It was not that long ago that anyone who suggested that animals had "personalities" was seen as anthropomorphizing. While that criticism is not without some merit, research has shown that it may not be correct in all or most circumstances.
Research is showing that it is not just the meal of a particular goose or fish or other animal, but the literal fate of the gaggle, school, or group that can change depending on the particular mix of "personality types" within said gaggle, school, or group. Now that the idea of individual animals having a version of personality (or, more formally, "behavioral type") has become acceptable in scientific discourse, the next wave of research is looking at the consequences of the mix of personalities in a group, with some interesting results. Research is showing that in flocks, schools, or herds, the melding of behavioral types can be a matter of life and death, thus nudging a species' evolutionary trajectory. An individual animal's chances of finding food or starving, wooing a mate or dying without offspring, dodging a predator's jaws or becoming lunch have been found to change with different blends of behavioral types, as has the fate of a group as a whole. [It is also for this reason that preservation of ecosystems, not individual species, is important. But, I digress.]
While psychologists describe human behavior (simplistically) in five dimensions (conscientiousness, agreeableness, neuroticism, openness and extraversion - see What Are The Classifications Of Human Behavior), personality testing for nonhumans tends toward simpler terms; nonhuman animals can be shy or bold, aggressive or docile, social or asocial, and so on. The key to distinguishing a personality amid all of a creature's behaviors is whether the individual responds consistently across time.
There are a multitude of examples that can be used to illustrate these points, but for the sake of brevity I will mention just one, Mosquito fish [they are described in the first reference noted below]. If you have an even passing acquaintence with biological methods of mosquito abatement, you know of Mosquito fish. Mosquito fish, it turns out, are perhaps too good at conquering worlds; the fish have been bred and coddled and carried just about everywhere. Local governments in California maintain supplies to pour into swimming pools of foreclosed homes, and farmers stock the fish in rice paddies. The fish certainly gobble mosquito larvae, but if they make it into wild waterways they will also savage just about any other insect larvae.
The fish are loved by researchers studying how behavioral types influence the spread of invasive species. Researchers have set up different mixes of social and asocial fish at one end of artificial waterways with miniature pools connected by swifter-flowing riffles to see how far and how fast populations would expand from pool to pool. Asocial fish did indeed flee the crowds and move readily into new territories. When a population builds up, the asocial mosquito fish get crowded and move on to ever newer territory.
By, combining these studies and computer simulations, researchers have concluded that the most potent mix of types for fueling invasions turns out not to be dominated by loners; they keep moving from one new territory to the next without really building up big numbers in one place. For the best (and in a sense worst) invasions by this invasive species, a substantial proportion of social fish need to be mixed in with the less social ones. Thus, the loners swim away from the crowd and move into open habitat. When the social fish numbers get high and some of them spill over, there are already others of their kind, albeit the loners, around to keep the new territory from feeling isolated or void of their species. The population with the social fish thus sends in a second wave to build up the next crowd. Invasions progressing this way bring big numbers of the incoming creatures pushing across the landscape. In this way, the mix of personalities can govern not only a group's movements, but also a whole species' range. This also impacts the life for the rest of the communities residing alongside the expanding species. Draw different personalities out of a hat and there goes not just dinner or neighborhood welfare, but the tenor of the entire ecosystem.
Research on this topic can be found at http://rspb.royalsocietypublishing.org/content/278/1712/1670.abstract, http://www.sciencedirect.com/science/article/pii/S0003347210004707, http://psycnet.apa.org/?&fa=main.doiLanding&uid=2005-15797-014, http://www.sciencedirect.com/science/article/pii/S0003347208002236, & http://www.esajournals.org/doi/full/10.1890/11-0701.1.