Responses of organisms to abiotic factors (This is an overview. Later we will go into more details with examples of plants and animals)

Biological timing responses These are governed by internal clocks and are: 1.Annual cycles: responses to yearly changes – seasons. 2.Daily cycles: responses to length of day and night. 3.Lunar cycles: monthly responses, related to the moon. Tidal cycles are also related to the moon

Definitions Biological rhythm – a repeating cycle of activity that typically includes times of activity and inactivity. Period of a rhythm – time taken for one cycle. Biological Clock – an internal timing system that continues without external cues. Exogenous rhythm – controlled by external stimuli. Endogenous rhythm – controlled by an internal clock.

(Definitions continued) Free running – when an organism continues to show a rhythm in the absence of environmental cues it is “free running”. Free running period – the time taken for one cycle of a free running rhythm. Phase shift – when the start time of a rhythm is changed (starts earlier or later). Entrainment – Resetting the biological clock. Occurs on a regular basis in response to a zeitgerber. Zeitgerber – (‘time giver’) agent that resets a biological clock (e.g. daylight, high tide, temp.)

(Definitions continued) Circa – ‘about’. Endogenous rhythms are usually slightly different from the natural cycle so natural rhythms are circa -. E.g. Circadian = ‘about a day’. Photoperiod – the length of a period of light (e.g. a day). Photoperiodic Responses – responses of organisms to day/night lengths.

Rhythms Natural cycle Length of cycle Biological rhythm Known Zeitgerbers Solar year days CircannualPhotoperiod Lunar month 29.5 daysCircamonthly Light of full moon Solar day24 hoursCircadian Light, temp., humidity Tidal12.4 hoursCircatidalTide Spring tides 14.7 dayscirca-semilunarSpring tides

Example Humans show circadian rhythms with a period of about 24 hours. We have a biological clock that is reset daily (entrainment) using daylight as a zeitgerber. People who have lived for a time in a cave with no zeitgerbers become free running with a free running period of about 24 ½ hours. After a long flight to Perth (which is about 4hr behind us), we suffer jet lag as we undergo a phase shift as our biological clock is entrained by the different start time of the photoperiod. The End