Stockholm university

Niklas Janz

About me

Professor in Evolutionary Insect Ecology.

I am Deputy Head at the Department of Biology Education (BIG), and director of studies at the Department of Zoology.


Currently, I give two courses, ”Ecology I” on the Bachelor program in Biology and the Master course ”Science in Biological Research and Investigation”. In addition to these I also give stray lectures on various other courses.

I am currently also director of studies at the Department of Zoology and program director of the Master program in Ecology & Biodiversity.


In my group, we study the ecological and evolutionary interaction between butterflies and their host plants.

Most of our research focuses on understanding the evolutionary dynamics of host plant repertoires, host shifts, and speciation. And in particular how these phenomena are interrelated.


Research projects


A selection from Stockholm University publication database

  • Host repertoires and changing insect-plant interactions

    2021. Mariana P. Braga, Niklas Janz. Ecological Entomology 46 (6), 1241-1253


    1. One of the main challenges faced by ecologists today is to understand and predict how species interactions will respond to the current environmental change. It is likely that these changes will have a stronger effect on phylogenetic lineages that depend on intimate and specialised ecological interactions, such as most herbivorous insects.

    2. In this review, we highlight the aspects that we consider are fundamental for understanding how species interactions change over time. We start by reviewing terminology and conclude that commonly used terms have undesired connotations with regard to what we know about how hosts are acquired and lost over time.

    3. Based on that, we suggest host repertoire as a better term to describe the use of multiple hosts than the host range or diet breadth, both of which fail to capture that host use is often non-contiguous and tend to emphasise the total number of hosts, while ignoring host identity.

    4. Another important issue that we highlight is the differentiation between fundamental and realised host repertoires, where the latter is the set of hosts that are actually used in nature, whereas the first includes all hosts that can potentially be used by the species. We show that the characterisation of the fundamental host repertoire is key to understanding how insect-plant interactions will respond to the ecological opportunities instigated by environmental change.

    Read more about Host repertoires and changing insect-plant interactions
  • Phylogenetic reconstruction of ancestral ecological networks through time for pierid butterflies and their host plants

    2021. Mariana P. Braga (et al.). Ecology Letters 24 (10), 2134-2145


    The study of herbivorous insects underpins much of the theory that concerns the evolution of species interactions. In particular, Pieridae butterflies and their host plants have served as a model system for studying evolutionary arms races. To learn more about the coevolution of these two clades, we reconstructed ancestral ecological networks using stochastic mappings that were generated by a phylogenetic model of host-repertoire evolution. We then measured if, when, and how two ecologically important structural features of the ancestral networks (modularity and nestedness) evolved over time. Our study shows that as pierids gained new hosts and formed new modules, a subset of them retained or recolonised the ancestral host(s), preserving connectivity to the original modules. Together, host-range expansions and recolonisations promoted a phase transition in network structure. Our results demonstrate the power of combining network analysis with Bayesian inference of host-repertoire evolution to understand changes in complex species interactions over time.

    Read more about Phylogenetic reconstruction of ancestral ecological networks through time for pierid butterflies and their host plants
  • Unifying host-associated diversification processes using butterfly-plant networks

    2018. Mariana P. Braga (et al.). Nature Communications 9


    Explaining the exceptional diversity of herbivorous insects is an old problem in evolutionary ecology. Here we focus on the two prominent hypothesised drivers of their diversification, radiations after major host switch or variability in host use due to continuous probing of new hosts. Unfortunately, current methods cannot distinguish between these hypotheses, causing controversy in the literature. Here we present an approach combining network and phylogenetic analyses, which directly quantifies support for these opposing hypotheses. After demonstrating that each hypothesis produces divergent network structures, we then investigate the contribution of each to diversification in two butterfly families: Pieridae and Nymphalidae. Overall, we find that variability in host use is essential for butterfly diversification, while radiations following colonisation of a new host are rare but can produce high diversity. Beyond providing an important reconciliation of alternative hypotheses for butterfly diversification, our approach has potential to test many other hypotheses in evolutionary biology.

    Read more about Unifying host-associated diversification processes using butterfly-plant networks
  • Embracing Colonizations

    2018. Sören Nylin (et al.). Trends in Ecology & Evolution 33 (1), 4-14


    Parasitehost and insectplant research have divergent traditions despite the fact that most phytophagous insects live parasitically on their host plants. In parasitology it is a traditional assumption that parasites are typically highly specialized; cospeciation between parasites and hosts is a frequently expressed default expectation. Insectplant theory has been more concerned with host shifts than with cospeciation, and more with hierarchies among hosts than with extreme specialization. We suggest that the divergent assumptions in the respective fields have hidden a fundamental similarity with an important role for potential as well as actual hosts, and hence for host colonizations via ecological fitting. A common research program is proposed which better prepares us for the challenges from introduced species and global change.

    Read more about Embracing Colonizations
  • Evolutionary history of host use, rather than plant phylogeny, determines gene expression in a generalist butterfly

    2016. Maria de la Paz Celorio-Mancera (et al.). BMC Evolutionary Biology 16


    Background: Although most insect species are specialized on one or few groups of plants, there are phytophagous insects that seem to use virtually any kind of plant as food. Understanding the nature of this ability to feed on a wide repertoire of plants is crucial for the control of pest species and for the elucidation of the macroevolutionary mechanisms of speciation and diversification of insect herbivores. Here we studied Vanessa cardui, the species with the widest diet breadth among butterflies and a potential insect pest, by comparing tissue-specific transcriptomes from caterpillars that were reared on different host plants. We tested whether the similarities of gene-expression response reflect the evolutionary history of adaptation to these plants in the Vanessa and related genera, against the null hypothesis of transcriptional profiles reflecting plant phylogenetic relatedness. Result: Using both unsupervised and supervised methods of data analysis, we found that the tissue-specific patterns of caterpillar gene expression are better explained by the evolutionary history of adaptation of the insects to the plants than by plant phylogeny. Conclusion: Our findings suggest that V. cardui may use two sets of expressed genes to achieve polyphagy, one associated with the ancestral capability to consume Rosids and Asterids, and another allowing the caterpillar to incorporate a wide range of novel host-plants.

    Read more about Evolutionary history of host use, rather than plant phylogeny, determines gene expression in a generalist butterfly
  • Ehrlich and Raven Revisited

    2011. Niklas Janz. Annual Review of Ecology, Evolution and Systematics 42, 71-89


    After almost 50 years of scrutiny, the ideas that Ehrlich and Raven presented in their classical paper on the coevolution between butterflies and plants are still very much alive. Much of this interest has involved the potential for codiversification, both in how the interaction itself diversifies and how the interaction affects modes and rates of speciation. Despite high levels of conservatism and specialization, diversification of the interaction appears to be mainly a consequence of host shifts, but this somewhat paradoxical conclusion can be understood by an appreciation of the ecological as well as genetic mechanisms behind host shifts. There are several ways that the interaction can influence speciation, with or without host-plant-based di-vergent selection on reproductive barriers. One current debate is over the relative importance of radiations following shifts to new adaptive zones and elevated rates of speciation in groups with plastic and diverse host use.

    Read more about Ehrlich and Raven Revisited
  • How specialists can be generalists: resolving the “parasite paradox” and implications for emerging infectious disease

    2010. Salvatore J Agosta, Niklas Janz, Daniel R Brooks. Zoologia 27 (2), 151-162


    The parasite paradox arises from the dual observations that parasites (broadly construed, including phy- tophagous insects) are resource specialists with restricted host ranges, and yet shifts onto relatively unrelated hosts are common in the phylogenetic diversification of parasite lineages and directly observable in ecological time. We synthe- size the emerging solution to this paradox: phenotypic flexibility and phylogenetic conservatism in traits related to resource use, grouped under the term ecological fitting, provide substantial opportunities for rapid host switching in changing environments, in the absence of the evolution of novel host-utilization capabilities. We discuss mechanisms behind ecological fitting, its implications for defining specialists and generalists, and briefly review empirical examples of host shifts in the context of ecological fitting. We conclude that host shifts via ecological fitting provide the fuel for the expansion phase of the recently proposed oscillation hypothesis of host range and speciation, and, more generally, the generation of novel combinations of interacting species within the geographic mosaic theory of coevolution. Finally, we conclude that taxon pulses, driven by climate change and large-scale ecological perturbation are drivers of biotic mixing and resultant ecological fitting, which leads to increased rates of rapid host switching, including the agents of Emerging Infectious Disease.

    Read more about How specialists can be generalists: resolving the “parasite paradox” and implications for emerging infectious disease

Show all publications by Niklas Janz at Stockholm University