Intrinsic and extrinsic heterogeneity in spatially subdivided habitats: Importance and interactions

Ecologists often investigate the effect of spatial heterogeneity on populations. However, heterogeneity of population abundance can also arise in a uniform environment with local interactions or dispersal, a phenomenon called spatial pattern formation. Thus two kinds of heterogeneity may exist in a single system: extrinsic environmental heterogeneity to which a population reacts, and intrinsic heterogeneity which is induced by the processes essential to the population itself. This dissertation takes a theoretical approach to investigate both kinds of heterogeneity and the interactions between them.; In Chapters I and II, I investigate spatial patterning using coupled map lattice models. In Chapter I, I delineate the conditions under which patterning arises, showing that thresholds of both dispersal fraction and dispersal distance exist, such that patterning is most likely when dispersal distances are short and a large fraction of the population disperses every generation. When dynamics are highly chaotic, the threshold for dispersing fraction is higher, but the incidence of patterning depends on dynamical complexity in complicated ways. Finally, I show that in some cases assuming uniform dispersal underestimates the incidence of patterning compared to more realistic dispersal functions (Gaussian and negative exponential).; Models of pattern formation have assumed a homogeneous environment and produced the interesting result that spatial patterning can occur even when environmental variation is absent. However, in nature, environmental variation is extremely common, which has hampered empirical tests of patterning theory. Thus in Chapter II, I ask whether and how patterning will be manifested in a heterogeneous environment by manipulating the contrast and scale of differences in patch quality. I show that dispersal-induced patterning is robust to random variability at high and low levels, and to patterned variation, if the scales of heterogeneity and dispersal are markedly different. More generally, when extrinsic and intrinsic sources of heterogeneity are combined, outcomes may differ fundamentally, depending on whether the scales of heterogeneity are similar or different.; In the first two chapters, the emphasis is strongly theoretical. In the third chapter, I integrate empirical investigations concerning the response of passively transported biota to arrangement of habitat. In Chapter III, I hypothesize that subdivided habitat may be more favorable than aggregated habitat for passively transported animals. In my spatially explicit, individual-based model of small stream invertebrates, habitat arrangement accounts for a third or more of the variability in survival and in success in finding habitat. However, the subdivision effect decreases for passive long-distance transport. Empirical measurement of parameters related to passive vs. active movement showed that a subdivision effect may be reasonably expected in natural systems.