Forest bat population dynamics over 14 years at a climate refuge: Effects of timber harvesting and weather extremes

<div><p>Long-term data are needed to explore the interaction of weather extremes with habitat alteration; in particular, can ‘refugia’ buffer population dynamics against climate change and are they robust to disturbances such as timber harvesting. Because forest bats are good indicators of ecosystem health, we used 14 years (1999–2012) of mark-recapture data from a suite of small tree-hollow roosting bats to estimate survival, abundance and body condition in harvested and unharvested forest and over extreme El Niño and La Niña weather events in southeastern Australia. Trapping was replicated within an experimental forest, located in a climate refuge, with different timber harvesting treatments. We trapped foraging bats and banded 3043 with a 32% retrap rate. Mark-recapture analyses allowed for dependence of survival on time, species, sex, logging treatment and for transients. A large portion of the population remained resident, with a maximum time to recapture of nine years. The effect of logging history (unlogged vs 16–30 years post-logging regrowth) on apparent survival was minor and species specific, with no detectable effect for two species, a positive effect for one and negative for the other. There was no effect of logging history on abundance or body condition for any of these species. Apparent survival of residents was not strongly influenced by weather variation (except for the smallest species), unlike previous studies outside of refugia. Despite annual variation in abundance and body condition across the 14 years of the study, no relationship with extreme weather was evident. The location of our study area in a climate refuge potentially buffered bat population dynamics from extreme weather. These results support the value of climate refugia in mitigating climate change impacts, though the lack of an external control highlights the need for further studies on the functioning of climate refugia. Relatively stable population dynamics were not compromised by timber harvesting, suggesting ecologically sustainable harvesting may be compatible with climate refugia.</p></div

Time varying survival parameters and their covariate (MaxTempSumm).

<p>The value plotted over year <i>n (n</i> = 1999–2011) is survival over the year beginning with the capture event of year <i>n</i> to year <i>n</i>+1. VrU = <i>V</i>. <i>regulus</i> in unlogged; VpM = <i>V</i>. <i>pumilus</i> males; VpFU = <i>V</i>. <i>pumilus</i> females in unlogged. Note that survival values are averages over modelled estimates assuming covariation with the weather covariates.</p

Covariates employed in modelling time dependence in survival and recapture.

<p>Covariates employed in modelling time dependence in survival and recapture.</p

Capture frequency and counts of individual bats per species (18 of these captures occurred in 2012, the final capture event, and eight marked bats were not re-released (deaths) at earlier capture events, so of 2087 captures, 26 did not constitute releases).

<p>Capture frequency and counts of individual bats per species (18 of these captures occurred in 2012, the final capture event, and eight marked bats were not re-released (deaths) at earlier capture events, so of 2087 captures, 26 did not constitute releases).</p

Plots of estimated abundance (●) and number of residents (▲) over all four species per site with respect to capture event, numbered as in Fig 3.

<p>Plots of estimated abundance (●) and number of residents (▲) over all four species per site with respect to capture event, numbered as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0191471#pone.0191471.g003" target="_blank">Fig 3</a>.</p

Survival structure in model φ(1)p(sex+ MaxTempSumm + AvMinTempDur).

<p>The two-letter species abbreviation indicate survival rates for that species, qualified if the rate was sex specific by adding M or F and treatment specific by adding R(regrowth) or U (unlogged), e.g., CmFU, designates a survival rate for female <i>Chalinolobus morio</i> in unlogged treatments. The absence of a qualifier means the survival rate is independent of that effect. Coincidences between survival rates are expressed with equality signs. All the survival parameters in the second column were modelled as logistic regressions on the covariate MaxTempSumm with the same regression coefficient but their own intercept, independently of whether they were newly or already marked. Note that survival rates <i>within the same year</i> only apply to the capture season, and should not be extrapolated beyond this period.</p

High elevation eucalypt forest in the climate refuge study area.

<p>(a) unlogged forest catchment showing tall emergent eucalypts above a dense rainforest understorey (b) 27 year old regrowth catchment showing dense stands of mixed eucalypts and rainforest species with a small 4WD trail used for harp-trapping bats.</p

Quantifying the demographic cost of human-related mortality to a raptor population

<div><p>Raptors are exposed to a wide variety of human-related mortality agents, and yet population-level effects are rarely quantified. Doing so requires modeling vital rates in the context of species life-history, behavior, and population dynamics theory. In this paper, we explore the details of such an analysis by focusing on the demography of a resident, tree-nesting population of golden eagles (<i>Aquila chrysaetos</i>) in the vicinity of an extensive (142 km²) windfarm in California. During 1994–2000, we tracked the fates of >250 radio-marked individuals of four life-stages and conducted five annual surveys of territory occupancy and reproduction. Collisions with wind turbines accounted for 41% of 88 uncensored fatalities, most of which were subadults and nonbreeding adults (floaters). A consistent overall male preponderance in the population meant that females were the limiting sex in this territorial, monogamous species. Estimates of potential population growth rate and associated variance indicated a stable breeding population, but one for which any further decrease in vital rates would require immigrant floaters to fill territory vacancies. Occupancy surveys 5 and 13 years later (2005 and 2013) showed that the nesting population remained intact, and no upward trend was apparent in the proportion of subadult eagles as pair members, a condition that would have suggested a deficit of adult replacements. However, the number of golden eagle pairs required to support windfarm mortality was large. We estimated that the entire annual reproductive output of 216–255 breeding pairs would have been necessary to support published estimates of 55–65 turbine blade-strike fatalities per year. Although the vital rates forming the basis for these calculations may have changed since the data were collected, our approach should be useful for gaining a clearer understanding of how anthropogenic mortality affects the health of raptor populations, particularly those species with delayed maturity and naturally low reproductive rates.</p></div

High quality nesting habitat for golden eagles in the Diablo Range, California.

<p>High quality nesting habitat for golden eagles in the Diablo Range, California.</p

The Altamont Pass windfarm as viewed from its southern boundary in April 1995.

<p>The dense configurations of small turbines in this photo are currently being replaced by fewer, larger, and more widely spaced machines.</p