In a progressively human-dominated planet (Venter et al., 2016), the fate of many species will depend on the extent to which they can persist in anthropogenic landscapes. In Western Europe, where only small areas of primary habitat remain (e.g. Sabatini et al., 2018), semi-natural areas are crucial habitats to many native species, yet they are threatened by the expansion of human activities, including agricultural expansion and intensification (Rigal et al., 2023). 

A new study by Mallet and colleagues (Mallet et al., 2023) investigates the extent to which bird species in the Camargue region are able to use the margins of agricultural fields as substitutes for their preferred semi-natural habitats. Located in the delta of the Rhône River in Southern France, the Camargue is internationally recognized for its biodiversity value, classified as a Biosphere Reserve by UNESCO and as a Wetland of International Importance under the Ramsar Convention (IUCN & UN-WCMC, 2023). Mallet and colleagues tested three specific hypotheses: that grass strips (grassy field boundaries, including grassy tracks or dirt roads used for moving agricultural machinery) can function as substitute habitats for grassland species; that reed strips along drainage ditches (common in the rice paddy landscapes of the Camargue) can function as substitute habitats to wetland species; and that hedgerows can function as substitute habitats to species that favour woodland edges. They did so by measuring how the local abundances of 14 bird species (nine typical of forest edges, 3 of grasslands, and two of reedbeds) respond to increasing coverage of either the three types of field margins or of the three types of semi-natural habitat. 

This is an elegant study design, yet – as is often the case with real field data – results are not as simple as expected. Indeed, for most species (11 out of 14) local abundances did not increase significantly with the area of their supposed primary habitat, undermining the assumption that they are strongly associated with (or dependent on) those habitats. Among the three species that did respond positively to the area of their primary habitat, one (a forest edge species) responded positively but not significantly to the area of field margins (hedgerows), providing weak evidence to the habitat compensation hypothesis. For the other two (grassland and a wetland species), abundance responded even more strongly to the area of field margins (grass and reed strips, respectively) than to the primary habitat, suggesting that the field margins are not so much a substitute but valuable habitats in their own right. 

It would have been good conservation news if field margins were found to be suitable habitat substitutes to semi-natural habitats, or at least reasonable approximations, to most species. Given that these margins have functional roles in agricultural landscapes (marking boundaries, access areas, water drainage), they could constitute good win-win solutions for reconciling biodiversity conservation with agricultural production. Alas, the results are more complicated than that, with wide variation in species responses that could not have been predicted from presumed habitat affinities. These results illustrate the challenges of conservation practice in complex landscapes formed by mosaics of variable land use types. With species not necessarily falling neatly into habitat guilds, it becomes even more challenging to plan strategically how to manage landscapes to optimize their conservation. The results presented here suggest that species’ abundances may be responding to landscape variables not taken into account in the analyses, such as connectivity between habitat patches, or maybe positive and negative edge effects between land use types. That such uncertainties remain even in a well-studied region as the Camargue, and for such a well-studied taxon such as birds, only demonstrates the continued importance of rigorous field studies testing explicit hypotheses such as this one by Mallet and colleagues. 

References

IUCN, & UN-WCMC (2023). Protected Planet. Protected Planet. https://www.protectedplanet.net/en 

Mallet, P., Béchet, A., Sirami, C., Mesléard, F., Blanchon, T., Calatayud, F., Dagonet, T., Gaget, E., Leray, C., & Galewski, T. (2023). Field margins as substitute habitat for the conservation of birds in agricultural wetlands. bioRxiv, 2022.05.05.490780, ver. 3 peer-reviewed and recommended by Peer Community in Ecology. https://doi.org/10.1101/2022.05.05.490780 

Rigal, S., Dakos, V., Alonso, H., Auniņš, A., Benkő, Z., Brotons, L., Chodkiewicz, T., Chylarecki, P., de Carli, E., del Moral, J. C. et al. (2023). Farmland practices are driving bird population decline across Europe. Proceedings of the National Academy of Sciences, 120, e2216573120. https://doi.org/10.1073/pnas.2216573120 

Sabatini, F. M., Burrascano, S., Keeton, W. S., Levers, C., Lindner, M., Pötzschner, F., Verkerk, P. J., Bauhus, J., Buchwald, E., Chaskovsky, O., Debaive, N. et al. (2018). Where are Europe’s last primary forests? Diversity and Distributions, 24, 1426–1439. https://doi.org/10.1111/ddi.12778 

Venter, O., Sanderson, E. W., Magrach, A., Allan, J. R., Beher, J., Jones, K. R., Possingham, H. P., Laurance, W. F., Wood, P., Fekete, B. M., Levy, M. A., & Watson, J. E. M. (2016). Sixteen years of change in the global terrestrial human footprint and implications for biodiversity conservation. Nature Communications, 7, 12558. https://doi.org/10.1038/ncomms12558 

Parasitism requires parasites and hosts to meet and is therefore conditioned by their respective dispersal abilities. While dispersal has been studied in a number of wild vertebrates (including in relation to infection risk), we still have poor knowledge of the movements of their parasites. Yet we know that many parasites, and in particular vectors transmitting pathogens from host to host, possess the ability to move actively during at least part of their lives.
So... how far does a vector go – and is this reflected in the population structure of the pathogens they transmit? This is the question addressed by Rataud et al. [1], who provide the first attempt at using capture-mark-recapture to estimate not only functional dispersal, but also detection probability and survival in a wild parasite that is also a vector for other pathogens.
The authors find that (i) functional dispersal of soft ticks within a gull colony is very limited. Moreover, they observe unexpected patterns: (ii) experimental displacement of ticks does not induce homing behaviour, and (iii) despite lower survival, tick dispersal was lower in nests not containing hosts than in successful nests.
These results contrast with expectations based on the distribution of infectious agents. Low tick dispersal within the colony, combined with host territoriality during breeding and high site fidelity between years should result in a spatially structured distribution of infectious agents carried by ticks. This is not the case here. One possible explanation could be that soft ticks live for much longer than a breeding season, and that they disperse at other times of year to a larger extent than usually assumed.
This study represents one chapter of a story that will likely keep unfolding. It raises fascinating questions, and illustrates the importance of basic knowledge of parasite ecology and behaviour to better understand pathogen dynamics in the wild.

References
[1] Rataud A., Dupraz M., Toty C., Blanchon T., Vittecoq M., Choquet R. & McCoy K.D. (2019). Evaluating functional dispersal and its eco-epidemiological implications in a nest ectoparasite. Zenodo, 2592114. Ver. 3 peer-reviewed and recommended by PCI Ecology. doi: 10.5281/zenodo.2592114

Dispersal is a key element of the natural dynamics of meta-communities, and plays a central role in the success of populations colonizing new landscapes. Understanding how demographic processes may affect the speed at which alien species spread through environmentally-heterogeneous habitat fragments is therefore of key importance to manage biological invasions. This requires studying together the complex interplay of dispersal and population processes, two inextricably related phenomena that can produce many possible outcomes. Stochastic models offer an opportunity to describe this kind of process in a meaningful way, but to ensure that they are realistic (sensu Levins 1966) it is also necessary to combine model simulations with empirical data (Snäll et al. 2007).

Morel-Journel et al. (2023) put together stochastic models and experimental data to study how population density may affect the speed at which alien species spread through a heterogeneous landscape. They do it by focusing on what they call ‘colonisation debt’, which is merely the impact that population density at the invasion front may have on the speed at which the species colonizes patches of different carrying capacities. They investigate this issue through two largely independent approaches. First, a stochastic model of dispersal throughout the patches of a linear, 1-dimensional landscape, which accounts for different degrees of density-dependent growth. And second, a microcosm experiment of a parasitoid wasp colonizing patches with different numbers of host eggs. In both cases, they compare the velocity of colonization of patches with lower or higher carrying capacity than the previous one (i.e. what they call upward or downward gradients).

Their results show that density-dependent processes influence the speed at which new fragments are colonized is significantly reduced by positive density dependence. When either population growth or dispersal rate depend on density, colonisation debt limits the speed of invasion, which turns out to be dependent on the strength and direction of the gradient between the conditions of the invasion front, and the newly colonized patches. Although this result may be quite important to understand the meta-population dynamics of dispersing species, it is important to note that in their study the environmental differences between patches do not take into account eventual shifts in the scenopoetic conditions (i.e. the values of the environmental parameters to which species niches’ respond to; Hutchinson 1978, see also Soberón 2007). Rather, differences arise from variations in the carrying capacity of the patches that are consecutively invaded, both in the in silico and microcosm experiments. That is, they account for potential differences in the size or quality of the invaded fragments, but not on the costs of colonizing fragments with different environmental conditions, which may also determine invasion speed through niche-driven processes. This aspect can be of particular importance in biological invasions or under climate change-driven range shifts, when adaptation to new environments is often required (Sakai et al. 2001; Whitney & Gabler 2008; Hill et al. 2011).

The expansion of geographical distribution ranges is the result of complex eco-evolutionary processes where meta-community dynamics and niche shifts interact in a novel physical space and/or environment (see, e.g., Mestre et al. 2020). Here, the invasibility of native communities is determined by niche variations and how similar are the traits of alien and native species (Hui et al. 2023). Within this context, density-dependent processes will build upon and heterogeneous matrix of native communities and environments (Tischendorf et al. 2005), to eventually determine invasion success. What the results of Morel-Journel et al. (2023) show is that, when the invader shows density dependence, the invasion process can be slowed down by variations in the carrying capacity of patches along the dispersal front. This can be particularly useful to manage biological invasions; ongoing invasions can be at least partially controlled by manipulating the size or quality of the patches that are most adequate to the invader, controlling host populations to reduce carrying capacity. But further, landscape manipulation of such kind could be used in a preventive way, to account in advance for the effects of the introduction of alien species for agricultural exploitation or biological control, thereby providing an additional safeguard to practices such as the introduction of parasitoids to control plagues. These practical aspects are certainly worth exploring further, together with a more explicit account of the influence of the abiotic conditions and the characteristics of the invaded communities on the success and speed of biological invasions.

REFERENCES

Hill, J.K., Griffiths, H.M. & Thomas, C.D. (2011) Climate change and evolutionary adaptations at species' range margins. Annual Review of Entomology, 56, 143-159. https://doi.org/10.1146/annurev-ento-120709-144746

Hui, C., Pyšek, P. & Richardson, D.M. (2023) Disentangling the relationships among abundance, invasiveness and invasibility in trait space. npj Biodiversity, 2, 13. https://doi.org/10.1038/s44185-023-00019-1

Hutchinson, G.E. (1978) An introduction to population biology. Yale University Press, New Haven, CT.

Levins, R. (1966) The strategy of model building in population biology. American Scientist, 54, 421-431. 

Mestre, A., Poulin, R. & Hortal, J. (2020) A niche perspective on the range expansion of symbionts. Biological Reviews, 95, 491-516. https://doi.org/10.1111/brv.12574

Morel-Journel, T., Haond, M., Duan, L., Mailleret, L. & Vercken, E. (2023) Colonisation debt: when invasion history impacts current range expansion. bioRxiv, 2022.11.13.516255, ver. 3 peer-reviewed and recommended by Peer Community in Ecology. https://doi.org/10.1101/2022.11.13.516255

Snäll, T., B. O'Hara, R. & Arjas, E. (2007) A mathematical and statistical framework for modelling dispersal. Oikos, 116, 1037-1050. https://doi.org/10.1111/j.0030-1299.2007.15604.x

Sakai, A.K., Allendorf, F.W., Holt, J.S., Lodge, D.M., Molofsky, J., With, K.A., Baughman, S., Cabin, R.J., Cohen, J.E., Ellstrand, N.C., McCauley, D.E., O'Neil, P., Parker, I.M., Thompson, J.N. & Weller, S.G. (2001) The population biology of invasive species. Annual Review of Ecology and Systematics, 32, 305-332. https://doi.org/10.1146/annurev.ecolsys.32.081501.114037

Soberón, J. (2007) Grinnellian and Eltonian niches and geographic distributions of species. Ecology Letters, 10, 1115-1123. https://doi.org/10.1111/j.1461-0248.2007.01107.x

Tischendorf, L., Grez, A., Zaviezo, T. & Fahrig, L. (2005) Mechanisms affecting population density in fragmented habitat. Ecology and Society, 10, 7. https://doi.org/10.5751/ES-01265-100107

Whitney, K.D. & Gabler, C.A. (2008) Rapid evolution in introduced species, 'invasive traits' and recipient communities: challenges for predicting invasive potential. Diversity and Distributions, 14, 569-580. https://doi.org/10.1111/j.1472-4642.2008.00473.x

The mean trophic level of catches in world fisheries has generally declined over the 20th century, a phenomenon called "fishing down the food web" (Pauly et al. 1998). Several mechanisms have been proposed to explain this decline including the collapse of, or decline in, higher trophic level stocks leading to the inclusion of lower trophic level stocks in the fishery. Fishing down the food web may lead to a reduction in the resilience, i.e., the capacity to rebound from change, of the fished community, which is concerning given the necessity of resilience in the face of climate change. 

The practice of adaptive harvesting, which involves fishing stocks based on their availability, can also result in a reduction in the average trophic level of a fishery (Branch et al. 2010). Adaptive harvesting, similar to adaptive foraging, can affect the resilience of fisheries. Generally, adaptive foraging acts as a stabilizing force in communities (Valdovinos et al. 2010), however it is not clear how including harvesters as the adaptive foragers will affect the resilience of the system.

Tromeur and Loeuille (2023) analyze the effects of adaptively harvesting a trophic community. Using a system of ordinary differential equations representing a predator-prey model where both species are harvested, the researchers mathematically analyze the impact of increasing fishing effort and adaptive harvesting on the mean trophic level and resilience of the fished community. This is achieved by computing the equilibrium densities and equilibrium allocation of harvest effort.  In addition, the researchers numerically evaluate adaptive harvesting in a tri-trophic system (predator, prey, and resource). The study focuses on the effect of adaptively distributing harvest across trophic levels on the mean trophic level of catches, the propensity for regime shifts to occur, the ability to return to equilibrium after a disturbance, and the speed of this return. 

The results indicate that adaptive harvesting leads to a decline in the mean trophic level of catches, resulting in “fishing down the food web”. Furthermore, the study shows that adaptive harvesting may harm the overall resilience of the system. Similar results were observed numerically in a tri-trophic community.

While adaptive foraging is generally a stabilizing force on communities, the researchers found that adaptive harvesting can destabilize the harvested community. One of the key differences between adaptive foraging models and the model presented here, is that the harvesters do not exhibit population dynamics. This lack of a numerical response by the harvesters to decreasing population sizes of their stocks leads to regime shifts. The realism of a fishery that does not respond numerically to declining stock is debatable, however it is very likely that there will a least be significant delays due to social and economic barriers to leaving the fishery, that will lead to similar results.

This study is not unique in demonstrating the ability of adaptive harvesting to result in “fishing down the food web”. As pointed out by the researchers, the same results have been shown with several different model formulations (e.g., age and size structured models). Similarly, this study is not unique to showing that increasing adaptation speeds decreases the resilience of non-linear predator-prey systems by inducing oscillatory behaviours. Much of this can be explained by the destabilising effect of increasing interaction strengths on food webs (McCann et al. 1998). 

By employing a straightforward model, the researchers were able to demonstrate that adaptive harvesting, a common strategy employed by fishermen, can result in a decline in the average trophic level of catches, regime shifts, and reduced resilience in the fished community. While previous studies have observed some of these effects, the fact that the current study was able to capture them all with a simple model is notable. This modeling approach can offer insight into the role of human behavior on the complex dynamics observed in fisheries worldwide.

References

Branch, T. A., R. Watson, E. A. Fulton, S. Jennings, C. R. McGilliard, G. T. Pablico, D. Ricard, et al. 2010. The trophic fingerprint of marine fisheries. Nature 468:431–435. https://doi.org/10.1038/nature09528

Tromeur, E., and N. Loeuille. 2023. Effects of adaptive harvesting on fishing down processes and resilience changes in predator-prey and tritrophic systems. bioRxiv 290460, ver 5 peer-reviewed and recommended by PCI Ecology. https://doi.org/10.1101/290460

McCann, K., A. Hastings, and G.R. Huxel. 1998. Weak trophic interactions and the balance of nature. Nature 395: 794-798. https://doi.org/10.1038/27427

Pauly, D., V. Christensen, J. Dalsgaard, R. Froese, and F. Torres Jr. 1998. Fishing down marine food webs. Science 279:860–86. https://doi.org/10.1126/science.279.5352.860

Valdovinos, F.S., R. Ramos-Jiliberto, L. Garay-Naravez, P. Urbani, and J.A. Dunne. 2010. Consequences of adaptive behaviour for the structure and dynamics of food webs. Ecology Letters 13: 1546-1559. https://doi.org/10.1111/j.1461-0248.2010.01535.x

Whether individuals are able to cope with new environmental conditions, and whether this ability can be improved, is certainly of great interest in our changing world. One way to cope with new conditions is through behavioral flexibility, which can be defined as “the ability to adapt behavior to new circumstances through packaging information and making it available to other cognitive processes” (Logan et al. 2023). Flexibility is predicted to be positively correlated with innovativeness, the ability to create a new behavior or use an existing behavior in a few situations (Griffin & Guez 2014). 
The post-study manuscript by Logan et al. (2023) proposes to test flexibility manipulability, and the relationship between flexibility and innovativeness. The authors did so with an experimental study on great-tailed grackles (Quiscalus mexicanus), an expanding species in the US, known to be flexible. 
The authors used serial reversal learning to investigate (1) whether behavioral flexibility, as measured by reversal learning using tubes of different shades, is manipulable; (2) whether manipulating (improving/training) behavioral flexibility improves flexibility and innovativeness in new contexts; (3) the type of learning strategy used by the individuals throughout the serial reversals.
The study described in this manuscript was pre-registered in Logan et al. (2019) and received in-principle recommendation on 26 Mar 2019 (Coulon 2019). One hypothesis from this original preregistration will be treated in a separate manuscript.
Among several interesting results, what I found most striking is that flexibility, in this species, seems to be a trait that is acquired by experience (vs. inherent to the individual). This opens exciting interrogations on the role of social learning, and on the impact of rapid environmental changes (which may force the individuals to experiment new ways to access to resources, for example), on individual flexibility and adaptability to new conditions. 
 
REFERENCES

Coulon A (2019) Can context changes improve behavioral flexibility? Towards a better understanding of species adaptability to environmental changes. Peer Community in Ecology, 100019. https://doi.org/10.24072/pci.ecology.100019

Griffin, A. S., & Guez, D. (2014). Innovation and problem solving: A review of common mechanisms. Behavioural Processes, 109, 121–134. https://doi.org/10.1016/j.beproc.2014.08.027

Logan C, Rowney C, Bergeron L, Seitz B, Blaisdell A, Johnson-Ulrich Z, McCune K (2019)
Is behavioral flexibility manipulatable and, if so, does it improve flexibility and problem solving in a new context? In Principle Recommendation 2019. PCI Ecology. http://corinalogan.com/Preregistrations/g_flexmanip.html

Logan CJ, Lukas D, Blaisdell AP, Johnson-Ulrich Z, MacPherson M, Seitz B, Sevchik A, McCune KB (2023) Behavioral flexibility is manipulable and it improves flexibility and innovativeness in a new context. EcoEcoRxiv, version 5 peer-reviewed and recommended by Peer Community in Ecology. https://doi.org/10.32942/osf.io/5z8xs