Population regulation: In class the other day we discussed how most populations do not exhibit exponential population growth - this type of growth is seen with species who are exploiting a new environment, such as the alien crabs and beetles which were unintentionally imported into the US. In general though most populations appear to exhibit either sigmoidal or oscillatory growth patterns, these growth patterns suggesting that some mechanism of density control is at hand. Question of the day: Given that a species has a 'choice', with respect to its population size ( and is not simply at the mercy of some chaotic catastrophic force such as a storm), is it in the interest of the species itself to control its' own population size or is it reasonable to assume that external controls dominate most species growth in the real world?. Factors that are known to act in a density-dependent way to regulate population size include both extrinsic factors, wherein one or more other species suppress the growth of 'our' population, or intrinsic where 'our' population attempts to regulates its own growth. We tend to assume from watching so many nature movies that in fact most populations out there in the wild are either controlled by their predators or competitors, but that humans and a few select superior species control their own number. In the following weeks we'll explore each of these forces in detail, but for now let's review what these potential regulatory controls are and how they may affect the integrity of our hypothetical population. Extrinsic vs. Intrinsic: a. Extrinsic: control factors are those that involve an interaction with one or more species of the community predation - it would appear to us that population's whose numbers were controlled by predators should be the most disadvantaged. Visions of the youngest and oldest members savagely attacked by snarling cats or packs of wolves leaves most of us with disturbing suggestions. Yet we have numerous examples where when humans have eliminated predators, resulting overpopulations of former prey fare not much better. Can you come up with such an example? Is it possible that the services of these predators actually put the prey population at advantage over the long run if in the process they help 'select ' out the less 'fit' individuals or through simultaneous predation of the prey's competitors help relieve overall stress on the population? parasitism- most of us can relate better to the idea of regulation through parasitism as most of us have at one time or another been victim of a particularly nasty parasite. Death itself via the parasite is not necessary for population control. The draining of an individual's resources may be enough to curb or reduce reproduction without killing the host. In most cases, though not all, parasitism as a regulator is only effective when either the population density is high already permitting easy transmission or when the population is already stressed out by other forces. Are there any "positive" results of population regulation by a parasite? Are there human examples of pop regulation by a parasite? interspecific competition; perhaps from a human perspective, population control by a competitor of another species seems the most 'fair'. A democratic type of 'selection of the fitness' with everyone having equal chance of survival based on their innate strengths. Can we view the case however where there is no clear cut winner in the competition ( both species compete and survive) as the best scenario, or perhaps the worse, with both species in constant conflict and with valuable resources used for competition rather than better survival. What factors would increase the competition? reduce it? b. Intrinsic controls involve the population's ability to response to its' own density and make the necessary adjustments. i. intraspecific competition: we are all familiar and accepting of this concept due to Darwin's theorem of survival of the fittest. When resources are abundant or changes in the environment stressful, this strategy 'makes' sense. However when resources are tight the loss of progeny appears wasteful, or when the environment is stable, when is to be gained by the loss of already tested genotypic stock? Perhaps the less genetically related we are ( or cognizant of the relationship) the easier it is to accept demise or forced non-reproduction of one's own population member or species due to our singular ow collective actions. Can you come up with an example of a 'gentile' means to enforce self-regulation of the population through means of competition? Let's look at intraspecific competition more closely- Exploitation: (scramble) each individual takes as much as it can of the resource base- Can you come with an example? Interference: (contest)- there is an interaction between the competing individuals; " sort of the winner is?" an example? Net result: interference- limited few win scramble- too many competing for the same resource destroy the resource base; all lose in the end What is territoriality an example of... exploitation or interference? Territoriality: only winners get space to mate or feed- once territory is recognized and accepted, energy cost is minimized; but if the winner is not truly accepted the cost over time may be high. What happens to the 'losers'? -> move out to less suitable area -> form a floating non-breeding sub-population ii. Immigration/emigration: when emigrants are accepted, certainly immigration to another population makes sense. If the new population has adequate resources, and the immigrant has something to offer via some fresh genetic stock, or services that population can use, immigration is a useful means to help alleviate high density populations. Yet there may be costs for both the immigrant and the accepting population. Can you suggest a few? a. Concept of metapopulations: It can be incredibly difficult for a population biologist to know where a population starts and ends unless there are close societal groupings, For example, in a large barn, are the clusters of mice subpopulations or distinct populations? How do you define where a population 'ends'? If in a mountain range we find distinct large clumps of maples, and given their pollen may move with the wind ( and thus lead to reproduction between widely spaced individuals) are all these groupings, 'subpopulations' forming one meta population or are they distinct populations as they are not competing directly for the same resources? This may appear to be a moot point but perhaps not. One example: you are wildlife manager responsible for a protected species. How protective must you get if one of the 'populations' starts dropping in number and you know that other 'populations' are stable or increasing in size? Another consideration involves the concept of the free ideal of habitat distribution..... b. Ideal free model of habitat distribution: S = suitability most suitable to least...............................> First fill the most suitable habitat till it overcrowds With overcrowding it becomes less suitable till it is as "suitable" as a less suitable surrounding site. This less suitable site eventually becomes less suitable as it fills till it matches the desirability of a still less popular surrounding site and so on it goes... How you view the population growth and its' regulation depends on when you happen to measure population density. Did you measure pop number when in the core site, in latter expanding sites? iii. Physiological & *behavioral regulation factors that affect reproduction & survival - in your text, find an example for each of these.... Endocrinal glands- hormone production which results in reduced reproduction; Immunological- stress due to increased social contact immunological depression & disease & death. Behavioral/genetic: aggressive behavior leads to passive not mating or immigrating away. Throughout this discussion of means of population regulation, you come to realize that no one factor decides it all- there is always the phenomenon of intercompensatory reaction: For example: if an external force such as climate radically changes the population size, a population will compensate by increasing or decreasing its' birth or death rates as need be. Birds which normally tightly regulate egg production will in bad years lay a second clutch. At times however if regulation is too tight, or beyond the control of the singular species ( via another species) that population may just die out. Are most populations in equilibrium? stabilizing at K? In today's environment it almost seems trite to ask this question. What sense does it make to ponder if populations ever reached their carrying capacities (K) given that we have so degraded or reduced the space that any other wild species survives in today? Yet we do need to better understand whether in fact population do stabilize and how. In trying to determine how much land we need to preserve to at least maintain the existence of some wildlife, we at the same time need to know what constitutes a valid and sustainable population number for the species in interest and what 'other' species we also need to help support which normally keeps the other in 'check' so that an near equilibrium is obtained. When we deal with outbreaks of alien, introduced species, how in fact can help regulate their population numbers? Unstable environment vs. Stable environment Another critical parameter to consider when trying to understand population regulation is where in fact does the species of interest derive from. Those species from seasonal/fluctuating temporary vs. constant as in the deep ocean, may use very different means to help regulate their population size. It has been surmised by ecologists that species from stable environments such as the tropical rainforests have a much more difficult time rebuilding their populations after much perturbation than those which come from environments which normally undergo radical shifts due to the environment Concept of "r" species- vs. "k-species"; After reading the text, come up with examples of species which are more likely to classified as an r or K species. An r species supposedly displays the following characteristics and a K species just the opposite: small size short-lived little parental care intra-sp. competition low high reproductive rate  These r like populations tend to have exponential growth and live in unstable environments. However not all species fit into this r or K prescribed mold. Groups like lichens or others that grow in "stressful" conditions may be small yet long-lived and so on. What is Grimes designation for such a species?