Population Ecology What drives population cycles? ESRM 450 Wildlife Ecology and Conservation

Transcription

1 Population Ecology What drives population cycles? ESRM 450 Wildlife Ecology and Conservation

2 Population Fluctuations Density dependence tends to push populations toward carrying capacity, K Consequently, populations do not grow indefinitely (over long term) Yet they often don t rest at K either i.e., density dependence doesn t always lead to a static equilibrium Instead, at least some variability around K is the rule

3 Population Fluctuations Fluctuations around K can be dramatic, sometimes exceeding 1000-fold leading many ecologists to - explore their occurrence in different species - and ask why they occur Population Size Time K

4 Irregular vs. Cyclic Population Fluctuations Population fluctuations can be erratic (irruptive) Often due to variation in density-independent environmental factors that have a large, immediate impact on population size (e.g., fires, catastrophes) Or they can be periodic (cyclic) Populations exhibiting periodic cycles hit peaks and valleys in abundance at regular intervals

5 Evidence for Cycles Records of gyrfalcons (Falco rusticolus) exported from Iceland in the mideighteenth century indicate dramatic and regular population fluctuations Popularity of falconry waned

6 Why Do Cycles Occur? One answer: Cycles are the result of time lags in responses of populations to their own density i.e., delayed density dependence

7 Intrinsic Mechanism For Population Cycles That is, cycles derive from intrinsic properties of populations No environmental (extrinsic) change needed Populations acquire momentum when high birth rates at low densities cause the populations to overshoot K Overshoot of K causes very low survival and birth rates, population falls well below K Recovery occurs when birth rates again reflect current, lowdensity, conditions Key point: Population cycles derive from time delays in the responses of birth and death rates to current environmental conditions nature of cycle depends on nature of time delay Instant updating, no cycle

8 The Importance of a Time Delay: An Empirical Example Laboratory populations of Daphnia at higher temperature (25 o C), Daphnia magna exhibits oscillations: period of oscillation is 60 days (delay days) At high density, reproduction stops; doesn t begin again until senescent individuals die off (takes at least 10 days; a slow decline) Next increase phase needs recruitment of young, fecund individuals at lower temperature (18 o C), the population fails to cycle, because of little or no time delay of responses Minimal senescent period (rapid die off) Pratt (1944) Biol Bull

9 Sources of Intrinsic Time Delays Daphnia example neat because is implicates demographic process as source of time delay High survival of senescent, non-reproductive adults under crowded conditions in warm environment Other sources of time delays might include Slow development (current generation a product of past environment) Food (nutrient) storage: holdover from period of plenty buffers population from current conditions, for a while

10 Why Do Cycles Occur? Another mechanism: Cycles are the result of time lagged influence of density-dependent extrinsic factors e.g., delayed density-dependent predation, parasitism, disease

11 An Extrinsic Mechanism For Population Cycles Prey Population Size Time Lag Predation Rate Time Because of time lag, prey population overshoots K before predation (parasitism) drives it back down; can t grow toward K again until predation (parasitism) rate drops off

12 A Cycle Caused by Extrinsic Factors Across boreal forests of North America Trapping records of the Hudson s Bay Company show cycles in populations of snowshoe hares (Lepus americanus) and Canada lynx (Lynx canadensis)

13 The Snowshoe Hare Cycle Fluctuations are dramatic, characterized by a roughly 10-year period Lynx cycle lags a year or two behind that of hare cycle # of Pelts

14 Mechanics of the 10-Year Hare Cycle The 10-year snowshoe hare cycle consists of three phases Increase Phase Decline Phase Low Phase Krebs et al. (1995) Science

15 Mechanics of the 10-Year Hare Cycle Increase phase Time of plenty (lots of winter browse) High productivity, rapid (exponential) population growth Before long (2-3 years), K is exceeded and, all the while, specialized predator (lynx) populations growing as well Krebs et al. (1995) Science

16 Mechanics of the 10-Year Hare Cycle Decline phase Huge momentum shoots hare populations well beyond K Food becomes limiting (see picture), predation heavy So, productivity and survival plummet, triggering decline Decline is swift (1-2 years), driven by time-lagged predation Krebs et al. (1995) Science

17 Mechanics of the 10-Year Hare Cycle Low phase Following decline, hares are scarce, allowing food to recover Predators die off, disperse so survival is high Yet, low phase can persist for 4-5 years Why? Answer seems to be stress (time lag); i.e., the ghost of predation risk Boonstra et al. (1998) Ecology

18 Southern Snowshoe Hare Populations Along the southern periphery of the species range, cycle doesn t seem to exist. Why? Still debated, but Apparently, increase phase never occurs Instead, hares perpetually suppressed by suite of generalist carnivores (never die off, disperse) No predation time lag also, fragmentation allegedly intensifies predation