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Zoology 369 Gilbert's Lecture 1.
Distribution of Populations
INTRODUCTION
Species "occur" in regions where their component populations are present in at least some local sites. Range maps do not reflect how patchily most species are distributed. Populations of a species are found where habitat conditions allow component individuals to survive and reproduce. Questions about where species occur as viable (self-sustaining) populations and why they occur in particular places are fundamental problems of ecology and may often be quite distinct from questions about abundance and population dynamics.
In the lecture I discussed two major classes of explanation, historical and ecological, for why species occur or do not occur in particular areas.
1) Historical: A given habitat may be perfect in all ways for a given species' requirements yet that species will be absent. One obvious example is the southern USA before red imported fire ants were accidentally introduced. These ants evolved in grasslands of southern South America and had no way to move on their own through thousands of miles of intervening tropical forests to get here. The example given in lecture is the observation that pine trees (genus Pinus) do not occur south of Lake Nicaragua except for introduced plantations. This is because the South American continent was isolated from North America during the evolutionary radiation and spread of Pinus, (which has its world center of diversity in Mexico). Moreover, volcanic mountains providing suitable habitats for pines appeared only recently in the Central American isthmus. Many crop plants and weeds are successful because they have been moved from continents of origin to suitable environments on other continents.
2) Ecological: Contemporary features of a site may allow or disallow the existence of viable populations of a species known to be present within range of natural dispersal and colonization events. These fall into two major categories, abiotic and biotic.
a. Abiotic factors are physical forces that affect physiology of growth, reproduction, and probability of individual survival. Read Stiling Chapter 7 for a broader perspective. Some abiotic phenomena are highly local while some can predictably characterize large regions as well as local sites. Under this category I mentioned:
- soil chemistry: This is one category of so called edaphic factors. Toxic soils derived from certain types of rock are not suitable for plants which haven't evolved appropriate metabolic solutions to the problem. For example serpentine soils of California are the last stronghold of native annual grasslands which otherwise have been extirpated by European grasses introduced by the Spanish. These exotic grasses cannot grow well enough on serpentine soil to survive the biotic forces present. (Why would they have difficulty evolving local adaptation?).
- temperature: You might have gathered from my discussion in lecture that delinquents with 12 gauge shotguns control the distribution of saguaro cactus (and vice versa). My diversion into Darwin awards distracted me from the point, which was that saguaro is a good example of temperature influencing local distribution of a population. These grand plants cannot survive over 36 hours below 32 degrees. Even small changes in local topography (e.g. low places where cold air might settle and persist) might account for discontinuities in saguaro distribution. Also, as one ascends up a mountain occupied by this cactus, an elevation will be reached at which the occurrence of freezing temperatures for extended periods is too regular for the survival of this plant. We could call it the saguaro line.(book chpt.7 for this and related examples).
Alexander Von Humbolt first noticed that certain vegetation trends over increasing latitude were repeated on very local transects as he climbed up mountains. Thus what is seen moving from boreal forest to treeless tundra in Canada can be seen on Cerro Potosi near Saltillo, Mexico as one passes through low pine trees past the treeline into tundra-like vegetation. As we proceed north or south from the equator the average elevation of treeline drops (it's much lower on California's Mt. Shasta than say Volcan Orizaba near Veracruz, Mexico). Temperature is definitely involved with the setting of treelines but the phenomenon involves a complex interaction of temperature, water balance and energy budgets of trees.
- soil moisture: This edaphic factor depends on a site's proximity to the water table or to soil properties that influence drainage. Soil moisture influences the distribution of
plants and ground-nesting animals like fire ants. For example, fire ants thrive in areas where moist soil is available within a meter of the surface. They are often found in seasonally flooded areas and are adapted to rafting entire colonies to higher ground as water rises.
*see chpt 7 for more
b. Biotic Factors in this context include the direct or indirect influence of surrounding organisms on the population characteristics of a given species. In my discussion I mentioned the following examples:
1) Golden-cheeked warbler and black-capped vireo, two locally threatened and endangered species of great interest in the Texas Hill Country. The Golden-cheeked warbler requires old growth juniper/oak woodland. It makes nests only from the oldest class of juniper trees and additionally requires deciduous trees nearby for finding caterpillars on new leaves in spring. UT grad student Tom Engels discovered why golden-cheeks are sometimes absent from otherwise suitable habitat: they abandon their nesting territories if they hear the vocalizations of blue jays. Blue jay, an eastern bird reaching its western distributional limit in this area, is a nest predator which expands into the natural habitats of its western cousin, the scrub jay (which apparently warblers tolerate) when well-meaning people in those areas place bird feeders in their back yards. For some reason, blue jays are one of those native species, like cockroaches, raccoons, whitetail deer, grackles and cowbirds, which often increase in suburban anthropogenic landscapes. This is a very nice example of behavior influencing both distribution and abundance of a population and was published in the journal, "Conservation Biology" (vol. 8 pp.286-290, 1994).
Although it is endangered in the same region, the black-capped vireo has a very different relationship to the environment. Its nesting habitat is the scrubby vegetation which occurs part way through the transition from open grassland (just after fire or clearings made for pastures) and dense woodland. The human species, as a biotic factor, can be both positive and negative in respect to the vireo. By actively suppressing natural fires we can reduce the suitability of former vireo habitat. By accidentally introducing the imported fire ant, we added a major source of mortality for vireo chicks. In rural areas, brown-headed cowbirds, brood parasites, are a major source of nest failure, and in urban areas feral house cats are important. So, whether we find black-capped vireo in a site depends on a complex set of biotic influences beyond nesting habitat and food supply.
2) The mystery of the Costa Rican "spider moth," Eudulophasia invaria, is an example from my own exploration of Corcovado National Park, Costa Rica. This is a tiny day-flying moth is rarely observed except in widely scattered patches of its larval herbaceous host plant, the toxic Spigelia. However, most patches of Spigelia show no signs of this moth. The mystery was partly solved when I found that the moth female is not satisfied to lay eggs on the hostplant. She will only lay eggs in silk webs such as spiders produce! Of course these webs must be on or very near the host plant so that newly hatched larvae can locate leaves of Spigelia. The spider moth is distasteful and rejected by predators such as spiders. The spider moth and golden-cheeked warbler are examples of what I call "habitat selecting" species. Adults can assess a patch based on the resources and other biotic factors observed there and choose to stay or continue searching.
3) California editha checkerspot butterfly, Euphydryas editha, was presented as an example of a "habitat correlated" species. At least this is my diagnosis for the populations of the species occuring on serpentine outcrops in the San Francisco Bay Area, such as Stanford University's research area, Jasper Ridge. In that region this endangered species only occurs where a community of serpentine-restricted resource plants co-occur. Female editha butterflies at Jasper Ridge locate Plantago, for depositing egg masses in early spring (March). However, in this Mediterranean climate, a brief spring is followed by a dry summer. Plantago dies and dries up before the checkerspot larvae have grown large enough to reach the third instar (most butterflies undergo 4-5 stages of growth separated by molting of old skin. These stages are called instars). Third instar is significant because it is in this stage that the checkerspot survives to the next growing season, and resumes feeding soon after Plantago seed germinate in response to the return of rains in late fall. UT Professor Michael Singer, then a grad student at Stanford, discovered that when the oviposition host, Plantago, dried up in late spring, larvae survived only if they managed to discover a secondary host, Orthocarpus, a parasitic plant.
When editha checkerspot females are searching for hosts during their brief (less than two week) lives it is not possible to perceive those spots where Orthocarpus will be available (it becomes conspicuous later). If you were to release a mated female editha haphazardly in the area, how would it recognize a place to lay eggs? Certainly it can't be based on the only plant it wants to place eggs on, since Plantago is wide spread off serpentine soil. Eggs or larvae transplanted to lush patches of this plant do not produce editha populations. If you released millions of mated females over the area you would later find that some "lucked-out" and happened to oviposit on Plantago associated with all the more cryptic biotic factors that can't be judged by the egg laying females. In those lucky places populations would appear. In other words, under these circumstances, the occurrence of editha checkerspot correlates with habitat conditions and not because of any behavioral choices by the insects based on direct assessment of resources. Such "habitat correlated" species probably use indirect indicators of habitat suitability such as the presence of other individuals, in deciding whether to "stay or go."
Using social cues to make decisions about dispersal can lead to some unexpected outcomes. For example we found that individuals in low density editha populations had a greater tendency to disperse away even when host plants were great. Some populations apparently went extinct because these behaviors accelerate any downward trend in density. This is an empirical example of a phenomenon similar to the so-called "Allee effect."
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