand, an insidefarm design that attracts biological controllers (e.g., flower strips or beetle banks) (Altieri, Ponti, and Nicholls 2005; Nicholls and Altieri 2002) and on the other, the existence of nearby sources of organisms with enough internal complexity to provide the necessary control agents (Rusch et al. 2016, 2010; Tscharntke et al. 2012).

So, metapopulation and metacommunity dynamics seem to be crucial for the long term survival of species in heterogeneous environments (Alfonzo et al. 2009; Aberg et al. 1995; Cantrell, Cosner, and Fagan 1998; Delin and Andren 1999; Griffon, Alfonzo, and Hernandez 2010; Griffon and Hernández 2014; Gustafson and Gardner 1996; Perfecto, Vandermeer, and Wright 2009; Sisk, Haddad, and Ehrlich 1997; Tejat et al. 2002; Vandermeer and Carvajal 2001; Vandermeer and Perfecto 2007). In general terms, the spatial structure of populations, along with processes of dispersal, migration and colonization, allows the emergence of dynamics that make possible the persistence and coexistence of species (Hanski 1994, 1998; Hanski and Gilpin 1997; Hanski et al. 1996; Leibold et al. 2004). Thus, spatial heterogeneity may enhance the configuration of the complex ecological networks needed for a successful ecological pest management program (Batáry et al. 2011; Fahrig et al. 2011; Rusch et al. 2016, 2010; Tejat et al. 2002; Tscharntke et al. 2002, 2012). For this reason, in the mathematical approach used here we also include the effect of spatial heterogeneity on population dynamics.

In short, the objective of this work is to find architectural features that promote the self-regulation of population densities in ecological networks associated to agroecosystems. To do this, we built mathematical models that represent ecological networks, both for a single community and for metacommunitarian systems. We must make clear that whenever we say ‘ecological network’, we are considering the potential presence of ‘all’ types of interactions, i.e., competition, mutualism, victim-exploiter, amensalism and commensalism.

Species survivals. Initial richness, for six different initial conditions (defined in Fig 3). Blue: survival percentage in one community (no spatial heterogeneity). Grey: survival percentage of one community in a metacommunitarian background. The curves are averages of 40 simulations for each initial condition.

Uniform perturbation (example). A persistent network obtained under the 20:10:10:60 initial condition is perturbed by a little increase in the densities of each population. Left: dynamics after the perturbation. Two populations reach very low (but non zero) densities. Right: network before and after the perturbation. Notice that both networks are the same.

The more relevant results of the models evaluated and discussed in this article can be summarized as follows: (i) The conditions under which persistent networks are obtained after the iterative process are very restricted. However, when persistent  networks are obtained, they are fundamentally resilient to perturbations. (ii) Mutualistic (and positive in general) interactions have an important and extensive effect under certain (very specific) conditions. (iii) Spatial heterogeneity increases the possibility of persistence in hypothetical communities. (iv) Ecological interactions that somehow have been neglected in the past (commensalism and amensalism, the forgotten sisters), may be: 1- More frequent than generally thought, and 2- Important for the persistence of communities.

https://bit.ly/2YPVlTO

.

.
Land-use history impacts functional diversity across multiple trophic groups 

Le Provost et al., 2019

Land-use change is a major driver of biodiversity loss worldwide. Although biodiversity often shows a delayed response to land-use change, previous studies have typically focused on a narrow range of current landscape factors and have largely ignored the role of land-use history in shaping plant and animal communities and their functional characteristics. Here, we used a unique database of 220,000 land-use records to investigate how 20-y of land-use changes have affected functional diversity across multiple trophic groups (primary producers, mutualists, herbivores, invertebrate predators, and vertebrate predators) in 75 grassland fields with a broad range of land-use histories. The effects of land-use history on multitrophic trait diversity were as strong as other drivers known to impact biodiversity, e.g., grassland management and current landscape composition. The diversity of animal mobility and resource-acquisition traits was lower in landscapes where much of the land had been historically converted from grassland to crop. In contrast, functional biodiversity was higher in landscapes containing old permanent grasslands, most likely because they offer a stable and high-quality habitat refuge for species with low mobility and specialized feeding niches. Our study shows that grassland-to-crop conversion has long-lasting impacts on the functional biodiversity of agricultural ecosystems. Accordingly, land-use legacy effects must be considered in conservation programs aiming to protect agricultural biodiversity. In particular, the retention of permanent grassland sanctuaries within intensive landscapes may offset ecological debts.

Importance of the drivers of multitrait diversity, mobility trait diversity, resource-acquisition trait diversity, and body size trait diversity. Relative effects (% R²), resulting from a model averaging procedure, were calculated for each group of predictors (i.e., land-use history, current land use, and the species pool). All predictors were scaled to interpret parameter estimates on a comparable scale. Note that for mobility, resource-acquisition, and body size trait diversity, we focused on animal traits and excluded plant traits from the analyses. Results were consistent considering spatial scales ranging from 500 to 1500 m radii surrounding the sampled grasslands.

https://www.pnas.org/content/early/2020/01/01/1910023117 

.

.
On a first approximation, and setting aside vertebrate chauvinism, it can be said that essentially all organisms are insects.

Robert May
. 

   
Third generation of Desert Locusts about to hit East and Central Africa.
Photo: oxfam https://bit.ly/2XEnTkK 
.

Water lily harvest in Vietnam.
Images: Pham Huy Trung, via @mediasocum on Instagram

.

.

 

For the Love of Corals

......An Ecology of Perhaps

https://critical-zones.zkm.de/#!/detail:for-the-love-of-corals

 

.

.

 

Increasing crop heterogeneity enhances multitrophic diversity across agricultural regions  
Sirami et al., 2020

Agricultural landscape homogenization has detrimental effects on biodiversity and key ecosystem services. Increasing agricultural landscape heterogeneity by increasing seminatural cover can help to mitigate biodiversity loss. However, the amount of seminatural cover is generally low and difficult to increase in many intensively managed agricultural landscapes. We hypothesized that increasing the heterogeneity of the crop mosaic itself (hereafter “crop heterogeneity”) can also have positive effects on biodiversity. In 8 contrasting regions of Europe and North America, we selected 435 landscapes along independent gradients of crop diversity and mean field size. Within each landscape, we selected 3 sampling sites in 1, 2, or 3 crop types. We sampled 7 taxa (plants, bees, butterflies, hoverflies, carabids, spiders, and birds) and calculated a synthetic index of multitrophic diversity at the landscape level. Increasing crop heterogeneity was more beneficial for multitrophic diversity than increasing seminatural cover. For instance, the effect of decreasing mean field size from 5 to 2.8 ha was as strong as the effect of increasing seminatural cover from 0.5 to 11%. Decreasing mean field size benefited multitrophic diversity even in the absence of seminatural vegetation between fields. Increasing the number of crop types sampled had a positive effect on landscape-level multitrophic diversity. However, the effect of increasing crop diversity in the landscape surrounding fields sampled depended on the amount of seminatural cover. Our study provides large-scale, multitrophic, cross-regional evidence that increasing crop heterogeneity can be an effective way to increase biodiversity in agricultural landscapes without taking land out of agricultural production.

(A) Traditional and (B) alternative representations of agricultural landscape heterogeneity, focusing either on seminatural heterogeneity or crop heterogeneity, are associated with distinct hypotheses. 

https://www.pnas.org/content/116/33/16442
.

.
Believe me who have tried. Thou wilt find something more in woods than in books. Trees and rocks will teach what thou canst not hear from a master.​

Bernard of Clairvaux (1841).
.

Self-perpetuating ecological–evolutionary dynamics in an agricultural host–parasite system 

Ives et al. 2020

Ecological and evolutionary processes may become intertwined when they operate on similar time scales. Here we show ecological–evolutionary dynamics between parasitoids and aphids containing heritable symbionts that confer resistance against parasitism. In a large-scale field experiment, we manipulated the aphid’s host plant to create ecological conditions that either favoured or disfavoured the parasitoid. The result was rapid evolutionary divergence of aphid resistance between treatment populations. Consistent with ecological–evolutionary dynamics, the resistant aphid populations then had reduced parasitism and increased population growth rates. We fit a model to quantify costs (reduced intrinsic rates of increase) and benefits of resistance. We also performed genetic assays on 5 years of field samples that showed persistent but highly variable frequencies of aphid clones containing protective symbionts; these patterns were consistent with simulations from the model. Our results show (1) rapid evolution that is intertwined with ecological dynamics and (2) variation in selection that prevents traits from becoming fixed, which together generate self-perpetuating ecological–evolutionary dynamics.

Field experiment showing eco–evo dynamics. Between mid-summer and autumn 2015 (panels on the left), the asynchronous harvesting treatment was applied in two hoop houses to maintain aphid habitat and increased parasitoid populations (top two panels) or synchronously to decrease parasitism pressure on aphids (bottom two panels). We counted aphids (solid and dashed black lines for the two hoop houses in each treatment) by visual inspection of 500 stems per cage; s.e.m. bars are given but in some cases are covered by the dots. Our index of parasitism (red lines) is the number of mummies as a proportion of the number of mummies and aphids. Peak parasitism rates (on 20 and 26 August and 2 September 2015) were higher in the asynchronous hoop houses (P = 0.007, Supplementary Information). The narrow panels give the estimated demographic rates (for the hoop houses in the panels above them) from the fitted model (Extended Data Fig. 8). The estimated daily parasitoid attack rates (a(t) in equation (2)) is given in red and the densityindependent relative aphid survival (z(t) in equation (2)) is given in blue, with solid and dashed lines corresponding to the two replicates. Note that the relative aphid survival is scaled to the maximum estimated survival in 2015, so values greater than 1 in 2016 imply higher survival than the 2015 maximum. The estimated proportion of resistant clones is given by black lines and the black points with s.e.m. give the proportion of Hamiltonella–APSE3 clones from the genetic symbiont surveys.

https://www.nature.com/articles/s41559-020-1155-0

.

Young seedling of Arabidopsis thaliana, with red-colored nuclei and green-colored cell membraneses 
Photo: Fernan Federici

Mutualism increases diversity, stability, and function of multiplex networks that integrate pollinators into food webs     
Hale et al., 2020

Ecosystems are composed of complex networks of many species interacting in different ways. While ecologists have long studied food webs of feeding interactions, recent studies increasingly focus on mutualistic networks including plants that exchange food for reproductive services provided by animals such as pollinators. Here, we synthesize both types of consumer-resource interactions to better understand the controversial effects of mutualism on ecosystems at the species, guild, and whole-community levels. We find that consumer-resource mechanisms underlying plant-pollinator mutualisms can increase persistence, productivity, abundance, and temporal stability of both mutualists and non-mutualists in food webs. These effects strongly increase with floral reward productivity and are qualitatively robust to variation in the prevalence of mutualism and pollinators feeding upon resources in addition to rewards. This work advances the ability of mechanistic network theory to synthesize different types of interactions and illustrates how mutualism can enhance the diversity, stability, and function of complex ecosystems.

Interspecific and intraspecific mechanisms of feeding and reproduction combine to describe pollination mutualisms and traditional trophic interactions. Biomass of plants with pollinators is partitioned into two pools, vegetation (purple node) and floral rewards (light purple node), coupled by intraspecific dynamic feedbacks (dashed arrows). Rewards production is proportional to vegetative biomass but subject to self-limitation such that reward productivity per unit biomass decreases with increasing rewards abundance. Producing rewards incurs costs (reduced vegetative productivity), which creates tradeoffs between producing rewards to attract pollinators and benefiting from the quantity (number of visits measured as feeding rate on rewards) and quality (conspecific feeding/total feeding) of pollinators’ reproductive services (purple arrow) that are required for vegetative production. At saturation, reproductive services allow plants with pollinators to potentially achieve a 25% higher per-biomass growth rate than that of plants without pollinators whose intrinsic growth rate is independent of consumers’ behavior. All plants are also subjected to competition from the plant community (not shown), which reduces per-biomass vegetative growth rate close to carrying capacity. Gray arrows show herbivores feeding on vegetation and pollinators feeding on rewards.

https://www.nature.com/articles/s41467-020-15688-w
.

.
The Sick Rose

O Rose thou art sick. 
The invisible worm, 
That flies in the night 
In the howling storm: 

Has found out thy bed 
Of crimson joy: 
And his dark secret love
Does thy life destroy. 


William Blake
.

.

Hortus Eystettensis
Edited by Basilius Besler (1561-1629), first published in Nuremberg in 1613.

.
Tim Ingold : The Sustainability of Everything
.

1728 kitchen garden layout  
.

Spring from Jamie Scott on Vimeo.

Crop Varietal Mixtures as a Strategy to Support Insect Pest Control, Yield, Economic, and Nutritional Services 
Lauren D. Snyder, Miguel I. Gómez and Alison G. Power

Most on-farm diversification strategies to enhance ecosystem services, such as insect pest control and yield, have focused on expanding crop species diversity. While polycultures often provide valuable services, logistical constraints with planting and harvesting can hamper implementation on large scales. An alternative diversification strategy is to increase within-field intraspecific crop diversity through the use of crop varietal mixtures. Here, we evaluate an interdisciplinary body of research to determine the potential for crop varietal mixtures to support food security by providing ecological, economic, and nutritional services. Previous literature has synthesized the link between varietal mixtures and yield and insect pest suppression services. We expand on prior analyses by considering hypotheses generated from species-level research and assessing whether they also provide a useful framework for predicting how varietal mixtures affect crop productivity and insect pest suppression. In addition, we evaluate the potential for varietal mixtures to increase farm resilience and growers' profits. While there is a growing effort to quantify the economic value of ecosystem services provided by agrobiodiversity in terms of enhanced yield or revenue, much less attention has been given to quantifying the production costs associated with diversification schemes. Consequently, we know little about the effect of diversification practices on farm profitability, the metric of ultimate importance to farmers. We address this issue by evaluating the ability of varietal mixtures to reduce production costs associated with other types of agrobiodiversity and outline areas for future research to better understand the profit implications of varietal mixtures. Further, we review evidence that varieties of some crop species differ in phytochemical content—a functional trait important for insect pest suppression and human dietary diversity—suggesting that varietal mixtures could be designed to simultaneously support insect pest control and human nutrition services. Given that little research has explicitly addressed the capacity for varietal mixtures to support human nutrition, we outline predictions for where we would expect to see the greatest nutritional impact of mixtures, providing a foundation for future human nutrition research. Taken together, our review suggests that varietal mixtures are a promising and logistically feasible strategy that could simultaneously support multiple services.

Conceptual framework for comparing the services (human nutrition, yield stability, insect pest control) and economic implications (labor, implementation effort, profits) associated with agricultural management practices. In this qualitative diagram, the level of service or economic implication is indicated along each axis; achieving greater distance along each axis indicates a stronger benefit. To illustrate predictions for how services and economic implications will vary with the level of diversification, we compare three hypothetical agriculture systems: a monoculture, growing a single crop variety in a field (closed circle); a polyculture, intermixing multiple crop species together in a field (open circle); and a varietal mixture, planting multiple varieties of the same crop species together in a field (open square). In general, polycultures enhance many services (Poveda et al., 2008; Letourneau et al., 2011), but production costs can be high (Gliessman, 1985; Tooker and Frank, 2012). In contrast, monocultures minimize costs, but are poor producers of some services (Altieri, 1999; Karp et al., 2012). We propose varietal mixtures could serve as an intermediate strategy that addresses some of the limitations associated with monocultures and polycultures. The symbol “?” represents predictions with the least amount of supporting evidence.

https://cutt.ly/Zyc6qaC

.

Daniel Martin Diaz
https://bit.ly/3aTOhvj

Soil fungal assemblage complexity is dependent on soil fertility and dominated by deterministic processes 

Guo et al., 2019.

• In the processes controlling ecosystem fertility, fungi are increasingly acknowledged as key drivers. However, our understanding of the rules behind fungal community assembly regarding the effect of soil fertility level remains limited.
• Using soil samples from typical tea plantations spanning c. 2167 km north‐east to south‐west across China, we investigated the assemblage complexity and assembly processes of 140 fungal communities along a soil fertility gradient.
• The community dissimilarities of total fungi and fungal functional guilds increased with increasing soil fertility index dissimilarity. The symbiotrophs were more sensitive to variations in soil fertility compared with pathotrophs and saprotrophs. Fungal networks were larger and showed higher connectivity as well as greater potential for inter‐module connection in more fertile soils. Environmental factors had a slightly greater influence on fungal community composition than spatial factors. Species abundance fitted the Zipf–Mandelbrot distribution (niche‐based mechanisms), which provided evidence for deterministic‐based processes.
• Overall, the soil fungal communities in tea plantations responded in a deterministic manner to soil fertility, with high fertility correlated with complex fungal community assemblages. This study provides new insights that might contribute to predictions of fungal community complexity.

 https://nph.onlinelibrary.wiley.com/doi/abs/10.1111/nph.16345
.

 Photo: @sporgasm
.

Pinus cembra
Illustrated in Kutschera and Lichtenegger’s 2009 ‘Wurzelatlas’.