Long-term decline in a salt marsh hare population largely driven by bottom-up factors

Abstract

The widespread decline of the European brown hare (Lepus europaeus) in Europe has been attributed to both bottom-up and top-down factors, as well as climate change. Few studies have attempted to study the relative importance of these factors considered simultaneously. In this study we tested the hypotheses that hare population density is regulated by bottom-up (food), top-down (predation) or abiotic factors including tidal floods and climatic conditions. We related data on hare population density on a relatively isolated island to changes in surface area of suitable vs. unsuitable vegetation for forage, predator densities, flooding parameters and climatic variables. During the study period (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012), hare numbers decreased from 580 to 219. Estimated population density was positively correlated with the cover of short, intermediate successional vegetation types and was negatively correlated with the cover of tall, late successional vegetation types. These findings corroborate results from earlier experimental studies that reported a strong aversion of hares to tall vegetation. Additionally, we found indications that raptor population density and unusually high floods also exerted negative effects on hare population density. We conclude that bottom-up factors (the availability of suitable forage) are the main regulators of the studied hare population. This suggests that the importance of bottomup effects has been underestimated and could explain leporid population decline in areas that have experienced a similar increase in tall, unsuitable vegetation. RÉSUMÉLe déclin généralisé des populations de lièvre d'Europe (Lepus europaeus) a été attribué à la fois à des facteurs ascendants et descendants, ainsi qu'au changement climatique. Peu d'études ont tenté d'évaluer l'importance relative de ces facteurs lorsque considérés simultanément. Dans la présente étude, nous avons testé les hypothèses selon lesquelles la densité de population de lièvre est régulée par des facteurs ascendants (nourriture), des facteurs descendants (prédation) et des facteurs abiotiques tels que les inondations dues aux marées et les conditions climatiques. Nous avons comparé les données de densité de population de lièvre d'une île relativement isolée aux changements de la superficie de végétation adéquate ou non pour le lièvre, de densité de prédateurs, des paramètres d'inondations et des variables climatiques. Pendant la période étudiée (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012), le nombre de lièvres a diminué de 580 à 219. La densité de population estimée était positivement corrélée au recouvrement de la végétation courte de milieu de succession et négativement corrélée au recouvrement de la végétation haute de fin de succession. Ces résultats corroborent ceux d'études expérimentales antérieures qui ont montré une forte aversion des lièvres pour la végétation haute. De plus, nos résultats sugg...

Top-down vs. bottom-up control on vegetation composition in a tidal marsh depends on scale

“…atherica also spread at the cost of low-statured species such as Puccinellia maritima and Festuca rubra [ 45 ]. The reduction of these edible plant species initially resulted in a decline of winter-staging Brent geese Brant bernicla [ 17 ], followed by a decrease in European brown hares Lepus europaeus [ 72 ]. Similarly to these studies, we observed a reduction in Greylag goose numbers that co-occurred with an increase in E .…”

Small herbivores slow down species loss up to 22 years but only at early successional stage

“…The eastern part of the saltmarsh (the study area) is only grazed by small herbivores, including spring staging Brent Geese ( Branta bernicla ), Barnacle Geese ( Branta leucopsis ), and year‐round present Brown hares ( Lepus europaeus ) and rabbits ( Oryctolagus cunniculus ). Hares and geese are the most important herbivores, while predators are rare in this system (Van De Koppel et al, ; Kuijper & Bakker, ; Schrama et al, ; Wal, Egas, et al, ; Van Der Wal et al, ). A natural successional gradient is present here, as the island expands naturally eastward (Olff et al, ): the eastern part of the island is younger than the western part, and different successional stages occur adjacent to one another, naturally separated by creeks (Figure ).…”

“…The effects of small herbivores on plant diversity may change along successional gradients. Small herbivores are usually selective grazers (Olff & Ritchie, ), and thus, changes in the identity of forage plants during vegetation succession may change the abundance of herbivores, particularly in systems where predators are rare (Schrama, Kuijper, Veeneklaas, & Bakker, ). The abundance of herbivores is sometimes, if not always, more important than herbivore size in regulating plant communities (Olofsson, Hulme, Oksanen, & Suominen, ).…”

“…Previous work from this saltmarsh indicates that hares, and to a lesser extent, geese, affect plant composition, particularly at early successional stages (Kuijper & Bakker, ). However, the hare population has declined by more than 50% in the last two decades (Schrama et al, ), while geese populations remain stable (Figure ).…”

Competition rather than facilitation affects plant performance across an abiotic stress gradient in a restored California salt marsh

“…Herbivore feeding preferences can affect plant performance and vegetation structure (Furbish & Albano 1994; Kuijper & Bakker 2008), and abiotic or biotic context can influence consumer pressure. For example, environmental stress can reduce forage quality or abundance, or predators can keep herbivores in check (Silliman & Bertness 2002; Crain 2008; Schrama et al 2015). Abiotic factors have long been considered the primary constraint on plant performance and community structure in salt marsh, but herbivore impacts have also been documented (Kotanen & Jefferies 1997; He & Silliman 2016).…”