* = Undergraduate mentees
Intensive land use activities, such as agriculture, are a leading cause of biodiversity loss and can have lasting impacts on ecological systems. Yet, few studies have investigated how land-use legacies impact phylogenetic diversity (the total amount of evolutionary history in a community) or how restoration activities might mitigate legacy effects on biodiversity. We studied ground-layer plant communities in 27 pairs of Remnant (no agricultural history) and Post-agricultural (agriculture abandoned >60 years ago) longleaf pine savannas, half of which we restored by thinning trees to reinstate open savanna conditions. We found that agricultural history had no impact on species richness, but did alter community composition and reduce phylogenetic diversity by 566 million years per 1000 m2. This loss of phylogenetic diversity in post-agricultural savannas was due to, in part, a reduction in the average evolutionary distance between pairs of closely related species, that is, increased phylogenetic clustering. Habitat restoration increased species richness by 27% and phylogenetic diversity by 914 million years but did not eliminate the effects of agricultural land use on community composition and phylogenetic structure. These results demonstrate the persistence of agricultural legacies, even in the face of intensive restoration efforts, and the importance of considering biodiversity broadly when evaluating human impacts on ecosystems.
2015 – Palkovacs EP, Fryxell DC, Turley NE, Post DM. Ecological effects of intraspecific consumer biodiversity for aquatic communities and ecosystems. In Aquatic Functional Biodiversity. Belgrano A, Woodward G (editors). Academic Press. link to book abstract
Community and ecosystem ecology have a long history of focusing on the species-level biodiversity, implicitly assuming that intraspecific biodiversity is unimportant for ecological dynamics. This perspective has been challenged by recent studies in aquatic systems that have directly examined the importance of intraspecific consumer biodiversity relative to species removal or replacement. Our meta-analysis reveals that, while species effects are stronger on average, intraspecific effects can be important, especially for ecosystem responses driven by the combined effects of consumption and nutrient excretion. Our meta-analysis reveals such effects across a diversity of organisms (fishes and invertebrates) and aquatic study systems (lakes, ponds, and streams), suggesting that intraspecific consumer biodiversity is broadly important for aquatic communities and ecosystems.
Agricultural practices such as breeding resistant varieties and pesticide use can cause rapid evolution of pest species, but it remains unknown how plant domestication itself impacts pest contemporary evolution. Using experimental evolution on a comparative phylogenetic scale, we compared the evolutionary dynamics of a globally important economic pest–the green peach aphid (Myzus persicae)–growing on 34 plant taxa, represented by 17 crop species and their wild relatives. Domestication slowed aphid evolution by 13.5%, maintained 10.4% greater aphid genotypic diversity and 5.6% higher genotypic richness. The direction of evolution (i.e. which genotypes increased in frequency) differed among independent domestication events but was correlated with specific plant traits. Individual-based simulation models suggested that domestication affects aphid evolution directly by reducing the strength of selection and indirectly by increasing aphid density and thus weakening genetic drift. Our results suggest that phenotypic changes during domestication can alter pest evolutionary dynamics.
2015 – Santangelo JS*, Turley NE, Johnson MTJ. Fungal endophytes of Festuca rubra increase in frequency following long-term exclusion of rabbits. Botany. (93) 233-241. online pdf supplemental data abstract
Plant – fungal endophyte interactions are common in nature and they can shape the ecology of plants. Vertically transmitted endophytes are hypothesized to serve as mutualists, protecting plants from herbivores. If this hypothesis is true, then we expect endophytes to be most abundant in the presence of herbivores and least abundant in their absence, assuming endophytes incur a cost to their host. We tested this prediction by studying the effects of intense rabbit (Oryctolagus cuniculus Linnaeus) grazing on grass–endophyte interactions at Silwood Park, UK. We examined seeds of red fescue (Festuca rubra L.) collected from 15 natural populations that were protected from rabbits for 0.3–21 years. Contrary to our prediction, the mean proportion of seeds with endophytes increased 1.84×, from 0.45 to 0.83, following 21 years of rabbit exclusion. To better understand the mechanisms driving this increase in frequency, we conducted a fully factorial greenhouse experiment where we manipulated the presence or absence of endophyte infection, intraspecific competition, and simulated grazing on F. rubra plants. In both damaged and undamaged treatments, infected plants produced approximately twice as much biomass as uninfected plants, and endophytes did not influence tolerance to herbivory. These results suggest that endophytes directly change plant growth but not compensatory responses to damage. In the absence of competitors, infected plants produced 2.17× more biomass than uninfected plants, whereas in the presence of competitors, infected plants produced only 1.55× more biomass than uninfected plants. This difference suggests that intraspecific competition might lessen the benefits of endophyte infection. Our results do not support the defensive mutualism hypothesis, but instead suggest that endophyte-induced plant growth is important in shaping the costs and benefits of endophytes in our system.
2015 – Turley NE, Johnson MTJ. Ecological effects of aphid abundance, genotypic variation, and contemporary evolution on plants. (178) 747-759 Oecologia. online pdf supplemental blog post about research abstract
Genetic variation and contemporary evolution within populations can shape the strength and nature of species interactions, but the relative importance of these forces compared to other ecological factors is unclear. We conducted a field experiment testing the effects of genotypic variation, abundance, and presence/absence of green peach aphids (Myzus persicae) on the growth, leaf nitrogen, and carbon of two plant species (Brassica napus and Solanum nigrum). Aphid genotype affected B. napus but not S. nigrum biomass explaining 20 and 7 % of the total variation, respectively. Averaging across both plant species, the presence/absence of aphids had a 1.6× larger effect size (Cohen’s d) than aphid genotype, and aphid abundance had the strongest negative effects on plant biomass explaining 29 % of the total variation. On B. napus, aphid genotypes had different effects on leaf nitrogen depending on their abundance. Aphids did not influence leaf nitrogen in S. nigrum nor leaf carbon in either species. We conducted a second experiment in the field to test whether contemporary evolution could affect plant performance. Aphid populations evolved in as little as five generations, but the rate and direction of this evolution did not consistently vary between plant species. On one host species (B. napus), faster evolving populations had greater negative effects on host plant biomass, with aphid evolutionary rate explaining 23 % of the variation in host plant biomass. Together, these results show that genetic variation and evolution in an insect herbivore can play important roles in shaping host plant ecology.
2014 – Turley NE. Ecological and evolutionary consequences of herbivory on plant communities. Ph.D. thesis, Department of Ecology and Evolutionary Biology, University of Toronto. pdf abstract
Understanding the consequences of species interactions is central to ecological and evolutionary research. My research focused on better understanding the evolutionary ecology of plant-herbivore interactions by asking two broad questions: 1) How do herbivores shape the composition and diversity of plant populations and communities? And, 2) How does genetic variation and ongoing evolutionary dynamics in herbivores influence plant ecology? To test the first question, I utilized long-term herbivore manipulations in the grasslands of Silwood Park, England. I tested ecological and evolutionary consequences of rabbit (Oryctolagus cuniculus) grazing on plant populations by collecting seeds from rabbit exclosures ranging in age from 4 months to >20 years and growing them in common environments. I found that rabbit exclusion caused evolution of multiple plant defensive traits in three plant species and influenced the costs and benefits of a mutualistic interaction between a grass and its fungal endophyte. In another experiment at Silwood Park I investigated how multiple groups of herbivores (rabbits, insects, and mollusks) and nutrient additions shaped plant species richness and phylogenetic diversity. I found that nitrogen addition strongly reduced species richness while rabbit grazing increased plant phylogenetic diversity. To test the second question, I conducted field experiments where I manipulated the genotypic composition of green peach aphid (Myzus persicae) populations on two host plant species. I found that genetic variation in M. persicae had large impacts on plant growth, that M. persicae populations changed in genotype frequency (i.e. they evolved) over 5 generations, and that in some cases faster evolving populations had larger negative impacts on plant growth. Together these results demonstrate how herbivory can concurrently shape the ecology and evolution of plants at multiple levels of organization and that herbivore genetic variation and evolution can have important consequences on plant ecology.
2014 – Turcotte MM, Turley NE, Johnson MTJ. The impact of domestication on resistance to two generalist herbivores across 29 independent domestication events. (204) 671–681. New Phytologist. online pdf supplemental abstract
The domestication of crops is among the most important innovations in human history. Here, we test the hypothesis that cultivation and artificial selection for increased productivity of crops reduced plant defenses against herbivores. We compared the performance of two economically important generalist herbivores – the leaf-chewing beet armyworm (Spodoptera exigua) and the phloem-feeding green peach aphid (Myzus persicae) – across 29 crop species and their closely related wild relatives. We also measured putative morphological and chemical defensive traits and correlated them with herbivore performance. We show that, on average, domestication significantly reduced resistance to S. exigua, but not M. persicae, and that most independent domestication events did not cause differences in resistance to either herbivore. In addition, we found that multiple plant traits predicted resistance to S. exigua and M. persicae, and that domestication frequently altered the strength and direction of correlations between these traits and herbivore performance. Our results show that domestication can alter plant defenses, but does not cause strong allocation tradeoffs as predicted by plant defense theory. These results have important implications for understanding the evolutionary ecology of species interactions and for the search for potential resistance traits to be targeted in crop breeding.
2014 – Didiano TJ*, Turley NE, Schaefer H, Everwand G, Crawley MJ, Johnson MTJ. Experimental test of plant defense evolution in four species using long-term rabbit exclosures. Journal of Ecology. (102) 584–594. online pdf supplemental data abstract
Plant defence traits have evolved over macro- and microevolutionary time-scales in response to herbivores. Although a number of studies have investigated the evolutionary impacts of herbivores over short time-scales, few studies have experimentally examined what defence traits most commonly evolve and whether multiple coexisting species exhibit similar evolutionary responses to herbivores. We addressed these questions using a long-term experiment at Silwood Park, England, United Kingdom, where we excluded rabbits from 38 grassland plots for <1–34 years. To assess the evolutionary impacts of rabbits on plant defence traits, we collected seeds from plots containing the following perennial species: Anthoxanthum odoratum (Poaceae), Festuca rubra ssp. rubra (Poaceae), Holcus lanatus (Poaceae) and Stellaria graminea (Caryophyllaceae). We then grew these plants in a common garden and measured defensive and morphological traits. We found some evidence for evolutionary change of defence traits in three of the four species following the exclusion of rabbits. We observed the clearest changes in F. rubra, which showed a 9% decline in tolerance to herbivory and a 26% decline in leaf number. We also observed weak evidence for a change in all grass species towards a more erect growth form suggesting that grazing selects for plants that grow close to the ground. Although our results are most consistent with evolution due to changes in the frequency of alleles and genotypes, we cannot rule out that epigenetic changes (e.g. methylation) or maternal environmental effects also contributed to or caused the observed long-term phenotypic responses. Our study provides an experimental test of the evolutionary effects of an ecologically important herbivore. We found evidence for plant defence evolution following >20 years of rabbit exclusion. However, the evidence was only strong in one species for multiple traits, weak in all three grass species for avoidance and absent in an herb species. This suggests that the evolutionary effects of an ecologically important herbivore on plants will be variable and difficult to predict in nature.
Edge effects are among the most important drivers of species interactions in fragmented habitats, but the impacts of edge effects on multitrophic interactions are largely unknown. In this study we assess edge effects on species interactions within an ant–plant mutualistic system—where ants protect plants against herbivory—to determine whether habitat edges alter the amount of protection ants provide. We focus on a single species of myrmecophytic plant, Solanum americanum, and experimentally manipulate ant access to study plants in large-scale fragmented habitat patches at the Savannah River Site National Environmental Research Park, USA. In this system, S. americanum commonly hosts honeydew-producing aphids that are tended by ants, and grasshoppers are the primary herbivores. We measured edge effects on the per-plant abundance of aphids and protective ants as well as the abundance of grasshoppers in each habitat patch, and we evaluated levels of ant protection against herbivory near and far from habitat edges. We found that ants provided significant protection to plants far from edges, where herbivory pressure was highest, despite the fact that aphids and ants were least abundant on these plants. Conversely, ants did not provide significant protection near edges, where herbivory pressure was lowest and aphids and ants were most abundant. We conclude that a strong edge effect on grasshopper abundance was a key factor determining the amount of protection ants provided against herbivory. Future studies of the impacts of habitat fragmentation on ant–plant mutualisms will benefit from studies of ant behavior in response to herbivory threats, and studies of edge effects on other species interactions may also need to consider how species’ behavioral patterns influence the interactions in question.
2013 – Turley NE, Odell WC*, Schaefer H, Everwand G, Crawley MJ, Johnson MTJ. Contemporary evolution of plant growth rate following experimental removal of herbivores. American Naturalist. (181) S21–S34. online pdf supplemental data abstract
Herbivores are credited with driving the evolutionary diversification of plant defensive strategies over macroevolutionary time. For this to be true, herbivores must also cause short-term evolution within plant populations, but few studies have experimentally tested this prediction. We addressed this gap using a long-term manipulative field experiment where exclosures protected 22 plant populations from natural rabbit herbivory for Rumex acetosa L. (Polygonaceae) from our plots and grew them in a common greenhouse environment to quantify evolved differences among populations in individual plant growth rate, tolerance to herbivory, competitive ability, and the concentration of secondary metabolites (tannins and oxalate) implicated in defense against herbivores. In 26 years without rabbit herbivory, plant growth rate decreased linearly by 30%. We argue that plant growth rate has evolved as a defense against intense rabbit herbivory. In contrast, we found no change in tolerance to herbivory or concentrations of secondary metabolites. We also found no change in competitive ability, suggesting that contemporary evolution may not feed back to alter ecological interactions within this plant community. Our results combined with those of other studies show that the evolution of gross morphological traits such as growth rate in response to herbivory may be common, which calls into question assumptions about some of the most popular theories of plant defense.
Habitat corridors confer many conservation benefits by increasing movement of organisms between habitat patches, but the benefits for some species may exact costs for others. For example, corridors may increase the abundance of consumers in a habitat to the detriment of the species they consume. In this study we assessed the impact of corridors on insect herbivory of a native plant, Solanum americanum, in large-scale, experimentally fragmented landscapes. We quantified leaf herbivory and assessed fruit production as a proxy for plant fitness. We also conducted field surveys of grasshoppers (Orthoptera), a group of abundant, generalist herbivores that feed on S. americanum, and we used exclosure cages to explicitly link grasshopper herbivory to fruit production of individual S. americanum. The presence of corridors did not increase herbivory or decrease plant fruit production. Likewise, corridors did not increase grasshopper abundance. Instead, patches in our landscapes with the least amount of edge habitat and the greatest amount of warmer “core” area had the highest levels of herbivory, the largest cost to plant fruit production as a result of herbivory, and the most grasshoppers. Thus habitat quality, governed by patch shape, can be more important than connectivity for determining levels of herbivory and the impact of herbivory on plant fitness in fragmented landscapes.
Broad-scale geographical gradients in the abiotic environment and interspecific interactions should select for clinal adaptation. How trait clines evolve has recently received increased attention because of anticipated climate change and the importance of rapid evolution in invasive species. This issue is particularly relevant for clines in growth and defense of plants, because both sets of traits are closely tied to fitness and because such sessile organisms experience strong local selection. Yet despite widespread recognition that growth and defense traits are intertwined, the general issue of their joint clinal evolution is not well resolved.
To address heritable clinal variation and adaptation of growth and defense traits of common milkweed (Asclepias syriaca), we planted seed from 22 populations encompassing the species’ latitudinal range in common gardens near the range center (New York) and toward the range edges (New Brunswick and North Carolina). Populations were differentiated in 13 traits, and six traits showed genetically based latitudinal clines. Higher-latitude populations had earlier phenology, lower shoot biomass, more root buds and clonal growth, higher root-to-shoot ratio, and greater latex production. The cline in shoot biomass was consistent in all three locations. Selection on phenology was reversed in New Brunswick and North Carolina, with early genotypes favored in the north but not the south. We found no clines in foliar trichomes or toxic cardenolides. Annual precipitation of source populations explained variation in phenology, clonal growth, root-to-shoot ratio, and latex. Across four traits measured in New Brunswick and North Carolina, we found garden-by-latitude (and garden-by-precipitation) interactions, indicating plasticity in genetically based trait clines.
In the two gardens with substantial herbivory (New York and North Carolina), northern populations showed higher resistance to insects. Resistance to aphids was driven by trichomes and water content, while resistance to monarch caterpillars was driven by latex. However, surveys of natural populations indicated that leaf damage and insect diversity on milkweed are low at the geographical extremes (New Brunswick and North Carolina) and higher toward the range center. We speculate that milkweed plants evolved clines in growth traits in response to climate, and that this set the template for tolerance to herbivory, which subsequently shaped the evolution of defensive traits.
2011 – Hersch-Green EI, Turley NE, Johnson MTJ. Community genetics: what have we accomplished and where should we be going? Philosophical Transactions of the Royal Society B. (366) 1453-1460. online pdf abstract
Research in community genetics seeks to understand how the dynamic interplay between ecology and evolution shapes simple and complex communities and ecosystems. A community genetics perspective, however, may not be necessary or informative for all studies and systems. To better understand when and how intraspecific genetic variation and microevolution are important in community and ecosystem ecology, we suggest future research should focus on three areas: (i) determining the relative importance of intraspecific genetic variation compared with other ecological factors in mediating community and ecosystem properties; (ii) understanding the importance of microevolution in shaping ecological dynamics in multi-trophic communities; and (iii) deciphering the phenotypic and associated genetic mechanisms that drive community and ecosystem processes. Here, we identify key areas of research that will increase our understanding of the ecology and evolution of complex communities but that are currently missing in community genetics. We then suggest experiments designed to meet these current gaps.
Papers in Preparation
Post DM, Turley NE, Bailey JK, des Roches S, Hendry AP, Kinnison MT, Palkovacs EP, Schweitzer JA, Vellend M. The ecological importance of biodiversity within species.
Turley NE, Davies TJ, Schaefer H, Crawley MJ. Long-term herbivore exclusion and nutrient addition interact to shape plant species richness and phylogenetic diversity.