IB114 The Role of Disease in Red Squirrel Conservation PDF

The Role of Disease in the Conservation of Red Squirrels in the UK IB114  The ecological replacement of red squirrels by grey squirrels Red squirrel native to UK Grey squirrel introduced to UK from North America...

The Role of Disease in the Conservation of Red Squirrels in the UK IB114  The ecological replacement of red squirrels by grey squirrels Red squirrel native to UK Grey squirrel introduced to UK from North America c.1900 Grey squirrel has replaced the red in most of UK  Competitive Replacement Greys are better competitors – Competition for food – Competition for habitat (Bryce et al 2001. Proc. Roy Soc) Shared Disease: Squirrelpoxvirus (SQPV) Squirrelpox: Harmless to grey squirrels Fatal to red squirrels  Characteristics of the Squirrel poxvirus interaction Probably introduced into the UK with the Grey squirrels – Disease was unrecorded before grey squirrel introduction (Sainsbury & Gurnell 1995) Grey Seroprevalence ~74% in England and Wales In contrast, disease seems to be absent in Scotland where declines in Red are less marked Little pathogenic effect on greys Highly Pathogenic to the Reds (Tompkins et al 2001: Proc Roy Soc)  Tompkins, D., White, A. & Boots, M. Motivation: to understand how competition and disease effect the rate of replacement of reds by greys. The Model combines the classical approaches in Competition modelling (Lotka, Volterra 1930s) Disease modelling (Anderson and May 1981) Model allows us to understand how the competition and disease mechanisms operate. Model with relatively few parameters that can be estimated from data.  Competition only model HG = Density of grey squirrels, HR = Density of red squirrels dH G (aG  qG ( H G  cR H R )) H G  bH G dt dH R (aR  qR ( H R  cG H G )) H R  bH R dt aG (aR) : Max reproduction rate of grey (red). b : adult mortality rate qG (qR) : Susceptibility to crowding of grey (red). KG (KR) : Carrying capacity of grey (red) Here qG = (aG – b) / KG , qR = (aR – b) / KR cR : competitive effect of reds on greys cG : competitive effect of greys on reds  Competition parameter estimates Intrinsic net growth rate (Okubo 1989) rG = 0.8 year-1, rR = 0.6 year-1 Adult mortality (Gurnell 1987, Wauters 2000) b = 0.4 year-1 Maximum reproduction rate aG = 1.2 year-1, aR = 1.0 year-1 Carrying capacity (Rushton 1997) KG = 80 per (5km)2 , KR = 60 per (5km)2 Competition coefficients (Bryce 2001) cG = 1.65 , cR = 0.61  Competition only results Start the model at the red carrying capacity and introduce 2 greys. Replacement takes approximately 15 years Squirrels per (5km)2 5 10 15 Years  Competition and disease model S – Susceptible, I – Infected, R – Removed (Immune) HG = SG + IG + RG, HR = SR + IR dS G (aG  qG ( H G  c R H R )) H G  bS G   S G ( I G  I R ) dt dI G   S G ( I G  I R )  bI G   I G dt dRG  I G  bRG dt dS R (a R  q R ( H R  cG H G )) S R  bS R   S R ( I G  I R ) dt dI R   S R ( I G  I R )  bI R   I R dt  : recovery rate from virus of infected greys  : virus transmission rate  : mortality rate due to virus  Disease parameter estimates Recovery rate from virus of infected greys (Tompkins 2001)  = 13 year-1 Virus transmission rate (Sainsbury 2000, Tompkins 2003)  = 0.7 year-1 (5km)-2 Mortality rate due to virus (Tompkins 2001)  = 26 year-1  Competition and disease results Start the model at the red carrying capacity and introduce 2 infected greys Replacement takes approximately 6 years Squirrels per (5km)2 Could the model explain the observed replacement speeds? Reynolds (1985) Annual Distribution Maps Presence of Squirrels reported 5 x 5 km squares  Patch Model System of 5 by 5 km squares Competition and disease model within each patch Patches linked by dispersal A fraction  = 0.2 individuals move to nearest patches each year. (Okubo 1989) Initial conditions assume red squirrels at carrying capacity in each patch. Introduce either 2 grey uninfected or 2 grey infected squirrels into 2 patches. Presence/absence threshold assumed Output from the model equivalent to the data Reds Observed Data Greys Reynolds, 1985, J. Anim. Ecol. Reds Competition and disease Tompkins, White, Boots. 2003, Ecol. Lett. Greys Reds Greys Competition-Only Tompkins, White, Boots. 2003, Ecol. Lett.  Conclusions: for squirrel system Disease increases the speed of replacement. Competition alone can not explain the observed replacement speed. The number of infecteds is small (and so difficult to observe in the field). 50 year from observation of disease (1930) to identify virus (1981). Further 14 years for potential impact suggested (1995). Squirrelpox is unequivocally linked to the replacement of red squirrels and a key threat to the species survival in the UK. Do these findings apply to other ecological systems? Other Examples Signal crayfish (right) replacing White-clawed crayfish (left) Harlequin Ladybird replacing native species Copyright: Rolf Hicker Photography White tailed deer Copyright: Peter Mirejovsky replacing moose and caribou Expansion of Squirrelpox in Scotland Percentage Chance of Seropositive Greys (from 10 simulations) - no grey control Identification of ‘pinch points  New Data – Epidemics in Red Squirrels Log Squirrel density (+1) Year  New Data – Epidemics in Red Squirrels Red squirre Log Squirrel density (+1) Grey squirre SQPV Year  Estimate of Transmission rates from the epidemic Get an estimate for transmission coefficient as Previous estimate from the model – Effectively saying what transmission rate fits the data Previous estimate was – so similar  New Data – Endemic in Grey Squirrels Very high prevalence  New Data – Endemic in Grey Squirrels Individual data, recaptures, Serology and Viremia Re-infection Viremic and seropositive Not seropositive after 1st infection  Key characteristics in the data Re-infections occur Once seropositive it is life-long but infection still occurs – Partial or waning protection – Viremic and seropositive Seropositive after second infection Also found in limited experimental data* * Fiegna, C. 2012. “Study of squirrelpox virus in red and grey squirrels and an investigation of possible routes transmission.” PhD diss., June  Competition and disease model S – Susceptible, I – Infected, R – Removed (Immune) HG = SG + IG + RG, HR = SR + IR dS G (aG  qG ( H G  c R H R )) H G  bS G   S G ( I G  I R ) dt dI G   S G ( I G  I R )  bI G   I G dt dRG  I G  bRG Lifelong perfect immunity dt dS R (a R  q R ( H R  cG H G )) S R  bS R   S R ( I G  I R ) dt dI R   S R ( I G  I R )  bI R   I R dt  : recovery rate from virus of infected greys  : virus transmission rate  : mortality rate due to virus SIR – partial immunity – seropositive at second challenge Susceptible First Infection Post infection susceptible Second infection Partially Immune  SIR – Partial immunity SIR – Partial immunity – seropositive at second challenge Prevalence 25% Time Partial Between Immunity Infection 19 days Recovery rate Time between infection =  Fit prevalence and time between infection for partial of waning immunity but only if seropositive at second infection Transmission 4x in Reds than Greys – Grey transmission lower than we thought R0 similar in mixed habitat (Red 1.86, for Grey 1.93) – Very different epidemiology – 25% prevalence in Grey, very low in Reds  Competition Original SIR Model New Model -partial infection -seropositive at second infection  Conclusions Models are really useful in gaining an understanding of infectious disease processes from field data Partial or waning immunity with seropositivity after multiple challenge Not partial immunity on its own Not recrudescence  Conclusions Strategic models often wrong but useful even when we have limited data SIR – should maybe not be the default simplest model SIRS or SI ? Understanding how immunity works is the key