Immediate or lagged responses of a red squirrel population to pulsed resources

Abstract

According to producer-consumer models, consumers should follow pulsed resources with a time lag. This view has been challenged by studies demonstrating that individuals may anticipate future resource pulses by increasing reproduction just before the pulse. We studied population fluctuations and reproduction in European red squirrels, Sciurus vulgaris, in relation to seed masting of the main food resource (the Norway spruce) in boreal coniferous forests between 1979 and 2013. Red squirrels are pre-dispersal seed predators, and previous studies have shown that they can anticipate the coming seed mast. We did not find any indication that anticipation of masting (year t ) increased red squirrel reproduction in the preceding spring to early summer. Instead, the reproductive output of the squirrels was highest in the spring following the mast, indicating that the population had to be at its largest size in the autumn after the mast (year(t+1)), when lots of subadults were around. However, we surmised, based on snow tracks and squirrel nest data, that the population crashed during the following winter (year(t+1)). These data reflected the adult population during winter, which peaked at the same time as the resource pulse. We can therefore conclude that the time lag between the resource pulse and the attainment of the peak number of squirrels was less than one year, and that the resource crash affected more juveniles and subadults than adults. The population increase overlapped with the occurrence of masting, but there was also a lagged response, supporting the classical view of producer-consumer models.

Red squirrels decline in abundance in the boreal forests of Finland and NW Russia

“…Due to global warming, when several species shift their area of distribution polewards (Parmesan 2006), we expect to see the more northern populations of red squirrels remain stable or increase as a result of milder winters and increased overwinter survival, which is a critical phase especially for subadults (Selonen et al 2015). We also aim to assess the impacts of climate change and deforestation on trends in red squirrel abundance, while taking into account predator abundance.…”

“…Red squirrels live on the northern edge of their range in the northernmost parts of our study area, Finland and north-west Russia. Dynamics of red squirrel populations follow the seed crop of the Norway spruce Picea abies and other conifers (Gurnell 1983, Andrén and Lemnell 1992, Kenward et al 1998, Wauters et al 2004, Selonen et al 2015. We therefore study the effect of change in winter temperature on the regional red squirrel population growth rate.…”

Siberian flying squirrels do not anticipate future resource abundance

“…Thus, if a female could anticipate the coming mast, its offspring would face the winter with optimal resource availability. In addition, variance in birch and alder catkin production [23] is comparable to variation in spruce cone production used by red squirrels [1, 14]. Furthermore, the Siberian flying squirrel are entirely dependent on trees, and very seldom move on the ground (North American flying squirrels, Glaucomys spp., move regularly on the ground, e.g.…”

“…To optimize reproduction with masting events, it is suggested that in European and North American red squirrels [1, 10], chipmunks [11] and fat dormice [12] a mother may increase reproductive output in summers before mast autumns. However, the role of this behaviour in population dynamics of the species remains uncertain [13, 14]. It also remains unclear how general the anticipation behaviour might be for rodents living in forest communities [15].…”

“…Earlier red squirrel and chipmunk studies have indicated that summer litters are produced in anticipation, whereas spring litters are less affected by future food conditions [10, 13, 14]. Thus, we focus our analysis on production of summer litters in flying squirrels.…”